Biomarker Records And Climate Reconstruction From Lake Qixing Since The Last Deglaciation

Global climate change,as a hot topic of both academia and the general public,not only has a profound impact on the environment,but also relates to the sustainable development of human society and environmental security.The study of paleoclimate is an important basis for understanding modern climate and predicting future climatic change patterns.During the period from the last deglaciation to the early Holocene,the climate system underwent huge and rapid changes,which has attracted widespread attention from the scientists.As the closest geological period to us,the climatic boundary conditions of the Holocene do not change significantly compares with the present,and understand its climate change and driving mechanisms is crucial for us to deal with the current global climate change.The semi-arid region of northern China is vast and located near the modern summer monsoon boundary,with large interannual climate fluctuations and a more sensitive response to global climate change.Compared with other terrestrial carriers,peat has the advantages of high resolution,rich biological information,continuous deposition and high chronological accuracy.Therefore,in this study we select the peat sediment of Lake Qixing in the semi-arid region of China,and establishe an accurate chronological frame using AMS¹⁴C dates on plant residues.The geochemical index such as n-alkanes,Long-chain n-alkanes?D and glycerol dialkyl glycerol tetraethers(GDGTs)were used to reconstruct the climate change and ecological environment of the Lake Qixing region since the last deglaciation and the corresponding mechanisms.The main conclusions are as follows:(1)Modern process research shows that different species of plants in the Qixing region have different distribution patterns of n-alkanes,terrestrial plants with C₃₁ as the dominant carbon and its regnant carbons mainly are C₃₁,C₂₉,and C₂₇.Aquatic plants with C₂₅ as the dominant carbon and its regnant carbons are C₂₇,C₂₅,and C₂₃.In comparison with the surface sediments of Lake Qixing,it was found that peat n-alkanes are mainly influenced by terrestrial plants.In addition,based on the comparison with the distribution characteristics of surface soil n-alkanes,it was found that the peat sediments contained more n-alkanes from aquatic plants,therefore,the peat n-alkanes of Lake Qixing may be influenced by both terrestrial sources and peat authigenesis.A study of modern processes of GDGTs revealed that both peat surface sediments and surface soils were dominated by br GDGTs-II series compounds.Br GDGTs-IIa in peat sediments accounted for 38%of the total br GDGTs,while br GDGTs-IIa in surface soils accounted for 20%of the total br GDGTs,but at the same time,the relative content of br GDGTs-?a in the compounds reached 24%.(2)The temperature and pH evolution history of Lake Qixing region since the last deglaciation were reconstructed based on br GDGTs.At the same time,?D of long-chain n-alkane from Lake Qixing was also used to reconstruct the?D changes of atmospheric precipitation since the last deglaciation,with the change of pH values responded to the changes of regional humidity.According to the trend of each parameter of the climatic environment,the climatic environment changes in Lake Qixing were divided into six stages:14.9-13.0 cal kyr BP temperature rised rapidly,regional humidity was low,and the climate was warm and dry;13.0-11.7 cal kyr BP humidity was still low,the average annual temperature droped by about 2°C,and the whole climate was cold and dry;11.7-7.5 cal kyr BP,the whole temperature was high but showed a decreasing trend,humidity was low,and the climate was warm and dry;7.5-5.5 cal kyr BP temperature droped rapidly and regional humidity rised rapidly to the highest state throughout the period,the climate was cold and wet;5.5-2 cal kyr BP temperature rose,but humidity droped rapidly,with better growth of aquatic plants and an increase in regional total organic carbon,and the climate was cool and dry;2-0 cal kyr BP climate shows a decreasing trend in both temperature and humidity,so the regional climate is cold and dry as a whole.(3)The temperature record of Lake Qixing since the last deglaciation shows that it experienced a cooling event with a drop of about 5°C during the Middle Holocene(7.5-5.5 cal kyr BP).In this study,we made a preliminary discussion on the driving mechanism that may have led to this anomaly.We concluded that the decrease of Total Northern Hemisphere Solar Insolation(NHSI;45°N)in the Middle Holocene was the dominant factor.At the same time,the transpiration of vegetation and the shorter distance of water vapor transport from the ocean to the land due to the highest sea level could increase the effective humidity in the atmosphere and provide favorable conditions for the formation of clouds.In addition,the overall concentration of greenhouse gases CO₂ and CH₄ was low in the Middle Holocene,which had less radiative forcing effect on temperature,and the stronger Siberian High during this period could bring more cold air,that is winter monsoon,which significantly reduced the regional winter and spring temperatures,and thus influenced the annual mean temperature.Therefore,the combination of these factors led to a significant cooling event in the Middle Holocene 7.5-5.5 cal kyr BP of the Lake Qixing region.