Article

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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... The ISM strongly influences the climate conditions of southwestern China, as it is located in the overland pathway of the ISM. Several lacustrine paleoclimate records have been synthesized in southwestern China during the past decades, including records from Lugu Lake (Wang et al., 2014), Xingyun Lake (Hodell et al., 1999;Chen et al., 2014;Wu et al., 2018), Tiancai Lake (Xiao et al., 2014a(Xiao et al., , 2014bZhang et al., 2017Zhang et al., , 2019, Erhai Lake (Shen, 2005;Tareq et al., 2011), Shudu Lake (Cook et al., 2011), Chenghai Lake , Lake Tengchongqinghai (Li et al., 2018;Tian et al., 2019) and Lake Ximenglongtan (Ning et al., 2019). These studies have attempted to reconstruct ISM variability and/or precipitation and identify their potential driving forces. ...
... Temperature and precipitation are the dominant signals of ISM strength. Yet, only a few studies in southwestern China have quantitatively reconstructed their past variabilities during the Last Glacial Maximum (Tian et al., 2019;Zhang et al., 2019), last deglaciation and Holocene Zhang et al., 2017;Wu et al., 2018;Feng et al., 2019;Ning et al., 2019). Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids of soil bacteria, that are widely (Li et al., 2018;Tian et al., 2019); 3. Tiancai Lake (Xiao et al., 2014a(Xiao et al., , 2014bZhang et al., 2017Zhang et al., , 2019; 4. Tianchi Lake (Jiang et al., 2019); 5. Xingyun Lake (Chen et al., 2014;Wu et al., 2018); 6. Lake Chenghai ; 7. Lake Ximenglongtan (Ning et al., 2019); 8. Lake Lugu (Wang et al., 2014); 9. Lake Shudu (Cook et al., 2011); 10. ...
... Yet, only a few studies in southwestern China have quantitatively reconstructed their past variabilities during the Last Glacial Maximum (Tian et al., 2019;Zhang et al., 2019), last deglaciation and Holocene Zhang et al., 2017;Wu et al., 2018;Feng et al., 2019;Ning et al., 2019). Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids of soil bacteria, that are widely (Li et al., 2018;Tian et al., 2019); 3. Tiancai Lake (Xiao et al., 2014a(Xiao et al., , 2014bZhang et al., 2017Zhang et al., , 2019; 4. Tianchi Lake (Jiang et al., 2019); 5. Xingyun Lake (Chen et al., 2014;Wu et al., 2018); 6. Lake Chenghai ; 7. Lake Ximenglongtan (Ning et al., 2019); 8. Lake Lugu (Wang et al., 2014); 9. Lake Shudu (Cook et al., 2011); 10. Erhai Lake (Shen et al., 2006;Tareq et al., 2011); 11. ...
Article
The Indian summer monsoon (ISM) significantly influences the climate of southwestern China; however, continuous quantitative temperature reconstructions since the Last Glacial Maximum are rare, which limits our understanding of ISM evolution. In this study, we analyzed the branched glycerol dialkyl glycerol tetraethers (brGDGTs) from a lacustrine sediment core from Qionghai Lake to reconstruct the paleoclimatology of southwestern subtropical China over the past 27.7 kyr. Our results suggest that the brGDGTs in Qionghai Lake sediments were probably sourced from catchment soils, with some contributions from in situ lacustrine production. We found that the temperature of the Last Glacial Maximum and Holocene Climatic Optimum was ~3 °C cooler and ~ 2.5 °C warmer than present-day conditions, respectively, based on brGDGTs-reconstructed mean annual air temperatures (MATs). Generally, temperature and precipitation showed synchronous paleovariability in ISM regions. Moreover, Northern Hemisphere summer insolation was the dominant control on climate variability in ISM regions on the precessional scale, while Atlantic Meridional Overturning Circulation activity, Intertropical Convergence Zone positioning, and glacial boundary conditions (ice sheet extent and sea surface temperature) contributed to climate shifts on millennial timescales.
... This Holocene winter warming trend could be forced by the retreat of remaining Northern Hemisphere ice sheets, increases in atmospheric greenhouse gas concentrations and boreal winter insolation (Baker et al., 2017;Meyer et al., 2015;Rao et al., 2020;Vasil'chuk et al., 2020). In monsoon-influenced terrestrial East Asia, current widely used biological proxies, such as chironomid and pollen, are regarded as summer temperature indicators because modern processes suggest that the optimal growth of chironomids and vegetation occurs in the summer (Wu et al., 2018;Zhang et al., 2017Zhang et al., , 2019. However, the stress imposed by limited moisture availability cannot be completely excluded for vegetation because of the co-occurrence of a hot climate and monsoon rainfall in summer months (Wu et al., 2018). ...
... However, the stress imposed by limited moisture availability cannot be completely excluded for vegetation because of the co-occurrence of a hot climate and monsoon rainfall in summer months (Wu et al., 2018). These temperature proxies constantly show a gradual cooling trend from the early to late Holocene (Wu et al., 2018;Zhang et al., 2017Zhang et al., , 2019, which is concurrent with the gradually decreasing summer monsoon intensities recorded by Chinese speleothem δ 18 O records and forced by declining boreal summer insolation during the Holocene (Dykoski et al., 2005;Wang et al., 2005). To better understand the Holocene temperature changes in different seasons, a new proxy representing different (or in addition to summer) seasonal temperatures is required for the Asian monsoon region. ...
... Meteorological data from the Liuku station (25°50′N, 98°51′E, 950 m a.s.l.), ∼88.5 km southwest of Tingming Lake, indicate that the annual mean temperature was 20.3°C and the SUN ET AL. Dykoski et al., 2005), Tiancai Lake (TC, Zhang et al., 2017Zhang et al., , 2019, Xingyun Lake (XY, Wu et al., 2018), 136 KL (Schulte & Müller, 2001), and 74 KL (Huguet et al., 2006). (b) Sampling sites for Tingming Lake. ...
Article
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Reconstructions of global surface temperature, dominated by records from the northern extratropics, show an apparent Holocene cooling trend after the early Holocene climatic optimum. However, model simulations suggest a global warming Holocene tendency. This “Holocene temperature conundrum” may be caused by the seasonal bias of paleotemperature proxies. Here we report a quantitative Holocene record with ∼100-year resolution based on branched glycerol dialkyl glycerol tetraethers from an alpine lake in southwestern China. Our reconstructed Holocene temperature record displays a steady long-term trend without distinct cooling or warming changes. Based on the temperature values and their evolution over time, our reconstruction is interpreted to present temperature changes in ice-free seasons from March to November. Unlike the often-documented Holocene cooling of regional summer temperatures driven by boreal summer insolation, this observed trend in our reconstructed temperatures is probably caused by slightly decreasing local ice-free season insolation and somewhat compensated by increasing atmospheric greenhouse gas concentrations. Our results demonstrate that the climatic drivers of ice-free season and summer temperature changes could be different and highlight the significance of elucidating the seasonality of proxies before using them for paleoclimate reconstructions.
... The Yunnan-Guizhou Plateau, located in southwestern (SW) China and on the southeastern (SE) margin of the Tibetan Plateau, is currently strongly influenced by the South Asian summer monsoon (SASM). Numerous studies have attempted to reconstruct the Holocene climate of the region (Brenner et al., 1991;Chen et al., 2014;Feng et al., 2019;Hillman et al., 2017;Hodell et al., 1999;Li et al., 2018a;Song et al., 2012;Wang et al., 2016;Xiao et al., 2014;Zhang et al., 2017aZhang et al., , 2018a. Most well-dated records show consistent temperature trends with a gradual increase from the onset of Holocene to 8000 cal years BP; high and relatively stable temperatures during 8000-5500 cal years BP; and a cooling trend, with fluctuations superimposed, since 5500 cal years BP (Wu et al., 2018;Xiao et al., 2020;Zhang et al., 2017aZhang et al., , 2020b. ...
... Numerous studies have attempted to reconstruct the Holocene climate of the region (Brenner et al., 1991;Chen et al., 2014;Feng et al., 2019;Hillman et al., 2017;Hodell et al., 1999;Li et al., 2018a;Song et al., 2012;Wang et al., 2016;Xiao et al., 2014;Zhang et al., 2017aZhang et al., , 2018a. Most well-dated records show consistent temperature trends with a gradual increase from the onset of Holocene to 8000 cal years BP; high and relatively stable temperatures during 8000-5500 cal years BP; and a cooling trend, with fluctuations superimposed, since 5500 cal years BP (Wu et al., 2018;Xiao et al., 2020;Zhang et al., 2017aZhang et al., , 2020b. However, with regard to changes in precipitation/moisture during the Holocene, two different patterns are indicated: (1) precipitation tracked changes in Holocene boreal summer insolation with maximum precipitation in the early Holocene. ...
... (e) Magnetic susceptibility of core LGH1 from the southern basin of Lake Lugu (Zhang et al., 2018a). (f ) Chironomid record, reflecting runoff into Tiancai Lake (Zhang et al., 2017a). (g) Pollen-based mean July temperature reconstruction from Xingyun Lake (Wu et al., 2018). ...
Article
Changes in moisture conditions or precipitation in the SE Tibetan Plateau during the Holocene have been studied using various environmental archives and proxies. However, due to different interpretations of the proxies and records, the pattern of Holocene precipitation/moisture variations in the region remains unclear. A lake-sediment-based reconstruction of runoff variations, which can directly and sensitively reflect changes in precipitation, provides the opportunity to reconstruct the evolution of moisture conditions in the SE Tibetan Plateau during the Holocene. In this study, we used a well-dated sediment core (LGH2) from Lake Lugu, a deep alpine lake charged mainly by precipitation on the lake surface and by runoff from the watershed, to reconstruct variations in runoff during the Holocene. In addition, 70 lake surface sediment samples were collected to examine the spatial variation of grain size. Endmember modeling analysis of the grain-size data was used to characterize the processes of sediment transport and runoff fluctuations. The carbonate content of core LGH2 shows that the lake level was generally high during 11,600–3100 cal years BP, and that the lake basin was closed after 3100 cal years BP and semi-closed since 90 cal years BP. Grain-size endmember EM 3, which represents the runoff input clastic materials, is used to reconstruct runoff fluctuations in the Lake Lugu watershed. The record indicates a gradual increase in runoff during 11,600–9000 cal years BP, stable and high runoff during 9000–2000 cal years BP, and weak runoff and a low lake level since 2000 cal years BP. Our reconstruction of runoff fluctuations tracks changes in regional temperature and tropical SSTs rather than in boreal summer insolation. This finding supports the hypothesis that increasing tropical SSTs strengthened ITCZ convection which enhanced the flux of water vapour from the ocean to the air, and hence the moisture supplies to SW China.
