Vegetation And Climate History Around Balikun Region From MIS3 To Early Holocene(~60-9 Ka BP)documented By Multi-proxy Data Of Balikun Lake

Arid western China,including Xinjiang,is mainly dominated by the Westerlies at present.It is one of the driest regions in the world,with scarce water resources,sparse vegetation cover,and vulnerable ecosystems?Qin et al.,2005;Narisma et al.,2007?.As the main content of the Arid Central Asia,the arid western China plays a vital role in the parts of atmospheric radiation balance and ocean fertilization,hence the global climatic variations?Maher et al.,2010;Martin et al.1991;Uno et al.2009?through its releasing and/or absorbing the aeolian dust of the Northern Hemisphere?Biscaye et al.,1997;Pye and Zhou 1989?.However,the evolution of vegetation and climate are poorly documented in arid western China at a longer temporal-scale beyond Holocene,thus our understanding of past arid ecosystems and their responses to changing climate is still insufficient.Furthermore,existing studies are faced with problems such as the frequent presence of erosional hiatuses?e.g.Rhodes et al.,1996;Zhao et al.,2013?and uncertainties in the chronology?Lai et al.,2014,Long et al.,2015;Song et al.,2015?,especially for terrace and loess deposits,masking our understanding of moisture evolution and its main controlling factor during the late Pleistocene.A synthesis of evidence from lake sediments indicates that the pattern of moisture evolution in arid western China exhibits an“out-of-phase”relationship with the pattern of monsoonal evolution in eastern monsoonal Asia on various time scales?Chen et al.,2008?.What's the climate pattern in the arid western China beyond Holocene,and how it responds to the global climate?What's the differences in mechanism between arid western China and monsoon dominated areas on various time scales?e.g.orbital or sub-orbital orbital?during the late Pleistocene.To answer these question,the first priority is to obtain longer palaeo-records with robust age control.MIS3 is an interstadial in the late Quaternary with the characteristics of substantially reduced,although not completely degraded,continental ice sheets that can be compared with the Holocene and present climate?Imbrie et al.,1989;Dansgaard et al.,1993;Voelker et al.,2002;North Greenland Ice Core Project Members,2004;Lisiecki and Raymo,2005?.The period also marks the entry of modern humans into Europe and the sub-sequent demise of the Neanderthals.The subsequent LGM is an interval of mostly cold climate conditions along with low sea level and vast ice masses in Europe and North America?Mix et al.,2001;Clark et al.,2009?.These two intervals thus are valuable materials for the comparison for modern climate,to better understand the climate changes between arid western China and eastern monsoonal Asia,as well as yield important data which can be used to test General Circulation Model output of past climate change.Compared with loess and fluvial terrace,lake sediments are usually versatile achieves for late Pleistocene vegetation and climate construction.Our target lake Balikun Lake is located in the westernmost portion of the Balikun basin,Xinjiang,north-west China.It is a closed-saline lake,and has continuous Quaternary deposits spanning the Late-Pleistocene interval?Ma et al.,2004?.This lake is thus an ideal setting to reconstruct palaeovegetation and palaeoclimatic changes during the late Pleistocene.A 62.53-m sediment core?BLK11A?was collected from the central of Balikun Lake in June 2011.The sediment core was frozen,and then transported to the laboratory at Lanzhou University?Lanzhou,China?.The cylindrical sediment core was split lengthwise,one half of the core was stored at 4°C for archival purposes and the other half was subsampled at 1-cm intervals for analyses.In this study,we focus on a segment of the core between 20.0 and 2.9 m?covering MIS3 to early Holocene?,to study the vegetation and climate history of MIS3.Multi-proxies?e.g.lithology,grain size,LOI,pollen and XRF-elements?are used to reconstruct the palaeovegetation and palaeoclimate,underpinned by a robust chronology(based on accelerator mass spectrometry?AMS?¹⁴C,optical stimulated luminescence?OSL?,and palaeomagnetism).Our three main results are as followings:1)Vegetation history from MIS3 to early HoloceneBased on the pollen data,the vegetation history from MIS3 to early Holocene around Balikun lake has been reconstructed as follows:1)A high relative abundance of thermophilous temperate tree pollen?e.g.Betula?accompanied by other warm-wet associated species?e.g.Ranunculaceae,Cyperaceae,Typha?suggesting warm and wet climate conditions during most of the early MIS3?59.0–51.7 ka BP?that resemble the“Holocene optimum”with an increase in effective moisture in most parts of arid western China;2)The middle to late MIS3?51.7–26.5 ka BP?was marked by the development of desert shrubs?e.g.Rhamnaceae,Nitraria,Zygophyllum?,together with low pollen concentrations of upland communities,indicating a continuous and gradually cooling and drying climate that resulted in reduced vegetation cover;3)The regional vegetation around Balikun during LGM was mainly dominated by Artemisia and Amaranthaceae,which are typical desert-steppe and/or desert taxa,indicating a progressive aridification in north-western