Coral records of Mid-Holocene sea-level highstands and climate responses in the northern South China Sea

Coral records of Mid-Holocene sea-level highstands and climate responses in the northern South China Sea

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Yuanfu Yue¹^,²^,^*, Lichao Tang¹^,², Kefu Yu¹^,²^,^*, Rongyong Huang¹^,²

Acta Oceanologica Sinica | 2024, 43(2) : 43 - 57

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Acta Oceanologica Sinica | 2024, 43(2): 43-57

• Marine Geology •

Coral records of Mid-Holocene sea-level highstands and climate responses in the northern South China Sea

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Yuanfu Yue¹^,²^,^*, Lichao Tang¹^,², Kefu Yu¹^,²^,^*, Rongyong Huang¹^,²

Affiliations

¹ School of Marine Sciences, Coral Reef Research Center of China, Guangxi University, Nanning 530004, China

² Guangxi Laboratory on the Study of Coral Reefs in the South China Sea, Nanning 530004, China

Published: 2024-02-25 doi: 10.1007/s13131-023-2264-9

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Abstract

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High-resolution sea-level data and high-precision dating of corals in the northern South China Sea (SCS) during the Holocene provide a reference and historical background for current and future sea-level changes and a basis for scientific assessment of the evolutionary trend of coral reefs in the SCS. Although sporadic studies have been performed around Hainan Island in the northern SCS, the reconstructed sea level presents different values or is controversial because the indicative meaning of the sea-level indicators were neither quantified nor uniform criteria. Here, we determined the quantitative relationship between modern living coral and sea level by measuring the top surfaces of 27 live Porites corals from the inner reef flat along the east coast of Hainan Island and assessed the accuracy of results obtained using coral as sea-level indicators. Additionally, three in situ fossil Porites corals were analyzed based on elevation measurements, digital X-ray radiography, and U-Th dating. The survey results showed that the indicative meanings for the modern live Porites corals is (146.09 ± 8.35) cm below the mean tide level (MTL). It suggested that their upward growth limit is constrained by the sea level, and the lowest low water is the highest level of survival for the modern live Porites corals. Based on the newly defined indicative meanings, 6 new sea-level index points (SLIPs) were obtained and 19 published SLIPs were recalculated. Those SLIPs indicated a relative sea level fluctuation between (227.7 ± 9.8) cm to (154.88 ± 9.8) cm MTL between (5 393 ± 25) cal a BP and (3 390 ± 12) cal a BP, providing evidences of the Mid-Holocene sea-level highstand in the northern SCS. Besides that, our analysis demonstrated that different sea-level histories may be produced based on different indicative meanings or criteria. The dataset of 276 coral U-Th ages indicates that coral reef development in the northern SCS comprised the initial development, boom growth, decline, and flourishing development again. A comparison with regional records indicated that synergistic effects of climatic and environmental factors were involved in the development of coral reefs in the northern SCS. Thus, the cessation of coral reef development during the Holocene in the northern SCS was probably associated with the dry and cold climate in South China, as reflected in the synchronous weakening of the ENSO and East Asian summer monsoon induced by the reduction of the 65°N summer insolation, which forced the migration of the Intertropical Convergence Zone.

Key words

northern South China Sea / Middle Holocene / sea-level highstand / Porites corals / climate response

Cite this Article

Yuanfu Yue, Lichao Tang, Kefu Yu, Rongyong Huang. Coral records of Mid-Holocene sea-level highstands and climate responses in the northern South China Sea[J]. Acta Oceanologica Sinica, 2024 , 43 (2) : 43 -57 . DOI: 10.1007/s13131-023-2264-9

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1 Introduction

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The global average sea level is showing a continuous upward trend because of global warming (Climate Change Center of the China Meteorological Administration, 2022). Sea-level rise and associated disasters are major threats to human survival, social and economic development, and the environment in low-lying coastal areas worldwide; consequently, it has become an important concern in international global change research (IPCC, 2021). For example, beach submergence (Zhang et al., 2011; Webster et al., 2016), accelerated coastal erosion (Zhang et al., 2004; Anderson et al., 2015), more extreme flood events in coastal areas (Kriebel et al., 2015; Vitousek et al., 2017; Taherkhani et al., 2020), and increased damage from destructive storms (Emanuel, 2005; Webster et al., 2005; Landsea, 2005; Klotzbach, 2006; Knutson et al., 2010; Cheng et al., 2019; Yue et al., 2019) have occurred. Therefore, sea-level rise has attracted widespread attention from the international community and governments of coastal countries. Although this crisis has been identified, a complete understanding of the history, patterns, and driving mechanisms of past sea-level changes is currently lacking. Therefore, definitive conclusions cannot be made about whether future sea-level rises will accelerate and reach high sea levels as in the Middle Holocene, which has seriously affected the accuracy of our predictions of future sea-level changes. More importantly, past climate change is the background for contemporary and future climate and environmental changes. To address global change, historical sea-level conditions and change patterns must be better understood, especially the changes in the Holocene as the changes that occurred in this period is closely associated with modern environmental changes. For instance, robust evidence has shown that the Mid-Holocene was a high-temperature period with a climate similar to the modern climate but much warmer (Gagan et al., 1998; Yu et al., 2005; Wei et al., 2007), and Mid- to Late-Holocene sea-level records from low-latitude regions serve as an important baseline of natural past sea-level change and global ice volume prior to the Anthropocene (Hallmann et al., 2018; Yue and Tang, 2023). Therefore, an in-depth understanding of past sea-level change processes and trends is required to further explore their relationship with climate change. This in turn will facilitate better prediction of future sea-level change scenarios and evaluation of their impacts, and is essential for human survival and social development.

In terms of historical sea-level changes, sea-level indicators that reflect sea-level changes must be identified (Wang, 1989; Nie, 1996) and the elevation and age of the indicators must be accurately measured to understand the historical process of sea level changes (Engelhart et al., 2011). In recent decades, with the improvement of research techniques and experimental devices, various sea-level indicators (such as foraminifera, diatoms, coral reefs, beach rocks, salt marshes, peat), reconstruction methods, and even modern observation records (~100 years, as far back as 1925 CE in Macao) have been used to broadly and intensively study historical sea-level changes in the South China Sea (SCS) along the coast, and large amounts of Holocene sea-level data have been published (Zhao and Yu, 2002; Yu et al., 2002, 2009; Zong, 2004; Huang et al., 2007; Shi et al., 2008; Yao et al., 2013; Xiong et al., 2018; Liu et al., 2020; Ma et al., 2021; Yan et al., 2021; Tao et al., 2022; Yue and Tang, 2023; Tang and Yue, 2023). For instance, Zong (2004) and Xiong et al. (2018) reconstructed the Holocene sea-level history for the lower latitudes of the Chinese coast based on ¹⁴C data and suggested that the Mid-Holocene sea level was similar to the modern mean sea-level (MSL) or changed little during the past 7 000 years. Conversely, both U-Th and ¹⁴C ages for Porites corals and Porites microatolls from records collected from the coastal Lei-Qiong area (an abbreviation for the Leizhou Peninsula and Hainan Island) in the northern SCS indicated a higher sea level above the modern MSL during the Mid-Holocene (Zhao and Yu, 2002; Huang et al., 2007; Shi et al., 2008; Yu et al., 2009; Yao et al., 2013).

