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Holocene high-resolution quantitative summer temperature reconstruction based on subfossil chironomids from the southeast margin of the Qinghai-Tibetan Plateau

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Abstract

This paper reports the first quantitative summer (mean July) temperature reconstruction based on subfossil chironomids from the southeast margin of the Qinghai-Tibetan Plateau (QTP) covering the end of the last deglaciation and the entire Holocene, spanning 11,800 cal yr BP to the present. The record is based on 223 chironomid samples throughout a 927 cm sediment core providing a temporal resolution of ~50 years per sample. We validate the record by applying several statistical reconstruction diagnostics and comparing with pollen and diatom records from the same sediment core. The record suggests the summer temperature varies by ~2.5 C across the entire period. A generally warmer period occurred between c.8500 and c.6000 cal yr BP and a cooling trend was initiated from c.5500 cal yr BP. The overall pattern broadly matches the summer insolation at 30N and the Asian Summer Monsoon records from the surrounding regions suggesting that summer temperatures from the southeast margin of the QTP respond to insolation forcing and monsoon driven variability on a multi-millennial time scale. Modifications of this overall trend are observed on the finer temporal resolution and we suggest that solar activity could be an important mechanism driving the centennial-scale variability. It may have a strengthened effect in the late Holocene when the monsoon influence weakened. Further detailed investigation is needed to disentangle these effects on the climate change over the highlands of southwestern China.

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... Variations in climate observed over this time interval likely represent the natural climate variability that might be expected during the present century without human influence. In the past decades, critical advances in paleoclimate studies of the Late Holocene in the Tibetan Plateau and its surrounding areas with various types of proxies have been achieved (e.g., Hong et al., 2003;Kramer et al., 2010;Wang et al., 2010;Chen et al., 2014;Bird et al., 2014;Xu et al., 2015;Hillman et al., 2017;Zhang et al., 2017;Li et al., 2018;Hillman et al., 2022;Zhang et al., 2023). However, there are contradictions among proxy climate reconstructions from the eastern and SE Tibetan Plateau region over the Late Holocene. ...
... For example, summer temperature reconstructed by alkenones from Hurleg Lake in the Qaidam Basin (Zhao et al., 2013) and BrGDGTs-based mean annual air temperature records from the Hongyuan peatland in the eastern Tibetan Plateau showed long-term climatic warming in the past 4000 years ago (Zheng et al., 2015). However, the chironomidbased record from Tiancai Lake on the SE margin of the Tibetan Plateau and a synthesis of fossil pollen records from the Tibetan Plateau indicate a cooling trend in summer temperature during the Late Holocene (Zhang et al., 2017;Chen et al., 2020). Moreover, conflicting results have been obtained using different lake sediment proxies for climate reconstructions of the same lake. ...
... As an area affected by the ISM, these phenomena comprehensively reflect the decreasing trend of the ISM strength during the past ∼4000 years. This trend is consistent with other records from the Tibetan Plateau, such as the quantitative summer temperature reconstruction based on subfossil chironomids from Tiancai Lake on the SE edge of the Tibetan Plateau (Zhang et al., 2017); the pollen-based mean July temperature reconstruction from Xingyun Lake on the SE edge of the Tibetan Plateau (Wu et al., 2018); summer temperature variations reconstructed from a synthesis of fossil pollen records from the Tibetan Plateau ( Figure 6) (Chen et al., 2020); and a decreasing trend over the past ∼4000 years that has also been reported by the temperature record from Xiada Co on the western Tibetan Plateau (Li et al., 2024). Our records are also well correlated with the June-August land temperature reconstruction and growing degree days (using a baseline of 5°C) reconstruction for North America and Europe ( Figure 6) (Marsicek et al., 2018). ...
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To better understand how global and regional-scale climate has changed, high-resolution records for environmental changes are still needed in southwestern (SW) China during the Late Holocene epoch. This study presents a well-dated high-resolution diatom analysis from a 1.66-m-long sediment core taken from Lake Cuogeda (CGD) on the southeast (SE) edge of the Tibetan Plateau to document environmental changes over the past ∼4000 years. Diatom and other geochemical proxies show that, from 3850 to 3430 cal yr BP (before present, 0 BP=1950 AD, 1900 to 1480 BC), the environment of Lake CGD is acidic, oligotrophic, and enriched with humic acids. And the lake ice cover duration is short during this period. During 3430–1550 cal yr BP (1480 BC-400 AD), Lake CGD has less humic acid and a relatively high pH environment. The ice cover duration is longer, and the temperature drops during this period. Our multi-indicator recorded two environment fluctuations at ∼2800 cal yr BP (850 BC) and 2210–1950 cal yr BP (260 BC-0 AD). From 1550 to 3.6 cal yr BP (400–1946 AD), the lake ecosystem changed to a higher pH condition and had a prolonged freezing time. From 3.6 cal yr BP (1946 AD) to the present, Lake CGD’s water was acidic, with an environment of shorter duration of ice cover and stronger lake water turbulence. Comparisons between the CGD records and other climate reconstructions underscore the relevance of the CGD record for regional and global environments. Comparisons indicate that the environment evolution pattern of SW China during the Late Holocene was greatly affected by solar radiation and North Atlantic sea surface temperature.
... High-resolution proxy-based studies have examined East Asian temperature and precipitation variations during the Holocene, as well as their correlation with global systems such as the Atlantic Meridional Overturning Circulation (AMOC) and the Intertropical Convergence Zone (ITCZ) (Chen et al., 2015;Dykoski et al., 2005;Park et al., 2023;Wu et al., 2012;Zhang et al., 2017a). However, other studies have identified discrepancies between climate modeling results and each proxy data records or differences in the spatial interpretations of these records (Chen et al., 2023;Zhang et al., 2017bZhang et al., , 2018. ...
... Discussions of temperature changes in the Northern Hemisphere have led to investigations of the "Holocene temperature conundrum" and the Holocene Climate Optimum (HCO) using proxy records and climate modeling approaches (Bova et al., 2021;Dong et al., 2022;Kobashi et al., 2017;Liu et al., 2014;Marcott et al., 2013;Marsicek et al., 2018;Rao et al., 2022;Renssen et al., 2009;Wu et al., 2018;Zhang et al., 2017aZhang et al., , 2017bZhang et al., , 2022. Europe and other Northern Hemisphere continents show HCO in the early to mid-Holocene owing to strong insolation and northern ice sheet retreat during the early Holocene (Liu Marcott et al., 2013;Renssen et al., 2009;Zhang et al., 2022). ...
... Particularly in East Asia, including China, a strengthened EASM occurred during the early Holocene due to strong insolation, leading to a positive correlation between temperature and precipitation that resulted in a warmer climate. However, during the mid-to-late Holocene, East Asia experienced a long-term cooling (Selvaraj et al., 2007;Zhang et al., 2017a;Zhao et al., 2011). Several studies have suggested that the occurrence of the HCO occurred during the mid-to-late Holocene (Bova et al., 2021;Kobashi et al., 2017;Marsicek et al., 2018;Zhang et al., 2022). ...
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The occurrence of extreme weather events in response to climate change has become a global concern. To prepare for the intensifying climate crisis, it is imperative to comprehend past extreme climate fluctuations, control factors, and triggers. In this study, we conducted a pollen-based quantitative reconstruction of the Northeast Asia paleohydroclimate. We collected 387 surface soil samples from 89 mountains in South Korea to produce a modern pollen assemblage dataset. As the dataset indicated that pollen samples from South Korea reflected the regional vegetation well, we applied a weighted modern pollen analogue technique to quantitatively reconstruct the mean annual temperature (Tann), annual precipitation (Pann), and summer precipitation (Psum) in the Gwangyang area. The reconstructed Tann increased by ca. 4000 cal BP and gradually decreased thereafter. Long-term Tann trends in the Gwangyang area, along the East Asian continental and Northwestern Pacific margins, appear to have been influenced by oceanic climate conditions, particularly the Kuroshio Current, whereas short-term trends were significantly correlated with the continental climate, the East Asian winter monsoon, and Bond events. Pann and Psum have gradually decreased during the Holocene, which is consistent with the induction of the East Asian summer monsoon by a northward shift of the Intertropical Convergence Zone. After ca. 5000 cal yr BP, the decrease in Pann slowed in association with strong typhoon activity of the ElNi˜no–Southern Oscillation. Analysis of hydroclimate proxy data from southern South Korea revealed wet events at ca. 8900, 7800, 5900, 4500, 3200, 1900, and 1300 cal yr BP, and dry events at ca. 9300, 8200, 6200, 5200, 2800, 2200, and 1400 cal yr BP. Notably, we also confirmed the occurrence of a flooding event in southern South Korea at ca. 8900 cal yr BP.
... In recent decades, many Holocene quantitative reconstructions of seasonal and annual temperatures for the TP have been produced using various proxies, like pollen (Herzschuh et al., 2014;Lu et al., 2011), chironomids (E. Zhang et al., 2017, δ 18 O in ice cores (Pang et al., 2020;Thompson et al., 1997), and biomarkers Zhao et al., 2013;Cheung et al., 2017). These reconstructions have provided crucial data for the elucidation of Holocene temperature changes. ...
... For example, annual temperatures based on brGDGTs (branched glycerol dialkyl glycerol tetraethers; Feng et al., 2022;Sun et al., 2022) indicate a gradual warming trend that resembles the δ 18 O temperature record from the Chongce ice core on the western TP, except for the last 2 ka (Pang et al., 2020). Second, an early to middle Holocene summer temperature maximum and a gradual cooling trend during the late Holocene are observed in pollen-, alkenone-, and chironomid-based temperature records (Herzschuh et al., 2014;Hou et al., 2016;Zhang et al., 2017;Zheng et al., 2015). Third, a prominent relatively cool middle Holocene (M. ...
... Additionally, lake water temperature reconstructions based on subfossil chironomids from Tiancai lake (Fig. 6f) (E. Zhang et al., 2017 and alkenones from Qinghai lake (Fig. 6g) show the same trends during the past 15 ka, as also shown by other pollen-based temperature records from the TP (Chen et al., 2020). Pollen, chironomids, and alkenones mainly respond to the growing season temperatures in middle and high latitudes, and thus the reconstructed temperature records are consistent with the variations in summer solar irradiance. ...
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Understanding Holocene temperature changes is vital for resolving discrepancies between proxy reconstructions and climate models. The intricate temperature variations across the Tibetan Plateau (TP) add complexity to studying continental climate change during this period. Discrepancies between model-based and proxy-based reconstructions might stem from seasonal biases and environmental uncertainties in the proxies. Employing multiple proxies from a single sediment core for quantitative temperature reconstructions offers an effective method for cross-validation in terrestrial environments. Here, we present an ice-free-season temperature record for the past 15 kyr from a shallow, freshwater lake on the eastern TP, based on brGDGTs (branched glycerol dialkyl glycerol tetraethers). This record shows that the Holocene Thermal Maximum lags the pollen-based July temperature recorded in the same sediment core. We conclude that the mismatch between the brGDGT-based and pollen-based temperatures is primarily the result of seasonal variations in solar irradiance. The overall pattern of temperature changes is supported by other summer temperature records, and the Younger Dryas cold event and the Bølling–Allerød warm period are also detected. A generally warm period occurred during 8–3.5 ka, followed by a cooling trend in the late Holocene. Our findings have implications for understanding the seasonal signal of brGDGTs in shallow lakes and provide critical data for confirming the occurrence of seasonal biases in different proxies from high-elevation lakes. To further investigate the significance of the brGDGTs and temperature patterns on the TP, we examined existing brGDGT-based Holocene temperature records, which interpret these compounds as indicators of mean annual or growing season temperatures. The existing and available temperature records show complicated patterns of variation, some with general warming trends throughout the Holocene, some with cooling trends, and some with a warm middle Holocene. We analyzed the possible reasons for the diverse brGDGTs records on the TP and emphasize the importance of considering lake conditions and modern investigations of brGDGTs in lacustrine systems when using brGDGTs to reconstruct paleoenvironmental conditions.
