RESEARCH ARTICLE
A westerly-dominated climate in Arid Central Asia during the Holocene revealed by ostracods from Bosten Lake in Xinjiang, China
2025 Volume 29 Pages 169-181
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2025 Volume 29 Pages 169-181
The westerlies, a major atmospheric circulation system, have a crucial role in driving climate and environmental changes in the middle latitudes of the Northern Hemisphere, particularly in arid Central Asia (ACA). However, the influence of variations in the westerlies on biota remains elusive due to less sedimentary archives. Here, we focus on Holocene ostracods from a core in the Bosten Lake in Xinjiang of the ACA area. We conducted a comparative analysis of our ostracod records, including radiocarbon dating results, with ostracod and salinity records from other cores within Bosten Lake, and suggested that alternative ostracod assemblages can serve as indicators of salinity changes in the lake over the past 8,000 years, which corresponded to westerlies dynamics. We propose that over the course of 8,000 years, precipitation patterns in the Bosten Lake region were influenced by intensified westerlies, which were associated with high sea surface temperatures (SSTs) in the North Atlantic. This climatic shift, in turn, led to changes in the local ostracod communities.
The westerly circulation associated with the Asian monsoon system is a major element of global atmospheric circulation, playing a pivotal role in climate and environmental changes in the middle latitudes of the Northern Hemisphere (Li and Zhu, 2024). Arid Central Asia (ACA) in the inland Asian continent including the Caspian Sea, the Iranian Plateau, the Tarim Basin, and the Mongolian Highland, where precipitation rates are typically below 1 mm/day, is heavily influenced by the westerlies, which bring most of the water vapor into the region. (Shi et al., 2021). Due to its arid climate, sparse vegetation cover, and fragile ecological environment, ACA exhibits high sensitivity to sudden shifts in rainfall patterns (Chen et al., 2016; Chen et al., 2019). ACA was characterized by a dry climate during the early Holocene around 7–8 thousand years ago when most lakes in the region were formed, a more humid climate and a rise in lake levels during the middle Holocene, and a moderately wet climate toward the end of the Holocene (Chen et al., 2019). However, the biological responses to these climate and environmental changes in this region remain poorly understood. Here, we investigated changes in the Holocene ostracod fauna through time in a core from Bosten Lake in Xinjiang, ACA. As the largest inland freshwater lake in China, Bosten Lake is significantly affected by the westerly wind circulation and provides a reliable record of climate changes in ACA during the Holocene (Chen et al., 2006). Ostracods are microcrustaceans with fully calcified bivalved carapaces that are found in a variety of aquatic habitats, including lakes, rivers, estuaries, and oceans, playing key roles in aquatic ecosystems as both prey for larger organisms and indicators of environmental conditions due to their sensitivity to changes in water quality (e.g. salinity and temperature) (Rodriguez-Lazaro and Ruiz-Muñoz, 2012; Villegas-Martín et al., 2023; Marchegiano et al., 2024). Due to their small size, morphological variability, ecological diversity, high fossilization potential, long geological history, and ease of extraction in large quantities from small sediment samples, ostracods are widely used to reconstruct Quaternary paleoenvironments. (Rodriguez-Lazaro and Ruiz-Muñoz, 2012). By investigating fossil ostracods from Bosten Lake, we aim to reconstruct the regional paleoenvironment throughout the Holocene and clarify the relationship between ostracod assemblages and the westerlies.
