The results of the dating of the six samples are shown in Table 1. The AMS ¹⁴C ages were calibrated to calendar ages using the MARINE 20 calibration curve (Heaton et al., 2020). We adopted the average reservoir age (−15 ± 38 years) around the South China Sea for the last thousand years (Southon et al., 2002; Dang et al., 2004; Fallon and Guilerson, 2008; Bolton et al., 2016). Yu et al. (2010) revealed that significant fluctuations in radiocarbon marine reservoir ages and regional marine reservoir corrections occurred in the South China Sea during the Holocene. Therefore, reservoir age corrections of 89 ± 59 years and 151 ± 85 years were applied for the periods between 2–3.5 kyr and 3.5–13 kyr BP, respectively (Table 2). The Bayesian age-depth model of core KZK01 was calculated using the R program BACON (Blaauw and Christen, 2011; Figure 2).

The grain size of KZK01 sediments is heterogeneous, with the mean grain size (Mz) ranging from 1.93 to 7.23 Φ (mean value: 4.89 Φ). the mean sorting coefficient (δi) is 1.74 φ, and the mean skewness (Ski) is 0.17. The major geochemical element composition of the sediments was dominated by SiO₂ and Al₂O₃, with the sum of the two accounting for more than 70% of the total chemical composition (Table 3) (see Supplemental Text for detail). The grain size and major composition varied significantly from bottom to top, and three sedimentary stages (U1, U2, and U3) could be distinguished from bottom to top (Figure 3). The three stages are described briefly below:

• U1 (12–9 cal kyr BP): the overall grain size in this stage was fine, with the average Mz value being 7 and δi value reaching 1.67. The grain size did not vary much. The contents of SiO₂, K₂O, Na₂O, and CaO tended to increase upward, whereas the contents of the other major elements tended to decrease upward.

The REE contents and major rare earth indicators of KZK01 sediments are shown in Figure 4 and the same three sedimentary stages (i.e., U1, U2, and U3) can be distinguished based on the vertical distribution of REEs. In U1, the average ∑REE value is high, reaching 233.15 (Table 4). ∑LREE/∑HREE (La/Yb)_N, and (La/Sm)_N exhibit an obvious increasing trend. In U2, the ∑REE value is lower than that in the previous stage. The major rare earth indicators, such as ∑LREE/∑HREE (La/Yb)_N, (La/Sm)_N, (Gd/Lu)_N, Ce/Ce*, and Eu/Eu*, are higher than those in the previous stage, and the values exhibit a tendency to increase. In U3, clear faults are seen separating this stage from the previous stage, indicating substantial changes in the sedimentary environment.

The composition of the Upper Continental Crust (UCC) was used to normalize the REEs in the KZK01 sediments. The distribution curves of U1 (Figure 5A) and U2 (Figure 5B) were relatively smooth and similar, with ∑LREE/∑HREE ratios of 8.65 and 9.44, respectively, which were similar to that of the UCC (9.45). The distribution curve of U3 (Figure 5C) showed a right-dipping negative slope pattern with a high left and a low right, and Eu exhibited a clear negative anomaly.

The clay minerals in Core KZK01 consisted mainly of illite, with moderate amounts of smectite, kaolinite, and chlorite. The average contents of illite, smectite, kaolinite, and chlorite were 60%, 21.15%, 10.48%, and 8.37%. The chemical index of illite varied from 0.03 to 0.89, with an average value of 0.26. The crystallinity of illite ranged from 0.16° to 0.27°Δ2θ, with an average value of 0.2°Δ2θ (Table 5). The vertical variation of the clay minerals was consistent with the variation observed in grain size, constants, and REEs, with significant variations at 9 cal kyr BP and 1.3 cal kyr BP. The same three sedimentary stages (i.e., U1, U2, and U3) could be distinguished. In U1, the smectite content, illite content, and illite crystallinity values tended to increase upward, whereas the kaolinite and chlorite contents decreased (Figure 6). In U2, the illite content, smectite content, illite chemical index, and smectite/(illite + chlorite) ratio decreased steadily, whereas the kaolinite and chlorite contents slowly increased. In U3, the lithology was coarse-medium sand with very low clay mineral content, which differed significantly from that of U2.

