Temperature variations since 1750 CE inferred from an alpine lake in the southeastern margin of the Tibetan Plateau

Quaternary International 436 (2017) 37e44

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Temperature variations since 1750 CE inferred from an alpine lake in the southeastern margin of the Tibetan Plateau Jingjing Li a, Lingyang Kong a, Huan Yang b, Qian Wang a, Xiangdong Yang a, Ji Shen a, Cheng Zhao a, * a

State Key Laboratory of Lake Sciences and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 October 2016 Received in revised form 8 December 2016 Accepted 18 December 2016 Available online 1 March 2017

High-altitude regions are thought to be very sensitive to global climate change, especially under current global warming. However, due to the lack of paleoclimate archives such as ice core and tree-rings above the modern tree line, limited paleoclimate data have been extracted to extend instrumental data over the past few centuries, which will help understand the interannual-to decadal-scale climate variability in high-altitude regions. Here we present a high-resolution (1e6 yr) quantitative temperature record from a remote alpine lake at the southeastern margin of the Tibetan Plateau, by applying a novel proxy based on branched glycerol dialkyl glycerol tetraethers (brGDGTs). Contrary to the often-documented warming trend over the past few centuries, but consistent with temperature record from the northern Tibetan Plateau, our data show a gradual decreasing trend of 0.3
C in mean annual air temperature from 1750 to 1970 CE. This result suggests a gradual cooling trend in some high altitude regions over this interval, which could provide a new explanation for the observed decreasing Asian summer monsoon. In addition, our data indicate an abruptly increased interannual-to decadal-scale temperature variations of 0.8 e2.2
C after 1970 CE, in terms of both magnitude and frequency, indicating that the climate system in high altitude regions would become more unstable under current global warming. © 2016 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Temperature reconstructions Alpine lake brGDGTs Climate variability

1. Introduction Current global warming has induced profound climatic and environmental changes in different regions. For example, in tropical areas, global warming induced an intensified variability of Indian Ocean Dipole (Abram et al., 2008), an increase in the frequency and intensity of tropical cyclone activities (Knutson et al., 2010), a strengthening rainfall triggered by the equatorial maximum of sea surface temperature (SST) (Xie et al., 2010). In the polar region, global warming caused significant warming of the Antarctic winter troposphere (Turner et al., 2006), accelerated declining of Arctic sea ice cover (Stroeve et al., 2007; Comiso et al., 2008), and increased the ice sheet mass loss of Greenland (van den Broeke et al., 2009). Recent studies also suggest that high-altitude regions are climatically and ecological sensitive to global climate change (Pepin et al., 2015). There is growing evidence indicating that the rate of global

* Corresponding author. E-mail address: [email protected] (C. Zhao). http://dx.doi.org/10.1016/j.quaint.2016.12.016 1040-6182/© 2016 Elsevier Ltd and INQUA. All rights reserved.

warming is amplified at high-altitude regions, which likely represent the influence of natural climate change as these sites usually locate in remote areas with very limited regional and local anthropogenic impacts (Borgaonkar et al., 2011; Catalan et al., 2013). In addition, climate responses of high-altitude regions to global warming could be spatially heterogeneous owning to the large geological and topographic characteristic discrepancy (Hobbs et al., 2011; Slemmons et al., 2013). Because of the dearth of highresolution, quantitative paleoclimate records over the past few centuries, understanding the spatial and temporal climate response to global warming is limited in remote high-altitude regions. In the last decade, Glycerol Dialkyl Glycerol Tetraethers (GDGTs), tetraethers membrane lipids synthesized by archaea and some bacteria (Schouten et al., 2013), have been proposed as a potential tool for quantitative temperature reconstructions in marine and ~ eda and Schouten, 2011). Unlike later on terrestrial archives (Castan the TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) proxy based on archaea-produced isoprenoid GDGTs (isoGDGTs) is mainly used in marine environment (Schouten et al.,

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J. Li et al. / Quaternary International 436 (2017) 37e44