... There are numerous paleotemperature reconstructions since the last deglaciation for the TP and the adjacent areas (Wang et al., 2015a;Hou et al., 2016;Li et al., 2017;Zhang et al., 2017;He et al., 2020;Zhao et al., 2021;Sun et al., 2022;Zhang et al., 2022). These records generally indicate a cooling trend of summer temperatures during the Holocene, although the patterns of mean annual temperature (MAAT) change on the TP remain controversial (Chen et al., 2020;Sun et al., 2022). ...
... Summer temperature changes have been reconstructed from chironomids Zhang et al., 2017), pollen (Chen et al., 2020), and alkenones (Wang et al., 2015b;Hou et al., 2016). Our brGDGTs-based temperature record from Xiada Co (Fig. 6(a)) demonstrates a similar trend to the pollen-based summer temperature record (Fig. 6(b)) (Chen et al., 2020), displaying a gradual cooling trend during the past 5000 years. ...
... In summary, although the absolute values are different, the trends of the temperature record from Xiada Co is consistent with most summer temperature records Zhang et al., 2017;Chen et al., 2020;Li et al., 2021b) and with some of the MAAT records (Chen et al., 2020;Pang et al., 2020;Zhao et al., 2021) from the TP-all demonstrating a cooling trend during the past 5000 years. However, several other MAAT records from the TP contrast with our temperature records, showing a marked warming trend since the middle Holocene (Thompson et al., 2006;Li et al., 2017;Wang et al., 2021;Yan et al., 2021;Sun et al., 2022). ...
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We present a quantitative mean annual air temperature (MAAT) record spanning the past 4700 years based on the analysis of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from a sediment core from Xiada Co, an alpine lake on the western Tibetan Plateau (TP). The record indicates a relatively stable and warm MAAT until 2200 cal yr BP; subsequently, the MAAT decreased by ∼4.4°C at ∼2100 cal yr BP and maintained a cooling trend until the present day, with centennial-scale oscillations centered at ∼800 cal yr BP, ∼600 cal yr BP, and ∼190–170 cal yr BP. MAAT decreased abruptly at ∼500–300 cal yr BP and reached its minimum for the past 4700 years. We assessed the representativeness of our record by comparing it with 15 published paleotemperature records from the TP spanning the past ∼5000 years. The results show divergent temperature variations, including a gradual cooling trend, a warming trend, and no clear trend. We suggest that these discrepancies could be caused by factors such as the seasonality of the temperature proxies, the length of the freezing season of the lakes, the choice of proxy-temperature calibrations, and chronological errors. Our results highlight the need for more high-quality paleotemperature reconstructions with unambiguous climatic significance, clear seasonality, site-specific calibration, and robust dating, to better understand the processes, trends, and mechanisms of Holocene temperature changes on the TP.
... Multiproxy reconstructions of Holocene temperature patterns in the TP and its surrounding areas are disputed. For example, the summer temperature reconstructions-based on alkenone form Qinghai Lake, fossil chironomids from Tiancai Lake, and pollen form Xingyun Lake, respectively-display overall cooling trends (Hou et al., 2016;Zhang et al., 2017;Wu et al., 2018). For high altitude lakes, ice-free-season temperatures, inferred from branched glycerol dialkyl glycerol tetraethers (brGDGTs), show a long-term warming trend in Tingming Lake and Cuoqia Lake Zhang et al., 2022b). ...
... during the Holocene (Zhang et al., 2022b). For example, a long-term cooling pattern occurs in the fossil chironomid record from Tiancai Lake, the alkenones record from Qinghai Lake, and the pollen record from Xingyun Lake (Hou et al., 2016;Zhang et al., 2017;Wu et al., 2018). Most of the MAAT records of the TP were reconstructed from the brGDGTs proxies, in which the existence of a warm-season bias remains disputed (Deng et al., 2016;Cao et al. 2018;Crampton-Flood et al., 2020). ...
... 9(g)). During the mid to late Holocene, a gradual temperature decline was recorded in our MAAT record and in summer temperature records from Qinghai Lake, Cuoqia Lake, Tiancai Lake, and Xingyun Lake (Hou et al., 2016;Zhang et al., 2017;Wu et al., 2018;Zhang et al., 2022b). This decline coincides with a decrease in local summer insolation (Laskar et al., 2004;Fig. ...
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Reconstructing Holocene temperature evolution is important for understanding present temperature variations and for predicting future climate change, in the context of global warming. The evolution of Holocene global temperature remains disputed, due to differences between proxy reconstructions and model simulations, a discrepancy known as the ‘Holocene temperature conundrum’. More reliable and quantitative terrestrial temperature records are needed to resolve the spatial heterogeneity of existing records. In this study, based on the analysis of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from a loess-paleosol sequence from the Ganjia Basin in the north-eastern Tibetan Plateau (NETP), we quantitatively reconstructed the mean annual air temperature (MAAT) over the past 12 ka. The MAAT reconstruction shows that the temperature remained low during the early Holocene (12–8 ka), followed by a rapid warming at around 8 ka. From 8 to 4 ka, the MAAT record reached its highest level, followed by a cooling trend from the late Holocene (4–0 ka). The variability of the reconstructed MAAT is consistent with trends of annual temperature records from the Tibetan Plateau (TP) during the Holocene. We attribute the relatively low temperatures during the early Holocene to the existence of ice sheets at high-latitude regions in the Northern Hemisphere and the weaker annual mean insolation at 35°N. During the mid to late Holocene, the long-term cooling trend in the annual temperature record was primarily driven by declining summer insolation. This study provides key geological evidence for clarifying Holocene temperature change in the TP.
... Pollen analysis has been extensively applied in the Yunnan region, primarily for the purpose of reconstructing historical vegetation and climate changes (Chen et al., 2014a;Chen et al., 2014b;Liao et al., 2021;Wang et al., 2014b;Wang et al., 2018;Xiao et al., 2014b;Yang et al., 2016b;Zhang et al., 2017;Zhang et al., 2019;Zhang et al., 2020). However, studies focusing on the response of vegetation to climate change and anthropogenic disturbances have been relatively rare (Chen et al., 2020), likely owing to the limited availability of robust climate records. ...
... However, studies focusing on the response of vegetation to climate change and anthropogenic disturbances have been relatively rare (Chen et al., 2020), likely owing to the limited availability of robust climate records. Recent research has unveiled regional climate patterns (Tian et al., 2019;Wang et al., 2019;Zhang et al., 2017;Zhang et al., 2020;Zhao et al., 2021). These patterns included a cold and arid phase during the Last Glacial Maximum (LGM; 27-18 cal kyr BP), a gradual warming period during the Last Deglacial Period (LDP; 18-11 cal kyr BP), and a warm and humid Holocene (11 cal kyr BP to the present). ...
... Vegetation typically tends to reach equilibrium with climate changes, including both long-term climate conditions and abrupt climate episodes (Birks and Berglund, 2018;Prentice et al., 1991). Recently, high-resolution climate reconstruction records have been reported (Tian et al., 2019;Zhang et al., 2017;Zhao et al., 2021). These records provide a basis for discussing the relationships between vegetation and climate. ...
... Early quantitative paleoclimate reconstructions mainly focused on sea surface temperatures (SSTs) in the ISM region (Sonzogni et al., 1998;Kudrass et al., 2001;Naidu and Malmgren, 2005;Huguet et al., 2006;Naidu, 2010, 2011;Mohtadi et al., 2014) and in recent years quantitative paleoclimate reconstructions based on terrestrial records have rapidly increased (Shen et al., 2006(Shen et al., , 2008Lu et al., 2011;Herzschuh et al., 2014;Opitz et al., 2015;Wu et al., 2018;Zhang et al., 2017Zhang et al., , 2019Tian et al., 2019;Liang et al., 2020;Sun et al., 2021;Wang et al., 2021;Yan et al., 2021;Zhao et al., 2021b;Zhao et al., 2021a;Zhang et al., 2022b). However, there are still relatively few high-resolution terrestrial quantitative paleoclimate records that span the last ~20 ka. ...
... However, there are still relatively few high-resolution terrestrial quantitative paleoclimate records that span the last ~20 ka. Moreover, the reconstructed climatic factors are also not the same, such as: July temperature based on subfossil chironomids (Zhang et al., 2017, mean annual temperature (MAT) and temperature in ice-free seasons based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) (Tian et al., 2019;Zhao et al., 2021b;Wang et al., 2021;Yan et al., 2021), and MAT, mean temperature of the warmest month (MTWM), and mean annual precipitation (MAP) based on pollen (Herzschuh et al., 2014;Opitz et al., 2015;Zhao et al., 2021a), etc. Among these published studies, even quantitative reconstruction results for the same climate factor based on the same proxy differed greatly. ...
... For example, brGDGT MAT reconstructions from Lake Tengchongqinghai (Fig. 9b, Tian et al., 2019;Zhao et al., 2021a), Lake Qionghai (Fig. 9c, Wang et al., 2021), and Hongyuan Peatland in southwestern China show this pattern. Zhang et al. (2017) suggested a generally warmer period between ~8.5 and ~ 6.0 ka based on subfossil chironomids from Lake Tiancai, southwestern China. The pollen record from Lake Tianchi in southwestern China revealed maximum monsoonal precipitation between 7.1 and 6.4 ka, coinciding with peak Holocene warmth (Jiang et al., 2019). ...
... Pollen, chironomids, alkenones and branched glycerol dialkyl glycerol tetraethers (brGDGTs) have been widely used to reconstruct the terrestrial palaeotemperature since the LGM (Chen et al., 2020;Chu et al., 2017;Marsicek et al., 2018;Sinninghe Damsté et al., 2011;Weijers et al., 2007;Zhang et al., 2017a;Zhang et al., 2019;Zhao et al., 2021). BrGDGTs are membrane lipids produced by some bacterial species that contain straight alkyl core chains with four or six methyl groups, and one or two cyclopentyl moieties formed by internal cyclization involving a methyl group (Schouten et al., 2013;Sinninghe Damsté et al., 2011). ...
... Several studies on quantitative temperature records extending to the LGM have been undertaken in southwest China, where ecosystem functions and biodiversity are sensitive to global climate change (Opitz et al., 2015;Tian et al., 2019;Zhang et al., 2022;Zhang et al., 2017a;Zhang et al., 2019;Zhao et al., 2021). However, the temperature responses to millennial-scale meltwater discharge and increase in atmospheric CO 2 have been muted. ...
... However, the temperature responses to millennial-scale meltwater discharge and increase in atmospheric CO 2 have been muted. For example, chironomid-based summer temperatures from Lake Tiancai display a significant cooling trend during the Holocene, whereas brGDGT-based temperatures from different altitudes show a gradual warming trend (Zhang et al., 2022;Zhang et al., 2017a;Zhang et al., 2019;Zhao et al., 2021). In addition, the most recent millennial abrupt cold event, the Younger Dryas (YD), is not identified in all of these temperature records. ...
