Holocene Climate Change Pattern Recorded Mainly By Diatom And Other Proxies From Bosten Lake

As one of the most vitally important principles to predict the future environment what we are depending closely,the understanding of Holocene climate changes has attached extensive attentions of the world.With the increasing wealth of high-resolution and precisely dated climatic proxy records such as oxygen isotopic data from speleothem and peat,the consensus on Holocene climate evolutionary pattern of the Asian Summer Monsoon(ASM)domains has been reached gradually. Comparatively speaking,as to the study of the westerly dominant area of inland Asia (namely,Arid Central Asia or ACA),high-resolution climatic proxy records are still scare.Furthermore,dating accuracy,timing scales and time resolution of those records are different.All of the inferior status quo of study would inevitably make it difficult to get a coherent knowledge of how climate changed in the ACA during the Holocene.In recent years latest studies have revealed that the climate evolutionary pattern of the ACA might show significant discrepancy with that of the ASM domains. However,either this discrepancy represents a local phenomenon or has a regional linkage with other areas under the same atmospheric circulation,which is still need to be tested and confirmed by more convincing climate records.Moreover,rapid climate changes with periodicity have been extensively explored in the Holocene studies on the ASM domains and the ASM marginal areas,but they have been rarely elaborated in study sites of the ACA.Therefore,to carry out some high-resolution analyses on well dated,highly sensitive proxy records in an selected ideal site would significantly help us to improve our understanding of the Holocene climate evolutionary process of the ACA,to evaluate whether rapid climate changes occurred in the ACA or not and to calculate the periodicity of these rapid climate changes if they did occur.Bosten Lake,the largest inland freshwater lake in China,locates at the middle part of the ACA.Controlled by the westerlies,Bosten Lake is an ideal site to understand the Holocene climate evolutionary process of the ACA.Furthermore,the high sedimentation rate of Bosten Lake during the Holocene has been shown in previous studies,which makes it possible to exact information from lake sediments to examine the pattern and process of rapid climate changes in the ACA.Diatoms,single-celled algae,are extremely abundant in many lake ecosystems. They are made of opaline silica,well preserved in sediments and highly sensitive to environmental changes.Therefore,shifts in diatom abundance and community composition in lake sediments can reflect rapid climate changes of short duration.Up to now,diatom fossil records have been performed for many lakes around the world and thus help to gain valuable information on Holocene palaeoenviromental and palaeoclimatic reconstructions in those lake areas.Judging from above evidence, diatoms are selected as the main proxy recorder of past environmental changes in our study.In order to get knowledge of the composition and distribution of modern diatom assemblages in Bosten Lake,a diatom survey was carried out systematically.A total of 35 diatom samples from surface lake sediment were collected and analysed and at least 400 diatom valves were counted under microscope for each sample.Then,ASM ¹⁴C dating was carried out on twelve dating materials picked out from BSTC001 of 9.52m in length and a 9.25m long parallel core,XBWu46.By the correlation of magnetic susceptibility between these two cores,all ASM ¹⁴C dates were aligned along a unified depth coordinate.It was expected that the carbon reservoir effect was of minor significance as the location of our coring site is near to the estuary of Keidu River where water circulation works efficient and fast.Besides,most of the dating materials are composed of terrestrial plant debris,which confirm the carbon reservoir effect can indeed be neglected.By comparison and analysis,seven ASM ¹⁴C dates from respective depths were screened out to develop the final age-depth model and calibrated into calendar years.Subsequently,sedimentation rates of BSTC001 were calculated by interpolating of these seven calendar ages,except for the uppermost section whose sedimentation rates were determined by the ¹³⁷Cs dating results.On the basis of sedimentation rates,a robust age-depth sequence for BSTC001 was eventually developed.Multiple proxy indicators were analyzed including diatoms(a total of 179 samples,400～600 valves per sample,with time resolution of 45 a), carbonate content(a total of 456 samples,with time resolution of 18 a),grain size(a total of 421 samples,with time resolution of 19 a),magnetic susceptibility(a total of 451 samples,with time resolution of 18 a),loss-on-ignition(a total of 443 samples, with time resolution of 18 a)and C/N ratio(a total of 263 samples,with time resolution of 30 a),which can provide help information of palaeoenvironment and palaeoclimate of the lake drainage during the past ca.8.0 cal ka BP.The main results of this study include the followings:1.By Correspondence Analysis(CA)of modern