Rapid Changes Of The Surface Hydrology Evolution In The Northern Okinawa Trough Since The Last Glacial Maximum

Based on the identification of planktonic foraminifera,analysis of Mg/Ca and oxygen isotope analysis of G.ruber from core PC-1 and NOKT-3 in the northeastern East China Sea(ECS),we reconstructed the surface hydrological evolution in the northern Okinawa trough since the Last Glacial Maximum(LGM)and identified the millennial Oscillations of climate.Also,The intensity of Kuroshio and the main factors affecting the sea surface salinity(SSS)of the northern Okiwana trough were studied too.By comparing with the record of oxygen isotope from the Greenland ice core,the East Asian Summer Monsoon(EASM)and sea surface temperature(SST)of the tropical Pacific,we discussed the response of the ECS to the high latitude climate in the northern hemisphere.According to Mg/Ca paleotemperature and oxygen isotope record of planktonic foraminifera(G.ruber),the sea surface hydrology change has been divided into three phases.In addition,the millennial oscillations of climate are recorded,including H2 event,H1 event,B/A warm interval,YD event and 8.2ka event,which shows that the ECS is closely related to the high latitude climate in the northern hemisphere.The evolution of precipetation in the middle and lower reaches of the Yangtze River between 6330 and 3890 a BP can be divided into three major stages based on G.quinqueloba content variation:(1)high precipetation during the period of 6330~4950 a BP,which corresponded to the Holocene optimum;(2)a decreased trend of precipetation from 4950 to 4600 a BP,implying termination of the Holocene optimum;(3)a marked decrease in the monsoon precipitation between 4600 and 3890 a BP indicating a arid cliamte condition.This precipetation pattern could be related to the general weakening and southeastward retreat of the East Asian summer monsoon resulted from the decreased solar-radiation in the northern hemisphere during the middle Holocene.The changes of SST?SSS and the characteristcs of planktonic foraminifera fauna indicated that the main axis of Kuroshio did not remove from the Okinawa Trough,but the intensity of it was greatly weakened.With the rise of sea level,the intensity of Kuroshio was significantly increased about 15 ka BP and further enhanced at the early stage of Holocene.The intensity of Kuroshio has fluctuated strongly since about 8 ka BP,which may be affected by the ENSO activity.During 5-3 ka BP,Pulleniatina minimum event(PME)happened.But our results show that there were not a cold climate condition corresponding to the PME during the period of late mid-Holocene.In addition,enhancement of the El Ni?o-like condition in the tropical Pacific region during this time window also can not fully explain the occurrence of PME in the northern Okinawa trough.The SSS of northern Okinawa trough is affected by many factors,including: sea level,ancient rivers on the ECS shelf and Changjiang dilution water.During the LGM(~24.3-18 ka BP),a large amount of fresh water was entered into the study area because the lower sea level caused the paleo-Yangtze and Yellow river estuary to be closer to the Okinawa trough.Therefore,the SSS in the northern Okiwana trough was significantly reduced.During the H1 event(~18-15 ka BP),the fresh water input from the ancient river remained the main factor affecting the SSS in the northern Okiwana trough.During about 15-12.6 ka BP,the SSS of the northern Okiwana trough is high,mainly because of the rising of sea level which caused the fresh water input of ancient river to reduced.During the YD event(~12.7-11.6 ka BP),the sea level was already quite high which caused the fresh water input from ancient river to have a weak effect on the SSS of the northern Okiwana trough.At the same time,the EASM was weak,leading to the relatively high SSS in the northern Okiwana trough.During about 11.7-6 ka BP,SSS of northern Okiwana trough fluctuated strongly which may be caused by the interaction of the enhanced EASM and Kuroshio.Since about 6 ka BP,SSS of northern Okiwana trough has been low and stable reflecting that the SSS of northern Okiwana trough was controlled by the EASM.Compared with the ice core and the EASM records,the surface hydrology changes in the northern Okiwana trough have a good correlation with both of them,but there are some differences.For example,the surface hydrology changes in the northern Okinawa trough lagged behind the ice core and the EASM.Moreover,the EASM and the hydrology of northern Okinawa trough were different in response to climate changes in the high latitudes of the northern hemisphere.The causes of these differences may be that millennial oscillations of climate in the high latitudes of the northern hemisphere affected the strengh of the EASM rapidly by the reorganization of atmospheric circulation,but influenced the SST of the tropical sea and ECS through the thermohaline circulation,and greenhouse gases have an important regulating effect on ice-atmosphere-ocean climate system.