REEs have similar chemical properties and low solubilities, and fractionation rarely occurs during the weathering of rocks and migration of terrigenous sediments. Weathered material typically inherits the REE characteristics of the parent rock. Therefore, the REE parameters of sediments are widely used to identify provenance (Mi et al., 2017; Su et al., 2017). ∑LREE/∑HREE and (La/Yb)_N values are indicators of REE fractionation. The correlation coefficients between the mean grain size and ∑LREE/∑HREE (r = 0.35, Figure 7A) and SiO₂ and (La/Yb)_N (r = 0.20, Figure 7B) in the study area are low, indicating that (La/Yb)_N and ∑LREE/∑HREE can eliminate the effects of sediment grain size and better preserve the chemical composition characteristics of the source rocks. Therefore, (La/Yb)_N and ∑LREE/∑HREE ratios are effective indicators for provenance tracing.

The REE distribution patterns in the sediments of Core KZK01 were compared with those in the sediments from the Mekong River, Red River, Pearl River, Changhua River, rivers in southwest Taiwan, eastern part of the Qiongzhou Strait and western part of Hainan Island, in addition to the surface sediments of the Beibu Gulf, as these are potential source areas for the Core KZK01 sediments. This comparison yielded valuable insights into the sedimentary processes and sedimentary provenance of the study region. We found that the REE patterns of U1 (mean) and U2 (mean) were closer to those of the nearshore sediments from the western part of Hainan Island, sediments from the eastern mouth of the Qiongzhou Strait, surface sediments from the Beibu Gulf, and river sediments from the southwestern part of Taiwan (Figure 8A); they exhibited a difference from those of other source areas (Figure 8B). Moreover, the partition curves of the sediments from the eastern mouth of the Qiongzhou Strait had the highest degree of similarity to those of U1 and U2. This indicated that during the period from 1.3 to 12 cal kyr BP, sediments from nearshore rivers and nearshore erosion on Hainan Island, sand transport from the Qiongzhou Strait, and sediments from rivers in Taiwan that were transported by ocean currents were the main sources of detrital material in Beibu Bay and that east-to-west sand transport in the Qiongzhou Strait may have been the most important source and sink pathway. These results are consistent with those of the material source analysis of SO-50 sediments in the Beibu Gulf by Zhang et al. (2018). The partitioning curve of U3 had an obvious left-high-right-low right-slope, and the partitioning curve was very similar to that of the parent rock on Hainan Island (Figure 8C) and differs greatly from those of other neighboring potential source areas (Figure 8D). This suggested that the sediments in the study area were mainly influenced by terrestrial material from Hainan Island during the last 1.3 cal kyr BP. This result is consistent with those of a previous study of clastic minerals in the Beibu Gulf (Dou et al., 2012) and analyses of sediment grain size trends (Xu et al., 2010).

