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Sediment source-to-sink process variations of sandy-muddy transitional beaches and their morphological indications
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Shaohua Zhao1, 2, 3, *, Feng Cai1, 2, 3, *, Hongshuai Qi1, 2, 3, Jianhui Liu1, 2, 3, Chao Cao1, 2, 3, Gen Liu1, Gang Lei1, 2, 3
Acta Oceanologica Sinica | 2023, 42(7) : 10 - 24
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Acta Oceanologica Sinica | 2023, 42(7): 10-24
• Beach Protection and Restoration around the South China Sea •
Sediment source-to-sink process variations of sandy-muddy transitional beaches and their morphological indications
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Shaohua Zhao1, 2, 3, *, Feng Cai1, 2, 3, *, Hongshuai Qi1, 2, 3, Jianhui Liu1, 2, 3, Chao Cao1, 2, 3, Gen Liu1, Gang Lei1, 2, 3
Affiliations
  • 1 Laboratory of Ocean and Coast Geology, Third Institute of Oceanography, Ministry of Natural Resources, Xiamen 361005, China
  • 2 Key Laboratory of Marine Ecological Conservation and Restoration, Ministry of Natural Resources, Xiamen 361005, China
  • 3 Fujian Provincial Key Laboratory of Marine Ecological Conservation and Restoration, Xiamen 361005, China
Published: 2023-07-25 doi: 10.1007/s13131-022-2077-2
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The clay mineralogy of 28 sandy-muddy transitional beach (SMT-Beach) sediments and surrounding mountain river sediments along the coasts of southeastern China was systematically investigated to reveal the sediment source-to-sink process variations of such beaches and their morphological indications. The results show that the clay mineral assemblages of these SMT-Beaches mainly comprise of almost equal illite (~30%), kaolinite (~28%), chlorite (~22%), and smectite (~20%) contents. From the surrounding mountain rivers to the SMT-Beaches, clay mineral assemblages show distinct spatial changes characterized by a large decrease (~40%) in kaolinite, whereas the other three clay minerals present relative increases, especially clear for smectite. The muddy sediment sources of SMT-Beaches inferred from the clay mineralogy are mainly derived from nearby mountain rivers coupled with long-distance transport and penetration of the Changjiang River. The sandy sediments of these beaches are predominantly sourced from nearby mountain rivers, the weathering products of surrounding rocks in both mainland and island environments, and erosion of the “Old Red Sand” and “Red Soil Platform”. However, the sandy sediment sources of the SMT-Beaches are largely reduced because of the remarkable decrease in the river fluvial supply associated with intensive human activities such as dam construction and coastal reclamation. Subsequently, the sandy sections of SMT-Beaches present clear erosion and have revealed by both time series remote sensing images and a compilation of published literature. In contrast, the muddy sediment supply of SMT-Beaches is temporarily stable and relatively constant, resulting in the landward migration of the mudflats with relative transgression or accumulation. These findings highlight that the natural evolution processes of SMT-Beaches have been greatly reshaped by intensive human activities.

sandy-muddy transitional beach  /  clay mineral  /  sediment source  /  human activity  /  fluvial discharge  /  morphological evolution
Shaohua Zhao, Feng Cai, Hongshuai Qi, Jianhui Liu, Chao Cao, Gen Liu, Gang Lei. Sediment source-to-sink process variations of sandy-muddy transitional beaches and their morphological indications[J]. Acta Oceanologica Sinica, 2023 , 42 (7) : 10 -24 . DOI: 10.1007/s13131-022-2077-2
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1 Introduction
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Coastal zones have been heavily populated and developed and contains 40% of the world’s population and 15 of the 20 megacities (population >10 million) (Luijendijk et al., 2018; Mentaschi et al., 2018 and references therein). Beaches, one of the most important natural resources in coastal zones, with 31% of the world’s ice-free shorelines (Luijendijk et al., 2018), possess high value corresponding to their travel and leisure, disaster prevention and mitigation, and ecological maintenance services, providing the key for the rapid development of coastal tourism and cities worldwide (Stronge, 2005; Houston, 2013; Cai, 2015, 2019). In particular, numerous sandy beaches, accounting for only 27% of the long coast of China, have played an important role in coastal tourism development and ecological civilization construction in recent years (Zhao et al., 2014; MNRC, 2019–2021). However, such limited but valuable beach resources are suffering from extensive human activities coupled with global climate changes (Luijendijk et al., 2018; Mentaschi et al., 2018; Vousdoukas et al., 2020), and half of these beaches show erosion and retreat to varying degrees (Cai, 2019), resulting in great challenges and uncertainty in the development of coastal tourism and cities in China and ecological protection.
Sandy-muddy transitional beaches (SMT-Beaches) are a special type of beach between sandy beaches and tidal mudflats and are characterized by the exposed mudflats during low tidal periods (Short, 1991, 2006; Zhao et al., 2020a, b). Despite their lower attraction compared to sandy beaches, widely distributed SMT-Beaches are still valuable for potential tourism development, especially those in some highly developed coastal cities located in/surrounding moderate to large river estuaries that lack sandy beach tourism space (e.g., many cities located in and surrounding the Changjiang River Estuary in China; Jiang et al., 2014; Huang et al., 2016). Moreover, quite limited attention has been given to these SMT-Beaches, mainly due to the exposed mudflats leading to less attractive sightseeing during low-tide periods and the possibility of these beaches being rich in various pollutants, indicating their relatively low value for tourism development. Consequently, studies on SMT-Beach morphodynamics and sedimentation processes are rare (Anthony et al., 2002, 2015; Tamura et al., 2010; Morio et al., 2016; Chang et al., 2016, 2017; Zhao et al., 2020a, b), and the potential development and usage of these beaches are further constrained due to this limited knowledge.
