Changes in detrital sediment supply to the central Yellow Sea since the Last Glacial Maximum

The sediment supply to the central Yellow Sea since the Last Glacial Maximum was uncovered through clay mineralogy and geochemical analysis of core 11YS-PCL14 in the Central Yellow Sea mud (CYSM). The core can be divided into four units: Unit 4 (700‒520 cm; 15.5‒14.8 ka), Unit 3 (520‒280 cm; 14.8‒12.1 ka), Unit 2 (280‒130 cm; 12.1‒8.8 ka), and Unit 1 (130‒0 cm; < 8.8 ka). Comparison of the clay mineral compositions, rare earth elements, and εNd values indicated 10 distinct provenance shifts in core PCL14. Moreover, the integration of clay mineralogical and geochemical indices showed different origins according to particle size. The late last deglaciation (Units 3 and 4) sediments originated from all potential provenance rivers, while the source of coarse sediments changed to Huanghe in Unit 3. Early Holocene (Unit 2) sediments were characterized by oscillating grain size, clay minerals, and moderate εNd values. In this period, the dominant fine sediment provenance changed from the Huanghe to the Changjiang, whereas coarse sediments most likely originated from western 15 Korean rivers. The Unit 1 CYSM sediments were sourced primarily from the Changjiang, along with minor contributions from the western Korean rivers. Possible transport mechanisms in the riverine sediment sources change and contributions to this include position shifts of river mouths, tidal stress evolution, and the development of the Yellow Sea Warm Current and coastal circulation systems. 20


Introduction 25
The Yellow Sea, located between the China continent and Korean Peninsula, is a semi-enclosed epicontinental shelf with a complex oceanic circulation system (Fig. 1). It is notable for its large amount of runoff and terrigenous sediment supplied from several adjacent rivers, including two of the world's largest rivers, the Changjiang and Huanghe, as well as from several smaller Korean rivers, including the Han, Keum, and Yeongsan River. Although most riverine sediments are trapped in estuaries and along coastal areas, some are deposited on adjacent shelves (Milliman et al., 1985;Milliman et al., 1987), forming several 30 shelf mud patch depositions such as Central Yellow Sea Mud (CYSM), Southeastern Yellow Sea Mud and Southwestern Cheju Island Mud (Fig 1). These deposits provide abundant information on paleo-environmental changes as well as sediment supply, marine hydrodynamics, and climate variation (e.g. Wang et al., 1999;Kim and Kucera, 2000;Li et al., 2014a;Cho et al., 2015;Kwak et al., 2016;Hu et al., 2018).
The provenance of CYSM sediments have attracted many researchers over the last three decades. Many studies have indicated 35 that CYSM sediments originated mostly from the Huanghe considering the large amount of sediment load carried by that river (Milliman et al., 1987;Lee and Chough, 1989;Liu et al., 2002;Yang and Liu, 2007;Shinn et al., 2007;Xiang et al., 2008).
On the other hand, other studies have used mineralogical, geochemical, and magnetic observations and determined that the CYSM was formed from a complex mixture of sediments from the Huanghe as well as the Changjiang and several Korean rivers (Zhao et al., 1990;Wei et al., 2003;Zhang et al., 2008;Li et al., 2014a;Wang et al., 2014;Koo et al., 2018). In addition, 40 recent studies using core sediments suggested that the provenance of CYSM changed mainly from Huanghe to Changjiang with minor contribution from the Korean rivers during the Holocene (Lim et al., 2015;Hu et al., 2018). However, the timing of the CYSM formation and the deposition environment prior to the Holocene are remains unclear.
Discrimination of sediment source and reconstruction of paleo-environmental changes can be undertaken based on grain size, clay mineralogy, and elemental signals. In particular, clay mineralogy and geochemistry have been utilized as a powerful tool 45 to trace provenance of the terrigenous fraction of marine sediments in the Yellow sea (Yang et al., 2002;Yang and Youn, 2007;Liu et al., 2007Liu et al., , 2010bDou et al., 2010;Hu et al., 2012;Wang and Yang, 2013;Li et al., 2014a;Koo et al., 2018).