Bosten Lake is situated in the southeastern part of the Yanqi Basin at the southern foothills of the Tianshan Mountains in Xinjiang, northwestern China (86°40′–87°26′E and 41°56′–42°14′N) (Figure 1). It is considered the largest freshwater lake in inland China with a surface area of 1,000 km2 (Fontana et al., 2019). Bosten Lake is the endpoint for its sole perennial tributary the Kaidu River, which contributes about 95% of its total water inflow. It is also the origin of the Kongque River, which flows south into the Tarim Basin (Zhang et al., 2010; Liu et al., 2013). Currently, the average elevation of Bosten Lake is 1,048 meters above sea level, while its average depth is approximately eight meters, reaching a maximum depth of ~17 meters (Zhang et al., 2010; Fontana et al., 2019). As part of the arid region of northwestern China, mean annual precipitation is about 70 mm with potential evaporation as high as 2,000 mm and a mean annual temperature of 8.4°C (Mischke and Wünnemann, 2006). The water salinity of Bosten Lake has been greater than 1.0 g/L since the 1970s (up to 1.8 g/L in the 1980s) and is currently about 1.3 g/L (Zuo et al., 2006). According to the lake water balance model and climate elasticity method, the proportions of lake water supply from different sources were as follows: during the period of 2003–2010, lake precipitation accounted for 12.45%, inputs generated by precipitation accounted for 63.86%, and glacier melting water accounted for 23.69% of the total lake water supply, and in terms of lake water loss, evaporation accounted for 35.34% and output accounted for 73.49% during the period of 2003–2015 (Yao et al., 2018). Climate-driven regime shifts contribute to the lake level changes of Bosten Lake, showing an inverse relationship with salinity, where decreased precipitation increases drought frequency. (Yao et al., 2018).
We studied a core (BLX-C) obtained in the southwest of Bosten Lake at the coordinates 41°54.16′N, 86°45.70′E from a depth of eight meters below the lake surface (Figure 1). The core has a length of about 720 cm. The sediments consisted primarily of gray clay and silt, with intermittent layers of purple-brown laminae and fossils of molluscs. The core sediments were sliced into one cm-thick samples for various analyses, of which 64 containing ostracods were used in this study. The chronology of the core is based on accelerator mass spectrometry (AMS) radiocarbon ages obtained from gastropod fossils and plant debris, which were visually identified and hand-picked for AMS dating. To establish a detailed age model, plant debris from the top section of the lake core sample was carefully selected using stereo microscopes. The Tandem Laboratory at Uppsala University in Sweden analyzed the gastropod fossils, and the Rafter Radiocarbon Laboratory at the Institute of Geological and Nuclear Sciences in New Zealand analyzed the plant and grass debris. All dating results (Table 1) were converted to calendar age using the software CALIB rev. 4.4 (calib.org/calib) (Stuiver and Reimer, 1993). Salinity values given without units use the practical salinity scale (Unesco, 1981).
| Depth (cm) | 14C age | δ13C (‰) | AD/BC (max, min) | Calendar BP (max, min) | 2 sigma area | materials |
|---|---|---|---|---|---|---|
| 2.2 | 990±60 | −2.00 | 1075 (959, 1191) | 875 (991, 759) | 0.968 | Gastropods |
| 24 | 665±70 | −11.50 | 1328 (1241, 1415) | 622 (709, 535) | 0.997 | Grass root |
| 25.5 | 807±45 | −6.24 | 1224 (1158, 1289) | 727 (792, 661) | 0.981 | Plant debris |
| 31.5 | 840±40 | −9.23 | 1217 (1156, 1278) | 733 (794, 672) | 0.876 | Plant debris |
| 33.5 | 892±40 | −6.00 | 1126 (1031, 1221) | 824 (919, 729) | 1.000 | Plant debris |
| 41.5 | 1105±40 | −9.40 | 942 (864, 1019) | 1009 (1086, 931) | 0.980 | Plant debris |
| 42.0 | 1130±65 | −6.10 | 897 (772, 1021) | 1054 (1178, 929) | 0.986 | Gastropods |