2002; Kim et al., 2008, 2010), indices based on bacteria-produced branched GDGTs (brGDGTs) are widely used in terrestrial systems. Analyses of brGDGTs in a set of global soil samples have revealed that the mean annual air temperature (MAAT) and soil pH regulate the distribution pattern of brGDGTs (Weijers et al., 2007). For instance, the Cyclisation ratio of Branched Tetraethers index (CBT, Eq. (1)) is controlled by soil pH, while the Methylation of Branched Tetraethers (MBT, Eq. (2)) correlates well with both soil pH and MAAT (Weijers et al., 2007). Recently the MBTeCBT index has been recalibrated and revised as MBT0 eCBT (Eq. (3)), a recalibration of extended soil dataset from Weijers et al. (2007) suggested that the new transfer function based on MBT0 eCBT led to more reasonable estimated temperatures (Peterse et al., 2012) (Eq. (4)). Following this original discovery, studies have been undertaken to examine the modalities of the paleotemperature proxy in lacustrine system (Tyler et al., 2010; Fawcett et al., 2011; Niemann et al., 2012; Heyng et al., 2015). These studies have shown the distributions of brGDGTs in lakes are also affected by temperature, although the relationships differ from that in soils due to the distinctive distributions of brGDGTs between lake sediments and  et al., 2009; Tierney and surrounding soils (Sinninghe Damste Russell, 2009; Loomis et al., 2011). On this basis, several global and regional lakeespecific temperature calibrations have been generated (Tierney et al., 2010; Pearson et al., 2011; Sun et al., 2011; Loomis et al., 2012; Günther et al., 2014). Despite the complexity sources of brGDGT in lake setting, the application of MBTeCBT to reconstruct temperature offering a potential proxy in paleolimnology especially when lake monitoring data and other proxies are limited. Owing to the active tectonic processes, high-altitude mountains in the eastern margin of the Tibetan Plateau is calved by steep valleys and displays dramatic topographic variations (Kirby et al., 2002). Therefore, it is an ideal place to study the spatially heterogeneous climate change and climate sensitivity in high-altitude regions (Günther et al., 2011). Previous paleoclimate studies in this area were mainly focused on reconstructing temperature and precipitation changes inferred by the ice core d18O and tree-ring analyses (Yang et al., 2009; Liu et al., 2011; Li et al., 2015a, 2015b), and tracing ecological changes by analyzing diatoms, pigments (Hu et al., 2014, 2016) and Cladocerans (Kong et al., 2016). So far there is very limited quantitative data being reported from this region, especially from locations above the modern tree line (Holmes et al., 2009). In this study, we use brGDGTs from a sediment core collected at an alpine lake to reconstruct quantitative temperature variations over the past ~260 years, with a sampling resolution of 1e6 years. We aim to investigate the interannual-to decadal-scale climate variations over the past few centuries for a region with limited human disturbance, and to better understand the nature of regional climate response to current global warming in high-altitude areas. 2. Materials and methods 2.1. Jiren Lake and modern climate setting Jiren Lake (29
4301600 N, 100
480 5800 E) is located in Ganzi Tibetan Autonomous Prefecture, Sichuan Province, southwestern China. It has an elevation of 4480 m above sea level (a.s.l), above the present tree line (Fig. 1a and b). The lake was formed by glacial retreat during the last deglaciation (Wang and Dou, 1998). Currently, the freshwater lake is still recharged snow meltwater originated from the nearby mountaintop with an elevation of >5000 m. There is a single outflow at the southwest shore (Fig. 1b) (Kong, 2015). The lake area is about 0.12 km2, and the catchment basin is about 4.71 km2. The mean water depth is 11.7 m, with the