In this study, we aimed to determine the sediment provenance and transport mechanism of CYSM using clay mineralogy and geochemistry multi-proxy. The purposes are to provide a broad insight into the supply of CYSM sediments and to reconstruct the paleo-environment since the last glacial maximum. 55

Oceanography
The Yellow Sea is characterized by a complex hydrodynamic system (Fig. 1), with two major circulation patterns. One is a basin-scale counterclockwise (cyclonic) gyre consisting of northward inflow via the Yellow Sea Warm Current (YSWC) in the central Yellow Sea and southward inflow via the Yellow Sea Coastal Current (YSCC) along the east coast of China 60 (Beardsley et al., 1985;Yang et al., 2003) (Fig. 1). The other is a clockwise gyre in the eastern part made up of the YSWC and southward inflow from the Korea Coastal Current (KCC) (Beardsley et al., 1985;Yang et al., 2003). The YSWC is one of the most important dynamic phenomena in the East China Sea and Yellow Sea. It is a branch of the Kuroshio Current that carries warm, salty water into the Yellow Sea roughly along the Yellow Sea Trough (Xu et al., 2009;Liu et al., 2010a;Wang et al., 2011Wang et al., , 2012. The Transversal Current (TC), identified in recent studies, separates from the KCC southwest of the Korean The vertical granularity, clay mineralogical, and geochemical characteristics of core PCL14 are plotted against the calibrated age on the y-axis in Fig. 3. The four clay minerals were dominated by illite (60.1-74.7%), followed by chlorite (12.0-22.6%), kaolinite (9.6-14.8%), and smectite (1.2-6.8%). The 87 Sr/ 86 Sr ratios ranged from 0.719 to 0.724 (mean 0.721) and the εNd values from −16.2 to −12.3 (mean −14.0). 110 Tables 2 and 3 list the detailed characteristics of the clay minerals and geochemistry in each unit and their main potential provenances (the Huanghe, Changjiang, and western Korean rivers). Each unit had distinct dissimilarities in clay mineral content and mean grain size, especially the sand content (Fig. 2). The Unit 2 sediments were 1.8-44.2% (mean 17.6%) sand with a mean grain size of 6.6 ϕ (10.3 μm) and Unit 4 sediments had a high sand content (8-58.7%, mean 26.3%) with a mean grain size of 6.0 Φ (15.6 μm). In comparison, Unit 1 contained only fine sediment with a mean grain size of 8.8 Φ (2.2 μm) 115 and Unit 3 sediments were clayey silt with a mean grain size of 7.3 Φ (6.3 μm). The downcore variation in the clay mineral composition showed that the illite content decreased gradually from Unit 2 to 3 and was constant in the other parts of the core.
Overall, the variations in the smectite and kaolinite+chlorite contents were opposite that of illite (Fig. 3). Units 3 and 4 had relatively constant compositions in terms of clay minerals, although their granularity was heterogeneous. The 87 Sr/ 86 Sr ratio was constant at the bottom and tended to increase in the upper part. The εNd value was low in Units 2 and 4. ΣLREE/ΣHREE 120 was low only in Unit 3, and was mostly constant.

Provenance discrimination based on clay mineralogy
Relative clay mineral contents and ratios can be used as powerful proxies for determining fine-grained marine sediment 125 provenance, especially in terms of the rivers from China and Korea that may contribute to CYSM (Yang et al., 2003;Choi et al., 2010;Li et al., 2014a;Xu et al., 2014;Lim et al., 2015;Kwak et al., 2016). Generally, Huanghe sediments are characterized by high smectite, and Changjiang sediments contain a lot of illite contents. Western Korean rivers (e.g. the Han, Keum, and Yeongsan) contain more kaolinite and chlorite than do Chinese rivers (Table 2).
A ternary diagram of smectite-(kaolinite+chlorite)-illite was utilized to determine the provenance of fine sediments in core 130 PCL14 (Fig. 4). Although Unit 4 and 3 sediments differed in granularity, they had similar clay mineral compositions and plotted near the center of the three possible provenance end-members, indicating that clay-sized sediments were supplied with constant amounts from all potential rivers to the study area during these periods (Fig. 4a). Unit 2 sediments overall were characterized by an increasing illite content (Figs. 3 and 4b). It means that the influence of Changjiang-derived materials began to increase during this period. However, Unit 2-2 sediments displayed an increase in smectite content with illite, and then 135 every clay mineral composition except illite decrease in Unit 2-1 (Fig. 4b). Variation of smectite content in Unit 2 appears to be closely related to the change in coarse sediments (Figs. 3 and 4b). The relationship between smectite and coarse grains was also observed in the early Holocene sedimentary unit of core YSC-1 (Li et al., 2014a) and nearby core EZ06-2 between ~14.1 and ~9.0 ka (Lim et al., 2015). Unit 1 sediments had clay mineral compositions quite similar to those of Changjiang sediments, indicating that they might be originate mainly from the Changjiang (Fig. 4b). 140 Consequently, clay mineralogical results were suggested that the finer detrital sediments in Units 3 and 4 were affected by all potential provenances. During Unit 2, the influence of the Changjiang increased gradually with temporary influx containing coarse particles and high smectite, and the later Unit 1 sediments were derived primarily from Changjiang inputs.

