| It is of great significance to investigate forcing mechanism of past climate and environment change in the hyperarid region in northwest China,because the ecology and environment in this region are vulnerable and it needs protect and restore under the environment changing.Yitang Lake is located in Dunhuang basin on the north side of Altynian Mountain and Qilian Mountain,with extremely widely distributed sedimentary strata as an important carrier of climate and environmental change in the Kumtag desert region.We drilled and acquired nearly 300 m core(40°18' N,94°58' E)from Yitang lake,and carried out detailed paleomagnetic and OSL measurements,which provide specific age control for this depositional sequence.We measured TOC(total organic matter content)and stable carbon isotopic composition(?13Corg.)for about 500 samples and TOC/TN(carbon-nitrogen ratio)for near 1000 samples,together with stable carbon isotope of modern plant samples.The results show that these sediments were deposited in the Quaternary,and we reconstruct the past vegetation and climate changes based on these proxy index sequences.This study fills the research of Quaternary geological record in the Northwest China.The sedimentary facies of Yitang Lake show the transition from fluvial sediment to lacustrine record,and the best estimate of bottom age for YTH core is ca 2.08 Ma according to the paleo-geomagnetism result.During this period TOC fluctuated in the range of 0-3%,reflecting low organic matter in this region,and for TOC/TN in 0-30 respectively,affected by total otganic content.It shows that the ?13Corg varied between-27.84 ‰ and-22.30 ‰ during 2.08 Ma and it can be compared with the TOC changes.The existing vegetation in YTH area,according to the latest vegetation survey,is dominated by desert vegetation and grassland vegetation,together with C3 and C4 plants.Based on the analysis of carbon isotopes in modern vegetation,it seemed that stable carbon isotope values increased gradually with the decrease of rainfall,also the proportion of C4 plants also raised.It is suggested that the distribution of existing plants(especially C4 plants)in this region is mainly affected by rainfall,followed by temperature,which is of great importance for analysis of core samples.The organic carbon of the sediments in this arid region is mainly controlled by the terrestrial vegetation from the exogenous input of the whole basin with the background of dry climate.The stable carbon isotopic composition of organic matter of these sediments is determined mainly by the ratio of terrestrial C3 and C4 plants within the whole basin of Yitang Lake,that is,the ?13Corg from C4 plant is more positive while for C3 plant it is more negative,and the ratio of C3/C4 can reflect vegetation and climate change of long time scales.On the basis of this interpretation and the ?13Corg changes,the paleoenvironment variation of the Yitang lake is divided into four phases over the Quaternary:It varied gradually from cold and wet to warm and dry and became more suitable for C4 vegetation growth during 2.08?1.82 Ma;between 1.82?0.24 Ma,the most positive stage of ?13Corg changes with the average value about-23‰,indicating that the vegetation is partially C4 plants but C3 driven,with the highest proportion of C4 plants along transition from fluvial facies to lake sediments;since 0.24 Ma,the climate was dry again with increasing C4 plants.Our new results show the paleocliamte evolution of Yitang Lake was closely associated with the Asian aridification and driven by the growth and decay of northern hemisphere ice sheets,and this paleoclimate inference respond to the northwest inland drought trend under global climate change. |
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