... They have irreplaceable advantages in reconstructing climatic and environmental changes (Shen et al., 2010). In recent years, many records of quantitative temperature and precipitation/relative humidity have been reconstructed based on lake sediments in southeastern Tibetan Plateau Liu X. et al., 2014;Zhang et al., 2017Zhang et al., , 2022Tan et al., 2018;Feng et al., 2019;Xu et al., 2019;Sun et al., 2021;Zhao et al., 2021a). Quantitative temperature reconstruction can not only understand the trend of temperature change in a long-time scale, but also obtain the absolute value and change amplitude of temperature more clearly. ...
... Frontiers in Ecology and Evolution 06 frontiersin.org of TOC and TN may indicate the gradually cold-dry climate conditions, in accord with the variation of our reconstructed temperature. Our reconstructed temperature is also consistent with previously limited regional temperature records (Duan and Zhang, 2014;Zhang et al., 2017;Wu et al., 2018). For instance, July temperature based on subfossil chironomids from Lake Tiancai showed an overall decrease with a rapid increase after 1970 AD, albeit with an abnormal value at 1950 AD and quite low resolution ( Figure 3B; Zhang et al., 2017). ...
... Our reconstructed temperature is also consistent with previously limited regional temperature records (Duan and Zhang, 2014;Zhang et al., 2017;Wu et al., 2018). For instance, July temperature based on subfossil chironomids from Lake Tiancai showed an overall decrease with a rapid increase after 1970 AD, albeit with an abnormal value at 1950 AD and quite low resolution ( Figure 3B; Zhang et al., 2017). Our absolute temperature has lower values than Lake Tiancai for the past 300 years, which can attribute to differences of reconstructed season and elevation (higher ~60 m). ...
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High-elevation lakes on the Tibetan Plateau have the advantage of sensitive response to climate changes. Multiple proxy records in lake sediments can provide a large amount of extractable information for paleoclimate reconstructions and assessing the position of recent global warming within the context of natural climate variability. In this study, we reconstruct the climatic and environmental changes over the past 300 years from a remote alpine lake (Lake Cuoqia) in the southeastern Tibetan Plateau using multiple proxies including branched glycerol dialkyl glycerol tetraethers (brGDGTs), n-alkanes, elements, fatty acids and their hydrogen isotopes. Due to ice-cover nature of lake surface during winter, brGDGTs mainly reflect the variation in warm-season temperature from March to October, supported by nearby instrumental data. Our reconstructed high-resolution temperature showed a continuous cooling trend between 1700 and 1950 AD, followed by a rapid warming afterward, in parallel with other proxies such as n-alkanes and fatty acids in the same core, which is also consistent with previously published regional temperature records. The hydrogen isotope (δD) of fatty acids, similar to regional tree-ring δ¹⁸O, can record the history of atmospheric precipitation isotope and further indicate the variations of regional relative humidity. Our record exhibited a long-term decrease since 1700 AD, in accord with the decreasing lake level inferred from the ratio of Fe/Mn. The combined pattern of reconstructed temperature and relative humidity showed consistent changes before 1950 AD toward to a gradually cold-dry trend, whereas started to decouple afterward. Before 1950 AD, the declined temperature and relative humidity are mainly driven by insolation and thermal contrast between the Indian-Pacific Ocean and south Asian continent. After 1950 AD, decoupling of temperature and relative humidity may be related to the increased regional evaporation and human-induced emission of greenhouse gases and aerosol.
... Modern observation indicates the warming rate in the TP is twice that of the global mean over past five decades . Many efforts have been made to reconstruct the paleotemperature both by qualitative Thompson et al., 1997) and quantitative studies (Cheung et al., 2017;Herzschuh et al., 2014;Hou et al., 2016;Wang et al., 2021b;Zhang et al., 2017a) in the TP and adjacent area. Generally, the summer temperature reconstructed by chironomids (Zhang et al., 2017a), pollen (Wu et al., 2018) and alkenone showed an overall cooling trend in phase with summer insolation during the Holocene despite of centennial-millennial oscillations superimposed. ...
... Many efforts have been made to reconstruct the paleotemperature both by qualitative Thompson et al., 1997) and quantitative studies (Cheung et al., 2017;Herzschuh et al., 2014;Hou et al., 2016;Wang et al., 2021b;Zhang et al., 2017a) in the TP and adjacent area. Generally, the summer temperature reconstructed by chironomids (Zhang et al., 2017a), pollen (Wu et al., 2018) and alkenone showed an overall cooling trend in phase with summer insolation during the Holocene despite of centennial-millennial oscillations superimposed. In contrast, the annual temperature reconstructions show complicated or even reversed results. ...
... As for the summer temperature in the TP, the studies based on different proxies including chironomids (Zhang et al., 2017a), pollen (Wu et al., 2018) and alkenones demonstrate a similar trend with an early to middle Holocene temperature optimum and a gradual cooling trend to present as the reconstruction based on pollen assemblage (Fig. 6b). Similarly, Chen et al. (2020) summarized Holocene glacier activity based on published 10 Be ages of moraines in the TP and surrounding mountains. ...
Article
Knowledge of paleotemperature variation in the Tibetan Plateau (TP) can provide potential insights to evaluate the future climate change under ongoing global warming and to improve our understanding of the controversial Holocene temperature changes between stacked proxy and model simulation. Here, we present a 115-yr-resolution record of bGDGTs (branched glycerol dialkyl glycerol tetraethers) based mean annual air temperature (MAAT) from Ngamring Co in the southern TP, spanning the past 18 ka. Our record reveals the MAAT generally responds to annual insolation and greenhouse gases changes, showing slight cooling during the deglaciation and remaining at low temperature until 6 ka BP, and then warming up to present. Combination with other records reveals the regional annual temperature shows a warming trend in the Holocene, which is in agreement with the model simulation rather than the stacked proxies. Importantly, the distinct cold interval during 10–6 ka BP is not present either in the simulation or in most of records for the adjacent region. We suggest such cold interval was a response to the positive North Atlantic Oscillation (NAO) conditions during the early Holocene, which led to a decrease in winter temperature and increased snowfall in the southern TP. Considering the consistent pattern of local annual and winter insolation over past 18 ka, and the high correlation between annual and winter temperature over the past 40 yr, we cautiously indicate that winter season may be crucial for the annual temperature in the TP. Additional quantitative records with clear seasonal signals, especially in winter, are needed to confirm our hypothesis.
... In October 2006, lake water pH was 7.45 and conductivity was 11 μS/cm (Chen et al., 2014). Bedrock in the catchment is primarily a quartz-poor granitoid (syenite) (Zhang et al., 2017). Vegetation is dominated by montane conifers such as Abies and Picea, which appear undisturbed by human activity and indicate that the lake sediment record is also unaffected by human land-use. ...
... The cyan shaded areas indicate cold periods, and the yellow shaded areas indicate warm intervals. YD indicates the Younger Dryas event; EH indicates early Holocene; LH indicates Late Holocene, (b) brGDGT MAAT record (blue line) with ±2σ uncertainty at Tingming Lake based on the Tiancai Lake site-specific brGDGT-temperature calibration (revised from Sun et al., 2021), (c) the percentage abundance of pollen genus Tsuga ; (d) percentage abundance of the diatom fragilarai capucina record and Cyclotella distinguenda from Tiancai Lake core TCK1 (Chen et al., 2014), (e) brGDGT inferred ice-free season temperature at Tingming Lake based on East African lakes calibration (Sun et al., 2021), (f) brGDGT inferred warm season temperature (from March to October) at Cuoqia Lake based on site-specific calibration (Zhang et al., 2022), (g) Chironomid-based mean July temperature (MJT) record from Tiancai Lake (Zhang et al., 2017), (h) Pollen-based summer temperature (MJT) record from Xingyun Lake (Wu et al., 2018), (i) CO 2 concentration from Dome C ice core (Lüthi et al., 2008), (j) winter season (DJF) (dashed line) insolation, (k) mean annual insolation (dotted line), and (i) summer season (JJA) (solid line) anomaly at 26°N (Laskar et al., 2004). and 2d) (Chen et al., 2014;Xiao et al., 2014). ...
... The Tiancai Lake brGDGT-based MAAT record does differ from chironomid-inferred mean July temperature (MJT) reconstructions at Tiancai Lake ( Figure 2g; Zhang et al., 2017), a pollen-based MJT temperature record from Xingyun Lake (Figure 2h; Wu et al., 2018) in southwestern China, and a brGDGT-inferred ice-free season temperature record from Cuoqia Lake (Figure 2f; Zhang et al., 2022) in southwestern China, with a generally overall warming trend in T MJJAS (temperature from March to October). In addition, the reconstructed MAAT record differs from the recent global synthesis temperature reconstructions from around NH which were obtained from biogenic proxies (Bova et al., 2021;Kaufman, McKay, Routson, Erb, Davis, et al., 2020;Marcott et al., 2013) and paleoclimate data assimilation (Osman et al., 2021). ...
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The Holocene temperature discrepancy between paleoclimate reconstructions and climate model simulations—known as the Holocene temperature conundrum—calls for new high‐quality Holocene temperature records at high elevations. Here, we present a quantitative Holocene mean annual air temperature record based on a site‐specific branched glycerol dialkyl glycerol tetraethers calibration from a small remote alpine lake on the southeastern Tibetan Plateau. The record reveals a temperature history comprising a relatively cool early Holocene (before 7 ka) followed by a warmer mid‐ to late‐Holocene (after 7 ka), which was likely linked to increasing local annual insolation and greenhouse gases. Three cold events punctuated the general warming trend ca. 10.4 ka, 3.7 ka, and 1.7 ka BP, and correspond closely in time to ice rafting events in the North Atlantic, and to episodes of volcanism and/or unusual solar activity. The entire Holocene temperatures are cooler than the previously identified anthropogenic warming from 1990–2015 AD.
... Geological archives, such as cave stalagmites, lake deposits and wetland/peatland records in south-western China, reveal that the regional hydroclimate is sensitive to glacial-interglacial East Asian and Indian Summer Monsoon (ISM) dynamics (Wang et al., 2005;An et al., 2011;Hong et al., 2014;Cheng et al., 2016;Wassenburg et al., 2021). ISM variation responses to hydroclimate on different timescales have been well documented using speleothem oxygen isotope (δ 18 O) records from cave stalagmites in Asia (Cai et al., 2015;Kathayat et al., 2017;Liu et al., 2020), as well as multi-proxy-based palaeoclimate reconstructions of lacustrine sediments from lakes in south-western China Zhang et al., 2017aZhang et al., , 2017bChang et al., 2018;Feng et al., 2019;Jiang et al., 2019;Peng et al., 2019;Tian et al., 2019;Hillman et al., 2020;Xiao et al., 2020;Zhang et al., 2020;Zhao et al., 2021aZhao et al., , 2021b. For example, speleothem δ 18 O records from Xiaobailong Cave indicate that ISM precipitation variability differs from that of the records in East China, owing to diverse climate forcings over different timescales (Cai et al., 2015). ...
... Previous studies have used lacustrine records from south-western China to evaluate the changes in precipitation (Chen et al., 2014a(Chen et al., , 2014bLi et al., 2018a) and temperature Feng et al., 2019;Ning et al., 2019) associated with lake ecosystem evolution triggered by ISM variations (Chen et al., 2014a(Chen et al., , 2014bWang et al., 2014Wang et al., , 2020a. Based on radiocarbon dating, previous studies on lacustrine records have successfully characterised ISM variability using multiple lake sediment proxies, including pollen (Xiao et al., 2014(Xiao et al., , 2020Zhang et al., 2020), diatoms (Li et al., 2018a), subfossil chironomids (Zhang et al., 2017a(Zhang et al., , 2017b, and lake-level fluctuations (Xu et al., 2020). Leaf wax hydrogen isotopes (δD wax ) in sediments from Lugu Lake (Zhao et al., 2021a) and the volcanic Tengchongqinghai Lake (Zhao et al., 2021b) have provided novel robust reconstructions of ISM rainfall. ...