| 44.5 | 1395±65 | −4.80 | 657 (538, 775) | 1294 (1412, 1175) | 1.000 | Gastropods |
| 49.5 | 1192±40 | −13.10 | 835 (766, 903) | 1116 (1184, 1047) | 0.825 | Plant debris |
| 53.5 | 1246±40 | −12.67 | 786 (686, 885) | 1165 (1264, 1065) | 1.000 | Plant debris |
| 57.5 | 1244±40 | −11.35 | 786 (686, 886) | 1164 (1264, 1064) | 1.000 | Plant debris |
| 65.5 | 1351±50 | −12.68 | 692 (609, 775) | 1258 (1341, 1175) | 1.000 | Plant debris |
| 67.5 | 1186±40 | −11.35 | 836 (768, 904) | 1114 (1182, 1064) | 0.811 | Plant debris |
| 113.5 | 1386±45 | −11.88 | 657 (596, 717) | 1294 (1354, 1233) | 0.932 | Plant debris |
| 150.0 | 2245±55 | −5.40 | −298 (398, 197) | 2247 (2347, 2146) | 0.976 | Gastropods |
| 169.5 | 1250±45 | −4.60 | 782 (678, 886) | 1168 (1272, 1064) | 1.000 | Gastropods |
| 173.0 | 2825±50 | −3.50 | −1009 (1127, 890) | 2958 (3076, 2839) | 0.927 | Gastropods |
| 284.0 | 2595±65 | −5.30 | −708 (899, 517) | 2657 (2848, 2466) | 0.993 | Gastropods |
| 304.0 | 2845±70 | −7.50 | −948 (1133, 834) | 2933 (3082, 2783) | 0.889 | Gastropods |
| 307.0 | 3195±55 | −4.10 | −1489 (1604, 1374) | 3438 (3553, 3323) | 0.974 | Gastropods |
| 343.5 | 2415±55 | −5.20 | −496 (596, 396) | 2445 (2545, 2345) | 0.648 | Gastropods |
| 352.5 | 2784±55 | −7.10 | −933 (1052, 813) | 2882 (3001, 2762) | 0.988 | Gastropods |
| 382.0 | 3945±70 | −3.10 | −2434 (2600, 2268) | 4383 (4549, 4217) | 0.937 | Gastropods |
| 393.0 | 4090±70 | −3.00 | −2636 (2785, 2486) | 4585 (4734, 44350) | 0.788 | Gastropods |
| 441.0 | 2905±55 | −6.30 | −1115 (1261, 968) | 3064 (3210, 29170) | 0.948 | Gastropods |
| 446.5 | 2845±65 | −6.60 | −1010 (1133, 886) | 2959 (3082, −2835) | 0.848 | Gastropods |
| 472.0 | 3965±70 | −2.30 | −2454 (2633, 2274) | 4403 (4582, 4223) | 0.954 | Gastropods |
| 547.5 | 2535±65 | −3.20 | −643 (804, 482) | 2592 (2753, 2431) | 0.933 | Gastropods |
| 561.0 | 3465±65 | −5.60 | −1783 (1949, 1616) | 3732 (3898, 3565) | 1.000 | Gastropods |
| 581.5 | 4065±60 | −5.30 | −2588 (2710, 2466) | 4537 (4659, 4415) | 0.770 | Gastropods |
| 591.5 | 4560±75 | −4.00 | −3231 (3385, 3077) | 5180 (5334, 5026) | 0.794 | Gastropods |
| 602.5 | 5130±75 | −7.00 | −3902 (4048, 3756) | 5851 (5997, 5705) | 0.949 | Gastropods |
| 699.5 | 6830±60 | −2.90 | −5716 (5807, 5624) | 7665 (7756, −7573) | 0.977 | Gastropods |
Accelerator mass spectrometry (AMS) 14C dates from Core BLX-C on grass root, plant debris and gastropods and converted calendar ages with 2δ maximum and minimum error ranges.
Ostracod processing followed the standard for fossil ostracod research (Gemery et al., 2017; Yasuhara et al., 2017). The counting method of Yasuhara et al. (2017) was employed, with each articulated carapace counted as two specimens and each disarticulated valve counted as one. The studied ostracod specimens were generally well preserved with translucent valves.
Statistical analyses were conducted using the software R version 4.4.0 (R Core Team, 2024). Non-metric multidimensional scaling (NMDS) was used to explore the relationships between ostracod assemblages. This method produced a two-dimensional ordination of our faunal assemblages while preserving the ranks of differences between data points (Borcard et al., 2011; Legendre and Legendre, 2012). We first constructed a Bray-Curtis dissimilarity matrix based on ostracod abundance data (Bray and Curtis, 1957). Based on this matrix, we constructed the NMDS overlaid with standard deviation ordination ellipses using the vegan package (Oksanen et al., 2020). The same package was used to perform a permutational multivariate analysis of variance (PERMANOVA) (Anderson and Robinson, 2001; Anderson and Walsh, 2013; Anderson, 2017) with 9999 permutations on the dissimilarity matrix to compare the assemblages’ structures. Pairwise PERMANOVA comparisons among assemblages were performed with the pairwiseAdonis package (Martinez-Arbizu, 2020). Significance level was set at α = 0.05 for all statistical analyses.