Sediment source-to-sink transport processes are highly important for understanding SMT-Beach morphodynamics. These beaches are either stable or unstable due to the balance between the sediment budget and coastal dynamics (Wright and Short, 1984; Komar, 1998; Aagaard et al., 2013; Flemming, 2020 and references therein), while the sediment budget is closely correlated to variations in sediment supply capacities from source areas. SMT-Beaches are normally composed of narrow high-tidal sandy beaches and wide dissipative low-tidal mudflats and the boundaries between these zones are called sand-mud transition (SMT) (Morio et al., 2016; Chang et al., 2016, 2017; Zhao et al., 2020a, b). The relatively coarse sediments of the upper sandy section are normally derived from fluvial sediments of surrounding rivers and direct weathering products of nearby bedrock, whereas the fine sediments of the lower muddy section can be sourced from long-distance transport, or even be transported across the whole continental shelf and deep-sea basin, as inferred from clay mineralogy research (Xu et al., 2009; Liu et al., 2016; Zhao et al., 2018a). Previous studies have suggested that SMT-Beaches are widely distributed in river estuaries with high sediment supplies (Anthony et al., 2002, 2015; Pereira et al., 2011; Chang et al., 2016, 2017; Zhao et al., 2020b). Therefore, against the background of a dramatic decrease in global fluvial discharge (Syvitski et al., 2005; Walling, 2006; Milliman and Farnsworth, 2011), revealing the sediment sources of SMT-Beaches and their variations plays a key role in understanding SMT-Beach sedimentation and morphodynamic processes and their near-future evolution trends.
Large numbers of SMT-Beaches are well developed along the coast of China, especially along the western mesotidal to macrotidal coasts of the Taiwan Strait (TS) in southeastern China (SE China), and these beaches are characterized by circuitous coastlines, widespread semi-enclosed bays and mountain rivers, high sediment supplies, and contrasting hydrodynamic conditions (Huang et al., 2016; Zhao et al., 2020a, b; Guo et al., 2021; Fig. 1). Such coastal areas provide natural laboratories for studying and investigating the morphodynamic processes of SMT-Beaches. Simultaneously, some sandy beaches along the long coasts of China with high tourism value suffer from sea-level rise, sand mining, coastal reclamation, and industrial pollution discharge. The resultant decreased sand supply and weakened coastal hydrodynamics tend to lead to sandy beach retreat/erosion or even transformation into SMT-Beaches. Thus, investigations of SMT-Beaches could also provide substantial evidence to conquer this transformation by matching the actual demands of SMT-Beach nourishment. In particular, various studies on the mineral assemblage and geochemistry of river and sea surface sediments have revealed the main sources of sediments in the TS and adjacent areas and their transport processes and potential controlling factors (Liu et al., 2008a, b, 2016; Xu et al., 2009, 2012; Horng and Huh, 2011; Fang et al., 2012; Li et al., 2019, 2021). These previous achievements have provided basic information and are highly convenient for revealing the sediment source-to-sink processes of SMT-Beaches.
This study focuses on contrasting clay mineral compositions from various SMT-Beaches to those of surrounding mountain rivers along the coasts of SE China. In combination with published data on clay minerals and heavy minerals, the sediment source-to-sink processes of these SMT-Beaches are systematically analyzed. Then, the relatively long-term morphological responses of SMT-Beaches to the largely decreased sediment supply are revealed by time series remote sensing images and further confirmed by a compilation of published literature. The results of this study highlight that the natural processes of SMT-Beach evolution have been largely disturbed by intensive human activities.
2 Materials and methods
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Mineral assemblage analysis is a powerful tool to reveal beach sediment sources, mainly when the source regions lie under contrasting bedrocks and/or prevailing climate, in which case chemical/physical weathering tend to product different clay/heavy mineral assemblages (Mehring and McBride, 2007; Hein et al., 2013; Parra et al., 2012; Carranza-Edwards et al., 2019; Morrone et al., 2020). To investigate the sediment sources of SMT-Beaches in the study area, totals of 64, 16, and 41 surface sediment samples were collected for clay mineral analysis from 28 SMT-Beaches, mudflats in adjacent bays, and nearby mountain rivers, respectively (Fig. 1, Table 1). Normally, according to their scales, 1–4 samples were obtained from each SMT-Beach and each bay, while 1–8 samples were obtained from each mountain river. Simultaneously, many previously published clay mineral data representing surface sediments and core-top sediments from the TS and surrounding mountain rivers were compiled to better synthesize the sediment sources of SMT-Beaches along the western coasts of the TS in SE China as well as the potential transport processes of these sediments (Xu et al., 2009, 2012, 2013, 2014; Chen et al., 2017; Liu et al., 2008b, 2016; Fang et al., 2018; Li et al., 2021; Zhao et al., 2021). By referring to published literature compilations on heavy minerals, geochemistry, and beach morphological variations (Xu, 1995, 1996; Hong and Chen, 2003; Fang et al., 2010, 2012; Ma et al., 2018; Gong, 2019; Zhao et al., 2020a, b; Guo et al., 2021), the sediment source-to-sink processes with changes are revealed, and their indications for SMT-Beach morphological variations are discussed.
