Holocene sea level trend on the west coast of Bohai Bay, China: reanalysis and standardization

Holocene sea level trend on the west coast of Bohai Bay, China: reanalysis and standardization

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Jianfen Li¹^,², Zhiwen Shang¹^,², Fu Wang¹^,², Yongsheng Chen¹^,², Lizhu Tian¹^,², Xingyu Jiang¹^,², Qian Yu³^,⁴, Hong Wang¹^,²^,^*

Acta Oceanologica Sinica | 2021, 40(7) : 198 - 280

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Acta Oceanologica Sinica | 2021, 40(7): 198-280

• Marine Geology •

Holocene sea level trend on the west coast of Bohai Bay, China: reanalysis and standardization

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Jianfen Li¹^,², Zhiwen Shang¹^,², Fu Wang¹^,², Yongsheng Chen¹^,², Lizhu Tian¹^,², Xingyu Jiang¹^,², Qian Yu³^,⁴, Hong Wang¹^,²^,^*

Affiliations

¹ Tianjin Center, China Geological Survey, Tianjin 300170, China

² Key Laboratory of Coast Geo-Environment, China Geological Survey, Tianjin 300170, China

³ Key Laboratory for Coast and Island Development (Nanjing University), Ministry of Education, Nanjing 210023, China

⁴ School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210023, China

Published: 2021-07-25 doi: 10.1007/s13131-021-1730-5

Outline

Abstract

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Using 110 newly revised Holocene sea level indicators categorized into three types, sediments (67), shelly cheniers (27) and oyster reefs (16), this paper firstly provides a Holocene relative sea level curve, based on multiple approaches of litho- and biostratigraphies and sedimentary faces analysis, for the west coast of Bohai Bay, China. Following considerations, including indicative meaning, the paleo tidal pattern and range and conversion from mean tidal level to mean sea level, an apparent relative mean sea level (RMSL) curve was further reconstructed. After systematical calibration using CALIB, those of the 48 reworked samples were further corrected for the residence-time effect. Similarly, the younger ages for another 35 samples were chosen at the subsample level. These result in a younger-oriented shift for about 0.5 ka. Three local spatial factors, including neotectonic subsidence (average rate about 0.1 mm/a), self-compaction of unconsolidated sediments (between a few decimeters to about 6 m) and subsidence due to groundwater withdrawal (between a few centimeters to about 2.5 m), were quantitatively corrected. Finally, the amended RMSL curve after eliminating all these local temporo-spatial factors is very likely to show non-existence of mid-Holocene highstand and imply potential influences of both ice-volume equivalent sea level and regional glacial isostatic adjustment. Although it is still unable to divide both global and regional factors, the slowdown of sea level rise, in 7.5–6.8 ka with a maximum height less than +1 m, may constrain the model effort in the near future.

Key words

cheniers / oyster reefs / sediments / temporospatial corrections / local RMSL

Cite this Article

Jianfen Li, Zhiwen Shang, Fu Wang, Yongsheng Chen, Lizhu Tian, Xingyu Jiang, Qian Yu, Hong Wang. Holocene sea level trend on the west coast of Bohai Bay, China: reanalysis and standardization[J]. Acta Oceanologica Sinica, 2021 , 40 (7) : 198 -280 . DOI: 10.1007/s13131-021-1730-5

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1 Introduction

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Local relative sea level changes contain rich information on the mechanisms that reflect the spatial and temporal variations of the global sea level picture and constrain history of ice melting and isostasy. However, such information is useful only if the resolution of the data (i.e., the sea level indicators, sea level index points) is sufficiently high.

Holocene sea level change on the west coast of Bohai Bay, China, represents a typical example. This issue has been investigated intermittently for the last four decades, initiated by litho- and biostratigraphical studies (Wang, 1964; Zhao et al., 1978; Yang et al., 1979). Zhao and Zhang (1984), Xia (1981), Xie (1986), Xu (1994), Wang (1994) and Xie et al. (2012) preliminarily reconstructed the history of relative sea level change for Bohai Bay and/or other sectors along the northern mainland coast of China. These studies were evaluated by Pirrazoli (1991) in his atlas of sea level curves. However, as Woodroffe and Horton (2005) have indicated, the spatial and temporal uncertainties of these early indicators were too large, in particular, these indicators do not meet the generally acknowledged criteria established by van de Plassche (1982, 1986), Murray-Wallace and Woodroffe (2014) and Shennan et al. (2015).

For example, ¹⁴C dates from bulk-samples were presented as measured values (without isotope fractionation correction) and were not systematically calibrated for the regional marine reservoir effect and fluctuations in the atmospheric ¹⁴C activity (Wang et al., 2004; Wang and Fan, 2005). The influence of residence time effects on reworked samples was not recognized or corrected for (Shang et al., 2016).

The vertical spatial accuracy of the early indicators was on the order of decimeters or meters (Zhao and Zhang, 1984; Wang, 1994). Moreover, the approaches used to measure the elevations of several of the indicators were oversimplified. For example, many researchers believed that shelly cheniers formed along shorelines at high tidal positions (Zhao et al., 1980; Zhao and Zhang, 1984; Liu and Walker, 1989; Xu, 1994). Therefore, half of the tidal range or 2 m should be subtracted from the elevation measured to determine the contemporary mean sea level (MSL) (Zong, 2004; Sun et al., 2015). However, other researchers have found that the subsurface of shelly cheniers may dip seawards, with a 1–2 m vertical difference between the rear and front ends (i.e., between the landward and seaward edges), into a muddy intertidal flat (Cai, 1981; Xia, 1981; Wang, 1994; Wang et al., 2000a, b, c; Su et al., 2011). Therefore, one must carefully consider the depositional position of a chenier indicator and its relationship with the tidal environment or use a wider vertical error range if one cannot accurately determine its depositional position (Su et al., 2011). Recently, several case studies were able to precisely define the subsurface slope of cheniers and the relationship between the chenier and the tidal range for such indicators found just above the chenier subsurface^{11^①Shang Zhiwen, Li Jianfen, Wang Hong, et al. 2016. Report of the China Coastal Depositional Records on Climate Changes (in 　Chinese), 74–99} (Su et al., 2011; Wang et al., 2011a; Li et al., 2016a, b).

Another method for determining the indicative meaning is using the top surface of oyster reefs, mainly those formed by Crassostrea gigas. Early studies suggested that the growth of C. gigas reefs was controlled by low tidal levels and the reefs cannot build above low tide. Therefore, half of the tidal range should be added to the elevation of the surface to reconstruct mean tidal level ( MTL) (Zhao and Zhang, 1984). However, recent studies have reconfirmed that the elevation of the top surface of C. gigas reefs is mainly controlled by MTL, but not low tides (Wang, 1994; Zhang, 2004; Wang et al., 2006, 2011a, b, c). This has been shown by conchologists such as Okutani (2000).

Other than cheniers and oyster reefs, most sea level indicators are embodied in sediments; these types of sea level indicators include basal peat layers, which previously have been used by the local researchers (Zhao and Zhang, 1984) and the authors (Wang, 1994; Li, 2010; Wang et al., 2011a). Recently, several new efforts to integrate results from the area were restricted to basal peat layers obtained from cores in the coastal lowland and event-typed pulses of relative sea level changes found in cores in the adjacent shallow sea (Tian et al., 2017; Wang et al., 2020b).

In this paper, we attempt to establish an improved data set of the Holocene sea level indicators of the area; and the materials were obtained from sediments, shelly cheniers and oyster reefs (we reevaluated our previous data in Li et al. (2015) with a set of newly obtained data for the analysis). On such a basis, we establish a new local relative mean sea level (RMSL) curve, with systematic considerations for the local factors affecting the RMSL and discussions for the spatio-temporal position of the turning-phase of sea level change and possible constraints for the glacial isostatic adjustment (GIA) influence on the local RMSL trend. Finally, we try to approach potential ability to identify future research directions.

2 Regional setting

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The Bohai Sea is an inner sea of China, including its three embayments, i.e., Liaodong Bay, Bohai Bay and Laizhou Bay (Fig. 1). Our study area, the coast of Bohai Bay, is located in the west Bohai Sea, with a muddy coastal lowland being distributed over the northeast part of the North China Plain (Fig. 1). Today, the coastal plain is generally lower than 5 m in elevation relative to National Vertical Datum 1985, with gentle slopes at around 1:10 000. There are intertidal muddy flats, 3−5 km wide, with an average gradient of about 1:1 000^{22^②Pei Yandong, Wang Hong, Li Fenglin, et al. 2008. Tianjin Coastal Investigation Report (in Chinese), 7–13} ( Shang et al., 2016).

Geologically, the basement of study area, from the northwest to the southeast, includes the Jizhong Depression, the Cangxian Uplift, the Huanghua Depression and the Chengning Uplift, which are a set of secondary units of the Bohai Bay Basin (Fig. 1). Relative to the surrounding fold zones, the basin has slowly subsided since Paleogene rifting, being filled with mudstone, sandstone and psepholite, 3 000−5 000 m thick, during the Paleogene and sandstone and mudstone, 1 000−5 000 m thick, during the Neogene; in the Quaternary, fluvio-lacustrine muddy and sandy sediments were deposited with a maximum thickness of approximately 500 m^{33^③Li Jijun, Zhai Zimei, Shen Jian, et al. 2009. The Report of Tianjin Urban Geological Survey (in Chinese), 32–260} ( Zhang et al., 2010; Chen et al., 2013; Huang et al., 2014).

Holocene sediments, about 15−25 m thick, superficially covered the entire study area and are divided into three parts: the lower and upper parts with flood plain and salt marsh-lagoonal facies, and the intercalated middle part with marine transgressive−regressive facies representing intertidal to shallow marine environments. The Holocene strata are mainly composed of silts and clays with a small amount of fine sands, showing a continuous stratigraphic sequence. All these coastal fine sediments are favorable for recording and preserving information of sea level changes. However, due to such very fine characteristics the late Pleistocene and Holocene brackish shallow aquifers in this coastal area were often semi-confined and confined with high levels of mineralization (Cao et al., 2016). It may affect the ages of sea level indicators concerned (see the following discussion of this paper).

Furthermore, we speculate that the Holocene tidal pattern in the Bohai Bay could be similar to present, and the paleo tidal range is also similar if not even smaller (Shennan and Horton, 2002). This estimation is supported by the spatio-temporal distribution of Holocene shorelines showing an approximately concentric−circle configuration with present (Wang et al., 2010; Li et al., 2015; Shang et al., 2016). Therefore, the indicative meanings of Holocene sea level indicators are likely determined reasonably based on the present tidal regime (Fig. A1 and Tables A1−3).

At present, the annual average precipitation in the study area is about 600 mm (max. 897 mm, min. 332 mm), but the annual mean evaporation reaches about 1 740 mm (max. 2 673 mm). This striking contrast results in the presence of the salt-tolerant Artemisia-Chenopodiaceae-Pinus assemblage (Xu et al., 1986), including scattered Cyperus rotundus (?) and Suaeda salsa in the upper part of the intertidal zone. Such high evaporation directly causes poor development of vegetation in the salt marsh. For the recent decades, Spartina alterniflora, artificially imported from Jiangsu Province about 900 km southeast of the study area, has been thriving in the upper part of the intertidal zone. This newly intruded species modified the original characteristics of coastal vegetation.

3 Materials and methods

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3.1 Data sets of sea level analysis

For the western Bohai Bay, the three major types of sea level indicators used in this study are sediments, cheniers and oyster reefs (Fig. 2). Two remarkable morphological features, the shelly cheniers of the Chenier Plain and the linear earthy mounds on the Oyster Plain, delineate the ancient shoreline positions and control the distributions of the sea level indicators (Wang et al., 2010; Shang et al., 2015; Shang et al., 2016) (Fig. 2).

Our previous work has determined 136 indicators from more than 530 radiocarbon-dated samples and their spatial positions (depth in cores and elevation/depth in outcrops) along the coast of Bohai Bay (Li et al., 2015). All the procedures for ensuring reasonable judgement of spatial and temporal information of relative sea level (rsl) indicators and further conversion to the corresponding RMSL have been conducted. Here, reference water level (RWL) and indicative range (IR) are based on van de Plassche (1986) and Shennan et al. (2015), with reference to the regionally generalized summary of Woodroffe and Horton (2005). Also, local residence-time corrections for both reworked shells and plant materials have been attempted (Shang et al., 2015; Shang et al., 2016; Li et al., 2015). Consequently, a RMSL band was obtained and compared preliminarily with the model-predicted regional relative sea level curves^{44^④Lambeck K. 2007. Paleo Reconstruction for China Sea. Lecture at Graduate School of Chinese Academy of Sciences, Beijing}^{55^⑤Okuno J. 2009. Glacio- and Hydroisostasy and Sea Level Changes since the Last Glacial Maximum. Lecture at East China Normal 　University, Shanghai} (personal communication, Lambeck, 2014; Li et al., 2015).

In this study, we abandoned 34 “limited” indicators from the original 136 data (Li et al., 2015). These include (1) those sea level indicative meanings are relatively less clear. For example, the indicator Yujialing/TD356 was taken from a pit of bridge-pier. It could be estimated only in intertidal to subtidal depth because pier digging did damage to original reef sequence (Li et al., 2015) but now it is cautiously thought such a depth range estimated is too wide. (2) A vertical range between basal peat layer and overlying clearly marine-influenced sediment, identified by firaminifera, ostrocoda or diatom, is too large. For example, definite marine evidence was found 1 m or even near 2 m above the indicators ZH2/4/98Y083 or CH19/BA06817 (Li et al., 2015). Therefore, their capability indicating the contemporaneous sea level elevation is relatively weak. Afterwards, on the other hand, 8 newly revised data are added. Finally, 110 indicators are represented in this study.

Furthermore, a number of additional quantitative considerations for the local tidal regimes with estimation for the paleo tidal patterns, conversion from mean tidal level to MSL, standardization of different elevation systems and corrections for the local factors affected spatial characteristics including neotectonics, self-compaction and groundwater abstraction are given in this study.

3.2 Determination of the validity of the sea level indicators

Based upon the studies on the Holocene basal peat in core sequence and the modern coastal vegetation zonation in the southern Changjiang River Delta, together with the present tidal information from the five local gauge stations there, the Holocene basal peat was formed between the highest high water (HHW) and mean high water (MHW) (Wang et al., 2012b, 2013). Following this judgement, we chose an average 4.10 m of the observed highest high tidal ranges (in period of 1960−1979, Tanggu Station) and an average of 5.16 m for the predicted astronomical tidal ranges (four stations in the west of Bohai Bay) (Liu et al., 1986) and mistakenly used an average 4.63 m of both values as an estimated approximate maximum range of the local HHW and the lowest low water (LLW) (Li et al., 2015). Then, to compare with the 2.5 m mean tidal range (Liu et al., 1986), a vertical difference of 1.05 m (i.e., a half of difference between HHW−LLW and MHW−MLW ranges) was obtained for the IR for the local basal peaty samples and the midpoint RWL is 1.775 m above MSL. Finally, an indicative meaning of (1.8±0.5) m (rounded from (1.775±0.525) m) was used to be subtracted from the peaty sample’s altitude to reconstruct RMSL (Li et al., 2015).

However, the following conditions should be considered in applying this method for the basal peat indicators. (1) The basal peat layers are a proxy of the vertical space between mean higher high water (MHHW) and MHW, or mean high water springs (MHWS) and MHW (van de Plassche, 1982; Shennan et al., 1983; Törnqvist et al., 1998; González and Törnqvist, 2009), but not between HHW and MHW. Even for Wang and her co-authors themselves, the IR, has been changed from HHW−MHW to MHWS−MHW now (Wang et al., 2018). (2) A potential mistake was not being aware of the difference between MTL and MSL and assuming they are equal (Li et al., 2015) even though the local MTL is actually approximately 15 cm lower than the MSL when higher tides are taken into consideration^{66^⑥DOE (Dagang Oil Exploration Bureau and State Information Center of Oceanography). 1991. Analytical Study on Marine 　Environmental Conditions of the West Bohai Bay (in Chinese), 1–11}^{77^⑦FDI (China Communications First Design Institute of Navigation Engineering and National Ocearnographical Information Center). 　2006. Analytical Report on the Waves, Tides and Storm Surges for the East Port Area of Tianjin Port (in Chinese), 1–70}.

In this study, we changed to use vertical space between MHHW and MHW as IR for the basal peat layers. Consequently, the reconstructed RMSL is about 0.5−0.7 m higher than using 1.8 m and the IR is much narrower than in our previous paper (Li et al., 2015) (Table A3). Taking the same peat layer in the bayhead area as an example, a newly reconstructed RMSL will be 54 cm higher because RWL+IR, (1.26±0.05) m, is subtracted in this study instead of subtracting (1.8±0.5) m. For the south periphery area, the difference is even larger, up to 0.7 m; i.e., compare the (1.1±0.03) m value in this study to the (1.8±0.5) m from Li et al. (2015) (Table A3).

For the other sediment indicators, either at approximately high waters, low waters or just in intertidal depth, their corresponding indicative meanings are given for bayhead and periphery, respectively (Table A3). In general, those formed at high waters, 1.3 m or 1.1 m should be subtracted; those formed at low waters, 1.15 m or 1.05 m should be added; those formed particularly at approximately mean lower low water (MLLW), 1.6 m or 1.45 m should be added, respectively, for bayhead or southeast periphery (Table A3). However, in our previous study, 1.5 m (a half of mean high tidal range) or 2.3 m (a half of the mistakenly recognized maximum tidal range of 4.63 m in Li et al. (2015)) were simply subtracted or added, respectively.

For the indicators, which are known formed in the intertidal depth but without tangible evidence to indicate their precise positions, we have indicated a vertical range of their intertidal depth. Therefore, its estimated RMSL is just their existing elevation with a large error of ±1 m. Such a treatment is the same as performed previously (Li et al., 2015).

In Li et al. (2015), the chenier samples collected from literature were commonly recognized to be from the intertidal zone only; therefore, no vertical compensation was needed. However, with further consideration on sedimentary facies now, the concrete sampling positions (either above the front-, the mid- or the rear-base) are determined when possible. As a result, for those formed above the mid- and rear-parts of the subsurface, certain compensations are given (Fig. 3; Tables A3 and A4). The oyster reefs’ samples were taken exactly from the reef-top enable to indicate mean tidal level directly. Then, adding compensation of 15 cm, a converted RMSL can be estimated, then with an error ±0.7 m as practically observed undulation of the flat topography (Tables A3 and A4).

3.3 Determination of the local tidal regime

Tidal terms used in this study refer to those from van de Plassche (1986), Lin and Sun (1987) and Shennan (2015). The present tides in the west coast of Bohai Bay have an unequal semidiurnal pattern⁶⁷ and thus, the “mixed tidal pattern” (Shennan, 2015) was used. The data of the present local tidal ranges, measured in four gauge stations from north to south along the west Bohai Bay, were collected and carefully collated (Figs 1 and A1; Tables A1 and A2).

Studies are rare about the local paleo tidal regime in the study area. Given an example of both the Humber and the Wash embayments, Shennan and Horton (2002) further corrected the MSL elevation of the sea level indicators, based on the modeled paleo tidal ranges of 6 ka and 3 cal ka BP, which are 2.5 m or 1.6 m, respectively, less than a half of the present high tidal ranges. However, Shennan and Horton (2002) also indicated that for those of open coasts, difference between the present and the paleo tidal ranges are relatively small. The paleo morphology of the west Bohai Bay, delineated as concentric-circles of the paleoshorelines determined by both the chenier and the earthy mound chains (Wang et al., 2010; Shang et al., 2015, 2016) (Fig. A1), was quite similar with its present configuration. Therefore, the present tidal ranges (Tables A1 and A2) were used for the Holocene period.

The present shoreline of Bohai Bay had been completed continuously during the last 1.5 cal ka BP, following the formation of Chenier I (Wang et al., 2010; Shang et al., 2016). Therefore, the indicative meaning of samples, younger than 1.5 cal ka BP and distributed along the present shoreline, are derived based on the present tidal regime. However, prior to 1.5 ka, the paleo Bohai Bay extended further inland, and the paleo bayhead directed slightly northwestwards to the Oyster Plain with an extensive south periphery (Fig. A1). Sea level indicators older than 1.5 ka should comply with the paleo tides and paleo morphology, with estimated tidal ranges given in Table A3.

A small difference that should not be ignored between MTL and MSL is approached in this study, although we were not aware of this difference in our previous work or in all the early literature. According to the tidal gauge station in Tanggu (Tianjin New Port), in the present bayhead area, the MTL between MHHW and MLLW is 15−16 cm lower than the local MSL, while the MTL between MHW and MLW is only 2−2.5 cm lower than the local MSL⁶⁷. On the other hand, the local MSL in Tanggu Station is only 2 cm lower than the national MSL of National Vertical Datum 1985⁶⁷. We think therefore that interchangeability between the local and the nationwide MSLs is satisfied in this study. However, when using either MHHW or MLLW as being the RWL, a difference of about 15 cm between MTL and MSL must be taken into account (Tables A2 and A3). For example, at bayhead area, if a sea level indicator is determined as MHHW or MHW, (1.3±0.5) m should be subtracted for restoring MSL; however, if an indicator was formed at MLLW, (1.6±0.5) m should be added for its contemporaneous MSL (Table A3). Another example is that the actually observed elevation of reef-top for each oyster reef is indeed a proxy of MTL elevation, and thus, 15 cm should be added to MSL reconstruction (Tables A3 and A4.3: see the notes of the indicator No. 95). In a word, simply using a half of tidal range as MSL as we did before would provide about 15−30 cm deviation during MSL reconstruction in the coast of Bohai Bay.

3.4 Unification of different elevation systems

During the last two decades, we have used the National Vertical Datum 1985 to determine the elevation of our own samples. However, many previously published data, especially those before the 1990s, were based on different local datum systems or the old national system. In our former paper (Li et al., 2015), we have converted all the elevation values of the previously published data, relative to 1951 or 1972 local datum systems, into the National Vertical Datum 1985 by subtracting 1.543 m or 1.668 m from those original values, respectively; while for those values of the 1956 national system, 0.029 m should be subtracted to the National Vertical Datum 1985, based on an authoritative document issued by SMG^{88^⑧SMG (Tianjin Administration of Surveying, Mapping and Geoinformation). 1998. Notice for Start Using the 1972 Tianjin Dagu 　Elevation System and Converting with the National Datum (in Chinese)}. In this paper, the same procedure is still performed for all the indicators used.

4 Results

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4.1 Sea level indicators from surficial/cored sediments

We use the vertical space between MHHW and MHW as the growing space for basal peat. Thus, the newly revised indicative meaning is (1.255±0.045) m (i.e., (1.26±0.05) m) for both the present and ancient bayhead areas and (1.105±0.025) m (i.e., (1.1±0.03)) m for the ancient southeast periphery (Table A3). Those basal peat layers, formed in the present and ancient periphery sectors, will be treated as the samples in bayhead but based on their own tidal properties given in Table A2, although only small differences exist in between the bayhead and periphery subareas. The indicative meanings for these basal peat layers are (1.1±0.03) m for the south periphery and (1.15±0.03) m for the north periphery (Table A3). Each indicator was processed using the same procedure but its own tidal property and there is no need to go into more details in the following context. Detailed descriptions can be found in Appendix A.

