Paleoenvironmental Evolution Of South Asia Revealed By Sediment Record In The Southern Bay Of Bengal Since The Late Eocene

Based on the terrigenous mass accumulation rate,clay mineralogy,major and trace element geochemistry,Sr-Nd-Pb isotopic geochemistry and Nd isotopic composition of planctonic foraminifera mixed species,clay-sized sediments from ODP Site 758 in the northern Indian Ocean,southern Bay of Bengal on the Ninetyeast Ridge since the late Eocene.According to these analysis methods,this study has reconstructed the evolution history of river systems in South Asia,and revealed the linkage of development of rivers,tectonic uplift activity and global climate change;It has been proposed the variation of the chemical weathering intensity in the Himalayas since the late Miocene,which has been the source region of clay-sized sediments,combined with the evolution history of the South Asian Monsoon reconstructed by the chemical weathering proxies,revealed that the linkage between the evolution history of chemical weathering history in the Himalayas and the South Asian Monsoon;The source of seawater in the northern Indian Ocean has also been used to establish its bottom water evolution history since the late Eocene.The ΔεNd proxy was proposed to measure the Himalayan weathering flux,and reconstruct the evolution history of silicate weathering in the Himalayas.The conclusion reveals the persistently increase of the Himalayan weathering flux during the Cenozoic.Provenance analysis indicates that sediment source have changed from the sediments mainly originated from the weathering of gneissic rock of the stable southern Indian continent and Sibumasu rocks of the west Burma block(37-22 Ma)to terrigenous material eroded from the eastern Himalaya with minor contributions from southern India and the Sibumasu rocks(22-8 Ma),and finally the Irrawaddy and Brahmaputra Rivers became the main sources of the Site 758 after ～8 Ma.Considering the timing of tectonic events and climate change in South Asia,this study indicated that the two stages of tectonic uplift in the Himalayas during the early and late Miocene(～23 and 8 Ma),respectively,are the main control mechanism of drainage reorganization in South Asia and provenance change in the southern Bay of Bengal.Therefore,This study suggest that the Brahmaputra and Irrawaddy Rivers might have been initiated and fully established at that two-stage time,respectively.Based on the chemical weathering proxies like major and trace element geochemistry,clay mineralogy,and the Sr-Pb isotopic compositions,the chemical weathering intensity of silicate minerals at Site 758 since the late Miocene have been suggested that it was weak during 7.8-3.8 Ma,and strong during 8.6-7.8 Ma,3.8-3.2Ma,2.4-1.8 Ma and 0.7-0 Ma.By comparing previous studies on the South Asian Monsoon intensity and the evolution history of the South Asian monsoon since the late Miocene reconstructed by the chemical weathering proxies in this study,it is suggested that the South Asian summer monsoon intensity became weak during7.8-3.8 Ma,and appeared the warm humid and cold dry alternate monsoon climate after ～3.8 Ma.Nd isotopic composition of planktonic foraminiferal mixed species combined with the paleoceanic history of the Indian Ocean indicates that,the source of bottom seawater in the northern Indian Ocean was dominated by AABW water mass from Eocene to Oligocene,and then gradually mixed with the weathering input of Himalayas,resulting in the gradual increase of ΔεNd until 12 Ma.The ITF water mass from the western Pacific Ocean mixed into Indian Ocean and had a great impact on the seawater composition for the northern Indian Ocean during 12-9 Ma.Based on the Nd isotope composition of planktonic foraminifera,combined with other previous works on the Nd isotopic composition of the carbonate fraction of the Indian Ocean sedimentary cores,this study reconstruct the evolution history of the Himalayan weathering flux during the Cenozoic.The Himalayan weathering flux has persistently increased since 35 Ma during the Cenozoic,implying the beginning of the uplift of the Himalayan region and its significant contribution to global cooling during the Cenozoic.