Abrupt Climatic Events Recorded By Stalagmite In Wanxiang Cave During The Last Deglaciation On Eastern Tibetan Plateau

Global climate change has attracted more and more attention from the international community.The study of paleoclimate evolution law is helpful for us to correctly understand the present climate change and make scientific prediction of the future climate change.Monsoon system is the link of the interhemispheric air-sea circulation system,and is also an important part of the earth’s climate system.At the same time,the Asian monsoon is the most important and active part of the global monsoon system.It is very sensitive to the global climate change,and the regional hydrological changes caused by it will directly affect the survival of human beings.Therefore,it is of great significance to study the Asian monsoon system both scientifically and practically.The last deglaciation was the period in the Quaternary when the climate was most similar to modern times and the ice sheet retreated on a large scale.The study and comparison of high-resolution paleoclimatic records during this period will help us to understand the characteristics of climate change at the millennium and submillennium scales and the phase relationships between different climate systems and their driving mechanisms,which can also provide reference for future climate change prediction.Wanxiang Cave is located at the edge of the region affected by the modern Asian monsoon,which is more seriously affected by the monsoon retreat and retreat,and the climate environment is more sensitive to the monsoon change.Therefore,the stalagmite of Wanxiang Cave is of great significance in the study of the Asian monsoon system,which can establish an accurate and reliable geochronological framework and high-resolution climatic information of monsoon precipitation for the study of climate change in the western monsoon marginal region which lacks high resolution climatic records.Based on 8 ²³⁰Th ages and 100 oxygen isotope data of stalagmite WX40C in Wanxiang Cave,we set up 21.08～12.42 ka B.P.evolution history of the Asian summer monsoon.The variation characteristics of the Asian summer monsoon during LGM warming,Heinrich1（H1）and B（?）ling-Aller（?）d（B/A）events are discussed respectively.By comparing with other high-resolution climate records,the regional differences and causes of monsoon response are analyzed,and the driving mechanism of monsoon response on the millennium scale is discussed.Finally,the following conclusions are obtained:（1）Theδ¹⁸O values of stalagmites in Wanxiang Cave had a significant shift in each case at 19.5～17.9 ka B.P.,17.3～14.1 ka B.P.and 14.1～12.7 ka B.P.,which correspond to LGM warming（～19 ka B.P.strong monsoon event）,H1 cold event and B/A warm event in theδ¹⁸O record of Greenland ice core records and and theδ¹⁸O record of other stalagmite records of Asian monsoon region,respectively.The WX40Cδ¹⁸O records show a good agreement with the millenium scale climate change records of Hulu Cave and Bittoo Cave in the Asian monsoon area,which can reflect the evolution characteristics of the Asian monsoon circulation on a large scale.The inverse correlation with the Flores stalagmite record in the Southern hemisphere at the millennium scale suggests a“See-saw”pattern in the northern and southern hemispheres.（2）During 19.5～17.9 ka B.P.,theδ¹⁸O records of the stalagmite WX40C in Wanxiang Cave indicate that a strong monsoon event occurred on a millennial scale.During this period,the records of GRIP ice core in Greenland indicate LGM warming,and the stalagmite records in each monsoon area indicate that the monsoon intensifies obviously.However,there are differences in the occurrence time andδ¹⁸O amplitude of stalagmite records,which are attributed to the dating error and resolution,and the stalagmite in Wanxiang Cave is greatly affection by seasonal precipitation and is far away from the water vapor source.（3）There is a remote correlation between the intensity of the Asian monsoon and the temperature of the northern high latitude on the millennium scale.However,WX40Cδ¹⁸O records at the end of H1 show a slow change characteristic different from the polar temperature abrupt change characteristic,which may be the response of the Asian summer monsoon to the slow warming of the southern hemisphere at this time.The internal structure characteristics of“V”shape in the stalagmite records of Wanxiang Cave and the variation patterns of segmented weakening and strengthening of monsoon intensity also exist in stalagmite records of other caves,indicating the consistency of stalagmite records in monsoon area.（4）We identify three climate fluctuation events in the stalagmite record of Wanxiang Cave,which are similar to multiple sub-millennium scale abrupt climate events in the GRIP ice core record of B/A warm period,namely CI-1a,CI-1b and CI-1c.CI-1d and CI-1e could not be shown because of the low resolution or the Asian summer monsoon had little effect on the stalagmite records.During the whole B/A warm period,the Asian summer monsoon continued to strengthen slowly and the climate fluctuation was relatively moderate.Theδ¹⁸O record of GRIP ice core continued to decline,and the oscillation of the northern high latitude climate was relatively severe,indicating that the monsoon system did not fully respond to the northern high latitude climate change on a long time scale.（5）Through comparative analysis of paleoclimatic records,we attribute this intensification of the 19.5～17.9 ka B.P.Asian summer monsoon to the combined effect of increased solar radiation intensity,the northward shift of Intertropical Convergence Zone（ITCZ）due to the intensification of the Atlantic Meridional Overturning Current（AMOC）,and Super-ENSO in the tropical Pacific.H1 events,in which the intensification of solar radiation at northern high latitudes acted as a“trigger”,Antarctic iceberg discharge events in the North Atlantic（Antarctic iceberg discharge events）weakened（strengthened）the AMOC,resulting in cooling（warming）in the Northern Hemisphere,southward（northward）ITCZ,and weakened（strengthened）Asian summer monsoon.The complete restoration and enhancement of the AMOC and the increase in greenhouse gas concentrations eventually led to the B/A event.The continuous intensification of the Asian summer monsoon during the B/A warm period was caused by the increased solar radiation and the Antarctic Cold Reversal event.