ZHANG Mingyu, LIU Xiting, 2), , XU Fangjian, LI Anchun, GU Yu, CHANG Xin, ZHUANG Guangchao, ZHANG Kaidi, BI Naishuang, 2), and WANG Houjie, 2)

Organic Carbon Deposition on the Inner Shelf of the East China Sea Constrained by Sea Level and Climatic Changes Since the Last Deglaciation

ZHANG Mingyu1), LIU Xiting1), 2),*, XU Fangjian3),*, LI Anchun4), GU Yu1), CHANG Xin1), ZHUANG Guangchao5), ZHANG Kaidi4), BI Naishuang1), 2), and WANG Houjie1), 2)

1),s,,266100,2),,266061,3),,570228,4),,,266071,5),,,266100,

The East China Sea (ECS), which is located in the transitional zone between land and ocean, is the main site for the burial of sedimentary organic carbon. Despite good constraints of the modern source to the sinking process of organic carbon, its fate in response to changes in climate and sea level since the last deglaciation remains poorly understood. We aim to fill this gap by presenting a high-resolution sedimentary record of core EC2005 to derive a better understanding of the evolution of the depositional environment and its control on the organic deposition since 17.3 kyr. Our results suggest that sedimentary organic carbon was deposited in a terrestrial environment before the seawater reached the study area around 13.1 kyr. This significant transition from a terrestrial environment to a marine environment is reflected by the decrease in TOC/TN and TOC/TS ratios, which is attributed to deglacial sea level rise. The sea level continued to rise until it reached its highstand at approximately 7.3 kyr when the mud depocenter was developed. Our results further indicate that the deposition of the sedimentary organic carbon could respond quickly to abrupt cold events, including the Heinrich stadial 1 and the Younger Dryas during the last deglaciation, as well as ‘Bond events’ during the Holocene. We propose that the rapid response of the organic deposition to those cold events in the northern hemisphere is linked to the East Asian winter monsoon. These new findings demonstrate that organic carbon deposition and burial on the inner shelf could effectively document sea level and climatic changes.

organic carbon; East China Sea; mud sediments; sea level changes; environmental evolution

1 Introduction

As classic environmental indicators, organic carbon isotope (δ13C) and TOC/TN ratios are commonly used proxies to indicate the source-to-sink process of the organic carbon and the evolution of the depositional environment constrained by sea level changes and climatic events (Emery., 1967; Meyers, 1997; Lamb., 2006; Hu., 2014). The reconstructed curves of ancient sea level were mostly based on the combination of biological and physical indicators, such as pollen, diatom, foraminifera, and grain size (Korotky., 1995; Boski., 2002; Barlow., 2013). However, few or even no microfossils in the sedi- ments may be found in some cases. Thus, geochemical indicators, δ13C and C/N of organic carbon, can serve as alternative indicators of the environmental evolution caused by sea level changes (Lamb., 2006; Yu., 2010; Sun., 2021). On the basis of previous studies (Meyers, 1997; Lamb., 2006; Yang., 2011b; Chen., 2022), terrigenous organic matter dominated by C3plants is characterized by lower δ13C values (?32‰ to ?21‰) and higher TOC/TN values (> 12). Meanwhile, marine organic matter with the influence of nitrogen-rich marine algae is characterized by higher δ13C values (?16‰ to ?23‰) and lower TOC/TN values (< 8). For example, Goslin. (2017) reconstructed the Holocene sea level curve by using geochemical indices of δ13C, TOC and TN, showing that it is highly consistent with the results for traditional paleontological fossils.

