INTRODUCTION
The effect of the relationship between the Asian monsoon (AM) and the mid-latitude westerlies on climate change in the Asian middle latitudes is an intricate yet crucial subject (Y. Wang et al., Reference Wang, Cheng, Edwards, He, Kong, An, Wu, Kelly and Dykoski2005; Chen et al., Reference Chen, Chen, Holmes, Boomer, Austin, Gates, Wang, Brooks and Zhang2010, Reference Chen, Xu, Chen, Birks, Liu, Zhang and Jin2015; P. Wang et al., Reference Wang, Wang, Cheng, Fasullo, Guo, Kiefer and Liu2017; G. Li et al., Reference Li, Wang, Zhang, Yan, Liu, Yang and Wang2022). Generally speaking, the AM and westerlies show different climate evolutionary patterns on long-term timescales as a result of alternative driving mechanisms (Nagashima et al., Reference Nagashima, Tada, Matsui, Irino, Tani and Toyoda2007; Chen et al., Reference Chen, Chen, Huang, Chen, Huang, Jin and Jia2019). The monsoon climate pattern in East Asia manifesting as the humid Early Holocene (EH) and drier LH is out of phase with the typical westerlies-dominated climate in arid Central Asia, which is characterized by dry EH and wet LH (Chen et al., Reference Chen, Yu, Yang, Ito, Wang, Madsen and Huang2008; An et al., Reference An, Colman, Zhou, Li, Brown, Jull and Cai2012; Wang and Feng, Reference Wang and Feng2013; Li et al., Reference Li, Han, Liu, Song and Wang2021; Gao et al., Reference Gao, Jiang, He, Hu, Shen and Zhang2023). Many studies in recent years suggested that the interaction between the westerlies and AM can be viewed as an explanation for the spatiotemporal difference in the precipitation pattern in the Asian middle latitudes (Nagashima et al., Reference Nagashima, Tada, Tani, Sun, Isozaki, Toyoda and Hasegawa2011; Chiang et al., Reference Chiang, Fung, Wu, Cai, Edman and Liu2015; Zhang et al., Reference Zhang, Jin, Lu, Park, Schneider and Latif2018; Li et al., Reference Li, Peng, Liu, Zhang, Ye, Han, Zhang, Xu and Li2020). On long-term timescales, the westerly jet path over East Asia is the main factor determining the location of the Asian summer monsoon rain belt (Sampe and Xie, Reference Sampe and Xie2010; Kong et al., Reference Kong, Swenson and Chiang2017; Herzschuh et al., Reference Herzschuh, Cao, Laepple, Dallmeyer, Telford, Ni and Chen2019). When earlier seasonal northward migration of the westerly jet axis occurs during the warm period, the Asian summer monsoon rainband shifts northward earlier, allowing more abundant precipitation in the northwestern margin of the Asian summer monsoon (Nagashima et al., Reference Nagashima, Tada and Toyoda2013). At the same time, despite weakening of the summer monsoon, the larger ice sheet and lower North Atlantic sea surface temperature during the cold period led to an increase in the meridional temperature gradient and southward migration of the westerlies, bringing more water vapor to the northwestern margin of the Asian summer monsoon (Lan et al., Reference Lan, Wang, Dong, Kang, Cheng, Zhou, Liu, Wang and Ma2021). Consequently, specific contributions and influencing mechanisms of the westerlies and AM to regional climate change remain controversial.
Climatic and environmental changes in the middle part of the Hexi Corridor, controlled by the westerlies and AM, are climatically sensitive and ideally located for paleoclimate research (Cheng et al., Reference Cheng, Chen and Zhang2013; Wang et al., Reference Wang, Li, Li and Cheng2013; Chen et al., Reference Chen, Wu, Chen, Zhou, Yu, Shen, Wang and Huang2016a). In the past few decades, a number of attempts have been made to investigate a range of environmental and climatic reconstructions in the Hexi Corridor and its surroundings based on various paleoclimatic archives, such as lake sediments (Shen et al., Reference Shen, Liu, Wang and Matsumoto2005; Zhao et al., Reference Zhao, Yu, Chen, Ito and Zhao2007; Chen et al., Reference Chen, Wu, Chen, Zhou, Yu, Shen, Wang and Huang2016a; Qiang et al., Reference Qiang, Song, Jin, Li, Liu, Zhang, Zhao and Chen2017), aeolian sediments (Lu et al., Reference Lu, Zhao, Mason, Yi, Zhao, Zhou, Ji, Swinehart and Wang2011; Liu et al., Reference Liu, Lai, Yu, Sun and Madsen2012; Sun et al., Reference Sun, Clemens, Morrill, Lin, Wang and An2012; Li et al., Reference Li, Peng, Liu, Zhang, Ye, Han, Zhang, Xu and Li2020), ice cores (Thompson et al., Reference Thompson, Mosley-Thompson, Wu and Xie1988; Yao and Thompson, Reference Yao and G.Thompson1992), and tree rings (Gou et al., Reference Gou, Deng, Gao, Chen, Cook, Yang and Zhang2015; Yang et al., Reference Yang, Qin, Bräuning, Osborn, Trouet, Ljungqvist and Esper2021). However, the debate continues about the climate evolutionary history due to nonuniform paleoclimate records from different positions (An et al., Reference An, Feng and Barton2006; Wang et al., Reference Wang, Liu and Herzschuh2010). Based on the processes and mechanisms of modern climate, the Asian summer monsoon plays a crucial role in climate change and water vapor transport in the Hexi Corridor (Fig. 1) (Li et al., Reference Li, Wang, Chen, Li, Zhou and Zhang2012). Therefore, whether the range of influence of the Asian summer monsoon has changed and how effective moisture transport by westerlies contributes to the middle part of the Hexi Corridor are vital issues in studying regional climate evolution in the northern margin of the QTP.
Figure 1. Circulation conditions and location of the study area (red star). The arrows represent the monthly mean 850 hPa (hectoPascals) wind field (m/s) in summer (JJA, June–July–August) during 1969–2018 (data from the NCEP/NCAR Global Reanalysis 1 dataset; Kalnay et al., Reference Kalnay, Kanamitsu, Kistler, Collins, Deaven, Gandin and Iredell1996).
