Paleoclimatic And Paleoenvironmental Evolutions During The Late Holocene:A Multi-Proxy Stalagmite Records From Shijiangjun Cave,Guizhou Province,China

Droughts and floods alternate frequently in karst areas of southwest China under the influence of the Asian summer monsoon,bringing immense effect on ecological environment and economy.However,this area lack of continuous,high resolution,multiple proxies to further investigate the ASM variability and its relations with hydrological and ecological environment in the late Holocene.In this thesis,a precisely dated stable isotope ratios(?¹⁸O,?¹³C),the trace element ratios?Mg/Ca,Sr/Ca?and the trace element concentrations?Fe,Al,Mn,Si?were analyzed in stalagmite?SJJ7?in Shijiangjun Cave,Southwestern China,for reconstructing the regional climatic and environmental evolutions during the late Holocene,the main results were summarized as follows:The evolution of Asian summer monsoon was reconstructed during 3.1?0.69 ka BP by using stalagmite?¹⁸O,with an average?¹⁸O resolution of about 2.5 years.On one side,several weaken Asian summer monsoon was recorded by stalagmite?¹⁸O,?¹³C positive excursions,such as,2.85 ka BP,2.40 ka BP,2.10 ka BP,1.91 ka BP,1.70 ka BP,1.20 ka BP,0.95 ka BP respectively,representing notable decadal to centennial fluctuations over the late Holocene.On the other side,normalized SJJ7 stalagmite?¹⁸O for extract the Pacific's signal during the late Holocene,supporting that the Chinese stalagmite?¹⁸O is controlled by ISM and EASM.We also noticed a significant negative correlation with sea surface temperature?SST?record of Western Pacific Warm Pool?WPWP?,imply ENSO?El Ni?o-Southern Oscillation?activity could influence Asian summer monsoon.Timing and structure of 2.8 ka BP event and Medieval Climate Anomaly?MCA?have been well determined in our?¹⁸O record with a high precision,provideing a robust basis for Asian summer monsoon evolutions.During 2.8 ka BP event,the stalagmite?¹⁸O structure between north and south China was different:a striking asymmetrical“V”structure for South China,and“W”for north China.However,during Medieval Climate Anomaly,the stalagmite?¹⁸O structure between north and south China both exhibited a symmetrical“M”,and monsoon intensity showed a rise-drop-rise trend,indicating the instability of monsoon during this time.In summary,the structure and amplitude of the climatic events were different in the period and region.The driving force for late Holocene remains a subject of debating,which differ from early Holocene climate change driving by high latitude,but affected by low latitude atmosphere-ocean circulation and sun activity.Solar activity as a major external forcing could modulate the variability of ASM in the millennium to centennial timescale.But at a decadal timescale,the variability of ASM was controlled by low latitude forcing,such as,Intertropical Convergence Zone?ITCZ?and ENSO.A multi-proxy of SJJ7 stalagmite reconstruct the hydroclimate of southwestern China.On the decadal to centennial timescales,?¹³C reflect the regional hydrological and local ecological environment.At centennial timescales,using detrend and cross-wavelet analysis,we observed?¹⁸O variations well matched with?¹³C,which means?¹⁸O and?¹³C controlled by the same mechanism.However,during 2.8 ka BP event?¹⁸O and?¹³C asynchronous changes might be attributed to the soil effective humidity.A larger positive amplitude in?¹³C changes reflect the degradation of the ecological environment,which attributed to ENSO and human activities.SJJ7 trace element?Sr/Ca,Mg/Ca?records generally exhibit coherent variations among themselves as well as with the corresponding?¹⁸O,?¹³C records,which reflects hydroclimatic variability may be inferred from carbon isotope and trace element records.Especially during 1170?1410 yr BP when calcite deposition,the trend Sr/Ca is rising,in contrast,dropping for Mg/Ca,which may be affected by mineral crystal.Generally,the various reasons for trace element ratio are complex,including prior calcite precipitation?PCP?,water-rock interaction?WRI?,the strength of soil leaching effect.There are seven obvious dark layers in SJJ7 stalagmite,XRD results show that its structure was aragonite.The trace metals of Si,Al,Mn,Fe in SJJ7 were analyzed with Avaatech XRF,we observed that Si,Al,Mn,Fe increased impulse and up to a peak in dark layers,corresponding with a?¹⁸O,?¹³C shift from heavy to light.Mn,Fe,Al and Si in dark aragonite layers increased,responding to the transition period of ASM.The reason may be attributed to local hydrology,soil dynamics.