China;4)An extremely cold and dry climate prevailing from 17.0?15.2 ka BP,correlated with Heinrich event 1?H1?,would explain the low vegetation cover found then;and 5)the warm and humid B?lling-Aller?d interstadial?BA:ca.15?ca.13 cal kyr BP?is clearly recorded in the Balikun region by the development of wetland herb communities?e.g.,Poaceae,Cyperaceae,Typha?.2)Balikun lake water-chemistry change from MIS3 to early HoloceneBased on the soluble salts,carbonate contents and the XRF-elements,the water-chemistry of Balikun Lake has been revealed as follows:1)Balikun lake changes from nearly fresh to brackish and finally to hypersaline,with evaporate minerals varying from carbonate to sulfate and finally halite,respectively.Specifically,the early MIS3 was characterized by low soluble salts,indicating a fresh water condition,when carbonate minerals were frequently precipitated.Lake level was higher deduced by the grain size data;2)Halite sedimentation took place during the middle MIS3,and it indicates a gradual cooling and drying trend that reduced in the lake area and vegetation cover.Lake level was similar to the present condition?¹ m?;3)High frequency of the water-chemistry and lake level were found during the late MIS3;4)Salt contents are lower during the MIS2,mainly due to a lower evaporation during this time,while the lake level maintained a lower status.From19.2 ka BP,water-chemistry varied frequently again,generally with carbonate precipitation during the warm periods?19.2–17.0 ka BP and 15.2–12.6 ka BP?,with silicate detritus dominated from watershed during the cold periods?H1,17–15.2 and MIS2/1,12.6–8.9 ka BP?.3)Watershed chemical weathering from MIS3 to early HoloceneBased on the grain size and XRF-elements,the watershed chemical weathering from MIS3 to early Holocene in Balikun Basin was recovered as follows:1)The Rb/Sr ratios can be used as an indicator for the chemical weathering in Balikun basin.Ti and Si are mainly distribute in fine particles,which was transported into the lake along with dust by wind.The early MIS3 was characteristic of high weathering intensity,linking with warm and wet climate.The high vegetation cover reduced the amount of dust into the lake;2)From the middle MIS3 to MIS2,watershed was dominated by physical weathering,more dust was carried to the lake along with the reduction in vegetation cover;3)The post-glacial climate was featured by abrupt increase in regional temperature and meltwater from the mountain glacial.Watershed chemical weathering was strengthened during¹9.2–17 ka BP and BA periods,while decreased during the cold H1 and MIS2/1 periods.4)Climate condition from MIS3 to early Holocene and its possible mechanismBased on a modern pollen dataset from China and Mongolia,the annual precipitation in Balikun region was reconstructed using the weighted averaging partial least squares?WA-PLS?methods.The results showed that annual precipitation during the periods of early MIS3,post-glacial and BA was higher,while decreased during the middle to late MIS3,LGM,H1 and MIS2/1.With the combination of vegetation history,water-chemistry changes,and watershed chemical weathering intensity,the climate condition in Balkun region from MIS3 to early Holocene can be summarized as follows:1)On orbital/sub-orbital time scales,the moisture evolution in arid western China region?including Balikun region?exhibits a“warm-wet and cold-dry”pattern,and follows the Northern Hemisphere summer insolation?NHSI?;2)The high early MIS3 NHSI induced a warm background across the Northern Hemisphere.A high relative abundance of thermophilous temperate tree pollen?e.g.Betula?accompanied by other warm-wet associated species occurred in Balikun basin,accompanying with high vegetation cover,high lake level,and high watershed chemical weathering intensity;3)From the middle to late MIS3,the annual precipitation reduced as the decrease in NHSI.Sparse desert vegetation developed and physical weathering intensity increased.Sulphate and halite minerals precipitated on the dry condition;4)The MIS2,especially the LGM,cold-dry climate dominated the Balikun region,desert and/or desert-steppe expanded,and Balikun Lake was relatively shallow and experienced high aeolian input;5)As the beginning of the post-glacial,regional temperature increased sharply,annual precipitation also increased,this warm-wet climate thus strengthened the watershed chemical weathering.In addition,the Atlantic climate also can affect arid western China as well as Balikun region via westerlies,for example,the Atlantic-related H1?BA?event had an obvious influence on the Balikun climate,leading to a cold?warm?and dry?wet?condition,lowing?increasing?the vegetation cover and lake level,while increasing?decreasing?the physical weathering intensity.Our new interpretations contradict earlier inferences of highest lake level and wettest conditions prevailing during the late MIS3,and we found that climate during the LGM in arid western China was cold and dry,rather than cold and wet as previous though,and we propose that the decline of northern high-latitude summer insolation and the increase of global ice volume from early MIS3 to MIS2 have exerted a remarkable influence on the evolution of vegetation communities and lake level in arid western China on orbital or sub-orbital time scales.