亚洲夏季风9.2ka事件的湖北落水洞高分辨率石笋记录

Variability of Asian summer monsoon during 9.2 ka event recorded by a stalagmite from Luoshui Cave,Hubei Province,Central China

基于湖北省落水洞一支长550 mm石笋的23个^230Th年龄、1099个氧同位素数据,重建了12.96~1.53 ka B.P.时段的平均分辨率为11 a的亚洲夏季风演化序列.该序列记录了在9.2 ka B.P.时存在一个显著的弱季风事件(称为"9.2 ka"事件),"RAMPFIT"法分析显示:"9.2 ka"事件的持续时间在9.8~9.2 ka B.P.之间,亚洲夏季风在"9.2 ka"事件中表现出开始时缓慢减弱(219 a)、 结束时迅速增强(78 a)的特征,事件内部存在两个次一级的振荡,曲线呈"两谷夹一峰"的不对称"W"型结构特征."9.2 ka"事件受太阳活动和北大西洋淡水注入的共同影响,此时太阳活动减弱,且发生淡水注入事件,导致温盐环流减弱,赤道辐合带南移,促使亚洲夏季风减弱.

Understanding the structure and forcing of the millennial-scale 9.2 ka event for deciphering the spatial and temporal changes of the Asian summer monsoon before and after the 9.2 ka event is a critical issue in Holocene climate study.Here,we present a high-resolution stalagmite record from Luoshui Cave(29°43'N,109°06'E)to characterize the low-latitude hydro-climate variability during the 9.2 ka event.Luoshui Cave is located in Xianfeng Town,Enshi City,Hubei Province,Central China.The entrance of cave is about 975 m above sea level,and overlain by about 30 m thick limestone.The average local annual precipitation is 1470 mm,most of which falls between April and September,accounting for 62%to 88%of the annual precipitation and belonging to the typical monsoon area.The relative humidity and temperature in the cave are about 100%and 14℃,respectively.The LS21 stalagmite is 550 mm in length and its diameter ranges from 30 mm to 40 mm,shaped like a cylinder.When halved and polished along the growth axis,the top of the stalagmite is broken at the depth of 50 mm.The stalagmite is composed of pure dark black and translucent calcite.The subsamples were drilled along the growth axis with a hand-held carbide dental drill.The 23 subsamples for U/Th date analysis were measured by multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)in the Isotope Laboratory of Nanjing Normal University.Uncertainties in the U-Th isotopic data and ^230Th dates are calculated at the 2σlevel.For stable isotopic measurements,a total of 1099 subsamples,about 30μg each,were drilled at 0.5 mm intervals.Oxygen isotopic analysis was conducted on a MAT-253 mass spectrometer fitted with a Kiel Carbonate Device in the Isotope Laboratory of Nanjing Normal University.Precision ofδ^18O is±0.05‰and the results adopt the VPDB(Vienna Pee Dee Belemnite)standard.The high-resolution oxygen isotope sequence and the ^230Th dates from the Luoshui Cave are used to reconstruct the Asian summer monsoon variability from 12.96 ka B.P.to 1.53 ka B.P.,with an average resolution of 11 a.Notably,the sequence is imprinted by a significant weak monsoon event(called the 9.2 ka event)at around 9.2 ka B.P.The result of RAMPFIT analysis of Luosuhi Cave record reveals that the 9.2 ka event took place in 580 years,with a gradual weakening of the Asian summer monsoon lasting for 219 years at the beginning and a rapid enhancement for 78 years at the end of the event.Besides,two sub-level oscillations are observed inside the 9.2 ka event,contributing to an asymmetrical"W"structure with two valleys and one peak.The 9.2 ka Asian monsoon weakening corresponds to the 6th peak in the North Atlantic ice rafted debris record,leading us to the assumption that the meltwater inflow into the North Atlantic could have caused the 9.2 ka event.Besides,strong solar signals are found on the decadal timescales(76 and 29 years)throughout the whole event.Therefore,we imply that the cooperation of solar activity decreasing and the freshwater flux,which led to the weakening of the Thermohaline Circulation and the southward movement of the Intertropical Convergence Zone,giving rise to the weakening of the Asian summer monsoon finally.