Paleoceanographic Records Of Core TX05 From The Southern South China Sea Over The Past 270 000 Years

The southern South China Sea(SCS)is geographically located in the margin of the western Pacific warm pool and in the tropics.Its critical location makes the southern SCS of a crucial place in the paleoceanographic study that leads to understanding of Quaternary evolution of oceanic circulation and its response to both local and global climate change in the entire SCS and in the western tropical Pacific region.Fossil planktonic foraminiferal shells are ones of indispensable materials in reconstruction of paleoceanographic parameters because of the close relationship of their faunal distribution and geochemical characteristics with seawater temperature,salinity,upper ocean structure and paleoproductivity.In this study,oxygen(?18O)and carbon(?13C)isotopes and Mg/Ca ratio are analyzed on shells of Globigerinoides ruber,Globigerinoides sacculifer and Pulleniatina obliquiloculata from sediments of a 778-cm long gravity core that was retrieved at Site TX05(6°20.6'N,111°16.2'E;water depth:1750 m)in the southern SCS.Absolute abundances of planktonic foraminiferal assemblages and relative abundance of some species are examined to give supplementary information.The core was sampled at 6 cm intervals in the upper 260 cm and at 10 cm intervals from 260 to 778 cm,resulting in a total of 91 sediment samples.G.ruber is used as an indicator of surface water,G.sacculifer is considered as a deep mixed-layer dweller and P.obliquiloculata a thermocline/subsurface water dwelling species.Isotopic profiles of these species are thus jointly investigated to reconstruct evolutionary history of upper ocean thermal structure and paleoproductivity.The stratigraphy of this core is constrained by five AMS 14C dating points and five oxygen isotope stratigraphic tie points.Together with countings of planktonic foraminiferal species,these isotopic data document a?270-kyr history of depth of thermocline(DOT)and paleoproductivity in the southern SCS since marine isotope stage(MIS)eight.The faunal counting results show that G.ruber(19.71%),G.sacculifer(20.78%),N.dutertrei(7.13%),P.obliquiloculata(12.01%)and Globigerinella calida(7.94%)are five dominant species in the planktonic foraminiferal assemblages at Site TX05 in the southern SCS over the past 270 000 years,and averagely contributed over 50%to the fauna in the total of 91 samples.The results show a larger glacial-interglacial amplitude of seawater temperature variation in the sea surface than in the thermocline,indicated by Mg/Ca records of G.ruber,G.sacculifer and P.obliquiloculata.The glacial-interglacial difference is?7.2?for sea surface temperature and?6.4? for thermocline temperature.The highest temperature of the surface sea water is 29.9?(MIS 5),and the lowest is 22.7?(MIS 6).The highest thermocline seawater temperature is 22.0?(MIS 7),and the lowest is 15.6?(MIS 2).Difference of shell ?18O between surface and subsurface dwellers(??18O(P.obiliquloculata-G.ruber))indicates a general shoaling trend of DOT,particularly remarkable since the penultimate deglaciation,and also shows shallower DOT in interglacials and deeper DOT in glacials.813C of all the species shows an overall increasing trend,while higher values occur in interglacials than in anteceding glacials.Depleted 813C of the three species occurred almost synchronously and mostly during transitions of glacials to interglacials,except for during the transition of MIS 8-7,possibly due to insufficient sampling resolution.?13C profiles among species indicate that primary productivity may increase with water depth until?50 m and then decrease in deeper water depths.In addition,difference of shell ?13C between species(??13C(G.r-P)),together with absolute abundance of planktonic foraminiferal assemblages and relative abundance of high-productivity species,indicates two stages of increasing in paleoproductivity interrupted during the penultimate deglaciation and suggests higher paleoproductivity in interglacials than in glacials.This study ascribes changes in DOT and paleoproductivity mainly to intensifying of East Asian winter and summer monsoon under the general background of gradually increasing glacial-interglacial difference.