The carbon cycle in the marginal sea plays an important role in the global carbon cycle (Bauer., 2013; Wu., 2022). As the transitional zone between land and ocean, the continental marginal sea accounts for only 20% of the ocean surface area. However, it stores 80% of sedimentary organic carbon, which strongly affects the glo- bal carbon cycle (Berner, 1982; Walsh, 1991; Allison., 2007; Hu., 2016). As one of the marginal seas in China, the East China Sea (ECS) is greatly affected by rivers (Bi., 2015; Yang., 2015), where organic matter is actively transported, deposited, and preserved (Milliman., 1984; Liu., 2006; Hu., 2012; Yao., 2014; Qiao., 2017). Rivers, mainly the Changjiang Ri- ver, carry a large amount of sediment into the ECS, there- by leading to the obvious terrigenous proportion of sedimentary organic matter on the ECS inner shelf (Lei., 2011; Yao., 2015; Zhou., 2018). Particularly since the Holocene sea-level highstand, a large number of fine- grained materials have been deposited on the ECS inner shelf, forming a mud belt with a width of 100 km and a length of 800 – 1000 km (Fig.1) (Liu., 2006; Xu., 2009). Under the influence of the western boundary current and the Asian monsoon, the ECS has complex dyna- mic conditions, forming a macro transport pattern of sto- rage during summer and transport during winter in the coas- tal area of the inner shelf. Despite the high sedimentation rate on the ECS inner shelf, organic carbon has a low pre- servation rate due to strong physical disturbance during the deposition process and the consumption of organic carbon during the early diagenetic process before being buried (Mc- Kee., 2004; Zheng., 2010; Bianchi, 2011; Liu., 2020). In sum, the ECS, especially the inner shelf, is a natural idea laboratory for studying sedimentary pro- cesses and sedimentary records of organic carbon.

TOC/TN and δ13C of TOC values have been successfully and widely used in recent decades to distinguish different sources of organic matter in marine sediments (Kao., 2003; Zhang, 2007; Li, 2012; Zhou., 2018) and reveal paleoclimatic and sea level changes (Mey- ers, 1997; Lamb., 2006; Liu., 2020). For example, Zhan. (2011) found that proxies of C/N and δ13C of core ZK9 could indicate the evolution of the Changjiang Estuary since 13 kyr. However, on the ECS inner shelf, most research on organic carbon is concentrated in surface sediments, while its fate on a long-term scale is still not well understood. On the basis of an analysis of the organic carbon-related indicators of core EC2005 (Fig.1) in the de- pocenter of the ECS southern inner shelf, combined with published data of grain size, total sulfur (TS), and AMS14C dating of core EC2005 (Table 1) (Xu., 2009, 2011; Liu., 2021), we demonstrate how organic carbon has responded to depositional evolution on the ECS inner shelf and global climatic changes since the last deglaciation.

Fig.1 Study area geological background map with core EC2005 location. (a) The circulation system of the ECS (Lian et al., 2016). The mud areas are referred from Yang et al. (2014). CDW, the Changjiang diluted water; ZMCC, Zhe-Min coastal current; TWC, Taiwan warm current; TSWW, Taiwan Strait warm water; KC, Kuroshio current. (b) Photos of core sediments of EC2005.

2 Regional Setting

As one of the largest marginal seas, the ECS connects the Eurasian plate and the Pacific plate, where the inner shelf is relatively wide at about 500 km with a depth of 60 m (Wang., 2008). The ECS has a unique and complex current circulation system (Yang., 2011a; Qiao., 2017), mainly including the Changjiang River diluted wa- ter (CDW), Zhe-Min coastal current (ZMCC), Taiwan warm current (TWC), and Kuroshio current (KC), which jointly affect the sediment transport, distribution, and deposition (Fig.1) (Liu., 2007; Yao., 2015; Liu., 2018a; Zhang., 2019, 2021). The Changjiang River transports a large amount of nutrient-rich freshwater to the inner shelf, combined with the invasion of TWC, resulting in high primary productivity in this area (108 – 997 mg m?2d?1, with an average of 425 mg m?2d?1), which constitutes the marine end-member of sedimentary organic matter (Gong., 2003; Jiao., 2007). The surrounding rivers also transport a large number of terrestrial materials, including organic carbon (Milliman and Syvitski, 1992), thus forming another important source of organic carbon in marine sediments. The Changjiang River, with a length of about 6300 km and a drainage area of 1.9 × 106km2, transports (2 – 5) × 106t terrestrial organic carbon to the ECS every year, which is of great significance to the ecological environment of the basin and the marine environment of the marginal sea (Bianchi and Allison, 2009; Milliman and Farns- worth, 2011; Yao., 2015). Other sources of sediments in the ECS include terrigenous materials from mountainous rivers (such as the Qiantang River, Oujiang River, and Min- jiang River) and Taiwan rivers (such as the Choshui River).