... The Yunnan-Guizhou Plateau, located in southwestern (SW) China and on the southeastern (SE) margin of the Tibetan Plateau, is currently strongly influenced by the South Asian summer monsoon (SASM). Numerous studies have attempted to reconstruct the Holocene climate of the region (Brenner et al., 1991;Chen et al., 2014;Feng et al., 2019;Hillman et al., 2017;Hodell et al., 1999;Li et al., 2018a;Song et al., 2012;Wang et al., 2016;Xiao et al., 2014;Zhang et al., 2017aZhang et al., , 2018a. Most well-dated records show consistent temperature trends with a gradual increase from the onset of Holocene to 8000 cal years BP; high and relatively stable temperatures during 8000-5500 cal years BP; and a cooling trend, with fluctuations superimposed, since 5500 cal years BP (Wu et al., 2018;Xiao et al., 2020;Zhang et al., 2017aZhang et al., , 2020b. ...
... Numerous studies have attempted to reconstruct the Holocene climate of the region (Brenner et al., 1991;Chen et al., 2014;Feng et al., 2019;Hillman et al., 2017;Hodell et al., 1999;Li et al., 2018a;Song et al., 2012;Wang et al., 2016;Xiao et al., 2014;Zhang et al., 2017aZhang et al., , 2018a. Most well-dated records show consistent temperature trends with a gradual increase from the onset of Holocene to 8000 cal years BP; high and relatively stable temperatures during 8000-5500 cal years BP; and a cooling trend, with fluctuations superimposed, since 5500 cal years BP (Wu et al., 2018;Xiao et al., 2020;Zhang et al., 2017aZhang et al., , 2020b. However, with regard to changes in precipitation/moisture during the Holocene, two different patterns are indicated: (1) precipitation tracked changes in Holocene boreal summer insolation with maximum precipitation in the early Holocene. ...
... (e) Magnetic susceptibility of core LGH1 from the southern basin of Lake Lugu (Zhang et al., 2018a). (f ) Chironomid record, reflecting runoff into Tiancai Lake (Zhang et al., 2017a). (g) Pollen-based mean July temperature reconstruction from Xingyun Lake (Wu et al., 2018). ...
Article
Full-text available
Changes in moisture conditions or precipitation in the SE Tibetan Plateau during the Holocene have been studied using various environmental archives and proxies. However, due to different interpretations of the proxies and records, the pattern of Holocene precipitation/moisture variations in the region remains unclear. A lake-sediment-based reconstruction of runoff variations, which can directly and sensitively reflect changes in precipitation, provides the opportunity to reconstruct the evolution of moisture conditions in the SE Tibetan Plateau during the Holocene. In this study, we used a well-dated sediment core (LGH2) from Lake Lugu, a deep alpine lake charged mainly by precipitation on the lake surface and by runoff from the watershed, to reconstruct variations in runoff during the Holocene. In addition, 70 lake surface sediment samples were collected to examine the spatial variation of grain size. Endmember modeling analysis of the grain-size data was used to characterize the processes of sediment transport and runoff fluctuations. The carbonate content of core LGH2 shows that the lake level was generally high during 11,600–3100 cal years BP, and that the lake basin was closed after 3100 cal years BP and semi-closed since 90 cal years BP. Grain-size endmember EM 3, which represents the runoff input clastic materials, is used to reconstruct runoff fluctuations in the Lake Lugu watershed. The record indicates a gradual increase in runoff during 11,600–9000 cal years BP, stable and high runoff during 9000–2000 cal years BP, and weak runoff and a low lake level since 2000 cal years BP. Our reconstruction of runoff fluctuations tracks changes in regional temperature and tropical SSTs rather than in boreal summer insolation. This finding supports the hypothesis that increasing tropical SSTs strengthened ITCZ convection which enhanced the flux of water vapour from the ocean to the air, and hence the moisture supplies to SW China.
... Over the past 2 decades, climate evolution in the ISM region since the Last Glacial Maximum has been reconstructed from various paleoclimatic archives, including speleothems and marine or lacustrine sediments (Dykoski et al., 2005;Rashid et al., 2007;Govil and Divakar Naidu, 2011;Saraswat et al., 2013;Contreras-Rosales et al., 2014;Q. Wang et al., 2014;Dutt et al., 2015;Wu et al., 2015;Kathayat et al., 2016;Zhang et al., 2017aZhang et al., , b, 2018Zhang et al., , 2019Li et al., 2018;Sun et al., 2019). These studies provide evidence of changes in ISM precipitation on orbital and millennial timescales, with a weakened ISM occurring dur-ing cold events and strengthened ISM occurring during warm intervals. ...
... In addition to precipitation, temperature is an important climatic factor, due to its significant effects on evaporation and regional hydrological cycle. There remains a lack of quantitative reconstructions of terrestrial temperature from the ISM region (Shen et al., 2006;Zhang et al., 2017aZhang et al., , 2019Wu et al., 2018;Feng et al., 2019;Ning et al., 2019;Tian et al., 2019). During the last deglaciation-Holocene transition, the climate of high latitudes in the Northern Hemisphere is punctuated by three abrupt, millennial-scale events: the Heinrich 1 (H1) cold event, the Bølling-Allerød (BA) warm period and the Younger Dryas (YD) cooling (Alley and Clark, 1999). ...
... These intervals are attributed to a variety of mechanisms including changes to orbitally controlled insolation, ice sheet extent, oceanic circulation and atmospheric greenhouse concentrations (Alley and Clark, 1999). The recent quantitative summer temperature proxy based on pollen and chironomids from southwest China has been developed to address the response of long-term temperature to the highlatitude climate changes (Zhang et al., 2017aWu et al., 2018). However, the magnitude of these temperature variations is not consistent, and further studies are required. ...
Article
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Over the past decades, paleoenvironmental studies in the Indian summer monsoon region have mainly focused on precipitation change, with few published terrestrial temperature records from the region. We analysed the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) in the sediments of Lake Chenghai in southwest China across the Pleistocene–Holocene transition, to extract both regional hydrological and temperature signals for this important transition period. The lake level was reconstructed from the relative abundance of crenarchaeol in isoGDGTs (%cren) and the crenarchaeol′/crenarchaeol ratio. The %cren-inferred lake level identified a single lowstand (15.4–14.4 ka cal BP), while the crenarchaeol′/crenarchaeol ratio suggests a relatively lower lake level between 15.4–14.4 and 12.5–11.7 ka cal BP, corresponding to periods of weakened ISM during the Heinrich 1 and Younger Dryas cold event. A filtered TetraEther indeX consisting of 86 carbon atoms (TEX86 index) revealed that lake surface temperature was similar to present-day values during the last deglacial period and suggests a substantial warming of ∼4 ∘C from the early Holocene to the mid-Holocene. Our paleotemperature record is generally consistent with other records in southwest China, suggesting that the distribution of isoGDGTs in Lake Chenghai sediments has potential for quantitative paleotemperature reconstruction.
... Previous studies have revealed the regularity of ISM evolution, mainly from marine records (Herzschuh et al., 2005;Gebregiorgis et al., 2016;Li et al., 2018), and continental records that are within the Holocene and last deglacial periods (Zhao et al., 2011;Cai et al., 2012;Berkelhammer et al., 2012;Xiao et al., 2014aXiao et al., , 2014bAhlborn et al., 2017;Zhang et al., 2017b;Kumar et al., 2018). Till now, there have been some debates about how the ISM relates to the East Asian Summer Monsoon (EASM) over timescales such as the Holocene (Herzschuh, 2006;Wang et al., 2010;Hillman et al., 2017;Zhang et al., 2017a). While in the Last Glacial Maximum (LGM), there is little research about the relationship between the ISM and EASM (Jiang et al., 2014;Huang et al., 2016). ...
... In fact, many lakes across SW China have provided quality data that advance our understanding of the ISM Xiao et al., 2014a;Zhang et al., 2017aLi et al., 2018). Lacustrine archives, providing the best sources of continental data, exhibit the local and regional climate responds to the global climatic changes (Street-Perrott and Harrison, 1985;Cheddadi et al., 1996;Shen, 2012). ...
... paleo-lake level changes. However, the quantitative reconstructions in climate indicators such as temperature and precipitation are few in this region Zhang et al., 2017a;Wu et al., 2018;Feng et al., 2019). It is essential to seek independent quantitative proxies that can trace changes in temperature and precipitation respectively. ...
Article
Over the past few decades, studies on the Indian Summer Monsoon (ISM) have been mainly based on paleo-marine records whilst the sparsity of land records limits our understanding of the ISM evolutionary process over continental landscapes. Here, we provide paleoclimate reconstructions based on n-alkanes and branched glycerol dialkyl glycerol tetraethers (brGDGTs) from a 50-ka lacustrine sediment core collected from Lake Tengchong Qinghai in southwestern subtropical China. Our results reveal that the botanical sources of sedimentary n-alkanes in this lake are mainly from aquatic macrophytes and terrestrial plants. The lake level evolution from the MIS 3 to the Holocene can be divided into four stages: high in the MIS 3, low in the LGM, high in last deglacial and low in mid and late Holocene. Our brGDGTs-based climate reconstructions indicate that the mean air temperature differences between the present-day and the LGM were up to about 3–4 °C. From MIS 3 to Holocene, there was a broadly and consistently synchronous trend between precipitation and air temperature in this ISM region. This is different from the asynchronous temperature and precipitation variations in the East Asian Summer Monsoon (EASM) regions in the period of last glacial termination. Our study shows that the climate in ISM regions was primarily driven by changes in June summer solar insolation at 30°N, and those glacial boundary conditions such as ice volume and sea surface temperature might also have affected the ISM.
... The declining trend of summer insolation in the North Hemisphere during the Northgrippian (Fig. 8d) (Berger and Loutre, 1991) led to a gradual decrease in the ISM. The reconstructed summer climate based on the pollen record from Muge Co is consistent with the summer temperature and monsoon precipitation records in the wider ISM regions of China, including the chironomid record from Tiancai Lake (Fig. 8e) (Zhang et al., 2017a), the pollen record from Xingyun Lake (Wu et al., 2018) in the southeastern QTP and stalagmite records in eastern Asia (Fig. 8f) (Dykoski et al., 2005). The notable feature of Muge Co record is that summer temperature and precipitation decreased synchronously after c. 4900 cal yr BP, which indicates that (Berger and Loutre, 1991), and (c) Sample scores of the PCA 1 of the Wuxu Lake pollen record, which was interpreted as a proxy for EAWM . ...
