Holocene Climate Change Documented By Loess Biomarkers In The Southern Loess Plateau

High-resolution quantitative reconstruction of Holocene climate is of great significance to explore the evolution of regional and global paleoclimate and the development of human society.Based on the biomarkers in the Weinan loess profile of the southern Chinese Loess Plateau（CLP）,We reveal the characteristics of local climate change during the Holocene.At the same time,the relationship between Holocene climate change in the southern CLP and human society is discussed based on cultural evolution and historical events.The quantitative reconstruction of temperature based on Glycerol Dialkyl Glycerol Tetraethers（GDGTs）in Weinan shows that the Holocene temperature has varied from 13.2℃to 20.2℃.The temperature increased gradually in the early Holocene,reached the maximum warmth in the Middle Holocene（7000a B.P.）,when the temperature in Weinan was about 6℃higher than that in modern times.Behind,the temperature continued to decrease and fluctuated sharply in the late Holocene,which is consistent with the global cold and warm periods.Through comprehensive comparison of indicators such as R_i/b,BIT,IR_6ME with previous studies,it is found that the East Asian summer monsoon（EASM）prevailed in the southern CLP during the Middle Holocene,bringing a warm and wet climate.In the late Holocene,the climate tended to be arid.This conclusion is supported by the short chain predominance and indicators of n-alkanes,that is,the high abundance of C₁₆ and C₁₈ and the low Carbon Preference Index（CPI）reflect the large input of microorganisms and significant pedogenesis,and the relatively warm and humid climatic background of this stage probably promoted the population growth of the early Yangshao culture.R_i/b indicates that there were several extreme drought events during the early and late Holocene.Of Which,the drought stage since 11500-12000a B.P.and 8200a B.P.correspond to the absence of cultural relics in the middle reaches of the Yellow River.It is speculated that the mid-Holocene climatic optimum（HCO）in the southern CLP promoted the transition from primitive agriculture to hoeing and ploughing agriculture in China,and the drought trend caused by the overall weakening of the EASM since the late Holocene and the extreme drought events promoted the change of dynasties to a certain extent.By contrast and periodic analysis,we suggest that the Holocene climate change in the southern CLP has been driven by multi-scale factors.The millennium scale climate change is mainly driven by Northern Hemisphere Summer Insolation（NHSI）,while the centennial scale climate change is related to solar activity and its induced Sea Surface Temperature（SST）variation in the North Pacific Ocean.