However, observational records are too short and the reconstructed sea levels in the northern SCS differ or are controversial due to the selection of different sea-level indicators and the use of different methods (Yao et al., 2013; Yue and Tang, 2023). Moreover, whether the exposed dead Porites corals are the product of high sea level in the Mid-Holocene or the result of tectonic uplift has not been determined (Yao et al., 2013). Consequently, simply providing a historical reference for the trend of sea-level changes in the context of modern climate change is not a reliable method and accurate prediction of future sea-level changes is difficult. Therefore, the historical sea-level in the northern SCS must be reconstructed by selecting more precise sea-level indicators and using more accurate dating methods to clarify the existing differences.

Furthermore, the northern coast of the SCS is one of the key areas of the transition from the SCS to land and is an area sensitive to global change, as represented by Hainan Island, a far-field island located in low-latitude regions far away from the former and present ice sheet margins. The coast of this area is affected by sea-level change and has developed a larger area of fringing reef in the Holocene (Yu, 2000; Zhao and Yu, 2002), which likely represents the northern margin and coral reef distribution belt of the SCS. Of particular significance are the microatolls (commonly Porites microatolls), which are intertidal individual colonies with relatively flat and dead upper surfaces surrounded by an annular rim of living coral (Woodroffe and McLean, 1990; Yu et al., 2009; Smithers, 2011; Meltzner and Woodroffe, 2015). Their upward growth limit is constrained by the sea level through prolonged subaerial exposure at the lowest spring tide/lowest low water (LLW), although polyps on the colony sides remain alive and continue to grow laterally, thereby adopting a microatoll form (Woodroffe and McLean, 1990; Yu et al., 2009; Smithers, 2011; Meltzner and Woodroffe, 2015). Typically, most microatolls develop from massive in situ corals in shallow waters, and Porites microatolls are particularly common, although intertidal Goniastrea and Platygyra colonies also form microatolls on many reefs (Smithers, 2011). In flat reef settings, the upper surfaces were limited by past sea levels (Woodroffe and McLean, 1990) and thus have the potential to serve as reliable sea-level indicators (Scoffin and Stoddart, 1978; Stoddart and Scoffin, 1979; Woodroffe and McLean, 1990; Smithers, 2011; Meltzner and Woodroffe, 2015). Additionally, the U-Th dating method has been successfully developed for dating corals, enabling high-precision and accurate dating of coral reefs in this area (Zhao and Yu, 2002; Yu et al., 2009; Yan et al., 2019). Under these conditions, combining fossil corals with appropriate dating methods can provide an effective tool for accurately reconstructing the timing and trajectory of local sea-level fluctuations and revealing how Holocene climate change affected coral reef growth.

Previous studies have either focused on the history of reef growth (Zhao and Yu, 2002; Huang et al., 2007; Yan et al., 2019; Chen et al., 2021) or the reconstruction of sea-level changes during certain periods of the Holocene in the northern SCS (Shi et al., 2008; Yu et al., 2009; Yan et al., 2021). Unfortunately, these studies only collected small amounts of data, did not include tectonic corrections, and rarely included Porites (microatolls) elevations that were directly established by surveys to a precise datum. Therefore, a comprehensive understanding of Holocene sea-level changes and coral reef development and responses to regional environment and climate is currently lacking.

To provide a comprehensive understanding of historical sea-level changes and a necessary scientific basis for the prediction of future sea-level changes, this study determines the quantitative relationship between modern living coral and sea level by measuring the upper surfaces of 27 modern live Porites corals along the east coast of Hainan Island, and produces new high-quality sea-level index points (SLIPs). Based on this, we focus on reconstructing a detailed Mid-Holocene sea-level history based on elevation measurements and U-Th ages for three in situ exposed fossil Porites corals and 19 recalculated SLIPs. Additionally, our new record and an additional 270 available published coral U-Th ages from the coral reef areas along the northern coast of the SCS were further analysed and compiled. These findings will provide critical insights into major episodes of coral reef development and their relationship with climate change and a context for understanding modern ecological states.

2 Regional setting

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The SCS is a semi-enclosed marginal sea located along southern mainland China and adjacent to the western Pacific Ocean (Fig. 1), and is connected to the Indo–Pacific Warm Pool. The Asian Monsoon system is composed of the South Asian (or Indian), western North Pacific, and East Asian subsystems (Wang et al., 2017). The SCS is under the effect of the East Asian Monsoon system, which is influenced by the East Asian Summer Monsoon (EASM) in summer (May–September) and East Asian Winter Monsoon (EAWM) in winter (October–April). As shown in Fig. 1a, in winter, the EAWM creates an anticlockwise pattern of circulation, with the wind pushing cooler coastal waters down though the Taiwan Strait, where they circulate west and southwards along the coast of China (Yu and Liu, 1993; Morton and Blackmore, 2001). As shown in Fig. 1b, in summer, under the influence of the EASM, the current flow in the SCS is reversed and water enters from the Java Sea via the Karimata Straits, travels to the central area, and exits through the Taiwan Strait (Yu and Liu, 1993; Morton and Blackmore, 2001). The coastal current is controlled by monsoon dynamics, and northeast and southwest winds alternate periodically in winter and summer. Additionally, this area suffers frequent and intense storms (Yue et al., 2019).

The northern SCS, including the Lei-Qiong area, Weizhou Island of Beibu Gulf, and Xisha Islands, is adjacent to the north edge of the Intertropical Convergence Zone (ITCZ) and represents a key pathway of the EASM and a major moisture source for EASM precipitation in summer. As shown in Fig. 1c, the coast of the Lei-Qiong area is located at the northern fringe of the tropics and occupies the main part of the northern coast of the SCS. Hainan Island is situated at the southern tip of China (Fig. 1c). Fringing reefs with a total length of approximately 200 km along Hainan Island are discontinuously distributed along the east and west coast of Hainan Island (Lu et al., 1984), including along the Wenchang, Qionghai, Danzhou, Dongfang, and Sanya areas (Fig. 1c). However, the coral reefs in this area and even worldwide have experienced severe bleaching due to increased human activities and global warming, with recent increases in frequency and severity being observed (Yu et al., 2012; Hughes et al., 2017; Barkley et al., 2018; Claar et al., 2018; Dalton et al., 2020; Romero-Torres et al., 2020).

Hainan Island has a tropical monsoon climate. Instrumental data from the Qionghai Meteorological Station shows that the mean annual (air) temperature is 24.62℃ for the period 1980–2010 CE (Fig. S1, Table S1). The instrumentally obtained sea surface temperature (SST) from four adjacent ocean stations in the Lei-Qiong sea area indicated that the mean annual SST is (24.85 ± 0.39)℃ (Yu, 2000), which is within the range suitable for coral growth and coral reef development on the coast (Guan et al., 2015). Notably, the SST in winter is lower than 20℃ and seasonal changes in SST moderate the growth of the coral reefs (Lu et al., 1984; Yu, 2000).