The clay mineral compositions were identified by X-ray diffraction (XRD) performed on oriented mounts of decarbonated clay-sized (<2 μm) particles using a PANalytical X’Pert PRO diffractometer at the State Key Laboratory of Marine Geology (Tongji University). The detailed preparation of these oriented mounts was consistent with extensive previous studies conducted in and/or surrounding the study area (Liu et al., 2008b, 2016; Zhao et al., 2018a; Fang et al., 2018). All samples were measured in triplicate under the conditions of air-drying, ethylene-glycol solvation for 24 h, and heating at 490°C for 2 h. The identification and interpretation of clay minerals were mainly conducted on 3 XRD diagrams according to the (001) basal reflections, and then the proportion of each clay mineral species was semiquantitatively obtained using the MacDiff software. The peak areas of basal reflection stresses at 15–17 Å, 10 Å, and 7 Å were associated with smectite (including mixed layers), illite, and kaolinite/chlorite, respectively, while the ratio of the 3.57/3.54 Å peak areas was further used to determine the relative proportions of kaolinite and chlorite. Considering the different calculation methods used to determine the relative clay mineral contents in previous studies, the weighing factors introduced by Biscaye (1965) were not used herein, and all the published data cited in this study were recalculated without the weighing factors. The accuracy of the semiquantitative evaluation of each clay mineral based upon the XRD method is ~5%. In addition, the illite crystallinity and illite chemical index were obtained from the half-height width of the 10 Å peak and the ratio of the 5 Å and 10 Å peak areas on the glycolated curve, respectively, providing two additional parameters to trace the sediment sources and their transport processess (Liu et al., 2008b, 2016).
3 Results
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3.1 Clay mineral assemblages of SMT-Beaches
The clay mineral assemblages of 28 SMT-Beach sediments are mainly composed of illite (12%–38%), kaolinite (15%–66%), chlorite (10%–28%), and smectite (1%–32%), with average values of 30%, 28%, 22%, and 20%, respectively. The illite crystallinity and illite chemistry index values vary in the ranges of 0.159–0.224°Δ2θ (~0.182°Δ2θ) and 0.297–0.540 (~0.419), respectively. The clay mineral compositions at each SMT-Beach contrasted with those obtained from different river estuaries and/or bays, and are characterized by relatively obvious changes in both kaolinite and smectite (Figs 2 and 3a; Table 2). The clay mineral assemblages of the SMT-Beach sediments collected from the Minjiang Estuary, Xinghua Bay (including Fuqing Bay), Meizhou Bay, Jiulong Estuary, and Zhangzhou bays vary in the corresponding proportions of kaolinite-illite-chlorite-smectite, with 42%:26%:21%:10%, 21%:34%:25%:21%, 27%:32%:23%:19%, 40%:20%:23%:17%, and 39%:21%:21%:19%, respectively. Generally, the SMT-Beach sediments from the Minjiang Estuary, Jiulong Estuary, and Zhangzhou bays presented similar clay mineral assemblages with higher kaolinite contents, while those from both Xinghua Bay and Meizhou Bay show quite similar clay mineral compositions with higher illite contents compared to the other three equal clay mineral species. In addition, the average illite crystallinity of the SMT-Beach sediments sampled from the Minjiang Estuary, Xinghua Bay, Meizhou Bay, Jiulong Estuary, and Zhangzhou bays are 0.201°Δ2θ, 0.175°Δ2θ, 0.173°Δ2θ, 184°Δ2θ, and 0.182°Δ2θ, while the average illite chemical index of SMT-Beach sediments sampled from the five regions are 0.406, 0.420, 0.439, 0.349, and 0.433, respectively (Table 2).
3.2 Clay mineral compositions of surrounding river samples
The clay mineral assemblages of the surface sediments collected from surrounding mountain rivers are mainly composed of dominant kaolinite (~49%), moderate illite (~22%) and chlorite (~19%), with minor smectite (~10%), according to the 41 samples analyzed in this study coupled with 30 published data points. The average illite crystallinity and the average illite chemical index of these sediments are 0.181°Δ2θ and 0.404, respectively. The kaolinite-illite-chlorite-smectite proportions in the surface sediments sampled from mountain rivers surrounding the Minjiang Estuary, Xinghua Bay, Jiulong Estuary, and Zhangzhou bays are 62%:18%:18%:1%, 61%:14%:14%:11%, 67%:18%:12%:4%, and 64%:15%:12%:9%, respectively (Fig. 2, Table 2). The average illite crystallinity of river sediments from these four regions are 0.201°Δ2θ, 0.174°Δ2θ, 171°Δ2θ, and 0.211°Δ2θ, while the average illite chemical indices of these sediments are 0.437, 0.420, 0.393, and 0.360, respectively. In addition, the surface sediments obtained from river estuaries in Sansha Bay, Xinghua Bay, and Meizhou Bay feature clay mineral assemblages with kaolinite-illite-chlorite-smectite proportions of 29%:32%:26%:13%, 23%:30%:22%:23%, and 30%:27%:22%:22%, respectively, with average illite crystallinity and the average illite chemical indices of 0.177°Δ2θ and 0.409, 0.173°Δ2θ and 0.399, and 0.168°Δ2θ and 0.418, respectively (Table 2).