maximum depth of 28.5 m (Fig. 1b). There is no direct human activity in the catchment basin, no fish or vertebrate has been found in Jiren Lake. The vegetation type surrounding the lake is alpine shrub, dominated by Rhododendron lapponicum (Fig. 1c). The bedrock is clastic rocks with lava (Kong, 2015). Currently, the study region is under the influences of Indian Summer Monsoon. Based on the instrumental data (1971e2014) from the nearest meteorological station in Litang County (~23 km to the northwest, 3949 m a.s.l), the mean annual air temperature (MAAT) is ~3.6
C. If we consider the altitude effect of 6
C temperature decrease with every 1000 m elevation increase, then the MAAT of Jiren Lake is ~0.4
C, the summer temperature is 7.4
C, while the winter temperature is 7.5
C (http://data.cma.cn/). The Litang instrumental data also indicate a mean annual precipitation of 730 mm, with near 90% of which occurs during the summer monsoon season (May to September) (Fig. 1d). 2.2. Sample collection and lab analysis A 38-cm-long sediment core JR was retrieved from the deepest site of the lake (water depth ~28 m) using a Hon-Kajak gravity corer in May 2014. The core is dominated by yellow brown silt in the upper 5 cm, and gray black ooze mud below 5 cm. The sediment core was sub-sectioned at every 5 mm interval in the field, and stored at <4
C immediately after the collection. In the laboratory, the core was analyzed for total organic carbon (TOC), total phosphorus (TP), the ratio of carbon to nitrogen (C/N) and the amount of titanium (Ti) (Fig. 5), see details in Kong (2015). The chronology is determined by 210Pb and 137Cs analyses and constructed by the combined models of constant rate of supply (CRS) and constant initial concentration (CIC), which yields the chronological uncertainty from 0.01 yr at the top to 7.8 yr at the bottom of the core (see details in Kong (2015), Fig. 3a), and to 0.28 yr of the top 50 years, while the resolution is 1e6 yr, on average 3e4 yr. The data of TOC, TP,C/N, Ti and the chronology can be reviewed in Kong (2015). 2.3. GDGT extraction and analysis Lipid extraction was generally followed He et al. (2014) with some modifications. Each sample (ca. 2 g) was ultrasonically extracted 4 times with the organic solvent of dichloromethane DCM/MeOH (9:1, v/v). The total lipid extract (TLE) was concentrated using N2 then added 6% KOH in methanol solution for further hydrolysis. The neutral lipids were extracted with n-hexane and then separated using column chromatography with silica gel as stationary phase and n-hexane and MeOH as flowing phase to yield the apolar and polar fraction. The polar fraction, containing the GDGTs, was filtered over a 0.45 mm PTFE filter with hexane/isopropanol (99:1, v/v) and dried under N2 prior to analysis using high performance liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (HPLCeAPCIeMS). GDGT analyses were performed with an Agilent 1200 series liquid chromatograph coupled with a triple quadrupole mass spectrometer by applying single ion monitoring (SIM) mode and m/ z 1302, 1300, 1298, 1296, 1294, 1292, 1050, 1048, 1046, 1036, 1034, 1032, 1022, 1020, 1018 and 744 to enhance sensitivity. Separation was achieved by using an Alltech Prevail Cyano column (150  2.1 mm, 3 mm), following the method of Li et al. (2016). Quantification was obtained by addition of an internal synthetic C46 GDGT standard (Huguet et al., 2006). The final quantification was semi-quantitative as the relative response factor between the individual GDGTs and the standard was not determined. The MBT and CBT indices were calculated following Weijers et al. (2007):

J. Li et al. / Quaternary International 436 (2017) 37e44

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Fig. 1. (a) Overview map showing the study location, modified from Kong (2015). (b) The bathymetry map of Jiren Lake, white star indicates the location of sampling site. (c) A photo of the catchment at the Jiren Lake. (d) Monthly variations of total precipitation (bars), mean temperature (line with circles) for Litang Meteorological Station (23 km from Jiren Lake to the northwest, 3949 m a.s.l) over the period of 1971e2014 CE.

