| The temperature variations have an essential impact on ecology environment.However,the patterns of Holocene temperature variations are complex,and it is urgent to clarify the past temperature change with a robust proxy.Pediastrum Meyen has a worldwide distribution in freshwater lakes and is easily identifiable.The requirements for a simple habitat and significant interspecific differences enable them ideal for responding quickly to changes in the water environment.Unfortunately,previous studies have often overlooked the interspecific differences within Pediastrum,despite employing it as a specific component of aquatic plant communities to assist in the interpretation of pollen assemblages.This oversight can be attributed to a lack of large-scale investigations into the distribution of modern Pediastrum,resulting in a poor understanding of the ecologies of different species.Bosten Lake is the largest inland freshwater lake in China,and a large amount of climate and environmental information was documented in the lake sediments.Although previous researchers have conducted many researches on its modern ecological environment and paleoenvironment changes from Bosten Lake,the Holocene diatom records are limited to lake near-shore sedimentary cores.The identification of Pediastrum species requires improvement,and there is a noticeable lack of research concerning their potential as proxies for past temperature variations in this particular lake.On the basis of the modern investigation of Pediastrum,the regional temperature variations were reconstructed by subfossil Pediastrum assemblages from central sedimentary core BST04H/BST19B of the Bosten Lake.In combination with the diatom assemblages and geochemical proxy at the same core,the paleoenvironment changes of the Bosten Lake since～8.2 kyr（1 kyr=1000 cal yr BP）were reconstructed.Subsequently,the impact of climate fluctuations on past human activities in northwest China was further discussed.The main findings are summarized below.（1）In this study,we investigated the occurrence of modern Pediastrum in 234surface sediment samples of water bodies（lakes and reservoirs）in China and western Mongolia.29 Pediastrum taxa（including species and varieties）were found in the surface sediments of 151 different water bodies.The spatial distribution characteristics of Pediastrum species and the environmental factors determining the community composition were identified using Canonical Correspondence Analysis（CCA）methods.The results demonstrate that the growing season temperature（TMar-Oct）of Pediastrum was the most significant environmental factors influencing the modern Pediastrum species composition and spatial distribution（independent interpretation variance was12.3%,p=0.001）.Furthermore,the transfer function between the 72-surface sediment Pediastrum dataset and TMar-Oct was established through multivariate statistical analysis.Additionally,we found Pediastrum can hardly survive in water with total dissolved solids（TDS）higher than 2-3 g/L.（2）Twelve subfossil Pediastrum taxa were identified in the 226 samples from combined cores BST04H and BST19B,Bosten Lake.The results show an exceptionally warm climate during the period of approximately 4.8-3.6 kyr indicated by the occurrence of thermophilic species P.simplex.This warming climate was further corroborated by a quantitative temperature reconstruction based on the sedimentary Pediastrum record using the Weighted-Averaging Partial Least Squares（WA-PLS）method.The climate cooled during 4.2-4.1 kyr and 3.6-3.5 kyr,as indicated by the decrease or absence of P.simplex.These temperature constructions were also verified by clumped isotopes(Δ47)in the carbonates of sediment cores and supported by paleoclimatic records from other sites.（3）A total of 239 diatoms taxa（63 genera,232 species and 7 varieties）were identified in 134 sedimentary samples from core BST19B,Bosten Lake.During certain periods（～6.1-5.3 kyr,4.8-3.2 kyr,1.1-0.4 kyr）,the warm/meso-eutrophic species（e.g.,Aulacoseira Group,Cavinula scutelloides,Staurosirella mutabilis,etc.）were the dominate species,which may be caused by the increase of Tianshan glacier melting,the enhancement of chemical weathering and detrital elements inputs in the catchment due to the higher temperature,and then result in rise of lake turbulence as well as the nutrient level of water body in Bosten Lake.During other periods（such as 5.3-4.8 kyr,3.2-1.5 kyr,0.4-0.05 kyr）,the diatom community compositions shifted to cold/oligo-mesotrophic dominate species（e.g.,Lindavia radiosa,Navicula peroblonga）.（4）Analysis of ostracod trace elements Mg/Ca and Sr/Ca was conducted in sediment core BST04H/BST19B.It was found that Sr/Ca was higher and decreased continuously during～8.3-4.8 kyr,indicating that water salinity was higher and showed a downward trend during this period,driving the change of the concentration of Pediastrum.Mg/Ca and Sr/Ca increased between 4.8 and 3.5 kyr,indicating an increase in salinity.After the 3.6-3.5 kyr cold event,the water salinity fluctuated widely,which could be attributed to the unstable climate conditions of the late Holocene.In addition,several high salinity events occurred（such as 2.7-2.5 kyr,2.2-2.0 kyr,1.7-1.4 kyr,1.1-0.4 kyr）,which were in close agreement with the lower concentration of Pediastrum and Candona.（5）The growth of Pediastrum is limited by the salinity levels in lakes,which are,in turn,influenced by the hydrology and climate of the catchment.This study synthesized Holocene subfossil Pediastrum records from arid central Asia（ACA）,marginal areas of the summer monsoon region of northern China and the Indian summer monsoon-dominant region.The results indicate that,in ACA,there were low concentrations of subfossil Pediastrum during the early Holocene,followed by an increasing trend in subfossil Pediastrum content during the middle-late Holocene（after～6 kyr）.This suggests aridity during the early Holocene and increased moisture during the middle-late Holocene.The independent record of Pediastrum abundance serves as a humidity proxy and is essentially consistent with the integrated pattern of Holocene effective moisture changes in the westerly region as established by previous studies.In the marginal areas of the summer monsoon region of China and the Indian summer monsoon-dominated region,the synthesized record of subfossil Pediastrum abundance is generally consistent with the climate change patterns in both regions.（6）Combined with archaeological data,it is found that the warm climate promoted prehistoric human occupation of the present relatively cold Altai Mountains and surrounding areas.The unusual cold events may have triggered southward prehistoric human migration out of the Altai region,which led to the widespread distribution of archaeological sites in the desert-oasis basin areas of the southern and northern Xinjiang.Moreover,it is speculated that a series of cultural changes in the Gansu-Qinghai regions in northwest China may be linked to the temperature variations in the middle and late Holocene.To sum up,this study further clarified the regional paleoclimate changes and the evolution history of the lake’s paleoenvironment were,and presents a novel approach by using Pediastrum as an independent paleoenvironmental proxy.The findings of this research reveal the climate changes during the middle to late Holocene in Xinjiang and provide essential evidence for the climate and environmental background of the cultural evolution along the Silk Road. |