4 Discussion
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4.1 Spatial variations in the clay mineral assemblages of surface sediments
The clay mineral assemblages of surface sediments determined herein indicate obvious spatial discrepancies from the upper-middle river drainage basin to the river mouths and then to the SMT-Beaches in and surrounding the estuary regions, featuring largely decreased kaolinite contents coupled with relatively increased illite and smectite contents (Figs 2, 3a and 4a; Table 2). Higher kaolinite (>60%) and lower smectite (<10%) contents are observed in the surface sediments of the upper-to-middle river basin, whereas the kaolinite contents largely decrease by a value of ~30% in the river estuaries, coinciding with synchronous ~10% increases in both the illite and smectite contents, which are particularly clear in Xinghua Bay. Simultaneously, from the river estuaries to the adjacent SMT-Beaches, the kaolinite contents still decrease, while both the illite and smectite contents increase unceasingly. However, there is no clear spatial discrepancy in either the illite crystallinity or the illite chemical index (Fig. 3b, Table 2). In addition, the analogous illite and chlorite present a good linear correlation (R2=0.67, Fig. 3a), which is consistent with previous studies on clay mineralogy in the TS, the adjacent East China Sea and the South China Sea (Liu et al., 2008b, 2016; Fang et al., 2018; Zhao et al., 2021), indicating their similar production processes from physical erosion (Wilson, 2004).
The clear spatial variations in the clay mineral assemblages of surface sediments from river basins to river estuaries and then to SMT-Beaches are mainly correlated with the sediment sources, differential settling, and tidal power. In the SE China drainage basin, the bedrock experiences long-term strong and thorough chemical weathering due to the low-relief and stable morphology coupled with stable tectonic settings and the warm and humid climatic conditions of the East Asian monsoon. In this case, widespread granitic rocks and sedimentary rocks preferentially produce and accumulate high contents of kaolinite (Liu et al., 2007b, 2016). Moreover, the strong monsoonal rainfall and scouring allow both illite and chlorite to form from the physical weathering of bedrock in the study area. When these clay minerals enter a river estuary or marine environment, different clay minerals can differentially settle due to their contrasting grain-sizes and chemical features, characterized by a large decrease in the kaolinite content and a relative increase in the smectite content (Gibbs, 1977; Liu et al., 2016). Kaolinite possesses large grain-sizes with a range of 0.5–8.0 μm and flocculates and settles quickly in alkaline seawater, probably explaining the 30% decrease in kaolinite observed from the drainage basin to the river estuary. By comparison, smectite with the relatively small grain-size (<1.0 μm) represents the fast chemical weathering production of andesitic-basaltic volcanic rocks (Wilson, 2004), and these sediments can be transported over long distances and favor deposition in calm environments. In further consideration of the limited distribution of volcanic rocks, the presence of smectite in the surface sediments of mountain rivers in the study area is quite rare (Xu et al., 2013; Liu et al., 2016; Fang et al., 2018). Consequently, the smectite increases from the upper-to-middle river basins to the SMT-Beaches in the river estuary could have other sources, in addition to those deduced from the differential settling effect. Furthermore, the estuaries of these mountain rivers are under strong macrotidal control, i.e., the average tide is larger than 4 m in the estuaries of the Minjiang River, Mulan River, and Jinjiang River, and the average spring tide is larger than 7 m (Zheng et al., 1982; Fu et al., 2013). Especially during flooding periods, large amounts of seawater enter the narrow river channels, delivering certain abundances of sediments and mixing with the fluvial sediments, thereby increasing the spatial diversity of clay mineralogy along the river routes and coastal areas.
4.2 Source-to-sink processes of SMT-Beach sediments
4.2.1 Mud sources and transport processes
According to the spatial distribution of clay minerals from rivers to SMT-Beaches described above, the muddy sediments of SMT-Beaches not only originate from nearby mountain rivers but can also be derived from other source areas through long-distance transport. Previous studies have reported that TS sediments mainly originate from numerous rivers on the Chinese mainland and Taiwan Island (Liu et al., 2008a, 2016, 2018; Xu et al., 2009, 2013; Horng and Huh, 2011; Fang et al., 2012; Li et al., 2019, 2021). In further consideration of the clay mineralogy representing different regions, the Chinese mainland can be divided into three source subregions, i.e., the Changjiang River, Zhejiang rivers, and Fujian rivers, as suggested by Zhao et al. (2021). Sediments sourced from the Changjiang River are characterized by major illite (~45%), moderate chlorite (~28%) and kaolinite (~22%), and minor smectite (~6%) contents (He et al., 2013; Zhao et al., 2018b); Fujian rivers provide principal kaolinite (~61%) with moderate illite (~18%) and chlorite (~19%) contents; Zhejiang rivers contribute relatively equal illite (~37%), chlorite (~24%), smectite (~23%), and kaolinite (~16%) contents; and Taiwan rivers deliver predominant illite (~56%) and chlorite (~36%) contents (Liu et al., 2008b, 2016; Fang et al., 2018). Generally, the sediment sources from the Taiwan and Fujian rivers are recognizable due to their contrasting clay mineral assemblages, while the Changjiang River and Zhejiang rivers supply similar clay mineral contents (Fig. 4b).