  ð½Ib þ ½IIbÞ CBT ¼ elog ð½Ia þ ½IIaÞ

MBT ¼

(1)

ð½Ia þ ½Ib þ ½IcÞ ð½Ia þ ½Ib þ ½Ic þ ½IIa þ ½IIb þ ½IIc þ ½IIIa þ ½IIIb þ ½IIIcÞ (2) MBT0

The revised was calculated according to the equation developed by Peterse et al. (2012):

MBT0 ¼

ð½Ia þ ½Ib þ ½IcÞ ð½Ia þ ½Ib þ ½Ic þ ½IIa þ ½IIb þ ½IIc þ ½IIIaÞ

(3)

The revised MBT0 eCBT calibration function explored by Peterse et al. (2012) was assigned in this study to reconstruct the mean annual air temperature:

MAATPeterse2012 ¼ 0:81  5:67  CBT þ 31:0  MBT0

(4)

The Branched and Isoprenoid Tetraether (BIT) index, indicating the soil input index was calculated following the equation of Hopmans et al. (2004):

BIT ¼

GDGTeIa þ GDGTeIIa þ GDGT  IIIa GDGTeIa þ GDGTeIIa þ GDGTeIIIa þ Cren

(5)

3. Results All analyzed samples from core JR contain brGDGTs and generally have consistent distribution patterns. Type brGDGT III is the most abundant components of brGDGTs in all sediment core samples, the percentage of III ranges from 46% to 51%, with the average of 48%, then followed by brGDGT II and I, while the average proportion of II and I is 39% and 13%, respectively (Fig. 2). Overall, the percentage of brGDGTs bearing no cyclopentane ring is higher than brGDGTs containing cyclopentane ring(s). For example, the percentage of IIIa (ranging from 45% to 50%, average 47%) is higher than other brGDGTs, then followed by IIa (30%) and Ia (10%). When comes to the individual brGDGTs containing cyclopentane rings, brGDGT IIb is the most abundant brGDGT containing cyclopentane rings, with a percentage of 8% (Fig. 2). The concentration of brGDGTs and crenarchaeol is normalized to TOC and shows variability along the lake sediment core. The amount of total brGDGTs varies from 2.3 to 315 mg/g TOC, with the average of 76.7 mg/g TOC.

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J. Li et al. / Quaternary International 436 (2017) 37e44

trend in MAAT from 1.9 to 1.1
C, with slight decadal-scale fluctuations on top. While after 1970 CE, our data show a pronounced interannal-to decadal-scale variations from 0.8 to 2.2
C. 4. Discussion 4.1. BrGDGTs based quantitative MAAT reconstructions and comparison with instrumental temperature records after 1970 CE

Fig. 2. The fractional abundance of individual brGDGTs in core JR.

Comparing to brGDGTs, the concentration of crenarchaeol is relative lower, ranging from 0.01 to 1.71 mg/g TOC. Our reconstructed MAATs at Jiren Lake range between 0.8 and 2.2
C, with an average value of 1.6
C during the past 260 years (Fig. 3b). Before 1970 CE, the record shows a gradually decreasing

The ratio of high brGDGTs and low crenarchaeol from core JR at Jiren lake is generally stable, leading to stable and high BIT values of about 0.99e1, all close to 1. This likely indicates that brGDGTs of the core are mainly derived from the catchment soils instead of the in situ production within the water column (Hopmans et al., 2004). Further, Jiren Lake is a flow-through lake likely having a very short residence time, this probably leads to higher contributions of soilderived brGDGTs than that of produced within lake water. Therefore, soil calibration(s) could be better than lake calibration(s) for MAAT reconstructions for this study. Indeed, we have tried a set of published global and regional soil (Weijers et al., 2007; Peterse et al., 2012; Yang et al., 2014) and modern lake calibrations (Tierney et al., 2010; Pearson et al., 2011; Sun et al., 2011; Loomis et al., 2012; Günther et al., 2014), to reconstruct MAAT variations at Jiren Lake for the last 260 years. Most of these calibrations result in more than 5
C differences between reconstructed temperatures and altitude-corrected

Fig. 3. Comparison between reconstructed and instrumental MAATs. (a) Age-depth mode for Jiren Lake sediment core (Kong, 2015). (b) Reconstructed MAATs from Jiren Lake based on the soil calibration proposed by Peterse et al. (2012) during the past 260 years. (c) The observed MAAT of Litang Meteorological Station (~23 km, 3949 m) from 1971 to 2014. (d) Represents the linear regression between the instrumental MAAT of Litang Meteorological Station from 1971 to 2014 and the year. (e) Represents the linear regression between the reconstructed temperatures of core JR from 1960 to 2010 and the year. (f) Reconstructed temperatures of core JR from 1960 to 2014. (g) The instrumental MAAT of Litang Meteorological Station from 1971 to 2014. The line between (f) and (g) represents the corresponding variations between reconstructed MAATs at Jiren Lake and the instrumental MAATs of Litang.

