The Spatiotemporal Difference Research Of Holocene Climate Optimum In Middle Latitude Asia

The Holocene refers to the time from the end of last glacial to now?11.5 ka?,it can be defined as modern interglacial.The Holocene climatic conditions are similar to the climatic conditions that we are experiencing.Therefore,the study of Holocene climate and environment evolution has important research value.The spatial difference of climate evolution during Holocene has always been focused on paleoclimate research,and the conclusions from different materials and proxies are different.In this study,many loess sections were chosen as the main material to study the spatial difference of climate evolution during Holocene in mid-latitude Asia,because the loess widely distributed in the mid-latitude Asia,include Chinese Loess Plateau?CLP?,Tianshan area in Xinjiang and Tajikistan.The magnetic parameters and geochemical parameters are applied as environment proxies,and the detailed age control is based on accelerator mass spectrometry?AMS?¹⁴C and optically stimulated luminescence?OSL?dating methods.The results show:1)The Holocene paleosol formed during the mid-Holocene in CLP and the loess deposited in early and late Holocene.In Xinjiang area,the Holocene paleosol formed in middle to late Holocene and the loess deposited in early Holocene.In Tajikistan the Holocene paleosol formed in late Holocene.2)The dry environment appeared during early and late Holocene in central CLP and the humid environment appeared during mid-Holocene.In western CLP,the dry environment also appeared during early and late Holocene,and the humid environment appeared during mid-late Holocene,however,the time of dry environment during early Holocene in western CLP is longer than center CLP,and the humid environment during Holocene in western CLP shorter the center CLP.The climate evolution history during the Holocene in Tajikistan and Xinjiang is similar,the dry environment appeared in early Holocene and the humidity increased from middle to late Holocene.3)The Holocene Climate Optimum generally occurred during the late Holocene in Tajikistan and Xinjiang area;during the middle-late Holocene in the western CLP;and during the mid-Holocene in the central CLP.we concluded that in the Holocene Climate Optimum occurred earliest in the central CLP,followed by the western CLP,and then by Tajikistan and Xinjian.This asynchroneity reflects differences in the response of regional climates to global climate change.4)The Holocene Climate Optimum in Xinjiang and central Asia occurred in the late Holocene due to increasing summer and winter precipitation,which was caused mainly by the strengthening of the Westerlies and by a negative phase of the Arctic Oscillation?AO?or North Atlantic Oscillation?NAO?.In the central CLP,the Holocene Climate Optimum occurred in the mid-Holocene because of changes in Northern Hemisphere summer insolation?NHSI?and Atlantic meridional overturning circulation?AMOC?.The Holocene Climate Optimum in western CLP occurred in mid-late Holocene may be related to shifts in the position of the East Asian summer monsoon?EASM?belt due to the changing longitude of the western Pacific subtropical high?WPSH?,and it also related to the intensity of the East Asian monsoon.A stronger EASM during the mid-Holocene?⁸-5 ka BP?,and a gradual eastward shift in the WPSH accompanied with a stronger La-Ni?a,enabling the EASM rain belt to reach the central CLP and resulting in significantly increased precipitation.However,the EASM rain belt may not have reached the western CLP,and therefore precipitation there remained low.In the mid-late Holocene?⁵-3 ka BP?,the westward extension of the WPSH accompanied with a gradual weakening La-Ni?a and stronger EASM led to a westwards expansion of the rain belt,which not only reached the central CLP but also the western CLP.Precipitation then remained high in the central CLP and increased in the western CLP.