Scatter plots of UCC-normalized REE fractionation parameters can be used for further source identification (Dou et al., 2010). Comparison of (La/Yb)_N (Figure 9A) and (Gd/Yb)_N and LREE/HREE and (Gd/Lu)_N (Figure 9B) for different source areas and shelves in the northern South China Sea showed that the U1 stage sediments in Core KZK01 were similar to those from the southwestern rivers of Taiwan, the Beibu Gulf, the Pearl River, and the eastern part of the Qiongzhou Strait. The U2 stage sediments were similar to those from the Beibu Gulf, the western rivers of Hainan Island, the eastern part of the Qiongzhou Strait, and Taiwan and the Pearl River. Stage U3 sediments were similar to the parent rock on Hainan Island and sediments from the western rivers of Hainan Island. Core KZK01 sediments differed greatly in REE characteristics from sediments of the Red River and Mekong River. The southwestern rivers of Taiwan, Pearl River, and Beibu Gulf contributed the most to the sediments deposited in the U1 stage. The Beibu Gulf, rivers of western Hainan Island, and eastern part of the Qiongzhou Strait contributed the most to the sediments deposited in the U2 stage, whereas the contribution of the Pearl River and the rivers in Taiwan gradually decreased. The southwestern rivers of Hainan Island and the parent rock of Hainan Island contributed the most to the sediments deposited during the U3 stage. Recent studies have found that sediments from the rivers in Taiwan and the Pearl River can be transported to the southeast Qiongnan shelf, thousands of meters away, or even to the Xisha Trough, which is thousands of kilometers away, under the action of the Guangdong littoral current and the deep-water currents during the winter (Liu et al., 2016; Yan et al., 2016; Xu et al., 2017; Xu et al., 2021). This further supports the contribution of sediments from the Pearl River and the rivers in Taiwan to the shelf and slope of the northwestern part of the South China Sea.

The use of clay minerals in marine sediments to study paleoclimate requires knowledge of the main source areas and transport pathways of each mineral (Gingele et al., 1998). The clay minerals in Core KZK01 originate from terrestrial sources, and the effects of secondary and diagenetic effects on these clay minerals is negligible. The main source areas in the study area include the Pearl River, the Red River, the Mekong River, the rivers of Hainan Island, the rivers of Guangxi, the rivers of Taiwan, and the rivers of northern Vietnam. Sediments exported from small rivers along the coast of Vietnam are mainly transported southward by the Vietnamese coastal current (Figure 1), and it is difficult for them to reach the study area, so their contribution to Core KZK01 is negligible. Coastal erosion in marine areas such as the Qiongzhou Strait and the western part of the Leizhou Peninsula may also provide a small amount of sediments (Xu et al., 2010), but the amount of these sediments and their clay mineral fractions are not known, so their contribution is ignored in this study.

The Red River is the largest source of sediments in the Beibu Gulf, delivering approximately 130 × 10⁶ t of suspended sediment (Milliman and Syvitski, 1992). Illite (44%) is the main clay mineral, followed by kaolinite (26%), chlorite (23%), and smectite (7%) (Liu et al., 2007). The Mekong River transports approximately 160 × 10⁶ t of suspended sediments to the South China Sea annually (Milliman and Syvitski, 1992). The clay minerals in these sediments are also dominated by illite, with an average content of 37% (Table 6), followed by kaolinite (31%), chlorite (23%), and smectite (approximately 10%) (Liu et al., 2016). The Pearl River transports 84.3 × 10⁶ t of suspended sediments to the South China Sea every year (Zhang et al., 2012). The clay minerals in the Pearl River sediments are mainly kaolinite (average of 46%, the same as below), followed by chlorite (31%), illite (18%), and smectite (5%). The Changhua River and Nandu River are the largest rivers on Hainan Island, and their annual average sediment transport is approximately 1.18 × 10⁴ t (Milliman and Farnsworth, 2011), which is lower than that of the Red River (130 × 10⁶ t) and Mekong River (160 × 10⁶ t). The fluvial sediments in Guangxi and Hainan have similar clay mineral assemblages. Taiwan is the main source of sediments in the South China Sea (Milliman and Farnsworth, 2011), with 176 × 10⁶ t of suspended sediments flowing into the South China Sea from Taiwan every year (Dadson et al., 2003). These sediments are rich in illite (average: 56%; some sediments contain 66% illite) and chlorite (36%), with less smectite and kaolinite. Approximately 130 × 10⁶ t of suspended matter is directly imported from Luzon Island every year (Milliman and Syvitski, 1992). Smectite is the predominant clay mineral, with an average content of 87%, whereas the average contents of illite, kaolinite, and chlorite are generally less than 10% (Liu et al., 2016).