... In the same lake, several independent cores (e.g. TCQH1 and TCQH4) reported by earlier studies also concluded that the AMS 14 C dates of plant fragments and bulk organic sediments were in stratigraphic order and increased steadily with core depth (Fig. 10d-e; Xiao et al., 2015;Zhang et al., 2017aZhang et al., , 2017bPeng et al., 2019;Zhang et al., 2020). In particular, the robustness of AMS 14 C dates from plant fragments and bulk sediments in core TCQH4 was verified by independent palaeomagnetic data and tephra age ( Fig. 10e; Zhang et al., 2020). ...
Article
As documented in several studies, lake sediments in south-western China are important archives to decipher Indian Summer Monsoon (ISM) variability through reconstructions of precipitation and temperature changes, based on multi-proxy analysis of lake sediments. However, lacustrine records on glacial–interglacial timescales in south-western China remain limited because of a lack of reliable long chronologies. In this paper, we provide a 53 ka chronology for the Beihai wetland (core TCBH14) deposits in combination with luminescence and radiocarbon dating of sediments for the upper 13.4 m of a 36.6 m long drill core. The reliability of our chronological framework was verified within two independent chronology sequences and validated by correlations with regional chronostratigraphy. Results suggest that luminescence dating is a feasible method that can provide reliable age controls for lake sediments in south-western China. The depositional history of the Beihai wetland was characterised by alternating lake and wetland facies. Lakes existed at 52–47 ka, 41–32 ka, 26–19 ka and 11–7 ka, whereas a wetland system prevailed at 47–41 ka, 32–26 ka, 19–11 ka, and after ~7 ka. The patterns and formation periods of alternate lake–wetland successions are consistent with speleothem δ¹⁸O and lake δDwax records in South Asia, indicating that the sedimentary evolution of the Beihai wetland and other lake–wetland systems in this region are closely linked to ISM-dominated hydroclimate changes. Correlation of sedimentation rates (SRs) of the studied core (TCBH14) with the cores from 11 additional lakes in south-western China suggest that higher SRs occurred during the periods 47–34 ka, 26–19 ka,15–3 ka, and the early Holocene (11–7 ka). In contrast, lower SRs existed during the periods 50–47 ka, 34–26 ka, the last deglaciation (19–15 ka and 12–11 ka), and the middle Holocene (7–5 ka). Thus, we infer that such quasi-synchronous variations of the SRs from these different lakes in south-western China were strongly controlled by the suborbital- or millennial-scale regional hydroclimate, keeping pace with the Northern Hemisphere insolation-induced ISM variability since the last glaciation.
... • C decade − 1 since the 1950s and have been suggested to be one of the most sensitive areas to global climate change (Kuang and Jiao, 2016;Liu and Chen, 2000). The quantitative temperature reconstructions based on lacustrine sediment records have been synthesized in the SE Tibetan Plateau during the past decades, including records from Lake Lugu (brGDGTs, Zhao et al., 2020), Lake Tiancai (diatom, Wang et al., 2021b;chironomid, Zhang et al., 2019;Zhang et al., 2017), Lake Cuoqia (brGDGTs, Zhang et al., 2022a), Lake Tingming (brGDGTs, Sun et al., 2021), Lake Chenghai (isoGDGTs, Sun et al., 2020), Lake Qinghai of Yunnan (brGDGTs, Tian et al., 2019;Zhao et al., 2020), Lake Ximenglongtan (brGDGTs, Ning et al., 2017), Lake Ximenco (pollen, Herzschuh et al., 2014), Lake Chenco (pollen, Lu et al., 2011), Hongyuan Peatland (brGDGTs, Yan et al., 2021) and Lake Yidun (pollen, Shen et al., 2006). These studies have attempted to quantitatively reconstruct temperature variability since the deglacial and Holocene period and identify their potential driving forces. ...
... As the mean summer temperatures of the three lakes fall within the range of the MJT gradient of the calibration lake dataset used for developing the transfer functions, they are well-suited for this study. The diatom sequence from Lake Tiancai during the Holocene has been previously published (Chen et al., 2014), and MJT of Lake Tiancai during the Holocene has been quantitatively reconstructed based on diatoms and chironomids (Wang et al., 2021b;Zhang et al., 2017). Samples for diatom analysis were prepared following Battarbee (1986). ...
... Therefore, we used modern summer water temperature of 31.2 • C for Lake Chenghai (Wan et al., 2005) and 20 • C for Lake Xingyun and Lake Qilu (Wang and Dou, 1998) in our calculations. In southwestern China, summer temperature changes of lake water derived by Chironomidae show an overall ~1 • C cooling trend (Zhang et al., 2017), we therefore also considered this effect. α aragonitewater and α calcite-water are the fractionation factors between aragonite or calcite and lake water, which can be further estimated using the following approximations (Kim et al., 2007;Hoefs, 2021): ...
... Indeed, the δ 18 O p records in those two caves closely resemble the δ 18 O s values, while the calculated δ 18 O p records for three lakes exhibit noticeable deviations from the original δ 18 O c records (Fig. 5), suggesting that elevational differences cannot be the dominant factor leading to the large range of δ 18 O c variations. In addition, given that Holocene summer temperature gradually decreased by 1 • C (Zhang et al., 2017) during the past 8 kyr, calculated δ 18 O w records still vary in a larger range than calculated δ 18 O p records (Fig. 5b-d). Thus, temperature changes during mineral precipitation should have a limited impact on the observed large δ 18 O c variations in the three lakes. ...
... The recently published pollen records from Tengchong Qinghai, Tiancai Lake, Lugu Lake and Tianchi reveal clearly that the Holocene climatic optimum occurred during the mid-Holocene in the western Yunnan Province (Xiao et al., 2015(Xiao et al., , 2017Shen and Xiao, 2018;Jiang et al., 2019;Zhang et al., 2020). The temperature reconstruction based on subfossil chironomids from the Tiancai Lake also suggests that a generally warmer period occurred between ~8.5 and ~6.0 cal kyrs BP, and the pollen records from the central Zoige Basin on the eastern Tibetan Plateau suggest that a warm and wet climate prevailed in the mid-Holocene (Zhao et al., 2011;Zhang et al., 2017). However, other research suggests that the climatic optimum occurred much earlier, during the early Holocene or at least the early to mid-Holocene, owing to an abrupt monsoonal intensification in one or several steps during the early Holocene in the southwest monsoon region (Thamban et al., 2007). ...
... In addition, the percentage of evergreen Quercus decreased again later around 10.0-9.0 cal kyrs BP, illustrating that the timing and duration of this interval are not consistent with the YD event, but may instead reflect a short cold pulse at the begging of the Holocene. Although some paleoclimatic records of different proxies from the southwest China have revealed the YD event (Xiao et al., 2015;Zhang et al., 2017;Shen and Xiao, 2018;Zhang et al., 2020), it does not seem to be clearly recognizable in the Ganchi pollen record. ...
Article
Reconstructing the vegetation history of southwest China since the Last Glacial Maximum is crucial for understanding the evolution of the Indian summer monsoon. In this study, we present a high-resolution palynological investigation from a 5 m core from the Ganchi peatland (southwest margin of the Sichuan Basin, southwest China) in order to reconstruct regional vegetation and climate over the last 25 kyrs. The succession, from bottom to top, comprises lake, wetland, and peat deposits, and wetland/aquatic pollen types are found to be a sensitive indicator of palaeoenvironmental change. Deciduous broad leaf forests dominated by Betula were present between 25-18 cal kyrs BP, suggesting a cold and dry climate during the Last Glacial Maximum. After 18 cal kyrs BP, temperature and summer insolation increased gradually, marking the Lateglacial transition. Thermophilous and hygrophilous species expanded between 11-8 cal kyrs BP, indicating a progressively warmer and wetter climate. From 8-4 cal kyrs BP, evergreen forests developed, while Tsuga reached its maxim abundance, implying a warm and humid climate coincided with the mid-Holocene climatic optimum. Results indicate that residual ice sheets may have impeded the northward shift of the Intertropical Convergence Zone, resulting in peak Holocene warmth lagging behind peak summer insolation forcing. Vegetation change was primarily controlled by climate, with no obvious evidence of anthropogenic disturbance until the last 1 cal kyrs BP. Our study shows that vegetation and climate in the southwest margin of the Sichuan Basin were controlled by summer insolation and ice sheet dynamics, and also influenced by sea surface temperature in the Indian Ocean.
... Subfossil remains of algae and invertebrates in the sediments of different water bodies represent important sources of information about their ecosystems' past. They also serve as essential resources for reconstructing the history of climate and anthropogenic modifications of the environment (Qin et al. 2013;Zhang et al. 2017;Nazarova et al. 2017;Sochuliakova et al. 2018;Tóth et al. 2019;Ibragimova et al. 2022). Studies based on the analysis of subfossil remains have been carried out over several decades, and its popularity continues to increase. ...
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Surface sediments of the mesotrophic dimictic Lake Valdayskoye (Novgorod region, Russia) were studied by analysing microscopic remains of algae, cladocerans, and other invertebrates. Surface-sediment samples were collected from 3 m depth intervals along a transect spanning a depth gradient from 0 to 57 m. The results show that remains of cladocerans, sponges and tintinnids were preserved predominantly near their production zones, despite some mixing and the transfer of individual remains over long distances. Specifically, sediments from the shallower part of the transect were most representative of subfossil Cladocera diversity. When comparing the results of the analysis of subfossil remains with limnological data, signs of the influence of the thermal stratification of the lake on the distribution and preservation of living organisms remains in surface sediments were revealed. A pronounced change in the taphocenosis composition was observed in the lower part of the metalimnion. Also, we detected a predominance of animal remains over algal remains in the metalimnion. Towards its lower boundary the relative abundance of sponge spicules sharply decreases and that of ciliate Codonella cf. cratera increased. An increase in the relative abundance of daphniid remains also occurred in the cladoceran taphocenosis. Cluster analysis identified two groups of samples based on their taphocoenosis, with a boundary between 12 and 15 m, at the lower part of the thermocline, the metalimnion/hypolimnion transition, in Lake Valdayskoye.
... A warming of 1 • C in summer temperature thus would cause a shift in δ 18 O carb of about +0.24 ‰ in isotopic equilibrium within the lake water (Kim et al., 2007). However, the amplitude of Holocene summer temperature variations was generally constrained within 1-2 • C based on pollen and chironomid records on the QTP (Chen et al., 2020;Zhang et al., 2017). It is therefore unlikely that the large range of variation in δ 18 O carb values are primarily caused by changes in temperature. ...
... Quantitative paleotemperature reconstructions based on modern relationships between aquatic biological proxies and the environment are a major endeavor of Quaternary research. These proxies include diatoms (Kumke et al., 2004;Wang et al., 2012Wang et al., , 2021Wang et al., , 2023Zou et al., 2021), chironomids (Kurek et al., 2009;Medeiros et al., 2015;Zhang et al., 2017;Rudaya et al., 2021), and Cladocera (Kattel et al., 2008;Dong et al., 2020). Pediastrum Meyen 1829 (Chlorophyceae) is a major group of primary producers that occurs in almost all freshwater ecosystems and they are often well-preserved in lake sediments (Komárek and Jankovská, 2001;Lenarczyk, 2014). ...