Situated in the marginal zone of the Asian summer monsoon vapor transport, the Heihe River basin is a key experimental field for clarifying the history of interaction between the westerlies and AM (Fig. 1). Consequently, the Beihaizi (BHZ) paleolake section from the Heihe River basin in the middle part of the Hexi Corridor enables exploration of past climate change and its driving forces in the northern margin of the QTP (Fig. 2). In paleoclimatology, geochemical proxies (TOC [total organic carbon], TN [total nitrogen], C/N [percent total nitrogen/percent organic carbon ratio], δ13Corg [organic carbon isotopes], and δ18O [carbonate oxygen isotopes]), magnetic susceptibility, and grain size are widely used to indicate changes in past temperature, moisture conditions, vegetation status, etc. In this paper, we expound on a general framework for reconstructing climate changes over the last 5334 years based on the analysis of multiple proxies in the BHZ sediments. In addition, paleoclimate models from the Paleoclimate Modeling Intercomparison Project 3 (PMIP3) were selected to visually investigate the relationship between climate and environment evolution with the westerlies and AM. A better understanding of the interaction between different atmospheric circulation systems will be of great significance for characterizing future variability of the hydroclimate in climate-sensitive areas during increased global warming.
Figure 2. (a) Locations and distribution of the BHZ section (red star) and other lake records (red dots) in the adjacent area. The climatological northern boundary of the East Asian summer monsoon (black dashed line) during 1965–2014 is from Wang et al. (Reference Wang, Liu, Kim, Webster and Yim2012). (b) Satellite image showing the Jinta Basin, the water system of the Beidahe River, and key geographic features mentioned in the text. The Beidahe River used to flow into the Heihe River in the town of Dingxin, northeast of the Jinta Basin. However, after the completion of the Yuanyangchi Reservoir and the Jiefangcun Reservoir in the 1970s, the amount of downstream water dropped sharply, making it impossible for the Beidahe River to reach Dingxin.
REGIONAL SETTING
The BHZ section (40°13′N, 98°45′E), located in Xiba town, Jinta County, Gansu Province in northwestern China, was generated from the Beihaizi paleolake in the Jinta Basin in the middle part of the Hexi Corridor (Fig. 2a). The Jinta Basin is the terminal area of the Beidahe River (Taolai River), which originates from the Qilian Mountains in the northern QTP and flows through the alluvial plain in the middle reaches of the Heihe River (Zhang et al., Reference Zhang, Zhang, Zhao and Ma2011). The Jinta Basin is surrounded by the Beishan Mountains and Jinta Nanshan in the north and south, respectively, and is connected to the Huahai Basin in the west and the Badain Jaran Desert in the east (Fig. 2). The terrain of the basin is higher in the south and lower in the north, and gradually inclines from the southwest to the northeast. Many studies have established that Quaternary fluvial and lacustrine sediments were deposited in the paleolake basin in the northern Jinta Basin (Dodson et al., Reference Dodson, Li, Ji, Zhao, Zhou and Levchenko2009; X. Li et al., Reference Li, Ji, Dodson, Zhou, Zhao, Sun and Yang2010, Reference Li, Sun, Dodson, Ji, Zhao and Zhou2011, Reference Li, Lin, Yu, Wang, Zhou, Liu and Liu2013; Wang et al., Reference Wang, Li, Li and Zhu2011; Feng et al., Reference Feng, Sun, He, Gao, Liu, Wu and An2020). At present, the Beihaizi paleolake has completely dried up, and Beihaizi wetland park was built at the northern margin of the alluvial fan of the Beidahe River. Most of the lacustrine plains in the paleolake basin are occupied by Tamarix and have developed gypsiferous gray-brown desert soil, meadow soil, and swamp soil (Li, Reference Li1998; Wang et al., Reference Wang, Li, Li and Zhu2011). As an arid area in northwest China, the study area has a typical continental climate with average annual temperature of 8°C, average annual precipitation of 54.4–77.7 mm, and potential evaporation of 3000 mm/year. Desert vegetation, including Tamarix chinensis, Agriophyllum squarrosum, Apocynum venetum, and Achnatherum splendens, is widely distributed in the study area (Zhang et al., Reference Zhang, Alexander, Gou, Deslauriers, Fonti, Zhang and Pederson2020). According to an analysis of modern climatology, the Asian summer monsoon rarely reaches the sampling site compared to the westerlies (Fig. 1). As a result of the climate complexity in the monsoon marginal zone, the sensitive location enables sediment proxies of the BHZ section to record the interaction between the westerlies and AM.
MATERIALS AND METHODS
Sample collection
The BHZ sedimentary samples were collected from bottom to top with a sampling interval of 12.5 cm for 527–252 cm and 2 cm for 252–16 cm, respectively (Fig. S1; Table S2). Since the uppermost 16 cm of the BHZ section is modern soil, no sampling was performed. Ultimately, the BHZ section (section depth = 527 cm, sampling depth = 511 cm) yielded 140 sedimentary samples.
AMS14C dating
Terrestrial plant residue is usually viewed as one of the most reliable materials for 14C dating (Zhang et al., Reference Zhang, Ming, Lei, Zhang, Fan, Chang, Wünnemann and Hartmann2006). However, considering the lack of available terrestrial plant residue for dating, organic matter obtained from the bulk sediments is often used for 14C dating (Sun et al., Reference Sun, Wang, Zhou, Shen, Cheng, Xie and Wu2009; Wen et al., Reference Wen, Xiao, Chang, Zhai, Xu, Li, Itoh and Lomtatidze2010; An et al., Reference An, Zhao, Tao, Lv, Dong, Li, Jin and Wang2011). Consequently, the bulk organic matter of 58 samples in the BHZ section was selected for Accelerator Mass Spectrometry Radiocarbon Dating (AMS14C) (Table S1). The OxCal v4.4.2 program (Bronk Ramsey, Reference Bronk Ramsey2009; Bronk Ramsey and Lee, Reference Bronk Ramsey and Lee2013) and IntCal20 atmospheric curve (Reimer et al., Reference Reimer, Austin, Bard, Bayliss, Blackwell, Bronk Ramsey and Butzin2020) provided calibration of radiocarbon 14C dates (0 BP = AD 1950). The AMS14C ages of the BHZ section were all measured in the Laboratory of Scientific Archaeology and Preservation of Cultural Relics, School of Archaeology and Museology, Peking University, China.