3 Materials and Methods

3.1 Core EC2005

The core EC2005, which is 60.2 m in length and 36.0 m in water depth, was drilled from the depocenter of the sou- thern Zhejiang-Fujian Province coastal mud belt on the ECS inner shelf (121?20?0.216?E, 27?25?0.216?N, Fig.1) in November 2005 by. The general recovery rate was 94.4%. Sixteen mixed benthic foraminiferas (0 – 41.0 mbsf) and peat samples (deeper than 41.0 mbsf) were selected to complete the dating at the Woods Hole Institute of Oceanography in the United States by using acce- lerator mass spectrometry AMS14C ages, and the calendar ages were calculated (Table 1) (Xu., 2009, 2011). Previous research indicated the presence of sand and silt layers in the middle and lower parts of core EC2005 (Fig.2a). However, above 28.06 mbsf (7.28 kyr), the grain size changes slightly and is mainly composed of clayey silt (Fig.2b), thereby representing the formation of the mud depocenter. The mean grain size of sediments in the core EC2005 ranges from 2.58 to 7.83 φ, with an average of 7.03 φ, showing a fine trend from the bottom to the top (Fig.2c). On the ba-sis of the linear age-depth model, the calculated sedimentation rate fluctuates from 0.03 to 2.27 cm yr?1(Fig.2d).

3.2 Methods

3.2.1 TOC and TN analysis

The TOC and TN contents in core EC2005 were mea- sured in the Institute of Oceanography, Chinese Academy of Sciences, using the acidification combustion method and sampling at an interval of 20 cm. First, freeze-dried sediment samples were ground into 150 mesh powders. Then, about 0.5 g of each sample was acidified with 1 mmol L?1HCl for 48 h. During this period, each sample was shaken four times to fully react the HCl with the carbonate components in the sample before the completely reacted samples were placed in a freeze dryer. The samples were then mixed evenly after drying. Finally, a high-precision analytical balance with a minimum weight of 0.0001 g was used to weigh an appropriate amount of sample into a tin cup, and a flash EA 1112 element analyzer was used to measure the total carbon and nitrogen contents. The content used in this article is the mass (%) of TOC or TN per 100 g of the bulk sediment. The repeated test of samples showed that the analytical accuracy of TOC and TN contents was higher than 0.05% and 0.03%, respectively; see details in Nan. (2014). These newly obtained high-re- solution data contribute to our discussion of the response of organic carbon burial to abrupt climate events and differ from the low-resolution data acquisition methods and mea- suring conditions that were previously reported by Liu. (2020).

3.2.2 Dry bulk density and sediment flux

A sample of the same volume was taken every 20 cm along the long axis of the cut core with a cutting ring (D 2.357 cm × H 0.983 cm). During sampling, the cutting ring was pressed down vertically on the mud surface, and this process was stopped when the cutting ring was filled with the sample; secondary compaction was prohibited. The sam- ple was dried at a constant temperature of 60℃ in an oven for 72 h and weighed. Then, its dry bulk density (DBD; ac- cording to Eq. (1)) was calculated after drying to constant weight. The mass accumulation rate (MAR) and TOC accumulation rate (TOCMAR) were calculated according to Eqs. (2) and (3), respectively.

Table 1 AMS14C dating ages of core EC2005 (Xu et al., 2009)

Fig.2 Lithologic and grain size characteristics of core EC2005. (a), Lithology with 14C dating points; (b), grain size composition; (c), mean grain size (φ); (d), sedimentation rates of core EC2005 (Xu et al., 2009, 2011).

whereandare the bulk weight and volume, respective- ly. Hence,is the sedimentation rate (cm kyr?1). TOC re- presents the TOC content (%).

4 Results

The TOC and TN contents in core EC2005 range from 0.11% – 0.83% and 0.01% – 0.08% respectively, with average values of 0.52% and 0.04% (Table 2). The TOC content at the bottom of the core (below 36.8 mbsf) fluctuates greatly with low values, while the content fluctuation at the top (above 28.1 mbsf) is relatively slight with high values (Fig.3a). TN content has a similar changing trend, showing an overall growth trend from bottom to top (Fig.3b).

The ranges of TOC/TN ratio and DBD are 7.04 – 52.59 and 0.87 – 1.56 g cm?3with mean values of 15.89 and 1.16 g cm?3(Table 2). The TOC/TN ratio and DBD show over all decreasing trends, and several intervals with significant fluctuations are observed below 28.1 mbsf (Figs.3c, 3d). Notably, both TOC/TN ratio and DBD show high values at the depth between 36.8 and 41.0 mbsf, with mean values of 25.21 and 1.39 g cm?3, respectively (Figs.3c, 3d). Another interval with significant fluctuation is found between 30.2 and 28.1 mbsf (Figs.3c, 3d).

