Records Of Monsoon And Upwelling In The Sumatra Sea During The Late Pleistocene

Sumatra was suited in the Northeast Indian Ocean, was in the area of Western Pacific Warm Pool(WPWP).As the main source of global heat and vapor, the WPWP acts as an engine of the global climatic system and, together with the High-latitude North Atlantic, constitutes the switch of the global environmental changes. Located in the joint of the three major atmospheric circulations, the WPWP is inseparable from the Monsoon activity, transport of Intertropical Convergence Zone(ITCZ) and El Ni(?)o-Southern Oscillation, so any changes of the upper sea structure and sea-surface temperature(SST) in this area often influence the global climatic change.Core BAR9432 (0°24.54'N,97°18.12'E) was retrieved during the 1994 BARAT campaign with the RV Baruna Jaya I from 2760m water depth at the Nias Forearc-Basin, offshore the western Sumatra in the northeastern Indian Ocean.Core BAR9443(6.457S and 104.293E; 2180m water depth) was obtained below the present South Java Current, offshore the southern portion of Sumatra in the eastern Indian Ocean.Compared with these two cores and based on analyses ofδ18O,δ13C, AMS14C ages, planktonic foraminiferal assemblages, on the sea surface temperature and salinity estimated by the planktonic foraminifera Modern Analog Technique, the reconstruct of paleoceanography records over the past ~132ka have the following results: Climate variation of the studied region was affected by glacial-interglacial sea-level fluctuation obviously, and followed by the 23ka orbit precession cycle. In short, the Paleoceanographical changes in the WPWP display the synchronous impact of the high-latitude icesheet forcing and the low-latitude tropical forcing. So the low-latitude processes such as the monsoon systems, emphasizing the significant role played by orbital forcing in the global climate system through low-latitudinal response.In Southwestern Sumatra during the last interglacial period when the sea-level was higher and similar to the Holocene, terrigenous material supply decreased, the paleoproductivity was low for the studied area. The mixed layer was thicker and coastal upwelling was absent, so with no evident Indian Southwestern Monsoon over the studied area. During the last glacial, when sea level was lower, increased terrigenous material induced a high paleoproductivity. On this basis, the higher paleoproductivity implied the Southwestern Monsoon was strengthened, when the summer insolation in the northern hemisphere strengthened, and the easterly Southwestern Monsoon Current transported the high salinity water of the Equatorial Indian Ocean to the studied area and increased its salinity. In addition, the salinity was higher in August than in February, the seasonal salinity difference between August and February was large. Under the influence of Australian Southeast Monsoon, the weaker Indian Monsoon Current and Equatorial Eddy Current could produce a relative obvious upwelling activity, which made the mixed layer thinner and led to a rise in paleoproductivity. Australian Southeast Monsoon boosted up the influence of Indian Southwestern Monsoon. When the summer insolation in the northern hemisphere decreased, the Indian Southwestern Monsoon was weak, upwelling activity was restrained and paleoproductivity was low. The Indian Northeastern Monsoon seem to be strengthened in the middle MIS 3 over the studied area. The sea surface salinity increased and the seasonal salinity difference between August and February minished due to the strong evaporation near the shore region.Climate variation of the area around the Sunda Strait(time resolution ca.20a) was not only affected by glacial-interglacial sea-level fluctuation obviously and followed by the 23ka orbit precession cycle, but also partly influenced by the influx of terrigenous supply and less saline Java Sea water. During the last glacial period 22~17kaB.P. , when sea level was lower, terrigenous nutrient supply increased, the water column was stratified and may have trapped nutrients in deeper water masses, upwelling activity was absent and nutrient levels in the surface waters were low. The Northwest Monsoon developed more humid conditions and the duration and intensity of the dry season reduced. During 17~14kaB.P. periods, the Southeast monsoon prevailed ,and the climate was cool and dry. Increased mixing of the surface waters led to a rise in paleoproductivity. The Southwest wind reduced, the precipitation limited induced high SSS. During 14~8kaB.P.periods, due to rapid sea level rising, inundated the Sunda shelf and enabled throughflow of warm and less saline Java Sea water through the Sunda Strait into the Northeast Indian Ocean, which made the mixed layer thickened. The SE Monsoon and NW Monsoon seem to be strengthened in Early Holocene, indicating that the biannual monsoonal system was mostly intense at this time. Between 8~3 kaB.P. , NW monsoon generate stronger seasonal rainfall, the advection of fresher Java Sea Water through the Sunda Strait are responsible for the low-salinity"Cap", which made the mixed layer thickened, and restrained upwelling activity. Since~3kaB.P. , the thermocline became shallower, and dissolution was high.