Linear regressions were used to investigate the relationship between the relative abundance of ostracods (Candona neglecta [C] + Limnocythere inopinata [L]) and NMDS1, which was derived from NMDS analysis in relation to several variables: hydrodynamic intensity, pollen ratio of Artemisia to Chenopodiaceae (A/C ratio), PC1, westerlies effect index, East Asian summer monsoon (EASM), and North Atlantic sea surface temperature (SST). Data for those variables were extracted from figures in relevant publications using the juicr package (Lajeunesse, 2021). The residuals from the NMDS1 models were normally distributed, and simple linear regressions were utilized. However, the abundance models did not meet the assumption of residual normal distribution. Therefore, robust linear regressions (RLM) using M-estimation (Huber, 1981) were applied utilizing the rlm function from the MASS package (Venables and Ripley, 2002). RLMs are not least square based, so pseudo-F was calculated as a measure of relative fit. Pseudo-F was calculated as a measure of model fit based on deviance reduction between full and reduced (intercept-only) models. Confidence intervals (2.5th and 97.5th percentiles) for intercept and slope were calculated in our models. For RLM models, confidence intervals were obtained by bootstrap resampling (1000 iterations) using the boot package (Davidson and Hinkley, 1997; Canty and Ripley, 2024).
In total, six genera including seven species and one indeterminate species were discovered in the studied core (Table S1). The species are Candona candida, C. neglecta, C. sp., Cyprideis torosa, Darwinula stevensoni, Fabaeformiscandona hyalina, Herpetocypris helenae, and Limnocythere inopinata (Figure 2). We further calculated relative abundance of the studied ostracods (Figure 3).
Candona candida, a typical cold-water species, is widely distributed across different aquatic environments, such as springs, streams, ponds, and lakes at various depths in both still and turbulent water conditions (De Deckker, 1979; Wilkinson et al., 2005). It has a limited tolerance to salinity levels higher than 5.3 (low in the mesohaline range) (Hiller, 1972; Usskilat, 1975) and cannot survive in water temperatures exceeding 18°C during the summer after prolonged exposure to high salinity conditions (Hartmann and Hiller, 1977). According to Fryer (1980, 1993), it is relatively tolerant to acidic waters with a pH below five. This species is often found in marshy vegetation (De Deckker, 1979).
Candona neglecta inhabits in a wide range of aquatic habitats including springs; ponds and brooks fed by springs; the shallow littoral zone down to great depths in lakes; small water bodies such as ponds, pools, ditches, and streams; underground waters (interstitial habitat and wells); and even temporary waters (Meisch, 2000). Candona neglecta shows a preference for cooler water but will survive in temperatures of about 20°C for short periods (Czajkowska, 2022). Studies conducted by Danielopol et al. (1985, 1993) demonstrated that Candona neglecta exhibits a tolerance for low levels of oxygenation in water and can withstand salinity levels ranging from 0.5 to 16.
Cyprideis torosa has an extensive geographical distribution spanning Eurasia and Africa and occurs down to a depth of around 30 m in diverse water bodies including coastal ponds, lakes, lagoons, estuaries, fjords, deltas, salt marshes and other marginal marine environments with a wide range of salinities (0.4–20) from almost freshwater to fully marine and even hypersaline water (>60) (Meisch, 2000; Frenzel et al., 2010; Wouters, 2017). Its optimal growth occurs within a salinity range of 2 to 16.5 (Meisch, 2000). While C. torosa prefers mud or sandy mud substrates, it can also be found thriving in habitats characterized by pure sand and algae, and it is characterized by tolerance of a wide temperature range from 0°C to 32°C (Frenzel et al., 2010; De Deckker and Lord, 2017).
Darwinula stevensoni is a benthic species occurring on both muddy and sandy substrates and prefers ponds, lakes, and slow streams but has also been reported from interstitial groundwaters (Meisch, 2000). It is tolerant of mesohaline environments with a maximum salinity of 15 (Meisch, 2000; Pérez et al., 2010). According to Frenzel et al. (2010), D. stevensoni exhibits tolerance to a wide range of salinities (typically 0–12 but up to 15) from freshwater to α-oligohaline with temperature from 0.1°C to 27°C (even 40°C) (thermoeuryplastic). It is capable of inhabiting depths ranging from 0 to 12 meters, with the highest concentration found at a depth of six meters (Meisch, 2000).
