A Multi-indicator Study Of High-resolution Stalagmite Records During The MIS7/6 Stage In The Asian Monsoon Region

The stalagmite has advantages of precise ²³⁰Th dating,high resolution,abundant climate proxies,and wide geopraphic distribution,thus being an important carrier for reconstructing the evolution of paleoclimate and palaeo-environment.At present,the research has made great progress during the Holocene and the Last Glacial cycle,but there are still many shortcomings in the research of MIS7/6 on millennial to centennial timescales.Exploring the climate change and its driving mechanism at different timescales in MIS7/6 is helpful to further understand the history of global climate evolution in the Late Quaternary and is of great importance to the prediciton of climate evolution and the impact of climate system.Based on 12 U/Th dating results and 1068 sets of oxygen and carbon isotopes data for stalagmite LS41 in Luoshui Cave,Hubei Province,we reconstruct the Asian monsoon evolution sequence with an average resolution of 26 years from 204.4 ka B.P.to 177.6 ka B.P.On the general trend,the LS41?¹⁸O record closely follow the rhythm of the 65°N summer insolation curve,with the variation characteristics of two peaks and one valley.This agrees with the fact that other stalagmite records in the Asian monsoon region also track the solar insolation curves during the overlapped interval,futher supporting the idea that the Asian monsoon evolution is driven by the Northern hemisphere summer insolation change on the precessional cycles.On the millennial timescales,six CIS events(B18?B21)and four weak monsoon events are observed in LS41?¹⁸O record,replicating well with?¹⁸O records of samples SB11 and LS26.Besides,two substages of B17(B17',B17'')are clearly registered in LS41,SB11 and LS26,indicating its reliable existence in the monsoon region of China.During the transition from MIS7.0 to MIS6.5,the LS41?¹⁸O record shows a gradual negative bais,which is consistent with the LS26 record,but different from the sharp shift in Sanbao Cave.It is probably affected by the time scale.Further comparison shows that these millennial-scale weak monsoon events and CIS events are consistent with the ice-rafted detritus peaks and the warmings in the synthetic Greenland temperature record,respetively,indicating their hemispheric significance.We suggest that the climate change in the Northern high latitudes could affect the Asian monsoon throuth the westerlies and the north-south movements of the ITCZ.The millennial-scale weak monsoon events correspond to the?¹⁸O records in Vostok ice core,the Antarctic warm events and the atmospheric CH₄concentration,suggesting that the Asian monsoon climate change on the millennial timescales may be the common product of the climate change in the Northern and Southern Hemispheres.The detrended oxygen and carbon isotope records on the millennial timescales are significantly correlated(r=0.48,n=1035,p<0.01),proving that the oxygen and carbon isotopes are probably controlled by the same driving factors.We imply that?¹³C is probably influenced by the change of available soil moisture,which is controlled by the monsoonal precipitation.Local parameters,trace metal(Sr/Ca and Ba/Ca)results,also show a positive correlation(r=0.29,n=526,p<0.01);besides,variations of trace elements are in accordance with oxygen and carbon isotopes,further indicating a same mechanism.Correlation analyses of oxygen and carbon isotopes with trace elements show that Sr/Ca is inversely correlated with?¹³C(r=-0.35,n=526,p<0.01)and?¹⁸O(r=-0.3,n=526,p<0.01),confirming that PCP is not the primary cause for controlling the variations of Sr/Ca.We suggest that Sr/Ca and Ba/Ca are mainly affected by vegetation development,soil weathering and microbial activities.When the monsoon intensifies(namely,the?¹⁸O is negative-shifted),the available soil moisture increases,leading to the increase in the rate of soil CO₂production and the decreasing in?¹³C values.Simultaneously,the vegetation overlying the cave develops,and the activity of microbe increases;these processes cause the enhancement in soil weathering and the increases in the ratios of Sr/Ca and Ba/Ca,and vice versa.