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Fig. 4. Comparisons of (a) Reconstructed MAATs from Jiren Lake, (b) reconstructed temperatures based on alkenone indices of Lake Sugan in the northern Tibetan Plateau (He et al., 2013a, 2013b), (c) d18O record from Xiaobailong Cave in Yunnan Province (Tan et al., 2016), (d) Tropical Indo-Pacific ocean SST anomalies (Wilson et al., 2006). The dash arrows in (a), (b), (c) and (d) show the linear trends of these records.

instrumental data for Jiren Lake, with the same variation pattern. Fortunately, we find MAAT reconstructed by the revised MBT0 eCBT global soil calibration proposed by Peterse et al. (2012) ranges between 0.8 and 2.2
C with the average value of 1.5
C for the period 1971e2014 CE, which is comparable with the altitude-corrected instrumental data which show variations between 1.0e3
C with the average value of 0.4
C. Our data did not capture the significant warming trend over the past several decades as shown in global observational data synthesis. There is also a warming trend toward the present in instrumental data from Litang Meteorological Station (Fig. 3c). This record shows a gradual increase in MAAT with ~3
C from 1971 to 2014 CE, with a linear regression shown in Fig. 3d. In contrast, our reconstructed MAATs at Jiren Lake show a slightly warming trend of ~0.2
C from 1971 to 2010 CE (Fig. 3e). Different slopes have been observed between the instrumental and reconstructed temperatures (Fig. 3d and e), probably indicating the spatially and vertically complex regional climate variations in high-latitude mountainous areas, and/or different warming rate between the instrumental and reconstructed MAATs. Further, we calculated the reconstructed MAATs based on other published calibrations as we mentioned above (Weijers et al., 2007; Tierney et al., 2010; Pearson et al., 2011; Sun et al., 2011, Loomis et al., 2012; Peterse et al., 2012; Günther et al., 2014), all of the slopes from these linear regression are < 0.01, lower than the instrumental record (0.043, Fig. 3d). However, some of them are higher than our reconstructed MAAT record (0.0063, Fig. 3c) calibrated by Peterse et al. (2012), suggesting that reconstructed temperatures based on these calibrations may capture the increasing trend from 1971 to 2010 CE. Although the reason for a decreasing trend of reconstructed MAATs between 2012 and 2014 is unknown and the extent of estimated MAATs and recorded MAATs over the period from 1971 to 2011 is

different, variation of reconstructed and recorded MAATs after 1970 CE is consistent. As shown in Fig. 3f, our data show minimum extreme temperatures around 2003 CE, 1997 CE, 1990 CE, 1985 CE, and maximum extreme temperatures around 2010 CE, 2007 CE, 2000 CE, 1992 CE, 1988 CE. Considering the relatively large chronological uncertainties originating from 210Pb analyses and the compression and movement of the soft mud of the core, these reconstructed temperature extremes correspond to the instrumental data from Litang Meteorological Station (Fig. 3g). Therefore, we argue that our reconstructed MAAT inferred from Jiren Lake could represent natural temperature variations for the past ~260 years in the study region.

4.2. Decreasing temperature from 1750 to 1970 CE in the eastern and northern Tibetan Plateau Our record indicates a slightly cooling trend from 1750 CE to 1970 CE, with a decreasing temperature ~0.3
C (Fig. 4a). This is quite different with the global warming (0.12
C, per decade) since the mid-20th century as indicated by the IPCC 5th assessment report. A number of temperature reconstructions in the Tibetan Plateau have also show a warming trend over the past 2e3 centuries (Yang et al., 2009; Liu et al., 2013; Wu et al., 2013). However, a recently published alkenone-based summer air temperature reconstruction at Sugan Lake in the northern Tibetan Plateau do show a gradual temperature decrease of ~1.0
C over the past 250 years (Fig. 4b) (He et al., 2013a, 2013b). Indeed, a study focusing on the synchronicity and discrepancy of temperature variations along the north and south of the Eastern margin of the Tibetan Plateau (ETP) indicated that temperature patterns in different regions displays various tendency at different timescales (Xu et al., 2014). This is probably caused by different sensitivity of temperature proxies