Proxies used
Organic carbon isotopes (δ13Corg), carbonate oxygen isotopes (δ18O), grain size (GS), total organic carbon (TOC), total nitrogen (TN), the percent total nitrogen/percent organic carbon (C/N) ratio, and magnetic susceptibility (MS) of the BHZ section were measured (Table S2). Eighty-one lake sediment samples were used for δ13Corg, δ18O, TOC, and TN testing. δ13Corg (‰) and δ18O (‰) values were measured, respectively, by the MAT253plus+FlashEA and MAT253plus+GasBench in the Beijing Createch Testing Technology Co. Ltd, Beijing, China. TOC (%) and TN (%) were measured by Vario-III elemental analyzer with an analytical error of <0.1% in the Analysis and Testing Center of Lanzhou University, Lanzhou, China. MS and GS analyses of 140 lake sediment samples were completed in the Key Laboratory of Western China's Environmental Systems (Ministry of Education), Lanzhou, China. MS was determined using the Bartington MS2 logger. In this study, frequency magnetic susceptibility percentages (χfd %) were used as MS parameter to indicate climate changes (Ji and Xia, Reference Ji and Xia2007). Grain-size distribution was measured using a Malvern Mastersizer 2000 particle analyzer, which has a measurement range of 0.02–2000 μm and a repeated error of <3%. Subsequently, we calculated three grain-size parameters, including mean GS (%), median GS (%), and modal GS (%). In addition, the division of sedimentary facies based on grain size, such as clay (<4 μm), silt (4–63 μm), and sand (>63 μm) can provide a good understanding of the sedimentary environment (Terry and Goff, Reference Terry and Goff2014). Nevertheless, grain-size data are still problematic because of the difficulty in differentiating sediment-transport mechanisms merely by sedimentary facies and grain size in finely laminated lacustrine deposits (X. Liu et al., Reference Liu, Zhang, Jiao and Mischke2016). Therefore, we also used the end-member modeling (EMM) method to more rationally decompose the GS distribution data into sensitive grain-size components of terrestrial, fluvial, and aeolian sediments, and to obtain valuable information on geological processes and paleoenvironments (Kranck et al., Reference Kranck, Smith and Milligan1996; Prins et al., Reference Prins, Bouwer, Beets, Troelstra, Weltje, Kruk, Kuijpers and Vroon2002; Weltje and Prins, Reference Weltje and Prins2003; Meyer et al., Reference Meyer, Davies, Vogt, Kuhlmann and Stuut2013; Dietze et al., 2014; Greig and David, Reference Greig and David2015).
Paleoclimate simulations
The equilibrium “time-slice” simulations for 6000 cal yr BP (MH) and pre-industrial (PI) from the PMIP3- (Palaeoclimate Modelling Intercomparison Project Phase 3) CMIP5 (Coupled Model Intercomparison Project Phase 5) were synthesized to understand the role of the atmosphere circulation system and analyze the dynamic mechanism of climate conditions in the BHZ section during the MH and LH. We applied a multi-model ensemble simulation from 11 models: BCC-CSM1-1 (Beijing Climate Center Climate System Model), CNRM-CM5 (Centre National de Recherches Météorologiques-Climate Model Version 5), CCSM4 (Community Climate System Model Version 4), CSIRO-Mk3-6-0 (Commonwealth Scientific and Industrial Research Organisation), GISS-E2-R (Goddard Institute for Space Studies), MIROC-ESM (Model for Interdisciplinary Research on Climate-Earth System Model), FGOALS-s2 (Flexible Global Ocean-Atmosphere-Land system model, Spectral Version 2), FGOALS-g2 (Flexible Global Ocean-Atmosphere-Land system model, Grid-point Version 2), IPSL-CM5A-LR (Institut Pierre Simon Laplace-Climate Model 5A-Low Resolution), MPI-ESM-P (Max Planck Institute Earth System Model-Paleoclimate), and MRI-CGCM3 (Meteorological Research Institute Coupled Global Climate Model Version 3). The variables used are precipitation, evaporation, meridional winds, and zonal wind, which are available at https://esgf-node.llnl.gov/search/esgf-llnl/. The name, affiliation, country, resolution, and references of the models are shown in Table 1. In view of the different horizontal resolution of the output data, we used the bilinear interpolation method to interpolate the output data of all models to a resolution of 1° × 1° to facilitate model aggregation.
Table 1. Basic information about climate models from PMIP3-CMIP5 used in this study.
* pr = precipitation; evp = evaporation; ua = zonal wind; va = meridional wind
RESULTS
Lithological description
Based on observations of sediment color and texture (Figs. S1, 3), the lithology characteristics from top to bottom can be approximately divided into eight parts: (1) 16–125 cm, light brown silt, large parts of which could be covered by wind-blown loess or aeolian sand; (2) 125–180 cm, light brown clay; (3) 180–230 cm, dark green clay; (4) 230–240 cm, light gray sand; (5) 240–300 cm, dark green silty clay; (6) 300–400 cm, gray clayey silt; (7) 400–460 cm, gray silty clay; (8) 460–540 cm, gray-black silty clay with humic mud.
Chronology
In arid and semi-arid regions, the reservoir effect may cause AMS 14C dating results of bulk organic matter in lacustrine sediments to be older than the real ages (Liu et al., Reference Liu, Dong, Yang, Herzschuh, Zhang, Stuut and Wang2009; Long et al., Reference Long, Lai, Wang and Zhang2011). However, more-reliable dating materials, such as plant residues or charcoal, cannot be found in the BHZ section, resulting in the inability to obtain an accurate age of the reservoir effect. Additionally, due to the lack of detailed investigation on the embankment of Beihaizi paleolake, it is impossible to quantify the age of drying of the Beihaizi paleolake. Therefore, surface sediments of the Beihaizi paleolake cannot be used to determine the modern reservoir age of the paleolake. Hence, we set the reservoir age as 2000 yr and preliminarily removed the reservoir effect from samples below ~125 cm, mainly in light of the median value of existing reservoir ages, which, in adjacent regions, is ca. 2500 yr in Huahai paleolake (Wang et al., Reference Wang, Li, Li and Cheng2013), ca. 2000 yr in eastern Juyan paleolake (Hartmann and Wünnemann, Reference Hartmann and Wünnemann2009), and ca. 1080 yr in lakes of the Badain Jaran Desert (Hofmann and Geyh, Reference Hofmann and Geyh1998; Yang et al., Reference Yang, Ma, Dong, Zhu, Xu, Ma and Liu2010). After removing the reservoir effect, the AMS 14C ages were calibrated (0 BP = AD 1950) using the OxCal v4.4.2 program and IntCal20 atmospheric curve (Table S1).
