ArticlePossible obliquity-forced warmth in southern Asia during the last glacial stage
Graphical abstract
Introduction
Southern Asia, including India monsoon-influenced southwestern China, was a major habitat and dispersal route for anatomically modern Homo sapiens following their “out of Africa” migration ~100,000 years ago (100 ka) [1], [2]. Characterizing climate changes in this monsoon-influenced region, in particularly over the last glacial cycle, may therefore provide important climatic context to this dispersal [2]. So far, long-term monsoon hydroclimate variations in broader Asian monsoon region have been established from a range of different records [3], [4], [5], showing strong 23,000-year precession cycles driven by boreal summer insolation [4]. Meanwhile, nearby low-latitude sea-surface temperature (SST) reconstructions display the “classical” ~100,000-year ice-age cycles [6], [7], [8], due to the influences of greenhouse-gas (GHG) forcing and showing in-phase with high-latitude climates [6]. Yet, hardly any sufficiently resolved and dated, and well-quantified land-based annual mean temperature records exist for Indian monsoon-influenced Southern Asia through the last glacial stage and until the present. For example, the most recent syntheses of global temperature over the last deglaciation [9], [10] and Holocene [11], [12] contain only three summer temperature records in southwestern China. These indicate an overall Holocene summer cooling associated with declining Holocene boreal summer insolation [13], [14], with associated reduction in Holocene summer monsoon intensities [3], [4], [5]. In contrast to this summer season trend, recent climate simulations indicate that many low-latitude land areas, including Indian-monsoon influenced Southern Asia, experienced an increase in Holocene annual mean temperatures [15]. This contrast between summer and annual mean temperature trends is likely dominated by similarly contrasting changes in Holocene summer and annual mean insolation [13], [14], [15], although increasing GHGs could also play a role [16]. According to astronomical calculations, annual mean insolation variations in low latitudes are modulated by obliquity, and therefore (1) are opposite to those at high latitudes, and (2) fluctuate with a fundamentally different periodicity than the precession-dominated variations in summer insolation [17]. Hence, detailed reconstructions of annual mean temperature records in Southern Asia are needed to obtain a comprehensive view of long-term climate changes. This, in turn, may offer valuable climatic context for understanding developments in Homo sapiens in Asia (Fig. S1 online) through the last glacial stage.
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane-spanning lipids produced by heterotrophic bacteria containing two C28 alkyl chains with 4–6 methyl substituents and 0–2 cyclopenthy moieties (Fig. S2 online) [18]. The utility in temperature reconstructions is based on the capacity of bacteria to alter the fluidity of their lipid membrane to adjust to colder conditions by producing more methyl branches, and vice versa [18]. Because of the consistent variations between lake-surface and air temperatures [19], [20] and the relatively weak seasonality in low-latitude areas [21], [22], [23], lacustrine brGDGTs-based indices have been proposed as proxies for low-latitude mean annual air temperature (MAAT) reconstructions [23], [24]. Meanwhile, long-chain (C27, C29, and C31) n-alkanes preserved in lake sediment derive mainly from leaf-wax lipids of terrestrial plants [25], [26]. Many studies have demonstrated that leaf-wax hydrogen isotope (δDwax, the weighted average δD values of C27-, C29-, and C31-alkanes) can be used to record isotopes of rainfall [27], [28], [29]. Specifically, in the Indian monsoon region, leaf-wax δD values have been used to record isotopes of Indian monsoon rainfall [27], [30], [31], [32]. The primary control on rainfall isotopes in the Indian monsoon region is the integrated summer monsoon rainfall between tropical ocean sources and the specific study site through Rayleigh distillation, with lower δDwax values indicating increased rainfall and stronger summer monsoon intensity, and vice versa [33], [34]. As both δDwax and high-precision U-series dated Chinese speleothem δ18O records reflect water isotope fractionation processes in the same monsoon rainfall [33], [34], and are similarly affected by processes in the hydrological cycle [35], analyzing brGDGTs and leaf wax δD from a single lake sediment core could not only help to constrain the age model of lake sediments beyond the time coverage of the radiocarbon dating method, but also help to identify the potential orbital-scale difference in seasonality between annual temperature and summer monsoon intensity over the last glacial stage.