Geochemical approaches 145
Geochemical proxies for provenance discrimination in the Yellow Sea have been  Koo et al., 2018). The chemical compositions of Korean and Chinese rivers differ, especially in their rare earth elements (REE) and Sr-Nd contents (Xu et al., 2009;Jung et al., 2012;Lim et al., 2014;Hu et al., 2018). These are essentially unaltered during weathering, transport, and sedimentation, and can be a powerful tool for tracing the provenance of the 150 terrigenous fraction of marine sediments (McLennan, 1989;Blum and Erel, 2003;Xu et al., 2009).
Recent studies have emphasized that in addition to the source rock, many other factors influence the geochemical composition of riverine and marine sediments, such as grain size, heavy mineral content, and biogenic component, especially in bulk sediment analysis (Yang et al., 2002;Song and Choi, 2009;Lim et al., 2015;Hu et al., 2018). For example, the major elements Fe and Mg were suggested to be useful proxies in the Yellow Sea (Lim et al., 2007). However, they are closely correlated with 155 particle size because they are abundant in clay minerals, making them unconformable for provenance tracing in bulk sediments ( Fig. 5a). In addition, Ca has a problem based on biogenic carbonate despite the poor correlation with grain size (Fig. 5a).
Trace elements also exhibit positive and negative correlations with grain size (Yang et al., 2002;Lim et al., 2014). To complement this, recent studies have suggested ratios of the binding of abundant elements at comparable grain sizes (e.g., the La/Sc and Zr/Th ratios) (Yang et al., 2002;Lim et al., 2014). However, we observed that these ratios and mean grain size were 160 strongly negatively correlated in our dataset (Fig. 5b), implying that these ratios are also unsuitable for studying provenance.
Normalization of REE values to upper continental crust (UCC) is a widely accepted method for discriminating the sediment provenances of various geological materials (Song and Choi, 2009;Xu et al., 2009;Lim et al., 2015). This method can better offset differences caused by grain size, and could be a useful geochemical proxy (Fig. 5c). In addition, the Nd isotope ratio of silicate particles is essentially unaltered during weathering, transport, and sedimentation and can be a powerful tool for tracing 165 the provenance of the terrigenous fraction of marine sediments (Blum and Erel, 2003;Hu et al., 2018). However, recent studies indicated that the Sr isotope composition in both Chinese and Korean riverine sediments was a function of grain size, with a higher 87 Sr/ 86 Sr in clay-dominated fractions than in silt-dominated fractions (Fig. 5d) (Hu et al., 2018). Therefore, we used only the UCC-normalized REE and εNd values for discriminating sediment provenance; these could be useful indicators for distinguishing the contributions of Chinese and Korean rivers. 170 Korean rivers are characterized by a high LREE and low εNd, while Chinese rivers have abundant MREE (middle REE) and εNd (Table 3, Fig. 6). Figs. 6 are discrimination plots using the REE and εNd values that clearly separate the Chinese and Korean rivers. In these plots, the REE values represented the source of both coarse and fine sediments because the analysis was performed with coarse grains. Unit 1 is generally close to the Changjiang with slightly influence of the Korean rivers, as well as the clay mineralogy (Figs. 4 and 6). Unit 4 sediments are plotted between China and Korean river end-members in all 175 discrimination plots (Fig. 6), consistent with the results for clay minerals, which suggests that the coarse sediments included in Unit 4 were from contributions from all potential rivers.