3 Materials and methods

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To uncover details of sea-level change in the northern SCS during the Mid-Holocene, it is necessary to establish the indicative meaning of live Porites corals for SLIPs. Firstly, the top surface elevation of 27 modern live Porites corals obtained from the inner reef flat of the east coast of Hainan Island were measured using real-time kinematic (RTK) global positioning system (GPS) during our field surveys at the lowest low water (LLW) in July 2020 CE (Figs 1c, d). This survey was expected to help improve our ability to accurately define the indicative meaning and potential uncertainty range of modern live Porites corals. Secondly, the top surface elevation of three in situ fossil Porites corals which are now exposed above the tide on the inner reef flat were also measured, and then sampled. Those Porites corals were further polished into 7-mm thick coral plates, and then selected for X-ray radiography analyses. After performing digital X-ray radiography analyses, six subsamples were selected for U-Th dating, and six new U-Th ages and SLIPs were obtained.

Additionally, a total of 276 published U-Th ages of corals from the coral reef areas along northern coast of the SCS were collected and recalculated, including 19 region’s previously published SLIPs. We recalibrated the 19 SLIPs from Porites corals using the newly defined indicative meanings, which were also subjected to tectonic correction, error analysis, and reliability assessment. The distribution of episodes of coral growth in the northern SCS during the Holocene was determined based on the order of U-Th ages and two standard deviations (~50-year intervals) of the age determination errors (± 2σ). Details on the data processing and methods are provided in the Supplementary Material.

4 Results

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4.1 Biological geomorphology

During our field investigations in July 2020, several biological-geomorphological zones along the coast of Tongguling (Wenchang) southward to Qingge (Qionghai) (Fig. 1c) were generally recognized from the fringing reef. For instance, from the sea to the land, we observed reef fronts with living coral clusters, an outer reef flat coral zone that was dominated by massive Porites and microatolls, a middle reef flat massive coral mixed zone, an inner reef-flat branch-coral/sand zone, and sandy beach (Fig. 2).

The slope of the reef front is dominated by several types of live corals. The outer reef flat contains many in situ massive dead Porites. A few live Porites, Porites microatolls, and Platygyra were observed in the low-lying areas, in addition to several other hermatypic corals, such as Montipora solanderi, Acropora austera, Facia speciose, Pocillopora, and Fungia fungites. The middle reef flat is wide and relatively flat, with accumulations of coral debris, mainly dominated by Acropora spp. Generally, live corals were not observed, while numerous in situ dead corals were identified. The upper surface of the inner reef flat, characterized by the presence of numerous dead Porites and Porites microatolls. The sandy beach is distributed with scores of granite outcrops and a small number of coral debris. The sand embankment is approximately 3 m higher than the average sea level. Many coconut trees and some villages and ponds are located behind the sand embankment.

4.2 Indicative meanings for the modern live Porites corals

Porites corals represent sea-level indicators that have a quantifiable vertical relationship to a reference tidal level, meaning that they can serve as indicators; thus, these corals were examined and quantified in this study. According to our survey, the surface elevations the modern live Porites corals derived from Tongguling along the coast of Wenchang, Hainan Island were within −51.60 cm to −34.94 cm with an average elevation of −43.09 cm that varied within ± 8.33 cm (corresponding to a maximum range of 16.66 cm). Namely, results show that the indicative meanings for the modern live Porites corals is 146.09 cm below the mean tide level (MTL), or 15.09 cm below the LLW (Fig. 3, Table S2). This finding suggested that their upward growth limit is constrained by the sea level, and the LLW is the highest level of survival for the modern live Porites corals. Additionally, our investigations discovered that the top surface of each Porites coral from this area has a significantly narrow growth range and is almost uniformly horizontal. Accordingly, a relatively consistent relationship between Porites corals and tidal levels emerges.

4.3 U-Th isotopic data and age

The U-Th isotopic data of the three in situ fossil Porites corals (Fig. 4) from the east coast of Hainan Island are listed in Table S3. The measured U concentration (×10⁻⁶) was variable and ranged from (2.820 7 ± 2.2) ×10⁻⁶ to (3.167 7 ± 2.3) ×10⁻⁶, with an average value of 2.997 9 ×10⁻⁶. The values of the initial δ ²³⁴U (T) varied between (147.2 ± 1.1)‰ and (151.2 ± 1.4)‰ (2σ, similarly hereafter), which were within the range for modern seawater (144 ± 2)‰ (Chen et al., 1986) and modern corals (156 ± 6)‰ (Stirling et al., 1995). Correspondingly, the six U-Th dating ages of our three in situ dead Porites ranged from (5 314 ± 14) cal a BP to (3 485 ± 16) cal a BP (years before 1950, similarly hereinafter), thus spanning 1829 years (Table S3). These dates have 2σ errors of ± 14 years to 27 years.

Additionally, over 360 published U-Th ages for the in situ dead Porites, Porites microatolls, branching corals, and coral-related debris were collected, although only 273 coral U-Th ages were finally selected after recalculation and reliability assessment. Therefore, a total of 276 coral U-Th ages, including our six new data points, were selected and compiled in this study.

Distribution of the δ ²³⁴U (T) variations (Fig. 5a), the U concentrations (Fig. 5b) and U-Th ages (Fig. 5c) as shown in Fig. 5 (Table S4). Their δ ²³⁴U (T) values varied from (143.7 ± 1.6)‰ to (156 ± 3)‰ with an average of 147.28‰, which was generally comparable to those of modern corals (Stirling et al., 1995) and seawater (Chen et al., 1986). The U concentrations was 3.072 1 ×10⁻⁶ on average, ranging from 1.872 ×10⁻⁶ to 4.163 9 ×10⁻⁶. Accordingly, those ages ranged from (7 463 ± 37) cal a BP to (−67 ± 2) cal a BP, with a 2σ error from ± 1 year to 319 years, thus spanning ~7 500 years.

4.4 Episodic reef growth

The 276 coral U-Th ages, including our six new data points, ranged from (7 463 ± 37) cal a BP to (−67 ± 2) cal a BP (Figs 5c, d, Table S4) and provided unique insights into past coral development, responses to recent disturbance regimes, and potential for future reef growth. The average two standard deviation of the determination errors (± 2σ) of all 276 U-Th ages was approximately 49.70 years. For this study, an average 50-year interval was used to categorize the coral U-Th ages; thus, a total of 57 intervals of more than 50 years with no coral U-Th age distributions but belong to 25 periods were identified. Furthermore, 40 gaps/hiatus were observed between two successive ages of more than 50 years with no coral U-Th age distributions, which were largely compatible with the previous 50 intervals. The 50 intervals were mainly distributed in the period of (7 463 ± 37) cal a BP to (7 062 ± 100) cal a BP (4 gaps), and the period of (4 103 ± 23) cal a BP to (650 ± 5) cal a BP (26 gaps). Based on the age distributions and the 50-year class interval, four distinguishable episodes punctuated by two switch-on episodes and two switch-off episodes were identified during the Holocene, as shown in Fig. 5d.