Fabaeformiscandona hyalina is found in both small permanent water bodies and the littoral zone of lakes with mud and phytal substrate at very shallow to shallow depths and is also reported in swampy and peaty ditches in open fields, swampy and muddy ponds, streams, and dead arms of rivers (Meisch, 2000; Frenzel et al., 2010). It can tolerate a salinity range of 0 to 4.4 from freshwater to α-oligohaline with temperatures from 1.2 to 24°C (Frenzel et al., 2010).
Herpetocypris helenae is known to inhabit a variety of aquatic environments, ranging from freshwater to slightly saline waters with a salinity range of 0.2 to 1.4, characterized by alkaline pH levels and temperatures ranging from 19.8 to 29.5°C (D’Ambrosio et al., 2016). Based on Meisch (2000), H. helenae has similar ecology to that of H. chevreuxi, which prefers slightly salty waters with a reported maximum salinity of three–four in small stagnant water bodies including the littoral zone of lakes, swamps, slow streams, and rivers. For H. helenae, in lentic (still) environments, the dissolved oxygen levels range from 19% to 84%, while in lotic (flowing) environments, the dissolved oxygen levels range from 100% to 108% (D’Ambrosio et al., 2016).
Limnocythere inopinata is found on both muddy and sandy substrates from a variety of water bodies including ponds, swamps, ditches, lakes, slow brooks and rivers, and rice fields (Meisch, 2000; Wang et al., 2021a). It also occurs in superficial interstitial habitats, slightly salty inland and coastal waters, and even in slightly brackish areas such as the Baltic Sea (Meisch, 2000). Limnocythere inopinata predominantly inhabits shallow waters down to a depth of around 6 m, but there have been rare instances in which it has been observed in deeper waters, such as at a depth of 64 m in the Baltic Sea (Meisch, 2000; Wang et al., 2021a). It exhibits a preference for salinity levels ranging from 0.5 to 9 and is capable of tolerating a wide range of temperatures from 0.5°C to 35°C but prefers warm waters (Meisch, 2000; Frenzel et al., 2010; Wang et al., 2021a). However, in the Tibetan Plateau, L. inopinata shows a distinct preference for polyhaline waters characterized by a salinity level exceeding 10 (Akita et al., 2016). Results from the Lake Jiang Co in northern Tibetan Plateau showed that the L. inopinata population in the lake prefers a narrow temperature range of 11–14°C and a narrow salinity range of 3.5–6.5 but is able to tolerate a wider temperature range of 11–17°C and a wider salinity range of 0.50–9.50 (Wang et al., 2021b).
Wei et al. (2013) divided the past climate of Bosten Lake into three stages as revealed by the lithology, organic carbon isotopic changes, and age models from the studied core (BLX-C). These are Stage III, which lasted from approximately 7665 years before present (BP) to 2050 BP and consisted of alternating periods of humid and dry climates; Stage II, which lasted from 2050 BP to 1190 BP and was a stable and humid climate period that spanned the Han, Gin, Southern and Northern, and early-mid Tang dynasties; and Stage I, which was a drought period that persisted for about 1,000 years from around 1190 BP to 250 BP and spanned the late Tang Dynasty to the end of the Ming Dynasty. According to these three stages, ostracods from the studied core were further divided into three Assemblages A, B and C, corresponding to Stage I, II and III, respectively. The species compositions of the three assemblages are dissimilar (PERMANOVA, Pseudo-F(2,63) = 10.261, SS = 3.923, p < 0.001; pairwise-PERMANOVA, p < 0.001). Assemblage C has a small overlap with both assemblage A, which is dominated by the species C. neglecta and L. inoppinata, and assemblage B, where the dominant species is D. stevensoni (Figure 4), indicating alternative changes of Assemblage A and Assemblage B, which is also supported by the relative abundance changes (Figure 3). Abundance was significantly negatively correlated with the pollen A/C ratio and the EASM and positively correlated with PC1 (Table S2). Conversely, NMDS1 showed a significant positive correlation with the pollen A/C ratio and the EASM but a negative correlation with PC1 (Table S2).