Interestingly, the clay-sized particles of Unit 2 were a composite of the Huanghe and Changjiang in Fig. 4, but the geochemical data were similar to Unit 4 (Fig. 6). This probably means that a significant amount of coarse sediments in Unit 2 was supplied from Korean rivers with a high LREE (Fig. 6a). The association between an increased impact of Korean rivers and coarse 180 sediments was identified in an isotope analysis before ~8 ka in core YSC-1 (Hu et al., 2018). Thus, the supply of smectite in clay-sized particles and sand grains is synchronic, but possibly has different sources. In addition, Unit 3 sediments, identified as the homogenous origin as Unit 4 in clay mineralogy (Fig. 4), are biased towards Chinese rivers (Fig. 6), especially close to the Huanghe. A scatter plot of clay mineral ratio vs. εNd distinguished three possible provenances for particles smaller than 63 μm (Fig. 6c). Unit 3 sediments in this plot are certainly plotted close to the Huanghe. This is caused by the many silt 185 fractions in Unit 3 and probably represents a relatively close supply from the Huanghe.
Consequently, the estimated sediment provenances in each unit based on the clay mineralogical and geochemical indices were as follows. During Unit 4, both coarse and fine sediments were influenced by all of these provenances. However, in Unit 3, silt-sized fractions were predominantly affected by the Huanghe. Unit 2 represented a period of great change in the sediment sources. The fine grains in the Unit 2-2 sediments were derived primarily from Chinese rivers, especially the Huanghe, while the Unit 2-1 samples were supplied mainly from the Changjiang, with minor contributions from the Huanghe and western Korean rivers. However, coarse sediments source in Unit 2 were identified as western Korean rivers based on geochemical indices. The source of CYSM sediments in Unit 1 was primarily the Changjiang.

Paleo-environmental implications for sediment provenance changes 195
The four units could be distinguished based on the characterization of the major sediment source changes in the CYSM over the last 15.5 kyr (Figs. 4-6). Identification of sediment sources is a useful method for understanding paleo-environmental dynamics and sediment transport mechanisms in the Yellow Sea since the late last deglaciation. The main factors that potentially influenced provenance changes in the Yellow Sea include pronounced sea-level fluctuations that regulate the positions of shorelines, paleo-river pathways, tidal stress amplitude, and the formation of modern ocean currents (Liu et al., 200 2004;Lim et al, 2007Lim et al, , 2015Choi et al., 2010;Wang et al., 2014;Yoo et al., 2015Yoo et al., , 2016. Here, we discuss how these complex processes have affected sedimentation in the CYSM during the last 15.5 kyr. The sea level during Units 3 and 4, which corresponds to the late last deglaciation (15.5-12.1 ka), was approximately 60-100 m lower than the present sea level (Li et al., 2014b). The high signatures of C/N values in Unit 4 indicated a significant influx of terrigenous materials (Badejo et al., 2016). Mixed deposits of fine and coarse sediments with high influx and sedimentation 205 rates (Figs. 2 and 3) allows us to infer Unit 4 as a delta or prodelta environment. The paleo-river pathways of potential provenances, recently reconstructed based on seismic profiles, merged around the study area and were connected to the East China Sea (Yoo et al., 2015(Yoo et al., , 2016. During sedimentation of Unit 4, sediments in the study area would have been affected most strongly by direct inflow from paleo-rivers, because the low sea level led to the exposure of shelves in and near the Yellow Sea (Li et al., 2014b). 210 Sediment fining during Unit 3 reflects an increase in distance between the river mouths and study area due to transgression, and the study area probably formed a mud flat during sedimentation of Unit 3. During this period, clay-sized particles were still supplied from all rivers (Fig. 4), while silt-sized particles were supplied only from the Huanghe (Fig. 5). The record for the same period in core EZ06-1 showed significant coarse sediments with a high sand content (Lim et al., 2015), indicating that the Huanghe was relatively close to the west side of the study area (Fig. 2). In addition, the substantial flux from the 215 Huanghe would have supported the distant movement of coarse grains.