Figure 5d shows that episode 1 contained only 2 coral U-Th ages (0.72%), and it started at (~7 436 ± 37) cal a BP and lasted until (7 062 ± 100) cal a BP. This episode likely represents the initial time of coral development in this area. According to the criteria of 50-year class intervals, no coral U-Th ages were distributed in the following periods: before 7 500 cal a BP, (7 450–7 200) cal a BP and (7 150–7 100) cal a BP. Among those 276 U-Th ages, 140 coral U-Th ages (56.88%) fall into episode 2, which lasted approximately 2 800 years from (7 062 ± 100) cal a BP to (3 371 ± 21) cal a BP. This result indicates thriving coral growth in the Mid-Holocene, which formed the geomorphic pattern of modern coral reefs. Nevertheless, the U-Th ages can be sorted into 22 gaps between two successive ages of more than 50 years during this period, namely (6 992 ± 19) cal a BP to (6 937 ± 37) cal a BP, (6 686 ± 24) cal a BP to (6 603 ± 41) cal a BP, (6 603 ± 41) cal a BP to (6 553 ± 23) cal a BP, (6 519 ± 179) cal a BP to (6 445 ± 19) cal a BP, (6 359 ± 17) cal a BP to (6 287 ± 19) cal a BP, (6 246 ± 19) cal a BP to (6 162 ± 19) cal a BP, (6 075 ± 39) cal a BP to (5 979± 22) cal a BP, (5 958 ± 19) cal a BP to (5 897 ± 22) cal a BP, (5 897 ± 22) cal a BP to (5 935 ± 20) cal a BP, (5 935 ± 20) cal a BP to (5 760 ± 66) cal a BP, (5 304 ± 30) cal a BP to (5 253 ± 15) cal a BP, (5 116 ± 36) cal a BP to (5 021 ± 18) cal a BP, (4 850 ± 39) cal a BP to (4 750 ± 33) cal a BP, (4 613 ± 34) cal a BP to (4 561 ± 53) cal a BP, (4 436 ± 21) cal a BP to (4 369 ± 12) cal a BP, (4 272 ± 15) cal a BP to (4 216 ± 19) cal a BP, (4 162 ± 14) cal a BP to (4 110 ± 17) cal a BP, (4 103 ± 23) cal a BP to (3 998 ± 35) cal a BP, (3 998 ± 35) cal a BP to (3 938 ± 23) cal a BP, (3 938 ± 23) cal a BP to (3 804 ± 15) cal a BP, (3 799 ± 22) cal a BP to (3 641 ± 16) cal a BP. According to the 50-year class interval criterion, the above-mentioned gaps mainly centered on the intervals of (6 500–6 450) cal a BP, (6 350–6 300) cal a BP, (6 050–6 000) cal a BP, (5 950–5 900) cal a BP, (5 100–5 050) cal a BP, (4 800–4 750) cal a BP, (4 100–4 000) cal a BP, and (3 750–3 650) cal a BP as shown in Fig. 5d. Similarly, episode 3 contained 23 coral U-Th ages (8.33%) and lasted approximately 2 740 years from (3 371 ± 21) cal a BP to (650 ± 5) cal a BP [(1420 ± 23) BCE–(1300 ± 5) CE]; additionally, over 13 turn-off phases were observed, which mainly centered on (3 350–3 150) cal a BP, (3 100–2 900) cal a BP, (2 850–2 800) cal a BP, (2 750–2 700) cal a BP, (2 600–2 500) cal a BP, (2 450–2 300) cal a BP, (2 250–2 200) cal a BP, (2 150–2 100) cal a BP, (1 950–1 800) cal a BP, (1 750–1 500) cal a BP, (1 450–1 100) cal a BP, (1 000–750) cal a BP, and 700–650) cal a BP. Similar to episode 2, episode 4 also featured a large distribution of U-Th dating ages, indicating the occurrence of reef growth during the last millennia. Episode 4 consisted of 94 coral U-Th ages (34.06%) within the period of (650 ± 5) cal a BP to (−67 ± 2) cal a BP [(1 300 ± 5) CE–(2 017 ± 2) CE], thus spanning approximately 600 years, and it indicated the formation of the modern coral reef geomorphologic framework. However, coral U-Th ages were not obtained in the periods of (550–500) cal a BP (1400–1450 CE) and (250–200) cal a BP (1700–1750 CE).

4.5 Age elevation

The three in situ fossil Porites corals collected from the east coast of Hainan Island were analyzed for elevation measurements and U-Th dating. Elevation data indicate a difference of (35.69–71.09) cm between historical levels and the modern MSL if the influence of tectonic uplift is not considered. Based on the newly defined indicative meanings (which were derived from the 27 live Porites corals), the height of the top surface of the in situ fossil corals (H_fossil), and the height corrected for tectonic uplift (H_tectonic), 6 new SLIPs and 19 recalculated SLIPs were produced for historical sea-level reconstruction (as seen in Table S3). Additionally, the elevation error of the newly added coral sample was 8.33 cm, as indicated by the 27 modern live Porites corals. The measuring instrument error of the RTK GPS was 5 cm, and the sampling error was 1 cm, thus, the total sea-level error of our new SLIPSs was ± 9.8 cm. Accordingly, the sea-level indicators are plotted in a time-altitude graph with the range of errors, as shown in Fig. 6. The 6 new and high-quality SLIPs were dated from (5 314 ± 14) cal a BP to (3 485 ± 16) cal a BP and indicated a decrease in relative sea level (RSL) from (198.33 ± 9.8) cm to (169.33 ± 9.8) cm MTL (similarly hereinafter). While the 19 recalculated SLIPs indicated the RSL during the period of (5 393 ± 25) cal a BP to (3 390 ± 12) cal a BP was (227.7 ± 9.8) cm to (154.88 ± 9.8) cm MTL.

After the elevations of the 6 new and 19 published SLIPs are adjusted by the indicative meanings and their ages were recalibrated, a time elevation RSL is produced as shown in Fig. 6. Overall, the 25 SLIPs indicated an RSL fluctuation between (227.7 ± 9.8) cm to (154.88 ± 9.8) cm from (5 393 ± 25) cal a BP to (3 390 ± 12) cal a BP. In detail, five stages of sea-level fluctuations can be identified from the distribution pattern of the age spectrum during the following periods. For instance, five SLIPs revealed that the sea level was (192.68 ± 9.8) cm between (5 393 ± 25) cal a BP and (5 253 ± 15) cal a BP. Four SLIPs revealed that the sea level was (221.91 ± 9.8) cm between (5 251 ± 30) cal a BP and (5 178 ± 18) cal a BP. Six SLIPs revealed that the sea level was (188.38 ± 9.8) cm between (4 893 ± 19) cal a BP and (4 518 ± 16) cal a BP. Five SLIPs revealed that the sea level was (211.32 ± 9.8) cm between (4 340 ± 20) cal a BP and (3 625 ± 10) cal a BP. Five SLIPs revealed that the sea level was (167.22 ± 9.8) cm between (3 613 ± 10) cal a BP and (3390 ± 12) cal a BP.

5 Discussion

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The high precision and accuracy of U-Th dating with a quantifiable vertical relationship to a reference tidal level enabled the reconstruction of both the time and trajectory of local sea-level fluctuations, and such dating also aids in understanding the responses of coral reef development to environment and climate change. In this study, we collected and analyzed three in situ fossil Porites corals from the east coast of Hainan Island in the northern SCS, from which six new and high-quality SLIPs were generated. The new data indicated that the RSL fluctuation ranged from (227.7 ± 9.8) cm to (154.88 ± 9.8) cm after tectonic correction in the SCS during the Middle to Late-Holocene. This finding adds to the debate on whether a higher Mid-Holocene sea-level highstand occurred. Coupled with an additional 270 published coral U-Th ages, our findings enabled the recognition and precise timing of different phases of coral reef initiation, growth, and turn-off during the Holocene as well as their relationship with climate change.