The upper peaty layer, directly overlying on the Holocene marine and brackish sediments, was much less developed and preserved than the basal peat in the study area. However, a number of proxies, including organic-rich soil horizons and peaty mud and hydromorphic soil horizons in lagoonal-salt marsh environments, were probably discontinuously influenced by high tidal levels. Therefore, the upper peaty layers and proxies were treated in the same way as the basal peat layers were in this study.

The depositional mechanisms of the in situ thin organic-rich layers intercalated with marine-influenced, brackish muddy sediments are similar to those of the upper peat layers; their indicative meanings further support these interpretations. For example, indicator No. 53, a darkish organic-rich muddy layer less than 5 cm thick, intercalated with brackish muddy sediments, has a foraminifera assemblage consisting of 12 tests of Nonion glabruum and a single test of Ammonia beccarii, indicating a relatively weak influence from high water levels. Therefore, such intercalated thin organic-rich layers are treated in the same way as the peat layers are in their RMSL reconstruction (Table A4.1).

The other sedimentary indicators are composed of various marker layers. Lithostratigraphical logging, facies analysis and micro paleontological studies can determine indicative meanings. For indicators formed at MHHW, 1.3 m was subtracted, with an error ±0.5 m; for indicators formed at MHW, 1.21 m was subtracted (Table A2). Here, we simply used (1.3±0.5) m for both MHHW and MHW for the bayhead samples (Table A3). For those samples formed at approximately MLLW, (1.6±0.5) m was added, while for those formed at approximately MLW, (1.15±0.5) m was added for bayhead samples (Tables A2 and A3). The indicative meanings of samples formed in an intertidal zone are the altitudes of the samples, with an error ±1 m. For example, sample No. 7, a thin layer of lagoon-salt marsh sediments in the ancient bayhead area was formed by a flooding event, revealed by its foraminifera assemblage (Li et al., 2004) (Table A4.1). Therefore, 1.3 m, the vertical difference between MHHW−MTL (Table A2) was subtracted to reconstruct the RMSL (note that the relationship between the local MTL and MSL is described in Section 3.3).

4.2 Sea level indicators associated with cheniers

In earlier studies, the subsurface (i.e., the base, the bottom) of a chenier was thought to be spread along or above the MHHW shoreline, on which reworked shells and shell hash, mixed with fine sand and mud, accumulated (Russell and Howe, 1935; Price, 1955; Wang, 1964; Otvos and Price, 1979; Reineck and Singh, 1980; Zhao and Zhang, 1981; Zhao et al., 1980; Gu et al., 1983; Li and Li, 1987; Zhao, 1989; Wang and Ke, 1989; Liu and Walker, 1989; Xu, 1994).

However, several other researchers argued that the subsurface of chenier formed in intertidal zone, especially during its initial stage (Woodroffe et al., 1983; Weill et al., 2012) and dipped seaward (Neal et al., 2002; Rodríguez-Ramírez and Yáñez-Camacho, 2008; Weill et al., 2012). The vertical difference between its seaward front and landward rear of the subsurface is 1−2 m (Rodríguez-Ramírez and Yáñez-Camacho, 2008; Weill et al., 2012) or 2−3 m (Hijma et al., 2017). On the coast of Bohai Bay, several previous studies indicated that the seaward front of the subsurface declines into the intertidal zone and is buried by late intertidal muddy sediments. The seaward front of the subsurface was >2 m lower than its landward rear, with slope ratio of 13‰−19‰ (Xia, 1981; Xie, 1986; Cai, 1981; IOG, 1985).

Figure 3 shows a comprehensive model of the local shelly cheniers based on our own investigations (Wang, 1994; Wang and van Strydocnk, 1997; Wang et al., 2000a, b, c; Su et al., 2011), which have provided more sedimentary evidence for the indicative meaning of the local cheniers.

Leveling investigations of the subsurface, using an Eijkelkamp gouge corer (25 mm or 50 mm in diameter) and a total survey station, revealed that the chenier subsurface dips seaward with slope ratio of 5‰−20‰ (maximum 22‰−27‰), based on 22 Eijkelkamp-core sections conducted perpendicular to five chenier chains (Cheniers IV-1, III, II-1, II-2 and I) at an average surveying distance of 105 m and a mean vertical difference between the seaward and landward ends of about 1 m^{99^⑨Shang Zhiwen, Su Shengwei, Wang Hong, et al. 2013. Report of New Discoveries on the Cheniers and Oyster Reefs in Coast of Bohai 　Bay (in Chinese), 1–104} (Su, 2012a; Su et al., 2011; Wang et al., 2000a, b, c; Wang et al., 2020a).

As described in the first section of this paper, correctly using shelly cheniers as sea level indicators requires knowledge of the sampling position. The subsurface of the chenier body tilts seawards and can be separated into three parts: the front-, mid- and rear-base. Only samples taken closely above the subsurface can be used because the subsurface and the sediments immediately above it have a connection to tidal levels. Therefore, most of our chenier samples were collected immediately above the subsurface and a few were collected from the very top of the underlying muddy sediments. Sediment above the front-part, i.e., the foot of the chenier, is wedged into intertidal muds and is ideal for sampling because this part was undoubtedly deposited in the intertidal zone and can be used as index point directly indicating intertidal depth. However, the sediments above the mid- and the rear-part of the subsurface need microfossil information for interpretation; e.g., the mid-part of Chenier III in Jugezhuang site (Li et al., 2016b; the indicator No. 71 for example in this study). Radiocarbon-dated shells, taken from the mid- to upper parts of chenier body, accumulated from strong waves during storm surges and generally are difficult to precisely and convincingly use to reconstruct contemporaneous sea levels. The previously dated samples collected from the mid- and upper parts of cheniers were distinguished (Wang and van Strydonck, 1997) and abandoned (Li et al., 2015) (Fig. 3).

Along the coast of Bohai Bay, many pits, dug two decades ago by the local fishermen for shrimp culture in the present muddy upper tidal flat, expose the buried foot of Chenier I in the bayhead and its south periphery (Figs 4a and b). This deposit contains shelly fragments (commonly a few millimeter to 1 cm) mixed with silt and clay and is poorly sorted, showing roughly wavy bedding of alternation between shell hash-dominated and mud-dominated layers (a few centimeter thick each), with peculiar penecontemporaneous gravity-deformation (irregular convolute-bedding). The color is dark and indicates a reducing environment (even shells and foraminifera tests were dip-dyed to a dark color).

Moreover, numerous reworked shell hash and broken shells, several of which are still distinguishable as Mactra veneriformis, Scapharca sp., Rapana, Solen sp., and Glossaulax sp., and other marine mulloscs, were mixed with the underlying muddy sediments beneath the front-base. Thus, the top of the underlying muddy sediments below the front-base indicated the intertidal depth. Both samples, taken from the chenier foot and the top of its underlying mud, indicated the intertidal depth and their indicative meaning is the sample altitude ±1 m (Table A3).

In contrast to the underlying mud immediately below the front-base, muddy sediments beneath the mid-part, especially beneath the rear-part, were less influenced by sea water. These muddy samples and the shell samples taken from above the mid- and rear-part are thus preferable for study of microfossil assemblages. Here, we present a case study. Continuous Eijkelkamp core sampling from the underlying mud upward to the lower part of shelly chenier was conducted across the mid-part of the subsurface (Fig. 3) in Section J341-13, Jugezhuang site, Chenier III (Li et al., 2016b; Su et al., 2011). Foraminifera from the top of the underlying mud resulted in 79−692 tests per10 g dry sample, composed of Ammonia beccarii vars.−Elphidium nakanokawaense−Nonion glabrium assemblage with A. granuloumbilica and A. takanabensis, which is quite similar to the present tidal flat assemblage of L3 between MHW and MHWN (Li et al., 2016a). For the samples immediately above the subsurface in this core, 2 016−6 979 tests per 10 g dry samples, mainly consisting of A. beccarii vars.−A. confertitesta assemblage with small amount of E. simples, A. granuloumbilica, A. anntetens, Pseudononionella variabilis, P. tuberculatum and so on, were found and are similar to the present tidal flat assemblages L2 and L1, found at approximately MHW−MHHW (Li et al., 2016a). Therefore, the samples taken immediately above the mid-part of subsurface can be roughly recognized as representative of approximately MHW. Correspondingly, the indicative meaning is calculated by subtracting (1.2±0.5) m (rounded from 1.21 m) from bayhead sites and subtracting (1.1±0.5) m (rounded from 1.08 m, Tables A2 and A3) from south periphery sites. For the samples taken immediately above the rear-base, (1.3±0.5) m or (1.1±0.5) m should be subtracted for the bayhead and south periphery sites, respectively (Tables A2 and A3).

4.3 Sea level indicators with oyster reefs

Crassostrea gigas has been found from the East Siberian coast, Hawaii, Japan, through the Chinese coast, to Vietnam and Thailand, and lives in shallow waters to the intertidal zone (Johnson and Foster, 1951; Kira, 1975; Scarlato, 1981; Chonglakmani et al., 1983; Wang, 1994; Okutami, 2000; Wang et al., 2006). Relative to the genus Ostrea, which prefers deeper water, the species of Crassostrea can grow from subtidal to intertidal depths in shallow bays, estuaries, tidal-influenced mouth-channels and lagoons (Stenzel, 1971; Hopkins, 1979). Crassostrea requires higher summer water temperatures to successfully propagate (spawning, fertilization and larval growth) since they are all nonincubatoric. However, they can tolerate extreme low temperatures in the winter season. Therefore, accumulating in intertidal zone or very shallow water is normal for the species of genus Crassostrea (Stenzel, 1971). C. virginica, crowded with dense clusters of living and dead specimens in the intertidal zone, can be seen along the coast from the southern Atlantic states to the Gulf, USA. An extreme example is in estuary near Wadmalaw Island, South Carolina, where the living reefs of C. virginica are exposed twice daily during normal low tides (Stenzel, 1971; Hopkins, 1979; Davis and Fitzgerald, 2004). We found that the top of living C. gigas reefs can reach an elevation of MTL in the intertidal zone of Dashentang, Tianjin (Wang et al., 2011c). Similarly, in the intertidal area of the Xiaomiaohong site, Yellow Sea, Jiangsu Province, about 900 km south of the study area, the modern C. gigas reefs grow to MTL (Zhang, 2004).

Figure 5 shows a comprehensive model of the local oyster reef build-up based on our investigations^{1010^⑩Wang Hong, Li Jianfen, Yan Yuzhong, et al. 2002. Formation and evolution of the oyster reefs on the Oyster Plain, Bohai Bay, Special 　Report for the Huaidianxiang Sheet (J50E005015, 1: 50, 000) (in Chinese), 1–57} (Wang, 1994; Wang et al., 2006, 2011b, c, 2012a; Fan, 2008, 2010; Liu, 2010; Li et al., 2020), which have provided more sedimentary and ecological evidence of its indicative meaning.

Bed 1. Thin layered and/or laminated alternation between muddy (clayey and silty) and fine sandy−silty sediments, brownish gray to black brown (10YR 6/1−3/1), mixed with scattered marine shells such as articulated Scapharca kagoshimensis, Mactra veneriformis, Dosinia sp., Cyclina sinensis, Potamocorbula laevis and their broken valves, and gastropoda shells Terebridae and Nassarius sp., etc. It was undoubtedly a marine environment.

Bed 2. Oyster reef, densely populated, vertical growing in situ articulated oysters, Crassostrea gigas shells, with horizontally intercalated layers of disturbed individuals (mainly still articulated but a few disarticulated due to reworking), indicates alternation between tranquil and relatively unstable environments during the reef building. Trapezium liratum shells, articulated, were the most common concomitant species; also less common are Rapana, Scapharca kagoshimensis, Ruditapes philippinarum, Cerithidea sp., Assiminea sp., Stenothyra sp. and occasionally crab chelae and even teeth of Actobates sp. Clayey mud filled in between the crowded individuals, representing about 10%−20% of the normal growth layers, but >50% of the disturbed horizontal layers.

Bed 3. Clayey and silty mud, brownish gray (10YR 6/1−4/1), with scattered broken shells and articulated larvae of Potamocorbula laevis. Corbicula fluminea, and Terebridae and Cerithidea sp. also existed. A few Solen sp. shells are articulated and in a vertical in situ position. Usually, this mud deposit coarsens upward, especially when found in small depressions on the reef-top, implying less sedimentation in such depressions to normal accretion on an open tidal flat. In the small depressions, about 30−50 cm thick, nonlaminated, organic-rich clayey mud, brownish gray to dark brown (10YR 5/1−3/4), accumulated in a stagnant reducing microenvironment (Fig. 6d). The ventral margin of the shells on the upper most top of reef body is still sharp and without truncated and eroded traces. This implies a quiet transition from reef to overlying mud (Fig. 6d). Coarsing upward is present and a gradual change to alternate laminae of clay and silt-very fine sand or silty-sandy lenses, commonly <1 cm thick and less than 5−10 cm long, irregularly appear in the muddy matrix. These alternations and/or irregular silty-sandy lenses are typically of tidal bedding structure (Reineck and Singh, 1980; Davis and Fitzgerald, 2004). Nevertheless, the reef-top was sharply eroded and the ventral margin of the upper most shells was clearly truncated and some shell individuals were even removed from/horizontally paved on the reef top (Fig. 6e). These two circumstances, the change from reef to the overlying mud either conformably or erosionally, occurred at approximately MTL. However, most of the reef top was only slightly eroded and the samples taken from the very top of reefs are still able to indicate MTL.

A total thickness of Bed 3, i.e., between the reef-top and the upper boundary of the tidal bedding structure^{1313^⑪Wang Hong, Wang Yunsheng, Yan Yuzhong, et al. 2002. Report of Baishuitou-Qikouzhen Sheets (J50E008015, J50E009015, 1: 　50 000) (in Chinese), 1–128} (the last irregular silty/sandy lamina, LISL; Wang et al., 2006), was observed to be 1.2−2 m in Biaokou, 1.1−2.2 m in Beihuaidian, about 1 m in Lingtou, 1.4−2.4 m in Dawuzhuang, about 2.1 m in Binhaihu and 1−1.3 m in Konggang sites. Generally, the average thickness is 1.6 m (Wang et al., 2011c). The local mean high tidal range is about 3−2.8 m from the bayhead to the two periphery sides (Tables A1 and A2) and the reef top may build to MTL; therefore, a range of 1.6 m is an accommodation space that the high tides may provide to a vertically depositional capacity from the mean tidal level (i.e., the reef top) upward to approximately MHHW. This is counterevidence showing that the top surface of reefs was essentially controlled by MTL. Thus, the top surface of C. gigas reefs can be directly used as a MSL indicator after conversion from mean tidal level to MSL (see Section 3.3), with an undulation error ±0.7 m (see below).

Bed 4. Muddy sediments, dull yellow orange to dull yellowish brown (10YR 6/3−5/4), garlic structure, freshwater Gyraulus sp., Bithynia sp., Metodontia sp. and brackish articulated Solen sp., Glauconome primeana were scattered in situ in the muddy sediments. This represents a lagoonal-salt marshy environment with a few sea water flooding events (e.g., indicator nos 7 and 8, Table A4.1), and sometimes, a short gap existed in between this layer and the underlying intertidal mud deposition (Li et al., 2004).

The top of local C. gigas reefs is often slightly undulated. Leveling investigations for 19 spots at three buried reef sites, including Dawuzhuang (Fig. 6a), revealed that the top undulations for each site are 1.28 m, 1.4 m and 0.46 m, respectively (Fan et al., 2005; Liu, 2010; Wang et al., 2011c; Wang, 2012). Based on the maximum undulation of 1.4 m, we therefore use ±0.7 m as a vertical error when using the top-elevation of the reefs as MSL indicator. In Xiaomiaohong site, the top-undulation is similarly as 0.5−1.0 m (Zhang, 2004). Recently, a choosing-the-site investigation for envisaged Oyster Reef Museum was carried out in Ninghe County, Tianjin (about 3 km east of Chaobaixinhe River, about 30 km down from the top-frame of Fig. 2). In total, 35 rotary drilling boreholes were implemented penetrating the reef body over an area of 240 m×120 m within an extensive reef. Observed elevations of the top surface revealed in all 35 cores was between −3.522 m and −2.260 m (National Vertical Datum 1985) and thus the maximum undulation is 1.26 m (Qin et al., 2017) (Fig. 6f). This result indicates that this newly investigated result is consistent with general estimate of l.4 m obtained by the previous studies.

4.4 Distribution of RMSL indicators

All the ¹⁴C ages of the 110 indicators are calibrated by CALIB Series 7 (Reimer et al., 2013) with ΔR=−178±50 a for the marine shell samples in Bohai Sea (Southon et al., 2002). For those nonconventional dates, which were obtained from the literature in the 1980s−1990s, lacking of δ¹³C values, CALIB 3 (Stuiver and Reimer, 1993a, b) is first used with either the local empirical mean value of −2.68‰ PDB for marine shells (Wang, 1994; Wang and van Strydonck, 1997; Li et al., 2015) or recommended δ¹³C values given by Mook and van de Plassche (1986) for other samples to fulfill the isotope fractionation correction and obtain their approximate conventional ages, and then, IntCal13 or Marine13 of CALIB Programmes 7 were used for further systematic calibration (Reimer et al., 2013; http://calib.qub.ac.uk/calib) (Table A4). The 2σ range of calibrated age and the median probability are listed simultaneously (Table A4). For graphical purposes (Törnqvist et al., 2015; Hijma et al., 2015), the median values are illustrated in this paper (Figs 7 and 8).

The spatial positions are depicted qualitatively by using a few different symbols. For instance, indicators of the peat layers, showing high waters between MHHW−MHW, are illustrated as “┬” in time-depth graph (Fig. 7a). Indicators that were formed around low waters, i.e., the lower part of the intertidal zone or the upper part of the contiguous subtidal environment, are illustrated as “┴”. Those indicators, which are known to be formed in intertidal depth only, are drawn as “□”. Some indicators may directly define the mean tidal level and must be converted to MSL (see Section 3.3), the top of oyster reefs demonstrates this case, being portrayed as “+” (Fig. 7a). For those marked layers, including archaeological proof (e.g., indicator No. 34, a layer in which a copper coin was found in Chuangganglu site), relevant symbols will be chosen from one of the above based on their facies descriptions. As the primary result that we actually observed, Fig. 7a serves as a foundation for the following explorations. First, based on the RWL and IR characteristics (Table A3) of each indicator, a temporospatial RMSL trend is derived (Fig. 7b, Table A4).

4.5 Corrections for residence time effects

Local residence time corrections (RTC) have been added to CALIB for reworked shells and several organic-rich bulk samples from our previous studies to decrease the age contamination (Shang et al., 2015, 2016; Li et al., 2015). In reworked marine shells, the average maximum residence time ranges from 1 100 cal a to 1 300 cal a, with the semi-empirical values of 600 cal a or 100 cal a for the original ¹⁴C dates older or younger than 1 cal ka BP, respectively, based on the age comparison between 47 subsamples from 17 single layers in the study area (Shang et al., 2016). Similarly, for the terrestrial organic-rich bulk samples, an average maximum range of the residence time is 1 320 cal a, with the semi-empirical values of 660 cal a or 100 cal a for the original ¹⁴C dates older or younger than 2 cal ka BP, respectively, based on the age comparison of 40 subsamples from 20 bulk ones (each bulk sample has two subsamples: >180 μm and <180 μm) (Li et al., 2015). In total, 48 marine and terrestrial samples were RTC treated directly (Table A4: Column 10).

As to those subsamples themselves, the younger/youngest ages were directly chosen, and 35 indicators were the case (Table A4: ticked “yes” in the Column 9). In reality, selecting the younger/youngest at the subsample level is a kind of RTC, as well. In total, of 110 indicators, 83 have been RTC corrected. Thus, a small shift in the temporal distribution towards a younger direction is revealed in Fig. 8a.

4.6 Correction for neotectonic subsidence

For a long time, some local researchers simply believed that the neotectonic subsiding rate should be equal to the sedimentation rate. As a result, a neotectonic subsiding rate of 1−2 mm/a was estimated according to the thickness of the local Pleistocene and Holocene sediments (Wang et al., 2003). Although this is not appropriate, quantitative estimation of the neotectonic subsidence has long been one of the thorniest problems for the local geologists.

Xu et al. (2005) found that during the Quaternary period, a total of five alluvial terraces were formed in the western and northern mountains surrounding the study area (Fig. 1), and the highest one had been raised to about 250 m above the present river surface. To a first order, this outcome might imply that an average neotectonic uplifting rate is about 0.1 mm/a in the mountain areas. Correspondingly, the local neotectonic subsidence in the contiguous plain, including our study area, might be at most about 0.1 mm/a.

During recent years, seismological and lithostratigaphical studies, spurred by petroleum exploration, offered a new perspective for better understanding the regional neotectonics. Following the studies of the postrift tectonic reactivation during the Neogene−Quaternary in the Bohai Bay Basin, by using the back-stripping method for 120 deep cores (Huang et al., 2014), Liu et al. (2016) indicated that the Bohai Bay Basin as a whole began to subside during this postrift phase until present day. As a depressional center of the whole basin, the Bozhong Depression, which is the east of the study area (Fig. 1), had an average subsiding rate of about 0.08 mm/a for the past 5 Ma (Pliocene and Pleistocene), while the Huanghua Depression was only about 0.035 mm/a (Huang et al., 2014; Liu et al., 2016). As to those secondary uplifting regions such as the Cangxian Uplift and Chengning Uplift, their subsiding rates were even lower than those of the Huanghua Depression (Fig. 1).

These estimations of neotectonic subsiding rates—being less than one-tenth of 1 mm/a—are definitely an order lower than what was previously acknowledged by some local researchers. Nevertheless, a correction for the neotectonics is an indispensable part of a sea level study in a relatively small area, and we therefore use 0.1 mm/a as a general subsiding rate to give an elevation correction of all the sea level indicators in this study (Fig. 8b, Table A4: the first line of Column 13).

4.7 Correction for the self-compaction of sediments

The actually measured data of the vertical distribution of the porosity in the study area consist of four varying values (Table A5)³. A decay in the local porosity shows that consolidation settlement nearly finished at approximately 400 m in depth, which coincides approximately with the average thickness of Quaternary sediments in the area^{1414^⑭Yang Jilong, Xiao Guoqiang, Wang Qiang, et al. 2015. Stratigraphical Comparision of Deep Drilling between Subsiding Coasts and 　Lacustrine Basins (in Chinese), 1–126}. It means that a decompacting correction is needed for the upper 400 m at least. On the other hand, based on these data³ and average depths of several key layers (Table A5), approximate porosity values are estimated for the surface sediment, the Holocene basal peat, the hard soil horizon formed during the Last Glacial Maximum (LGM), and the base of Quaternary sediments as well (Table A6).

This approach is different from Pico et al. (2016), who used 1.68 km, a general estimate of total sediment thickness for the Bohai Sea and Yellow Sea region, as well as 63% of the surface porosity of clay sediment (Φ₀) and an exponential attenuation length of 3.7 km (i.e., 1/0.27 km) for the clay sediment column (z₀), both based on Guillocheau et al. (2012). Pico et al. (2016) then obtained a decompaction value of 8.04 m for an exemplified Core SYS-0701 at a depth of about 19.3 m (−52 m below sea surface and about 115 km out of the nearest present continental shoreline in the Yellow Sea (Liu et al., 2010)). Our result shows that a correction value is 5.62 m for the same depth of 19.3 m (Appendix B), which is less than the value of 8.04 m from Pico et al. (2016). Geographically, our study area is quite different with Core SYS-0701; therefore, the estimated value of 5.62 m for our study area seems to be reasonable.