A ternary diagram plotting the clay mineral assemblages of SMT-Beaches and the above four potential source areas indicates that the muddy sediments of these SMT-Beaches have obvious multiple sources (Fig. 4b). The analogous illite and chlorite contents plot at one end-member in the ternary diagram, while the smectite and kaolinite contents are recognized as the other two end-members. The clay mineral data representing all these potential source areas are plotted on the line between the kaolinite end-member and illite + chlorite end-member; the Fujian and Taiwan rivers are located on the two other sides, while both Zhejiang rivers and the Changjiang River are plotted in the central areas. All the clay mineral data of SMT-Beaches mainly fall in the areas constrained by the Fujian rivers, Zhejiang rivers, and the Changjiang River, indicating that the muddy sediment sources of the SMT-Beaches may mostly originate from these three sources.
To further determine the muddy sediment sources of SMT-Beaches, the clay mineral data obtained in this study were coupled with more than 1 000 available samples retrieved from the TS and surrounding areas (including some core-top samples, Xu et al., 2009, 2012, 2013, 2014; Chen et al., 2017; Liu et al., 2016; Fang et al., 2018; Li et al., 2021; Zhao et al., 2021; Fig. 1b) to reveal the spatial distribution of the TS clay minerals (Fig. 5). Both kaolinite and smectite are mainly distributed in the western TS and present quite similar spatial distributions, with especially high contents in the river estuaries and semi-enclosed bays (Figs 5a, c). In contrast, the spatial distribution of high illite and chlorite contents reflects the middle-to-eastern TS surrounding the Taiwan Island, with relatively low values in the western TS (Fig. 5b). Simultaneously, this result is also well correlated with the estimated TS clay contributions from each source region. Using the linear separation method for illite crystallinity proposed by Liu et al. (2008b, 2016) and applied in a sedimentary study in the East China Sea by Fang et al. (2018), the clay contributions of the TS muddy sediments from the Fujian rivers, Taiwan rivers, and Changjiang River (including the Zhejiang rivers) were obtained (Fig. 6). Taiwan rivers contribute most clay minerals to the middle-to-eastern TS (Fig. 6c), while the Fujian rivers and Changjiang River provide the most clay minerals along the coasts of the western TS (Figs 6a, d), implying potentially strong effects on the SMT-Beach morphology.
In view of the spatial distribution of both clay minerals and their contributions from various sources and the corresponding sediment supply capacity (Figs 5 and 6), the clay minerals of SMT-Beaches mainly originate from Fujian rivers and the Changjiang River, with a minor proportion from Zhejiang rivers. This observation indicates obvious provenance control and long-distance transport by oceanic currents, which are especially clear in the Minjiang Estuary, Xinghua Bay, and Meizhou Bay. Fujian rivers contribute 17–20 Mt/a fluvial sediments with high contents of kaolinite to the TS (Xu et al., 2013; Jia et al., 2018). In particular, most of these sediments are trapped in river estuaries due to differential settling and hindered by the strong Zhe-Min Coastal Current (ZMCC) and/or Taiwan Warm Current (TWC) (Ma et al., 2018; Li et al., 2019; Fig. 1), providing an important source for SMT-Beach muddy sediments. By comparison, the Changjiang River could be another significant source for SMT-Beach muddy sediments due to its larger sediment supply (470 Mt/a and 130 Mt/a for annual sediment discharge before and after the Three Gorges Dam, Milliman and Farnsworth, 2011; MWRC, 2002–2020; Dai et al., 2018; Dai, 2021) than those of both Fujian rivers and Zhejiang rivers (~13 Mt/a, Jia et al., 2018). Approximately 30% of these sediments can be transported southward by the ZMCC after a special process characterized by “deposition in summer and transport in winter” (Yang et al., 1992, Liu et al., 2018; Jia et al., 2018). These long-distance transported sediments, with high illite crystallinity and chemical index and high illite and chlorite contents, disperse in the inner shelf of the East China Sea and penetrate the river estuaries and bays along their transport route (Figs 1a, 5d, 6a, and 6b; Xu et al., 2009; He et al., 2013; Zhao et al., 2018b), even reaching the middle-to-southern areas of the TS (Liu et al., 2007a). Simultaneously, some proportion of Zhejiang river-sourced sediments are also transported southward by the ZMCC and mixed with the Changjiang River-sourced sediments, contributing some portion of muddy sediments to the SMT-Beaches. It must be pointed out that Taiwan rivers with large fluvial discharge (60–150 Mt/a) were found to provide limited muddy sediments for SMT-Beaches in this study, mainly due to the constrained sediments in the middle-to-eastern TS by the ZMCC and the TWC (Xu et al., 2009, 2013; Figs 5b and 6d).