5.1 Implications for Mid-Holocene sea levels

Elevations of the upper limit of coral growth have been determined for 27 modern Porites corals from Tongguling along the coast of Wenchang, Hainan Island. These corals appear to have had a planar upper surface typical of the microatoll growth form. Such a growth form is important because it implies that LLW constrained the upward growth of the coral. Corals that do not have such an upper flat surface may not be limited by low water levels and therefore are not indicators of former sea levels. Furthermore, the Porites habitat along the east coast Hainan Island lie at (−43.09 ± 9.8) cm in elevation, which is largely comparable with that observed for microatolls (−30.02 ± 16.2) cm from Cocos Island (Smithers and Woodroffe, 2000). This finding suggested that the conditions for establishing a close relationship between the upper coral growth limit and sea level are very good and indicated that live Porites corals is a precise diagnostic sea-level indicator, and therefore serves as a modern sea-level indicator in this study. Typically, microatolls include a single colony of massive Porites up to several meters in diameter, and reef flats are commonly formed by the dead upper surfaces of in situ massive Porites. Accordingly, it is reasonable to infer that any height changes in the upper surfaces/rims of different ages within an individual Porites in a relatively stable tectonic area should reflect sea-level variation.

The study area lies further away from the plate boundary and has been regarded as relatively stable tectonically (Zhao, 1983; Huang et al., 1995; Zong, 2004), although a tectonic uplift rate of (0.02–0.05) mm/a has been reported from the coral reef area in Leizhou Peninsula in the northern SCS since the Holocene (Yu et al., 2002; Zhan et al., 2007), which is much lower than the growth rate of coral reefs (1 mm/a). Therefore, combined with the elevations of dated in situ Porites (microatolls), we inferred that the large in situ fossil Porites (microatolls) exposed at MTL from the northern SCS were not the product of tectonic uplift. Rather, a plausible interpretation for this situation is the contemporaneous high sea level. In other words, the micro-geomorphology of these in situ fossil Porites (microatolls) suggested that the elevation differences between the upper surfaces mainly resulted from sea-level variations. Additionally, an earthquake occurred in 1605 in Puqian (Hainan Island) that resulted in a large subsidence range in the outer bay of Puqian Bay and a small subsidence range in the inner bay with obvious spatial differences (Zhang et al., 2008), and it would lead to a lowering of the sea level without a need for any RSL change. Although this situation is feasible, we consider it unlikely because tidal adjustments would likely manifest as a more gradual change in the RSL curve, which is not the case in our previous study (Yue and Tang, 2023). Overall, the tectonically corrected sea level presented an average 16.05 cm decrease relative to the current record (Table S3). Greater differences were observed with age, and vice versa.

In theory, the RSL should increase in the far-field due to a small amount of ice melting after the final melting of the Laurentide and Fennoscandian ice sheet between 7 000 cal a BP and 4 000 cal a BP (Lambeck et al., 2014; Bradley et al., 2016). In this study, six new SLIPs and the recalibration of the previously published SLIPs were generated for tectonic correction and reliability analysis. As shown in Fig. 6, the corrected ages and altitudes of the SLIPs were plotted. It showed that the reconstructed RSL was was (192.68 ± 9.8) cm MTL between (5 393 ± 25) cal a BP and (5 253 ± 15) cal a BP, followed by a rise to (221.91 ± 9.8) cm between (5251 ± 30) cal a BP and (5 178 ± 18) cal a BP, and decline to (188.38 ± 9.8) cm between (4 893 ± 19) cal a BP and (4518 ± 16) cal a BP, then rise to (211.32 ± 9.8) cm between (4340 ± 20) cal a BP and (3 625 ± 10) cal a BP, and finally fall to (167.22 ± 9.8) cm between (3 613 ± 10) cal a BP and (3 390 ± 12) cal a BP. Comparison between the sea level derived from the 6 new SLIPs and 19 recalculated SLIPs and the predictions of the glacial isostatic adjustment (GIA), e.g., ICE-4G model (Wang et al., 2012) and the outputs of the final ice-volume equivalent sea level (ESL) model from Bradley et al. (2016) offer us an opportunity to infer causal mechanisms of the Mid-Holocene sea-level history.

As shown in Fig. 7a, the reconstructed sea-level elevations were roughly consistently along the predicted Mid-Holocene sea level, which were above the ICE-4G model predictions but below the output of ESL model. These reconstructed sea-level and predicted elevations may imply a significant regional uplift due to neo-tectonic movement. Alternatively, a greater amount of ice melting is required for the Mid-Holocene period. For instance, considering the northern SCS is located near the active subduction zone, the mantle viscosity coefficient is low, and the melted ice water is significantly affected by weak viscosity regions, which produced faster response rate in the middle Holocene crustal activity, thus reducing the RSL (Bradley et al., 2016). Over all, this likely suggested that both GIA and ESL exerted a significant influence in the RSL history in this far-field region. Additionally, we noted that this is contrary to Malay Peninsula (Tam et al., 2018), which is comparable with the GIA model results. It was likely considered as the differential responses caused likely by the difference in the upper mantle viscosity between these two landmasses (Xiong et al., 2020), or refinements are needed in ice-history models to fully capture our RSL data.

5.2 Regional correlation and difference analysis of sea level

The results from the east coast Hainan Island indicated that the RSL was (227.7 ± 9.8) cm to (154.88 ± 9.8) cm between (5393 ± 25) cal a BP and (3390 ± 12) cal a BP recorded by the in situ fossil massive Porites corals. It further testified to the existence of a higher Mid-Holocene sea-level highstand in the northern SCS. Indeed, this scenario was generally consistent with other findings from the coast surrounding the SCS, although a certain degree of variability was observed in the timing and the height varied according to location.

For instance, both the U-Th and ¹⁴C ages for Porites corals and Porites microatolls records collected from the coastal Lei-Qiong area in the northern SCS indicated a (100–400) cm higher sea level above the modern MSL during the Mid-Holocene (Nie et al., 1996; Zhao and Yu, 2002; Huang et al., 2007; Shi et al., 2008; Yu et al., 2009; Yao et al., 2013). Additionally, evidence for a Holocene transgression was recorded in Thailand and Vietnam between 8 000 cal a BP and 7 000 cal a BP, and it reached a peak value close to 150 cm around 6 000 cal a BP (Tanabe et al., 2003; Stattegger et al., 2013). Similar patterns have been observed by comparison with distal records, e.g., the RSL was between 140 cm and 300 cm above present values along the east coast of Peninsula Malaysia at 7 000 cal a BP (Parham et al., 2014), and it then reached 150 cm around 6 500 cal a BP (Zhang et al., 2021). Similar patterns were also observed in Belitung Island, Indonesia, on the Sunda Shelf, where a Mid-Holocene highstand of 180 cm occurred between 6 800 cal a BP and 6 600 cal a BP (Meltzner et al., 2017). Recently, highstand sea-level observation data were reported for the period of (6 100–6 000) cal a BP along the western Sarawak coast of Borneo (Majewski et al., 2018). Besides, a case in point is the sea level around 5 140 cal a BP (uncorrected ¹⁴C age) in the Hongkong was about 190 cm higher than the present-day was reported (Davis et al., 2000). Similar patterns were also observed in the Penghu Islands, where indicated a highest Holocene RSL about 240 cm above the present sea level at about 4 700 years ago (Chen and Liu, 1996). Even a level of approximately (3 ± 2.5) m above the present mean sea level was recorded from corals in Ishigaki Island at around 5 000 cal a BP (Hongo and Kayanne, 2010).