... The notable feature of Muge Co record is that summer temperature and precipitation decreased synchronously after c. 4900 cal yr BP, which indicates that (Berger and Loutre, 1991), and (c) Sample scores of the PCA 1 of the Wuxu Lake pollen record, which was interpreted as a proxy for EAWM . (Berger and Loutre, 1991), and selected ISM records, including (e) summer temperature reconstruction based on subfossil chironomids from Tiancai Lake (Zhang et al., 2017a), and (f) speleothem δ 18 O record from Dongge Cave used as a proxy for precipitation (Dykoski et al., 2005). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) ...
... summer climate fluctuatations in southwestern China are likely to correspond to the changes in summer insolation forcing (Gray et al., 2010). It is worth noting that the summer insolation maximum at 30°N occurred between 12,000 and 10,500 cal yr BP, while the Holocene optimum period recorded from most lakes in China is about 8000 and 7000 cal yr BP (Wu et al., 2018;Xiao et al., 2014;Zhang et al., 2017a). The presence of residual ice sheets at high latitudes in the Northern Hemisphere during the Greenlandian may have delayed the onset of the optimum climatic period in southwestern China (Xiao et al., 2009). ...
... The technique is then compared to conventional WA-PLS regression model. Here, we hypothesize that BRTs should outperform WA-PLS-based climate reconstruction in Lake Tiancai; 3) Finally, the diatom-based BRT climate reconstruction is compared with the chironomid-based reconstruction of past mean July temperature and pollen records from Lake Tiancai (Zhang et al., 2017) with the aim to improve the robustness of the method for future climate reconstructions. The prepared reconstructions are evaluated based on the compositional distance between fossil samples and their closest modern analogues in the training set. ...
... For Lake Tiancai, a chironomid-based transfer function was constructed previously (Zhang et al., 2017). To assess the consistent features and between-reconstruction variability, we prepared the reconstructions of MJT, based on the sedimentary diatom record of Lake Tiancai. ...
... The reconstructions extend back to the LGM. We compared the BRT and WA-PLS diatom-based MJT reconstruction results with the chironomid-based MJT transfer function model (Zhang et al., 2017). The models are evaluated based on the compositional distance between fossil samples and their closest modern analogues in the calibration dataset. ...
Article
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Quantitative paleoclimate reconstructions based on biological fossils over glacial-interglacial timescales are a major source of information on long-term climate variability. However, such reconstructions can present major methodological challenges, as calibration methods based on modern climatic and biological patterns may yield biased results, especially when data are particularly sensitive to the influence of secondary environmental variables in the training set or fossil assemblage. A machine-learning technique, boosted regression tree (BRT), is compared with a weighted averaging-partial least squares (WA-PLS) regression in a diatom-based summer (mean July) temperature transfer function with a calibration set of 273 samples collected from China. Using these calibration methods and a fossil diatom sequence from Lake Tiancai (southwest China; 3898 m a.s.l.), the summer temperatures covering the last glaciation and entire Holocene (18000 cal yr BP to present) are reconstructed. The record is validated by applying several statistical reconstruction diagnostics and compared with pollen and diatom records from the same sediment core. The results suggest that the BRT-based calibration model, which produced a coefficient of determination (r² = 0.85) and root-mean-square error of prediction (RMSEP = 1.65 °C) offers substantial improvements over WA-PLS. The record shows that the summer temperature varies by ∼2.5 °C across the entire period. The temperature evolution inferred by diatom-based BRT model is characterized by an early cool phase (with a MJT of 9.3 °C) prior to 10000 cal yr BP, followed by a clear mid-Holocene thermal maximum (∼1.1 °C above the present-day temperature) from 6000 to 3500 cal yr BP and a subsequent cooling that ended at 1500 cal yr BP. This contrasts with the chironomid-based temperature transfer function in the same sequence showing a warmer period between 8500 and 6000 cal yr BP. The overall pattern of diatom-based summer temperature reconstructions broadly matches the climate change inferred from pollen records from Tiancai Lake and other records from the surrounding regions, suggesting that summer temperatures respond to insolation forcing and the variability of the southwest monsoon. We argue that the BRT-based climate model potentially outperforms the other models in southwest China when using a large and non-linear calibration set of diatoms for reconstructing long-term temperature.
... ka BP based on pollen record from Lake Tiancai (Xiao et al. 2014) vs. 8.5-6.0 cal. ka BP inferred from the chironomid assemblage from the same site (Zhang et al. 2017b). The available records are primarily recovered from the northern and eastern parts of the YP. ...
... The highest composition of Altingia trees appeared between 7.5 and 5.2 cal. ka BP, which indicates a period of higher temperature, and this result is roughly consistent with the chironomids-based temperature reconstruction from Lake Tiancai (Zhang et al. 2017b). ...
Article
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The evolution of the Indian Summer Monsoon (ISM) has been widely investigated and traditionally interpreted as consequences of orbital forcing change and the migration of the Intertropical Convergent Zone (ITCZ). The potential influence of sea-surface temperature (SST) on ISM variation at millennial time scales has been progressively proposed in recent studies. Towards a better understanding of Holocene evolution of ISM, especially the potential response to variations in SST, a 643 cm continuous sediment sequence covering the past 11.1 ka was recovered from Beihai Wetland in western Yunnan Plateau. Fossil pollen analysis revealed that evergreen broadleaved forest dominated the regional vegetation in the early Holocene, which represented a warm and humid period associated with strong ISM. Expansions of deciduous broadleaved forest and grassland from 5.2 cal. ka BP to the present, together with the appearance of abundant plant remains, indicate the formation of the wetland system. Ordination analysis of the pollen assemblages further confirmed the abrupt vegetation turnover at ca. 5.2 cal. ka BP, which is consistent with the abrupt decline in reconstructed mean annual precipitation (Pann, from ca. 1400 to 1100 mm), and indicates an abrupt decline in the summer monsoon. We performed a climate regime shift test on various records from the ISM domain and eastern Africa and found this phenomenon as well, which could not be explained by changes in solar insolation or the migration of the ITCZ. A mid-Holocene cooling was detected from the Indian Ocean, especially a decreased SST in the tropical Indian Ocean ca. 5.5 cal. ka BP. Cooling of the tropical Indian Ocean possibly caused the abrupt retreat of the summer monsoon by restraining the duration of the monsoonal season and the convergent process, which was supported by an abrupt monsoon decline based on results from a minimal concept model.
... Modeled simulations of Holocene summer temperatures show that summers were cooler over regions directly influenced by the Laurentide ice sheet during the early Holocene, whereas for areas elsewhere in the Northern Hemisphere, summer temperatures were dominated by orbital insolation (Renssen et al., 2009). Proxy-based reconstructions of Holocene temperature, however, have shown that both the timing and magnitude of the thermal maximum differed substantially among regions (Shi et al., 1993;Kaufman et al., 2004;Renssen et al., 2009;Zhang et al., 2017aZhang et al., , 2017b. The lack of summer temperature reconstructions from low-latitude land regions makes it difficult to establish temporal patterns of Holocene summer temperature variations across the broader landscape and to determine the underlying forcing mechanisms of temperature change. ...
... Many tectonic (fault) lakes are distributed across the Yunnan Plateau. Although a substantial amount of paleoenvironmental research has been conducted on these lakes, including vegetation and climate reconstructions (Sun et al., 1986;Hodell et al., 1999;Shen et al., 2006;An et al., 2011;Cook et al., 2013;Chen et al., 2014a;Xiao et al., 2014;Zhang et al., 2017a), the regional patterns of Holocene climate and vegetation changes in the region are still poorly understood. For example, some studies detected an early Holocene climatic optimum (Cook et al., 2013;Chen et al., 2014a), whereas others suggest a middle or even late Holocene climatic optimum (Xiao et al., 2014). ...
Article
Proxy-based reconstructions of Holocene temperature show that both the timing and magnitude of the thermal maximum varied substantially across different regions. Given the 'Holocene temperature conundrum', it is becoming increasingly important to reconstruct seasonal temperature variations. As a major component of the global monsoon system, the Indian summer monsoon (ISM) transports moisture and heat from the tropical oceans to higher latitudes and thus it has substantial socioeconomic implications for its regions of influences. We developed a well-dated, pollen-based summer temperature record (mean July; MJT) for the last 14,000 years from Xingyun Lake in southwest China, where the climate is dominated by the ISM. MJT decreased during the Younger Dryas, increased slowly to high values during 8000e5500 yr BP, and decreased thereafter. The MJT record differs from that inferred using carbonate oxygen isotopes (d 18 O) from the same sediment core. The latter record reflects variations in monsoon precipitation, with highest precipitation during the early Holocene (11,000e6500 yr BP). We propose that summer temperature and precipitation in southwest China were decoupled during the early Holocene. Both MJT and monsoon precipitation decreased after the middle Holocene, tracking the trend in boreal summer insolation. We suggest that greater cloud cover, associated with high precipitation and generated by a strong summer monsoon, may have depressed early Holocene temperatures that would otherwise be driven by greater summer insolation. Melting ice sheets in high-latitude regions and high concentrations of atmospheric aerosols during the early Holocene may also have contributed, in part, to the relatively cool summer temperatures.
... The past 2000 years, or the Common Era, is an important period during which Earth's climate system has transitioned from being forced solely by natural factors to also being forced by anthropogenic greenhouse gases (IPCC, 2013). To better understand how global and regional-scale climate has changed during this period, the International Past Global Changes (PAGES) project set up the '2k Network' (called PAGES 2k hereafter) in 2009, with the goal of generating a global array of continental-scale temperature recon-Moreover, for some continents like Asia (Cook et al., 2013), about 60% records in PAGES 2k network are based on high-resolution tree-ring data (PAGES 2k Consortium, 2013, 2017, which are not effective recorders of low-frequency multi-centennial scale temperature variability (Moberg et al., 2005;Cook et al., 2013). In particular, the Tibetan Plateau is the most extensive high-elevation area in the world. ...