Article
The green alga Pediastrum is an ecologically significant component of freshwater ecosystems, but its potential as an independent proxy for quantitative paleoenvironmental reconstruction is relatively unexplored. Here we use multivariate statistical analysis to explore the relationship between the Pediastrum composition of lake surface sediments and environmental variables. The findings highlight the mean growing season air temperature between March and October (T Mar-Oct) as the most important factor determining the Pediastrum composition. We then establish the first transfer function between the Pediastrum taxa and T Mar-Oct , which enables us to provide a quantitative temperature reconstruction for the past ~8.2 kyr based on sedimentary subfossil Pediastrum records from Bosten Lake in Xinjiang, China. Our findings reveal an exceptionally warm climate during ~4.8-3.6 kyr, with temperatures ~4 • C higher compared to those during ~8.2-4.8 kyr. This unusually warm climate is verified by the carbonate clumped isotopes record of the same core and other paleoclimate records. These findings contribute to our understanding of past temperature changes in Central Asia and provide a basis for future research on long-term lake ecosystem dynamics under past climatic warming.
... The annual temperature records include pollen-inferred temperature at NalengCo Lake, and brGDGT-inferred temperature at Lugu Lake, Qionghai Lake and Ximenlongtan Lake (Opitz et al., 2015;Ning et al., 2019;Zhao et al., 2021a). The summer temperature records include chironomid-inferred temperature at Tianchi Lake and Heihai Lake, pollen-inferred temperature at Xingyun Lake (Chang et al., 2017;Zhang et al., 2017a;Wu et al., 2018). We fully follow the original interpretation of reconstructed temperature in the previous literature, and whether seasonal bias of some reconstructed temperature exists would be further discussed below. ...
... Their use as a temperature indicator has resulted in the production of quantitative estimates of past temperature changes in continental locations, notably across much of the northern hemisphere (e.g. Self et al. 2011;Fortin et al. 2015;Heiri et al. 2015;Zhang et al. 2017), often with links to archaeological contexts (e.g. Taylor et al. 2017;Blockley et al. 2018;Wooller et al. 2018). ...
Chapter
Lake and peat sediments are frequently used to reconstruct palaeoclimate records from all continents using a variety of proxy techniques. This chapter explores the advances that have occurred in the last 20 or more years for using lake and peat sediments to reconstruct past climate change. It demonstrates the impact these advances have had for palaeoclimate research, and implications for archaeology. Fossilized insect remains are often abundant in a wide range of sediments, including lake, river, and peat deposits. Fire is a natural disturbance in many ecosystems, and lake sediments often contain charcoal that can be used to infer fire dynamics. Ombrotrophic bogs have accumulated deep peat deposits throughout the last 10 000 years in temperate latitudes and elsewhere around the world. The chapter focuses on the research that underpins estimates of past palaeoclimate variability as inferred from peatlands.
... Cai et al., 2015;Dykoski et al., 2005;Zhao et al., 2015), lake sediments (e.g. Hillman et al., 2017Hillman et al., , 2020Hodell et al., 1999;Li et al., 2018;Sun et al., 2019;Wang et al., 2020;Wu et al., 2018;Xiao et al., 2014a, b;Zhang et al., 2017;, and peats (e.g. Huang et al., 2016;Wei et al., 2012). ...
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Frequently occurring mega-droughts under current global climate change have attracted broad social attention. A paleoclimatic perspective is needed to increase our understanding of the causes and effects of droughts. South-western (SW) China has been threatened by severe seasonal droughts. Our current knowledge of millennial-scale dry and wet phases in this region is primarily based on the variability of the Indian summer monsoon. However, water availability over land does not always follow patterns of monsoonal precipitation but also depends on water loss from evaporation and transpiration. Here, we reconstructed precipitation intensity, lake hydrological balance and the soil water stress index (SWSI) for the last 27 000 years. Grain size, geochemical and pollen records from Yilong Lake reveal the long-term relationships and inconsistencies of dry–wet patterns in meteorological, hydrological and soil systems in the central Yunnan region, SW China. Our results show that the long-term trends among precipitation, hydrological balance and soil moisture varied through time. The hydrological balance and soil moisture were primarily controlled by temperature-induced evaporation change during periods of low precipitation such as the Last Glacial Maximum and Younger Dryas. During periods of high precipitation (the early to late Holocene), intensified evaporation from the lake surface offset the effects of increased precipitation on the hydrological balance. However, abundant rainfall and the dense vegetation canopy circumvented a soil moisture deficit that might have resulted from rising temperature. In conclusion, the hydrological balance in the central Yunnan region was more sensitive to temperature change while soil moisture could be further regulated by vegetation changes over millennial timescales. Therefore, under future climate warming, the surface water shortage in the central Yunnan region may become even more serious. Our study suggests that reforestation efforts may provide some relief to soil moisture deficits in this region.
Article
The patterns of Holocene temperature change can provide important insight for understanding the recent global warming and assessing future global climate change. However, the mismatch in Holocene mean annual temperature between proxy records and climate models remains controversial, which is likely caused by potential seasonal bias from the geological records. Here, 64 surface sediment samples are analyzed to examine the seasonal bias of brGDGTs reconstruction. Subsequently, we compile 23 brGDGT-based seasonal and annual temperature records from different regions in China and compare them with the TraCE-21 ka simulation results in the same geographical location. The linearly analysis shows a higher correlation between MBT′ 5me and MAF (mean temperature of months above freezing) than that of MAT (mean annual air temperatures), suggesting the reconstructed temperature based on brGDGTs from aquatic sources may be biased toward MAF. The regionally stacked results indicate that MAF in the Qinghai-Tibet Plateau and Northern China warmed from ~12 to 7 ka BP and then cooled thereafter, while an overall warming trend occurred in Southern China. These patterns of MAF change can be well reproduced by the model simulations. The summer and autumn temperature patterns during the Holocene in the Northern China are primarily driven by local insolation and modulated by ice sheets and greenhouse gas concentrations. The retreat of ice sheets in the early and middle Holocene, coupled with the increase in greenhouse gases, is considered the main driving factor for the long-term warming trend of the Holocene in southern China, whereas orbital-induced summer temperature change might be the major driving factor of MAF change in the Qinghai-Tibet Plateau. Our results suggest that the patterns of seasonal temperature change are basically matched between proxy records and climate models during the Holocene.
Article
Holocene climate change’s impact on ecosystems has attracted considerable interest. Although human activities increasingly influenced ecosystems during the Holocene, many paleo-records primarily reconstruct natural environmental changes, often neglecting the pre-instrumental human influenceon ecological responses. To understand ecosystem responses comprehensively, it is essential to consider anthropogenic impacts alongside natural factors. Here, we present a Holocene chironomid record from Qinghai Crater Lake in Tengchong, southwestern China, examining the timing and magnitude of natural and anthropogenic impacts on the lake ecosystem throughout the Holocene. Our results reveal that early to mid-Holocene chironomid communities, especially in terms of species richness, were mainly driven by rising summer temperatures associated with increased solar radiation. By the late-Holocene, the weakening Indian Summer Monsoon led to fluctuating lake levels and intensified human impacts, impacting both chironomid species richness and community stability. Around 0.8 ka BP, enhanced human disturbances shifted the chironomid community composition from mesotrophic taxa dominance (e.g., Cladotanytasus, Macropelopia) to eutrophic taxa dominance (e.g., Chironomus, Polypedilum). Over the past few centuries, human influence has further increased chironomid species richness but has led to declining community stability.
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Climate and human activity are two important factors in regulating organic matter (OM) accumulation in the lake environment. However, when and how anthropogenic impacts have affected lacustrine OM accumulation in southwest China during the late Holocene have not yet been well defined. Here, a 16.3-kyr n-alkane record derived from Erhai Lake was used to trace OM sources and explore their connections to climate and human activity. The n-alkane distributions indicated that the dominant sediment sources shifted from terrestrial and aquatic plants to algae in the late Holocene. OM accumulation was closely related to catchment soil erosion, sediment transport, and deposition processes regulated by climate conditions before 5.0 cal. kyr B.P., following the patterns that stronger monsoon precipitation favoured more terrestrial and less aquatic OM input, and vice versa. From 5.0 to 2.0 cal. kyr B.P., the synchronous downwards trends in terrestrial OM input and precipitation intensity indicated that climate remained a major driving force for OM accumulation. However, sediment sources experienced large-magnitude and centennial-scale oscillations between allochthonous and autochthonous inputs, reflecting early human impacts appeared and lake ecosystems retained the self-regulated ability to recover from the basin-wide early moderate human disturbances. Afterwards, the increased (decreased) OM contributions from terrestrial (aquatic) plants contradicted the weakening monsoon precipitation since 2.0 cal. kyr B.P., indicating a dominant effect of human activities on OM accumulation. This change was accompanied by highly improved algae productivity and gradually elevated lacustrine trophic status, and the lake ecosystem eventually shifted into another state largely deviating from its climate-driven background due to intensified deforestation and agricultural cultivation. Regional comparison indicated that anthropogenic disturbances have temporal differences in southwest China. This study will further improve our understanding of past climate-human-environment interactions in southwest China.
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Under the background of global warming, the frequency and intensity of extreme climate have increased, especially extreme high temperatures. In order to correctly predict the changes in the extreme high temperatures in summer in China in this century, it is urgent to deepen the understanding of the characteristics and physical mechanisms of the extreme high temperatures in summer on the centennial timescale. Many researchers have explored the mechanism of the influences of the variability of the solar cycle on climate change, while the mechanism of the influences of the centennial variation of solar activity on climate change remains elusive. Here, we use the outputs from the Control (CTRL) experiment, Total solar irradiance and Orbital (TSI_ORB) experiment, and Orbital (ORB) experiment from Nanjing Normal University-Holocene (NNU-Hol) experiments to study the extreme high temperatures in summer in China during the Holocene. On the basis of verifying the consistency of the centennial period between the TSI (TSI_ORB minus ORB plus CTRL) experiment and the reconstructed data, we compared the centennial variation characteristics of the summer extreme high temperature in the CTRL experiment and the TSI experiment. It shows that under the modulation of total solar irradiance, the centennial spatial pattern of the summer extreme high temperatures changed from dipole mode to uniform mode, with 300-year and 500-year periodicity, compared to the influence of only internal variability. On the centennial time scale, the greatest difference is located in northeast China. The subsidence movement and the reduction of cloud cover caused by the anticyclone under the control of high-pressure lead to the increase of downward solar radiation, thus making a positive center is showed in northeast China on the impacts of total solar irradiance. Furthermore, the center of the Rossby wave train in the barotropic structure of the upper circulation related to the summer extreme high temperature significantly moves northward. This barotropic structure is composed of continuous pressure ridges from Eurasia to North America and the North Atlantic, which is conducive to the increase of the summer extreme high temperatures. Furthermore, we investigated the underlying physical mechanisms. Under the influence of total solar irradiance, the Pacific Decadal Oscillation (PDO) with the same centennial cycle as extreme high temperatures lead to obvious subsidence movement and increase of radiation flux, causing an increase in extreme high temperatures over northeast China.