During the past ~8,000 years, Assemblage A, dominated by Candona neglecta and Limnocythere inopinata, and Assemblage B, dominated by Darwinula stevensoni, appeared alternately in Bosten Lake. Similarly, minimal abundances of D. stevensoni were observed when C. neglecta and L. inopinata shells were at their peak, and vice versa, in another core from the lake (XBWu46: Mischke and Wünnemann, 2006). Generally, L. inopinata is a salinity-dependent species (McCormack et al., 2019), and C. torosa is a typical brackish water ostracod. Both species were associated with Assemblage A (Figure 2), indicating a relatively high salinity. Darwinula stevensoni occurs mostly in waters with salinity between one and four, indicating a relatively low salinity. Thus, the change from ostracod Assemblage A to B suggests a variation in the salinity levels of Bosten Lake. Similar salinity changes in Bosten Lake were also observed for the past 8,000 years in the cores XBWu46 and BSTC2000 (Mischke and Wünnemann, 2006; Zhang et al., 2010; Figure 1). According to the ostracod assemblages and NMDS results, three intervals with relatively high salinity were identified: ~7,800–6,000 cal year BP (H1), ~4,500–3,500 cal year BP (H2), and ~1,100–0 cal year BP (H3) (Figure 5). Among these three intervals, H2 and H3 are consistent with two distinct millennial-scale episodes of weak lake hydrodynamic intensity spanning 4,700–3,500 cal year BP and 1,200–500 cal year BP based on the grain size of suspended lacustrine silt collected from the core BST04H (Xie et al., 2021), which are also supported by relatively low A/C ratio in the same core, meaning a smaller steppe component in regional vegetation and lower humidity (dry) (Huang et al., 2009; Xie et al., 2021; Figure 5). Even though hydrodynamic intensity was high during the H1interval, examination of pollen assemblages (A/C ratio and Ephedra percentage) suggests that the climate in the Bosten Lake area was relatively dry between 8,000 and 6,000 cal year BP (Huang et al., 2009; Figure 5). Xie et al. (2021) considered the increased hydrodynamic intensity during 8,200–6,000 cal year BP a result of higher early summer temperatures, which led to greater water supply due to the melting of snow and ice in the mountainous regions of the catchment area. Furthermore, X-ray fluorescence scanning data of a sediment core from Lake Xiaolongchi in the central Tianshan Mountains in ACA also showed three dry climate periods over the last 8,000 years, which could be correlated with H1, H2, and H3 in the Bosten Lake area (He et al., 2023; Figure 5). In summary, three periods (H1, H2, and H3) with relatively high salinity in lakes occurred during a dry climate in ACA.
Coincidentally, a global or near global event called the 4.2 ka event happened between 4300 and 3900 years BP during the H2 interval, which is also a time-stratigraphic marker horizon defining the base of the Meghalayan Stage/Age and the Upper/Late Holocene Subseries/Subepoch (Walker et al., 2018). The 4.2 ka event was characterized by extreme global drought and cooling that was hypothesized to be associated with collapse of the civilizations around the world, such as the Old Kingdom in Egypt, the Akkadian Empire in Mesopotamia, and the Liangzhu culture in the lower Yangtze River area (Gibbons, 1993; Li et al., 2018; Railsback et al., 2018; Walker et al., 2018; Wang et al., 2023). However, the underlying factors that triggered this event, as well as its broader implications and links to other climatic shifts on a regional to global scale, remain controversial (Walker et al., 2012). Even so, it is thought that the 4.2 ka event could be associated with the southward movement of the Inter-Tropical Convergence Zone (ITCZ) (Mayewski et al., 2004) and/or the cooling of North Atlantic surface waters (Bond et al., 1997). Additionally, in the Pacific region, tropical ‘deep’ waters cooled sufficiently to trigger the onset of the modern El Niño Southern Oscillation (ENSO) regime during the mid-Holocene (Gomez et al., 2004), which can suppress and weaken the Asian monsoon, leading to extensive drought conditions (Fisher et al., 2008; Fisher, 2011). However, it is uncertain whether the high salinity in Bosten Lake during the H2 interval was directly caused by the 4.2 ka event, even though warmer and drier conditions were prevalent in the region between 4200 and 2800 BP, as suggested by lake records (Jiang et al., 2022a).