... The freshwater lake is oligotrophic, surrounded by hills rising 200 m above the lake surface (Fig. 1b, c). Bedrock in the catchment is primarily a quartz-poor granitoid (syenite) (Zhang et al., 2017). Forests primarily comprise montane conifers such as Abies and Picea, which appear undisturbed by human activity (Xiao et al., 2014) and indicate the lake sediment record is also unaffected by human land-use. ...
Article
Documenting climate change over the past 2000 years, or the Common Era, is critical to understand how the climate system evolved from one controlled by natural forcings alone to one influenced by anthropogenic greenhouse gas forcing. However, global and regional temperature changes during this interval are still poorly understood, largely due to the limited geographical coverage and scarcity of data. Here we report a well-dated, quantitative, mean annual air temperature (MAAT) record with ∼10-yr resolution for the Common Era derived from a sediment core collected at a small alpine lake in remote subtropical southwestern China, and based on a site-specific temporal calibration between down-core analyses of brGDGTs and instrumental data for the interval 1959–2015 AD. The record reveals distinct multicentennial-scale temperature fluctuations, including a relatively cold interval from 0–800 AD, followed by warmer temperatures during the so-called Medieval Climate Anomaly (MCA; 800–1400 AD), cooling during the Little Ice Age (LIA; 1400–1900 AD), and abrupt and rapid warming into the late-20th-century after 1900 AD. Superimposed on these large-scale features are three short-term cold events, centered on 250–310 AD (∼0.06 °C), 570–650 AD (∼0.87 °C), and 1800–1823 AD (∼0.83 °C), respectively. The temperature variations captured in the record are supported by historical documents and existing regional and global paleoclimate records, and correspond in time to a number of the major Chinese dynastic transitions. Importantly, the record captures large-magnitude (up to 4 °C) centennial-scale temperature fluctuations, documents cold conditions during the first millennium AD, and demonstrates that the post-1989 AD warmth is greater than any other time during the past 2000 years. The results suggest that high-elevation areas have been more sensitive than low-elevation regions to climate variations during the Common Era.
... Chironomid-inferred summer temperature increased by 2.1 • C in Tiancai Lake at the beginning of the Holocene Fig. 7. A comparison of oxygen isotopes of GISP2 from Greenland (A; Grootes et al., 1993), speleothem oxygen isotopes from Dongge Cave (B; Dykoski et al., 2005), July insolation at 30 • N (C; Berger and Loutre, 1991), synthesized Holocene climate indices in Central Asia (D; Wang et al., 2010), reconstructed temperature across China (E; Fang and Hou, 2011), pollen-based reconstructed MJT from Xingyun Lake (F; Wu et al., 2018), percentage of Tsuga from Lugu Lake (G; Wang et al., 2016), chironomid-inferred MJT from Tiancai Lake (H; Zhang et al., 2017) with cladoceran PCA1 (I) from Cuoqia Lake, SW China. . ...
... 8.2 cal kyr BP (Fang and Hou, 2011;Xiao et al., 2014a). The solar radiation peak at 30 • N resulted in an abrupt intensification of the regional monsoon (Xiao et al., 2014a;Zhang et al., 2017;Wu et al., 2018). Previous palaeoclimate reconstruction records in SW China based on chironomid showed that mean July temperature was up to 8.1 • C between ca. ...
Article
The study of alpine lake ecosystem ontogeny provides an understanding of how climate dynamics and the associated catchment changes impact on lake functioning. Here, a high-resolution subfossil Cladocera sedimentary record, together with geochemistry proxies, from an alpine, treeline lake (Cuoqia Lake) in SW China were analyzed to understand the lake ecosystem response to changes in the regional Indian summer monsoon (ISM) climate and the local mountain environment since the Last Glacial Maximum (LGM, ca. 19.5 cal kyr BP to present). The results show that the cladoceran record from Cuoqia Lake clearly responds to long-term climate change and significant climate events. The lake was depauperate in cladoceran species throughout the late glacial period (ca. 16–11 cal kyr BP) but Daphnia longispina-group and Chydorus sphaericus were the pioneer taxa when the lake was formed. The rapid proliferation of open-water and littoral cladoceran taxa began with increasing water temperature. The Younger Dryas (YD) cold event was recorded by high abundance of the cold-tolerant taxon C. sphaericus during ca. 12–11 cal kyr BP. At the onset of the Holocene (ca. 11 cal kyr BP), Alona spp. and macrophyte-associated taxa became dominant while numbers of C. sphaericus decreased dramatically. In the mid- to late-Holocene, the productivity of Cuoqia Lake seemed to have increased as reflected by higher Cladocera concentration and the appearance of Ceriodaphnia spp. and raptorial P. pediculus which are common in high nutrient waters. Partial Redundancy Analysis (partial-RDA) results highlight the strong effect of indirect climate change (mediated via catchment processes) on cladoceran assemblages during the Holocene. Abrupt shifts in sediment geochemical proxies and regional catchment vegetation at ca. 11 cal kyr BP indicate that substantial nutrient inputs, treeline advancement, and soil development in catchment resulted from the intensified ISM. Therefore, the cladoceran community changes suggest a long-term direct response of the alpine lake ecosystem to climate change since the LGM, but one that was strongly mediated by catchment-vegetation processes, such as the treeline shifts.
... Moreover, SST results differ significantly due to the influence of upwelling (Levitus and Boyer, 1994), ocean currents (Prell et al., 1980) and seasonality in the growth of planktic foraminifera (e.g., Govil and Divakar Naidu, 2011). New quantitative terrestrial records based on pollen (Chen et al., 2014;Herzschuh et al., 2014;Wu et al., 2018) and chironomids Zhang et al., 2017) from the Tibetan Plateau (TP) and Yunnan have been developed to address the issue of long-term climate change in Indian Monsoon affected areas of southwestern China. Vegetation changes, however, are often controlled by the amount or the seasonality of precipitation and by ecological factors rather than temperature (e.g., Contreras-Rosales et al., 2014) and chironomids, which are a key proxy, are also sensitive to in-lake variables, such as pH, dissolved oxygen and lake productivity (Brooks and Birks, 2004;Zhang et al., 2006Zhang et al., , 2007. ...
... variations (Herzschuh et al., 2014;Zhang et al., 2017). More and new paleo-temperature proxies are required to clarify these issues. ...
Article
Here we investigate the sources of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in Lake Ximenglongtan from southwestern China and present a brGDGTs-based Holocene (∼9.4 cal kyr BP) temperature reconstruction. The similarities in the distributions of brGDGTs between surface sediments and catchment soils indicate that soil is the dominant source of brGDGTs. This contrasts with the recent emphasis on lacustrine GDGTs in other regional studies. Holocene temperature evolution is characterized by an early cool phase (with a mean annual air temperature (MAAT) of 12.5 °C) prior to 7.6 cal kyr BP, followed by a rapid warming towards the local thermal maximum (MAAT = 13.8 °C) from 7.6 to 5.5 cal kyr BP and a subsequent long-term cooling that ended at 1.5 cal kyr BP. Temperature changes after 1.5 cal kyr BP show high variability and low correspondence to global climate events such as the Medieval Warm Period. Overall Holocene temperature variation has been primarily controlled by boreal summer insolation changes. The larger amplitude (1.5–4.0 °C) in annual cooling during the early Holocene in MAAT contrasts with summer temperature records from this region (range of cooling 0.3–1.0 °C) and indicates the likely influence of lower winter temperature on MAAT variation. We suggest that substantial remnants of the Northern Hemisphere ice-sheets during the early Holocene enhanced the winter westerlies and East Asian winter monsoon (EAWM), and decreased the winter temperature in southwestern China, leading to a cold early Holocene climate.
... Pollen records from Lugu Lake in Yunnan Province reflected that after 3.3 ka, the vegetation type obviously changed and the pollen concentration decreased, indicating a cooler and drier climate (Zheng et al., 2014). In addition, records from Lake Tiancai, Lake Xingyun and Lake Qinghai in southwestern China indicated a decreasing trend of temperature and humidity after about 3 ka Wu et al., 2018;Zhang et al., 2017). Our records are overall consistent with other records where Indian summer monsoon dominates, and acted in concert with the decreasing summer insolation and weakening ISM (Fig. 9). ...
Article
Knowledge of the chemical weathering history under the subtropical monsoonal climate will help to resolve the discrepancies regarding how climate conditions constrain chemical weathering. Major element and rare earth element (REE) compositions of the Qionghai Lake sediments in Sichuan Province, southwestern China were analyzed, aiming to provide information on sediment source discrimination and history of chemical weathering intensities and its responses to variations of Indian summer monsoon (ISM) and paleoclimate conditions. As revealed by the A-CN-K diagram and other provenance proxies including TiO2/Zr ratio, Th/Sc vs. Zr/Sc and La/Th vs. Hf diagrams, as well as the REE characteristics, the source of Qionghai Lake sediments was felsic rocks. Additionally, these sediments were subjected to small changes in provenance, sedimentary sorting and recycling, and weak diagenesis after deposition and/or metasomatism, indicating original signals were maintained on the intensity of chemical weathering of source area. The chemical weathering intensities inferred from CIA (chemical index of alteration) values have significantly fluctuated over the past 28 kyr, reflecting three stages of paleoclimate evolution, which is consistent with grain size characteristics, carbonate contents, low-frequency magnetic susceptibilities, TOC and biomarker proxy (Paq). The CIA values changed significantly during the typical warm and cold climate events including the Last Glacial Maximum, Heinrich 1 cold event, Younger Dryas and Holocene Climatic Optimum, and generally exhibited an increasing trend in amplitude correspondingly, in concert with the climate shifting from cold and dry to warm and humid conditions. Generally warm and humid climate conditions suggest enhanced chemical weathering intensity, whereas weak chemical weathering denotes cold and dry climate in the ISM region. In addition, sequential variations of the CIA values during the past 28 kyr may reflect changing chemical weathering processes, in responses to the varying intensities of Indian summer monsoon and the climate conditions forced by the Northern Hemisphere solar insolation.