Additionally, the high smectite contents of SMT-Beaches along these estuaries and bays are mainly derived from northern regions, although this is still under debate. All rivers surrounding the TS and the Changjiang River contribute limited smectite (Fig. 4), while Zhejiang river-sourced sediments have relatively high smectite contents but small fluvial discharges of only ~13 Mt/a (Jia et al., 2018; Fig. 5a), making it difficult for these rivers to disperse and distribute smectite throughout the whole inner shelf of the East China Sea. In contrast, a recent study suggested that the widespread smectite in the mud wedge of the East China Sea could also partly originate from the fluvial and/or resuspended sediments of the Huanghe River and its abandoned delta (Zhao et al., 2018b). The Huanghe River delivers the largest sediment amount into the ocean worldwide (~1 100 Mt/a, Milliman and Farnsworth, 2011), with a much higher smectite content (~40%) than those of the Changjiang River and Zhejiang rivers (Yang, 1988; Yang et al., 2003). Then, the resuspended sediments of the old Huanghe River delta are carried by southward coastal currents associated with the winter monsoon and arrive at the Changjiang River Estuary. These sediments are then mixed with the Changjiang River-sourced clay minerals and are further transported by the ZMCC over long distances, penetrating into river estuaries and semi-enclosed bays along the western coasts of the TS under differential settling (Xu, 1985; Zhao et al., 2018b; Fig. 5a) and contributing a certain amount of clay minerals to SMT-Beaches. In addition, the smectite sampled in SMT-Beaches in the southern Zhangzhou bays may be derived from fast weathering processes of nearby volcanic rocks in South China delivered by northward coastal currents in summer (Liu et al., 2019). To summarize, the smectite sources of SMT-Beaches in the study areas are still unclear and thus need further investigation with new methods to better constrain this issue. Nevertheless, most smectite in the analyzed SMT-Beaches is mainly transported by the ZMCC from the northern regions.
4.2.2 Sand sources and transport processes
The sandy sediments of SMT-Beaches along the SE China coast are mainly derived from the surrounding mountain rivers, weathering of bed rocks in both the Chinese mainland and islands, and erosion of the “Old Red Sand” and “Red Soil Platform”. According to previous studies on heavy minerals, magnetic minerals, and geochemical compositions, the sandy particles of sediments collected from coastal areas of SE China are characterized by short transport distances (References in Table 3). In view of the contrasting sedimentary and morphological features between estuarine and bay SMT-Beaches (Zhao et al., 2020a, 2020b), their sandy sediment sources could also be different. The widespread small to mesoscale mountain rivers along the western coast of the TS, including the Minjiang River, Jiulong River, Jinjiang River, Jiaojiang River, Mulan River, and Zhangjiang River, provide important sediment sources for these well-developed SMT-Beaches in/near river estuaries (Wang and Ma, 1987; Yan, 1988; Hong, 1993; Chen et al., 1994; Xu, 1995, 1996; Hong and Chen, 2003; Fang et al., 2010, 2012; Ma et al., 2018). Otherwise, for SMT-Beaches located in semi-enclosed bays with limited sediment supplies, the erosion products of bedrock, “Old Red Sand”, and “Red Soil Platform” under long-term warm and humid climatic conditions provide the majority of the sandy sediments (Chen, 1992; Chen and Guo, 1994; Gu et al., 2003; Xu et al., 2004; Yang and Wang, 2009; Hong, 2008; Zhang et al., 2008; Su et al., 2018). Furthermore, the stable sandy bodies and/or shallow sandy flats in the coastal areas can also contribute some sandy sediments to these SMT-Beaches under landward waves and/or currents.
The sandy sediments of SMT-Beaches along the SE China coast from various sources are under differential transport processes but are mainly hindered by both the ZMCC and the TWC and then deposited in coastal areas. These river-sourced sandy sediments are delivered by fresh water into the ocean and then dispersed under the influence of waves and tides. Simultaneously, the erosion products of bedrock from the Chinese mainland and islands are mainly carried out to the ocean by rainfall-induced water flows, while the “Old Red Sand” and “Red Soil Platform” are eroded and collapse under waves and coastal currents and are then brought into the ocean, providing another sediment source for SMT-Beaches. Nevertheless, all of these sandy particles are hindered by both the ZMCC and the TWC and are further constrained to be deposited in river estuaries and semi-enclosed bays (Yan, 1988; Chen, 1992, 1993; Xu, 1994, 1995; Horng and Huh, 2011; Fang et al., 2012; Chen et al., 2018; Ma et al., 2018; Li et al., 2019).
4.3 SMT-Beach morphological responses to the decreased sediment supply
Sufficient sediment supplies provide the basic material basis for SMT-Beach evolution along the SE China coast. According to the above discussion, the muddy sediments of these SMT-Beaches are sourced mainly from Fujian rivers and the Changjiang River (including a minor proportion from Zhejiang rivers), while the sandy sediments originate mostly from Fujian rivers and the erosion products of nearby regions. Despite the potentially different sandy sediment sources between estuarine and bay SMT-Beaches, the much wider mudflat section than sandy section of each SMT-Beach (Zhao et al., 2020a, b) indicates that riverine fine sediments are quite important. Consequently, changes in the sediment discharge of rivers could play a key role in the morphological evolution of SMT-Beaches.