The above-mentioned past sea level records indicated that the formation and appearance of the Mid-Holocene sea-level highstand in the northern SCS was basically synchronous with the surrounding areas of the SCS. In addition, the tropical Pacific Ocean (Dickinson, 2004), east coast of India from Cape Comorin to Rameswaram (Banerjee, 2000), South Africa coastline (Ramsay, 1996), and southern Brazil (Angulo et al., 1999) have also confirmed the existence of a Mid-Holocene highstand sea-level, which suggests that the regional changes in RSL were unprecedented in modern times.

What’s more, we noted that sea-level reconstructions based on different methods or criteria presented significant differences and uncertainties and were not comparable in terms of the time, phasing, and amplitude of the highstand. In terms of dating methods, the radiocarbon content of terrestrial organisms differs from that of marine organisms due to the effect of the marine carbon pool (Hall and Henderson, 2001), and a difference of approximately 400 years in radiocarbon age has been observed between terrestrial and marine organisms of the same age. Additionally, ¹⁴C dating of coral provides dates that are (~200–1 000) years younger than their corresponding U-Th dating ages due to spatial and temporal variability in the marine reservoir effect as well as the influence of the surrounding environment and late diagenesis (Eisenhauer et al., 1993; Ingram and Southon, 1996; Yu et al., 2002, 2010; Bard et al., 1993, 1998; Hua et al., 2020). Moreover, the carbon pool effect has different effects in different time periods (Yu et al., 2010).

In terms of reconstruction methods, values at 100 cm below the tidal datum (Nie et al., 1996) or the mean lower low water of the local gauge station (Yao et al., 2013) were defined as the upper limit of coral growth. Even the elevation data were expressed in terms of altitude without providing any datum plane or reference to the upper surface of modern live Porites corals and the Porites microatolls (Shi et al., 2008; Yu et al., 2009). These findings suggest that the quantifiable vertical relationship of a sea-level indicator to a reference tidal level, i.e., indicative meaning, was not a quantifiable or uniform criterion, which may be the main reason for the different sea-level histories for the same region. Thus, controversies regarding sea-level changes will inevitably arise. For instance, the indicative meaning of the 27 live Porites corals in this study can be expressed as (146.09 ± 9.8) cm below MTL (comparing the height with tidal ranges of nearby Qinglan tide gauge), or 15.09 cm below LLW, even (−43.09 ± 9.8) cm (relative to 1985 National Elevation Benchmarks). Significantly, it corresponds to a reconstructed RSL of (227.7 ± 9.8) cm to (154.88 ± 9.8) cm MTL, or (81.61 ± 9.8) cm to (8.79 ± 9.8) cm between (5 393 ± 25) cal a BP and (3 390 ± 12) cal a BP as shown in Fig. 5. Obviously, based on different criteria, the same sea-level indicators, e.g., our 6 new and 19 recalculated SLIPs, support the following different findings: no obvious higher-than-present sea-level highstand or higher sea-level highstands in the mid-late Holocene is found from the northern SCS. In any case, our analysis demonstrates that mid-late Holocene sea-levels highstand did exist.

In addition, age and vertical errors (Yue and Tang, 2023), differential GIA (Xiong et al., 2020), and vertical tectonics (Yao et al., 2013) should be considered as significant factors in sea-level reconstruction, although the study area is in a relatively tectonically stable region on the northern coast of the SCS (Zhao, 1983; Huang et al., 1995; Lu, 1997; Zong, 2004).

5.3 Possible influences on coral reef growth

The worlds coral reefs are increasingly impacted by natural climatic and environmental change and anthropogenic disturbance; thus, understanding historical causes and mechanisms of reef switch-on and switch-off states is of critical importance (Perry and Smithers, 2011; Yan et al., 2019). Reports have indicated that the initiation of coral reefs and evolutionary states (switch-on and switch-off) reflect the long-term synergistic effects of several factors, e.g., sea-level fluctuations and climate change (Montaggioni, 2005; Yu et al., 2012; Foster and Attrill, 2021). However, their potential to influence the state of reef-building at timescales meaningful to contemporary reef ecology are poorly documented. Therefore, to identify possible climatic and environmental drivers of switch-on and switch-off events, we compared coral age distributions to regional paleoenvironmental conditions during the Holocene, such as the reconstructed RSL (Fig. 7a), storm records from Lingyang Reef (Yue et al., 2019) (Fig. 7b), the reconstructed temperature (relative to the present) from China (Wang and Gong, 2000) (Fig. 7e), subtropical evergreen broadleaved percentage (Yue et al., 2012) (Fig. 7f), the sea surface temperatures (SSTs) of Core MZ01 from the continental shelf of the East China Sea (Pan et al., 2020) (Figs 7g, h), reconstruction of El Niño occurrences from Laguna Pallcocha, Ecuador (Moy et al., 2002) (Fig. 7i), δ ¹⁸O records (‰, VSMOW) from the stalagmite of Dongge Cave (Wang et al., 2005) (Fig. 7j), and the 65°N summer insolation in July in the Northern Hemisphere (Berger and Loutre, 1991) (Fig. 7k).

Based on the distribution of coral U-Th ages, it can be inferred that the development of coral reefs in the northern SCS experienced initial development, boom growth, bust decline, and flourishing development again. For instance, our dataset clearly demonstrated a distinct period (Episode 1) between before (7062 ± 100) cal a BP during the early Mid-Holocene when coral reefs on the northern SCS experienced the first switch-on phase (Figs 7b, c). Significantly, there is evidence that a period of rapid rise of RSL from (−49.3 ± 0.8) m to the present height occurred from 10 500 cal a BP to 7 000 cal a BP (Xiong et al., 2018). We thus inferred that coral reefs initially developed in the northern SCS at the onset of the Mid-Holocene as a response to progressive transgression of the shelf associated with the postglacial sea-level rise (Zong, 2004; Zong et al., 2012; Xiong et al., 2018), although the coral ages were sporadically prsented. As the decay of ice sheets and glaciers caused post-glacial sea-level rise during this interval of global warming (Clark et al., 2012) and marine inundation occurred across the extensive upper continental selves (Smith et al., 2011), favorable conditions were provided for the development of reef in the Lei-Qiong area along the northern SCS. Additionally, increases of temperature (Wang and Gong, 2000) and a high percentage of subtropical pollen assemblages (Yue et al., 2012) in the early Holocene (as shown in Figs 7e, f) also suggested an interval of global warming and warm climate that favored the initiation of coral reef in this area. At the same time, contemporaneous records of coral widely distributed in Malaysia and other places (Dickinson, 2004; Horton et al., 2005; Majewski et al., 2018) likely indicate a universal development stage of coral reefs during this period.