... Since systematic observations of climate only extend back to the nineteenth century, knowledge of past climate changes largely relies on various types of proxy records, e.g., treering data Yang et al., 2014Yang et al., , 2019, ice cores (Yao et al., 2006(Yao et al., , 2018, historical documents (Ge et al., 2003;Luterbacher et al., 2004;Zheng et al., 2006;Hao et al., 2015Hao et al., , 2017, speleothems (Cheng et al., 2019), lake sediments (Chen et al., 2009;Zhang et al., 2017), and corals (Han et al., 2019). Over the past two decades, substantial efforts have been made to reconstruct past temperature changes in regional (Yang et al., 2002;Ge et al., 2010;Shi et al., 2012;PAGES-2k-Consortium, 2013;Wang et al., 2014Wang et al., , 2015Büntgen et al., 2016;Luterbacher et al., 2016;Zhang et al., 2018;Ljungqvist et al., 2019a), hemispheric (Esper et al., 2002;Mann et al., 2009;Shi et al., 2013;Neukom et al., 2014), and even global scales (Neukom et al., 2019a(Neukom et al., , 2019b. ...
Article
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The temperature variability over multidecadal and longer timescales (e.g., the cold epochs in the late 15th, 17th, and early 19th centuries) is significant and dominant in the millennium-long, large-scale reconstructions and model simulations; however, their temporal patterns in the reconstructed and simulated temperature series are not well understood and require a detailed assessment and comparison. Here, we compare the reconstructed and simulated temperature series for the Northern Hemisphere (NH) at multidecadal and longer-term timescales (>30 years) by evaluating their covariance, climate sensitivity and amplitude of temperature changes. We found that covariances between different reconstructions or between reconstructions and simulations are generally high for the whole period of 850–1999 CE, due to their similar long-term temporal patterns. However, covariances between different reconstructions or between reconstructions and simulations steadily decline as time series extends further back in time, becoming particularly small during Medieval times. This is related to the large uncetainties in the reconstructions caused by the decreased number of proxy records and sample duplication during the pre-instrumental periods. Reconstructions based solely on tree-ring data show higher skill than multiproxy reconstructions in capturing the amplitude of volcanic cooling simulated by models. Meanwhile, climate models have a shorter recovery (i.e., lag) in response to the cooling caused by volcanic eruptions and solar activity minima, implying the lack of some important feedback mechanisms between external forcing and internal climate processes in climate models. Amplitudes of temperature variations in the latest published tree-ring reconstructions are comparable to those of the multiproxy reconstructions. We found that the temperature difference between the Medieval Climate Anomaly (950–1250 CE) and the Little Ice Age (1450–1850 CE) is generally larger in proxy-based reconstructions than in model simulations, but the reason is unclear.
... Low diversity of benthic macroinvertebrates such as chironomid larvae on the plateau is mainly caused by harsh macroclimatic conditions, the alpine topography and the partially isolated spatial distribution of aquatic habitats (Meng et al., 2016). Air temperature is the most important environmental factor that controls the distribution of chironomids on a regional scale (Heiri et al., 2011;Zhang et al., 2017;Luoto & Ojala, 2018). The mean elevation of the Hanggay Mountain region (HF) exceeds 2000 m a.s.l., and MJT is cooler than that in the Orkhon River basin. ...
Article
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We analyzed the taxonomic and functional diversities of chironomid communities on the northern Mongolian Plateau to understand the responses to multiple environmental pressures. The 27 lakes studied were classified into three groups based on significant heterogeneity of habitats. A total of 65 chironomid taxa were identified with the dominant taxa being Chironomus pallidivittatus-type, Cricotopus sylvestris-type and Glyptotendipes pallens-type. Our redundancy analysis revealed that temperature, salinity and oxygen concentrations are the main factors influencing the functional feeding traits of the chironomids. Taxonomic and functional alpha diversities show a significant positive relationship across the lakes. Taxonomic alpha diversity is more responsive to the environmental variables in low-altitude freshwater lakes. In contrast, functional alpha diversity contributes to a better understanding of the relationship between chironomid communities and environmental factors in high-altitude freshwater and low-altitude brackish lakes. The decomposition components of overall beta diversity respond differently to taxonomy and function. The turnover component predominates in taxonomic beta diversity, while the nestedness component plays a more important role in functional beta diversity. Our study suggests that taxonomic and functional beta diversities and their partitioning components can be used as a reference for the aquatic biodiversity protection and ecological management in arid and semi-arid regions.
... Lu et al., 2011a), chironomids (e.g. Zhang et al., 2017a) and alkenone-based indexes (e.g. Hou et al., 2016), while mean annual temperature is always reflected by ice core d 18 O records (e.g. ...
... As discussed above, the cliff paintings were probably created with a higher river water level, that is, a humid climate condition. It seems contradictory that ancient people created this giant cliff paintings in the cold and dry late Holocene, but not in the warm-humid early and middle Holocene in the southern China (Dykoski et al., 2005;Sheng et al., 2017;Wang et al., 2005;Zhang et al., 2017). The fact should be that the motivation of the cliff paintings rooted in the social development and the relationship between human and environment. ...
Article
Human activities in the historical period were determined by the natural environment and social characteristics. The Zuojiang Huashan Rock Art Cultural Landscape in Guangxi is the most important World Heritage for prehistorical culture in southwestern China. However, knowledge about the natural environment and associated historic human–climate interactions during the period of cliff painting creation remains scarce, which has been an obstacle to understanding the process of regional civilization. Here, we report a pollen record from the core distributing region of Huashan cliff paintings to interpret the natural environment and climate changes, and further to discuss how and why ancient people create these giant cliff paintings based on the relationships of human–climate interactions. Pollen results imply an existence of a mixed broad-leaf and conifer forest in the woodland landscape at Huashan region before 1860 cal. yr BP, after which pine forests were more prevalent. During the cliff painting period from 2370 to 2115 cal. yr BP, broad-leaf and conifer trees dominated the tree layer and provided the key source materials for painting. It can be reasonably assumed that people used stake or ladder to climb the cliffs and created the cliff paintings during a humid climate condition from 2560 to 2000 cal. yr BP. Furthermore, the humid climate during 2700 and 2400 cal. yr BP had been interrupted by short-term drought episodes. These turbulent climate episodes probably played the role of disasters and impacted on the human activities, leading to the social–political tensions and the out-break of war. During the Qin and Han Dynasties, social–political changes relieved the pressure from natural climate changes and led to a more stable society, in which the creating of cliff paintings was gradually fading. Our results manifest the necessity to analyze the natural environment and social–political background in combination in this study on ancient human activities.
... The Tibetan Plateau (TP), known as the "Water Tower of Asia" and "the Third Pole", is highly relevant to Asian monsoon systems (East Asian monsoon and Indian monsoon), and global climate change due to its massive extension at high altitudes (Krause et al., 2010;Song et al., 2013). Instrumental data have recorded rapid warming during recent decades in this region, approximately three times faster than that of the global average (Zhang et al., 2017). Long-term records derived from paleoclimatic proxies are invaluable for improving our understanding of the natural range and causes of variability in regions compared with instrumental records. ...
Article
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Luminescence dating technology has been used for chronological constraints on lacustrine sediments due to the ubiquitous materials (e.g., quartz and feldspar) as dosimeters, and a relatively long dating range, compared with the commonly used radiocarbon dating method. However, quartz dating on the Tibetan Plateau may suffer from dim and unstable luminescence signals. In the current study, we investigate a lake-related outcrop from the shore of Cuoe Lake on the central Tibetan Plateau. Both coarse-grained quartz and K-feldspar fractions were extracted, and OSL and post-IR IRSL signals were measured from these fractions, respectively. Combining the stratigraphy analysis and dating results, this study shows that: (1) quartz appears to be unsuitable for dating because of very dim natural signals and even anomalous fading (average g -value: 4.30 ± 2.51 %/decade). The suitability of the applied pIRIR protocol measured at 150°C (pIRIR 150 ) for K-feldspar samples was confirmed by a set of luminescence tests; (2) compared with the luminescence-based chronology, the ¹⁴ C age of shells from the same sediment layer yielded older age by ~7 ka, which is likely attributed to hard water reservoir effect in Cuoe Lake; (3) the lake level reached its peak and maintained high-stand during the early Holocene (~9.4–7.1 ka). This study highlights the applicability of K-feldspar luminescence dating when the counterpart quartz OSL is insensitive and encounters anomalous fading.
... More recently, with the use of records of molecular biomarkers, chironomids, and pollen from lacustrine sediments, as well as tree rings, Holocene temperature variations in different periods have been reconstructed. For example, Hou et al. (2016) reconstructed Holocene summer temperature based on alkenone from the sediments of Qinghai Lake; Zhang E L et al. (2017) used chironomids from Tiancai Lake in Yunnan to reconstruct July temperatures; and Huang et al. (2013) produced a temperature reconstruction for Dajiuhu. However, reconstructed temperatures exhibit substantial regional differences across the vast region of China. ...
Article
The natural environment provides material essentials for human survival and development. The characteristics, processes, regional differentiation and forcing mechanisms of the elements of the natural environment (e.g. geomorphology, climate, hydrology, soil, etc.) are the main objects of research in physical geography. China has a complex natural environment and huge regional differentiation and therefore it provides outstanding reserach opportunities in physical geography. This review summarizes the most important developments and the main contributions of research in the physical geography and human living environment in China during the past 70 years. The major topics addressed are the uplift of the Tibetan Plateau and the evolution of its cryosphere, the development of fluvial systems, the acidification of the vast arid region of the Asian interior, variations in the monsoon and westerly climate systems on multiple timescales, the development of lakes and wetlands, the watershed system model, soil erosion, past human-environment interactions, biogeography, and physical geographic zonality. After briefly introducing international research developments, we review the history of research in physical geography in China, focusing on the major achievements and major academic debates, and finally we summarize the status of current research and the future prospects. We propose that in the context of the national demand for the construction of an ecological civilization, we should make full use of the research findings of physical geography, and determine the patterns and mechanisms of natural environmental processes in order to continue to promote the continued contribution of physical geography to national development strategies, and to further contribute to the theory of physical geography from a global perspective.
... The Yunnan-Guizhou Plateau, located on the southeastern (SE) margin of the Tibetan Plateau, is strongly influenced by the subtropical monsoon -South Asian summer monsoon (SASM). The palaeoclimate in this region has been studied using chironomids (Zhang et al., 2017;Chang et al., 2018), diatoms , pollen (Xiao et al., 2014;Yang et al., 2016), pyrogenic carbon (Zhang et al., 2018b), lithology (Guo et al., 2013), and geochemical methods (Zhang et al., 2018a;Feng et al., 2019) in several alpine lakes. However, the occurrence and potential palaeoclimatic significance of aeolian deposits preserved within lacustrine systems have not been explored in this area. ...