For these estuarine SMT-Beaches along the SE China coast, the observed decreases in their sandy section width mainly correlate with reduced sediment supplies (Figs 7a, b) associated with decreased sediment discharges of mountain rivers surrounding the TS under extensive human activities (Walling, 2006; Syvitski et al., 2009). Most beaches along the Chinese mainland coast developed during the high sea-level period following the last transgression in the mid-Holocene (~6.0–5.5 ka BP) (Lei et al., 2014; Cai, 2019), which is closely associated with sufficient sediment supplies from tremendously small rivers to world large rivers. However, dramatic decreases in river sediment discharges all over the world under extensive and increasing human activities (such as dam construction, river sand mining, conservation of water and soil, and anthropogenic river walls) (Walling, 2006; Syvitski et al., 2009; Milliman and Farnsworth, 2011; Wu et al., 2020) have greatly reduced sediment supplies to coastal beaches, and this is especially obvious for these estuarine SMT-Beaches along the SE China coast. As the main sediment suppliers, the large decreases in the fluvial sediment discharges of both the Minjiang River and Changjiang River since the 1990s have resulted in an imbalance between the sediment budget and hydrodynamics, leading to obvious erosion in river deltas and adjacent coastal regions (Figs 7c, d, and 8a; MWRC, 2002–2020; Chen et al., 2010; Yang et al., 2011; Gao et al., 2019). As the sediment discharge decrease in the Minjiang River was mainly due to dam construction, the sandy sediment sources of SMT-Beaches have greatly decreased, resulting in landward SMT migration coupled with sandy section width reductions (Fig. 8b). Based on time series remote sensing images, the sandy section width of Longsha Beach in the Minjiang Estuary was reduced by 10 m from 2004 to 2020 (Figs 7a, b). Additionally, the decreased sediment supply led to the obvious retreat/erosion of sandy beaches in the study area, with a minor portion of sandy beaches becoming SMT-Beaches (Liu et al., 2011; Cai, 2019).
Simultaneously, for these bay SMT-Beaches along the SE China coast, the decreases in their sandy sediment sources are mainly correlated with human activities, such as hard seawall construction, coastal reclamation, and sand mining, resulting in sandy sections with reduced widths (Figs 7e-h). Solid seawalls and coastal reclamation have hindered transport of the erosion product of bedrock, “Old Red Sand”, and “Red Soil Platform” to coastal beaches, while coastal sand mining and/or sand stealing have also reduced their sandy sediment sources. According to time series images, the sandy section width of Yulou Beach in Fuqing Bay decreased by 1–3 m from 2009 to 2017 (Figs 7e, f), while that of Lieyu Beach in Dongshan Bay decreased by 3–5 m from 2008 to 2019 (Figs 7g, h). In particular, these observations have also been confirmed by personal communications with many local people and viewed by the landward SMT migration of Jingfeng Beach in Meizhou Bay and Jiangyin East Beach in Xinghua Bay (Liu et al., 2011; Zhao et al., 2020b).
By comparison, the muddy sediment sources of all SMT-Beaches along the SE China coast present temporarily limited changes, and the mudflats of SMT-Beaches are mainly stable or are even expanding. This trend means that the SMT boundaries of SMT-Beaches are migrating landward and reducing the widths of their upper sandy sections, leading to the retreat of SMT-Beaches. During past decades, sediment discharges of Chinese mainland rivers have largely decreased (MWRC, 2002–2020; Wu et al., 2020; Figs 7c, d), while the transport capacity of the ZMCC has likely been reduced synchronously, mainly due to the weakened East Asian winter monsoon (D’Arrigo et al., 2005). However, the muddy sediments delivered to the SE China coast are not clearly decreasing, and the sedimentation rate along the northern coasts of the study region has even increased to a certain degree (Gao et al., 2019). These observations are mainly correlated with the rapid source-to-sink transfer between the Changjiang River and its subaqueous delta under intensive human activities. In response to thousands of dam proliferations, the sediment discharge of the Changjiang River was greatly reduced from ~470 Mt/a in the 1950s to ~110 Mt/a after the construction of the Three Gorges Dam (the world’s largest dam; Yang et al., 2011; Dai and Liu, 2013; Dai et al., 2016; Dai, 2021), leading to strong erosion of its mid-lower riverbed, river catchment, coastal areas, subaqueous delta, and its distal muddy deposits (Yang et al., 2011, 2018; Dai et al., 2018; Gao et al., 2019). Then, these eroded regions with coarsened grain-sizes in both the suspended and surface sediments were converted into sediment source areas, e.g., the previous sinks like middle-lower riverbed and shoals in the estuarine channels have now become sediment sources (Dai et al., 2018), providing sufficient sediments to maintain the stability and/or expansion of Zhe-Min coastal mudflats (Gao et al., 2019), even the largely decreased sediment supply of the Changjiang River including Zhejiang and Fujian rivers. Simultaneously, the weakened East Asian winter monsoon could reduce the wave height along coastal areas (Chien et al., 2014), resulting in relatively more fine sediment deposition and maintenance of the stability and/or expansion of the studied SMT-Beach mudflats. Nevertheless, due to the decreasing sediment supply of rivers undergoing long-term “starvation”, the eroded sediments from the mid-lower riverbed, delta and distal muds cannot provide sufficient sediments to support widespread mudflat stability or expansion along the SE China coast. Consequently, when the erosion-deduced sediment supplies for mudflats reach a threshold value, SMT-Beaches are expected to retreat due to insufficient sediment sources in the future.