Compared with the last glacial-interglacial cycle, the Mid-Holocene has been known as a brief period of relative stable climate, which included a stabilized sea-level and warmer SST stage (Gagan et al., 1998; Stott et al., 2004). In situ massive corals and branching corals were extensively distributed on the reef flat at Lei-Qiong, as shown in episodes 2 and 4, indicating that coral species have increased significantly. Of particular significance is episode 2, which mainly centered on the period (7 062 ± 100) cal a BP to (3 371 ± 21) cal a BP (Figs 7b, c). Firstly, it basically overlaps the Mid-Holocene sea-level highstand predicted by the ICE-4G model (Lambeck et al., 2014; Bradley et al., 2016) and recorded by our high-precision U-Th dating SLIPs, as shown in Fig. 7a. With this in mind, we are convinced that the Mid-Holocene was optimal for coral growth, and such growth was generally well correlated to the stabilization and culmination of the post-glacial global sea level (Zhao and Yu, 2002). In addition, the flourishing of corals in episode 2 almost perfectly corresponds to the Holocene thermal maximum (HTM). As shown in Fig. 7k, this period well coincided with a high stage of summer insolation at 65°N (Berger and Loutre, 1991). Higher temperatures (relative to the present) from China (Wang and Gong, 2000) and SST records from western tropical Pacific (Stott et al., 2004) indicated a rapid warming scene. Nearby SST records during the period of 6 500 cal a BP to 6 100 cal a BP derived from Porites corals in Sanya indicated that the average minimum winter and maximum summer monthly SSTs were approximately (0.5–1.4℃) and (0–2.0℃) higher than at present in the northern SCS (Wei et al., 2007). This finding suggested that warm winters are occurring more frequently while the warm climate favors thriving coral in this area and is probably conducive to the formation of a stable coral reef ecosystem.

Moreover, the switch-on episode is also consistent with other records from the subtropical zone, such as the high percentage of subtropical pollen record of SZY in Fujian (Yue et al., 2012) (Fig. 7f). As shown in Fig. 7j, the δ ¹⁸O record of speleothem and broader syntheses identified a strengthening of the EASM (Dykoski et al., 2005; Wang et al., 2005; Cai et al., 2021) induced by the strong insolation forcing a northward migration of the ITCZ (Haug et al., 2001). The monsoon circulation caused by the enhancement of solar radiation in the Northern Hemisphere reached the strongest stage during the period of ~9 000 cal a BP to 6 000 cal a BP (Sirocko et al., 1993). Furthermore, episode 2 formed prior to the weakening of EASM (Dykoski et al., 2005; Wang et al., 2005) and was also well correlated with the stronger ENSO activities (Moy et al., 2002), as seen in Fig. 7i. Moreover, the latest research points out that the shrinkage of the EAWM is associated with increased ENSO events since the Mid-Holocene (Wu et al., 2019). Consequently, the strengthening of the EASM drives more warm water masses northward, which is beneficial for coral migration to high latitude areas. For example, in our dataset, the number of coral U-Th ages from Weizhou Island and Leizhou Peninsula at relative high latitudes in Episode 2 and 4 increased significantly, which may be a positive response to these climate changes. On the contrary, the strengthening of the EAWM drives colder coastal currents southward, results in cooler winter SST and a larger SST seasonality, which inhibits the growth of corals, as seen in episode 3. Except that, it is reported that typhoon caused significant damage to reef associated depths on the passage of the atoll, where currents generated by the typhoon produced the strongest energy (Yang et al., 2015). After storm surges as seen in Fig. 7b (Yue et al., 2019), significantly fewer corals were recorded as seen in Episode 3, likely suggesting that the storm surge disrupted coral growth and inhibited reef development. Therefore, it must be emphasized that climate change can potentially impact coral reefs through several key mechanisms, such as sea-level rise, SST (rising global temperatures), ENSO, EASM and storm surges, although it is tempting to ascribe such changes to a single environmental cause.

5.4 Implications for climate events

However, it must be pointed out that the HTM was influenced by climatic cooling events. For example, the well-documented δ ¹⁸O records from stalagmites from Dongge Cave (Dykoski et al., 2005; Wang et al., 2005) and ice core records indicate the weakening of the EASM (Rasmussen et al., 2006). Reef corals are well-known as excellent archives for recording the climatic and environmental history of tropical oceans because corals growing at their latitudinal limits may be the most sensitive to global climatic changes (Yu, 2012). Many organisms tend to be limited to specific thermal ranges (Drinkwater et al., 2010). SST is a primary controlling factor of coral reef distribution and coral reef ecosystems (Yu, 2000; Lough, 2012). It is worth noting that higher winter SSTs would favor the development of coral reef ecosystems (Yu, 2000). On the contrary, cold winter SSTs caused a decrease and/or cessation of coral skeletal extension (Fallon et al., 1999). Therefore, it is reasonable to infer that the coral growth hiatuses that occurred in episodes 1 and 2 during the Mid-Holocene, as observed from the northern SCS based on the criteria of 50-year class intervals, should be a response to cooling events. Coincidentally, the timing of these reef growth disturbance with hiatuses were generally consistent with the above multiple cooling events recorded in stalagmites (Dykoski et al., 2005; Wang et al., 2005; Cai et al., 2021) when the gaps/hiatuses between two successive ages more than 50 years were considered.

Additionally, as shown in Fig. 1, this area is not only greatly affected by the EAWM and coastal current but also suffered cool water intrusions in winter and received inputs mainly from the Pearl and local rivers and coastal runoff. Cold coastal current and/or terrestrial inputs from nearby rivers that may bring more suspended sediments likely further prevent the growth of coral reefs. Furthermore, a detailed ecological, micro-structural, and skeletal Sr/Ca study from Leizhou Peninsula suggested high-frequency winter cooling and reef coral mortality during the period (7 500–7 000) cal a BP (Yu et al., 2004), which was comparable to the strong upwelling intensity during this period and almost synchronous with the hiatus of coral growth. Importantly, a similar pattern was also observed in Kodakara Island in the northwestern Pacific (Hamanaka et al., 2012) as well as Ovalau and Moturiki islands in Fiji in the southwest Pacific (Nunn, 2000). As shown in Fig. 7, the periods of (7 150–7 100) cal a BP, (6 500–6 450) cal a BP, (6 350–6 300) cal a BP, (6 050–6 000) cal a BP, (5 950–5 900) cal a BP, (5 150–5 000) cal a BP, (4 800–4 750) cal a BP, (4 100–4 000) cal a BP and (3 750–3 600) cal a BP corresponded particularly well to the rapid decline of SST and subsurface water temperature (Pan et al., 2020) and further emphasized that the weakening EASM induced by insolation forced a southward migration of the ITCZ in the Northern Hemisphere (Haug et al., 2001). Therefore, a plausible interpretation for the cessation of the developmental stage or the switch‐off period was the cold events that occurred during those periods within Mid-Holocene. Meanwhile, they also appeared to be synchronous with North Atlantic Holocene cooling events 3–4 (Bond et al., 1997, 2001); thus, they likely have a global connection.

Nonetheless, other interpretations appear to be persuasive, such as the rapid sea-level rise outpacing reef growth (Webster et al., 2018) and abrupt decline leading to subaerial exposure. Regional common causes may also have had an influence, such as coral bleaching during ENSO warm phases on inter-annual time scale or during positive PDO phases or sharp phase transitions on interdecadal time scales (Yu et al., 2012), increased nearshore deposition of terrigenous sediment, which may be related to increased tropical-cyclone frequency and intensity or early human activities (Nunn, 2000), and unknown local factors. Finally, it is also difficult to rule out the omission of sampling. After all, only 276 U-Th ages have been compiled; thus, including all in situ coral samples in this area is difficult.

