The Evolution Of Indian Monsoon Recorded By Planktonic Foraminifera Since The MIS25 Period In The Andaman Sea

The Indian monsoon,as an important part of the global climate,carries the heat and moisture from the tropical oceans across the equator to high latitudes.It has an important response and driving role in global climate change,playing a significant role in global-scale water vapor transport,atmospheric circulation and ocean circulation.The formation,evolution and driving mechanisms of the Indian monsoon have received much attention in global climate research due to its huge impact on human economy and livelihood.The Andaman Sea,as the core area of Indian monsoon precipitation,has a key role in the study of Indian monsoon.The planktonic foraminifera assemblage in the seafloor sediments has well preserved paleoclimate and paleoenvironmental information,and has a clear indication of surface seawater temperature and salinity changes.In this paper,a qualitative and quantitative analysis of the planktonic foraminiferal assemblage in the core sediments at 57.99 m above station U1448 in the Andaman Sea is carried out.The framework for stratigraphic dating since MIS25 was initially established by combining sedimentation rates and natural gamma rays(NGR)with biological and volcanic ash age events as control points,dominated by foraminifera abundance changes.A study of the assemblage characteristics of planktonic foraminifera found a total of five dominant species,Globorotaloides ruber,Globigerina bulloides,Globigerinitaglutinata,Pulleniatian obliquiloculata,Neogloboquadrina dutertre,with seven common species.In the analysis of palaeoenvironmental indicators based on planktonic foraminifera assemblages,alternative indicators that can effectively indicate the variation in summer wind strength in India were obtained-foraminifera accumulation rate and the difference in mean salinity of August surface water at stations MD81349 and U1448.A comprehensive discussion of palaeoenvironmental indicators,compared with previous studies on the Indian monsoon,explores the evolutionary history of Indian summer winds at long time scales since MIS25 and during the interglacial ice period,further clarifying the main driving mechanisms of cyclical changes in the Indian monsoon.The results show that the intensity of the Indian monsoon was weak during the middle Pleistocene transition and enhanced but fluctuating during the late middle Pleistocene on longer time scales.The strongest and most stable Indian summer winds in the late Pleistocene-Holocene,and the overall Indian summer wind intensity showed a gradual strengthening trend,and with significant interglacial cyclic patterns.Also the periodic variation of the Indian monsoon in the study area since MIS25 is mainly influenced by the 100 ka orbital eccentricity and 41 ka The slope is co-driven.220 ka BP since the Indian monsoon is more significantly driven by the 23 ka age difference.