diatom assemblages,it shows that salinity/conductivity was the most influential environmental factor on the components and distribution of diatom assemblages in different lake zones. According to their different tolerances of high conductivity,two diatom assemblages can be divided,which are Fragilaria group preferring lower conductivity/salinity and indifferent to water depth and Mastogloia simtii-elliptica group preferring higher conductivity/salinity and low water depth.This result of CA consists with that of Principal Correspondence Analysis(PCA).2.Evolution of palaeoclimate and palaeoenvironment in the Bosten Lake drainage during the past ca.8.3 cal ka BP was successfully established mainly on the basis of diatom and other multiple proxy indicators.The results show that the climate was dry before the early middle Holocene and relatively wet during the rest of Holocene.Six evolutionary stages could be segmented as follows:From 8.3 to 7.8 cal ka BP,lacustrine deposits began to present in the sediment core which indicate stable standing water gradually covered the drilling site.A wet and humid climate phase can be inferred by the low carbonate content and sparse diatom assemblages mainly composed of Fragilaria sp.preferring cold and fresh water during this period.From 7.8 to 5.9 cal ka BP,generally warm and dry conditions were shown by the increase of freshwater-brackish and brackish-saline diatom taxa,high abundance of mesoeuthermal diatom taxa and the decrease of freshwater diatoms;relatively high value of carbonate content,grain size and C/N ratio,but low magnetic susceptibility.From 5.9 to 4.8 cal ka BP,lake level increased under humid climate conditions which could be deduced by the high abundance of facultative planktonic freshwater diatom;low percentage of carbonate content,finer grain size and low value of loss-on-ignition,but high value of magnetic susceptibility.After 5.6 cal ka BP,the high abundance of cold-water diatom taxa can be linked with the decrease of water temperature due to cold climate conditions.From 4.8 to 2.7 cal ka BP,a generally dry climate prevailed in the lake drainage,which is indicated by the increase of abundance of freshwater-brackish and brackish-saline diatom taxa as well as high carbonate content.Superimposed on this broad trend of climate changes, some strong oscillations in temperature and humidity are suggested by frequent fluctuations in the abundance of cold-water diatom taxa and freshwater diatom taxa in a large scale.For instance,a pronounced cold-humid at 4.0 cal ka BP can be inferred by the increase of freshwater diatom taxa preferential of cold water conditions.From 2.7 to 0.12 cal ka BP,freshwater diatoms became the dominate taxa of Bosten Lake associated with high abundance of cold-water diatom taxa,low carbonate content,finer grain size,low C/N ratio as well as high magnetic susceptibility,which indicate cold and wet climate conditions in the study area.After 0.12 cal ka BP,the rapid increase of freshwater-backish and brackish-saline diatom taxa would be linked with falling of lake level under arid climate conditions or/and intense human impact.3.Seven rapid climate changes were clearly documented in diatom assemblages from BSTC001 core,namely,ca.7.9 cal ka BP,ca.6.8 cal ka BP,5.7～4.8 cal ka BP,ca.4.0 cal ka BP,3.1～2.9 cal ka BP,2.0～1.6 cal ka BP and 0.7～0.4 cal ka BP. All these climate events broadly correspond to the cooling of Northern Hemisphere,the advance of glaciers and the outbreak of ice-rafting events in North Atlantic during the Holocene,and consist with humid(cold)periods inferred from other records from the ACA.In addition,by MTM-Multitaper Method of Spectrum Analysis,it can be detected that these rapid climate changes own cycles of 2500a and 500a approximating to the periodicity of solar activity and North Atlantic Thermohaline Circulation.4.The climate evolution pattern of dry early Holocene,mild middle Holocene with rapid climate changes and wet late Holocene as shown in multi-proxies of BSTC001 from the Bosten Lake corresponds with the results from other archives of the Tarim Basin,Xinjiang and even wider range of the ACA.The quasi-antiphase pattern of climate evolutionary pattern of the ACA against the ASM domains exhibits that the effective moisture increased in the ACA with the decreased strength of ASM and vice versa.5.The Holocene climate evolutionary process of the ACA reflected by multi-proxies of BSTC001 was primarily driven by the change of summer insolation of Northern Hemisphere.In addition,it was strongly regulated by the temperature changes of Northern Hemisphere and the variation in the frequency/intensity of ENSO activity on longer timescale.As to the rapid climate changes of millennial and centennial scale,solar activity and North Atlantic Thermohaline Circulation should be the main driving force.The variation of the strength of westerly and displacement of its path due to the NAO anomaly may help these factors to perform their functions on the climate evolution pattern of ACA.Additionally,the decrease of land and surface water evaporation during the cold phase might contribute to increase the effective humidity in the ACA.