6 Conclusions

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This study has reconstructed the Mid-Holocene relative sea-level history in the relatively tectonically stable northern SCS with two specific objectives: quantifying the sea-level indicative meanings of Porites corals and producing new high-quality SLIPs. In this study, we surveyed the growth range of 27 modern live Porites and reported six newly added U-Th-dated SLIPs derived from three Porites along the east coast of Hainan Island in the northern SCS. The investigations showed that the top surface elevations of those live Porites are (146.09 ± 9.8) cm below MTL or 15.09 cm below LLW, (−43.09 ± 9.8) cm and almost horizonal in a narrow range, which suggested that their upward growth limit is constrained by the sea level, and the LLW is the highest level of survival for the modern live Porites corals. It further suggested that Porites coral is a precise diagnostic sea-level indicator that can provide a firmer basis for extrapolating historical sea levels. Additionally, based on the newly defined indicative meanings, the six new SLIPs coupled with 19 recalculated SLIPs indicated that the RSL varied from (227.7 ± 9.8) cm to (154.88 ± 9.8) cm above MTL during in the period of (5 393 ± 25) cal a BP to (3 390 ± 12) cal a BP, which provided support for the existence of Mid-Holocene sea-level highstands in the northern SCS. This scenario was generally consistent with other findings from the coast surrounding SCS, and indicates that the formation and appearance of the sea-level highstand in the northern SCS in the Mid-Holocene was synchronous with that in the surrounding areas of the SCS, which likely presented a local background. Furthermore, regional correlations and difference analyses of the reconstructed sea levels suggested that the quantifiable vertical relationship of the sea-level indicator to a reference tidal level, which represents indicative meaning, was not a quantifiable or uniform criterion and thus may represent the main reason for different sea-level histories for the same region. Thus, controversies about sea-level changes will inevitably arise.

Moreover, our six new data points coupled with the additional 270 coral U-Th ages depicted a history of coral reef development in the northern SCS during the Holocene, which experienced initial development [before (7 062 ± 100) cal a BP], boom growth (7 062 ± 100) cal a BP to (3 371 ± 21) cal a BP, bust decline (3 371 ± 21) cal a BP to (650 ± 5) cal a BP and flourishing development again [since (650 ± 5) cal a BP]. Furthermore, 56.88% of the coral U-Th dating ages fell in the Mid-Holocene, suggesting a peak period of coral reef development that was probably associated with a relatively stable period of sea level and the Holocene thermal optimum. Comparisons with the other climate records indicated that climate change can potentially impact coral reefs through several key mechanisms. For instance, the strengthening of EASM drives more warm water masses to move northward, which is beneficial for coral migration to high latitude areas and the boom growth of coral reef. On the contrary, the bust decline of coral reef featured by reef growth disturbance with hiatuses during the Mid-Holocene broadly corresponded with the synergistic effects of synchronous low temperature and rapid global cooling events. In summary, we inferred that the development/cessation of coral reef in the northern SCS was probably closely associated with the dry and cold climate in South China, which was reflected in the synchronous strengthening/weakening of the ENSO and EASM induced by the 65°N summer insolation, which forced the migration of the ITCZ.

Acknowledgements

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We thank Tingli Yan for assistance in the fieldwork and samples collection. We are thankful to the editor, and the anonymous reviewers for their constructive comments and suggestions.

Funding

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The National Natural Science Foundation of China under contract Nos 42366002 and 41702182; the National Key R&D Program of China under contract No. 2017YFA0603300; the Guangxi Scientific Projects under contract No. 2018GXNSFAA281293.

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doi: 10.1007/s13131-023-2264-9

Receive Date：2023-07-14
Online Date：2025-11-17
Published：2024-02-25

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Received：2023-07-14
Accepted：2023-09-06

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The National Natural Science Foundation of China under contract Nos 42366002 and 41702182; the National Key R&D Program of China under contract No. 2017YFA0603300; the Guangxi Scientific Projects under contract No. 2018GXNSFAA281293.

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¹ School of Marine Sciences, Coral Reef Research Center of China, Guangxi University, Nanning 530004, China

² Guangxi Laboratory on the Study of Coral Reefs in the South China Sea, Nanning 530004, China

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* E-mail: yuanfu.yue@gxu.edu.cn

kefuyu@scsio.ac.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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Fig. 1. A geographic map of South China Sea, showing the location of Hainan Island. a. Surface-ocean circulation patterns in the South China Sea during the East Asian Summer Monsoon (EASM) and b. the East Asian Winter Monsoon (EAWM)), modified after Yu and Liu (1993) and Morton and Blackmore (2001). c. Coral reefs are distributed along the coast of Lei-Qiong. d. The elevation survey and sampling during the field investigation from Wenchang southward to Qionghai along the east coastal Hainan Island in July 2020.

Fig. 2. Cross-section sketches showing distribution of biological-geomorphological zones of typical fringing coral reefs based on our field investigation in July 2020 on the eastern coast of Hainan in the northern SCS.

Fig. 3. Precise height measurement results of the top surface elevation of the 27 live Porites corals from Tongguling along the coast of Wenchang, Hainan Island. It showed that the upper surface elevations of those live Porites corals were 146.09 cm below the MTL on average. The depth range of live Porites corals heights is relative to 1985 National Elevation Benchmarks and the predicted tidal levels at Qinlan Port tide gauge. The mean tide level (MTL) lies 103 cm above 1985 National Elevation Benchmarks, the mean low water (MLW) is 58 cm, the mean high water (MHW) is 147 cm, the highest high water (HHW) is 315 cm, the lowest low water (LLW) is −28 cm, the mean tidal range is 89 cm, and the maximum tidal range is 255 cm (China Nave Hydrographic Office, 2022).

Fig. 4. Images showing macro-structures and preservation status of Holocene fossil Porites corals on the east coastal Hainan in the northern SCS. The diameters in Images a, b and c are 101.5 cm, 253 cm, and 191 cm, respectively.

Fig. 5. Distribution of the δ ²³⁴U (T) variations (a), the U concentrations (b) and U-Th ages (c) for the 276 in situ dead Porites, Porites microatolls and coral-related debris from the northern SCS. Episodes of corals growth from the northern SCS during the Holocene based on the U-Th ages order and its’ two standard deviations (50-year interval) of the age determination errors (±2σ) (d).

Fig. 6. U-Th age-elevation data for our 6 new SLIPs and 19 recalculated SLIPs from east costal Hainan Island in the northern SCS. Vertical uncertainties are uniformly ± 9.8 cm (±2σ).

Fig. 7. Comparison of Mid-Holocene RSL data with the predictions of the ICE-4G model (Wang et al., 2012) and the outputs of the final ice-volume equivalent sea-level model from Bradley et al. (2016) (a), storm records from Lingyang Reef (Yue et al., 2019) (b), coral U-Th ages distribution pattern derived from the in situ dead corals on the coast of northern SCS (c, d), the reconstructed temperature (relative to the present) from China (Wang and Gong, 2000) (e), subtropical evergreen broadleaved percentage (Yue et al., 2012) (f), the sea surface temperatures (SSTs) of Core MZ01 from the continental shelf of the East China Sea (Pan et al., 2020) (g, h), reconstruction of El Niño occurrences from Laguna Pallcocha, Ecuador (Moy et al., 2002) (i), δ ¹⁸O records (‰, VSMOW) from the stalagmite of Dongge Cave (Wang et al., 2005) (j), and the 65°N summer insolation in July in the Northern Hemisphere (Berger and Loutre, 1991) (k).

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