Finally, all sea level indicators in this study are corrected for self-compaction by adding an appropriate value (Fig. 8c, Table A4: the second line of Column 13). A thickness of the modern artificially backfilling material is certainly not considered for this calculation. However, for those indicators taken from the cores drilled in the shallow sea, the thickness of the overlying water column should be included because it will play a similar function for compaction as the sediment overburden on the layers of the indicators. Such indicators include Nos 18, 19, 38, 106, 107 and 108.

The self-compaction of sediments can largely alter the altitude of relative sea level indicators, especially for thick peat layers that may be reduced by 90% of their original volume (Shennan and Horton, 2002). Shennan and Horton (2002) then suggested to select thin peat layers that lay on the Pleistocene sandy sediments as sea level indicators. However, in our study area, the whole late Pleistocene and Holocene sediments are mostly composed of muddy sediments. Compaction may influence all the sediments at various depth ranges, within which the Holocene sea level indicators must be involved.

4.8 Correction for subsidence by groundwater withdrawal

Although land subsidence had started sporadically in the urban area of Tianjin since the 1920s by the first ground water withdrawal (Wang et al., 2003), extensive subsidence began only after 1975 in the whole low-lying coast of Bohai Bay³^{1515^⑮IGM (Institute of Geoenvironment Monitoring). 2016. Report on Ground Subsidence of Key Areas in North China Plain, CGS 　internal Report (in Chinese), 1–240} (Xie et al., 2019). Isobath map of land subsidence for the past four decades of about 1975−2015, with high resolution lines as 100 mm or 200 mm spacing, indicates that the maximum subsidence has accumulated more than 3 m in this period in some crucial areas¹⁵^{1616^⑯GSC (Tianjin Office of Ground Subsidence Control). 2015. The 2014 Annals of Tianjin Land Subsidence (in Chinese), 1–20}. Therefore, compensation for land elevation is needed for those indicators from the boreholes and pits, drilled or dug after 1975 in the subsiding places shown by GSC¹⁶ and IGM¹⁵. For example, Borehole BQ2, from which indicator No. 24 was obtained, is located in the isopach line 1 600 mm and was drilled in 2000 with the borehole elevation +1.57 m measured during the drilling process. If considering that the subsiding started in 1975, then the actual subsidence of this borehole until the drilling year of 2000 should be 1.0 m; i.e., the land surface of this borehole from its existing +1.57 m National Vertical Datum 1985 should be compensated to +2.57 m, and the elevation of indicator No. 24 should be correspondingly added by this value approximately. All the indicators influenced by this subsidence have been corrected (Fig. 8d, Table A4: the third line of Column 14).

To date, all the local factors have been removed step by step from the original RMSL pattern in Fig. 7b. As a result, a solid black line, arbitrarily hand-drawing in Fig. 8d, shows preliminarily a local RMSL curve in the study area.

5 Discussion

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5.1 Turning-phase of Holocene sea level rise

In total, 21 indicators were found in the period of about 8.3−5.8 ka. They can be separated into two groups. The first group is not necessary for RTC, including two articulated, vertically stood razor shell indicators, Nos 59 and 82, and seven oyster indicators, Nos 95−98 and 106−108. Similarly, indicators Nos 3 and 67 are classified to this group because they are macroplant samples.

The second group is composed of redeposited samples and is RTC processed. This group includes eight organic-rich sediment indicators, Nos 53−56, 60−61, and 63−64. Their younger ages, given by their subsamples >180 μm, were used. Additionally, indicators Nos 1 and 44, reworked shell or peaty mud, fall under to this group. The ten RTC processed indicators of the second group, together with the first group, comprise a hand-drawn line for this period (Fig. 9: black line). However, if considering the ages contributed by the subsamples <180 μm of the second group, the further obtained approximate ages for their bulk samples (i.e., a mathematic mean of the ages of the two subsamples), and the originally non-RTC processed ages of the indicators Nos 1 and 44, then another curve by integrating Group 1 may be achieved (Fig. 9: red line). A comparison between the two lines shows an obvious geochronological uncertainty, and a trend that is about 0.5 cal ka younger likely exists. As a result, a change from slowdown to stop (i.e., nearly horizontal) seems to occur at about 6.8 cal ka BP rather than about 7.3 cal ka BP and the slowdown is likely to start from about 7.5 ka (Fig. 9).

After eliminating all the local temporospatially affected factors, an amended RMSL pattern illustrated by a solid black curve in Figs 8d, 9 and 10 only remains; theoretically, combined contributions of the eustatic change and the regional isostatic adjustment only remain, as well.

Lambeck et al. (2014) and Bradley et al. (2016) discussed the ice-volume equivalent sea levels (ESLs) and gave the crucial heights of about −5.4 m or about −5.8 m, respectively, at about 7 cal ka BP, which can be roughly recognized as the beginning of the slowdown and nearlly stopped at about 2 ka (Lambeck et al., 2014). However, such two time boundaries are about 7.5 ka and about 6.8 ka, respectively, aforementioned for the study area. Note all the ESLs are not RTC corrected. Among all the 968 original observation data (Lambeck et al., 2014), nearly 450 belong to various sediments, shells, beahrocks and cheniers, of which some ages are likely contaminated by redeposition and/or different at the subsample level. If we would do the RTC for these data, then a beginning of the slowdown, constrained by both ice sheet melting history and isostasy, would be even younger.

Additionally, the progressively improved GIA-modeled projections emphasized a mid-Holocene highstand, i.e., about 2−3 m higher than that at present for the west coast of Bohai Bay⁴ (personal communication, Lambeck, 2014). Bradley et al. (2016) also discussed a set of projected sea level curves being even about 4−6 m higher at approximately 6 cal ka BP for the same region. However, our RMSL curve is remarkably lower than the predicted curves after about 6 cal ka BP but also obviously higher than both the later predicted curve of Lambeck and Robby (personal communication, 2014) (Fig. 10: the light green one) and the band of Bradley et al. (2016) before about 6−7 cal ka BP (Fig. 10). Such double misfits are perhaps interesting for the existing explanations (Lambeck et al., 2014; Bradley et al., 2016) for the north coast of China. It may imply that comparing that the projected GIA-contributions⁴ (personal communication, Lambeck, 2014; Bradley et al., 2016) after 6 ka were likely overestimated but were underestimated before that for the region (Fig. 10).

5.2 Uncertainties given by spatial corrections

It is worthwhile to pinpoint that the monotonically seaward-dipping characteristic of the chenier subsurface in the coast of Bohai Bay is different with the convex form of subsurface due to the sink-down of chenier body at the Louisiana coast (Reineck and Singh, 1980: p.352). This difference for the subsurface form may imply that postdepositional self-compaction of the chenier itself and the immediately underlying sediments in the study area was not as strong as that at the Louisiana coast. For instance, indicator No. 71 (Table A4.2) has a compensation of 1.56 m for self-compaction based on its sampling depth of 4 m (Table A5). In total, among 27 chenier indicators, most of them are estimated to experience a self-compaction of about 0.5−2 m (Fig. 8c, Table A4.2: the second line of Column 13). However, if considering the fact that the subsurface was not of a convex type (Fig. 3), then the vertical compensations of about 0.5−2 m may be overestimated. If so, it may explain why the chenier indicators lie at much higher positions, often about 2−3 m above the present MSL (Fig. 8c). On the other hand, due to lacking practically tested data at this time, the compensation for subsidence by groundwater withdrawal that we used is only linear-transmitted to the depth at which indicators existed. Therefore, such compensation may also be overestimated. Nevertheless, if considering two such possible overestimations, it is even much more unfavorable for model-projections of the mid-Holocene highstand.

The early Holocene sea level rise was quickly with an average rate of about 1.4−0.7 cm/a in period of 9−7 cal ka BP (Lemback et al., 2014). Thus the vegetation pavement, forming the basal peat layer later on, may further horizontally extend inland from its original range between MHHW−MHW (Tables A2 and A3), at most, for about 1 km, considering intertidal gradient (the present gradient, see Section 2) if vegetation growth and sediment supply kept pace with the sea level rise. Nevertheless, on the other hand, high sea waters may not often reach such extended end. This is likely a reason that why top part of basal peat layer and/or muddy sediment immediately-above-basal-peat-layers lack definite paleo microbiological evidence for sea water existence and such basal peat layers sometimes can only be used as “limited” indicators.

5.3 Uncertainties given by temporal corrections

Twenty organic-rich bulk samples, taken from six cores from DC01 eastward to Q7, about 70 km apart (Shang et al., 2018), were all separated into two subsamples with boundary of 180 μm. Of them, 19 bulk samples, taken from last five cores, located in the range of Holocene marine transgression, show all δ¹³C values are in between −19.2‰ and −27.0‰ PDB for the subsamples <180 μm, but δ¹³C values of subsamples >180 μm are −24.5‰ to −28.3‰ PDB (Shang et al., 2018). All these 19 subsamples of <180 μm have older AMS ages than subsamples >180 μm, with older range of 170−8 530 cal a (an average 1 320 cal a) (Li et al., 2015; Shang et al., 2018). Thus, much heavier and much older for the subsamples <180 μm must have a regional significance. The heavier δ¹³C the closer to the sea is obedient to marine influence (Törnqvist et al., 2015). However, the regional marine reservoir age of about 220 a, caused ΔR= −178±50 a (Southon et al., 2002), is unable to explain why the age-difference between the two subsample groups is so large.

Here, hydrogeological approach is likely to tackle this puzzle. Due to very fine sediments, semi-confined and confined groundwater aquifers exist in the coastal lowland of the study area, which may contain isolated water bodies as old as, at least, several thousand years (Cao et al., 2016; personal communication, Han, 2019). The finer subsamples have larger absorption and their ages can be relatively older.

6 Conclusions

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In this study, a total of 110 indicators are classified into three categories: sediments (67), cheniers (27) and oyster reefs (16). Thorough examinations of the indicative meaning for each indicator have been performed with particular descriptions for cheniers and oyster reefs. With reasonable estimation for the paleo tidal pattern and relationship between MTL and MSL, procedures from the observed rsl to RMSL have been completed.

RTC caused more than two-thirds of the whole data to have a younger-oriented shift. Three additional corrections for spatial factors such as neotectonics, self-compaction and groundwater withdrawal are further scrutinized. Consequently, about 0.1 mm/a has been used as the local neotectonic subsiding rate; a few decimeters to nearly 6 m have been compensated for self-compaction on different depths; a few centimeters to nearly 2.5 m have been reimbursed for subsidence by groundwater withdrawal. Finally, a local RMSL curve, eliminated all the local factors, has been speculated, and a slowdown is in between about 7.5−6.8 cal ka BP. None of the obvious mid-Holocene highstand was found, and a likely stable RMSL at <+1 m lasted from about 6.8 ka until about 3 ka before the present.

This new attempt for Holocene sea level reconstruction on the west coast of Bohai Bay may provide a new constrained platform for further modeling efforts in the coming future.

Acknowledgements

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The authors acknowledge the numerous researchers provided original data during the last decades, among whom Zhiren Xie (Nanjing Normal University, Nanjing) is particularly thanked for his consistent encouragement. We greatly appreciate Kurt Lambeck (The Australian National University) for his guidance in aspect of the GIA predicted curves in the study area, and acknowledge Lei Huang (Northwest University, Xi’an) and Sarah L. Bradley (Utrecht University) for discussions of the regional neotectonics and isostatic approach. Thanks are given to Shu Gao (Nanjing University, Nanjing) for his suggestions of significant reorganizing of the manuscript. Two anonymous reviewers are truly indebted for their valuable review and constructive comments.

Funding

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The National Natural Science Foundation of China under contract Nos 41372173, 41476074 and 41806109; the China Geological Survey Project under contract Nos DD20189506 and DD20211301.

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doi: 10.1007/s13131-021-1730-5

Receive Date：2020-04-02
Online Date：2026-03-03
Published：2021-07-25

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Received：2020-04-02
Accepted：2020-08-14

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The National Natural Science Foundation of China under contract Nos 41372173, 41476074 and 41806109; the China Geological Survey Project under contract Nos DD20189506 and DD20211301.

Affiliations

¹ Tianjin Center, China Geological Survey, Tianjin 300170, China

² Key Laboratory of Coast Geo-Environment, China Geological Survey, Tianjin 300170, China

³ Key Laboratory for Coast and Island Development (Nanjing University), Ministry of Education, Nanjing 210023, China

⁴ School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210023, China

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*E-mail: whong@cgs.cn

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Tables A1. The type of tides and tidal information collected from four gauge stations in the Bohai Bay⁶⁷ (Liu et al., 1986; HPL, 2007; EBC, 1989; HPL, 2007; Sun and He, 2013)

Tidal ranges	Northeast periphery (Northeast of Jiyun River Mouth)	Bayhead (Jiyun River Mouth−Napai River Mouth)		Southeast Periphery (Southeast of Nanpai River Mouth)
Tidal ranges	Jianhekou Station	Tanggu Station	Qikou Station	Dakouhe Station
Note: All the average values are calculated by this study and reaffirmed in Table A2.
Mean tidal range (MHW−MLW)	about 2.3 m	(1) 2.51 m, (2) 2.28 m, (3) 2.48 m, (4) 2.40 m	(1) 2.51 m, (2) 2.3 m	(1) 2.34 m, (2) 2.15 m, (3) 2.30 m
average: about 2.42 m		average: about 2.26 m
Mean high tidal range (MHHW−MLLW)	2.80 m	(1) 3.02 m, (2) 2.82 m, (3) 2.9−2.8 m	(1) 3.11 m	(1) 2.88 m, (2) 2.7 m, (3) 2.79 m
average: about 3.0 m		average: about 2.79 m
Mean high water (MHW)	about +1.13 m	(1) +1.13 m, (2) +1.20 m, (3) about +1.23 m	(1) +1.23 m, (2) about +1.23 m	about +1.08 m
average: about +1.21 m
Mean higher high water (MHHW)	about +1.18 m	(1) +1.26 m (2) +1.34 m	(1) about +1.23 m, (2) about +1.36 m	+1.13 m
average: about +1.3 m
Mean low water (MLW)	about −1.12 m	(1) −1.15 m, (2) about −1.17 m		about −1.07 m
average: about −1.16 m
Mean lower low water (MLLW)	about −1.52 m	(1) −1.56 m, (2) −1.66 m		−1.47 m
average: about −1.61 m
Observed highest high weter (HHW)		(1) +3.31 m, (2) +3.15 m, (3) +3.11 m average: +3.19 m

Tables A2. Summary of the tides and geographic distribution for both the present and Holocene tidal situations in the Bohai Bay region, based on Table A1 and paleo geomorphology described in the main text and Fig. A1

Tidal properties	The present tides and bay morphology			Analogous Holocene tides and paleo bay morphology
Tidal properties	northeast periphery (Northeast of Jiyun River Mouth)	bayhead (Jiyun River Mouth−Nanpai River Mouth)	southeast periphery (Southeast of Nanpai River Mouth)	paleo bayhead (Nanpai River Mouth−north most limit of the data)	south periphery (Southeast of Nanpai River Mouth)
Mean range	2.3 m	2.42 m	2.26 m	2.42 m	2.26 m
Mean high range	2.80 m	3.0 m	2.79 m	3.0 m	2.79 m
MHW	+1.13 m	+1.21 m	+1.08 m	+1.21 m	+1.08 m
MHHW	+1.18 m	+1.3 m	+1.13 m	+1.3 m	+1.13 m
MLW	−1.12 m	−1.16 m	−1.07 m	−1.16 m	−1.07 m
MLLW	−1.52 m	−1.61 m	−1.47 m	−1.61 m	−1.47 m

Tables A3. The indicative meanings (RWL and IR) of the Holocene sea-level indicators on the west coast of Bohai Bay

Types of indicators	Modern and ancient bayhead	Modern and ancient southeastern periphery	Modern northeastern periphery (rarely used) ¹⁾
Note: All the values, based on Table A2, were rounded to the nearest 5 or 10. ¹⁾ Indicative meanings for the northeastern periphery are also listed though this coastal sector is rarely dealt with in this study. ²⁾ Elevation of the top of oyster reef indicates mean tidal level (MTL) directly and 0.15 m is added to reconstruct a corresponding MSL.
Peaty layers	subtracting (1.26±0.05) m	subtracting (1.1±0.03) m	subtracting (1.15±0.03) m
Other sediments	(1) when indicating MHHW or MHW: subtracting (1.3±0.5) m, (2) when indicating MLLW: adding (1.6±0.5) m, (3) when indicating MLW: adding (1.15±0.5) m, (4) when indicating intertidal zone only: sample’s altitude ±1 m	(1) when indicating MHHW or MHW: subtracting (1.1±0.5) m, (2) when indicating MLLW: adding (1.45±0.5) m, (3) when indicating MLW: adding (1.05±0.5) m, (4) when indicating intertidal zone only: sample’s altitude ±1 m	(1) when indicating MHHW or MHW: subtracting (1.15±0.5) m, (2) when indicating MLLW: adding (1.5±0.5) m, (3) when indicating MLW: adding (1.1±0.5) m, (4) when indicating intertidal zone only: sample’s altitude ±1 m
Cheniers	(1) front-part: sample’s altitude ±1 m, (2) above the mid-subsurface: subtracting (1.2±0.5) m, (3) above the rear-subsurface: subtracting (1.3±0.5) m	(1) front-part: sample’s altitude ±1 m, (2) above the mid-subsurface: subtracting (1.1±0.5) m, (3) above the rear-subsurface: subtracting (1.1±0.5) m	(1) front-part: sample’s altitude ±1 m, (2) above the mid-subsurface: subtracting (1.1±0.5) m, (3) above the rear-subsurface: subtracting (1.2±0.5) m
Oyster Reefs ²⁾	(The reef-top-elevation+0.15 m) ± 0.7 m	−	−

A4.1 The Holocene relative mean sea level (rmsl) indicators, derived from the sediments, on the west coast of Bohai Bay

