Foraminiferal Assemblages In The Southwest Indian Ocean Sediments And Their Paleoenrivonmental Implications Since Late Pleistocene

The water of tropical Indian Ocean and South Atlantic get exchanged because of the Agulhas System.The strength of Agulhas Return Current is related to Indonesian Throughflow.Planktonic foraminifera is sensitive to environmental changes,so the planktonic foraminiferal assemblages could show the changes of water masses.Therefore,understanding the planktonic foraminiferal assemblages and their relationship with environment of surface sediment in Indian Ocean is important to clarify paleoenvironment evolution.This study collected the census data of planktonic foraminiferal of south Indian Ocean which obtained from surface sediment on sea floor by former researchers.We divided Indian Ocean into eight blocks based on the cluster analysis result of planktonic foraminiferal assemblages.The analysis of the relationship between planktonic foraminiferal assemblages and environmental factors in every blocks show that the planktonic foraminiferal assemblages are related to current transportation,the spacies gradually change from warm-water assemblages G.ruber,G.sacclifer and N.dutertrei to cold-water assemblages G.bulloides,G.inflata and N.pachyderma.sin along Indonesian Throughflow-South Equatorial Current-Madagascar Current-Agulhas Current-Agulhas Return Current.The planktonic foraminifera we collected from southwest Indian Ocean Agulhas Current are mainly subtropical zone and transitional zone species(G.bulloides,G.inflata,N.pachyderma.dex),the relative abundance distribute in small scale area depended on temperature and other hydrologic factors.The results show that the planktonic foraminiferal assemblages reveal surface marine environment information and are not effected by seafloor hydrothermal process.The deposition rate of MC03,MC04,MC05 from Southwest Indian Ocean Agulhas Current are low.The deposition rate of MC04 is only 0.6cm/ka due to the strong erosion around.Percentage change of surface warm water spacies show high value in MIS1,MIS3 and low value in MIS2.The spacies are mainly warm-water spacies G.ruber,G.sacculifer in site MD77150 which located above the path of Indonesian Throughflow in the equator 90°E.After comparing the sea surface temperature in two different areas we found that the influence of Indonesian Throughflow is much stronger in MIS1 and the sea surface temperature is raising in Southwest Indian Ocean Agulhas Return Current.The high temperature influence of Indonesian Throughflow to Agulhas Return Current decrease after long distance transportation.After comparing the sea surface temperature change of MC03,MC05 we found that Subtropical Front of MIS1 possibly locate between MC03,MC05 and the Subtropical Front may be the reason of great difference of water temperature and salinity between south and north.The large fluctuation of temperature in MIS1 is possibly because of the migration of the position of Subtropical Front.