Article
Accumulations of aeolian deposits in a range of sedimentary environments provide evidence for past changes in atmospheric circulation patterns over a range of spatiotemporal scales. As yet, there have been relatively few studies of aeolian records in Southwestern (SW) China, and thus there is hitherto untapped potential of using such records to reveal regional atmospheric circulation. Rare earth element (REE) and grain-size analyses were carried out on a well-dated sediment core from Lake Lugu, SW China, to establish the provenance of aeolian deposits preserved within the lake during the last 32 kyr. The core sediments (LGH2) possess high δEuN values (0.73–0.87) and exhibit a moderate REE abundance (156–274 ppm). The δEuN-∑REE pattern of the Lake Lugu sediments are distinct from those of adjacent geographical areas, but are shown to overlap with the sediments from the Kashmir Valley and northwestern (NW) Deccan Plateau indicating a long-distance dust source. The grain-size endmember for the aeolian deposits shows a gradually increasing trend from 32 kyr B.P. to the onset of the Holocene, followed by a sharp decrease around 9 kyr B.P., with very low values observed during the Holocene optimum (8.3–2 kyr B.P.). Aeolian deposition resumed from 2 kyr B.P. and has continued to the present day. Comparisons with both local and regional palaeoclimate records show that both the local climate and East Asian monsoon (EAM) have not influenced aeolian deposition in Lake Lugu. Rather, changing vegetation cover in the north India and NW Deccan Plateau provided the sediment supply, with entrainment governed by variations in the Southern Winter Westerlies in the dry season (north India), and the South Asian summer monsoon (SASM) in the rainy season (NW Deccan Plateau).
... Alpine lakes in southwestern China, close to the Tibetan Plateau, are strongly influenced by the Asian monsoon. Sedimentary records in these lakes can provide long-term insights into past climate dynamics over Southeast Asia ( Xiao et al., 2014;Wang et al., 2016a;Zhang et al., 2017;Li et al., 2018), with important socio-economic or environmental ramifications ( Wang et al., 2005). Generally, the weak Asian monsoon during the late-glacial was replaced by a more intensified monsoon after the onset of the Holocene. ...
Article
Sedimentary records of inorganic elements and pigments over the last 12,000 years are used to assess major changes in limnological conditions of Tiancai Lake (a small treeline lake, SW China), in response to Holocene climate variations. Algal communities shifted from the dominance of cyanobacteria and cryptophytes in the early Holocene, towards siliceous algae in the mid-Holocene and chlorophytes in the late Holocene. Algae responded to a combination of climate-mediated vegetation and soil development associated with allochthonous inputs of dissolved nutrients and organic matter, and sediment infilling. Decreases in Al, Pb, Cu and Zn from the early Holocene probably resulted from soil podsolization and the sequestration of these elements within soils. Changes in Mn and Fe were likely linked to redox condition in catchment soils and water column. Synchronous peaks in Ti, Ba, Ca, Sr, Na, K and Mg, median grain size and magnetic susceptibility coincided with the troughs in the chemical index of alteration, indicating that episodic cold events enhanced upland bedrock erosion and transported unleached and coarse detritus into the lake. These cold events broadly correlate with Holocene ice-rafting events in the North Atlantic. Although the cold events altered the influx of minerogenic elements by regulating upland bedrock erosion, climate-mediated vegetation and soil development led to a muted impact on primary producers. Holocene algal community shifts were subtle, reflecting the relative abundance of P (derived from weathering) and N (derived from soils) throughout the record, with the most pronounced effects on the lake biota being benthic expansion which occurred in response to sediment infilling.
... Furthermore, the winter and summer temperature reconstruction differed considerably at interannual (~1850-1853, ~1856-1859, ~1905-1910 and ~1970-1974), inter-decadal (~1718-1723, ~1763-1780, ~1879-1938 and ~1983-1995), multi-decadal (~1718-1770, ~1830-1841, ~1885-1900 and ~1930-1950) and centennial (~1718-1750, ~1790-1827, ~1889-1942 and ~1968-2005) scales, exhibiting phase mismatches and even inversions over multiple time periods (Fig. 7a-h). Given the climate sensitivity of the TP to changes in drivers (Wang et al., 2014a;Wang et al., 2015;Zhang et al., 2017), these differences and similarities in the T JJA and T DJF reconstructions, mainly on shorter timescales, may reflect the fact that the region's climate was subject to different external driving factors and internal variability. ...
Article
The annual mean temperature on the Tibetan Plateau (TP) has strongly increased over the past few decades, with larger warming in winter than in summer. Whether this different amplitude of change between seasons has persisted over longer time-scales in the past remains poorly understood, limiting our understanding of the mechanisms responsible. Here, we apply multivariate regression analysis and ensemble empirical mode decomposition (EEMD) to decompose winter (TDJF) and summer (TJJA) temperature reconstructions over the 1718–2005 CE period for the southeastern TP to investigate similarities and differences between winter and summer temperature changes, over multiple time-scales, as well as the driving factors behind the seasonal differences. The results reveal that the TDJF and TJJA reconstructions were significantly correlated throughout the study period, with the magnitude of the TDJF variations approximately six times greater than the TJJA variations. When the two reconstructions were decomposed over multiple time-scales, it was found that the consistency between winter and summer temperature reconstructions only existed at inter-annual scale. Assessing the driving factors, the main contributions to the TJJA and TDJF changes at the inter-annual and inter-decadal scales appear to be mainly the El Niño–Southern Oscillation (ENSO). The Pacific Decadal Oscillation (PDO) contribution was important to TJJA and TDJF changes at multi-decadal scales. Furthermore, we found that orbital parameters and the Atlantic Multidecadal Oscillation (AMO) was a major contributor to the changes in TJJA and TDJF at centennial scales, respectively. Both the TJJA and TDJF have a significant long-term increasing trend since c. 1850, mainly attributed to anthropogenic forcing. The detected similarities and differences between TDJF and TJJA at multiple time-scales provide new perspectives on the understanding the mechanisms behind climate change on the Tibetan Plateau and even entire East Asia.
... The Holocene (the last ∼11,400 years) has received much research attention (e.g., Zhou et al., 1991;Mayewski et al., 2004;Renssen et al., 2009), partly because it provides a basis for assessing ongoing and future changes in terrestrial ecosystems (Sun et al., 2006). Numerous studies, including of Holocene climate change (e.g., Feng et al., 2005;Wang et al., 2005;Zhang et al., 2017), lake evolution (e.g., Zhang et al., 2004;Hartmann and Wünnemann, 2009), vegetation change (Xiao et al.,2004;Zhao et al., 2011) and aeolian activity (Jia et al., 2015;Hu et al, 2017) have been conducted in China. It is now clear that climatic patterns differed between different regions (An, 2000;Feng et al., 2006;Chen et al., 2016). ...
Article
A high-resolution lacustrine-peat record from the desert-loess transitional zone in Northern China was obtained to reconstruct Holocene environmental change in the region. AMS ¹⁴C dates are used to provide a chronology. The results indicate that the site was a desert environment before 12.2 cal kyr BP, and was then occupied by a paleolake which started to shrink, with a wetland occurring from 6.2 to 3.0 cal kyr BP. Subsequently, the site became a seasonally water-filled depression. Based on the lithology and measurements of grain size and total organic carbon content, the climate changed from arid to humid at 12.2 cal kyr BP, and became more humid after 8.3 cal kyr BP. From 6.2 to 3.0 cal kyr BP, precipitation decreased but the climate remained at an optimum. After 3.0 cal kyr BP, the climate was dry overall but with several humid intervals. A comparison of paleoclimatic records from lacustrine and aeolian deposits from the region reveals a discrepancy about the nature of the early Holocene climate, and we conclude that this is because lacustrine sediments responded more sensitively to precipitation than aeolian deposits when the temperature was low. The environmental evolution of the region was synchronous with changes in the Asian summer monsoon (ASM), but temperature also played a key role in the early Holocene.
... Although this temperature reconstruction is too distant from Lake Neor to allow a detailed comparison, this study clearly shows that a cooling trend was initiated from ca. 5500 cal yr BP in the Tibetan Plateau. Zhang et al. (2017) suggest that the overall pattern of summer temperatures broadly matches the summer insolation at 30°N and the Asian Summer Monsoon records from the surrounding regions. This trend reflects the general Neoglacial cooling observed from 5000-4000 cal BP onwards at many locations over the Northern Hemisphere (eg. ...
Thesis
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Iran is located at the interface of several major climate systems and biogeographic field. The region was also one of the early centers of Neolithization and plant and animal domestication. In spite of this, Iran remains little documented from the paleoecological point of view and many paleoenvironmental questions remain unanswered: including for example, the effects of seasonality on the functioning of ecosystems, the environmental context during Neolithization, or the role of climatic versus anthropogenic factors in ecosystem changes. The aim of this thesis is to answer these questions using, for the first time, the analysis of chironomid fossils on several sedimentary sequences from Iranian sites located in different natural and anthropogenic contexts. The analysis of Neor Lake (NW Iran) revealed that chironomids are an excellent bioindicator of ephemeral aquatic conditions of wetland in relation to variations in summer temperatures and precipitation seasonality. This study highlights the alternation of dry and wet periods, reflecting the lake's hydrological variations during the Lateglacial and Holocene periods. The climate aridity at the beginning of the Holocene would be related to a strong seasonality of precipitation and in particular a decrease in spring precipitations, fruit of complex interactions between the main regional climatic systems. The wet phases at Neor are due to lower summer temperatures causing less evaporation of the lake. These lower summer temperatures can be partly explained by strengthening of Siberian cold air masses. Results from Lake Arjan (SW Iran) also reveal a major hydrologic change at the Lateglacial-Holocene transition. In Arjan, the Younger Dryas is characterized by open water and deep water conditions. At the start of Holocene, the lake becomes shallow and highly vegetalized, as evidenced by the diversification of littoral taxa associated with aquatic macrophytes. The Rouzian peat bog (SW Iran), situated in an anthropized context, in the north of Persepolis, also presents hydrological variations most likely to be related to the local development of human societies. The increase in aridity between 2500-2000 BP could have prompted the Achaemenids to set up water management systems affecting the wetlands, causing an increase of moisture and water level in the peat bog. The Ardashir pond, located in a Sasanian archaeological site situated in the Firuzabad plain (SW Iran), shows a significant hydrological change around the 5th century AD, as evidenced by the decrease of open water taxa in favor of coastal taxa associated with aquatic macrophytes. Once again, this hydrological change seems to have been caused by human intervention, especially by the extension and the development of the coastal areas of the pond. In conclusion, our work shows that chironomids fossil are a promising paleoecological tool in Iran. Independently of the palynological or geochemical analyzes, chironomids allow to rebuild the hydrological changes and to reveal the impact of the climate or the man on the hydrosystems of the semi-arid zones. Technical limitations of chironomid analysis as a paleoecological tool can be overcome by the implementation of transfer functions based on chironomids. The latter would allow robust quantitative paleoclimatic reconstructions to enhance our knowledge of hydroclimatic and hydrological changes in southwest Asia.