	Basic information			Temporal distribution								Spatial distribution
No.	Locality, stratigraphy and environment, material dated, sampling depth / ground surface elevation / (averaged) sample elevation	Coordinate	Lab code	Measured ¹⁴C date	δ¹³C /‰PDB	Conventional ¹⁴C age/a BP	Calibrated ¹⁴C age: median probability/2σ range/cal a BP	Status of material dated	Sub-sample	RTC /cal a	Age of rmsl: single value /range/cal ka BP)	Elevation of the observed rmsl/m	Corrections of tectonics/ self-compa- ction/water withdrawal /m	Elevation of rmsl after corrections for the three local factors /m
1	Zhakou, shelly layer, single valves of undetermined shells, about 2/-/about -1.75 m	39.4°N, 117.8°E	GC175A	5 410±250	-2.68	5 769±253	6 375/ 6 966-5 789	reworked	no	-600	5.775/ 6.36-5.19	-1.75±1	+0.58/ +0.78/ +0.20	-0.19±1
2	Lizigu, swamp, upper peaty layer, mud, articulated Arconaia contorta (-), about 2 m/about +1 m/-1 m	39.5°N, 117.4°E	CG1394	3 990±80	-27	4 199±90	4 715/ 4 473-4 446, 4 893-4 525, 4 960-4 898	in situ (-)	no	-	4.715/ 4.960-4.446	-2.26±0.05	+0.47/ +0.78/ +0.12	-0.89±0.05
3	idem, marine sediments, wood branch on the top, 2.3 m/about +1 m/about -1.3 m	CG--	6 680±110	-27	6 696±117	7 567/ 7 355-7 333, 7 387-7 374, 7 791-7 414	reworked	no	-	7.567/ 7.791-7.333	-2.60±0.5	+0.75/ +0.90/ +0.12	-0.83±0.5
4	Maomaojiang, intertidal shelly layer, single valves of Mactra veneriformis mainly, about 2 m/-/about 0 m idem, single valves of M. veneriformis mainly, about 2.3 m/-/about 0 m	39.4°N, 117.7°E	CG187, TD5	5 320±75, 6 350±105	-2.68	5 679±85, 6 709±112	6 274/ 6 481-6 019; 7 395/ 7 604-7 162	reworked	no	-600	5.674/ 7.00-5.42	0±1.5	+0.56/ +0.78/ +0.12	+1.46±1.5
5	Dawuzhuang, oyster reef, Section 9, the bottom of the overlying mud, semi-carbonized wood branch, about 5 m/-/-2.23 m	39.4°N, 117.9°E	BA 091202	-	-	4 810±35	5 520/ 5 558-5 471, 5 604-5 568	reworked	no	-	5.520/ 5.604-5.471	-2.23±0.7	+0.55/ +1.95/ +0.52	+0.79±0.7
6	Panzhuang, the upper peat, peaty mud, 0.5-1 m/about +2 m/+1.25 m	39.4°N, 117.5°E	ZK525, CG189	1 805±125, 1 275±95	-27	1 773±131, 1 243±103	1 697/ 1 988-1 402; 1 158/ 1 327-956	in situ	no	-660, -100	1.047/ 1.33-0.85	-0.01±0.05	+0.10/ +0.29/ +0.09	+0.47±0.05
7	Xingtuo, lagoon-salt marsh, garlic-structured clay, tests of Pseudononinella variabilis and Ammonia becarii vars., 0.58 m/+1.6 m/+1.02 m	39.3°N, 117.6°E	AA 45906	-	-5.4	2 301±54	2 134/ 2 339-1 892	in situ (-)	no	-	2.134/ 2.339-1.892	-0.28±0.5	+0.21/ +0.22/ +0.70	+0.85±0.5
8	idem, bottom of garlic-structured clay, articulated shells of Glauconome primeana, 2-2.1 m/+1.6 m/-0.45 m	AA 45905	-	-4.8	3 428±49	3 505/ 3 680-3 345	in situ	no	-	3.505/ 3.680-3.345	-0.45±0.5	+0.35/ +0.78/ +0.70	+1.38 ±1.5
9	Xingtuo Pit, the lower portion of intertidal zone, fine sandy-silty mud, articulated Mactra veneriformis shells, 3.8-4.0 m/about +1.6 m/about -2.3 m	AA 45904	-	-4.5	4 169±52	4 489/ 4 720-4 275, 4 766-4 753	in situ	no	-	4.489/ 4.766-4.275	-0.7±0.5	+0.45/ +1.52/ +0.87	+2.14±0.5
10	Core H1, basal peat, peaty mud, 16.57-16.63 m/+1.6 m/-15.0 m	39.3°N, 117.6°E	00Y078	8 562±100	-27	8 530±108	9 520/ 9 793-9 270, 9 816-9 808, 9 865-9 848, 9 885-9 877	in situ	no	-660	8.860/ 9.22-8.61	-16.26±0.05	+0.88/ +4.81/ +0.87	-9.7±0.05
11	Core H3, intertidal muddy sediments, single valves of Potamocorbula laevis, 4.4 m/+1.315 m/-3.09 m	39.3°N, 117.7°E	Beta 358164	3 640±30	-6.4	3 950±30	4 191/ 4 377-4 009	reworked	no	-600	3.591/ 3.77-3.41	-3.09±0.5	+0.36, +1.46, +1.40	+0.13±0.5
12	Core HD21, lagoon-salt marsh, articulated Saliqua pulchella, 1.1-1.2 m/+0.71 m/-0.44 m	39.4°N, 117.6°E	AA 45902	-	-7.6	1 579±48	1 310/ 1 473-1 173	in situ	no	-	1.310/ 1.473-1.173	-1.74±0.5	+0.13/ +0.40/ +0.45	-0.76±0.5
13	Core NP3, salt marsh, Potamocorbula laevis fragments, 1.1 m/+2.134 m/+1.03 m	39.2°N, 118.0°E	Beta 305307	1 390±30	-0.8	1 790±30	1 524/ 1 679-1 369	reworked	no	-600	0.924/ 1.08-0.77	-0.12±0.5	+0.09/ 0/ +1.72	+1.69±0.5
14	idem, upper part of shallow sea, Nassarius variciferus, 9.3-9.4 m/+2.134 m/-7.22 m	Beta 305310	4 710±30	-6.3	5 020±30	5 558/ 5 690-5 437	reworked	no	-600	4.958/ 5.09-4.83	-5.72±0.5	+0.49/ +2.82/ +1.72	-0.69±0.5
15	idem, charcoals from a shell hash layer, 17.75-17.87 m/+2.134 m/-15.65 m	BA08832	-	-	7625±40	8 417/ 8 483-8 371, 8 520-8 490, 8 536-8 532	reworked	no	-	8.417/ 8.536-8.371	-15.65±0.5	+0.84/ +4.87/ +1.72	-8.22±0.5
16	Core HG81, basal peat, peaty mud, 16.65-16.68 m/about +2 m/-14.67 m	39.2°N, 117.8°E	06Y084	8 160±250	-27	8128±253	9 040/ 9 539-8 430	in situ	no	-660	8.380/ 8.88-7.77	-15.93±0.05	+0.84/ +4.83/ +1.05	-9.21±0.05
17	Core Beining Park, basal peat, plant debris (-), 16.89-17.09 m/about +2.46 m/about -14.53 m	39.2°N, 117.2°E	ZK601-I	8 035±120	-27	8035±120	8 902/ 9 271-8 592	in situ	no	-	8.902/ 9.271-8.592	-15.79±0.05	+0.89/ +4.93/ +0.37	-9.6±0.05
18	Core CH114, basal peat, peaty mud, 12.0-12.1 m/about -3.90 m/about -15.95 m	39.1°N, 117.9°E	BA 081875	-	-	8415±35	9 455/ 9 351-9 320, 9 522-9 400	in situ	no	-660	8.795/ 8.86-8.66	-17.21±0.05	+0.88/ +4.61/ +1.12	-10.6±0.05
19	Core CH115, basal peat, charcoals and carbonized twigs, 17.41-17.43 m/-3.90 m/about -21.32 m	39.1°N, 117.9°E	BA 081878	-	-	8 805±35	9 833/ 9 946-9 682, 10 008-9 993, 10 127-10 063	in situ (-)	yes	-	9.833/ 10.127-9.682	-22.58±0.05	+0.98/ +5.75/ +1.12	-14.73±0.05
20	Core HDZ, basal peat, peaty mud, 18.05-18.15 m/about +2.5 m/-15.6 m	39.1°N, 117.7°E	TD408	8 120±160	-27	8 088±165	8 998/ 9 436-8 587	in situ	no	-660	8.338/ 8.78-7.93	-16.86±0.05	+0.83/ +4.67/ +0.17	-11.19±0.05
21	idem, basal peat, peaty mud, 18.81-19.0 m/about +2.5 m/-16.41 m	39.1°N, 117.7°E	TD409	8 645±130	-27	8 613±136	9 644/ 9 357-9 316, 9 962-9 399, 10 044-9 985, 10 152-10 052	in situ	no	-660	8.984/ 9.49-8.65	-17.67±0.05	+0.90/ +4.90/ +0.17	-11.7±0.05
22	Core Chentangzhuang, basal peat, peaty mud, 13.29-13.49 m/about +1.96 m(-)/-11.43 m	39.1°N, 117.21°E	CG256	8 825±140	-27	8 793±146	9 858/ 10 199-9 539	in situ	no	-660	9.198/ 9.54-8.88	-12.69±0.05	+0.92/ +4.15/ +0.37	-7.25±0.05
23	Papadi, Shanggulin, lagoon-salt marsh behind shelly chenier, the upper peat, peaty mud, about 1.2 m/about +2 m/+0.8 m	38.81°N, 117.1°E	98Y076	1 827±80	-27	1 795±90	1 721/ 1 904-1 529, 1 925-1 906	in situ	no	-	1.721/ 1.925-1.529	-0.46±0.05	+0.17/ +0.47/ +0.75	+0.93±0.05
24	Core BQ2, transitional zone, Potamocorbula laevis, 16.50 m/+1.57 m/-14.93 m	38.81°N, 117.51°E	BA 04544	-	-	7 955±40	8 611/ 8 841-8 430	reworked	yes	-	8.611/ 8.841-8.430	-14.93±1	+0.86/ +4.78/ +1.0	-8.29±1
25	Chuanganglu Pit, Section 7, Shell Bed B, articulated Potamocorbula laevis, 4.8/-/about -2.91 m	38.8°N, 117.5°E	Beta 363624	2 070±30	-3.7	2 420±30	2 271/ 2 427-2 114	reworked	yes	-	2.271/ 2.427-2.114	-2.91±1	+0.22/ +1.87/ +2.0	+1.18±1
26	Chuanganglu Pit, Section 4, carbonized, fine plant twig, about 4/-/-2.46 m	38.8°N, 117.5°E	Beta 352331	2 830±30	-26.0	2 810±30	2 911/ 2 999-2 844	reworked	no	-	2.911/ 2.999-2.844	-3.76±0.5	+0.29/ +1.56/ +2.0	+0.09±0.5
27	idem, Section 4, the bottom of Shell Bed A, articulated Mactra chinensis, about 4.4/-/-2.86 m	38.8°N, 117.5°E	Beta 352328	1 320±30	-1.3	1 710±30	1 436/ 1 558-1 300	reworked	yes	-	1.436/ 1.558-1.300	-2.86±1	+0.14/ +1.72/ +2.0	+1±1
28	Chuanganglu Pit, Section 5, the mid-upper part of Shell Bed A, articulated Sinonovacula constricta, about 4/-/-2.46 m	38.8°N, 117.5°E	Beta 352325	1 570±30	-6.6	1 870±30	1 620/ 1 786-1 482	in situ	yes	-	1.620/ 1.786-1.482	-3.76±0.5	+0.16/ +1.56/ +2.0	-0.04±0.5
29	Chuanganglu Pit, Section 1, coarse shell hash layer at the lower part of Shell Bed 1, Nassarius sp., about 4.2/-/about -2.4 m	38.8°N, 117.5°E	Beta 335910	1 520±30	-3.0	1 880±30	1 631/ 1 795-1 494	reworked	yes	-	1.631/ 1.795-1.494	-2.4±1	+0.16/ +1.64/ +1.5	+0.9±1
30	idem, Section 1, thin shelly lamina at the upper part of Holocene marine muddy sediments, single valve of Potamocorbula laevis, about 4.5/-/-2.7 m	38.8°N, 117.5°E	Beta 335912	2 080±30	-1.2	2 470±30	2 339/ 2 516-2 149	reworked	yes	-	2.339/ 2.516-2.149	-2.7±1	+0.23/ +1.75/ +2.0	+1.28±1
31	Chuanganglu Pit, Section 2, coarse shelly layer on the top of Shell Bed A, articulated Mactra venerifomis, about 4/-/about -2.2 m	38.8°N, 117.5°E	Beta 335907	1 410±30	-1.0	1 800±30	1 536/ 1 686-1 387	reworked	yes	-	1.536/ 1.686-1.387	-3.5±0.5	+0.15/ +1.56/ +2.0	+0.21±0.5
32	idem, Section 2, the bottom of Shell Bed A, single valve of Potamocorbula laevis, about 5/-/about -3 m	Beta 335911	2 470±30	-2.2	2 840±30	2 794/ 2 934-2 692	reworked	no	-600	2.194/ 2.33-2.09	-3.0±0.5	+0.22/ +1.95/ +2.0	+1.17±0.5
33	Chuanganglu Pit, Section 6, Shell Bed C, articulated Cyclina sinensis, about 5.5/-/-3.7 m;	38.8°N, 117.5°E	Beta 352323	2 100±30	-1.6	2 480±30	2 355/ 2 542-2 156	reworked	yes	-	2.318/ 2.542-2.093	-3.7±1	+0.23/ +1.87/ +2.0	0.4±1
	idem, Section 6, Shell Bed C, articulated Potamocorbula laevis, about 5.5/-/-3.7 m;	Beta 363617	2 090±30	-5.9	2 400±30	2 246/ 2 392-2 093
	idem, Section 6, Shell Bed C, articulated Mactra chinenesis, about 5.5/-/-3.7 m	Beta 363620	2 090±30	-1.4	2 480±30	2 355/ 2 542-2 156
34	idem, Section 6, Shell Bed C, copper coin, about 5.5/-/-3.7 m	-	-	-	-	-	in situ	-	-	2.119/ 2.171-2.068	-5.0±0.5	+0.21/ +1.87/ +2.0	-0.92±0.5
35	idem, Section 6, Shell Bed D, Dupliaria dussumierii (-), about 6/-/-4.15 m	Beta 363623	1 950±30	+0.4	2 370±30	2 217/ 2 341-2 060	reworked	yes	-	2.217/ 2.341-2.060	-4.15±1	+0.22/ +2.04/ +2.0	+0.11±1
36	Core BT113, single valve of Potamocorbula laevis, 4.4 m/+1.436 m/-2.96 m	38.8°N, 117.5°E	Beta 296005	1 180±30	-0.8	1 580±30	1 310/ 1 436-1 180	reworked	no	-600	0.710/ 0.83-0.58	-2.96±1	+0.07/ +1.52/ +1.50	+0.13 ±1
37	idem, fragments of Potamocorbula laevis, 16.70 m/+1.436 m/-15.26 m	Beta 297742	7 520±40	-6.0	7 830±40	8 467/ 8 599-8 341	reworked	no	-600	7.867/ 8.0-7.74	-15.26±1	+0.78/ +4.70/ +1.50	-8.28±1
38	Core BT115, basal peat, peaty mud, 11.1 m/-6.1 m/-17.2 m	38.7°N, 117.7°E	BA 091538	-	-	8 190±40	9 137/ 9 269-9 024	in situ	no	-660	8.477/ 8.61-8.36	-18.46±0.05	+0.85/ +3.26/ +0.87	-13.48±0.05
39	Core ZW15, basal peat, plant debris (subsample >180 μm), 12.6 m/+1.631 m/-10.97 m	38.7°N, 117.2°E	Beta 356208	7 450±40	-25.0	7 450±40	8 271/ 8 358-8 186	in situ	yes	-	8.271/ 8.358-8.186	-12.23±0.05	+0.83/ +3.90/ +0.95	-6.55±0.05
40	Core G15, basal peat, peaty mud, 17.95 m/about +2.46 m/-15.49 m	38.7°N, 117.4°E	ZK1465	8 580±130	-27	8 548±136	9 548/ 9 177-9 140, 9 223-9 203, 9 926-9 236, 10 117-10 069	in situ	no	-660	8.888/ 9.46-8.48	-16.75±0.05	+0.89/ +5.20/ +0.22	-10.44±0.05
41	idem, basal peat, peaty mud, 18.65 m/about +2.46 m/-16.19 m	ZK1466	9 140±120	-27	9 108±127	10 288/ 10 595-9 894, 10 650-10 624	in situ	no	-660	9.628/ 9.99-9.23	-17.45±0.05	+0.96/ +5.41/ +0.22	-10.86±0.05
42	Core BQ1, marine muddy sediments, intercalated coarse shelly layer, Corbicula fluminea, 18.22 m/+3.404 m/-14.81 m	BA 04542	-	-	8 620±40	9 462/ 9 576-9 305	reworked	no	-600	8.862/ 8.97-8.70	-13.21±0.5	+0.88/ +4.98/ +1.12	-6.23±0.5
43	Core TP23, single valve of Potamocorbula laevis, 5.7 m/+1.848 m/-3.85 m	38.7°N, 117.4°E	BA 091539	-	-	3 450±35	3 528/ 3 682-3 376	reworked	no	-600	2.928/ 3.08-2.77	-3.85±1	+0.29/ +1.50/ +1.35	-0.71±1
44	idem, peat layer, peaty mud, 14.1 m/+1.848 m/-12.25 m	BA 091542	-	-	7 610±40	8 407/ 8 462-8 350, 8 477-8 467, 8 513-8 495	in situ	no	-660	7.747/ 7.85-7.69	-13.51±0.05	+0.77/ +3.80/ +1.75	-7.19±0.05
45	idem, peat layer, peaty mud, 14.2 m/+1.848 m/-12.35 m	BA 091543	-	-	8 160±40	9 096/ 9 149-9 010, 9 254-9 162	in situ	no	-660	8.436/ 8.59-8.35	-13.61±0.05	+0.84/ +3.83/ +1.75	-7.19±0.05
46	Core Q7, lagoon-salt marsh (-), upper peat layer, peaty mud (<180 μm), 1.3 m/+3.458 m/+2.16 m	38.7°N, 117.5°E	Beta 358054	450±30	-20.4	530±30	540/ 559-510, 630-600	in situ	no	-100	0.440/ 0.53-0.41	+0.9±0.05	+0.04/ +0.12/ +1.0	+2.06±0.05
47	idem, single valve of Ruditapes philippinarum, 16.3 m/+3.458 m/-12.84 m	Beta 357152	7 360±40	-2.7	7 730±40	8 371/ 8 515-8 219	in situ	no	-600	7.771/ 7.91-7.62	-12.84±1	+0.78/ +4.44/ +1.0	-6.62±1
48	idem, peaty layer, plant debris (>180 μm), 17.2-17.23 m/+3.458 m/-13.76 m	Beta 357153	8 040±40	-28.0	7 990±40	8 868/ 8 666-8 662, 9 005-8 705	in situ	yes	-	8.868/ 9.005-8.662	-13.76±1	+0.89/ +4.70/ +1.0	-7.17±1
49	idem, basal peat, organic matter (<180 μm), 18.85 m/+3.458 m/-15.39 m	Beta 357157	9 130±40	-24.6	9 140±40	10 287/ 10 411-10 226	in situ	no	-1320	8.967/ 9.09-8.91	-16.65±0.05	+0.96/ +5.17/ +1.0	-9.52±0.05
50	Core QX01, lagoon-salt marsh, organic mud (<180 μm), 5.52 m/+5.16 m/-0.36 m	38.7°N, 116.8°E	Beta 329647	4 260±30	-22.5	4 300±30	4 858/ 4 892-4 829, 4 908-4 899, 4 917-4 913, 4 960-4 924	in situ	no	-1320	3.538/ 3.64-3.51	-1.66±0.5	+0.35/ +1.76/ +1.0	+1.45±0.5
51	idem, lagoon-salt marsh, organic mud (<180 μm), 6.35 m/+5.16 m/-1.19 m	Beta 329644	4 990±50	-23.6	5 010±50	5 750/ 5 900-5 644	in situ	no	-1320	4.430/ 4.58-4.32	-2.45±0.05	+0.44/ +1.82/ +1.0	+0.81±0.05
52	idem, lagoon-salt marsh, organic mud (<180 μm), 7.2 m/+5.16 m/-2.04 m	Beta 329643	5 090±30	-25.0	5 090±30	5 813/ 5 831-5 748, 5 912-5 843	in situ	no	-1 320	4.493/ 4.59-4.43	-3.34±0.5	+0.45/ +2.11/ +1.0	+0.22±0.5
53	idem, lagoon-salt marsh, plant debris (>180 μm), 8.2 m/+5.16 m/-3.04 m	Beta 329641	5 820±30	-24.6	5 830±30	6 647/ 6 732-6 554	in situ	yes	-	6.647/ 6.732-6.554	-4.34±0.5	+0.66/ +2.45/ +1.0	-0.23±0.5
54	idem, lagoon-salt marsh, basal peat, plant debris (>180 μm), 8.7 m/+5.16 m/-3.54 m	Beta 329642	6 020±40	-24.3	6 030±40	6 875/ 6 763-6 755, 6 981-6 778	in situ	yes	-	6.875/ 6.981-6.755	-4.8±0.05	+0.69/ +2.62/ +1.0	-0.49±0.05
55	idem, lagoon-salt marsh, basal peat, plant debris (>180 μm), 9.16 m/+5.16 m/-4.0 m	Beta 329645	6 160±40	-27.4	6 220±40	7 117/ 7 133-7 006, 7 250-7 140	in situ	yes	-	7.117/ 7.250-7.006	-5.26±0.05	+0.71/ +2.77/ +1.0	-0.78±0.05
56	idem, lagoon-salt marsh, basal peat, plant debris (>180 μm), 11.39 m/+5.16 m/-6.23 m	Beta 329640	7 010±30	-25.3	7 010±30	7 855/ 7 771-7 763, 7 935-7 786	in situ	yes	-	7.855/ 7.935-7.763	-7.53±0.5	+0.78/ +3.22/ +1.0	-2.53±0.5
57	Core QX03, peaty layer, plant debris (>180 μm), 2.9-2.92 m/+4.38 m/+1.47 m	38.7°N, 116.9°E	Beta 353792	2 280±30	-20.6	2 350±30	2 357/ 2 461-2 326	in situ)	no	-	2.357/ 2.461-2.326	+0.21±0.05	+0.23/ +0.74/ +0.90	+2.08±0.05
58	idem, peaty layer, plant debris (>180 μm), 4.9-4.91 m/+4.38 m/-0.53 m	Beta 353794	3 370±30	-24.0	3 390±30	3 634/ 3 699-3 569	in situ	no	-	3.634/ 3.699-3.569	-1.83±0.5	+0.36/ +1.52/ +0.90	+0.95±0.5
59	idem, articulated Sinonovacula sp., 7.28 m/+4.38 m/-2.9 m	Beta 353808	6 440±40	-9.5	6 690±40	7 380/ 7 497-7 255	in situ	no	-	7.380/ 7.497-7.255	-2.9±1	+0.74/ +2.13/ +0.90	+0.87±1
60	idem. gleysoil (-), plant debries (>180 μm), 7.39-7.40 m/+4.38 m/-3.01 m	Beta 353796	NA	NA	5 930±30	6 752/ 799-6 671, 6 844-6 816	in situ	yes	-	6.752/ 6.844-6.671	-3.01±1	+0.67/ +2.17/ +0.90	+0.73±1
61	idem. organic mud, plant debris (>180 μm), 8.63-8.65 m/+4.38 m/-4.26 m	Beta 353798	6 440±40	-26.7	6 410±40	7 350/ 7 420-7 271	in situ	yes	-	7.350/ 7.420-7.271	-5.52±0.05	+0.73/ +2.59/ +0.90	-1.3±0.05
62	Core QX02, intercalated organic mud (<180 μm), 5.68 m/+3.57 m/-2.11 m	38.6°N, 117. 0°E	Beta 332792	5 430±30	-24.0	5 450±30	6 247/ 6 300-6 204	in situ	no	-1320	4.927/ 4.98-4.88	-3.41±0.5	+0.49/ +1.78/ +0.95	-0.19±0.5
63	idem, intercalated organic mud, plant debris (>180 μm), 7.27 m/+3.57 m/-3.70 m	Beta 333329	6 360±30	-25.7	6 350±30	7 283/ 7 218-7 176, 7 331-7 240, 7 373-7 356, 7 413-7 390	in situ	yes	-	7.283/ 7.413-7.176	-5.0±0.5	+0.73/ +2.10/ +0.95	-1.22±0.5
64	idem, plant debris (>180 μm), 8.98 m/+3.57 m/-5.41 m	Beta 333330	6 620±30	-26.3	6 600±30	7 494/ 7 522-7 434, 7 564-7 532	in situ	yes	-	7.494/ 7.564-7.434	-6.71±0.5	+0.75/ +2.71/ +0.95	-2.3±0.5
65	Core Yugong 3, basal peat, peaty mud, 14.72-14.92 m/about +1.26 m/-13.56 m	38.5°N, 117.6°E	CG70	9 120±180	-27	9 088±184	10 240/ 9 640-9 635, 10 711-9 661	in situ	no	-660	9.580/ 10.05-8.97	-14.82±0.05	+0.96/ +4.59/ +0.06	-9.21±0.05
66	Core 8-1, lagoon-salt marsh behind chenier, gleysol horizon, Gyraulus sp. and Assiminea sp., 1.55-1.59 m/+2.80 m/+1.23 m	38.2°N, 117.8°E	AA 45899	-	-7.0	2 869±56	2 827/ 3 018-2 689	in situ	yes	-	2.827/ 3.018-2.689	+0.13±0.03	+0.28/ +0.60/ +0.15	+1.16±0.03
67	Core LL1, basal peat, mainly charcoals, 15.22-15.35 m/+2.80 m/-12.48 m	03Y141	7 550±230	-27	7 550±233	8 368/ 8 981-7 944	in situ	no	-	8.368/ 8.981-7.944	-13.58±0.03	+0.84/ +4.44/ +0.15	-8.15±0.03
No.	Notes
Explainations of the indicative meaning, RTC, the three spatial corrections and changes from the previous work (Li et al., 2015)
1	It is a shelly layer intercalated in intertidal muddy sediments (SQG, 1980). Two estimated sampling elevations were -2 m and -1.5 m (Han and Meng, 1996), so an average -1.75 m is used in this study. RTC was given (Li et al., 2015). “□” -1.75 m, msl (-1.75±1) m while error was ±1.5 m in Li et al. (2015).
2	An organic mud was formed in lagoon-swamp left by marine regression. This thin muddy layer, with fresh water mollusks including this dated Arcomaia contorta (-) and antler of Elaphodus davidianus, laid on the marine influenced sediments with oyster and razor clam shells (Zhao et al., 1989). Sampling depth and corresponding elevation were estimated on local topographic map (Zhao et al., 1989). “┬” -1m and (1.26±0.05) m (Table A3) should be subtracted for restoring msl, i.e., msl (-2.26±0.05) m. However, msl was (-2.5±0.5) m in Li et al. (2015) because MTL and MSL were confused at that time.
3	This thin twig was mixed into top part of marine sediments, buried by the upper peat layer, from which the sample CG1394 was obtained (Zhao et al., 1989). “┬” -1.3 m, (1.3±0.5) m should be subtracted (Table A3), i.e., msl (-2.6±0.5) m. However, 1.5 m was chosen in Li et al. (2015).
4	It was a shelly layer intercalated into intertidal muddy sediments (Han et al., 1980; Peng et al., 1980; Peng et al., 1984; Li and Zhao, 1990). The samples, taken from the lower to bottom part of the layer, indicate intertidal environemnt. However, the two ages are more than 1 100 cal a different and only sampling depths were given in the literature but without elevation. Correponding elevation has to be converted from topographic map. “□” 0 m, msl (0±1.5) m (Li et al., 2015).
5	This twig, 10 cm-long and 1.5 cm in diameter, was found mixed in the intertidal muddy sediments, immediately (only 1 cm) above the reef top. Foraminifera (abbreviated as “forams” below) analysis indicates that the muddy sample (10 g, dry), taken exactly from the same position with this twig, contains 14 different species and total 796 tests, mainly Ammonia beccarii vars., Elphidium nakanokawaense and E. magellanicum dominantly, indicating sea water influence in intertidal zone closing to estuary. Upwards, 10-20 cm above the reef-top, the mud consists of 18 species and about 1 600 tests, including A. beccarii, E. nakanokawaense and A. takanabensis. Both forams samples indicate the mid- to upper part of intertidal zone (Liu, 2010). Moreover, sedimentary facies analysis shows the mud sediments, overlying on the reef, remained intertidal environment for its lower 1.5 m thickness and further upward gradually changed to lagoon and salt marsh environment (Liu, 2010; Wang, 2012; Wang et al., 2011c). Elevation of the sampling position, -2.229 m, was given by Total Station measurement relative to the National Vertical Datum 1985 (Liu, 2010) (see the sea level indicator BA110343 for further details). So, “□” -2.23 m and directly indicates msl (-2.23±0.7) m. This reconstructed msl is same as Li et al. (2015) while the vertical error is changed from ±1.5 m to ±0.7 m in this study. RTC is not necessary because it is a macro plant fossil.
