| The climate during the past 2000 year is an important time window for evaluating and projecting future climate change,and also a key issue for understanding the paleoclimatic changes on decadal-to centennial time scales.Some international scientific groups such as PAGES,IPCC and Asia 2K pay close attention on high-resolution paleoclimatic data during the past 2000 years,and expect to obtain high-resolution paleoclimate change data in different climate regions and develop a global data system.In desert area,there are no high-resolution records from tree ring and stalagmites.The lake sediment can provide high-resolution paleoclimate and paleoenvironmental information,valuable for the data sparse region in desert.In this studya we focus on sediment core from Lake Tabu Jigede in the center of the Badain Jaran Desert.Based on the methods such as 210Pb,137Cs and OSL data,we established chronology for the sedivent in the Lake Tabu Jigede.Among them,210,Pb and 137Cs dating have been used for the past 120 years;OSL dating data has been used for the whole core.We analyzed n-alkane,compound-specific carbon isotope,and ratio of pristane to phytane(Pr/Ph)in the sediment of Lake Tabu Jigede.The n-Alkanes are derived from waxes,fatty acids and hydrocarbons in organisms,and they are chemically stable.N-alkanes with different chain lengths can "mark" their material source(Biomark),so n-alkanes and compound-specific carbon isotopes have become important indicators of paleoclimate and paleoenvironment.However,due to differences in lakes,regional vegetation,and climate background,the interpretation of n-alkanes and monomeric carbon isotopes should be careful.Lake Tabu Jigede is located at an extreme dry region,therefor ground water may be a major source of lake water.Long-chain n-alkanes in the sediment are mainly from terrestrial vascular plants around lake catchment,and an indicator of climate in our study lake,while the short-and mid-chain n-alkanes are primarily derived from algae and aquatic plants in the lake,may imply climatic change in the region of ground water source.Based on the distribution characteristics of the sediment indicators of Lake Tabu Jigede,we speculate that the n-alkane average chain length(ACL)may be a drought proxy,higher ACL value in the lake sediments represents the more arid environment and the lower effective humidity,while the lower ACL represents the more humid environment and the higher effective humidity.The aquatic macrophyte versus aquatic macrophyte and terrestrial plant ratio(Paq)may imply change in lake level,higher Paq represents the prosperity of aquatic plants,the lake level is high and the climate is humid.The low values of Paq imply the climate is drought.The Pr/Ph is an indicator reflecting the sedimentary oxidation-reduction environment of lakes,high Pr/Ph of values imply a oxidation condition,a low lake level,while low Pr/Ph is a reduction condition,a high lake level.The carbon isotopes of long-chain n-alkane indicate changes in regional effective humidity,which are more positive in dry periods and more negative in wet periods.According to the proxies as above,the paleoclimate change in the sedimentary sequence of Lake Tabu Jigede could be divided into five stages:The Roman Era(AD 0-300 years),the climate was stable,but was dry and cold.The Dark Age(AD 300-980),the climate changed frequently characterized in several intervals with dry-warm and wet-cold conditions.The Medieval Warm Period(AD980-1350),the climate was warm and dry,and evaporation might be also high,the lake level was low.The Little Ice Age(AD 1350-1900),the climate was humid,and the temperature was generally low.Among them,the climate is relatively dry from 1850 AD to 1900 AD and 1350 AD to 1500 AD and the water level of the lake was at a higher position.In the modern warm period from 1900 to the present,the lake began to shrink.It may be related to the increase of temperature and evaporation in this area. |
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