Article
We present a sub-centennial resolution summer temperature record from the SE Tibetan Plateau (Tiancai Lake, SW China, 3900 m a.s.l.) derived from a chironomid stratigraphy covering the last c.19.5 ka. The record highlights the interaction between tropical and high-latitude climate forcing through the changes in atmospheric circulation during the last deglaciation. The scale of the last glacial maximum (LGM) cooling is consistent with other tropical mountain regions at c.5 °C and a rapid recovery of temperatures at 19.0 ka is related to changes in adiabatic lapse rates at the end of the glaciation. The overall pattern of change shows that North Atlantic deglaciation climate events (the Heinrich 1 and Younger Dryas cooling events and the Bølling–Allerød warm period) are all recorded, but the influence of the events decline as the deglaciation progresses. We relate these patterns to North Atlantic Deep water ventilating the Southern Ocean and to the consequent movements of the Intertropical Convergence Zone (ITCZ) in the Indian Ocean Basin as transmitted through changes in the southern mid-latitude circulation.
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Identifying targets for chimeric antigen receptor-modulated T lymphocyte (CAR-T) therapy against solid tumors is an urgent problem to solve. In this study, we showed for the first time that the receptor tyrosine kinase, AXL, is overexpressed in various tumor cell lines and patient tumor tissues including triple negative breast cancer (TNBC) cell lines and patient samples, making AXL a potent novel target for cancer therapy, specifically for TNBC treatment. We also engineered T cells with a CAR consisting of a novel single-chain variable fragment against AXL and revealed its antigen-specific cytotoxicity and ability to release cytokines in a TNBC cell line and other AXL-positive tumors in vitro. Furthermore, AXL-CAR-T cells displayed a significant anti-tumor effect and in vivo persistence in a TNBC xenograft model. Taken together, our findings indicate that AXL-CAR-T cells can represent a promising therapeutic strategy against TNBC.
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Long-term, high-resolution temperature records which combine an unambiguous proxy and precise dating are rare in China. In addition, the societal implications of past temperature change on a regional scale have not been sufficiently assessed. Here, based on the modern relationship between chironomids and temperature, we use fossil chironomid assemblages in a precisely dated sediment core from Gonghai Lake to explore temperature variability during the past 4000 years in northern China. Subsequently, we address the possible regional societal implications of temperature change through a statistical analysis of the occurrence of wars. Our results show the following. (1) The mean annual temperature (TANN) was relatively high during 4000–2700 cal yr BP, decreased gradually during 2700–1270 cal yr BP and then fluctuated during the last 1270 years. (2) A cold event in the Period of Disunity, the Sui-Tang Warm Period (STWP), the Medieval Warm Period (MWP) and the Little Ice Age (LIA) can all be recognized in the paleotemperature record, as well as in many other temperature reconstructions in China. This suggests that our chironomid-inferred temperature record for the Gonghai Lake region is representative. (3) Local wars in Shanxi Province, documented in the historical literature during the past 2700 years, are statistically significantly correlated with changes in temperature, and the relationship is a good example of the potential societal implications of temperature change on a regional scale.
Article
Analysis of lake sediment geochemistry is valuable for understanding past hydroclimatic changes via the reconstruction of changes in catchment erosion, chemical weathering, redox processes and the intensity of evaporation. In this study, we analyzed the geochemical composition, magnetic susceptibility and carbonate content of the Holocene sediments of Lake Chenghai in the Jinsha River valley, southwest China. Our aims were to determine the influence of Holocene hydroclimate evolution and geo-hazards on lacustrine sedimentation. The results show that the hydroclimate was generally humid during 7.57–4.65 cal ka BP, and that subsequently it became relatively dry during 4.65–1.37 cal ka BP. Our results also reveal that the centennial scale variability of the lake level during the Holocene is forced by changes in solar activity and oceanic–atmospheric feedback. In addition, the interval of 3.31–1.95 cal ka BP was the driest, due to highly variable and intense El Niño events, and the southward migration of the intertropical convergence zone. Furthermore, three large landslide events occurred at 7.30, 6.36 and 0.76 cal ka BP around Lake Chenghai, which are correlated with unusually large floods during the period of weakened monsoon activity, and/or with earthquake events.
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We present a sediment record from Lugu Lake, a large and deep alpine lake in southwestern China, spanning the last c.2500 cal yr BP. This multi-proxy study focussed on subfossil chironomids but also included analyses of chemical elements using inductively coupled plasma-atomic emission spectrometry (ICP-AES), magnetic susceptibility, total organic carbon (TOC), total nitrogen (TN) and grain size. The chironomid assemblage is dominated by Procladius, Tanytarsus gracilentus-type and Polypedilum nubeculosum-type throughout the core. The record reveals that these chironomid taxa responded to two periods of catchment erosion between c.1700-1100 cal yr BP and c.1920 A.D. to the present. We provide evidence which suggests that the recent erosion episode is caused by human activities; however, the earlier event (c.1700-1100 cal yr BP) is likely related to increased regional precipitation, possibly linked to the strengthening of the Indian summer monsoon in the late Holocene. It is notable that the chironomids, through their varied ecologies, are able to detect the human-induced changes as well as natural climate changes, for instance, enhanced precipitation. Coupling palaeoecological studies using chironomids with more traditional catchment erosion indicators is thus a powerful tool for reconstructing past environmental and climate change.
Article
A continuous sediment record since 12.3 cal ka bp from Lake Wuxu (south‐eastern Tibetan Plateau) was investigated in terms of the Holocene evolution of the Indian Summer Monsoon. The molar C/N ratio and stable C isotope were used to identify the source of the organic matter as well as climate conditions. The evolution of Lake Wuxu was summarized wihtin two periods. During the first period (early to mid‐Holocene), the lake received increased fluvially transported materials, reflecting variation in the summer monsoon with solar insolation. The lake level declined and water residence time increased because of reduced river discharge during the second period (late Holocene) corresponding to a weakening of the summer monsoon. The organic material revealed a major contribution from lake primary productivity, which showed identical patterns with a high‐resolution isotope record from Dongge Cave, as well as total solar irradiance. Our record from Lake Wuxu indicates that the Holocene evolution of the Indian Summer Monsoon has been driven by the solar forcing at decadal/centennial to millennial time scales. Furthermore, an abrupt decline in the monsoon was detected at around 4.0 cal ka bp, which is probably caused by an increased frequency of EI Nino‐Southern Oscillation events.
Article
Wildfires are sensitive to climate change, but their response to changes in temperature and precipitation on long timescales is still disputed. In this study, we present a ~9.4 ka black carbon mass sedimentation rate (BCMSR) record from Lake Ximenglongtan (XMLT), southwestern China, to elucidate the Holocene fire regime and its linkages to climatic conditions. The results indicate that the regional fire activity was low during the early Holocene (before 7.6 cal ka BP), increased notably at 7.6 cal ka BP, and continued to increase gradually during the mid- to late Holocene until 2.2 ka. The episodes of higher fire occurrence reflected by higher BCMSR over the last 2.2 ka might be more likely related to the intensified human activities. The cool and humid climate during the early Holocene limited the spread of fire, while warming and drying at ~7.6 cal ka BP triggered higher fire occurrence. Instead of temperature, changes in precipitation dominated fire regime variation during the mid- to late Holocene. On millennial timescales, we suggest that Holocene fire variability has been predominantly controlled by the combined effects of Northern Hemisphere (NH) summer and winter insolation that influenced monsoonal precipitation and fire season temperature, respectively. Indian Ocean Dipole (IOD) events may also have affected fire incidence through influencing monsoon intensity.
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.
Article
Many remote montane ecosystems are experiencing biogeochemical changes driven by warming climate and atmospheric pollution. Compared with circumpolar and temperate lakes, the responses of subtropical montane lakes to these external stressors have been less investigated. Here we present sedimentary multi-proxies records (i.e. chironomids, elements and stable isotope of carbon and nitrogen) in 210Pb-dated cores from two montane ponds (central China). Before the 1900s, low biomass and the dominance of opportunistic species (e.g. Chironomus anthracinus-type) in both ponds might be in response to cold and harsh condition. Thereafter, chironomid communities in both ponds experienced pronounced shifts. Nutrient-tolerant/warm-adapted species (e.g. Chironomus sp., Polypedilum nubeculosum-type and Endochironomus impar-type) proliferated and biomass increased synchronously after the 1900s, suggestive of favorable condition for chironomid growth. Redundancy analyses revealed that changes in chironomid communities in both ponds were significantly correlated with rising temperature and δ15N depletion. Prolonged growing season and nitrogen subsidy would increase primary productivity, and hence enhancing food availability for chironomids. Catchment-mediated indirect effects of warming and nitrogen deposition, such as hydrological changes and terrestrial organic matter inputs, would impose further influences on chironomid communities. Taken together, the combined effects of climate warming and nitrogen deposition have caused significant shifts in primary consumers of these montane ponds, and imposed cascading effects on structure and function of subtropical montane aquatic ecosystems.
Article
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A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format. === Open access article: https://rdcu.be/b3y6w ===
Article
High-resolution pollen and charcoal records from Dianchi in central Yunnan Province, southwestern China are presented in this study and reveal variations in vegetation, fire, lake environments, and climate over the last 20,000 years (20 ka). The results show that the climate during the period 20.0–18.0 ka was relatively cold, and rainfall in winter and early spring was relatively lower than the present, which belongs to the Last Glacial Maximum (LGM) . After 18.0 ka, the temperature increased gradually, indicating the onset of deglacial warming in central Yunnan Province, though the mean annual temperature before 10.8 ka was still at least 1.5 °C lower than today. The climate began to obviously warm from ∼10.8 ka and was at least 2 °C higher than today and relatively humid between 10.0 and 5.4 ka, denoting the early-Holocene Thermal Maximum. A major vegetation change around Dianchi occurred at ∼5.4 ka and was most likely caused by the climate cooling and drying with no obvious evidence for cultivation activities except for some possible evidence of human fire use or selective deforestation from the pollen assemblage. Significant cultivation activities in Yunnan Province indicated by the first obvious increases in Poaceae (>40 μm) and Poaceae (25–40 μm) pollen percentages began at ∼2.4 ka. In central Yunnan Province, there was a continuous deglacial warming that occurred between 18.0 and 12.5 ka without marked interruption during the interval of Heinrich Event 1 (H1). Additionally, the Holocene Thermal Maximum occurred in the early Holocene. The results from this study are different from the western region of southwestern China where the paleoclimate studies have clearly detected the H1, the B/A, and the YD events, and revealed a mid-Holocene climatic optimum.