6	This terrigenous, 50-60 cm thick, peaty layer is 0.5-1 m below the ground surface and the sample was taken from the top part of the layer (IOA, 1983; Peng et al., 1980). Elevation of ground surface, about +2 m, is estimated from local topographic map by this study. The sampling position indicating “┬” about +1.25 m; two samples give a time range of 1988-956 cal a BP. After RTC it becomes about 1.33-0.85 cal ka BP. Contemporaneous msl was (-0.01±0.05) m, i.e., subtracting (-1.26±0.05) m (Table A3). However, (1.8±0.5) m was subtracted in Li et al. (2015).
7	The sampling layer shows a sudden increase of forams tests from less than a few hundred 60 cm below to about 3 630 at this position, where Pseudononinella variabilis and Ammonia beccarii vars. dominated, of which about 2 000 tests were picked out for AMS dating. On the other hand, 30 cm above, forams tests decreased to several tens only (Li et al., 2004; Wang et al., 2010). So, it indicates a flooding intrusion event of high sea water. “┬” +1.02 m, msl (-0.28±0.5) m, i.e., 1.3 m (an indicative meaning for sample deposited at MHHW in paleo bayhead, Table A3) is subtracted from +1.02 m. However, it was regarded as an equivalent of the upper peat, so 1.8 m was subtracted for msl reconstruction in Li et al. (2015).
8	The shells still kept their vertical growth position and were articulated (Li et al., 2004; Wang et al., 2010), and indicating intertidal muddy environment with freshwater input (Zhang, 2008; Okutani, 2000). RTC is not necessary for these in situ shells. “□” -0.45 m, msl (-0.45±0.5) m (Li et al., 2015).
9	Based on sedimentary facies analysis, forams study (Li et al., 2004) and the ecological description for this species (Zhang, 2008; Okutani, 2000), it was in the middle to lower intertidal and the upper subtidal zones. Five forams samples, from 2.8-4.8 m in depth, show 23 species and about 450 tests in average for each sample and maximum 30 species and 4 800 tests, mainly Quinqueloculina akneriana rotunda, Elphidium magellanicum, E. nakanokawaense, Ammonia beccarii vars. (Li et al., 2004). Ostrocoda of 2.5-4.5 m were mainly composed of Sinocytherdea impreaa, Tanella opima, Spinileberis sinensis, Eucyther serrata, Laxoconcha binhaiensis and totally 46 species with about 1 800 valves in average for each sample, indicating mid- to lower part of intertidal zone) (Lin et al., 2004). “┴” -2.3 m, msl (-0.7±0.5) m, i.e., adding (1.6±0.5) m (Table A3). RTC is not necessary for the in situ articulated shells. In Li et al. (2015), msl was (-0.8±0.5) m because 1.5 m was simply added.
10	Only rarely marine influenced Ostracoda Neomonoceratina dongtaiensis and Sinocytheridea impressa were found in the middle to upper part of this basal peat layer (Lin et al., 2004) but without forams (Li et al., 2004). Forams were found only from about 1 m above this layer with 48 tests of Ammonia beccarii indicating Holocene marine transgression started (Li et al., 2004). So, the sampling position roughly shows MHHW, i.e., “┬” -15.0 m, msl (-16.26±0.05) m ((1.26±0.05) m is subtracted, Table A3). After RTC, time range 9885-9270 cal a BP is corrected to 9.22-8.61 cal ka BP. However, (1.5±0.5) m was mistakenly subtracted from -15 m in Li et al. (2015).
11	Facies analysis and microbiological studies indicate that the sediments in depth 12-3 m were deposited in Holocene marine environments while this sample, at 4.4 m, was formed during the late Holocene marine regression. Microbiological sample at 4.8 m (40 cm below this sample), show (1) diatom: Coscinodiscus spp. and Diplonesis bombus; (2) forams: Quinqueloculina seminula and Massilina inaequalis; (3) ostracoda: Albileberis sinensis, Bicornucythere bisanensis, Metacytheropleron elliptica, Neomonoceratina dongtaiensis, Sinocytheridea impressa and Stigmatocythere dorsinoda. On the other hand, these analyses did at depth 4.0 m (40 cm above) also found marine fauna, in which diatom Coscinodiscus spp. reached 20 valves^①. Ground elevation of the borehole was firstly estimated as about +1.5 m, afterwards the actually measured value +1.315 m was obtained¹. “□” -3.09 m, msl (-3.09±0.5) m. RTC was given (Li et al., 2015).
12	+0.71 m of the ground elevation in this site was checked out from 1:10 000 map^-. This articulated in situ razor clam was found just above the depositional boundary between lagoon-salt marsh and underlying open intertidal flat^-. “┬” -0.44 m, msl (-1.74±0.5) m (Table A3). However, 1.8 m was subtracted in Li et al. (2015) when mistakenly made it as an equivalent as the upper peat.
13	Facies analysis indicates that the sedimentary environment of U5, from which this sample was taken, had changed from open intertidal to lagoon-salt marsh, caused by continuously increased accumulations of fluvial sediments (Chen et al., 2014). Forams show both Ammonia beccarii vars. and Elphidium magellanicum decreased but E. hugesi foraminosum, E. kiangsuensis, Pseudononionella variabilis and Cribrononion porisuturalis increased. Ostracoda assemblage shows Albileberis sheyangensis, Bicornucythere bisanensis, Neomonoceratina dongtaiensis, N. triangulata and Trachyleberis niitsumai disappered but Sinocytheridea impressa and Loxoconcha binhaiensis increased. All these indicate the sampling position was in the upper part of intertidal depth in lagoon-salt marsh environment (Chen et al., 2016).“┬” +1.03 m. msl (-0.12±0.5) m, i.e., (1.15±0.5) m should be subtracted because it is located in the northeast periphery (Table A3). However, it was mistakenly regarded as an equivalent of the upper peat, so 1.8 m was subtracted for msl reconstruction in Li et al. (2015).
14	This is a 10 cm-thick shell hash layer in marine facies U4, also with single valves of Chlamus forreri and other marine shells. Forams show A. beccarii vars. was domimant, while E. magellanicum decreased but Protelphid turberculatum and Quinqueloculina akneriana increased (Chen et al., 2014, 2016), indicating the lower limit of MLLW. “┴” -7.22 m. msl (-5.72±0.5) m, i.e., (1.5±0.5) m should be added because it is in the northeast periphery (Table A3). RTC was given. Although (1.5±0.5) m was also added in Li et al. (2015), it was considered as a half of MHHW-MLLW as a general value for the whole coast of the west Bohai Bay. However, there was lack of awareness of difference between MTL and MSL, and tidal regime difference beween bayhead and pheriphery at that time in Li et al. (2015).
15	The charcoal grains, >180 μm, picked out from a shelly layer, consisting of C. pestigris, P. laevis and Dosinia sp., intercalated into U3. It was formed in intertidal zone (Chen et al., 2014, 2016). RTC is not needed because of large charcoal grains. “□” -15.65 m, msl (-15.65±0.5) m while was (-15.65±1.5) m (Li et al., 2015).
16	Although terrigenous Bithynia sp. and Gyraulus sp. shells were found about 30 cm below this layer, broken Crasostrea shell and other marine shell fragments appeared 60 cm above it (Liu, 2007). So, it can be roughly recognized at MHHW level, i.e., “┬” -14.67 m, msl (-15.93±0.05) m (subtracting (1.26±0.05) m, Table A3), though msl was (-16.4±0.5) m in Li et al. (2015), i.e., subtracting (1.8±0.5) m when the sampling elevation was roughly determined as -14.6 m. RTC was given (Li et al., 2015).
17	Although it was immediately under the marine sediments the ground elevation +4 m (IOA, 1983) is doubtful, i.e., without definite indication of which elevation system was used. This study guesses it should be the 1950 Dagu System (the local system). So, 1.543 m is subtracted from the original +4 m. Thus, “┬” -14.53 m, msl (-15.79±0.05) m, i.e., subtracting (1.26±0.05) m (Table A3) though (1.8±0.5) m was subtracted in Li et al. (2015). RTC is not needed because the sample was described as plant peat (IOA, 1983) though it was given in Li et al. (2015).
18	Scattered fragments of marine diatom Coscinodiscus spp. were found in this peat layer, i.e., upper part of the diatom Assemblage Zone 1. This basal peat layer was eroded at 11.2 m by overlying marine mixed deposits of black-brown shell hash of Chlamus farreri (-), Crassostrea sp. and Anomia chinesis, with silty sand, consisting of diatom Zone 2, including Cyclotella striata/stylorum, Coscinodiscus perforates, C. subconcavus, Actinoptychus undulates, C. argus, C. radiates, Thalassionema nitzschioides and Grammatophora oceanica (Shang, 2011). Sedimentary facies analysis and diatom assemblages indicate this basal peat had been slightly influenced by sea water as salt marsh environment and then eroded by shallow sea deposits⁹ (Shang, 2011). So, “┬” -15.95 m. msl (-17.21±0.05) m, i.e., (1.26±0.05) m should be subtracted because at bayhead (Table A3). RTC was given (Li et al., 2015). However, (1.8±0.5) m was subtracted in Li et al. (2015).
19	This 2 cm-thick bulk sample, then subdivided into two subsamples, was taken from a dark muddy peaty layer in depth of 17.4-17.6 m. This layer consists of Gyraulus sp., Cincinnatia alticonula, Stenothyra glabra and Assiminea lutea, and seeds, semi-carbonized plant debris, and fresh water Ostracoda Cypris decaryi, Candonirlla albicans, Ilyocypris radiate and I. subbiplicata but without forams, indicating this is fresh water peat (unpublished data of the authors’ group). However, Assiminea sp. appeared in overlying mud at 15.05-17.4 m, indicating the beginning of sea water influence. This subsample of charcoals and twigs is about 1 330 cal a younger than another subsample: 11 167 cal a BP/BA081877, shells of Gyraulus sp. and Bithynia sp. (Li et al., 2015). On the other hand, this peaty layer was gradually formed from the underlying hard mud, i.e., the late Pleistocene soil horizon formed in LGM (Last Glacial Maximum). This in situ characteristic and being much younger than another subsample mean that RTC is not necessary. “┬” -21.32 m and (1.26±0.05) m should be subtracted (Table A3). So, msl (-22.58±0.05) m. However, (1.8±0.5) m was subtracted in Li et al. (2015).
20	Although marine sediments existed immediately above this basal peat layer, elevation of the borehole is approximately estimated only (Li and Zhao, 1990; Wang and Tian, 1999). So, “┬” -15.6 m, msl (-16.86±0.05) m (Table A3). However, it was (-17.4±0.5) m in Li et al. (2015) because (1.8±0.5) m was subtracted. RTC was given (Li et al., 2015).
21	Comparing with indicator TD408, this is a lower peat layer (Wang and Tian, 1999) and also enables to approximately indicate arrival of rising water. So, “┬” -16.41 m, msl (-17.67±0.05) m (i.e., subtracting (1.26±0.05) m), though it was (-18.21±0.5) m (i.e., subtracting (1.8±0.5) m) in Li et al. (2015). RTC was given as subtracting 660 cal a from two end dates of 10 152 and 9 316 cal a BP, respectively (Li et al., 2015).
22	This basal peat layer was covered by marine muddy sediments with a marine shell hash layer of 12.7-13 m and also a shell-concentrated lamina was just above this basal peat (Luo et al., 1983). However, the borehole elevation of +3.5 m is estimated to be the local datum, so 1.543 m shoule be subtracted in this study to be +1.96 m. As to the Borehole elevation of +1.6 m in Peng et al. (1980), it is probably wrong because Luo et al. (1983) was personally responsible for this drilling work. So, “┬” -11.43 m, msl (-12.69±0.05) m (Table A3), though it was -13.23 m (i.e., subtracting 1.8 m) in Li et al. (2015). RTC was given (Li et al., 2015).
23	A 10 cm-thick, in situ, black clayey peaty mud, as the upper peat layer, was revealed by a transverse section cutting Chenier II-1 in this site. The layer covered from chenier-crest landward to its lee side till the chenier rear-end (Fig. 4f). Such a tilted peaty layer is +1.7 m high on the chenier-crest and +0.7 m high on the rear-end. This sample was taken at about +0.8 m. Sedimentary analysis indicates this peaty sample was formed above the level of poured high sea waters (Wang et al., 2000c). So, “┬” +0.8 m and msl (-0.46±0.05) m, i.e., subtracting (1.26±0.05) m (Table A3). However, in Li et al. (2015), 1.8 m was subtracted and was perceived as reworked deposit and RTC of 660 a was given.
24	It is a 33 cm-thick transitional layer in depth of 16.63-16.3 m in between the Holocene marine transgressive sediments and the underlying terrigenous muddy sediments. This subsample is about 610 cal a younger than another subsample, freshwater Arconaia contorta shell: 9 225 cal a BP/BA04543 (Li et al., 2015) and, therefore, RTC is not necessary. This indicates intertidal environment, i.e., “□” -14.93 m, msl (-14.93±1) m while was (-14.93±1.5) m (Li et al., 2015).
25	It is also a beach face layer and about 130 cal a younger than another subsample of single valve of Mactra chinensis: 2 397 cal a BP/Beta363625 (Li et al., 2015; Shang et al., 2015; 2016). The sampling position is about 75 cm below the average top elevation of -2.16 m. So, “□” -2.91 m, msl (-2.91±1) m. RTC is not necessary. The reconstructed msl is more accuracy than (-3±1.5) m in Li et al. (2015).
26	It is a totally 30 cm-thick, horizontally alternative deposits between shelly hash laminae and muddy laminae with very fine, carbonized herbaceous twigs. It was formed in between the Shell Bed A and the overlying salt marsh-lagoonal mud (Shang et al., 2016), indicating a remained influence of sea water. It might indicate high waters rather than the intertidal as Li et al. (2015) used. The sampling position is 30 cm below the average top elevation of -2.16 m. So, “┬” -2.46 m and msl should be (-3.76±0.5) m, i.e., (1.3±0.5) m is subtracted (Tables A2 and A3). However, it was believed as intertidal environment and reconstruceted msl was (-2.46±1.5) m in Li et al. (2015). RTC is not necessary because it is macro plant though RTC calibration was given in Li et al. (2015).
27	This is a beach face in intertidal depth, Shell Bed A, and about 1 260-0 cal a younger than another two subsamples of single valves of Moerella sp.: 2 695 cal a BP/Beta352329, and Potamocorbula laevis: 1 436 cal a BP/Beta352330 (Li et al., 2015; Shang et al., 2015, 2016). The sampling position is 70 cm below the average top elevation of -2.16 m. Thus, “□” -2.86 m, msl (-2.86±1) m though msl was (-2.86±1.5) m in Li et al. (2015). RTC is not necessary (Li et al., 2015).
28	This in situ razor clam is still in vertical position and articulated. The species is in lower, or middle to lower, intertidal environment based on conchologists such as Okutani (2000) and Zhang (2008). However, sedimentalogically, this individual probably survived in a small depression with poured high waters in the Shell Bed A, top layer of the Holocene marine sediments (Shang et al., 2016), therefore, it may roughly indicate high position of the intertidal environment rather than its normally growing environment. It was 210-70 cal a younger than another two subsamples of the same layer as Umbonium sp.: 1 830 cal a BP/Beta352327, and single valve of Scapharca kagoshimensis: 1 689 cal a BP/Beta352326 (Li et al., 2015). The sampling position is about 30 cm below the average top elevation of -2.16 m (Li et al., 2015; Shang et al., 2015, 2016). So, “┬” -2.46 m and msl (-3.76±0.5) m (i.e., subtracting (1.3±0.5) m, Table A3), while msl was (-2.46±1.5) m in Li et al. (2015) because the conchlogists’ idea was simply followed. RTC is not necessary (Li et al., 2015).
29	This shell bed, as a cover of the Holocene marine sediments, was formed in intertidal zone and it is about 390 cal a younger than another subsample of single valve Potamocorbula laevis: 2019 cal a BP/Beta335908 (Shang et al., 2015, 2016). So, “□” -2.4 m, msl (-2.4±1) m (Table A3), though it was (-2.4±1.5) m (Li et al., 2015). RTC is not necessary.
30	It was in the intertidal flat during the Holocene marine regression period and is about 470 cal a younger than another subsample of single valve of Corbicula: 2 812 cal a BP/Beta335909 and RTC is not necessary. Elevation of the outcrop was Total Station and RTK measured relative to the National Vertical Datum 1985. “□” -2.7 m, msl (-2.7±1) m while it was (-2.7±1.5) m (Li et al., 2015).
31	It was a 10 cm-thick, coarse shelly layer, having high-angle foreset bedding, in the top part of Shell Bed A, Section 2, and is about 650-290 cal a younger than another two subsamples of single valve of Scapharca kagoshimensis (1 830 cal a BP/Beta335913) and shell hash (2 198 cal a BP/Beta335914) (Shang et al., 2016; Li et al., 2015). In this pit (about 200 m×120 m, cf., Shang et al., 2016), top elevations of the Holocene marine sediments and the ground surface were Total Station measured and then connected with the National Vertical Datum 1985 by RTK, showing the average elevation (-2.16±0.2) m (simply as -2.2 m) for top of the Holocene marine sediments (12-spot average, but only six sites were shown in Shang et al. (2016) and +1.93 m of the local ground surface elevation, unpublished data of the authors’ group). In this paper, elevations of sea level indicators in this pit were then calculated as a vertical distance from the average top elevation (-2.16±0.2) m (or (-2.2±0.2) m). As to this sample, it was probably formed by MHHW. “┬” -2.2 m. So, (1.3±0.5) m should be subtracted and msl (-3.5±0.5) m. RTC is not necessary compared with another two subsamples (Li et al., 2015). However, msl was (-2.2±1.5) m because it was approximately regarded as intertidal depth and without further compensation for msl reconstruction in Li et al. (2015).
32	It was formed in intertidal environment (Li et al., 2015; Shang et al., 2016). Sampling position is about 80 cm lower than the average top elevation of -2.16 m in this site. “□” about -3.0 m, msl (-3.0±0.5) m. RTC was given (Li et al., 2015). The indicative meaning was same while error was ±1.5 m in Li et al. (2015).
33	These three articulated shells, as three subsamples, taken from the same thin shelly layer in the shell beach, Shell Bed C, show the exactly same or highly overlapping ages of their 2σ range, and are about 1 130 cal a younger than the fourth subsample of single valve of Dosinia corrugate: 3 450 cal a BP/Beta352324 (Li et al., 2015; Shang et al., 2015, 2016). The sampling position is about 1.5 m below the average top elevation of -2.16 m (Li et al., 2015). This layer is considered as shelly beach in intertidal environment. So, “□” -3.7 m, msl (-3.7±1) m. Such three subsamples give the same ages and are 1 150 cal a younger than the fourth one, RTC is therefore not necessary (Li et al., 2015). In Li et al. (2015), vertical error was ±1.5 m.
34	This coin was found mixed with shells and muds at the same beach face layer, Shell Bed C (Li et al., 2015). Based on the two independent experts, this coin was cast during 221-118BC, i.e., 2 171-2 068 cal a BP. Numerous shards of pottery, 8 pottery fishnet sinkers and a 37 cm-long artifact of edge-truncated cattle-shoulder, as a spade/cooking utensil (-), were found altogether with this coin in this layer (Shang et al., 2015, 2016). The archaeological evidence indicates this beach face was soon exposed above MHHW during 221-118 BC (Shang et al., 2015, 2016). So, “┬” -3.7 m, and (1.3±0.5) m should be subtracted (Table A3). As a result, msl (-5.0±0.5) m. However, 1.8 m was subtracted in the light of the peat layer in Li et al. (2015).