Comparison of the clay mineral assemblage of Holocene samples from Core KZK01 with those in modern surface sediment samples from the aforementioned potential source areas showed that the clay minerals in the core originated from multiple sources (Figure 10A). The average smectite content of the study core was 21.15%, whereas the smectite content was less than 10% in all potential source areas except Luzon. It was found that differential sedimentation and sorting of clay minerals may have contributed to the high relative smectite content in marine sediments (Chen et al., 2017b). Since the average particle size of smectite (0.4 μm) is much smaller than that of illite (2–4 μm), chlorite (1–10 μm), and kaolinite (1–2 μm), the physical sorting effect of particle size will cause illite, chlorite, and kaolinite to preferentially undergo deposition close to the estuary, whereas smectite is easily transported to areas further away from the estuary.

In general, the smectite content exhibits enrichment as the distance from the estuary increases, a trend that has been confirmed in studies of clay minerals in the Beibu Gulf, (Jin et al., 2019), the northeastern part of the South China Sea (Liu J. G et al., 2010), and the Mississippi River (Johnson and Kelley, 1984). Thus, Luzon may be the main source area of smectite in the eastern part of the Beibu Gulf. Clay minerals exported from Luzon Island were carried into the South China Sea by a branch of Kuroshio as well as the north-east oriented deep South China Sea Current, and the contents of illite, chlorite, and kaolinite, which had large grain sizes, decreased rapidly as the transportation distance increased, owing to their large grain sizes. In contrast, smectite was more readily suspended in seawater owing to its small grain size, and its relative content gradually increased as the transportation distance increased. Subsequently, these suspended clay minerals were driven by the winter surface currents and littoral currents to the Beibu Gulf (Liu et al., 2016), where they were transported toward the northeast under the influence of counterclockwise circulation in the Beibu Gulf and eventually gradually settled in the eastern part of the Beibu Gulf (Figure 1).

In addition, Quaternary basaltic volcanic rocks are distributed across large areas in the northern part of Hainan Island (Liang et al., 2021). Chemical weathering of volcanic rocks, which can occur as long as there is a sufficient supply of water and does not depend on specific climatic conditions, is an important source of smectite (Liu et al., 2007), For example, up to 40% of smectite develops in Iceland near the North Pole and in the islands of West Antarctica (Rateev et al., 1969). The average smectite content of sediments from the rivers in Hainan is 6%–9% (Liu et al., 2016; Jin et al., 2019). The detrital material transported by the Nandujiang River is transported by littoral currents and surface currents through the Qiongzhou Strait and deposited in the Beibu Gulf (Figure 1); sediments from the Wanquanhe River can also be transported into the Beibu Gulf through littoral currents. However, the sediment flux of rivers in Hainan is very low (1–2 Mt/a), and its contribution to the sediments in the Beibu Gulf is very limited. Therefore, smectite mainly originates from Luzon Island, followed by Hainan Island.

The illite content at the study site is 52%–76%, and the average content of illite in the Pearl River, Taiwan River, Mekong River, and Red River is 31%–56% (Table 5). Thus, the illite in the study area could potentially originate from all of these source areas. Although the composition of clay minerals in the Thai Binh River and Ma River in northern Vietnam is dominated by illite (average: 54%) (Liu et al., 2016), the sediment transport of these rivers is too low (∼4 Mt/a) (Milliman and Farnsworth, 2011), and the contribution of illite to the study area is limited compared with the contributions of the Red River and the rivers in Taiwan. The illite content of rivers in Luzon Island, Hainan, and Guangxi ranges from 1% to 17% (Table 5), and the contribution of these sources to the illite in this area is the lowest. The provenance of illite minerals can be further determined by comparing the illite petrochemical indices and crystallinity of these potential provenance samples (Gingele et al., 2001). The illite crystallinity index can be used to reflect the degree of crystallization of illite—the smaller the crystallinity index, the better the degree of crystallization (Ehrmann, 1998). The results (Figure 11; Table 5) show that the crystallinity index of illite in Core KZK01 is 0.2° Δ2 θ, indicating that the illite in the Beibu Gulf has excellent crystallinity and is rich in aluminum, meaning that it originates from areas in which chemical weathering is dominant. This value is very similar to the crystallinity indexes of Red River illite (average of 0.19° Δ2 θ) and Pearl River illite (average of 0.17° Δ2 θ) (Liu et al., 2007). The mineralogical characteristics of illite from most Taiwanese river samples and some Red River samples are in good agreement with those from the research station. However, the illite geochemical index (average 0.62) and crystallinity (average 0.22° Δ2 θ) of the Pearl River are significantly larger than those from the study area, indicating that illite from the Pearl River does contribute to the illite in the eastern part of the Beibu Gulf. Therefore, we conclude that the rivers in Taiwan are the most likely sources of the illite in Core KZK01, followed by the Red River. A small amount of illite also originates from rivers in Hainan.