Figure 3 shows the distributions of calibrated 14C ages and the stratigraphic characteristics of the BHZ section. Based on the integrated analysis of the chronology, we determined that the age can be divided into three parts: the upper, middle, and bottom layers. The ages of the upper layer are in the LH, the ages of the middle layer are disordered, and the ages of the bottom layer basically belong to the MH except for the two abnormally old ages. Meanwhile, the slight fluctuations of TOC and mean GS in the upper and bottom layers indicate relatively stable deposition conditions, which suggests that the dates of these two layers are reliable and are likely less contaminated. However, the TOC and mean GS in the middle layer vary greatly, which is related to the reworking effect, the input of “old carbon,” and deposition instability (Sun et al., Reference Sun, Wang, Liu and Clemens2010). To this end, we first eliminated the abnormally old ages at 402–389.5 cm, 352–238 cm, and 182–150 cm in the middle layer (Table S1). The abnormally young age at 230 cm was also discarded because of its irregularity in the overall distribution of the chronology results and its distinct inconsistency with establishment of the Holocene age.
Figure 3. Age-depth model, lithology, and variation of mean GS and TOC with depth of the BHZ section.
In this study, the traditional mathematical fitting method is used to establish the age-depth model (X. Liu et al., Reference Liu, Zhang, Jiao and Mischke2016; Li et al., Reference Li, Peng, Liu, Zhang, Ye, Han, Zhang, Xu and Li2020). The logarithmic curve was finalized as the age-depth model of 32 dates due to having the highest correlation after eliminating anomalous ages (y = 1200.2•ln(x) − 2187.3) (Y. Zhao et al., Reference Zhao, Yu, Chen, Ito and Zhao2007; C. Zhao et al., Reference Zhao, Yu, Zhao, Ito, Kodama and Chen2010; Wu et al., Reference Wu, Zhou, Zhang, Chen, Li, Wang and Chen2020) (Fig. 3), which provides a continuous sedimentary record since 5334 cal yr BP. The chronology conducted by calibrated age-depth model covers ca. 1281–5334 cal yr BP.
Sedimentary environment based on GS and MS
We analyzed the variations of six grain-size proxies with depth (Fig. 4a). The average median GS, mean GS, modal GS, clay, silt, and sand are 56.03 μm, 37.69 μm, 74.93 μm, 21.48%, 52.09%, and 26.43%, respectively. The GS distributions in the BHZ section are mainly composed of silt fractions, with contribution percentages of 14.68–81.81%. The overall variations of four grain-size proxies (median GS, mean GS, modal GS, and sand) exhibit generally similar trends, which is in contrast to two finer-grained fractions (clay and silt). In addition, there is an obvious shift in six grain-size proxies at ~230 cm and ~125 cm. The MS in the BHZ sediments exhibits significant fluctuations (Fig. 4a). The silt- and clay-dominated sediments at 230–125 cm have a high MS value as a whole, which indicates that high MS corresponds to the finer GS with magnetic minerals.
Figure 4. Results of GS and MS analyses. (a) Variations in mean GS, median GS, mode GS, clay, silt, sand, and MS from the BHZ section. Dashed lines indicate obvious grain-size shifts. (b, c) EMM results of the BHZ sediments: (b) grain-size distributions of four EMs; (c) abundance variations (%) of four EMs in a synthetic data set of the BHZ sediments.
As shown in Figure 4b, EMM analysis in the BHZ section yields an optimal model with four EMs characterized by a single peak mode. The peak of EM1 at 1–10 μm indicates deep lacustrine deposits with fine grain size. In contrast, EM4 has a narrow peak at 100–400 μm, exhibiting stronger transport capacity and a high-energy deposition environment. Both EM2 and EM3 have a broad mode in the silt component, characterized by peaks of 10–30 μm in the fine-grain part and of 50–100 μm in the coarse-grain part, respectively. The endmember sequences generated by the abundance variations of all sediment samples at four EMs are shown in Figure 4c. The boundary of sedimentary facies at ~230 cm and ~125 cm is still very obvious, which is consistent with the variation in the six grain-size proxies (Fig. 4a). The variation in coarse grain size, which is dominant in EM4, has a similar trend to the mean GS, median GS, mode GS, and sand content. The deep lacustrine endmember EM1 primarily highlights the lake-dominated sedimentary environment at ~230–125 cm and 527–400 cm.
Combined GS proxies, MS, and EMM analysis show that the sedimentary environments of the BHZ section can be divided into four intervals: (1) aeolian deposits (16–125 cm) with an increasing trend of coarse-grained composition and decreasing trend of clay and silt, among which the sand content slightly decreased and the silt content slightly increased in fluctuation at 16–75 cm; (2) increased clay and silt and decreased coarse-grained composition (125–230 cm), belonging to lacustrine deposits and containing organic matter and carbon; (3) lakeside deposits with dominant sand content (230–240 cm); and (4) relatively small-amplitude variations (240–527 cm) with slightly increased coarse-grained composition and decreased fine-grained composition and clay, from deep lacustrine at 400–527 cm to shallow lacustrine deposit at 240–400 cm.
Geochemical and isotopic results
The average values of TOC, TN, C/N, δ13Corg, and δ18O in the BHZ sediments are 1.35%, 0.04%, 36.29%, −23.6‰, and −9.33‰, respectively (Fig. 5). The C/N ratio is 16.80–80.67% and is mostly >20%, indicating that the organic matter in the BHZ section was mainly contributed by terrestrial organisms. Meanwhile, the range of δ13Corg is between −25‰ and −22‰, most of which is within the δ13Corg distribution range of C3 plants (Farquhar et al., Reference Farquhar, Ehleringer and Hubrick1989; Bowen, Reference Bowen1991). As a result, it can be accurately determined, through the composition of the C/N and δ13Corg, that the organic matter in the BHZ section mainly comes from C3 land plants (Fig. 5c). By analyzing the scatter correlation diagrams of TOC, C/N, and δ13Corg, the higher TOC values are accompanied by the higher C/N values and the negative δ13Corg values (Fig. 5a, b). Figure 5d shows that all the proxies shift significantly at ~230 cm and are characterized by high TOC, TN, and C/N values, negative δ13Corg values, and positive δ18O values at 230–125 cm. At the same time, TOC, TN, and negative bias of δ13Corg show a slightly increasing trend above ~75 cm.
