A High-resolved Record Of Laminar Stalagmite Over The Past 1000 Years From Southwestern China

Global climate change in the past millennium has been the focus of research by scholars at home and abroad.The comparative study of the Medieval Warm Period and the Current Warm Period has a good indication of the evolution of the Asian summer monsoon under the background of current climate warming.Chinese scholars have contributed a large number of high-resolution stalagmite records to the study of climate change in the past millennium,which created favorable conditions for the study of this period.There are widespread doubts about the division of climate stages,such as the Medieval warm period(MWP),the Little Ice Age(LIA)and the Current Warm Period(CWP).The evolution model of the Asian summer monsoon in each stage and the control mechanism of it in each stage have been controversial.In addition,the weakening of the Asian summer monsoon in the past 100 years is also a very important direction in monsoon research,and the influence of human activities on the Asian summer monsoon is relatively obscure.The solution of the above problems will help us with the study of the Asian summer monsoon in the past millennium,and will also provide a basis for the prediction of the Asian summer monsoon in the future.In this study,the stalagmite TK22-1 collected from Tiankeng Cave in Southwest China(Youyang Autonomous County,Chongqing)is taken as the research object,and the high-resolution evolution history of Asian summer monsoon in the past millennium is established by using accurate²³⁰Th dating,stalagmite fluorescence lamina dating and oxygen isotope index.We use the technique of laminar dating to date the beginning stage of LIA,which provides a reference for the calibration at this stage later.Through comparative studies with high-resolution climate reconstruction indexes,meteorological observation results,and various paleoclimate records around the world,this article has conducted an in-depth discussion on the evolution characteristics and driving mechanism of the Asian summer monsoon over the past millennium.Conclusions are as follows:(1)There are some differences in the fluctuation amplitude of Asian summer monsoon in different periods and different regions,mainly in two modes.In mode 1,in Southwest China,the Asian summer monsoon is the strongest in MWP period,and the Asian summer monsoon is weaker in the early stage of LIA and CWP.In mode 2,in southern China,the Asian summer monsoon is the strongest in CWP period,and the intensity of Asian summer monsoon in MWP period is similar to that in LIA period.(2)On an interdecadal scale,the Tiankeng Cave stalagmite TK22-1?¹⁸O sequence recorded 9significant weak monsoon events.Weak monsoon events during the MWP and CWP periods are similar in time to the weakening of solar radiation and cooling events in the northern hemisphere,which indicates that a significant drop in solar radiation has a greater impact on the intensity of the monsoon.During the LIA period,there is a certain difference between the Asian summer monsoon and solar radiation.Changes in the atmospheric circulation during this period may have a greater impact on the Asian summer monsoon.(3)The stalagmite?¹⁸O records of Tiankeng Cave indicate that the initial stage of LIA is from1340 to 1460 AD,and the relative age of laminar dating is 119±2 years.In addition,the intensity of the Asian summer monsoon continued to weaken during the transition from MWP to LIA,and was accompanied by significant changes in the two phases.At this stage,the evolution of the Asian summer monsoon and the two-stage weakening of solar activity and the continued negative North Atlantic Oscillation Index were caused by the decrease of North Atlantic sea temperature and the increase of sea ice extent.(4)The Asian summer monsoon has a clear weakening trend in the past 100 years,accompanied by two significant weak monsoon events.The results show that the influence of human activities on the Asian summer monsoon cannot be ignored.