Article
The patterns and mechanisms of Holocene temperature variability in China remain highly controversial, possibly due to the lack of sufficient high-resolution records and few systematic compilations and reviews of published records. Here, we used branched glycerol dialkyl glycerol tetraethers (brGDGTs) to generate a Holocene mean annual air temperature (MAAT) record from Lake Yangzonghai (YZH) in southwest (SW) China. Our record, largely ruling out the effect of seasonal bias, displays a long-term cooling of ~1.5℃ since 10 cal ka BP, indicating that the discrepancy of Holocene temperature records (such as long-term cooling vs warming trends) in China cannot be attributed to the seasonal bias in temperature proxies. We compiled 67 published Holocene temperature records from China. These temperature records exhibit a mathematically consistent pattern with a relatively warmer mid-Holocene and colder late Holocene in the Asian summer monsoon (ASM) domain, whilst records show a relatively colder mid-Holocene and warmer late Holocene in the westerlies domain. This spatial heterogeneity of Holocene temperature variability is attributed to the modulation of monsoon systems, including the warming effect of ASM and the cooling effect of the East Asian winter monsoon (EAWM). In the ASM domain, the dominant pattern with the relatively warmer mid-Holocene and colder late Holocene is primarily controlled by the persistent weakening of the warming effect of the ASM and the concurrent strengthening of the cooling effect of the EAWM during the mid-to-late Holocene. In the westerlies domain, the warming effect of the ASM disappears, the cooling effect of the EAWM is confined to the north of the Tibetan Plateau due to the blocking effect of the Tibetan Plateau. This dominant pattern with the relatively colder mid-Holocene and warmer late Holocene is driven by the increases in atmospheric greenhouse gases (GHG) radiative forcing and Northern Hemisphere Annual Insolation (NHAI) forcing during the mid-to-late Holocene.
Article
The Holocene, the most recent interglacial, provides an important time window for evaluating current global warming and predicting future temperature changes. With the development of new temperature proxies and improvements in climate models, significant progress has been made in understanding Holocene temperature changes. However, a major debate persists about whether global temperatures during the Holocene followed a pattern of gradual warming from the end of the Last Glacial Maximum, which culminated in a temperature maximum in the early to middle Holocene, followed by gradual cooling in the late Holocene (the thermal maximum mode); or whether there was a continuous warming trend that continued to the present day (the continuous warming mode). Significant discrepancies exist between different proxy records as well as between proxy records and models, which have resulted in the “Holocene temperature conundrum” that has challenged paleoclimatologists for the past decade. Here, we summarize the progress made to date in the study of Holocene temperature change via proxy reconstructions, climate model simulations, and paleoclimate data assimilation. We emphasize that the current research has limitations in terms of the multiplicity and seasonality of proxy records, the spatial heterogeneity of temperature records, and the incorporation of feedback processes (e.g., vegetation, cloud-radiation feedback) in climate models. These limitations have hindered a comprehensive understanding of the processes and mechanisms of Holocene temperature changes. To solve the “Holocene temperature conundrum”, it is necessary to strengthen theoretical research on climate proxies from the perspective of the underlying processes and mechanisms, elucidate the seasonal response of various temperature proxies, emphasize regional differences in temperature changes, and expand quantitative temperature reconstructions to areas with limited records. However, it is also necessary to improve the simulation performance of complex feedback processes in climate models, reduce simulation errors, and advance the research on data assimilation of Holocene continental temperature records, which may ultimately lead to the optimal integration of paleoclimate records and simulations.
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In this paper, we apply standard zooarchaeological methods and novel osteological approaches to analyse faunal remains from five Middle–Late Holocene sites in the southern Tibetan Plateau (STP). Framed by direct radiocarbon dates on taxonomically classified bioarchaeological remains and compared with published palaeoclimate data, our findings revealed a three-stage process of agro-pastoral development in the STP ca. 5.5 to 1.0 ka. In the first phase, habitation was restricted to the lower southeastern part of the plateau and human subsistence essentially based on foraging and low-level pig–millet farming. With the onset of colder and drier climatic conditions ca. 3.8 ka, the study area witnessed a growing human presence at higher elevations in its central and western parts, together with a shift towards bovid husbandry and barley cultivation, that is, agricultural practices that originated in west Asia; these were likely introduced to the STP following the eastern margin of the TP and/or arrived by sub-Himalayan transfer. Climate and ecological degradation might have contributed to the decline of local game in favour of cold-and-dry-tolerant pastoral livestock and crops. Our work shows that Middle–Late Holocene climate change, ecological change, human subsistence shifts, and prehistoric cultural transmissions are intimately connected.
Thesis
In order to better understand the weathering-climate feedback, a series of studies was carried out in the Changjiang (Yangtze) Basin. First, the operation of the Three Gorges Dam (TGD) changed downstream sediment geochemical compositions. Before 2003, large amounts of upstream-derived sediments deposited and experienced further weathering in the mid-lower basin. Since 2003, strong riverbed sediment resuspension changed the roles of the mid-lower reaches from important sinks to major sources of sediments delivered to the sea. As a consequence, an intensified weathering intensity of downstream sediment was observed.Second, dissolved δ7Li exhibit a conservative behavior in the Changjiang Estuary. During the process of SPM transported offshore, weathering product flocculated and deposited, while primary minerals resuspended. The combined effect resulted in the UCC-like SPM δ7Li values. There is a Li isotope fractionation between the dissolved load and exchangeable phase. However, this process shows negligible impact on δ7Li of dissolved load and particulates. Third, weathering response to climate is complex in large river basins. During the Younger Dryas, the positive excursion of clay δ7Li is consistent with enhanced incongruent weathering, in response to temperature cooling. The minor variation of clay δ7Li at 11 – 2 ka likely indicate stable weathering and mile climate variation. Over the last 2 kyr, the fluctuation of clay δ7Li is mostly caused by intensification of human activities. In summary, this thesis sheds new lights on the weathering-climate feedback over geologic past, and provides perspectives on how human activities affect natural processes.
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The increased frequency of flooding on the Tibetan Plateau as a result of global warming has affected the lives of millions of people along the river valleys. The relationship between human settlement, climate change, and the frequency of flooding at long time scales is still unclear primarily due to the lack of a robust chronology for flooding deposits. In this study, AMS ¹⁴C dating, single-grain quartz OSL dating, and single-grain K-feldspar pIRIR dating using pIR50IR170 and pIR200IR290 signals were applied to the fluvial-paleosol sequence at the archaeology site Shalongka (SLK20) in the upper Yellow River valley on the northeastern Tibetan Plateau, where Paleolithic, Neolithic, and Bronze Age cultural layers were found. Utilizing luminescence characteristics analysis and a comparison of quartz OSL ages, K-feldspar pIRIR ages, and charcoal ¹⁴C ages, the reliability of single-grain quartz OSL and K-feldspar pIRIR ages was tested. Our results indicate that single-grain K-feldspar pIRIR170 dating with the minimum age model (MAM) can be used to date Holocene fluvial deposit that have been poorly bleached. Based on the age-depth model, proxy data of the SLK20 grain size and magnetic susceptibility demonstrate that Yellow River overbank flooding increased during the periods 9.9–9.1, ∼8.0, 7.6–6.3 and 5.0-3.4 kyr BP; during the intervals between these periods, Yellow River overbank decreased and paleosols developed. Human occupations, as indicated by the appearance of cultural layers at the SLK20 site during the ∼8.1, ∼7.8, 6.3–5.0 and 3.4–2.0 kyr BP periods, occurred when paleosol developed. The variation in East Asian summer monsoon (EASM) intensities caused a moist and warm mid-Holocene climate at the SLK20 site, which was conducive to human settlement. Flood events caused by EASM precipitation changes during the mid-Holocene significantly influenced human settlement at the SLK20 site. During the middle to late Holocene, as EASM decreased in the region, the improvement of subsistence strategies likely dominated human settlement on the northeastern Tibetan Plateau.
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The origin of the temperature divergence between Holocene proxy reconstructions and model simulations remains controversial, but it possibly results from potential biases in the seasonality of reconstructions or in the climate sensitivity of models. Here we present an extensive dataset of Holocene seasonal temperatures reconstructed using 1310 pollen records covering the Northern Hemisphere landmass. Our results indicate that both summer and winter temperatures warmed from the early to mid-Holocene (~11–7 ka BP) and then cooled thereafter, but with significant spatial variability. Strong early Holocene warming trend occurred mainly in Europe, eastern North America and northern Asia, which can be generally captured by model simulations and is likely associated with the retreat of continental ice sheets. The subsequent cooling trend is pervasively recorded except for northern Asia and southeastern North America, which may reflect the cross-seasonal impact of the decreasing summer insolation through climatic feedbacks, but the cooling in winter season is not well reproduced by climate models. Our results challenge the proposal that seasonal biases in proxies are the main origin of model–data discrepancies and highlight the critical impact of insolation and associated feedbacks on temperature changes, which warrant closer attention in future climate modelling. The study reconstructed Holocene seasonal temperatures using 1,310 pollen records covering the Northern Hemisphere landmass, and show that both summer and winter temperatures peaked at ~7 ka BP, but with significant spatial variability.
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As one of the paramount components of Asian monsoon and global atmospheric circulation system, the ISM (Indian summer monsoon) plays a vitally important role in the natural environment, ecological balance, and cultural development. Moreover, understanding the ISM could provide valuable insights into global hydrological, atmospheric, and carbon cycles. In recent decades, many ISM records based on different archives have been established within the Holocene, however, issues still remain: the TP (Tibetan Plateau) and the Indian subcontinent are often analyzed separately, despite the both are strongly influenced by the ISM, impeding comprehensive understanding of the monsoon and its object relationship with the East Asian summer monsoon (EASM). Here, covering the both regions, 65 published paleoclimate records within the Holocene have been collected, and conclusions are drawn as follow: (1) There are two evolution patterns of the ISM during the Holocene. The first pattern with the Holocene Optimum (HO) at ~10–6 ka BP, mainly distributing in the Arabian Sea and its surrounding areas, the Indian subcontinent, western central TP, northern TP, and southern China. The second pattern with the HO at ~8–3 ka BP, mainly distributing in the Bay of Bengal and its surrounding areas and southern TP. (2) The delay between the optimum of the second pattern and insolation maximum may be caused by the variation of high-latitude ice cover, although the underlying mechanism concerning the spatial distribution of the two patterns is still unclear. (3) The ISM and the EASM evolve synchronously, and share similar pattern divergence. Arguments over this issue mainly owe to the selected records originating from different patterns. This study makes up the gap in the pattern divergence of monsoon evolution, deepening the understanding of the ISM, even entire Asian monsoon system, and therefore is of great significance for future climate prediction.
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Climate variability during the Holocene has become a widespread concern, and recent studies have compared the annual mean climate variability of the orbital forcing in the Holocene. Little is known about the response of the JJA (June to August) and DJF (December to February) mean surface temperature and precipitation to orbital forcing at different latitudes. In this study, using the outputs of the climatic sensitivity experiments of orbital forcing in models (NNU and TraCE), we compared the linear trends and spatial patterns of JJA and DJF mean surface temperature and precipitation at different latitudes. In addition, we compared the results of the models with the proxy data. The results of the models suggest that the decreased linear trends of JJA mean surface temperature occur in the middle and high latitudes of the Northern Hemisphere (NH-MHL) and tropics, but in the middle and high latitudes of the Southern Hemisphere (SH-MHL), which show increasing trends. The increased linear trends of the DJF mean surface temperature appeared in the NH-MHL, tropics, and SH-MHL. Moreover, the annual mean surface temperature shows increasing trends in the tropics and SH-MHL, and the decreasing trends of annual mean surface temperature arise in the NH-MHL. As for the results of precipitation, the linear trends of JJA and DJF mean precipitation in the low latitudes of the Northern Hemisphere (NH-LL) show decreasing trends. In the low latitudes of the Southern Hemisphere (SH-LL), the JJA mean precipitation shows increasing trends, whereas the DJF results increase first then decrease. By comparing the temperature proxies, we found that simulation results appear in agreement with most temperature proxies, however, they do not match few temperature reconstructions. Furthermore, the trends of JJA and annual mean temperature in the tropics cannot reproduce the reconstructed temperature. We note that the results of NNU are more similar to the temperature reconstructions than those of the TraCE. In summary, we analysed the linear trends of JJA and DJF mean surface temperature and precipitation at different latitudes, and compared the results of models with temperature reconstructions. Our results provide a reference for paleoclimatic research.
