Late Quaternary Climate Change Of Northeastern Tibetan Plateau From Stable Carbon And Oxygen Isotopes Of Carbonate In Loess Deposite

East Asian summer monsoon?EASM?is an important component of Earth's climate system,which significantly influences the activities of billion-plus populations over East Asia.Therefore,EASM variability and associated physical mechanisms are the focus and frontier of Earth system science,and have been drawing much attention.Long-term climate changes provide an important background for understanding present changes of EASM,and therefore is beneficial for its projection.However,orbital timescale variability and forcing mechanisms of the EASM from paleoclimatic reconstructions remain uncertainty.Precession-band variance was not the dominated frequency in loess records,while eccentricity-band misses in speleothem records.Whether loess deposit can record monsoon variability on a precession timescale largely depends on sedimentation rate and post-depositional processes that can attenuate the variability of the loess proxies.In this study,we chose the upper 55 m of the loess core Panzishan?PZS??36.649°N,101.844°E,2728 m?retrieved from Xining Basin in the northeastern Qinghai-Tibet Plateau where the dust sedimentation rate is high.A total of272 samples were taken at an interval of 20 cm in this section.The depth-age model was constructed based on linear interpolations,and detail correlations between the grain size series of loess and the grain size time series of Luochuan loess.We present the mean grain size,magnetic susceptibility,carbonate content,dolomite content,?¹³C of carbonate and?¹⁸O of carbonate of the loess-paleosol samples to reconstruct the EASM changes.The carbonate content and dolomite content are negatively related to rainfall.The?¹³C of loess carbonate is related to soil respiration flux which was determined by the EASM intensity,therefore it is an ideal proxy of the EASM from the northeast of the Qinghai-Tibetan Plateau.The?¹⁸O of carbonate is affected by both of precipitation and the circulation of water vapor.When there is more precipitation,the value of oxygen isotope is more negative;whereas the value of oxygen isotopic composition is positive.Compared with magnetic susceptibility,carbonate content,dolomite content and?¹³C can detect the same order of warm and wet events during the past 280 ka.The mismatches of climate changes in the various proxies are mainly caused by the different sensitivity of proxies to climate changes.Our results show that these proxy records exhibit significant orbital timescale fluctuations,link the EASM with the high-latitude ice sheets in Northern Hemisphere and temperature.In addition,our spectral and wavelet analyses of the proxies time series detect a distinct 23 ka cycles,corresponding well with the precision cycle and frequency of speleothem?¹⁸O.These suggest that climate change in the northeastern Qinghai-Tibetan Plateau is not simply a response to high-latitude ice sheets and temperature;low-latitude solar radiation changes modulated by precession probably be the main controlling factor for monsoon precipitation changes in the northeastern Qinghai-Tibetan Plateau.