Figure 5. (a–c) Linear regressions (a) between TOC and C/N, (b) TOC and δ13Corg, and (c) δ13Corg and C/N; R and P-values represent the value of each linear regression and the level of significance, respectively. (d) Variations in TOC, TN, C/N, δ13Corg, and δ18Ocarb with depth.
DISCUSSION
Proxy Premises
The GS distribution of lake sediments can provide direct information on the lake level, hydrodynamic conditions, and sedimentary environment (Chen et al., Reference Chen, An and Head1999; Peng et al., Reference Peng, Xiao, Nakamura, Liu and Inouchi2005; Xiao et al., Reference Xiao, Fan, Zhou, Zhai, Wen and Qin2013). Generally, coarse GS indicates a dry climate in which the lake is shrinking and shallower in arid and semi-arid regions on long-term timescales (C. Liu et al., Reference Liu, Vandenberghe, An, Li, Jin, Dong and Sun2016; X. Liu et al., Reference Liu, Vandenberghe, An, Li, Jin, Dong and Sun2016; Hu et al., Reference Hu, Sha, Ma, Kong and Wang2017; Wu et al., Reference Wu, Zhou, Zhang, Chen, Li, Wang and Chen2020).
The MS in lake sediments documents the sizes and types of magnetic minerals (Wu, Reference Wu1993). Recently, comprehensive analyses of surface sediments in the Qilian Mountains confirmed that high MS values are usually enriched in fine-grained fractions and are positively correlated with regional precipitation (Y. Li et al., Reference Li, Peng, Hao, Zhou and Li2022; Peng et al., Reference Peng, Li, Liu, Han, Zhang, Feng, Chen, Ye and Zhang2022). Some paleoclimate studies also claimed that high MS values are positively related to silt and clay content and indicate a humid environment (Zhang et al., Reference Zhang, We and Wang1998; Zhao et al., Reference Zhao, Wang, Li, Cheng, Li. and Li2005).
The TOC in lake sediments, usually an indicator of vegetation coverage and primary productivity in watersheds and lakes, is widely applied to paleoclimate research (Aravena et al., Reference Aravena, Warner, MacDonald and Hanf1992; Zhong et al., Reference Zhong, Xue, Cao, Zheng, Ma, Jun, Cai, Zeng and Liu2010). In arid and semi-arid regions, moisture conditions are the main limiting factors for plant growth, therefore, a humid climate usually corresponds to higher values of TOC (Y. Li et al., Reference Li, Wang, Li and Cheng2011).
C/N values reflect the ratio of aquatic to terrestrial organic matter and can be used to determine the source of organic matter in lake sediments (Meyers and Lallier-Vergès, Reference Meyers and Lallier-Vergès1999). During intervals of low lake levels, exposed lakeshore shoal zones are beneficial to terrestrial plant growth, thus increasing the contribution of terrigenous organic matter in lake sediments, which will lead to higher C/N ratios (Wu et al., Reference Wu, Zhou, Chen, Yu, Zhang and Sun2015).
Organic matter δ13Corg in lake sediments is mainly derived from terrestrial plants and aquatic plants, which can be identified by composition of the δ13Corg and C/N. Meanwhile, photosynthetic types of C3-like and C4-like carbon fixation also exist in aquatic plants (Liu et al., Reference Liu, Li, An, Xu and Zhang2013). Relevant reports of modern plants and surface sediments pointed out that climate elements (i.e., temperature and precipitation) are dominant factors controlling C3/C4 relative abundance in arid and semi-arid regions (Rao et al., Reference Rao, Chen, Zhang, Xu, Xue and Zhang2012, Reference Rao, Guo, Cao, Shi, Jiang and Li2017; Zhao et al., Reference Zhao, Wu, Fang and Yang2017; Y. Li et al., Reference Li, Peng, Liu, Zhang, Ye, Han, Zhang, Xu and Li2020, Reference Li, Peng, Hao, Zhou and Li2022).
The carbonate δ18O in lake sediments is mainly controlled by the δ18O composition of lake water during carbonate precipitation. In closed lakes of arid and semi-arid regions, the lake water δ18O is primarily affected by the precipitation/evaporation (P/E) or inflow/evaporation (I/E) ratios, so δ18O records in lacustrine carbonate sediments can provide references to past hydrological and dry/wet changes (Lister et al., Reference Lister, Kelts, Zao, Yu and Niessen1991; Qiang et al., Reference Qiang, Chen, Zhang, Gao and Zhou2005; Yu et al., Reference Yu, Ricketts and Colman2009).
Evidence of lake and climate evolution in the BHZ section
The results of all proxies in the BHZ section show that there is a clear transition at ~230 cm, which is represented by a decrease in sand content (Fig. 4a) and an increase in organic matter (Fig. 5d) and is in line with the characteristics of lacustrine deposits (Berner, Reference Berner1981). Combined with the age-depth model (Fig. 3), it is apparent that the age at 230–125 cm is approximately late MH (ca. 4400 to ca. 3500 cal yr BP). The change with depth of all climate proxies from the BHZ section is shown in Figure 6, reconstructing lake evolution since 5334 cal yr BP. TOC, C/N, MS, δ18O, and clay fluctuate sharply in the late MH. At about 4400–3500 cal yr BP, compared with overlying horizons, the BHZ section has negative δ13Corg, positive δ18O, and high clay, TOC, C/N, and MS. Meanwhile, TOC, MS, and negative bias of δ13Corg and δ18O show a slightly increasing trend since the late LH.
Figure 6. (a–f) Comparison of climate proxies (δ18O, δ13Corg, C/N, TOC, Clay, MS) from the BHZ section. The black lines and purple shading indicate the running average and error bar of climate proxies, respectively. Green shading represents the remarkably humid condition; arrows indicate wetting trends in the LH.