35	This Shell Bed D is another beach face layer and about 0.45 m below the Shell Bed C and was also intercalated into fine sandy-/muddy marine sediments. About 420 cal a younger than another subsample of single valve Dosinia corrugate: 2 639 cal a BP/Beta363622 (Li et al., 2015; Shang et al., 2015, 2016). “□” -4.15 m, msl (-4.15±1) m, while the error was ±1.5 m in Li et al. (2015).
36	It was taken from the upper part of U5, consisting of abundant Potamocorbula laevis, and Ammonia confertitesta, Quinqueloculina akneriana rotunda, A. beccarii vars., A. annectens; Sinocytheeridea impressa, Bicornucythere bisanensis and Neomonoceratina dongtaiensis. It was in intertidal environment. The overlying sediments, at least 2 m thick, were still affected by high waters as salt marsh (Chen et al., 2012a, b, 2016). The borehole elevation was measured by Total Station and connected with the present-day National Datum. So, “□” -2.96 m, msl (-2.96±1) m. RTC was given. The sampling elevation was miscaculated as -2.94 m (Li et al., 2015).
37	This is the U2, 16.9-16.0 m, deposited in the estuary or tidal flat with fresh water influence, see notes of the previous indicator. The underlying U1, 20.0-16.9 m, fine sand and silt with land snail Cathaica sp. and carbonized plant debris, showing terrestrial environment (Chen et al., 2012a, b, 2016). So, this sample indicates intertidal environment, i.e., “□” -15.26 m, msl (-15.26±1) m. RTC was given (Li et al., 2015). However, the vertical error was ±1.5 m in Li et al. (2015).
38	Definitely marine influenced shelly hash layer with Scapharca kagoshimensis was found only 0.8 m above this peaty layer. This basal peat was formed in coastal salt marsh based on seismic investigations and sedimentary analysis (Tian et al., 2017). So, it was just above high waters. “┬” -17.2 m, msl (-18.46±0.05) m (Table A3). RTC was given (Li et al., 2015).
39	11.3-11.45 m: marine shell layer, with >10 cm fragmentary shells of Crassostrea sp., eroded the underlying sediments. 11.45-12.6 m: mud, without definite marine-influenced evidence. So, this sample, taken from the peaty layer of 12.6-12.7 m, approximately shows that corresponding msl must be lower. It is about 630 cal a younger than another subsample (portion <180 μm): 8 899 cal a BP/Beta355822, so RTC is not necessary (Li et al., 2015). “┬” -10.97 m, and (1.26±0.05) m was subtracted (Table A3), so msl (-12.23±0.05) m. Howwver, (1.8±0.5) m was subtracted in Li et al. (2015).
40	This basal peat layer was immediately overlaid by Holocene marine sediments with suddenly increasing of forams and marine ostracoda, 100-200 individuals, respectively, per sample (Wang et al., 1986). However, the borehole elevation could be only approximately estimated as +4 m relative to the local datum and so 1.543 m is therefore subtracted by this study to convert to the National Vertical Datum 1985 system. So, “┬” -15.49 m, and (1.26±0.05) m should be subtracted (Table A3), i.e., msl (-16.75±0.05) m. RTC was given (Li et al., 2015). However, the ground elevation was estimated as +2 m of the local datum and 1.8 m was subtracted by Li et al. (2015) and thus, reconstructed msl was even about 2.5 m lower than this study.
41	It was taken from the lower part of the same basal peat layer with the sample ZK1465 (Wang et al., 1986). This sample was even less marine-influenced than the overlying one (ZK1465), So, “┬” -16.19 m, msl (-17.45±0.05) m, i.e., subtracting (1.26±0.05) m (Table A3). However, msl was (-19.96±0.5) m in Li et al. (2015), see the notice of indicator ZK1465. RTC was given (Li et al., 2015).
42	It was taken from the lower part of Bed 2. This bed, yellowish brown (10YR 4/3) silt, is composed of Potamocorbula laevis, Scapharca subcrenata, Arcopsis sp., Sinonovacula aonstricta and Nassarius sp., with brackish water species such as Corbicula fluminea, Parafossarulus exiguous, Assiminea latericea. Forams increased to >5 000 tests while marine ostracoda reached to >1 760 valves per sample. This was a fluctuation of sea level rise named as Ib^④, formed in estuary in intertidal depth to subtidal depth (Yan et al., 2006a, b). So, it may indicate “┴” -14.81 m, msl -13.21 m, i.e., adding (1.6±0.5) m (Table A3). RTC was given (Li et al., 2015). However, (1.5±0.5) m, a half of mean high tidal range, was simply added in Li et al. (2015).
43	It was deposited on the top part of Holocene marine sediments, with abundant shell hash mixed in mud, indicating intertidal environment following marine regression process (unpublished data of the authors; Sun et al., 2011). “□” -3.85 m, msl (-3.85±1) m (Table A3). RTC was given (Li et al., 2015).
44	This and the following data all have a few forams, including Elphidium nakanokawaense and Ammonia beccarii, which suggest a coming of sea water (unpublished data of the authors’ group). So, “┬” -12.25 m, msl (-13.51±0.05) m, i.e., (1.26±0.05) m should be subtracted (Table A3). RTC was given (Li et al., 2015). However, (1.8±0.5) m was subtracted in Li et al. (2015).
45	This is same as the overlying BA091542 (unpublished data of the authors’ group). “┬” -12.35 m, msl (-13.61±0.05) m, i.e., (1.26±0.05) m should be subtracted (Table A3). However, 1.8 m was subtracted in Li et al. (2015). RTC was given (Li et al., 2015).
46	Many shelly hash and scattered articulated Mactra chinensis (-) were also found in this peaty layer and the layer was considered as the upper peat layer, i.e., “┬” +2.16 m. So, msl (+0.9±0.05) m (Table A3). RTC was given (Li et al., 2015). However, (1.8±0.5) m was subtracted in Li et al. (2015).
47	Forams at the sampling position of 16.3 m show less than 50 tests but rapidly booming upward to >500 tests at 16 m. It indicates an ongoing rising-fluctuation of sea level. Unfortunately, there is only a test-counting but without species identification. So, it is better to think it was in intertidal zone, i.e., “□” -12.84 m, msl (-12.84±1) m. RTC was given (Li et al., 2015). The sampling position was considered at low tidal environment and 1.5 m, as a half range between MHHW and MLLW, was added to restore msl as (-11.34±0.5) m in Li et al. (2015). However, difference between MTL and MSL was not realized three years ago.
48	As an intercalated thin peaty layer, it was found in the lower part of Holocene marine muddy sediments. Based on facies analysis, this layer was in salt marsh in intertidal depth with forams tests nearly 350 a sample. Although it was perhaps reworked peaty-concentrated layer, this subsample (>180 μm) is about 420 cal a younger than another one (i.e., the portion <180 μm): 9 287 cal a BP/Beta358055 (Li et al., 2015). Therefore, RTC seems to be not necessary. “□” -13.76 m, msl (-13.76±1) m (Table A3), though the error was ±1.5 m in Li et al. (2015).
49	It immediately overlay on the early Holocene terrigenous, yellowish brown mud, with a small amount of forams (<50 tests), indicating onset of marine transgression. “┬” -15.39 m, msl (-16.65±0.05) m, i.e., (1.26±0.05) m should be subtracted (Table A3). RTC should be given. This is a new indicator in this study.
50	Forams in Zone VI were nearly disappeared from 5.6 m upwards, only very few Ammonia beccarii vars. and Nonion glabrum remained in euryhaline and brackish environment as a small depression where only very high sea waters may enter. Facies analysis indicates this was in transition between Holocene marine environment and overlying salt marsh (Wang et al., 2015). So, it can be used to indicating a position of MHHW, i.e., “┬” -0.36 m, thus msl (-1.66±0.5) m (subtracting (1.3±0.5) m, Table A3). Only the portion <180 μm was dated and the empirical estimation of 1 320 cal a should be subtracted for RTC.
51	This sample was taken from the very bottom of Forams Zone VI. Comparing to huge amount of forams as >17 000 tests at 6.6 m and >38 000 at 6.8 m of Zone V-2, howevr, this sample is composed of 57 tests as being Ammonia beccarii dominant only (Wang et al., 2015), and CaCO₃ illuviation existed at 6.1-6.3 m. Such a calcium precipitation was abundant but without clear boundary showing short and not mature condition. All these suggest this sample was formed at around MHHW level, perhaps a small depression in saltmarsh environment with an increased evaporation upwards. So, “┬” -1.19 m, msl (-2.45±0.05) m (subtracting (1.26±0.05) m, Table A3). However, 1.8 m was subtracted in Li et al. (2015). RTC was given to subtracting 1 320 cal a (Li et al., 2015).
52	Formas Zone V-2. a salt marsh-lagoonal environment occasionally inundated by high water from open bay and has Ammonia beccarii, Pseudononion minitum, Nonion glabrium and Cribrononion porisuturalis (Wang et al., 2015). Stratigraphical transition from the immediately underlying hydromorphic soil to this faint, darkish layer also indicates this seawater reoccupied environment. So, “┬” -2.04 m and (1.3±0.5) m should be subtracted (Table A3), i.e., msl (-3.34±0.5) m. However, in the early study (Li et al., 2015) this sample was believed as in the intertidal depth, so no further vertical compensation was given. RTC was given (Li et al., 2015).
53	It is a faint, darkish layer belonging to the start of Forams Zone V-1. A very small amount of forams of Nonion glabrum (12 tests) and Ammonia beccarii (1 test) was found in a 20 g dry sample taken from this layer, showing brackish environment (Wang et al., 2015). This subsample was about 360 cal a younger than another subsample (portion <180 μm): 7 005 cal a BP/Beta331456 (Li et al., 2015). “┬” -3.04 m, msl (-4.34±0.5) m (subtracting (1.3±0.5) m, Table A3). It was classified to the “upper peat layer” in Li et al. (2015), or more precisely, a layer transformed from the immediately underlying hydromorphic gley horizon in salt marsh-lagoon environment behind shoreline and was occasionally inundated by high water.
54	A faint darkish layer in the bottom of Zone IV, 8.7-7.8 m. Two samples were without forams and only one sample had 13 tests, and colour become yellowish with weakly mottling structure, all indicating increased terrestrial influence, i.e., sea level drops for a short time. So, this sample was in the upper part of intertidal zone in between MHHW and MHW and can be treated as an intercalated organic-rich layer. It was about 210 cal a younger than another subsample (portion <180 μm): 7 088 cal a BP/Beta331457 (Li et al., 2015). So, “┬” -3.54 m and (1.26±0.05) m (Table A3) should be subtracted, i.e., msl (-4.8±0.05) m. However, 1.8 m was subtracted in Li et al. (2015).
55	Zone III, 9.2-8.7 m in depth and 2 forams samples show 68 and 338 tests, respectively, and Nonion glabrum dominated (Wang et al., 2015;). About 1 050 cal a younger than another subsample (portion <180 μm): 8 162 cal a BP/Beta331458 (Li et al., 2015). Downwards, core logging and forams study of Zone II, 10.6-9.2 m, having 7 forams samples with tests in between 0-18, showing a saltmarsh occasionally influenced by high water. So this sample can also be roughly treated as an intercalated organic-rich layer. “┬” -4.0 m, msl (-5.26±0.05) m, though (1.8±0.5) m was subtracted in Li et al. (2015).
56	Although freshwater diatoms were still dominant such as Eunotia spp. and Synedra unla, more marine and brackish species, including planktonic Cosinodiscus spp. and Actinocyclus spp., were found. The freshwater diatoms also changed from benthic (e.g., Eunotia spp. and S. unla) to planktonic taxa (e.g., Melosira spp.). On the other hand, forams in this sample downward to the basal peat at about 13.1 m, 8 010 cal a BP (Beta329646) were very rare for 4 forams samples (0-3 tests per sample). However, forams sadenly boomed from above 11.2 m upward (Wang et al., 2015). Therefore, this sample itself can be reconnized as being highwater influenced and indicating an arrival of sea water to the area. This subsample is about 160 cal a younger than another subsample (portion <180 μm): 8 016 cal a BP/Beta331455 (Li et al., 2015). So, “┬” -6.23 m, msl (-7.53±0.5) m (i.e., subtracting (1.3±0.5) m, Table A3) and RTC was given (Li et al., 2015). However, 1.8 m was subtracted in Li et al. (2015).
57	It was a peaty layer, with fragments of Gyraulus sp. and small CaCO₃ concretions, formed in lagoon-salt marsh environment and may indicate a maximum height of MHHW (unpublished data of the authors’ group). RTC is not necessary because only the plant debris was used for dating. “┬” +1.47 m, and (1.26±0.05) m (Table A3) should be subtracted because it shows influence by MHHW. So, msl (+0.21±0.05) m. However, (1.8±0.5) m was subtracted in Li et al. (2015).
58	A dark organic layer, 4.9-5.1 m, was formed in salt marsh-lagoonal muddy environment and this sample was taken from the very top of the layer. Forams in this layer have >500 tests a 20 g dry sample and then sharply decreased upward to less than 50 in the overlying sediments, implying an end of seawater fluctuation upwards. So, this layer was probably influenced by MHHW. “┬” -0.53 m, subtracting (1.3±0.5) m to restore msl, i.e., msl (-1.83±0.5) m. RTC is not needed (Li et al., 2015). However, 1.8 m was subtracted in Li et al. (2015).
59	6-7.9 m: mud, dull yellow orange (10YR 7/2), clearly mottling structure with scattered Fe/Mn nodules of mm scale. This is different with both underlying and overlying brownish gray mud (10YR 5/1) and indicating more oxidized environment. Conchologically, this razor clam usually lives in mid- and lower intertidal zone with fresh water input, according to Zhang (2008) and Okutani (2000). The ¹³C value, -9.5‰PDB, of this clam shell supports freshwater poured in. On the other hand, forams had suddenly booming from about 7.4 m, i.e., 12 cm below the clam position, upward, indicating this articulated razor shell was also seawater influenced. So, most probably, this razor clam lived in lagoon-salt marsh within intertidal depth where both high tidal waters and freshwater can pour into. So, “□” -2.9 m, msl (-2.9±1) m. However, it was considered to be survived by high waters and 1.8 m was subtracted for msl reconstruction in Li et al. (2015). Indeed, survived by high waters was true but it was in intertidal depth.
60	A 1 cm-thick gleysol horizon, light grey, 10YR 7/1, was in muddy sediment. Forams started to bloom from this position upward till 6.7 m in depth, within which five forams samples all show >500 tests, indicating seawater came from this horizon. 820 cal a younger than another subsample (part <180 μm): 7 572 cal a BP/Beta355244 (Li et al., 2015). So, this horizon can be considered as a sudden but endurable filling of high sea waters into lagoon-salt marsh (see explanation of Beta 353808). “□” -3.01 m, msl (-3.01±1) m. However, this age is about 630 a younger than the overlying razor clam (Beta353808). A doubt remains with this reversal time sequence. It is a new indicator in this study.
61	Forams analysis indicates a sudden high sea water fluctuation with >500 tests a sample at about 8.8 m, i.e., only a decimeter lower than this sampling position. It means this sample was high tide influenced. It is 955 cal a younger than another subsample (<180 μm): 8 305 cal a BP/Beta355245 (Li et al., 2015). “┬” -4.26 m, as being an equivalent of basal peat, so (1.26±0.05) m should be subtracted, msl (-5.52±0.05) m. RTC is not needed. It is a new indicator in this study.
62	This sample was taken from a thin peaty layer intercalated in between two mainly marine influenced beds (Wang et al., 2015). A forams sample, just 2 cm below this sample, shows 2 tests only. Another three forams samples, 6.2 m, 6 m and 5.8 m in depths, have foraminifera tests 5 600, 6 016 and 304, respectively. Upwards, tests were found as much as 5 360, 7 456 and 8 512 in depths of 5.6 m, 4.6 m and 4 m, respectively. These clearly indicate only the sampling position at around 5.68 m was nearly nothing but both the underlying and overlying were fully abundant with forams. It suggests that this sample was in a short term during which sea water was little bit far away. Thus, this peaty layer was formed above high waters. So, “┬” -2.11 m, msl (-3.41±0.5) m (i.e., subtracting (1.3±0.5) m, Table A3). The sample is <180 μm and RTC correction of 1 320 cal a was given (Li et al., 2015). However, (1.8±0.5) m was subtracted in Li et al. (2015).
63	The sampling position was almost no forms found. However, 40 cm above, forams increased to 9 000 tests a 20 g dry sample (Wang et al., 2015). About 460 cal a younger than another subsample (<180 μm): 7 739 cal a BP/Beta332793 and RTC is not necessary (Li et al., 2015). “┬” -3.70 m, msl (-5.0±0.5) m, i.e., an intercalated organic layer formed at high waters (Table A3).
64	The diatom assemblage at 9.0-8.39 m shows from more benthic to more planktonic species upwards, indicating marine inundation had started (Wang et al., 2015). However, abundance of foraminifera had decreased from >500 tests in about 9.2 m to disappeared in this sampling depth of 8.98 m, then reoccurred at about 8.6 m (i.e., about 40 cm above this sample). So, it can be roughly considered as only influenced by high waters. “┬” about 5.41 m, and (1.3±0.5) m should be subtracted because is was an intercalated organic layer indicating high waters (Table A3). So, msl (-6.71±0.5) m. This is about 460 cal a younger than another subsample (<180 μm): 7952 cal a BP/Beta332794 (Li et al., 2015), so RTC is not needed. This is a new indicator in this study.
65	It is composed of euryhaline forams, brackish ostracoda and molluscan shells (Peng et al., 1984) and so it was influenced by high sea waters. The borehole elevation of +2.8 m (Peng et al., 1984) was classified to the National Vertical Datum 1985 in Li et al. (2015). Nevertheless, in this study, it is reconsidered as the local datum only and 1.543 m has to be subtracted. So, “┬” -13.56 m, msl (-14.82±0.05) m, i.e., subtracting (1.26±0.05) m (Table A3). RTC was given (Li et al., 2015).
66	This is about 10 560 cal a younger than another subsample of bulk organic mud (gley horizon): 13 477 cal a BP/KIK12257 (Wang et al., 2004; Li et al., 2015), from which the in situ gastropoda shells were picked out and dated (Wang et al., 2003). “┬” +1.23 m, msl (+0.13±0.03) m, i.e., subtracting (1.1±0.03) m because it was located in the ancient southwest periphery as an upper peat layer (Fig. A1 and Table A3). However, 1.8 m was subtracted in Li et al. (2015). RTC correction is not necessary (Li et al., 2015).
67	This basal peat was eroded by marine muddy sediments, in which Potamocorbula sp., Scapharca subcrenata, Mactra veneriformis, Assiminea sp., Nassarius sp., Corbicula sp. and Ammonia beccarii, Elphidium advenum and Quinqueloculina seminula were found (Li et al., 2006). “┬” -12.48 m, msl (-13.58±0.03) m, i.e., (1.1±0.03) m was subtracted as it was basal peat in the ancient southwest periphery (Fig. A1 and Table A3). RTC seems to be not necessary because it was in situ and mainly charcoals though it was given in Li et al. (2015).

