Karst Environmental Changes During The Middle And Late Holocene:A Multiproxy Stalagmite Records From Feilong Cave,Guizhou Province,China

Rocky desertification is a major environmental problem in the karst regions of Southwest China and seriously impacts the ecology and socioeconomic development.Human activities and climate change have been proposed as the main factors responsible for the development and expansion of rocky desertification.However,there are many problems in geological history carriers in this region during the middle and late Holocene,such as low resolution and few use of multiple indicators in geological history records,which also provide a difficult problem to understand the relationship between the changes of the Asian summer monsoon and the evolution of regional hydrology and ecological environment.In this study,the history of the Asian summer monsoon（ASM）since 6200 yr BP is reconstructed based on ²³⁰Th dating,δ¹⁸O andδ¹³C data,the element ratios（Mg/Ca,Sr/Ca）of a stalagmite from Feilong Cave in the Guizhou Province of Southwest China.The evolution of Asian summer monsoon was reconstructed during 6200～680 yr BP by using stalagmiteδ¹⁸O.On the side,positiveδ¹⁸O andδ¹³C excursions record eleven interdecadal–centennial weak summer monsoon events at 1000-870,1660-1460,2040-1900,2780-2460,3020-2860,3250-3120,3650-3370,4273-3980,4820-4480,5370-5180,and 5840-5560 yr BP.On the other side,theδ¹⁸O records of stalagmite FL2 and other stalagmite records in the ASM region exhibit an overall positive trend during the MLH,indicating that the ASM gradually weakened as the Northern Hemisphere temperature and summer insolation gradually decreased on a suborbital scale.The ITCZ is the main source of water vapor and heat in the Asian monsoon region and is caused by evaporation and convective activity in the tropical Indian and Pacific Oceans.Ocean evaporation is directly influenced by the air temperature,sea-surface temperature and wind.The north-south shifts in the ITCZ′s position directly influence the amount of water vapor in the source region of the ASM.When the ITCZ moves southward,the convection of the ascending branch of the Hadley circulation in the Northern Hemisphere weakens,resulting in a weaker or more southerly ISM.Based on the hydroclimatic indexδ¹³C and trace elements（Sr/Ca,Mg/Ca）of FL2 stalagmite,the change state of hydroclimatic environment in southwest China was reconstructed.FL2 stalagmiteδ¹³C is controlled by surface vegetation type and abundance,soil biology,CO₂yield,and local hydrological address conditions.The synchronous bias ofδ¹⁸O andδ¹³C indicates that the degradation of karst ecological environment in the region responds to climate change.The trace element ratio（Sr/Ca,Mg/Ca）of FL2 stalagmite has obvious correlation with the overall variation ofδ¹⁸O andδ¹³C,that is,theδ¹⁸O andδ¹³C bias correspond to the increase of Sr/Ca and Mg/Ca ratios,which reflects the weakening of the Asian summer monsoon and the decrease of regional precipitation during the middle and late Holocene.As a result,Sr/Ca and Mg/Ca ratios increase.The Sr/Ca and Mg/Ca ratios of FL2 stalagmites reflect many influencing factors,such as pre-carbonate deposition,water-rock interaction and soil leaching.It is indicated that the final atmospheric precipitation,karst water and cave drip migration exerts a profound influence on the deposition of trace elements（Sr/Ca and Mg/Ca）in stalagmites under the dual influence of climate and karst environment.In addition,frequent dynasty changes in China since 3000 yr.BP profoundly influenced the karst ecological environment.During the Medieval Warm Period,the Jing-Kang event（JK event）resulted in the fall of the Northern Song Dynasty and led to a large-scale migration into southern China.As a consequence,the population increase in the Yangtze River basin enhanced land use,accelerated the disappearance of forests,and finally resulted in the deterioration of the regional environment.We conclude that climate change and anthropogenic activity may have jointly triggered and/or exacerbated rocky desertification in Southwest China.In this study,the evolution of the ASM and the regional karst environment during the middle to late Holocene（6270-670 yr.BP）was reconstructed based on U-Th dating and theδ¹⁸O andδ¹³C data of a stalagmite from Southwest China.During this period,the ASM experienced a weakening trend in response to decreased insolation,a southward migration of the ITCZ,and frequent El Ni（?）o events,as well as a superimposed series of interdecadal–centennial scale weak summer monsoon events.The significant positive correlation between theδ¹⁸O andδ¹³C values of the stalagmite suggests that the karst ecological environment is sensitive to ASM changes.A comparative analysis of several stalagmite records from this region reveals that climatic events at～5500 yr.BP,4200 yr.BP,2800 yr.BP and during the MWP are consistently recorded,characterized by synchronous increases in theδ¹⁸O andδ¹³C values.These results indicate the changes in the karst hydrological conditions and the degradation of vegetation and soil biological processes in Southwest China induced by ASM weakening.Therefore,the weakening of the summer monsoon and the sustained growth and southward migration of the Chinese population may have eventually caused the deterioration of the ecological environment in the karst regions of Southwest China since the middle Holocene.