Lake salinity changes are associated with the hydrological cycle, which is affected by climate change, resulting in changes in the frequency and intensity of precipitation events, evaporation rates, and snow accumulation and melting patterns (Sorg et al., 2012; Jeppesen et al., 2015; Rusuli et al., 2015; Zhou et al., 2015; Jiang et al., 2022b). Precipitation is the primary source of water supply for Bosten Lake (Yao et al., 2018), and the water vapor that contributes to precipitation in the studied area originates from distant water bodies, including the North Atlantic Ocean, Arctic Ocean, Mediterranean Sea, Black Sea, and Caspian Sea, and the mid-latitude westerlies transport this water vapor to ACA (Guan et al., 2019). There is a correlation between high lake salinity and low precipitation (Harris, 2009; Gu et al., 2015), which is thought to be caused by weaker westerlies in the Bosten Lake area. Accordingly, we correlated the studied ostracod assemblages with the dimensionless westerlies effect index (WEI) from the central Qinghai-Tibet Plateau (CQTP) (Figure 5; Chen et al., 2022). In the studied core (BLX-C), all three intervals (H1, H2, and H3) are correlated with lower WEI values, suggesting a westerly influence, and show an in-phase relationship with the EASM (Kaboth-Bahr et al., 2021) (Figure 5), despite the phase relationship between the westerly and Asian summer monsoon circulations exhibiting in-phase transitions on orbital time scales, out-of-phase transitions on suborbital time scales, and anti-phase transitions on millennium timescales (Li and Zhu, 2024). In ACA, there were more wet and dry events during the Holocene, and drought conditions throughout ACA were also considered to be associated with weaker westerlies causing less water vapor to be transported into the area (e.g. Jin et al., 2012; Xu et al., 2019). In a thorough review of nine Holocene paleoclimatic records in ACA, Liu et al. (2023) identified five wet events (around 6,500 cal year BP, 4,700 cal year BP, 3,300 cal year BP, 1,800 cal year BP, and 400 cal year BP) and five dry events (around 7,600 cal year BP, 5,300 cal year BP, 3,900 cal year BP, 2,600 cal year BP, and 1,100 cal year BP) and concluded that the main driving forces behind these abrupt wet events were decreased SSTs in the North Atlantic and negative phases of the North Atlantic Oscillation (NAO), resulting in strengthened mid-latitude cyclonic activity and increased delivery of water vapor to ACA via the westerlies. However, the studied three dry periods with relatively high salinity in lakes could be generally correlated with high SSTs in the North Atlantic but not the positive phases of NAO, which need further investigation.
Seven species and one indeterminate species belonging to six ostracod genera were identified in a 720-cm sediment core (BLX-1) from Bosten Lake in Xinjiang, northwestern China. The identified species were Candona candida, C. neglecta, C. sp., Cyprideis torosa, Darwinula stevensoni, Fabaeformiscandona hyalina, Herpetocypris helenae, and Limnocythere inopinata. Combining the ostracod and salinity records from core BLX-1 and adjacent cores in Bosten Lake, we discovered an alternation of ostracod assemblages associated with salinity changes. These salinity changes could indicate the dry and wet climate change and be correlated with westerlies variations during the past ~8,000 years. Higher salinity suggested dry climate in ACA and was correlated with stronger westerlies, and vice versa. We suggest that the intensified westerlies associated with high SSTs in the North Atlantic had an impact on precipitation patterns in the Bosten Lake region during the past 8,000 years, which ultimately led to changes in the ostracod assemblages in the area.
We thank Wong Pin Pin, Rachel for technical support.
Table S1, Taxonomic list of ostracods recovered from BLX-C; Table S2, Results of the robust linear regression analysis for abundance and simple linear regression analysis for NMDS1 examining relationships with hydrodynamic intensity, pollen A/C ratio, PC1, WEI, EASM, and North Atlantic SST. The F-statistic was calculated based on 1 and 62 degrees of freedom. SE = Standard Error; CI = Confidence Interval.
H.W., L.L. and M.Y. initiated the study. H.W. and M.Y. were primarily responsible for the taxonomic aspects. P.J.J. performed the comparative and analytical work. H.W., K.W. and P.J.J. collected data and prepared the figures. L.L. and K.W. collected data and contributed to the discussion. All authors contributed to the writing of the paper.