The average chlorite content in the study area is 8.4%, and the chlorite contents of all potential source areas are within the range of 10%–30% (Table 5), with no obvious differences. The source areas cannot be determined from the mineral assemblage. It is generally accepted that chlorite in the South China Sea indicates climatic conditions characterized by strong physical weathering (Boulay et al., 2005). A previous study on clay mineral assemblage characteristics of Core MD05-2901 in the northern part of the South China Sea found that chlorite was mainly supplied by the Mekong River and the Red River (Liu et al., 2007). Jin et al. (2019) suggested that chlorite in surface clay minerals in the Beibu Gulf mainly came from the Red River and the Pearl River. Therefore, we concluded that the chlorite in Core KZK01 was mainly supplied by the Red River, followed by the Pearl River. The clay mineral composition of the rivers in Guangxi and Hainan is not dominated by chlorite with average chlorite contents of only 16% and 6%, respectively. Moreover, the rivers in Guangxi and Hainan discharge less than 3 Mt/a of suspended sediment annually (Table 6), and their contributions to the chlorite in the study area are limited compared with those of the Red River (130 Mt/a) and the Pearl River (69 Mt/a).

Kaolinite is formed as a result of intense chemical weathering and leaching of rocks in a warm and humid acidic environment. The spatial distribution of kaolinite in marine surface sediments is closely related to the supply of terrigenous materials and the climate (Aoki, 1976). Kaolinite is prone to flocculation when it enters alkaline seawater from rivers (Liu Z. F et al., 2010), so a large amount of kaolinite is deposited in the estuary and coastal zone. Previous studies have found that the kaolinite content is very high in the northern part of the Beibu Gulf (Jin et al., 2019), especially near Guangxi, the Qiongzhou Strait, and Hainan Island, and that the coastal area with high kaolinite contents runs parallel to the coastline. Hainan Island and Guangxi are located in tropical and subtropical regions, respectively, with a warm and humid climate, stable structure, and dominant chemical weathering, which are very conducive to the formation and preservation of kaolinite. Therefore, the clay minerals in sediments from the rivers of Hainan Island and Guangxi are mainly composed of kaolinite, with average contents of 76% and 65%, respectively (Table 5). Kaolinite is deposited in the Beibu Gulf when these rivers empty into the sea. Therefore, we inferred that Hainan Island and Guangxi are the main sources of kaolinite.

The Beibu Gulf region has experienced significant provenance changes during the Holocene. Past changes in paleoclimate and relative sea level may have played a crucial role in shaping the provenance of sediments deposited in the gulf.