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The Tibetan Plateau (TP) is one of the most sensitive areas to global climate changes. Quantitative paleotemperature reconstructions on the TP since the Last Deglaciation provide a prominent opportunity not only for assessing the position, but also for better understanding the mechanism of recent warming. In this study, we first present a well-dated, high-resolution (~70 years), ice-free-season temperature (from March to October, TM-O) record over the past 19 ka from a small alpine lake on the southeastern TP based on brGDGT proxy. Our reconstructed TM-O record displays a long-term ~4 °C warming trend during the past 19 ka with a deglacial increase of ~3 °C and Holocene increase of ~1 °C. To better understand the pattern and mechanism of postglacial temperature changes on the TP, we review 16 published paleotemperature records since the Last Deglaciation. The results show a general warming pattern during the Last Deglaciation but divergent trends of seasonal temperatures during the Holocene with a gradual cooling pattern in summer temperature, an overall warming pattern in winter temperature, annual temperature, and TM-O as well as a warming-cooling-warming pattern in TMJJAS (temperature from May to September). Data-model comparison indicates that the long-term warming trend in deglacial temperatures are primarily driven by rising atmospheric greenhouse gases (GHGs) on the TP. In contrast, Holocene temperature changes are mainly controlled by local seasonal insolation and additional radiative forcing of GHGs on the TP, thereby resulting in divergent patterns of seasonal temperature changes. Our study highlights the necessity of taking into account the seasonal bias when reconstructing temperatures, especially in high latitudes and high altitudes where the freezing occurs.
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A chironomid-based calibration training set comprised of 100 lakes from south-western China was established. Multivariate ordination analyses were used to investigate the relationship between the distribution and abundance of chironomid species and 18 environmental variables from these lakes. Canonical correspondence analyses (CCAs) and partial CCAs showed that mean July temperature is one of the independent and significant variables explaining the second-largest amount of variance after potassium ions (K+) in 100 south-western Chinese lakes. Quantitative transfer functions were created using the chironomid assemblages for this calibration data set. The second component of the weighted-average partial least squares (WA-PLS) model produced a coefficient of determination (rbootstrap2) of 0.63, maximum bias (bootstrap) of 5.16 and root-mean-square error of prediction (RMSEP) of 2.31 ∘C. We applied the transfer functions to a 150-year chironomid record from Tiancai Lake (26∘38′3.8 N, 99∘43′ E; 3898 m a.s.l.), Yunnan, China, to obtain mean July temperature inferences. We validated these results by applying several reconstruction diagnostics and comparing them to a 50-year instrumental record from the nearest weather station (26∘51′29.22′′ N, 100∘14′2.34′′ E; 2390 m a.s.l.). The transfer function performs well in this comparison. We argue that this 100-lake large training set is suitable for reconstruction work despite the low explanatory power of mean July temperature because it contains a complete range of modern temperature and environmental data for the chironomid taxa observed and is therefore robust.
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The climate of the Tibetan Plateau is influenced by the monsoons and the Westerlies due to land-sea thermal contrasts. However, few quantitative temperature reconstructions have been made for the region, and this hinders our ability to validate climate models and to understand the coupling process between monsoon, ocean and ice sheets through time. Here we report a quantitative temperature reconstruction for the last deglaciation from a terrace of Lake Dangxiong Co in the southwestern Tibetan Plateau. Based on the unsaturation index of long-chain alkenones, the reconstructed mean annual air temperature during the late Last Glacial Maximum was about 0.2 °C, while the Bølling/Allerød (B/A) period was characterized by a rapid increase in temperature from 0.8 °C to 2.4 °C, (2.9) followed by an abrupt decrease in temperature to −0.9 °C in the early Younger Dryas (YD). The alkenones and n-alkanes record indicates that Dangxiong Co experienced a cold and wet climate in the early part of the last deglaciation after the Last Glacial Maximum, a warm-dry B/A, and a cold and dry YD. Our temperature reconstruction confirms that the thermal and dynamical effects of the Tibetan Plateau are one of the main elements contributing to abrupt changes in the Indian Summer Monsoon.
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Two sediment cores from the western Arabian Sea, NIOP905 and 74KL, surface temperature (SST) variations over the last 23 kyr. Two organic well-established UK'37 based on long-chain unsaturated ketones synthesized by haptophyte algae and the newly proposed TEX86 derived from the membrane lipids of Crenarchaeota. Comparison of NIOP905 and 74KL core top data with present-day SST (0–10 m) values indicates that both proxies yield temperatures similar to local annual mean SSTs. However, TEX86 and UK'37 SST down-core records derived from the same cores differ in magnitude and phasing. The alkenone SST record of NIOP905 shows small changes in SST (0.5 degree C) over the last 23 kyr, while that of core 74KL shows a 2 degree C increase from the Last Glacial Maximum (LGM) (23–19 calendar (cal) kyr B.P.) through the Holocene (the last 11.5 cal kyr B.P.) synchronous with changes in the Northern Hemisphere. In contrast, the TEX86 records of both cores show a large increase in SST from 22–23 degree C in the LGM to 28–30 degree C during Termination I (19–11.5 cal kyr B.P.), decreasing to present-day annual means of 26 degree C. A cold phase between 14.5 and 12 cal kyr B.P. that may correspond to the Antarctic cold reversal is also observed. This implies a Southern Hemisphere control on tropical SST reconstructed by the TEX86, possibly related to SW monsoon. Our results suggest that the application of both TEX86 and U37 give different but complementary information on SST developments in past marine environments.
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We present the results of pollen analyses from a 1105-cm-long sediment core from Wuxu Lake in southwestern China, which depict the variations of the East Asian winter monsoon (EAWM) and the Indian summer monsoon (ISM) during the last 12.3 ka. During the period of 12.3 to 11.3 cal ka BP, the dominance of Betula forest and open alpine shrub and meadow around Wuxu Lake indicates a climate with relatively cold winters and dry summers, corresponding to the Younger Dryas event. Between 11.3 and 10.4 cal ka BP, further expansion of Betula forest and the retreat of alpine shrubs and meadows reflect a greater seasonality with cold winters and gradually increasing summer precipitation. From 10.4 to 4.9 cal ka BP, the dense forest understory, together with the gradual decrease in Betula forest and increase in Tsuga forest, suggest that the winters became warmer and summer precipitation was at a maximum, corresponding to the Holocene climatic optimum. Between 4.9 and 2.6 cal ka BP, Tsuga forest and alpine shrubs and meadows expanded significantly, reflecting relatively warm winters and decreased summer precipitation. Since 2.6 cal ka BP, reforestation around Wuxu Lake indicates a renewed strengthening of the ISM in the late Holocene; however, the vegetation in the catchment may also have been affected by grazing activity during this period. The results of our study are generally consistent with previous findings; however, the timing and duration of the Holocene climatic optimum from different records are inconsistent, reflecting real contrast in local rainfall response to the ISM. Overall, the EAWM is broadly in-phase with the ISM on the orbital timescale, and both monsoons exhibit a trend of decreasing strength from the early to late Holocene, reflecting the interplay of solar insolation receipt between the winter and summer seasons and El Niño Southern Oscillation strength in the tropical Pacific.
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Numerical dating of glacial deposits is important for understanding Quaternary glacial evolution. Optically stimulated luminescence (OSL) dating is one of the techniques widely used on such sediments. Owing to the short distances traveled before deposition, the incomplete bleaching of luminescence signals in glacial sediments may introduce serious dating problems vis-à-vis glacial and any associated sediments. Here, we report a comparison of OSL ages obtained from the fine (4-11μm) and medium (38-63μm) grain size fractions of quartz extracted from glaciofluvial sediments and from glacial tills in the Basongcuo catchment near the eastern Himalayan syntaxis. Initially, four glacial stages were identified based on field observations of moraine distribution and geomorpho-stratigraphic relations. A total of 39 OSL samples were then collected from glaciofluvial sand layers or lenses and from till. Quartz grains in the fine (4-11μm) and medium (38-63μm) size fractions were extracted from each sample, and dated using the single-aliquot regeneration (SAR) protocol. The modern supraglacial sediment sample was dated to ~0.2 (fine grain) to ~0.7 (medium grain) ka, suggesting that the sediment was not completely bleached on deposition. Contrary to previous experience suggesting that coarse grains are usually better bleached than fine grains prior to deposition, our results show that estimated OSL ages for fine grains are generally younger than those for medium grains. This suggests that the two fractions may have come from different sources and thus have different bleaching histories, and that fine-grained quartz may be more suitable for OSL dating of these materials. Applying the minimum age model to data from medium-grained quartz yields ages close to those obtained from fine-grained quartz, suggesting that both can be used for dating glacial advances. The OSL dates suggest that glaciers in the studied area advanced at 0.1-1.3ka, ~7.5ka and 11-13ka, and were furthest advanced between ~16 and ~30ka. The timing of the local Last Glacial Maximum (LGM) is broadly consistent throughout the eastern and central Himalaya and frequent glacial advances after the LGM. All suggest that glaciers in the humid eastern Himalayan syntaxis are sensitive to global cooling.
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Analyses of diatoms, grain size, magnetic susceptibility, total organic carbon, and total nitrogen were applied to a 9.26m long sediment core, spanning the last 12.2kyr, from a small treeline lake (Tiancai Lake, similar to 3898m a.s.l.) in southwest China. Diatom assemblages are dominated by Cyclotella distinguenda, Aulacoseira species, and small fragilarioid taxa, all of which are sensitive to changes in water pH and light conditions that are probably related to vegetation development and runoff processes triggered by variations in the Asian monsoon. High abundances of C. distinguenda and Pseudostaurosira brevistriata reflected cold and dry climates during the Late Glacial (12.2-11.4kyrBP). In the early Holocene (11.4-9.4kyrBP), a steep decline in C. distinguenda and a visible increase in Aulacoseira alpigena responded to a strengthening monsoon intensity. The persistent increases in A. alpigena mirrored strong monsoon intensity in the middle Holocene (9.4-4.6kyrBP). After 4.6kyrBP, the reduction of A. alpigena was related to weak monsoon intensity in the late Holocene. The main trends of diatom evolution show a general correspondence to variations in solar insolation. Three visible excursions, with an increase in P. brevistriata and a drop in A. alpigena, centered at around 8.4, 2.5, and 0.3kyrBP, correlate with low sunspot numbers and known cold events in the North Atlantic. Some similarities and correlations between the Holocene diatom data, the North Atlantic record, and solar insolation indicate that variations in the Asian monsoon response to changes in solar forcing and the North Atlantic climate.
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It is currently suggested that climate change on the northeastern Tibetan Plateau (TP) was influenced alternately by the monsoon and the Westerlies. However, the mechanisms driving Holocene climate change on the TP remain unclear, since the extent of the influence of individual atmospheric circulation systems has not yet been clearly defined because of the shortage of high-quality paleoclimatic records. This is especially true in the central TP, where only a few ice core and paleolimnological records are available. Here, we present a decadal-resolution temperature record from Dagze Co in the central TP for the past 2000 years, based on the unsaturation index of long-chain alkenones, using an updated temperature calibration, and a record of precipitation isotopes from compound-specific isotope ratios of leaf waxes. The centennial-scale variation of the temperature and precipitation isotope records captures well-known climatic events over the past 1000 years, for example, the ‘Little Ice Age’, which was cooler and drier than the ‘Medieval Warm Period’. However, the relationship between temperature and the precipitation isotope records differed during the interval at 2000–1000 cal. yr BP compared to the past 1000 years, probably because of changes in precipitation seasonality and the additional influence of the Westerlies on the central TP. In addition, the temperature records exhibit a prominent 210-year cyclicity, suggesting a possible influence of solar radiation on temperature variability.