Based on our climatic/environmental interpretations of paleoclimate proxies, we can reconstruct climate evolutionary history since 5334 cal yr BP. During ca. 4400 to ca. 3500 cal yr BP, the clay and silt content in the BHZ section increased while the sand content decreased and the median GS is small (Fig. 6e), indicating weak hydrodynamics under high lake levels and humid environments in the late MH. Meanwhile, high MS values in the late MH confirm the development of fine grain size and more magnetic minerals under humid climate conditions in the BHZ section (Fig. 6f). Under humid climate conditions, the high lake level resulted in weak hydrodynamics, which is conducive to the development of fine grain size and more magnetic minerals (Hu et al., Reference Hu, Sha, Ma, Kong and Wang2017). TOC values are relatively high in the late MH, suggesting higher primary productivity within the lake basin in the context of a humid climate (Fig. 6b, d). Compared to 5334 to ca. 4400 cal yr BP, low C/N values in the late MH may indicate that a rising lake level was beneficial to the growth of aquatic organisms, thereby increasing the proportion of aquatic plants in the organic matter of lake sediments (Fig. 6c) (Lan et al., Reference Lan, Xu, Liu, Sheng, Zhao and Yu2013; Wu et al., Reference Wu, Zhou, Chen, Yu, Zhang and Sun2015). Sedimentary δ13Corg data in the BHZ section are mostly within the range of C3 plants (Fig. 6b). Many previous studies showed that the δ13Corg value of terrestrial C3 plants is driven by effective moisture and is more significantly correlated with precipitation than with temperature (Zheng and Shangguan, Reference Zheng and Shangguan2007; Kohn, Reference Kohn2010; Zhou et al., Reference Zhou, Li, Zhang and Wang2013). Our previous results of surface sediments in the Qilian Mountains demonstrated that the δ13Corg of C3 plants is more negative with increasing precipitation (Li et al., Reference Li, Peng, Liu, Zhang, Ye, Han, Zhang, Xu and Li2020). In addition, the δ13Corg values of C3-like plants, such as Cladophora, in lake sediments become more negative with increasing water depth (Liu et al., Reference Liu, Li, An, Xu and Zhang2013). Thus, we suggest that negative δ13Corg values indicate a relatively humid climate in Beihaizi paleolake, specifically manifesting as increased aquatic C3 plants with high lake levels in the late MH and increase of terrestrial C3 plants caused by increase in effective moisture since the late LH (Fig. 6b). Given the positive relationship between δ18O and evaporation intensity (Horton et al., Reference Horton, Defliese, Tripati and Oze2016), the positive δ18O in the MH reflects enhanced evaporation by increased insolation and high air temperature (Kaufman et al., Reference Kaufman, McKay, Routson, Erb, Dätwyler, Sommer, Heiri and Davis2020) (Fig. 6a), which raises atmospheric humidity to enhance regional precipitation. Ultimately, high regional precipitation, indicated by GS, MS, and geochemical proxies, may have offset evaporative losses from the lake surfaces, resulting in a climate and environment with higher effective humidity (high P/E or I/E ratio) in the late MH (Herzschuh et al., Reference Herzschuh, Borkowski, Schewe, Mischke and Tian2014). With the temperature decline since the MH, the δ18O depletion in the late LH probably mirrors a positive moisture balance (i.e., a high P/E or I/E ratio as the result of weakened evaporation) (Qiang et al., Reference Qiang, Song, Jin, Li, Liu, Zhang, Zhao and Chen2017).
The synthesis of all paleoclimate proxies confirms a humid late MH in the BHZ section. At the same time, although the grain size of lake sediments has not become significantly finer since the late LH, TOC, MS, δ13Corg, and δ18O proxies indicate that the surrounding vegetation environment improved and the organic matter content in the sediments increased. We argue that despite the overall transition from a lacustrine sedimentary environment in the MH to an aeolian sedimentary environment in the LH, there is still a slight wetting trend during the late LH in the BHZ section.
Paleoclimate ensemble simulation of precipitation, evaporation, and corresponding effective moisture can give more intuitive evidence. Precipitation and evaporation during the MH are higher than during the PI in the BHZ section, and the magnitude of precipitation is greater (Fig. 7a, b). This further corroborates the results of proxy reconstruction (i.e., high evaporation represented by δ18O depletion and high precipitation indicated by GS, MS, and geochemical proxies in the BHZ section). To clarify the moisture difference, we calculated the effective moisture represented by precipitation minus evaporation (P−E) (Fig. 7c). The result indicates that effective moisture during the MH in the BHZ section was higher than during the PI, which corresponds to the high P/E or I/E ratio.
Figure 7. Multi-model ensemble results from PMIP3-CMIP5. (a–c) Difference of annual mean precipitation (mm/month), evaporation (mm/month), and effective moisture (mm/month) between the MH and PI, respectively. The red dot represents the BHZ section.
Possible causes of regional climate evolution
Changes in atmospheric circulation patterns are important factors in affecting regional climate evolution and hydrological conditions. The climate and environmental records from the east part of the Hexi Corridor generally show monsoonal characteristics: a humid climate in the EH and MH, and gradually drying climate in the LH (Liu et al., Reference Liu, Shen, Wang, Wang and Liu2007; Qiang et al., Reference Qiang, Song, Chen, Li, Liu and Wang2013). However, the middle part of the Hexi Corridor, located in the monsoon-westerly transition zone, presents a complex Holocene climatic change pattern that is not the same as either the monsoon-affected region or the westerlies-affected region (Zhao et al., Reference Zhao, Yu, Chen, Ito and Zhao2007; Yan and Wünnemann, Reference Yan and Wünnemann2014; Li et al., Reference Li, Peng, Liu, Zhang, Ye, Han, Zhang, Xu and Li2020). Based on the paleoclimate ensemble simulation, we analyzed the climate regime anomalies between MH and PI to visually investigate the relationship between climate evolution in the BHZ section from the middle part of the Hexi Corridor and westerlies/AM. The southerly wind in the averaged and differential field at 850 hPa (hectoPascals) means that the Asian summer monsoon in the MH is more intense than that in the PI (Fig. 8a, b), which is consistent with the reconstruction of the East Asian summer monsoon from speleothem δ18O records in south China (Yuan et al., Reference Yuan, Cheng, Edwards, Dykoski, Kelly, Zhang and Qing2004; Wang et al., Reference Wang, Cheng, Edwards, An, Wu, Shen and Dorale2001). Meanwhile, the increase in summer precipitation also indicates the remarkable contribution of intensified Asian summer monsoon to local precipitation in the BHZ section, further causing the MH humid climate (Fig. 8b). In comparison to the averaged wind fields at 200 hPa (Fig. 8c), the same wind direction and increasing winter precipitation in the winter differential field indicate that the westerlies strengthened in the winter during the PI compared to the MH (Fig. 8d). In addition, the differential wind field of the Asian winter monsoon between the PI and MH, unlike the averaged wind field, indicates a weaker Asian winter monsoon during the PI (Fig. 8e, f). In the LH, weakening of the Asian winter monsoon and then the increase in the winter water vapor supplement (Zhang et al., Reference Zhang, Liu, Chen, Fu, Xie and Chen2022), caused by the increase in winter insolation (Berger and Loutre, Reference Berger and Loutre1991), favored the improvement of vegetation growth and the preservation of organic matter (Xiao et al. Reference Xiao, Nakamura, Lu and Zhang2002; Peng et al., Reference Peng, Li, Liu, Han, Zhang, Feng, Chen, Ye and Zhang2022). Moreover, the increased insolation gradient was conducive to the gradual strengthening of westerlies in the LH, thereby increasing moisture transport to the BHZ section and the middle part of the Hexi Corridor (Jin et al., Reference Jin, Chen, Morrill, Otto-Bliesner and Rosenbloom2012; Zhang et al., Reference Zhang, Jin, Huang and Chen2016). Consequently, we suggest that the slight wetting trend in the BHZ section since the late LH results from the intensification of westerlies and the weakening of the Asian winter monsoon.