Throughout the whole sediment core, the MAR and TOCMARrange from 38.65 – 2553.96 and 16.62 – 1865.90 g cm?2kyr?1, respectively, with mean values of 891.04 and 476.33 g cm?2kyr?1(Table 2). At the bottom, the accumulation rates of both continue to be low and do not fluctuate significantly until 36.8 mbsf and are then followed by a dis- tinct maximum for both at depths between 34.0 and 36.6 mbsf (Figs.3e, 3f). The overall fluctuation of MAR and TOCMARis similar and affected by the sedimentation rates (Figs.2d, 3e, 3f).

5 Discussion

5.1 Controlling Factors of TOC Content and TOC/TN Ratio

The ECS is a typical marginal sea that is under the influence of large rivers with complex hydrodynamic conditions, which influence the source, age, and activity of or ganic carbon (Schmidt., 2010; Yao., 2015; Sun., 2020). Under such depositional conditions, the sedimentation dynamics, grain size sorting, and particle density could influence the distribution, deposition, and burial of organic carbon in marginal seas (Bianchi., 2007; Sam- pere., 2008; Wakeham., 2009). The burial of TOC has been linked to the sedimentation rate (Zheng., 2010). However, our results show no obvious correlation between the TOC content and sedimentation rate (Fig.4a). This weak correlation indicates that on a long-term scale, the control of sedimentation rate on the preservation of or- ganic carbon is complex due to many other factors. The transport and preservation process of sedimentary organic carbon in the marginal sea is selective, being mainly controlled by sediment grain size, density, specific surface area, and other factors (Mayer, 1994; Bianchi., 2007; Wake- ham., 2009). Previous studies found that fine-grained mud sediments on the shallow surface sediments are conducive to the preservation of organic matter (Zhang., 2009), which also exists in the core EC2005. Our results show that a fine grain size corresponds to a high organic carbon content, which is reflected by a good correlation between grain size and TOC content (ln () = 0.18? 1.97,2= 0.41,< 0.01; Fig.4b).

Table 2 Maximum, minimum, and mean values of TOC contents, TN contents, TOC/TN ratios, DBD, MAR, and TOC accumulation rate (TOCMAR) in the sediments

Fig.3 Variation of TOC contents (a), TN contents (b), TOC/TN ratio (c), DBD (d), MAR (e), and TOCMAR (f) against the depth of core EC2005. The dotted lines represent their respective mean values.

DBD is another important parameter used to calculate material flux in sedimentology (Shi., 2003), which is the key link to correctly understanding the sediment budget, source-to-sink process, and geomorphic evolution. Density fractions can affect the distribution of organic matter in marginal sea sediments (Wakeham., 2009). Previous studies on surface sediment on the ECS inner shelf indicate that OC mainly occurs in the mesodensity fractions (2.0 – 2.5 g cm?3) up to 77.3%, while the content of OC in the less dense fractions (< 1.6 g cm-3) and high-density fractions (> 2.5 g cm?3) is relatively low (Wang., 2015). The selective sorting and transportation of components with different densities, in turn, affect the redistribution and bu- rial of OC in marginal sediments. Interestingly, a correlation also exists between the DBD and TOC content in core EC 2005 (2= 0.31,< 0.01; Fig.4c). When the DBD is 1.0 – 1.1 g cm?3, the TOC content is mainly concentrated at 0.5% – 0.6%, but when the DBD increases to about 1.4 g cm?3, the TOC content is more scattered (Fig.4c).

We also find that the TOC and TN contents in core EC 2005 are not linearly correlated but logarithmically correlated (= 0.14ln() + 0.99,2= 0.61,< 0.01). More than one-third of the samples fall within the range of TOC content of 0.5% – 0.6% and TN content of 0.04% – 0.06% (Fig.4d). The observed correlation pattern between TOC and TN contents is likely to be caused by significant changes in material source and/or diagenetic alternation, resulting in a strong bending of the fitting line between TOC and TN contents (Fig.4d).