Illite crystallinity and the smectite/(chlorite + illite) ratio are often used as indicators of climate change (He et al., 2022). During the deposition of U1 (12–9 cal kyr BP), the mean value of illite crystallinity was 0.22°Δ2θ, which was greater than the mean value of 0.20°Δ2θ during the deposition of U2 (Table 5), indicating that the climate was more temperate and humid during the U1 stage. During this period, illite crystallinity and the montmorillonite/(chlorite + illite) ratio increased (Figures 12A,B), the East Asian summer monsoon intensified (Figure 12E), rainfall increased, and the warm and humid climate was conducive to the formation of smectite. In addition, although the sea level has risen (Figure 12G), the Qiongzhou Strait had not yet opened (Ni et al., 2014), which hindered the transportation of sediments from the regions to the east of the strait into the study area. The detrital material during this phase mainly transported from nearby sources, and the fine-grained material was likely to be transported to Luzon sediments via the South China Sea Warm Current. The variation trend of illite crystallinity was similar to the trend observed for the strength of East Asian summer winds in Dongge Cave (Wang et al., 2005; Dykoski et al., 2005; Figure 12E), the trend observed for the strength East Asian winter winds in Huguangyan Maar Lake (Yancheva et al., 2007; Figure 12F), as well as the trend of change in the sporadic wetness index (Zhao et al., 2009) in the Chinese monsoon area. The illite crystallinity index during the deposition of U1 clearly records extreme cold events such as Bond8, Bond7, Bond5 (Bond et al., 2001), and the Younger Dryas events, reflecting the regional response to global climate change.

During the deposition of U2 (9–1.3 cal kyr BP), frequent changes in lithology occurred (Figure 12C), indicating a more turbulent depositional environment. The smectite/(illite + chlorite) ratio and illite crystallinity decreased relative to the previous phase and exhibited a trend of decreasing values. During this period, the East Asian summer monsoon weakened, the winter monsoon strengthened, and the sea level continued to rise (Figure 12G). At the same time, the Qiongzhou Strait opened (Ni et al., 2014; Cui et al., 2015), and the southwest coastal current strengthened, which increased the transport of sedimentary material from Taiwan. However, this trend changed at 2.78 cal kyr BP, when the climate changed from dry and cold to warm and humid. This is consistent with the Ti element intensity observed in Guangdong Huguangyan Maar Lake, stacked curves of drift ice and oxygen isotopes in Dongge Cave in South China (Figures 12D–F), and the relative rise in sea level of the northwestern part of the South China Sea since 2.78 cal kyr BP (Siddall et al., 2003; Figure 12g). Illite crystallinity is particularly sensitive to the record of cold events, and the Bond2, Bond3, and Bond4 cold events recorded in the North Atlantic ice floe debris elicited a relatively good response in the study area, indicating that illite crystallinity is a good proxy for reconstructing climate.

During the deposition of U3 (1.3 cal kyr BP to the present), the mean value of illite crystallinity was 0.17°–0.18° Δ2 θ before and after the 1.3 ka point, indicating a dry and cold climate dominated by physical erosion at this time. A previous study showed that the sea level along the northern coast of the South China Sea began to fall around 1.2 ka (Duong et al., 2020), the average age of the Late Holocene outcrops of beach rocks on Weizhou Island in the northern part of the Beibu Gulf was 1.3 ka, and the analysis of fossilized mangrove sporophytes showed a rapid sea recession in Qinzhou Bay at 1.1 ka (Xia et al., 2019). At this time, the shoreline of the Beibu Gulf underwent comprehensive repair and consolidation, and the present-day appearance of the Beibu Gulf took form (Huang et al., 2022). After the Late Holocene ice advance peaked at 1.3 kyr, the illite crystallinity curve continued to rise, which corresponded well with the precipitation record represented by the oxygen isotope curves of Dongge Cave stalagmites (Dykoski et al., 2005; Wang et al., 2005). This indicated a reversal of the climate change trend from dry and cold to warm and wet, which is also corroborated by the Ti intensity curves of the Huguangyan Maar Lake (Siddall et al., 2003). Moreover, sporulation characteristics showed that the sporulation content of woody and herbaceous plants remained essentially unchanged compared with that in the previous period and that the abundance of ferns was slightly increased (Lv et al., 2003), suggesting that the climate has improved since 1.3 kyr.