Figure 8. Multi-model ensemble results from PMIP3-CMIP5. Averaged wind field (m/s) at 850 hPa (a, e) and 200 hPa (c) isobaric in summer and winter during the MH, and the differential wind field at 850 hPa (b, f) and 200 hPa (d) isobaric in summer and winter between the MH and PI. Prep represents the precipitation, and the scale and shade indicate the difference of precipitation (mm/month) between PI and MH. JJA = June–July–August; DJF = December–January–February.
High-resolution and precisely dated speleothem and lake records from south and north China, the typical East Asian summer monsoon region, indicated that a strong summer monsoon and a humid climate occurred during the Early and Middle Holocene, and a weakened summer monsoon and a drier climate prevailed during the Late Holocene (Xiao et al., Reference Xiao, Wu, Si, Liang, Nakamura, Liu and Inouchi2006; Cosford et al., Reference Cosford, Qing, Eglington, Mattey, Yuan, Zhang and Cheng2008; Chen et al., Reference Chen, Xu, Chen, Birks, Liu, Zhang and Jin2015) (Fig. 9c–e). In the area influenced by the westerlies, most of the paleoclimate records documented a dry EH to a wetter MH and a moderately humid LH (Fig. 9f–h) (Chen et al., Reference Chen, Yu, Yang, Ito, Wang, Madsen and Huang2008, 2016b; Liu et al., Reference Liu, Herzschuh, Shen, Jiang and Xiao2008; Wolff et al., Reference Wolff, Plessen, Dudashvilli, Breitenbach, Cheng, Edwards and Strecker2017), which is out of phase with records in AM regions. Hence, regional climate evolution and hydrological conditions on millennial timescales could differentiate between the effects of the AM and the westerlies. Figure 9 illustrates the similarity between the Asian summer monsoon pattern (Fig. 9c–e) and our records (Fig. 9a, b), which all indicate that the Asian summer monsoon induced by summer insolation controlled MH climate changes (Liu et al., Reference Liu, Shen, Wang, Wang and Liu2007). Additionally, the westerlies-controlled proxy records (Fig. 9f–h) indicate that precipitation has increased gradually since the LH and generally is correlated with the slight wetting trend in the BHZ section.
Figure 9. Intersite comparisons of regional paleoclimate records. (a, b) δ13Corg and clay content, respectively, in the BHZ section of the Beihaizi paleolake in this study; (c) TOC content in Daihai Lake (Xiao et al., Reference Xiao, Wu, Si, Liang, Nakamura, Liu and Inouchi2006); (d) stalagmite δ18O in Lianhua Cave, eastern China (Cosford et al., Reference Cosford, Qing, Eglington, Mattey, Yuan, Zhang and Cheng2008); (e) pollen-based quantitative precipitation reconstruction from Gonghai Lake, northern China (Chen et al., Reference Chen, Xu, Chen, Birks, Liu, Zhang and Jin2015); (f) stalagmite δ18O in Uluu Cave, Central Asia (Wolff et al., Reference Wolff, Plessen, Dudashvilli, Breitenbach, Cheng, Edwards and Strecker2017); (g) χARM/SIRM ratio in the Xinjiang Loess (Chen et al., Reference Chen, Jia, Chen, Li, Zhang, Xie, Xia, Huang and An2016b); (h) lake level change of Wulungu Lake (Liu et al., Reference Liu, Herzschuh, Shen, Jiang and Xiao2008). ARM = anhysteretic remanent magnetization; SIRM = saturation isothermal remanent magnetization.
Comparison of paleoclimate records between the BHZ section and other sites in the adjacent region
The Holocene climate evolution history in the Hexi Corridor and its surroundings has been widely reported and many high-resolution records from adjacent areas have been presented. Multivariate statistics from the terminal lake of Heihe River–the eastern Juyan paleolake indicate a climate optimum at 5400–4000 cal yr BP (Herzschuh et al., Reference Herzschuh, Tarasov, Wunnemann and Hartmann2004; Hartmann and Wünnemann, Reference Hartmann and Wünnemann2009), which is essentially coincident with our results of the late MH humid climate (ca. 4400 to ca. 3500 cal yr BP). The interpretation of climate history in the Badain Jaran Desert suggested that the wet MH climate prevailed in the entire Badain Jaran Desert (Yang et al., Reference Yang, Scuderi, Paillou, Liu, Li and Ren2011). In addition, the humid climate period (ca. 4400 to ca. 3500 cal yr BP) in the BHZ section is coincident with the 4.2 ka events. Holocene climatic records of the East Asian summer monsoon indicated that the 4.2 ka event was characterized by cold, dry conditions in China with less monsoonal moisture reaching northern China (Wang et al., Reference Wang, Cheng, Edwards, He, Kong, An, Wu, Kelly and Dykoski2005; Tan et al., Reference Tan, Cai, Cheng, Edwards, Gao, Xu, Zhang and An2018). The climatic conditions in the BHZ section during the time were, however, different from previous views. In fact, we argue that the relatively low resolution compared to the stalagmite records means that climate reconstruction in the BHZ section is not suitable for accurately indicating 4.2 ka events. The synthesis of evidence from lake sediments in arid Central Asia indicates that the moisture evolution pattern is characterized by a moderately wet LH, providing a reference for the slight wetting trend in the late LH in the BHZ section (Chen et al., Reference Chen, Yu, Yang, Ito, Wang, Madsen and Huang2008). Collectively, although studies presented in this section brought up contradictory ideas about Holocene climate evolution, the humid climate in the MH and LH also is similar to the published records found in northern and western China (Zhou et al., Reference Zhou, Head and Deng2001).