In Unit 2 (12.1-8.8 ka), corresponding to the early Holocene, the sea level was approximately 20−60 m lower than at present (Li et al., 2014b). The Unit 2 period was thought to be cold and dry (Badejo et al., 2016) and was characterized by oscillating grain sizes and clay mineral and geochemical compositions (Fig. 3). In addition, increasing and decreasing trends of grain size with sand content, S/I ratio divided into two subsections (Fig. 3). This variation is also reported in the surrounding YSC-1 (Li 220 et al., 2014a) EZ06-1, and EZ06-2 cores. In this period, the low sea level led to the seaward progradation of the shoreline and formation of a thin sand layer (generally < 3 m) called the transgressive deposit throughout the Yellow Sea (Cummings et al., 2016). The change in the coastline configuration caused shifts of the tidal fields therein, with tidal currents being more energetic than at present (Uehara and Saito, 2003;Lim et al., 2015), which supplied coarse grains to the central Yellow Sea. In addition, the bottom stress in the Unit 2 period was stronger around the Korean Peninsula (Uehara and Saito, 2003), which caused most 225 of the coarse sediment to be of western Korean river origin (Fig. 6). The supply of fine sediments from the Huanghe was temporarily strengthened during sedimentation of Unit 2-2, but weakened in Unit 2-1 (Fig. 4). This could be due to a change in distance between the Huanghe and study area as the sea level rose. In addition, the paleo-Changjiang Shoal moved northeastward into the Yellow Sea at ~12 ka (Li et al., 2000) and may have contributed some materials to the study area (Lim et al., 2015). The reduction in Huanghe-derived materials due to the increased distance could be strengthen the influence of 230 the Changjiang in Unit 2-1. https://doi.org/10.5194/os-2020-60 Preprint. Discussion started: 26 June 2020 c Author(s) 2020. CC BY 4.0 License.
Since sedimentation of Unit 1 (< 8.8 ka), the sea level rose slowly from −20 m to the present level (Li et al., 2014b). The tidal field of the Yellow Sea became similar to that of the present (Uehara and Saito, 2003), leading to obviously decreasing contributions from sea bed erosion. A modern-type circulation in the Yellow Sea may have developed between 8.47 and 6.63 ka, characterized by an increase in bottom-water salinity (Kim and Kucera, 2000). The clay minerals and geochemical 235 composition generally point to the Changjiang, with minor influence from the western Korean rivers (Figs. 4 and 6), which is consistent with the reported 'multiple origin' concept (Wei et al., 2003;Li et al., 2014a;Lim et al., 2015;Koo et al., 2018).
Therefore, the formation of the CYSM and modern ocean circulation could have been synchronic around ~8 ka. The timing of mud patch formation in the central Yellow Sea was inferred to be 9~8 ka with low tidal bottom stress (< 0.35 N/m 2 ) (Uehara and Saito, 2003), which is consistent with our results. 240 The YSWC played a major role in the unique passage of sediment into the study area since the Unit 1 (Li et al., 2014a;Lim et al., 2015;Koo et al., 2018). The Changjiang Diluted Water can spread some finer sediments to Cheju and nearby areas (Hwang et al., 2014;Kwak et al., 2014;Li et al., 2014a;Lim et al., 2015;Koo et al., 2018). And then, fine-grained materials could be carried northward along the YSWC path to the CYSM, where the weak tidal-current system and cyclonic eddies provided favorable environment for the formation and maintenance of muddy sedimentations Lim et al., 2015). 245 Meanwhile, barrier effect of oceanic fronts and strong coastal currents restricts to enter the sediments from the Huanghe and western Korean rivers into the CYSM (Li et al., 2014a;Koo et al., 2018). However, some fine-grained particles from western Korean rivers may influence the CYSM through the Transversal Current (Hwang et al., 2014;Koo et al., 2018).

Conclusions 250
The purpose of this study is to better understand the CYSM in terms of provenance changes and transport mechanisms and to reconstruct the paleo-environment of the Yellow Sea since late last deglaciation using clay mineralogy and geochemical indices from core PCL14. The major conclusions are as follows.
Core PCL14 provides a continuous record of the late last deglaciation to Holocene in the CYSM. The core could be divided mainly into four units: Unit 4 (700-520 cm; 15.5-14.8 ka), Unit 3 (520-280 cm; 14.8-12.1 ka), Unit 2 (280-130 cm; 12.1-255 8.8 ka), and Unit 1 (130-0 cm; < 8.8 ka). The integration of clay mineralogical and geochemical data distinguished the CYSM sediments into different provenances by grain size. In fine particles, Unit 3 and 4 sediments originated from all possible provenances in the Korea and China, after which the sediment source is gradually shifted to the Changjiang. The inflow of Huanghe-derived fine sediments temporarily increased during Unit 2. On the other hand, the origin of coarse sediments changed in order of all possible rivers (Unit 4), Huanghe (Unit 3), and western Korean rivers (Unit 2). Change in sediment 260 supply are closely related to variations in sea level, positions of paleo-river mouths and tidal stress. Meanwhile, our data suggest that the formation of modern CYSM mud deposition began around ~8 ka with modern ocean circulation and the CYSM sediments are composed mainly of the Changjiang.