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J. Li et al. / Quaternary International 436 (2017) 37e44

Fig. 5. Comparisons of (a) Reconstructed MAATs from Jiren Lake, (b)The result of planktonic Cl flux, (c) PCA1 of cladoceran assemblage, (d) Total organic carbon (TOC), (e) Total phosphorus (TP), (f) The ratio of carbon to nitrogen (C/N), (g) The amount of titanium (Ti) from the same core at Jiren Lake analyzed by Kong (2015), (h) Reconstructed Asian (Shi et al., 2015), Northern Hemisphere, and global surface temperature anomalies (Mann et al., 2008).

derived from different natural archives, and various natural and anthropogenic forcings, like solar irradiance, volcanic aerosol, greenhouse gas, and land use change (Jones and Mann, 2004). A recent well-dated stalagmite d18O record from southwestern China revealed a decreasing trend in monsoon precipitation during the last 220 years (Fig. 4c) (Tan et al., 2016). This is also supported by other tree-ring based data from Nepal Himalaya (Sano et al., 2012), southern (Grießinger et al., 2011) and southeastern Tibetan Plateau (Wernicke et al., 2015). It has been argued that the decreasing monsoon precipitation in southwestern China is induced by a decreased land-ocean thermal contrast, which is attributed to a warming trend in the Indo-Pacific Ocean SST over the past two centuries (Wilson et al., 2006) (Fig. 4d). As a decrease in the land-ocean thermal contrast could be caused either by the ocean warming or a land cooling, or both of them. Our record based on the eastern Tibetan Plateau lake sediment core may provide a new evidence of continental cooling and thus further support the idea of decreasing monsoon precipitations over the past 2e3 centuries. 4.3. Multi-proxy evidence of increased temperature variability in the eastern Tibetan Plateau since 1970 CE Our data also reveal a clear evidence of an abrupt increase in interannual-to decadal-scal temperature variability since ~1970 CE, in terms of both frequency and magnitude (Fig. 5a). From 1750 to 1970 CE, the reconstructed MAAT gradually decreased about 0.3
C, without obvious interannual-to decadal-scale variations. Right after 1970 CE, our data display about 0.8e2.2
C, with obvious interannual-to decadal-scale fluctuations. Other ecological proxies

obtained from the same sediment core, which sensitive to temperature variations, also show an increased variability around 1970 CE. For instance, the flux of Planktonic cladoceran (Planktonic Cl), an indicator of lake trophic state (Chen et al., 2010) and tempera ska and Zawisza, 2011; Nevalainen and Luoto, 2012) ture (Szeroczyn (Fig. 5b), show an increased interannual-to decadal-scale variations during the period from 1970 to 2014 (Kong, 2015). In addition, PCA1 values obtained from principal component analyses of cladoceran assemblage, the proxy of the major ecological trend and mainly influenced by temperature, indicate highest variations after 1970 CE than prior to 1970 CE (Fig. 5c) (Kong, 2015). The increased temperature variability likely reflects a climate signal, rather than other environmental or ecological variations within or outside the lake. Within the lake, the total organic carbon (TOC) content shows a stable high value after 1970 CE, with a bit larger fluctuations in some decades before (Fig. 5d). The total phosphorus (TP) (Fig. 5e), a proxy widely used as nutrient availability, was kept at a stable low level before 1970 CE and then rapidly increased thereafter, but without any interannual-to decadal-scale variations. Outside the lake but within the catchment basin, the ratio of carbon to nitrogen (C/N) (Fig. 5f), indicator of relative contributions of terrestrial and aquatic organisms (Lobbes et al., 2000), generally shows a gradual reducing trend over the past ~ 260 years with obvious variations before 1970 CE. Concentrations of titanium (Ti), a proxy widely used for chemical erosions of soils and bedrocks in the catchment basin (Shen et al., 2013), present a gradual declining trend over the past ~260 years, without any short-scale variations as well (Fig. 5g). The shift in climate variability recorded in our record likely corresponds to an accelerated global warming after 1970 CE (Mann