The difference in climate interpretation is a function of the complexity of Holocene climate change near the northwest margin of the Asian summer monsoon (Zhao et al., Reference Li, Liu, Zhao, Ji and Zhou2007). The results of palynological reconstruction from the Dunde ice core implied that the enhanced Asian summer monsoon may have extended northward beyond the current northernmost edge to the Dunde ice core and the westernmost part of the Tibetan Plateau, leading to the high palynological concentration in the Holocene (Liu et al, Reference Liu, Yao and Thompson1998). The comprehensive analysis of carbonate oxygen isotope records suggested a significant influence of the winter monsoon and westerlies on climate and environmental change in Sugan Lake over the past 2700 years (Zhou, Reference Zhou2007).
Considering the factors and mechanisms controlling the humid climate in the MH in the eastern Juyan paleolake, several studies assumed that the interplay of the East Asian summer monsoon and the westerly waves resulted in increased runoff, refilling of the aquifer, and the increase of lake-internal organic carbon production in a lacustrine environment (An et al., Reference An, Porter, Kutzbach, Wu, Wang, Liu, Li and Zhou2000; Wünnemann et al., Reference Wünnemann, Hartmann, Janssen, Zhang, Madsen, Chen and Gao2007; Hartmann and Wünnemann, Reference Hartmann and Wünnemann2009). Previous studies in the Badain Jaran Desert have proposed that the periodic change of westerlies and the AM was an essential factor in controlling the humid period (Yang et al., Reference Yang, Liu and Xiao2003). Further investigation concluded that the strengthened East Asian summer monsoon triggered wetter climate in the Badain Jaran Desert in the Holocene, which was linked with increasing insolation (Yang et al., Reference Yang, Ma, Dong, Zhu, Xu, Ma and Liu2010). The climate simulations of Feng and Yang (Reference Feng and Yang2019) demonstrated that the Asian summer monsoon brought abundant moisture to the Alashan Sand seas and the eastern Beihaizi paleolake in the period 8.2–4.2 ka BP, which caused high precipitation and the humid climate. Moreover, the Holocene climate change revealed by elemental geochemical records of Tiaohu Lake in the northwestern Jinta Basin may be jointly affected by the AM and westerlies (X. Li et al., Reference Liu, Li, An, Xu and Zhang2013). Reconstructed effective moisture changes in Huahai Lake near the BHZ section were influenced by the AM, westerlies, and evaporation (Wang et al., Reference Wang, Li, Li and Cheng2013). The significant retreat of lake level and the lower deposition rate in Yanchi Lake since the MH indicated the monsoon effects on the northwest margin of the Asian summer monsoon (Y. Li et al., Reference Li, Wang, Li, Zhou and Zhang2013). The Holocene paleoclimate records of Qinghai Lake show the evolution of climate and environment controlled by the Asian summer monsoon (Lister et al., Reference Lister, Kelts, Zao, Yu and Niessen1991; Shen et al., Reference Shen, Liu, Wang and Matsumoto2005; Liu et al., Reference Liu, Shen, Wang, Wang and Liu2007). On account of the complex climate of the northwestern margin of the summer monsoon, the pollen results of Hurleg Lake indicate that the Holocene humidity variability in the northeastern QTP was mainly controlled by westerlies and regional topography (Fan et al., Reference Fan, Ma, Wei and An2014). Analysis of Hala Lake sedimentary cores found that the fluctuation of water levels in the Middle to Late Holocene was mainly controlled by westerly water vapor (Yan and Wünnemann, Reference Yan and Wünnemann2014).
In summary, the interaction between the westerlies and AM has a vital role in climate change in the middle part of the Hexi Corridor in the northern margin of the QTP. Our results demonstrate that strengthening Asian summer monsoon and increasing effective humidity dominated the late MH humid climate. On the other hand, the slight wetting trend since the late LH is mainly linked to enhanced westerlies and weakened Asian winter monsoon. Consequently, this study provides information for evaluating the interaction between the westerlies and AM and emphasizes the complex climate pattern under the control of diverse atmospheric systems in the northern margin of the QTP and the challenges for future predictions.
CONCLUSIONS
A new multiple-proxy dataset, including grain size, δ13Corg, δ18O, TOC, TN, C/N ratio, and MS, from the BHZ section taken in the Beihaizi paleolake was used to reconstruct climate change in the middle part of the Hexi Corridor in the northern margin of the QTP since 5334 cal yr BP. The analysis of all proxies indicated that high lake levels with humid climate occurred in the late MH (ca. 4400 to ca. 3500 cal yr BP). Despite the overall transition to an aeolian environment in the LH, there has been a slight wetting trend since the late LH.
Simulated precipitation, evaporation, and effective moisture based on the PMIP3-CMIP5 multi-model ensemble simulations verified the climate history and its possible causes in the MH and LH. In the MH, the Asian summer monsoon steadily strengthened, which favored high lake levels and humid climate. Increase of the surrounding vegetation since the late LH might result from enhanced westerlies, weakening Asian winter monsoon, and low evaporation. This climate pattern is different from those in the AM region or the westerlies-affected region and suggests a significant effect of the interaction between the AM and westerlies on Holocene climate change in the Hexi Corridor. The reconstruction and dynamic analysis of the lake and climate evolution presented in this study contribute to understanding their links with the AM and westerlies. Future work should focus on extending the AM and westerly records across multiple time scales in the northern QTP.
Supplementary Material
The supplementary material for this article can be found at https://doi.org/10.1017/qua.2023.37
Acknowledgments
This research was supported by the National Natural Science Foundation of China (Grant No. 42077415); the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (Grant No. 2019QZKK0202); the 111 Project (BP0618001). Data and software are available at https://doi.org/10.5281/zenodo.6407798.