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Ireland provides a unique setting for the study of past climates, as its climate is dominated by westerly airflow from the North Atlantic and readily responsive to changes in North Atlantic circulation patterns. Although there has been substantial research on Ireland’s past environments, quantitative palaeolimnological research, especially chironomid-based research, has been lacking. In order to further develop chironomid-based palaeolimnological investigations, a calibration set was constructed to determine the dominant environmental controls on modern chironomids in western Ireland. Chironomid subfossils were collected from surface sediments of 50 lakes. The lakes were characterised with 36 environmental variables, including physical attributes, lake water characteristics, lake sediment characteristics and land cover within each catchment. In this exploratory study, no specific environmental variable was targeted and lakes were chosen to span gradients of latitude, elevation, depth and trophic status. Redundancy analysis showed that six environmental variables—mean July air temperature, lake depth, dissolved organic carbon, and percentage catchment land cover of agriculture, peat bog and scrubland—captured a large and statistically significant portion of the variance in the chironomid data. July temperature and agricultural land cover were the most dominant environmental variables, with July temperature proving the most suitable for inference model development. A classical weighted-averaging model was developed to estimate July air temperature, with a coefficient of determination (r jack 2 ) of 0.60 and root mean square error of prediction (RMSEP) of 0.57 °C. Results suggest that summer temperature is the dominant influence on chironomid distribution across a wide variety of lake types, and the relatively small RMSEP should allow for more accurate reconstructions of Ireland’s relatively subdued Holocene temperature fluctuations.
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We report mineral magnetic measurements (magnetic susceptibility, anhysteretic and isothermal remanence, magnetic hysteresis and thermomagnetic properties) from a 2.46-m-long sediment core taken in Xingyun Lake, Yunnan, China. Results demonstrate that magnetic minerals in the lake sediment are dominated by stable single-domain (SSD) ferrimagnetic minerals derived mainly from in-washed catchment soil. Magnetic susceptibility increases sharply during the period AD ~500–1050, and the grain size of magnetic minerals decreases. These changes are accompanied by increases in the concentrations of Fe, Ti, Zr and Si, and by decreased tree pollen percentages. δ18O measures in speleothems from South China demonstrate that climate was relatively uniform during this interval. Documentary evidence, however, indicates a large increase in the human population of Yunnan Province at that time. We conclude that the late Holocene sediment record from Xingyun Lake reflects a dramatic increase in soil erosion that was caused by a growing human population, which in turn was responsible for increased deforestation and agricultural activity. Our results from Xingyun Lake are consistent with sediment magnetic records from Erhai Lake, located near our study site. We therefore conclude that the interval AD ~500–1050 corresponded to a regional-scale increase in the intensity of human activity on the Yunnan-Guizhou Plateau.
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[1] Annual, summer, and winter sea surface temperatures (SSTs) in the western Arabian Sea were reconstructed through the last 22 kyr using artificial neural networks (ANNs) based on quantitative analyses of planktic foraminifera. Down-core SST estimates reveal that annual, summer, and winter SSTs were 2, 1.2, and 2.6°C cooler, respectively, during the last glacial period than in the Holocene. A 2.5°C SST increase during Termination 1A (hereinafter referred as glacial to Holocene transition) in the western Arabian Sea. The study reveals a strong seasonal SST contrast between winter and summer from 18 to 14 calendar kyr owing to the combined effect of weak upwelling and strong cold northeasterly winds. Minor or no seasonal SST changes were noticed within the Holocene period, which is attributed to the intense upwelling during the summer monsoon. This causes a lowering of SST to values similar to those of the winter season in analogy with the present day. A 3°C rise in winter SSTs during the glacial to Holocene transition coincides with a strengthening of the monsoon, suggesting a link between winter SST and monsoon initiation from the beginning of the Holocene. Strikingly, annual, summer, and winter SSTs show a cooling trend from ∼8 ka to the present day, implying tropical cooling in the late Holocene.
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Sedimentological data and hydrogen isotopic measurements of leaf wax long-chain n -alkanes (δ Dwax) from an alpine lake sediment archive on the southeastern Tibetan Plateau (Paru Co) provide a Holocene perspective of Indian summer monsoon (ISM) activity. The sedimentological data reflect variations in lake level and erosion related to local ISM rainfall over the Paru Co catchment, whereas δ Dwax reflects integrated, synoptic-scale ISM dynamics. Our results indicate that maximum ISM rainfall occurred between 10.1 and ∼5.2 ka∼5.2 ka, during which time there were five century-scale high and low lake stands. After 5.2 ka, the ISM trended toward drier conditions to the present, with the exception of a pluvial event centered at 0.9 ka. The Paru Co results share similarities with paleoclimate records from across the Tibetan Plateau, suggesting millennial-scale ISM dynamics were expressed coherently. These millennial variations largely track gradual decreases in orbital insolation, the southward migration of the Intertropical Convergence Zone (ITCZ), decreasing zonal Pacific sea surface temperature (SST) gradients and cooling surface air temperatures on the Tibetan Plateau. Centennial ISM and lake-level variability at Paru Co closely track reconstructed surface air temperatures on the Tibetan Plateau, but may also reflect Indian Ocean Dipole events, particularly during the early Holocene when ENSO variability was attenuated. Variations in the latitude of the ITCZ during the early and late Holocene also appear to have exerted an influence on centennial ISM rainfall.
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We present arguments and evidence against the hypothesis that a large impact or airburst caused a significant abrupt climate change, extinction event, and termination of the Clovis culture at 12.9 ka. It should be noted that there is not one single Younger Dryas (YD) impact hypothesis but several that conflict with one another regarding many significant details. Fragmentation and explosion mechanisms proposed for some of the versions do not conserve energy or momentum, no physics-based model has been presented to support the various concepts, and existing physical models contradict them. In addition, the a priori odds of the impact of a >4 km comet in the prescribed configuration on the Laurentide Ice Sheet during the specified time period are infinitesimal, about one in 1015. There are three broad classes of counterarguments. First, evidence for an impact is lacking. No impact craters of the appropriate size and age are known, and no unambiguously shocked material or other features diagnostic of impact have been found in YD sediments. Second, the climatological, paleontological, and archeological events that the YD impact proponents are attempting to explain are not unique, are arguably misinterpreted by the proponents, have large chronological uncertainties, are not necessarily coupled, and do not require an impact. Third, we believe that proponents have misinterpreted some of the evidence used to argue for an impact, and several independent researchers have been unable to reproduce reported results. This is compounded by the observation of contamination in a purported YD sample with modern carbon.
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Lateglacial and early-Holocene mean July air temperatures have been reconstructed, using a chironomid-based inference model, from lake-sediment sequences from Abernethy Forest, in the eastern Highlands of Scotland, and Loch Ashik, on the Isle of Skye in north-west Scotland. Chronology for Abernethy Forest was derived from radiocarbon dates of terrestrial plant macrofossils deposited in the lake sediments. Chronology for Loch Ashik was derived from tephra layers of known ages, the first age-depth model of this kind. Chironomid-inferred temperatures peak early in the Lateglacial Interstadial and then gradually decline by about 1 °C to the beginning of the Younger Dryas (YD). At Abernethy Forest, the Lateglacial Interstadial is punctuated by three centennial-scale cold oscillations which appear to be synchronous with the Greenland Interstadial events GI-1d, when temperatures at Abernethy fell by 5.9 °C, GI-1c, when temperatures fell by 2.3 °C, and GI-1b, when temperatures fell by 2.8 °C. At Loch Ashik only the oscillation correlated with GI-1d is clearly defined, when temperatures fell by 3.8 °C. The start of the YD is clearly marked at both sites when temperatures fell by 5.5 °C at Abernethy Forest and 2.8 °C at Loch Ashik. A warming trend is apparent during the late-YD at Abernethy Forest but at Loch Ashik late-YD temperatures became very cold, possibly influenced by its close proximity to the Skye ice-field. The rapidly rising temperatures at the YD - Holocene transition occur about 300 years earlier at both sites than changes in sediment lithology and loss-on-ignition. The temperature trends at both sites are broadly similar, although between-site differences may result from the influence of local factors. Similar climate trends are found at other sites in the northern British Isles. However, the British summer temperature records differ in detail from trends in the oxygen-isotope records from the Greenland ice-cores and from other chironomid-inferred temperature records available from Scandinavia, north-west Europe and central Europe, which suggest important differences in the influence of climatic forcing at regional scales.
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Two sediment cores from the western Arabian Sea, NIOP905 and 74KL, were analyzed to determine sea surface temperature (SST) variations over the last 23 kyr. Two organic molecular SST proxies were used, the well-established U37 K′ based on long-chain unsaturated ketones synthesized by haptophyte algae and the newly proposed TEX86 derived from the membrane lipids of Crenarchaeota. Comparison of NIOP905 and 74KL core top data with present-day SST (0–10 m) values indicates that both proxies yield temperatures similar to local annual mean SSTs. However, TEX86 and U37 K′ SST down-core records derived from the same cores differ in magnitude and phasing. The alkenone SST record of NIOP905 shows small changes in SST (∼0.5�C) over the last 23 kyr, while that of core 74KL shows a ∼2�C increase from the Last Glacial Maximum (LGM) (23–19 calendar (cal) kyr B.P.) through the Holocene (the last 11.5 cal kyr B.P.) synchronous with changes in the Northern Hemisphere. In contrast, the TEX86 records of both cores show a large increase in SST from 22�–23�C in the LGM to 28�–30�C during Termination I (19–11.5 cal kyr B.P.), decreasing to present-day annual means of ∼26�C. A cold phase between 14.5 and 12 cal kyr B.P. that may correspond to the Antarctic cold reversal is also observed. This implies a Southern Hemisphere control on tropical SST reconstructed by the TEX86, possibly related to SW monsoon. Our results suggest that the application of both TEX86 and U37 K′ give different but complementary information on SST developments in past marine environments.
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Southeastern Tibet is the type area for monsoonal maritime glaciers in the Qinghai–Tibet Plateau with more than 4000 mountain glaciers covering an area of approximately 8000 km2. Based on moraines in the Boduizangbo River Valley, glacial advances have been recognized for the Guxiang, Baiyu, and Yuren Glaciations. Cosmogenic radionuclide surface exposure, ESR, and radiocarbon dating methods are used to estimate moraine ages. The dates suggest that glacial advances occurred during MIS-6, MIS-2 and MIS-1. Moraines of possible MIS-4 or MIS-3 age are also present, but have not yet been dated. During the Guxiang Glaciation, glaciers converged in the Boduizangbo Valley and formed a glacier approximately 100 km long, reaching the Parlongzangbo River Valley. During the Baiyu Glaciation the glacier was about 80 km long, yet did not reach the main valley of the Parlongzangbo River. During the Yuren Glaciation, glaciers advanced only within tributary valleys feeding the Boduizangbo River. Based on glacier extent, reconstructed equilibrium line altitudes during the Guxiang and Baiyu Glaciations were approximately 800 m and 600 m lower, respectively than at present. Mean annual air temperatures were depressed by approximately 6.6–7.8 °C compared to present values, estimating a 40% decrease in precipitation during glaciations.
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