Article
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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.
Article
The climatic significance of the carbonate content and composition of lake sediments remains unclear. In this study, we analyzed the glycerol dialkyl glycerol tetraethers and mineral contents of Alake Lake sediments to reconstruct temperature and moisture changes over the northeast Tibetan Plateau since 8000 cal yr BP. We identified warm (cold) and wet (dry) conditions during the Roman Warm Period and Holocene Climate Optimum (Early Neoglacial Cooling Period) accompanied by a dominance of aragonite (monohydrocalcite). Based on our results and previous laboratory experiments, we suggest that monohydrocalcite is predominantly deposited in colder, drier, and higher Mg/Ca conditions relative to that of aragonite. The carbonate mineral composition should therefore be fully considered when sediment carbonate contents are used to reconstruct paleotemperature/moisture changes.
Article
Studies of the abrupt climate events that punctuated the warm Holocene epoch allow us to understand the climate system, forecast future climate change, and interpret the cultural transformations. Paleoclimate records around the world show manifestations of prominent climate anomalies between 6000 and 5000 calibrated years before present (cal BP). However, a comprehensive review of these anomalies remains unavailable in China. In this article, we collected, compiled, and analyzed proxy climate evidence from 49 sites recorded by a variety of natural archives, including lacustrine sequences, speleothems, peat profiles, an ice core, and marine sediments, to document these changes. Our synthesis results indicate that pronounced climatic anomalies generally characterized by cold and dry conditions occurred during 6000–5000 cal BP, which were associated with weakening of the Asian summer monsoon. However, these anomalies show a lack of absolute synchronicity or equality by various proxy records because of the different proxy sensitivities and/or dating uncertainties. Such 6000–5000 cal BP climatic anomalies also have possible equivalents in proxy records from various regions around the world. Comparison of paleoclimate records with climate forcing time series suggests that both the reduced solar activity and a long-term trend of orbitally-driven insolation change are the main driving mechanisms for these short-lived climate changes. More quantitative and higher-resolution palaeoclimatic reconstructions are needed to fully capture the magnitude, timing, and mechanism of the 6000–5000 cal BP climatic anomalies.
Article
Orbital-scale global climatic changes during the late Quaternary are dominated by high-latitude influenced ∼100,000-year global ice-age cycles and monsoon influenced ∼23,000-year low-latitude hydroclimate variations. However, the shortage of highly-resolved land temperature records remains a limiting factor for achieving a comprehensive understanding of long-term low-latitude terrestrial climatic changes. Here, we report paired mean annual air temperature (MAAT) and monsoon intensity proxy records over the past 88,000 years from Lake Tengchongqinghai in southwestern China. While summer monsoon intensity follows the ∼23,000-year precession beat found also in previous studies, we identify previously unrecognized warm periods at 88,000−71,000 and 45,000−22,000 years ago, with 2−3 °C amplitudes that are close to our recorded full glacial-interglacial range. Using advanced transient climate simulations and comparing with forcing factors, we find that these warm periods in our MAAT record probably depends on local annual mean insolation, which is controlled by Earth’s ∼41,000-year obliquity cycles and is anti-phased to annual mean insolation at high latitudes. The coincidence of our identified warm periods and intervals of high-frequent dated archaeological evidence highlights the importance of temperature on anatomically modern humans in Asia during the last glacial stage.
Article
Lake sediments are widely applied to investigate palaeoclimatic and palaeoenvironmental changes on the Tibetan Plateau (TP). Radiocarbon dating of various materials is one of the most commonly used methods to constrain the timing of lake sediment deposition, but it is problematic for most lakes on the TP due to carbon reservoir effects. Because palaeomagnetic secular variations (PSV) share similar patterns at the regional scale, PSV-stratigraphic correlation can provide an alternative approach to date lake sediments. Here, we present a PSV-depth model for a sediment core taken from Guozha Co, a glacial meltwater-fed lake on the northwestern TP. To establish reliable chronological constraints on the sediment core, we compared our PSV-depth model with the PSV stack of East Asia, PSV records of Tangra Yumco and Lake Baikal. The establishment of this new chronology at Guozha Co is a major step toward dating lake records more widely in the TP to reveal palaeoclimatic changes at the world's ‘Third Pole’.
Article
In large river basins, the relationship between silicate weathering and climate variability on various temporal scales is still highly debated. This study presents clay Li-Nd isotopes and elemental compositions of bank sediments from the Changjiang (Yangtze) River, and of sediments from core CM97 recovered from its delta. In the modern basin, clay δ7Li and εNd values of bank sediments range from −4.1‰ to 0.9‰ and from −13.1 to −10.7, respectively. In core CM97, clay δ7Li and εNd values vary from −2.5‰ to −1.1‰ and from −12.7 to −11.9. We infer that the variations of clay Li isotope compositions are mostly indicative of weathering intensity changes in the Changjiang basin, and are little influenced by mineralogical sorting, lithology and early diagenesis. Over the last 13 kyr, three periods with distinct clay mineralogy and geochemical compositions can be identified. During the Younger Dryas, the positive excursion of clay δ7Li (by ∼1.4‰) is consistent with enhanced incongruent weathering of silicates (i.e., weakened weathering intensity) in the mid-lower Changjiang basin, likely due to climate cooling. The minor variation of clay δ7Li (average −1.6 ± 0.2‰) at 11–2 ka confirms the small changes of weathering conditions, which is consistent with mild climatic variations. Over the last 2 kyr, isotopic fluctuations are best explained by sediment source changes, which might be caused by the intensification of human activities in the mid-lower basin. Overall, this study supports a rapid response of silicate weathering to hydroclimate changes on millennium timescale in large catchments. It also provides new insights on Li isotope fractionations at the continental scale during the Quaternary period.
Article
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
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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.
Article
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.
Article
Full-text available
Long-term, high-resolution temperature records which combine an unambiguous proxy and precise dating are rare in China. In addition, the societal implications of past temperature change on regional scale have not been sufficiently assessed. Here, based on the modern relationship between chironomids and temperature, we use fossil chironomid assemblages in a precisely-dated sediment core from Gonghai Lake to explore temperature variability during the past 4000 years in northern China. Subsequently, we address the possible regional societal implications of temperature change through a statistical analysis of the occurrence of wars. Our results show that: (1) the mean annual temperature (TANN) was relatively high from 4000–2700 cal yr BP, decreased gradually from 2700–1270 cal yr BP, and then fluctuated drastically during the last 1270 years. (2) A cold climatic event in the Era of Disunity, the Sui-Tang Warm Period (STWP), the Medieval Warm Period (MWP) and the Little Ice Age (LIA) can all be recognized in the paleotemperature record, as well as in many other temperature reconstructions in China. This suggests that our chironomid-inferred temperature record for the Gonghai Lake region is representative. (3) Local wars in Shanxi Province, documented in the historical literature during the past 2700 years, are statistically significantly correlated with changes in temperature, and the relationship is a good example of the potential societal implications of temperature change on a regional scale.
Article
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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 (r²bootstrap) 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.
Article
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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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Abstract Available from http://www.agu.org
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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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The IntCal09 and Marine09 radiocarbon calibration curves have been revised utilizing newly available and updated data sets from 14C measurements on tree rings, plant macrofossils, speleothems, corals, and foraminifera. The calibration curves were derived from the data using the random walk model (RWM) used to generate IntCal09 and Marine09, which has been revised to account for additional uncertainties and error structures. The new curves were ratified at the 21st International Radiocarbon conference in July 2012 and are available as Supplemental Material at www.radiocarbon.org. The database can be accessed at http://intcal.qub.ac.uk/intcal13/. © 2013 by the Arizona Board of Regents on behalf of the University of Arizona.
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Radiocarbon dating is routinely used in paleoecology to build chronologies of lake and peat sediments, aiming at inferring a model that would relate the sediment depth with its age. We present a new approach for chronology building (called "Bacon") that has received enthusiastic attention by paleoecologists. Our methodology is based on controlling core accumulation rates using a gamma autoregressive semiparametric model with an arbitrary number of subdivisions along the sediment. Using prior knowledge about accumulation rates is crucial and informative priors are routinely used. Since many sediment cores are currently analyzed, using different data sets and prior distributions, a robust (adaptive) MCMC is very useful. We use the t-walk (Christen and Fox, 2010), a self adjusting, robust MCMC sampling algorithm, that works acceptably well in many situations. Outliers are also addressed using a recent approach that considers a Student-t model for radiocarbon data. Two examples are presented here, that of a peat core and a core from a lake, and our results are compared with other approaches.
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Hydrography of the Bay of Bengal is highly influenced by the river runoff and rainfall during the southwest monsoon. We have reconstructed δ18Osw, sea surface salinity and sea surface temperature (SST) changes in the Bay of Bengal by using paired measurements of δ18O and Mg/Ca in a planktonic foraminifera species Globigerinoides ruber from core SK218/1 in the western Bay of Bengal in order to understand the rainfall variability associated with southwest monsoon over the past 32kyr. Our SST reconstructions reveal that Bay of Bengal was ∼3.2°C cooler during the LGM as compared to present day temperature and a ∼3.5°C rise in SST is documented from 17 to 10ka. Both SST and δ18Osw exhibit greater amplitude fluctuations during MIS 2 which is attributable to the variability of NE monsoon rainfall and associated river discharge into the Bay of Bengal in association with strong seasonal temperature contrast. On set of strengthening phase of SW monsoon was started during Bølling/Allerød as evidenced by the low δ18Osw values ∼14.7ka. δ18Osw show consistently lower values during Holocene (with an exception around 5ka), which suggests that the freshening of Bay of Bengal due to heavy precipitation and river discharge caused by strong SW monsoon. Results of this study signify that the maximum fluctuations of the NE monsoon rainfall during MIS 2 appear to be controlled by the strong seasonality and boundary conditions.