Furthermore, the stability and development of SMT-Beaches are also closely correlated with global climate changes and other human activities, which tend to reduce the widths of the upper sandy sections. In view of global warming, sea-level rise, and increases in both the frequency and strength of tropical typhoons and winter storms, the erosion of coastal beaches is increasingly difficult to recover (Masselink et al., 2016; Luijendijk et al., 2018; Mentaschi et al., 2018; Vousdoukas et al., 2020). Particularly, under such an obviously decreasing sediment supply, the coastal regions of SE China are suffering 1.5–2.0 typhoons every year (Wang, 2004; Chien et al., 2014), resulting in the clear erosion of coastal beaches near typhoon-loading areas. Simultaneously, the sea-level rise along the coast of China has reached a value of 3.4 mm/a, while the local sea-level along the SE China coast is expected to rise with values of 65–160 mm during the next three decades (MNRC, 2021), tending toward widespread beach retreat, according to the Bruun Rule. Additionally, some beaches also suffer from sand stealing, anthropogenic pollution, and weakened hydrodynamics under coastal engineering, leading to obvious changes in their sediment source-to-sink processes (Mao et al, 2007; Fu and Liu, 2016; Huang et al., 2016; Zhang et al., 2017; Gong, 2019). The resulting sandy sediment supply decreases coupled with increasing muddy sediment supplies could further lead to coastal beach retreat with landward SMT migration.
To summarize, according to the sediment source-to-sink process analysis conducted in this study, the morphological evolution of SMT-Beaches indicates close correlations with sediment discharge variations in both small-moderate mountain rivers and the Changjiang River (Fig. 8). Most SMT-Beaches along the SE China coast present an overall muddying tendency characterized by landward SMT migration. The upper sandy sections are narrowing along with decreasing sandy sediment sources, while the lower mudflats indicate relative stability or even expansion associated with sufficient muddy sediment supplies at present (Fig. 8b). Despite these findings, the sand-mud interaction mechanism present during the morphological changes in SMT-Beaches is still unclear, and future studies analyzing in situ observations over various time scales are required to better address the issues associated with SMT-Beach morphological evolution and muddying processes, as well as the potential controlling factors.
5 Conclusions
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The clay mineralogy of surface sediments from SMT-Beaches and surrounding mountain rivers and semi-enclosed bays along the coast of southeastern China was investigated to constrain SMT-Beach sediment source-to-sink processes and their morphological indications. The following conclusions were obtained in this study.
(1) The clay minerals of SMT-Beaches are mainly composed of dominant kaolinite (15%–66%) and moderate illite (12%–38%), chlorite (10%–28%), and smectite (1%–32%) contents. From surrounding mountain rivers to SMT-Beaches, the clay mineral assemblages show distinct spatial changes characterized by large decreases in kaolinite, while illite and smectite present relative increases.
(2) The muddy sediments of SMT-Beaches are mainly sourced from nearby mountain rivers and long-term transport from the Changjiang River. The sandy sediments of these beaches are mainly sourced from nearby mountain rivers, the weathering products of surrounding rocks on both the Chinese mainland and islands, and erosion of the “Old Red Sand” and “Red Soil Platform”.
(3) The morphological processes of SMT-Beaches are closely correlated to the sediment supplies of surrounding mountain rivers and the distal Changjiang River. Under decreased sediment discharges and intensive human activities, the sandy sections of SMT-Beaches mainly present clear erosion trends, whereas the muddy sediment supplies of SMT-Beaches are temporarily stable and relatively constant, resulting in the landward migration of the mudflats with relative transgression or accumulation.
Acknowledgements
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We would like to thank Haitao Xu, Xiang Zhou, Chengtao Wang, and Jixiang Zheng from the Third Institute of Oceanography, MNR for their cooperation in collecting sediments. Yanli Li, Yijie Wang, and Xuan Lv from the State Key Laboratory of Marine Geology, Tongji University are thanked for their help with the laboratory analysis.
Funding
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  • The National Natural Science Foundation of China under contract Nos 41930538, 42076211 and 42076058; the Scientific Research Foundation of the Third Institute of Oceanography, Ministry of Natural Resources under contract Nos 2022017 and 2019006; the China Postdoctoral Science Foundation under contract No. 2019M652248.
References
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Year 2023 volume 42 Issue 7
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doi: 10.1007/s13131-022-2077-2
  • Receive Date：2022-04-28
  • Online Date：2025-11-22
  • Published：2023-07-25
Article Data
Affiliations
History
  • Received：2022-04-28
  • Accepted：2022-06-22
Funding
The National Natural Science Foundation of China under contract Nos 41930538, 42076211 and 42076058; the Scientific Research Foundation of the Third Institute of Oceanography, Ministry of Natural Resources under contract Nos 2022017 and 2019006; the China Postdoctoral Science Foundation under contract No. 2019M652248.
Affiliations
    1 Laboratory of Ocean and Coast Geology, Third Institute of Oceanography, Ministry of Natural Resources, Xiamen 361005, China
    2 Key Laboratory of Marine Ecological Conservation and Restoration, Ministry of Natural Resources, Xiamen 361005, China
    3 Fujian Provincial Key Laboratory of Marine Ecological Conservation and Restoration, Xiamen 361005, China

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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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