Deepwater Redox Changes In The Okinawa Trough And Its Paleoenvironmental Implications Since 17 Ka

Understanding the evolution of paleo-redox condition in the middle south Okinawa Trough(OT)since the last deglaciation its response to the deep water circulation,surface productivity is of great significance to study the paleo-environment of the East China Sea.However,the changes of redox condition of deepwater in the OT since the last degalcial remain unclear.Core OKT-3 was collected from the middle south Okinawa Trough(26.018°N,125.282°E;water depth 1 792 m),within the influence of Kuroshio.Based on AMS¹⁴C age framework,molybdenum isotope,grain size,element geochemistry,content of organic carbon,nitrogen and organic carbon isotope analysis of core OKT-3,combining with published data,we aim to discuss the changes in sedimentary redox conditions,the organic matter source and its response to paleoenvironmental evolution since the last deglaciation.The organic component of Core OKT-3 is composed of terrestrial and marine organic matter.Terrestrial organic matter is mainly derived from the Changjiang and Taiwan Rivers,which is controlled by sea level change,the Kuroshio Current,and climate variation.The Changjiang-sourced organic matter of Core OKT-3 showed a decreasing trend during 17.3?4 ka B.P.,consistent with the decreasing contribution of Changjiang-derived detrital sediments.In contrast,the Taiwan-sourced organic matter,which is transported by the Kuroshio Current,showed an increasing trend.It is worth noting that the Changjiang-sourced and Taiwan-sourced organic matter did not exhibit synchronous changes with the Kuroshio Current during 4?1.5 ka B.P.,which may be controlled by summer monsoon.Marine organic matter from Core OKT-3showed two peaks during B(?)lling-(?)ller(?)d and Pre-Boreal periods,with low values during the Younger Dryas period.Consistency between marine organic matter and primary productivity suggests that North Pacific Intermediate Water(NPIW)acts as a conveyor belt transporting nutrient from the North Pacific to the Okinawa Trough.NPIW passes through the Kerama Gap to the Okinawa Trough and carries nutrients to the surface water by way of upwelling.During 17.3?11.5 ka,The Mn/Al ratio of OKT-3 displayed lowest values,the?^98/95Mo profile exhibited relatively positive values and Mo and Cr were slightly enriched,suggesting development of suboxic depositional environment within the Okinawa Trough.This suboxic deepwater condition was mainly controlled by weak water mass exchange and larges amounts of fresh water input.In addition,increased decomposition of organic matter as a result of high export production may intensify originally suboxic deepwater condition.During 11.5?9.5 ka,the?^98/95Mo profile exhibited relatively negative excursion,Mo and Cr showed decreasing enrichment factors and Mn/Al ratio increased gradually,suggesting there occurred a deepwater redox condition transitional period from suboxic to oxic.The obvious peak of marine organic matter,U and Ni in the Okinawa Trough,synchronous with widespread enhanced primary productivity in North Pacific and its marginal sea during 11.5?9.5 ka(PB period),suggested the influence of NPIW within the OT was gradually increasing and became an important influence on the paleo-oceanic environment of the trough,including the bottom water redox condition,primary productivity and organic matter preservation.The negative excursion for?^98/95Mo profile,obvious increase in the content of Mn and nearly no enrichment for Mo and Cr at approximately 7 ka,suggesting development of oxic depositional environment within the Okinawa Trough.This was related with enhanced deepwater ventilation triggered by intensive intrusion of NPIW and South China Intermediate Water(SCSIW)through Kerama Gap with Philippine Sea and the channel east of Taiwan and reduced freshwater input.Our study suggested better deepwater ventilation and poor organic matter preservation condition likely switched OT to a regional soure of venting CO₂to atmosphere during sea level highstand.