The TOC/TN ratio of bulk sediments is a commonly used indicator to distinguish the relative proportions of terrestrial and marine sources in coastal depositional environments (Yu., 2010; Zhan., 2011; Yao., 2015; Zhao., 2020). Our results suggest that the TOC/ TN ratio of core EC2005 is less affected by the changes in TOC content but has a very high fitting degree with TN content (2= 0.87; Figs.5a, 5b). Organic matter will be degraded in the preservation process, leading to changes in TOC/TN ratios because of the difference in the activity of organic carbon and organic nitrogen (Zhu., 2013). For example, our previous study showed that the sulfate reduction process might affect the mineralization path of organic carbon (Liu., 2019, 2021). However, no corre- lation exists between TOC/TN ratio and TS content (Fig.5c), which indicates that the TOC/TN ratio in core EC2005 is less affected by the diagenetic process, and the change in depositional environment is the main controlling factor. The sedimentation rate could influence the sulfate reduction rate (Liu., 2020) and thus might contribute to changes in the TOC/TN ratio. However, our results suggest no such correlation (Fig.5d), further confirming that the digenetic process plays a minor role in controlling TOC/TN ratios of core EC2005.

Fig.4 Relationship between the TOC content with sedimentation rate (a), mean grain size (φ) (Xu et al., 2009, 2011) (b), DBD (c), and TN content (d). The red lines indicate the fitting results.

Fig.5 Relationship diagrams of TOC/TN ratios with TOC (a), TN (b), TS content (Liu et al., 2021) (c), sedimentation rate (d), DBD (e), and mean grain size (φ) (Xu et al., 2009, 2011) (f). The red ones are the fitting lines.

Interestingly, we found that the TOC/TN ratio showed an obvious exponential correlation with the DBD (2= 0.62,< 0.01; Fig.5e). When the TOC/TN ratio increases, that is, when the terrestrial material increases, the DBD also be- comes heavier; when the TOC/TN ratio decreases, that is, when the marine material increases, the DBD also decreases. The main factors that affect the DBD are the grain size and mineral composition of sediments (Shi, 2003; Xue., 2020), which control the density of particulate matter and porosity. Generally, a high clay content corresponds to a low DBD, and vice versa (Flemming and Delafontaine, 2000; Jia., 2003). Accordingly, the TOC/TN ratio also shows a certain linear correlation with grain size (2= 0.47,< 0.01; Fig.5f), a fine grain size – that is, high clay content – corresponds to a low TOC/TN ratio, while a coarse grain size – that is, high content of coarse-grained minerals (., quartz, feldspar) – corresponds to a high TOC/TN ratio. Therefore, the DBD and TOC/TN ratios are affected by the grain size and mineral composition of sediment, which could be indicative of the evolution of the sedimentary environment where they were deposited.

5.2 Depositional Environment Evolution Since the Last Deglaciation

The age of the bottom of core EC2005 was 17.3 kyr (Fig.2) when the sea level was about 130 m lower than the modern sea level (Liu., 2004; Lambeck., 2014). At the beginning of stage I (60.2 – 41.0 mbsf, 17.3 – 13.1 kyr BP), the bottom was about 30 m higher than the sea level at that time, which indicates that the seawater had no impact on the study area until 41.0 mbsf at 13.1 kyr (Xu., 2011). The C/S ratio is also a classical proxy that is widely used to distinguish marine and terrestrial environments (Berner and Raiswell, 1984; Raiswell., 2018) because freshwater environments generally lack sulfate ions, which limits the formation of sulfide, resulting in a C/S ratio higher than 10 (Wei and Algeo, 2020). Hence, the marine and terrestrial environment can be distinguished by C/S = 2.8 ± 1.5 (± 0.8) (Berner and Raiswell, 1984). Under such a terrestrial depositional environment, the grain size, TOC/TN ratio, TOC/TS ratio, and DBD show respective terrigenous signals and fluctuate significantly (Figs.6a – d).

Fig.6 Depositional evolution of the inner shelf of the East China Sea. (a), mean grain size (φ); (b), TOC/TN ratio; (c), DBD; (d), TOC/TS (Liu et al., 2021). The dotted line is 2.8, representing the boundary between freshwater and marine environment. (e), carbon isotope compositions (δ13C values) of organic matter (Liu et al., 2020). (f), sedimentation rates (Xu et al., 2009, 2011). (g), RSL, relative sea level (Lambeck et al., 2014). (h), grain-size composition with depth in core EC2005. I, terrestrial environment; II, mid-upper intertidal environment; III, mid-lower intertidal environment; IV, inner shelf.