J. Li et al. / Quaternary International 436 (2017) 37e44

et al., 2008; Shi et al., 2015), as shown in global and regional temperature anomalies (Fig. 5h). Also, the recent IPCC report indicate that global temperature has been increased in an unprecedented rate over the past ~50 years. Indeed, global warming may be responsible for a more variable climate in many regions. For instance, a study of European remote mountain lakes has indicated that a higher frequency of temperature variations under recent global warming (Battarbee et al., 2002). In addition, the global warming induced the increased multidecadal-scale moisture variability in the northeastern North America after 1920 (Zhao et al., 2010), the intensification of the frequency and strength of Indian Ocean Dipole events in tropical region in the 20 century (Abram et al., 2008), and the strengthening North Atlantic Oscillation (NAO) variability since 1920 (Goodkin et al., 2008). Thus, the increased temperature variability recorded in Jiren Lake likely represents the climate response to global warming in some highaltitude regions, and suggests a more unstable and variable climate with a continue increase in global temperature. 5. Conclusions In this study, we present a quantitative temperature record from a remote alpine lake at the southeastern margin of the Tibetan Plateau, by using brGDGTs-based MBT0 eCBT proxy. Our highresolution (1e6 yr) reconstruction is similar to alkenone-based reconstruction inferred from a lake from the northern Tibetan Plateau, with a gradual decreasing trend of 0.3
C in MAATs over the period from 1750e1970 CE, which is different from the oftendocumented warming trend over the past few centuries. Our result may indicate a gradual cooling in some high altitude regions, and provide a new evidence to explain the observed decreasing Asian summer monsoon over this time period. Our data also indicate an abruptly increased interannual-to decadal-scale temperature variations of 0.8e2.2
C after 1970 CE, in terms of both magnitude and frequency. This probably suggests the climate system in high altitude regions would become variable under a global warming. Acknowledgements We would like to thank Shijin Zhao from China University of Geosciences, Wuhan for LC-MS maintenance. Dr. Liangcheng Tan and Dr. Rob Wilson are acknowledged for providing valuable data. This work was supported by the Program of Global Change and Mitigation (#2016YFA0600502), Natural Science Foundation of China (Grants No. 41472315 to C. Zhao and 41502173 to J. Li), Jiangsu Planned Projects for Postdoctoral Research Funds to J. Li and “Hundred Talents Program” of NIGLAS (#Y3BR013064) to C. Zhao. References Abram, N.J., Gagan, M.K., Cole, J.E., Hantoro, W.S., Mudelsee, M., 2008. Recent intensification of tropical climate variability in the Indian Ocean. Nat. Geosci. 1, 849e853. Battarbee, R.W., Grytnes, J.-A., Thompson, R., Appleby, P.G., Catalan, J., Korhola, A., Birks, H.J.B., Heegaard, E., Lami, A., 2002. Comparing palaeolimnological and instrumental evidence of climate change for remote mountain lakes over the last 200 years. J. Paleolimnol. 28, 161e179. Borgaonkar, H.P., Sikder, A.B., Ram, S., 2011. High altitude forest sensitivity to the recent warming: a tree-ring analysis of conifers from Western Himalaya, India. Quat. Int. 236, 158e166. ~ eda, I.S., Schouten, S., 2011. A review of molecular organic proxies for Castan examining modern and ancient lacustrine environments. Quat. Sci. Rev. 30, 2851e2891. s, S., Wolfe, A.P., Smol, J.P., Rühland, K.M., Anderson, N.J., Catalan, J., Pla-Rabe  Kopa cek, J., Stuchlík, E., Schmidt, R., Koinig, K.A., 2013. Global change revealed by palaeolimnological records from remote lakes: a review. J. Paleolimnol. 49, 513e535. Chen, G., Dalton, C., Taylor, D., 2010. Cladocera as indicators of trophic state in Irish

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