A4.2 Holocene relative mean sea level (rmsl) indicators, derived from the shelly cheniers, in the west coast of Bohai Bay

	Basic information			Teomporal distribution								Spatial distribution
No.	Locality, stratigraphy and environment, material dated, sample-depth / ground surface elevation / (averaged) sample elevation	Coordinate	Lab code	Measured ¹⁴C date	δ¹³C /‰PDB	Conventional ¹⁴C age/a BP	Calibrated ¹⁴C age: median probability/2σ range/cal a BP	Status of material dated	Sub-sample	RTC /cal a	Age of rmsl: single value/ range/cal ka BP	Elevation of the observed rmsl/m	Corrections of tectonics /self-compaction/water withdrawal/m	Elevation of rmsl after corrections for the three local factors/m
68	Qingtuozi, shelly chenier, Section 1, Umbonium sp., 2.90/-/about−0.20 m	39.1°N, 117.8°E	Beta 305314	1 940±30	−3.6	2 290±30	2 123/ 2 292−1 972	reworked	no	−600	1.523/ 1.69−1.37	−1.4±0.5	+0.15/ +1.13/ +1.57	+1.45±0.5
69	idem, shelly chenier, Section 4, single valves of Potamocorbula laevis, about 2/-/about +0.22 m		Beta 305312	2 180±30	−0.2	2 590±30	2 505/ 2 677−2 341	reworked	no	−600	1.905/ 2.07−1.74	−0.98±0.5	+0.19/ +0.78/ +1.57	+1.56±0.5
70	Xinlicun, chenier, single valves of undetermined shells, about 1.5/-/about +1 m	39.1°N, 117.4°E	GC 172(2)	3 040±120	−2.68	3 399±127	3 476/ 3 827−3 138	reworked	yes	−600	2.876/ 3.23−2.54	−0.2±0.5	+0.29/ +0.58/ +0.25	+0.92±0.5
71	Jugezhuang, shelly chenier, Eijkelkamp core Site JN341-2, very bottom of chenier, single valves of Mactra veneriformis, about 4/-/−0.4 m	39.0°N, 117.3°E	BA 101095	−	−	3 350±25	3 417/ 3 285−3 270, 3 560−3 290	reworked	yes	−	3.417/ 3.560−3.270	−1.0±0.5	+0.34/ +1.56/ +1.75	+2.65±0.5
72	idem, Site JN341-2, very top of the underlying mud, single valves of M. veneriformis and Scapharca kagoshimensis, about 4/-/about −0.6 m		BA 101097	−	−	3 500±25	3 590/ 3 753−3 438	reworked	yes	−	3.590/ 3.753−3.438	−1.2±0.5	+0.36/ +1.56/ +1.75	+2.47±0.5
73	Yucenzi, shelly chenier, bottom, single valve of Crassostrea sp., about 1.5/-/about +0.39 m	38.9°N, 117.0°E	BA 110865	−	−	4 415±35	4 808/ 4 967−4 611	reworked	yes	−	4.808/ 4.967−4.611	−0.81±0.5	+0.48/ +0.58/ +2.36	+2.61±0.5
74	Banqiao, shelly chenier, Eijkelkamp core, just above the subsurface in its front side, Nassarius sp., 2.4−2.5 m/+2.311 m/about −0.14 m	38.9°N, 117.5°E	Beta 305294	1 490±30	−6.0	1 800±30	1 536/ 1 687−1 386	reworked	no	−600	0.936/ 1.08−0.78	−0.14±0.5	+0.09/ +0.94/ +2.19	+3.08±0.5
75	Apple Garden, Shanggulin, shelly chenier, top of the underlying mud, single valves of Scapharca kagoshimensis and Mactra veneriformis, about 4/-/about −1.5 m	38.8°N, 117.5°E	UtC7038	−	−1.70	1 850±35	1 597/ 1 760−1 430	reworked	yes	−	1.597/ 1.760−1.430	−1.5±0.5	+0.10/ +1.56/ +1.56	+1.72±0.5
76	idem. chenier, above the front-base, Umbonium sp. and Terebridae, -/-/about −1 m		UtC2240	−	0.0	2 350±100	2 190/ 2 478−1 894	reworked	yes	−	2.19/ 2.478−1.894	−1±0.5	+0.22/ +1.56/ +1.56	+2.34±0.5
77	Papadi, Shanggulin, shelly chenier, middle part, articulated Mactra veneriformis, 1.2 m/about +2 m/+0.8 m	38.8°N, 117.5°E	UtC7037	−	−4.03	2 125±40	1 919/ 2 096−1 701	reworked	no	−	1.919/ 2.096−1.701	−0.5±0.5	+0.19/ +0.47/ +1.56	+1.72±0.5
78	idem. shelly chenier, bottom, just above its subsurface at its seaward side, Umbonium sp., 2 m/about +2 m/0 m		98Y075	2 410±140	−2.68	2 769±146	2 705/ 3 080−2 319	in situ	no	−	2.28/ 2.66−1.90	0±0.5	+0.23/ +0.78/ +1.56	+2.57±0.5
79	Gongnongcun, Shanggulin, shelly chenier, top of the underlying mud, articulated Sinonovacula constricta, about 4/-/about −0.5 m	38.8°N, 117.5°E	03Y79	−	−	850±80	605/ 767−468	in situ	no	−	0.605/ 0.767−0.468	−0.5±0.5	+0.06/ +1.56/ +1.56	+2.68±0.5
80	Qikou, shelly chenier, lower part, single valves of undetermined shells, 5.32 m/+6.22 m/about +0.9 m	38.6°N, 117.6°E	SH217	2 000±70	−2.68	2 402±81	2 250/ 2 524−1 986	reworked	no	−600	1.650/ 1.92−1.38	+0.9±0.5	+0.16/ +1.81/ +0.17	+3.04±0.5
81	idem, shelly chenier, about 0.5 m above the rear-base of subsurface, single valves of undetermined shells, -/-/about +1 m		CG73	2 020±100	−2.68	2 379±108	2 225/ 2 573−1 917	reworked	no	−600	1.625/ 1.97−1.31	+0.2±0.5	+0.16/ +0.78/ +0.17	+1.31±0.5
82	Zhaizhuang, shelly chenier, top of the underlying mud, articulated Sinonovacula constricta, 4−5 m/-/about −3 m	38.6°N, 117.2°E	CG328	5 130±85	−2.68	5 489±94	6 070/ 6 295−5 845	in situ	no	−	6.070/ 6.295−5.845	−3±0.5	+0.61/ +1.75/ +0.09	−0.55±0.5
83	Zhangjuhe, shelly chenier, bottom, shells, 1.8/-/about +0.6 m	38.6°N, 117.6°E	YS265	2 495±65	−2.68	2 854±76	2 819/ 3 064−2 609	reworked	no	−600	2.219/ 2.46−2.01	−0.7±0.5	+0.22/ +0.70/ +0.15	+0.37±0.5
84	Houtangpu, shelly chenier, Eijkelkamp core, chenier foot above its front-base, Umbonium sp. and Terebridae, −2/-/about −0.5 m	38.5°N, 117.6°E	UtC2237	−	+0.7	820±90	586/ 762−426	reworked	no	−100	0.486/ 0.66−0.32	−0.5±0.5	+0.05/ +0.78/ +0.15	+0.48±0.5
85	Zhaojiapu, a thin shelly layer remained in the present-day muddy intertidal flat, shell hash, 0.1 m/-/about +0.48 m	38.5°N, 117.6°E	SH200	1 030±60	−2.68	1 389±72	1 121/ 1 281−937	reworked	no	−100	1.021/ 1.18−0.83	+0.48±0.5	+0.10/ 0/ +0.10	+0.68±0.5
86	Jiajiapu, shelly chenier, Eijkelkamp core, lower part, Scapharca kagoshimensis, Corbulidae and Nassarius sp., about 4/-/about +1.8 m	38.5°N, 117.6°E	UtC2239	−	−0.1	810±70	579/ 709−453	reworked	yes	−	0.579/ 0.709−0.453	+0.6±0.5	+0.06/ +1.56/ +0.11	+2.33±0.5
87	Jilingbo, shelly chenier, undetermined shells, 1.45/-/about +0.5 m	38.4°N, 117.7°E	SH247	2 860±70	−2.68	3 219±81	3 255/ 3 482−2 988	reworked	no	−600	2.655/ 2.88−2.39	−0.8±0.5	+0.26/ +0.56/ +0.21	+0.23±0.5
88	Wuditai, shelly chenier, undetermined shells, 1.1/-/about +1.1 m	38.4°N, 117.5°E	GC-?	3 920±120	−2.68	4 279±127	4 629/ 4 969−4 243	reworked	no	−600	4.029/ 4.37−3.64	−0.2±0.5	+0.40/ +0.43/ +0.10	+0.73±0.5
89	Yangjiapu, shelly chenier, undetermined shells, 1.05/-/about +1.1 m	38.4°N, 117.7°E	YS248	2 205±70	−2.68	2 564±81	2 477/ 2 724−2 253	reworked	no	−600	1.877/ 2.12−1.65	−0.2±0.5	+0.19/ +0.41/ +0.09	+0.49±0.5
90	Qianmiaozhuang, shelly chenier, middle-upper part, single valves of Meretrix meretrix, about 2.5/-/about +1.8 m	38.3°N, 117.4°E	CG 176(1)	4 205±105	−2.68	4 564±112	5 013/ 5 319−4 675	reworked	yes	−	5.013/ 5.319−4.675	+0.5±0.5	+0.50/ +0.97/ +0.05	+2.02±0.5
91	idem. shelly chenier, lower part, single valves of Meretrix meretrix and Mactra veneriformis, about 3.3/-/about +0.35 m		CG177	4 740±105	−2.68	5 099±112	5 648/ 5 336−5 334, 5 370−5 349, 5 906−5 384	reworked	no	−600	5.048/ 5.30−4.73	−0.85±0.5	+0.50/ +1.29/ 0.05	+0.99±0.5
92	Laolangtuozi, Pit 1, shelly chenier, top of the underlying mud, Terebridae and Nassarius sp., about 5/-/about +0.7 m	38.3°N, 117.8°E	AA 45901	−	−1.6	1 687±52	1 416/ 1 566−1 272	reworked	no	−600	0.816/ 0.96−0.67	−0.6±0.5	+0.08/ +1.94/ +0.12	+1.54±0.5
93	idem, Pit 1, shelly chenier, bottom, Terebridae and Nassarius sp., about 5/-/about +0.8 m		AA 45900	−	−0.8	1 455±47	1 191/ 1 321−1 034	reworked	no	−600	0.591/ 0.72−0.43	−0.5±0.5	+0.06/ +1.94/ +0.12	+1.62±0.5
94	idem, Pit 4, shelly chenier, bottom, fragments of Potamocorbula sp., Corbicula and Dosinia sp., about 4/-/+1.7 m	99Y007	2 740±80	−2.68	3 099±90	3 104/ 3 358−2 840	reworked	no	−600	2.504/ 2.76−2.24	+0.4±0.5	+0.25/ +1.56/ +0.12	+2.33±0.5
No.	Notes
	Explainations of the indicative meaning, RTC, the three spatial corrections and changes from the previous work (Li et al., 2015)
68	Redeposited on the middle part of subsurface of chenier (Su, 2012; Shang et al., 2013). Elevation of the sampling position was measured by Total Station connected with the National Vertical Datum 1985 benchmark (Su, 2012). “┬” −0.20 m, msl (–1.4±0.5) m, i.e., subtracting (1.2±0.5) m as it was above the mid-subsurface (Table A3). However, it was roughly treated as being formed in intertdal depth only and thus reconstructed msl is (−0.2±1.5) m in Li et al. (2015). RTC was given (Li et al., 2015).
69	Redeposited on the middle part of subsurface of chenier (Su, 2012; Shang et al., 2013). Elevation of the sampling position was measured by Total Station connected with the National vertical Datum 1985 benchmark (Su, 2012). “┬” +0.22 m, msl (−0.98±0.5) m, i.e., subtracting (1.2±0.5) m because it was above the mid-subsurface (Table A3). RTC was given (Li et al., 2015). However, it was simply judged to be just in intertidal depth in Li et al. (2015).
70	Detailed information is lacking except its approximate elevation It is treated to be taken from above the mid-subsurface, so (1.2±0.5) m should be subtracted, i.e., msl (−0.2±0.5) m (Table A3). However, it was simply considered at intertidal zone in Li et al. (2015). RTC was given (Li et al., 2015).
71	It was just above the chenier subsurface at its middle to seaward side. 720−310 cal a younger than another 3 subsamples: 3 731 cal a BP/BA101094, 3 766 cal a BP/BA101093 and 4 140 cal a BP/BA101112 (Su, 2012; Shang et al., 2016). Forams assemblages, for the lower part of chenier and the immediately underlying muddy sediments, indicate that the uppermost 1 m of the muddy sediments was dominantly composed of Ammonia beccarii, A. confertitesta, A. granuloumbilica, Elphidium simplex, A. granuloumbilica and Pseudononionella variabilis, based on four forams samples. Upwards in the mud column to closing the chenier subsurface, the assemblage shows size of the reworked individuals increased, i.e., tests become bigger and much broken. These suggest the chenier subsurface was subject to erosion and close to MHW environment (Li et al., 2016b; Su, 2012). Elevation of the ground surface was Total Station measured and then connected to the National Vertical Datum 1985 (Su, 2012). “┬” −0.4 m, msl (−1.0±0.5) m, i.e., 0.6 m could be subtracted because the sampling position is just in between the forepart and the mid-subsurface (the former is without vertical compensation but 1.2 m should be subtracted for the latter, Table A3). However, 1.8 m was subtracted in Li et al. (2015) since the compensation value of 1.8 m for basal peat was misused.
72	The sampling position was in between the mid-subsurface and the forepart. 720−340 cal a younger than another 3 subsamples: 3 927 cal a BP/BA101096, 4 089 cal a BP/BA101113 and 4 313 cal a BP/BA101098 (Su, 2012; Shang et al., 2016). Forams assemblages (Li et al., 2016; Su, 2012) and indicative meaning can be referred to the indicator BA101095 aforementioned. “┬” −0.6 m, msl (−1.2±0.5) m (Table A3). However, 1.8 m was subtracted in Li et al. (2015) because the compensation value of 1.8 m for basal peat was misused.
73	This age is 510 cal a younger than another subsample of Nassarius sp.: 5 318 cal a BP/BA110864 (Shang et al., 2016; Li et al., 2015; Su, 2012) though Crassostrea shells are usually able to bear stronger or longer reworking processes. Total Station measurement, connecting with RTK leveling, was given for elevation of the sampling position, which is only about 10 cm above the mid-subsurface (Su, 2012). So, (1.2±0.5) m should be subtracted, i.e., msl (−0.81±0.5) m (Table A3). However, the sampling position was just simply determined as intertidal depth in Li et al. (2015).
74	It was just above the subsurface at chenier’s seaward margin, indicating intertidal environment (Su, 2012; Li et al., 2015). “□” −0.14 m, msl (−0.14±0.5) m (Table A3) and RTC was given (Li et al., 2015). The vertical error was ±1.5 m in Li et al. (2015).
75	The sample, lying on the top of the underlying mud, is only 10 cm beneath the chenier subsurface at its fore-base (Wang et al., 2000c). This age is about 590−900 cal a younger than another two, 2 190 cal a BP/UtC2240 and 2 505 cal a BP/Beta305320, obtained from the bottom of the chenier at its fore-base (Li et al., 2015; Shang et al., 2013a). Elevation of the sampling position, about −1.5 m, was measured relative to a local benchmark. Considering all the three were reworked, this age as the youngest is chosen by this study indicating “□” at −1.5 m, so msl (−1.5±0.5) m (Table A3). RTC is not given because the age used was much younger than another two. Notes 6 ages of the chenier, including 2 190 cal a BP and 2 506 cal a BP, in this site are 550−1 100 cal a older than this age from the underlying mud. In general, ages in chenier may be older than those from the underlying mud because of reworking, but the time-gap of 550−1 100 cal a is too much. Therefore, it raised doubt that this age, 1 597 cal a BP, may be slightly younger. Thus, the following age, UtC2240, is still retained though both ages give self-contradictory time determination for the two indicators.
76	Another sample was taken from the similar position but age of 2 505 cal a BP/Beta305320. Therefore, RTC is not necersary. Elevation of subsurface at the fore-base here is −1.4 m and this sample was taken from about −1 m (Su et al., 2011). “□” −1 m, msl (−1±0.5) m (Table A3).
77	The dated M. veneriformis specimen, with very fresh shell surface decoration and still articulated, were mixed with wrecked single valves of Scapharca sp., Dosinia sp. and Meretrix sp. Although the dated M. veneriformis shells were also reworked, they must not be far away from their original living place and were quickly dumped into the chenier in here (Wang et al., 2000c). So, RTC is not necessary. Ground elevation is about +2 m relative to a local benchmark and the chenier subsurface is 2 m below it in this profile. The sampling position is about 0.8 m above the chenier subsurface (unpublished data of the authors’ group). The sample could be transported by MHHW and so (1.3±0.5) m should be subtracted, i.e., “┬” +0.8 m, msl (−0.5±0.5) m. Comparing to Li et al. (2015), this indicator is newly added in this study. Notes that this sample was not taken immediately from above the subsurface of chenier. Instead, it was taken from 0.8 m above it, therefore, it was piled up by high waters, i.e., subtracting (1.3±0.5) m, Table A3.
78	The shells dated still keep their glittered surface ornamentation and were lay in situ as thin shelly laminae intercalated with clayey laminae at the very bottom of chenier at fore-base in this site (Wang et al., 2000c). Concomitant were Terebridae shells and also with fresh decoration surface. It is considered to be formed during initial stage of chenier at intertidal depth in semi-closed bay environment (Li et al., 2015). Elevation of the sampling position was measured relative to a local benchmark. However, in this site two subsamples, UtC7037 and 98Y077 of articulated Mactra veneriformis shells from the same bulk sample, were separately dated by two labs and the 98Y077 is about 420 cal a older than UtC7037 (Wang et al., 2000c; Li et al., 2015). Another two pairs, given also by our inter-laboratory comtrastive test at subsample-level, show the Y is about 1 260 or about 5 180 cal a older than the UtC results, respectively, in Biaokou Oyster Reef^⑩. As a result, 400 a are subtracted. Based on Table A3, msl is (0±0.5) m though it was (0±1.5) m in Li et al. (2015).
79	Although 8 shell samples were dated in this site (Wang et al., 2007), only this in situ articulated razor clam sample is a most believable indicator. Conchologically, this species lives in mid to lower intertidal depth (Zhang, 2008; Okutani, 2000). The sampling elevation is estimated based on the measurement in Apple Garden, about 2 km north along the same chenier ridge (Shang et al., 2013). “□” −0.5 m, msl (−0.5±0.5) m (Table A3) though it was (−0.5±1.5) m in Li et al. (2015).
80	The sample, with definite sampling depth and elevation, was taken from the very bottom of chenier and its sampling position is closed to the present-day high tidal shoreline (Xu, 1994; Xu et al., 1986). Normally, chenier foot wadges seaward below the shoreface of late muddy sediments, so this suggests the sampling position is above the fore-base along its traverse section (i.e., a section perpendicular to the strike direction of chenier chain; Wang and van Strydonck, 1997). So, msl (+0.9±0.5) m (Table A3), though it was (+0.9±1.5) m in Li et al. (2015). RTC was given (Li et al., 2015).
81	Landward subsurface in the sampling position is about +1 m (Wang, 1994), while the sampling position is about 0.5 m above it (Zhao et al., 1980), i.e., about +1.5 m. So, msl (+0.2±0.5) m, i.e., subtracting (1.3±0.5) m (Table A3), while it was (−0.4±0.5) m in Li et al. (2015) because elevation was mistakenly cited from literature. RTC was given (Li et al., 2015).
82	Three radiocarbon-dated shell samples are available in this site (Zhao and Zhang, 1981). Based on Zhao and Zhang (1981) and personal discussion with the author, only this in situ articulated razor clam sample is believable but the another two were picked up from the upper part of chenier. However, Zhao and Zhang (1981) only indicated this razor shell was taken 4−5 m below the ground surface, while radiocarbon lab gave the sampling elevation as +1.5 m (Peng et al., 1984). However, local topographic map shows the ground surface here is about +1.5 m. So, Peng et al.’ estimation is wrong (we guess Peng et al. confused ground surface elevation with the sampling elevation for this sample) and thus this study uses the sampling elevation as about −3 m (considering sampling depth of 4−5 m). “□” −3 m, msl (−3±0.5) m (Table A3). However, sampling elevation was +1.5 m in Li et al. (2015) because simply accepted Peng et al.’ +1.5 m. RTC is not necessary (Li et al., 2015).
83	This sample was taken from the very bottom of chenier, i.e., just above the subsurface, with an approximate sampling elevation of +0.6 m (Xu, 1994). Beneath the subsurface here, underlying mud is composed of Ammonia beccarii vars., A. annectens, Candoniella albicans and pollen assemblage Chenopodiaceae-Pinus-Pteredum. On the other hand, A. beccarii vars., A. annectens and Chenopodiaceae-Querecus were found from the surficial muddy sediment along the present-day high tidal shoreline in the same site. So, the bottom material of chenier here was formed at high tidal environment influenced by fresh water input (Xu and Liu, 1991; Xu, 1994). “┬” +0.6 m, msl (−0.7±0.5) m, i.e., (1.3±0.5) m was subtracted (Table A3). RTC was given. However, Li et al. (2015) put the sampling position simply in intertidal depth without further compensation for msl reconstruction.
84	The sampling position is about 1 m above the chenier subsurface at its seaward margin of −1.5 m in elevation. Chenier foot was fully wedged into muddy tidal flat. Sediments in this part were composed of alternation between shelly hash laminae and muddy laminae, with penecontemporaneous deformation structure, and much dark-staining downwards, indicating intertidal environment (Wang et al., 2000a, b). The subsurface tilted seaward for about 2.8 m within about 100 m distance from its landward to seaward margin (Wang et al., 2000a, b; Su, 2012). So, “□” −0.5 m, msl (−0.5±0.5) m (Table A3) and RTC was given (Li et al., 2015).
85	Nowadays, it was found as a linear shoal, only 10 cm thick, consisting of well-sorted, very fine shell hash, about 2 200 m out of the present-day shoreline. This remaining shoal is about 70−80 m wide and about 10 cm beneath the intertidal flat surface of clayey silt and floating mud. Elevation was measured relative to local bench mark (Xu et al., 1986; Xu, 1994). This shoal was probably a remnant of Chenier S-T (Sui-Tang) (Wang et al., 2000b; Shang et al., 2016). RTC was given. “□” +0.48 m, msl (+0.48±0.5) m. However, msl was (+0.48±1.5) m in Li et al. (2015).
86	Due to landward digging by local fishermen from the sea side of chenier here, the core portion of chenier was exposed and a bulk sample was taken from the lower part, about 1 m above the subsurface. Elevation of subsurface in the sampling position is about +0.8 m based on comparison with the present-day MHHW shoreline which is 31 m east of the sampling pit (Wang, 1994). This is similar with subsurface elevation of +0.657 m measured by Xu and Liu (1991) and Xu (1994) in this site. So, “┬” +1.8 m, msl (+0.6±0.5) m (Table A3). This subsample is 100 cal a younger than another subsample of Umbonium sp. and Terebridae: 679 cal a BP/UtC2238 (Wang, 1994; Wang and van Strydonck, 1997; Shang et al., 2016; Li et al., 2015). However, the elevation of sampling position was roughly determined as (+2±1.5) m and thought to be msl directly in Li et al. (2015).
87	It was taken from lower-middle part of chenier and elevation was approximately obtained from topographic map (Xu, 1994). So, “┬” +0.5 m, msl (−0.8±0.5) m (Table A3). RTC was given. As indicator YS265, the sampling position was only simply recognized as intertidal depth in Li et al. (2015).
88	It was taken from the lower part of chenier because still having 0.4 m-thick shelly sediments below this sampling poition, and elevation was approximately obtained from topographic map (Xu, 1994). So, “┬” +1.1 m, msl (−0.2±0.5) m, i.e., (1.3±0.5) m was subtracted (Table A3). RTC was given. However, this sampling position was simply believed as intertidal depth (Li et al., 2015).
89	The chenier ridge was occupied by local residents long ago (Xu, 1994), implying that this site was a relatively high mound. The sampling position is about +1.1 m in elevation (Xu, 1994). “┬” +1.1 m, msl (−0.2±0.5) m (Table A3). RTC was given. In Li et al. (2015), reconstructed msl was −0.7 m because 1.8 m was subtracted from +1.1 m. This was misused when considering the sampling layer is an equivalent of the upper peat.
90	This is about 1 160 cal a younger than another subsample of Crassostrea gigas shell CG176(2) and the sampling position is about +1.8 m (Peng et al., 1980; Zhao et al., 1980; Xu, 1994), which is believed to be the position of MHHW. “┬” +1.8 m, msl (+0.5±0.5) m, i.e., (1.3±0.5) m should be subtracted (Table A3). However, 1.8 m was subtracted in Li et al. (2015) due to misuse of the upper peat.
91	Peng et al. (1980) gave the age and sampling elevation of about 0 m, which is about 0.5 m above the chenier subsurface which lay on the underlying grayish marine silty mud. However, Zhao et al.(1979) and Xu (1994) both gave another approximate elevation of the sampling position as +0.7 m. This study uses average +0.35 m of both, though it was 0 m in Li et al. (2015), i.e., only accepted Peng et al. (1980). The original authors mentioned above did not indicate the sample is from the chenier’s landward or seaward part (i.e., rear part or front foot of chenier). Considering its huge scale excavated by the local fishermen (unpublished data of our group), the sample could be taken from middle of subsurface and speculating the sample, being 0.5 m above the subsurface, was formed in the upper part of intertidal depth, though was only in intertidal zone simply in Li et al. (2015). So, “┬” +0.35 m and (1.2±0.5) m (Table A3) should be subtracted, i.e., msl (−0.85±0.5) m. RTC was give in Li et al. (2015).
92	This underlying mud is dull yellowish brown (10YR 5/3) clay, indicating much oxidized environment in the upper part of intertidal flat¹ (Wang et al., 2003). However, the authors thought previously it was formed just in intertidal depth (Li et al., 2015), but now it is further restricted to the upper part of intertidal zone. So, “┬” +0.7 m and (1.3±0.5) m should be subtracted (Table A3), i.e., msl (−0.6±0.5) m. However, msl was reconstructed as (+0.7±1.5) m in Li et al. (2015). RTC was given (Li et al., 2015). Elevations of three indicators, including the following two, in this site were leveled relative to a local benchmark with National Vertical Datum 1985 (Wang et al., 2003).
93	This gastropoda sample, picked from fine shelly hash, immediately lying on the chenier subsurface, is about 10 cm above the sample AA45901¹. It was thought to be formed in intertidal depth (Li et al., 2015) but now in this paper is restricted to the upper intertidal zone. So, “┬” +0.8 m, msl (−0.5±0.5) m (Table A3). RTC was given (Li et al., 2015).
94	The sample was taken just above the subsurface at the mid- to rear part of the chenier¹⁷. So, “┬” +1.7 m, msl (+0.4±0.5) m, i.e., (1.3±0.5) m should be subtracted (Table A3). However, 1.8 m was subtracted from the sampling elevation in Li et al. (2015) because it was incorrectly treated as an equivalent of the peat layer. RTC was given (Li et al., 2015).