At the beginning of stage II (41.0 – 36.8 mbsf, 13.1 – 12.3 kyr BP), the grain size, TOC/TN ratio, and DBD are all mu- tated and represented by extreme high/low values (Fig.6a – c). The grain size became coarser, with an average value of 6.43 φ in stage II, while the TOC/TN ratio and DBD rea- ched high values up to those at the core bottom with average values of 25.21 and 1.39 g cm?3, respectively, indica- ting some terrestrial signals, which is also supported by low δ13C values of organic matter, with an average value of ?24.54‰ (Liu., 2020). In contrast, the TOC/TS ratios suggested more marine signals (Figs.6d – e), which decreased below 2.8 rapidly, indicating the first seawater intrusion to the core site (Liu., 2021). At the same time, coarse-grained sediments increased significantly, indicating a more energetic environment caused by tidal current (Fig.4h), which is also found in the surrounding MZ02 core (Liu., 2017). All the above evidence demonstra- tes the beginning of transgression in stage II. During this period, amounts of terrigenous sediments were deposited in a mid-upper intertidal environment, where the sediments were affected by strong tidal currents.

During stage III (36.8 – 28.1 mbsf, 12.3 – 7.3 kyr BP), the sedimentation rate decreased significantly, and the content of clay in sediments increased (Figs.6f, h). Compared with stage II, grain size, TOC/TN ratio, and DBD decreased significantly with relatively constant values (Figs.6a – c), indicating that the water depth in the study area increased further to form a mid-lower intertidal environment (Dong., 2018). At the end of stage III, the sea level reached its highstand (Fig.6g), which is documented by changes in grain size, TOC/TN ratio, and DBD (Figs.6a – c).

During stage IV (28.1 – 0.0 mbsf, 7.3 – 0.0 kyr BP), the grain size, TOC/TN ratio, and DBD further decreased with a relatively stable level (Figs.6a – c). In contrast, the sedimentation rate fluctuated significantly (Fig.6f), suggesting that the change in sedimentation rate could not affect these environmental indicators. During this stage, the average value of TOC/TN is 10.82, and the δ13C value is high up to ?23.47‰, indicating an increased proportion of marine components in sediments. Meanwhile, the depocenter of the ECS inner shelf began to form at 7.6 – 7.0 kyr after reaching its sea level highstand during the Holocene (Xiao., 2006; Liu., 2007; Li., 2014; Liu., 2018a).

The reconstruction of the above environmental evolution provides an opportunity for us to evaluate the applicability of the combination of C/S and C/N ratio as a paleosalinity proxy (Liu., 2021). Our previous studies also showed that TOC/TS can well indicate the evolution of the marine and terrestrial environment on the ECS inner shelf since the last deglaciation (Liu., 2021, 2022). This study shows that TOC/TN is also sensitive to the evolution of the environment. Therefore, we propose that the scatter plots of C/S and C/N can effectively distinguish the marine and terrestrial environments (Fig.7). According to the diagram, the core sample can be classified easily into two end members, which represent the marine and terrestrial environment, respectively. Sediment from 60.2 – 41.0 mbsf represents a terrestrial sedimentary environment with high TOC/TS ratios and a large range of C/N ratios, while the sediments from 41.0 – 0 mbsf represent marine sedimen- tary environments with low TOC/TN and TOC/TS ratios (Fig.7). However, we also find that this diagram cannot distinguish between brackish water and marine environment, thus needing additional multiple indicators, such as Sr/Ba and B/Ga (Wei., 2018). In conclusion, we suggest that the TOC/TN and TOC/TS diagrams can be used to distinguish marine and terrestrial environments in the first order, especially without facies markers in the sedimentary sequence.

5.3 Response of Organic Carbon Deposition to Abrupt Climate Events

The continuous and rapid deposition on the mud belt of the ECS inner shelf provides us with an opportunity to study the paleoclimate (Gao., 2015). Previous work mostly revealed the climate changes since the Holocene sea-level highstand (Zheng., 2010; Liu., 2013; Chang., 2015). However, research on climate changes extending to the last deglaciation is rare. Most of the terrigenous sediments in the mud belt of the ECS inner shelf originate from the Changjiang River and are transported southward under the coastal current driven by the East Asian winter monsoon (EAWM), forming a mud depocenter (Liu., 2006). Thus, a natural step is to think that sedimen- tary grain size can reflect the ability of sediment transport (Wang., 2014). The most common approach in research is to use sensitive grain size to trace the evolution of the EAWM (Dong., 2021). However, recent studies show that it needs to be carefully considered when using sensitive grain size to trace the evolution of the EAWM (Tu., 2017; Shi., 2022). Except for grain size, indicators related to organic carbon in mud sediments also show potential application in indicating the evolution of the EAWM (Zong., 2006; Yang., 2011b; Chang., 2015; Hao., 2017).

