Variability In East Asian Summer Monsoon During Mid-late Holocene Recorded By Stalagmite ?¹⁸O Sequence From Liaoning Province

Several climate shifts in Middle-late Holocene,Causing the change of precipitation conditions that triggerred drought or floods,which influence the survival and development of human seriously.As a result,it is particularly important to reveal the variation of EASM(East Asian Summer Monsoon).Here two stalagmites MD11 and MD12 from cave Miaodong Benxi,in Liaoning province,are selected to reconstruct a dtailed history of East Asian monsoons covering period between 4.09 ka BP and 2.84 ka BP.Combined with other research reports to understand the interal structure of "2.8 ka event",the driving mechanism on centennial timescale surrounding the events in late Lolocene are discussed.The research results are as follows:(1)The variability of MD11 and MD12 ?180 is explained to be a proxy index of EASM and(or)local summer monsoon precipitation.Overall,these ?180 profiles apear to a remarkable change on decadal-millennial timescale.(2)During 4.09-3.85 ka BP,the EASM is relatively stable,similar to some stalagmite records from southern China,indicating that the well known "4.2 ka" cold event(failure EASM)maybe occurred earlier than 4.1 ka BP at least.(3)The "2.8 ka event" event recorded in MD stalagmite occurred at about 2.75?2.65 ka BP,implies that EASM weakened slowly and then strengthed all of a sudden,with two secondary oscillation cycles.It is indicated that solar activity is the main controlling factor of 2.8 ka weak monsoon event,but the physical mechanism is still unclear.