In addition, paleoclimate models can infer the driving force behind reconstructed climate changes. TraCE 21 ka (TraCE) is the first state-of-art transient simulation of the global climate over the period of the last 21 ka using fully coupled NCAR CCSM3 with T31 spatial resolution (3.75° × 3.75°) [36], forced by realistic climatic forcings that comprise orbital insolation [37], atmospheric GHG [38], meltwater discharge/AMOC changes [39], and continental ice sheets (ICE-5G) [40]. Modifications of coastlines and bathymetry are performed at 13.1, 12.9, 7.6, and 6.2 ka for the Barents Sea, the Bering Strait, Hudson Bay, and the Indonesian throughflow, respectively [41], [42]. In spite of the limitation of insufficient knowledge about meltwater discharge from continental ice sheets, TRACE 21 ka has been found to reproduce paleoclimatic variations in considerable detail through parallel sensitivity experiments across climate change events [41], [42]. Simulations include both the general climate transition from the glacial to interglacial state [10], [43], and the main climate change events such as H1, BA, and YD during the last deglacial [42], [44], [45].
In this study, we report both brGDGTs and δDwax records over the past 88,000 years, inferred from exactly the same sample set of a lake sediment core collected in Hengduan Mountains, southwestern China (Fig. S1 online). Together with the advanced TraCE climate simulations, we aim to (1) reconstruct both MAAT and summer monsoon intensity variations since the last glacial stage at a terrestrial site in Indian monsoon-influenced southern Asia; (2) identify the potential orbital-scale seasonal difference between annual temperature and hydroclimate in this region; (3) infer the driving mechanism behind obtained paleoclimate changes; (4) provide a more reliable paleoclimate background for the living conditions of anatomically modern humans.
Section snippets
Study site
Both Lake Tengchongqinghai (25°07′N, 98°34′E; 1885 m asl) and Lugu Lake (27°41′N, 100°45′E; 2685 m asl) are located in the Hengduan Mountains at the southeastern margin of the Tibetan Plateau (Fig. S1 online), within the influence of the Indian summer monsoon [46]. Lake Tengchongqinghai is close to the Yushuping Archaeology Site, where anatomically modern humans lived at around 30 ka [47] (Fig. S1 online). Lake Tengchongqinghai is a small freshwater crater (maar) lake with a surface area
Evaluation and validation of MAAT reconstruction
The results from both modern and down-core lake sediments in southwestern China are consistent with those from East African Lakes (Fig. S5a, c online). Results from soils in both lake catchments are also consistent with those from global soils (Fig. S5b, d online). This suggests that brGDGT distributions in southwestern China follow closely the global or broader-scale lacustrine/soil brGDGT patterns with separation of 5-methyl and 6-methyl isomers (Fig. S5 online).
However, brGDGT distributions
Conclusion and implication
We present the first, highly-resolved record of annual mean temperature in Indian-monsoon influenced Southern Asia that covers the entire last glacial cycle. It demonstrates that orbital-scale changes in annual mean temperature were decoupled from summer monsoon fluctuations in the region. In particular, we recognize two key intervals (88–71 and 45–22 ka) of elevated annual mean temperature that are close to the full glacial-interglacial range. With the output of TraCE simulations and comparing
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This work was supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDB40010200 and XDA2009000004), the Program of Global Change and Mitigation, Ministry of Science and Technology of China (2016YFA0600502), and the National Natural Science Foundation of China (41877293, 41672162, 41977381, and 41472315). We thank the editors and three anonymous referees for their thoughtful reviews and constructive suggestions. We thank Le Yang from Nanjing Institute of Geography
Author contributions
Cheng Zhao and Ji Shen designed the study. Cheng Zhao, Xiaoqiang Yang, and Enlou Zhang collected sediment cores. Zhengyu Liu and Jun Cheng performed climate simulations and analyses. Xiaoping Feng, Xiaoshuang Sun, Can Zhang, and Tianlong Yan performed lab work and data analyses. Shi-Yong Yu performed statistical analyses. Cheng Zhao and Eelco J. Rohling led the writing of the paper. All authors contributed to the discussion of the results and writing of the manuscript.
Cheng Zhao received his Ph.D. degree in Earth and Environmental Sciences from the Department of Earth and Environmental Sciences, Lehigh University (USA) in 2009. After graduation, he worked at the University of Hong Kong and the University of Southampton (UK). Since 2013, he has been working in Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences as a full professor. His research interest includes lake sediments and climate changes, as well as stable and organic
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Cheng Zhao received his Ph.D. degree in Earth and Environmental Sciences from the Department of Earth and Environmental Sciences, Lehigh University (USA) in 2009. After graduation, he worked at the University of Hong Kong and the University of Southampton (UK). Since 2013, he has been working in Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences as a full professor. His research interest includes lake sediments and climate changes, as well as stable and organic geochemistry.
Ji Shen received his bachelor degree from Nanjing University in 1985, and Ph.D. degree from the Department of Geosciences in Nanjing University in 1991. Since then, He has been working in Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences and promoted as a full professor in 1996. In 2020, he joined the School of Geography and Ocean Science in Nanjing University. His research interest focuses on lake sediments and environmental changes.