Fig.7 Scatter plots of TOC/TS (Liu et al., 2021) and TOC/ TN (this study) ratios.

The deglaciation process from the last glacial maximum to the Holocene was interrupted by two cold events, namely, the Heinrich stadial 1 (HS1; 18 – 14.6 kyr BP) and the Young- er Dryas (YD; 12.8 – 11.5 kyr BP) (Barker., 2009). These sudden cold events have been recorded in the North Atlantic, North America, North Pacific, Asia, tropical regions, and even the Southern Hemisphere (Bond., 1993; Hall and Mccave, 2000; Wang, 2001; Shakun and Carlson, 2010; Fiedel, 2011; Liu., 2018b). These cold events during the last deglaciation have been linked to the effect of a slowdown of the Atlantic meridional overturning circulation (McManus., 2004), which affected the global monsoon system (Dykoski., 2005). The sedimentary record of Chinese Loess indicates that the winter monsoon strengthened during these cold events (Sun, 2011). However, few reports on the ECS inner shelf have been conducted. In core EC2005, the grain size and TOC/ TN ratio show extremely high values during HS1 and YD intervals (Figs.8c – d), indicating the increase in terrestrial organic carbon and coarse-grained sediments. This condition occurred because of the intensified physical erosion and river transportation due to the strong winter monsoon. These two stages correspond well to the weakening period of the summer monsoon, which is likely to be affected by the weakening of the AMOC during the HS1 and YD (Figs.8a, b), indicating that the abrupt change of the EAWM during the last deglaciation is related to high-latitude cold events.

During the Holocene, the organic carbon deposition in the inner shelf is mainly subject to the transport of coastal current driven by the EAWM (Xiao., 2006; Liu., 2007). Our results indicate that TOCMARincreased significantly at approximately 7.3, 5.5, and 1.4 kyr, which cor- responds well to the cold events recorded by hematite- stained grains in the North Atlantic during the Holocene (Figs.8e, f). During these periods, the increase in sedimentation rate and organic carbon flux indicates that the trans- port capacity of coastal current was enhanced, thereby further documenting the strengthening of the EAWM (Dong., 2020). Therefore, our results show that the deposition and burial process of organic carbon can quickly respond to the abrupt climate.

Fig.8 Deposition of organic carbon in response to climatic changes during the last deglaciation and the Holocene. (a), δ18O record in Dongge Cave, China (Dykoski et al., 2005). (b), sedimentary 231Pa/230Th, indicating the strength of the Atlantic Meridional overturning circulation (McManus et al., 2004). (c), TOC/TN. (d), mean grain size (φ) (Xu et al., 2009, 2011). (e), hematite-stained grains, indicating the changes in Holocene drift ice (Bond et al., 2001). (f), TOC accumulation rate (TOCMAR) of core EC2005.

6 Conclusions

As an important sink of organic carbon, the mud depocenter of the marginal sea could provide abundant information on environmental evolution controlled by sea level and climatic changes. With the use of a high-resolution se- dimentary record of core EC2005, the main conclusions on organic carbon deposition and burial since the last deglaciation are as follows:

1) Indicators related to the organic carbon (., TOC/ TN, TOC/TS) and DBD of sediments are mainly controlled by environmental evolution, which could be used to identify marine and terrestrial strata in geological history.

2) The relevant indicators of sedimentary organic carbon of core EC2005 indicate that the depositional evolution from land to sea on the ECS inner shelf is mainly control- led by the deglacial sea level rise.

3) Organic carbon deposition could respond quickly to abrupt cold events during the last deglaciation (HS1 and YD) and the Holocene (‘Bond events’) in the Northern Hemisphere, indicating strong EAWM during these cold periods.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 41976053) and the Shandong Province Funds for Excellent Young Scholars (No. ZR2021YQ26).

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(June 27, 2022;

August 29, 2022;

September 6, 2022)

? Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2023

E-mail: liuxiting@ouc.edu.cn

E-mail: xufangjian@hainanu.edu.cn

(Edited by Chen Wenwen)