Response Of Deep-sea Diatoms From Northern South China Sea To The Evolution Of East Asian Monsoon Since The Last Deglaciation

Monsoon is a typical phenomenon in low to mid-latitudes which plays a very important role in changing the regional climate,environment,politics,economy as well as the way of people's living and the mode of production.Consequently it has been one of the hottest topics of global change research on formation,evolution,and development of the monsoon system.Great efforts have been made related to the study of East Asian Monsoon(EAM)with samples from both inlands and deep seas.But it seems irreconcilable among various records of surface seawater temperature(SST)and surface seawater salinity(SSS)from the northern South China Sea(SCS),which is also decoupled from terrestrial records.Thus,the history of climatic change in the northern SCS since the last deglaciation and whether the monsoonal records from inlands and deep seas are comparable to each other remain open to debate.The upper part(0-398 cm)of core MD05-2904 from the northem SCS was applied to carry out diatom analysis in order to investigate how diatoms responded to climatic change around the northern SCS over the last 15 kaBP.Based on the results of diatom analysis,an extensive comparison was made between the monsoonal records from core MD05-2904 and other sites.Primary conclusions are specified as follows:1.A total of 98 diatom taxa belonging to 46 genera had been identified from the upper part (0-398 cm)of core MD05-2904.The dominant diatoms were tropical/subtropical planktonic diatom taxon Thalassionema nitzschioides and Thalassionema nitzschioides var.parva,which were accompanied with small numbers of freshwater species,coastal species,and oceanic species.2.A program of Principal Component Analysis(PCA)was employed here and the first two main environmental factors controlling the characters of sedimentary diatom assemblages in the northern SCS were attributed to SST and SSS.According to the change of SST and SSS,five periods can be confirmed corresponding to Bφlling/Allerφd stage,Younger Dryas stage,early, middle,and late Holocene individually.Bφlling/Allerφd stage was characterized by an increasing SST and a decreasing SSS.During the Younger Dryas stage,SST decreased slightly at the first phase while SSS rose up dramatically.SST was higher in the mid-Holocene which was named Megathermal,and it kept rising or dropping in the early and late Holocene respectively.SSS kept declining sine the early Holocene,but this trend stopped at ca.1.2 kaBP.3.A total of 148 samples from the seafloor of the western Pacific were utilized to ascertain the distribution of Thalassionema nitzschioides and Thalassionema nitzschioides var.parva in waters with different SST.We confirmed Thalassionema nitzschioides var.parva is a warm water species and Thalassionema nitzschioides is a cosmopolitan and purposed the ratio of[T. nitzschioides var.parva /(T.nitzschioides+nitzschioides var.parva)],which is short for Rparva, can be regarded as a SST-index as to document SST changes in the northern SCS.The Rparva-based SST record is consistent with that of PCA analysis.4.Diatom-based SST record in the northern SCS is not only in concert with the terrestrial SST records but also comparable to other SST records around the world,which indicates that temperature change in the EAM area could be involved in both regional and global climatic mechanisms(such as monsoon and solar radiation respectively).5.Diatom-based SSS record was a combined result mainly due to monsoonal precipitation, sea-level change,and runoff from Pearl River.The influence of sea-level change and runoff from Pearl River on SSS declined sine the mid to late Holocene while the effect of monsoonal precipitation had been pervasive over the last 15 kaBP.Consequently monsoonal precipitation might be the most influential factor for the change of SSS around the northern SCS.Comparison between SSS records from East China Sea(ECS),southern and northern SCS,Sulu Sea,and western equatorial Pacific shows that SSS record from the northern SCS responded directly to the change of monsoonal precipitation.6.Younger Dryas event has also been recorded in both our diatom-based SST and SSS record. But change of SST was not as conspicuous as that of SSS.7.Consistency of climatic records between the inlands and deep seas around the EAM area suggests that northern SCS has been under the EAM regime sine the last deglaciation like the inlands.