A4.3 Holocene relative mean sea level (rmsl) indicators, derived from the oyster reefs, in the west coast of Bohai Bay

	Basic information			Temporal distribution								Spatial distribution
No.	Locality, stratigraphy and environment, material dated, sample-depth / ground surface elevation / (averaged) sample elevation	Coordinate	Lab code	Measured ¹⁴C date	δ¹³C /‰PDB	Conventional ¹⁴C age/a BP	Calibrated ¹⁴C age: median probability/2σ range/cal a BP	Status of material dated	Sub- sample	RTC /cal a	Age of rmsl: single value/ range/cal ka BP	Elevation of the observed rmsl/m	Corrections of tectonics /self-compa- ction/water withdrawal /m	Elevation of rmsl after corrections for the three local factors /m
95	Mengzhuang oyster reef, articulated C. gigas, 4.5 m/-/about −2.5 to −3.3 m (?)	39.4°N, 117.8°E	TD342	5 070±115	−2.68	5 429±122	6 008/ 6 280−5 704	in situ	no	−	6.008/ 6.280−5.704	−2.65±0.7	+0.60/ +1.75/ +0.37	+0.07±0.7
96	Dawuzhuang, oyster reef, Section 6, top, articulated C. gigas, about 4.5/-/about −2.58 m	39.4°N, 117.9°E	BA 091172	−	−	5 360±35	5 926/ 6 096−5 752	in situ	no	−	5.926/ 6.096−5.752	−2.43±0.7	+0.59/ +1.75/ +0.60	+0.51±0.7
97	Dawuzhuang, oyster reef, Section 1, top, articulated C. gigas, 0−0.3 m below the top surface, about 5/-/−3.22 to −3.5 m	39.4°N, 117.9°E	BA 110343	−	−	5 625±35	6 223/ 6 365−6 061	in situ	yes	−	6.223/ 6.365−6.061	−3.07±0.7	+0.62/ +1.95/ +0.60	+0.1±0.7
98	Shizhuang, oyster reef, top (?), articulated Crassostrea gigas, about 4/about +2.5 m/−1.4 to −2 m;	39.4°N, 117.5°E	TD344	5 860±95	−2.68	6 219±103	6 879/ 7 161−6 613	in situ	no	−	6.414/ 7.161−5.614	−1.55±0.7	+0.64/ +1.56/ +0.11	+0.76±0.7
	Jiangzhuang-Shizhuang oyster reef, articulated C. gigas, about 4/about +2.5 m/−1.4 to −2 m		CG181	5 020±140	−2.68	5 379±146	5 950/ 6 267–5 614	in situ	no	−
99	Lingtou, oyster reef, top, articulated C. gigas, 3.1−3.3 m/+0.87 m/−2.23 to −2.43 m	39.3°N, 117.6°E	BA 110339	−	−	3 980±30	4 234/ 4 404−4 069	in situ	yes	−	4.234/ 4.404−4.069	−2.08±0.7	+0.42/ +1.25/ +0.62	+0.21±0.7
100	Zengkouhe, oyster reef, top, articulated C. gigas, 2.8 m/about +0.5 m/about −2.3 m	39.3°N, 117.6°E	BA 110317	−	−	3 965±30	4 212/ 4 399−4 047	in situ	no	−	4.212/ 4.399−4.047	−2.15±0.7	+0.42/ +1.09/ +0.67	+0.03±0.7
101	Biaokou, oyster reef, top, articulated Trapezium liratum, 4.2 m/+2.38 m/−1.82 m	39.3°N, 117.6°E	UtC7036	−	−1.23	3 865±35	4 067/ 4 251−3 882	in situ	no	−	4.067/ 4.251−3.882	1.67±0.7	+0.41/ +1.64/ +0.56	+0.94±0.7
102	Yujialing, oyster reef, articulated C. gigas, about 3.1/-/about −1.4 to −1.6 m	39.2°N, 117.7°E	TD356	1 830±100	−2.68	2 189±108	2 000/ 2 296−1 720	in situ (?)	no	−	2.000/ 2.296−1.720	−1.5±1.5	+0.20/ +1.21/ +0.52	+0.43±1.5
103	Konggang, oyster reef, a transition zone between reef-top and the overlying mud, articulated Trapezium liratum, about 5.5/-/−2.85 to −2.90 m	39.1°N, 117.4°E	BA 101110	−	−	3 800±25	3 980/ 4 140−3 825	in situ	yes	−	3.980/ 4.140−3.825	−2.7±0.7	+0.40/ +1.87/ +1.05	+0.62±0.7
104	idem, reef-top, articulated C. gigas, about 5.5/-/about −2.9 to −3.0 m; idem, the same individual of BA101108	39.2°N, 117.4°E	BA 101108, Beta 305302	− 3 440±30	− −3.9	3 800±25, 3 790±30	3 980/ 4 140−3 825; 3 967/ 4 140−3 811	in situ	yes	−	3.973/ 4.140−3.811	−2.78±0.7	+0.40/ +1.87/ +1.05	+0.54±0.7
105	Binhaihu, oyster reef, top, articulated C. gigas, about 3.3/-/about 3.15 to −3.5 m	39.2°N, 117.6°E	BA 110340	−	−	2 265±30	2 088/ 2 276–1 939	in situ	yes	−	2.088/ 2.276−1.939	−3.16±0.7	+0.21/ +1.29/ +0.75	−0.91±0.7
106	Core CH79, oyster reef, articulated C. gigas, 8.75−8.80 m/−3.4 m/about −12.18 m	39.2°N, 117.9°E	BA 08822	−	−	6 935±35	7 588/ 7 702−7 466	in situ	no	−	7.588/ 7.702−7.466	−10.58±0.5	+0.76/ +3.78/ +1.87	−4.17±0.5
107	idem, articulated C. gigas, 9.90−9.95 m/−3.4 m/−13.33 m		BA 08823	−	−	7 275±40	7 912/ 8 048−7 762	in situ	no	−	7.912/ 8.048−7.762	−11.73±0.5	+0.79/ +4.13/ +1.87	−4.94±0.5
108	idem, articulated C. gigas, 11.50−11.55 m/−3.4 m/−14.93 m		BA 08824	−	−	7 625±35	8 268/ 8 388−8 146	in situ	no	−	8.268/ 8.388−8.146	−13.33±0.5	+0.83/ +4.63/ +1.87	−6.0±0.5
109	Beitang, oyster reef, articulated Crassostrea gigas, about 2/-/about −1 m	39.1°N, 117.7°E	ZK507-I	975±85	−2.68	1 334±94	1 067/ 1 281–850	in situ	no	−	1.067/ 1.281−0.850	−0.85±0.7	+0.10/ +0.78/ +0.19	+0.22±0.7
110	Tongju, oyster reef (?), single valve of Crassostrea gigas, about 2/-/about +1 m	38.6°N, 117.2°E	CG180	4 460±160	−2.68	4 819±165	5 327/ 5 679−4 857	in situ (?)	no	−	5.327/ 5.679−4.857	+1.15±0.7	+0.53/ +0.78/ +0.10	+2.56±0.7
No.	Notes
	Explainations of the indicative meaning, RTC, the three spatial corrections and changes from the previous work (Li et al., 2015)
95	The sample is about 1 m below the reef top (Wang et al., 1991) while the top elevation is confused as −1.5 m (Wang et al., 1991), −2.3 m or −1.5 m (Han and Meng, 1996). This study uses average of about −2.8 m as the sampling elevation. The age is considered approximately as its top age. So, msl (−2.65±0.7) m while it was (−2±1.5) m in Li et al. (2015). Usually, the reef-top can grow up to MTL but 15 cm lower than the corresponding msl in the study area (see Section 4.3 and Table A3). Thus, all the oyster indicators here have been treated by adding 15 cm to the observed reef-top elevation in order to fulfil the conversion from MTL to msl (the same below).
96	Elevation of this sampling position was not measured and we then use −2.58 m, the 7-spot-average of the reef-top elevations in this site (Li et al., 2015). msl (−2.43±0.7) m.
97	In this big pit, elevation measurement was given by using Total Station connected with the National Vertical Datum 1985. As a result, elevations of the 7 spots on the reef top were obtained, among which the top surface at this sampling position is −3.22 m. Furthermore, totally, 19 top-spots, including another two reef sites in Binhai Lake and Konggang, were leveled and showed that an average undulation of the reef-top is ±0.7 m (Fan et al., 2005a; Liu, 2010; Wang et al., 2011c, 2012a; Shang et al., 2013). This is similar to 0.5−1 m of the top undulation of the modern living oyster reef in Xiaomiaohong site, Jiangsu Province (Zhang, 2004). In this study, ±0.7 m has been used as an error range for the top undulation for all oyster reefs (Li et al., 2015). So, msl (−3.07±0.7) m (Table A3). However, it was (−3.4±0.7) m in Li et al. (2015) because −3.4 m is a rounding value of middle point of the sampling range of −3.22 m to −3.5 m.
98	C. gigas were with coexisting Trapezium sp., Assiminea sp. and Cerithidea sp. shells (Peng et al., 1980). Five independent investigations were carried out and five ages, dated by four different labs, were obtained for these two closed reef sites (Peng et al., 1980; ICP, 1987; IOA, 1983; Li and Zhao, 1990). Ages of TD344 and CG181 listed are the oldest and youngest, respectively, among the five (Li et al., 2015). Elevations of the reef top were quite different from about +0.5 m (ICP, 1987), −1.5 m (Wang et al., 1991), −1.4−−1.5 m (Han and Meng, 1996) to about −2 m (Wang et al., 2011c). Considering less precise estimation for the elevations and the inherent undulation of reef top, this study uses −1.7 m, an average of the latter three (Li et al., 2015). So, msl (−1.55±0.7) m in this study (Table A3) while it was (−1.7±1.5) m in Li et al. (2015).
99	This individual was taken from the top of reef in this site. Elevation of ground surface was leveled relative to a local benchmark (Li et al., 2015) and msl (−2.08±0.7) m (Li et al., 2015).
100	The ground elevation is +0.71 m of Core HD21, about 1 km west of this reef site and a 7-spot-average, given by RTK leveling in 2016, is +0.385 m, about 0.5 km north of this reef site (Qin et al., 2017). This is flattened lagoon-salt marsh lowland² (Qin et al., 2017; see indicator No. 12). So, the ground elevation in this reef site is about +0.5 m approximately. This sample was taken from the top of the reef, which is 2.8 m below the ground surface^① (Shang et al., 2013). So, the reef-top is −2.3 m and msl (−2.15±0.7) m. However, the sampling elevation, about −3.1 m in Li et al. (2015), was only reckoned by an estimation of elevation difference between the reef-tops of Biaokou and this site (the latter is based on Wang et al., 1991).
101	T. liratum is a most important associated species in the C. gigas reefs in the study area and this sample was taken from 5 cm below the reef top. Ground elevation was leveled relative to the local benchmark (Wang et al., 2006; Li et al., 2015). msl (−1.67±0.7) m (Li et al., 2015).
102	Top and/or upper part of oyster reef in this site was destroyed by bridge construction and the reef-bottom was not dug out (Wang et al., 1991). So, it is impossible to know from which part of reef column the sample was taken exactly. Approximately, it is considered to be in middle part of the reef body in this site and was in intertidal depth. So, ‘□’ −1.4−−1.6 m, msl (−1.5±1.5) m.
103	The sample, in the bottom of transitional zone, was immediately above the reef-top. Another in situ subsample is articulated Ruditapes philippinarum, which was a later burrowed animal showing reasonably 480 cal a younger age: 3 500 cal a BP/BA101109 (Wang, 2012; Wang et al., 2012a; Li et al., 2015). MTL (−2.85±0.7) m while msl should be (−2.7±0.7) m.
104	The top elevation fluctuated between (−2.70 to −3.16) m based on leveling at 3 spots in this site and the average is −2.93 m (Wang, 2012). A bulk sample, as a part of the ligamental resilium of LV of the same individual, taken from the reef-top, was divided and separately AMS dated by two labs and gave the exact same ages (Wang, 2012; Wang et al., 2012a; Li et al., 2015). msl (−2.78±0.7) m (Li et al., 2015).
105	The top part of reef in this site is composed of intact oyster individuals and oyster hash, also fragments of Rapana venosa, and Trapezium liratum, Mitrella bella (?), Pseudoliotia pucchella, Assiminea latericea, Barbatia sp., Potamocorbula laevis, Ruditapes philippinarum. Forams were Ammonia confertitesta (dominant species), Quinqueloculina akneriana rotunda, Protelphidium granosum, Elphidium nakanokawaense. Ostracoda were Sinocytheridea impressa, Loxoconcha binhaiensis and so on. These indicate intertidal environment with occasionally input of freshwater³ (Fan et al., 2008; unpublished data of the authors’ group). Nine spot levelling indicates the top elevation is in between −2.96 to −4.36 m³ and average −3.31 m has been used (Li et al., 2015). So, msl (−3.16±0.7) m. Another individual taken from the top is about 290 cal a older: 2 380 cal a BP/06Y082³.
106	The reef body, 8.45−11.63 m below sea floor, was revealed by this underwater borehole. This sample is only about 30 cm below the top of this buried reef. It therefore may roughly indicate environment of the reef-top. Two microbiological samples, 8.5−9.0 m, of Assemblage Zone 4-1, having huge amount of forams of about 3 000−4 000 tests per 20 g dry sample, mainly Ammonia beccarii vars., A. annectens and E. limpidum, indicating shallow sea environment closing to estuary. Also, forams assemblages of 4 samples, Zone 3, taken from the overlying muddy sediments, mainly A.beccarii vars. and P. tuberculatum with Q. akneriana rotunda, E. magellanicum, Cribrononion incertum and Buccella frigida, indicating shallow sea environment (Li, 2010). So, ‘┴’ −12.18 m and (1.6±0.5) m as a compensation value for MLLW (Table A3) should be added for restoring msl, i.e., msl (−10.58±0.5) m, though 1.5 m, as a half of mean high tidal range, was simply added in Li et al. (2015).
107	This sample, taken from the middle part of the reef column, belongs to the foraminifera assemblage Zone 4-2 in 9.6−11.5 m. Forams can reach 3 000−4 000 tests of 20 g dry sample, mainly Pseudoeponides compressum, Ammonia beccarii vars., P. enderssen and P. tuberculatum, A. convexidorsa, indicating estuary environment in lower intertidal to upper subtidal depth with salt and fresh water mixture (Li, 2010). ‘┴’ −13.33 m and (1.6±0.5) m (Table A3) should be added for restoring the contemporaneous msl. So, msl (−11.73±0.5) m, though 1.5 m was added in Li et al. (2015).
108	This articulated shell, taken from the reef-bottom, gives an initial age of this reef. Concomitant forams, 3 000−4 000 tests per 20 g dry sample, were composed of mainly Ammonia beccarii and shallow sea species such as A. flevensis, Elphidium limpidum, Pseudoeponides enderssen and A. convexdorsa, indicating estuary environment in lower intertidal to upper subtidal depth (Li, 2010). Downwards, two forams samples, taken from the underlying mud, 12−14.9 m, show mainly A. beccarii vars., indicating marine influence had started from this underlying mud and sea water become deeper upwards. So, ‘┴’ −14.93 m, msl (−13.33±0.5) m, i.e., (1.6±0.5) m should be added (Table A3). However, msl was (−13.43±0.5) m because 1.5 m was simply added in Li et al. (2015).
109	It seems to be taken from the reef top though more detailed information, including elevations for both ground surface and reef top, were deficient in original literature (Zhao et al., 1979). So, msl (−0.85±0.7) m and RTC is not necessary (Li et al., 2015).
110	This possible oyster reef, or at least an oyster bank, existed exceptionally in Chenier Plain (Peng et al., 1980) and with a numerous Trapezium liratum (Wang et al., 2007), one of a few coexisting species within C. gigas reefs in the area. So, it is probably a less developed reef occurred in Chenier Plain. ‘□’ +1 m, MTL +1 m and msl +1.15 m. The oyster sample is considered in situ and RTC is not necessary (Li et al., 2015). It was thought to be chenier with a vertical error of ±1.5 m (Li et al., 2015). Now it is reclassified to oyster reef and vertical error is changed to ±0.7 m as the local reef top undulation.

B1 Estimated porosity for this study, modified from Li et al.³

Key layer	Average depth/m	Porosity/%
Surface	0	60
Basal peat	18	50
Hard soil horizon, LGM	21	45
Subsurface of sediments	400	30

B2 Relationship of self-compaction and depth

Depth/m	Self-compaction, Δ (m), model	Self-compaction Δ (m), empirical
0	0.00	0.00
5	1.94	1.91
10	3.39	3.42
15	4.66	4.67
20	5.79	5.77
25	6.83	6.84
30	7.79	7.79
35	8.69	8.71
40	9.53	9.57

Fig. 1. Basement tectonic map of the study area, depicted by a black rectangle, mainly belongs to the Bohai Bay Basin (Zhang et al., 2010; Huang et al., 2014; Liu et al., 2016). Four tidal gauge stations, located at the mouths of four rivers, are shown by blue solid squares. Inset: 1. Liaodong Bay; 2. Bohai Bay; 3. Laizhou Bay (modified from Li et al., 2015).

Fig. 2. Map showing the geographic distribution of the sea level indicators in the coastal lowland and contiguous shallow sea area of the west coast of Bohai Bay (modified from Li et al., 2015).

Fig. 3. Schematic illustration of a sedimentary model of the shelly cheniers in the Chenier Plain, west coast of Bohai Bay. Bar F, exposing the foot, was excavated in the wall of a shrimp pond and further Eijkelkamp penetrated, as shown in Figs 4a and b, while above the mid- or the rear-part can be seen in Figs 4c or f, respectively.

Fig. 4. Photos showing the shelly cheniers and underlying and overlying muddy sediments in the Chenier Plain, the west coast of Bohai Bay. a. Chenier I, Houtangpu site, its crest covered by Jujuba shrubs, was perched on the surrounding muddy coast and formed the present native shoreline since last about 1 cal ka BP (Wang et al., 2000b). An excavation wall of a shrimp pond (bar F in Fig. 3), dug in the upper part of the present muddy tidal flat, shows the sediments above the front-base of the shelly chenier. b. Details of the wall in Fig. 4a. Thin layers, alternatively consisting of shell fragments and muddy shell hash, were revealed in this wall section. The indicator No. 84 (Table A4.2) was taken from the lower part of the wall-section. Eijkelkamp augering revealed a meter thick, muddy shell hash sediments, which show the very bottom of the chenier above its front-base and the underlying tidal mud. c. Chenier II-2 and the underlying mud, Gongnongcun, Shanggulin site, subsurface of the mid-part of chenier was visible about 30 cm below hammer. d. The underlying mud, below the subsurface in Fig. 4c, was further Eijkelkamp augured to a depth of 102 cm (from left-top to right-bottom), showing intertidal environment. The indicator No. 79 (Table A4.2), articulated in situ Sinonovacula constricta shells, was taken from the top of this underlying mud. e. The top of the underlying mud beneath the front-base, Chenier II-2, Apple Garden, Shanggulin site, was mixed with reworked single valves of Mactra veneriformis, Scapharca kagoshimensis and gastropoda Rapana, see the shells washed clean in a wooden box at right side, i.e., the indicator No. 75 (Table A4.2). f. Chenier II-1, Papadi, Shanggulin site, the rear-part of chenier body, showing fade-out landward (to right side), was covered by a thin darkish upper peat layer (i.e., indicator No. 23, Table A4.1). The geographical distribution of the sites here can also be found in Shang et al. (2016).

Fig. 5. Schematic illustration of a sedimentary model for Holocene Crassostrea gigas reefs in the Oyster Plain, northwest coast of Bohai Bay. a. Reef body and its underlying and overlying muddy sediments. The reefs, about 2−6 m thick, were mainly composed of C. gigas shells, with a binary structure of the normal vertical building-ups of almost exclusively vertical posed and articulated individuals, and the horizontally intercalated muddy layers mixed with distorted individuals laying up horizontally. Such cyclicity, mostly repeated 6−7 times in the Oyster Group III only, about 5.8−4 cal ka BP, is a remarkable characteristic, although whether this characteristic was a response to sea level fluctuations still remains an open question (Wang et al., 2006). In this study, we picked the shell samples from the reef-top only as indicating MTL (mean tidal level). b. Two extreme and opposite patterns of the transition from reef top to the overlying upper intertidal muddy sediments: Bed 3(i) tranquil accumulation occurred first in the small depressions on the top surface and the shell-margin (ventral margin) of the individuals at the very top was still intact; Bed 3(ii) erosion and then deposition, the shell-margin on the very top was sharply truncated and a few individuals were removed by strong tides and then paved horizontally.

Fig. 6. Buried Holocene Crassostrea gigas reefs, Oyster Plain, the northwest coast of Bohai Bay. a. Section 1, Dawuzhuang site, one of the largest reefs in Oyster Plain, was intermittently excavated from the 1990s to 2010s. By the end of digging work in the mid-2010s, the totally excavated area was about 150 m×100 m. Following such excavations, 11 sections were set up to study the reef itself and the overlying and underlying muds by our group (Fan, 2008, 2010; Liu, 2010; Wang et al., 2011c). Leveling shows the reef-top at 7 sections undulated in between −3.22 m and −1.94 m (undulation range 1.28 m), with an average elevation −2.48 m of National Vertical Datum 1985 (unpublished data). The indicator No. 97 was taken from the top part in this reef (Table A4.3). b. Biaokou site (Wang et al., 1995, 2006). The most concomitant species, Trapezium liratum, indicator No.101, was taken from 5 cm below the top surface in this reef (Table A4.3). c. Zengkouhe site. Dr. D. Surge, North Carolina University, stood on the bottom of excavating pit and Dr. Shang, one of the authors, stood with total station on a temporarily piled-dyke during the 2006 summer field work. Note a thin white line illustrating the boundary between the reef and the overlying muddy sediments, while the subsurface of reef was still a few decimeters below the pit bottom^{1111^⑫Shang Zhiwen, Fan Changfu, Wang Hong, et al. 2007. Comprehensive Report to Tianjin Paleo Coast and Wetland State Natural 　Conservation Area: Second-Phase Geological Investigations for Oyster Reefs (in Chinese), 1–55}^{1212^⑬Shang Zhiwen, Li Jianfen, Wang Hong, et al. 2016. Report of the China Coastal Depositional Recoreds on Climate Changes (in 　Chinese), 1–100}. The indicator No. 100 was taken from this reef (Table A4.3). d. The first type of transition (see 3(i) in Fig. 5b) from reef to the overlying intertidal mud revealed by Section 2, Dawuzhuang site, after carefully cleaning, no truncation was found from the ventral margin of the very top shells (except the one in the front of box ruler was artificially cut by cleaning). The lower part of overlying mud without laminar bedding structure is due to the stagnant qranquil microenvironment (i.e., the small depression), and gradually changed upwards to irregularly interbedded clayey and silty-fine sandy laminae, which are typically intertidal bedding structures. e. The second type of transition (see 3(ii) in Fig. 5b), Beihuaidian site. The shell margin was obviously truncated and even a few individuals were removed and paved on the boundary by strong floodings. f. A result for the Oyster-Museum-choosing-site investigation conducted in an area between Biaokou and Zengkohe sites. A three-dimensional diagram, based on 35 cores (the red vertical lines) in the area of 120 m×240 m, shows the reef-top (yellow) and reef-bottom (blue) depths, respectively. Ground surface, leveled against National Vertical Datum 1985, is shown by red color. The elevation of the reef-top observed undulates between −3.52 m and −2.26 m (Qin et al., 2017).

Fig. 7. The primary evidence of the sea level change on the west coast of Bohai Bay. a. Distribution of the observed relative sea level (rsl) indicators, with the various symbols indicating different water levels as high (┬), low (┴), intertidal (□) and the mean tidal (+). b. Distribution of the relative mean sea level (RMSL) indicators, derived quantitatively from their originally observed rsl with the specific indicative meanings, shown as the three major types of sediments, cheniers and oyster reefs.

Fig. 8. A series of temporospatial calibrations for the observed RMSL indicators on the west coast of Bohai Bay, China. a. Correction for the local residence time effects (RTC) for the 48 indicators, and the younger-age-selection at the subsample level for another 35 indicators are added to Fig. 7b. To date, two steps for time calibrations, CALIB+RTC corrections, have been completed in this study. b. A correction for the neotectonic subsidence, by compensating 0.1 mm/a, is added to Fig. 8a. c. A correction for the self-compaction of unconsolidated sediments is added to Fig. 8b. d. Further correction for the subsidence due to groundwater withdrawal is added to Fig. 8c. To date, all the local factors influencing the temporospatial distribution of the RMSL indicators have been corrected. Then, a solid black line describes approximately a local RMSL curve in the study area.

Fig. 9. Comparison of two inferred RMSL patterns shows that after RTC. A black curve is about 0.5 ka cal a younger than a red one without RTC in the time period of about 7.3−6.8 ka.

Fig. 10. A diagram illustrates our RMSL curve (black solid line) in this study, the ESL curve of Lambeck et al. (red solid line), the model-predicted RMSL curves of Lambeck⁴ and Lambeck et al. (personal communication, 2014) (two green lines), and a RMSL band (light green shadow), redrawing from Fig. 4a-BB (Bohai Bay) of Bradley et al. (2016).

Fig. A1. Comprehensive diagram shows tidal gauge stations and the paleo bayhead evolution since the early mid-Holocene. Bayhead migrated gradually from the Oyster Plain in about 7 ka BP southeastward to the Chenier Plain and, finally, the present bayhead is located in the Jiyun River−Nanpai River section. Thin red lines, 0.58 and 0.64 of $[(H_{{\rm O}_1}+H_{{\rm K}_1})/H_{{\rm M}_2}] $ in the shallow sea area, show less notable semidiurnal pattern^⑥. Dotted+solid blue lines on the coastal lowland are paleoshorelines, determined by chains of both cheniers in the Chenier Plain and earthy mounds in the Oyster Plain. The earthy mound chains were alternated with the buried oyster reefs (Wang et al., 2006, 2010). Two red bars are approximate boundary separating modern bayhead and both sides of periphery while the thick black lines are rough boundary between the speculated ancient bayhead and peripheries.

B1 Regression fit of the local porosity and depth, based on several key layers in Table B1.

B2 Relationship between self-compaction and depth in the west coast of Bohai Bay, for depth ≤25 m only.

B3 An Illustration showing comparison between decompacting value trends. For this study, differences are less than 1 m for shallow sediments normally less than 25 m in depth. Red circles: consolidation settlement is 400 m; green circles: consolidation settlement is 1 000 m. Blue circles are drawn by using slightly different values comparing with the red circles for porosity estimates as 45% for the basal peat, 40% for the hard soil horizon and 30% for the base of Quaternary sediments.

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