Research Of The Paleoceanography Of The Western Philippine Sea Over The Past 2.36Ma

This thesis discusses paleoceanography of the Western Philippine Sea over the last 2.36 million years on the basis of stable oxygen and carbon isotopes on planktonic foraminifers (δ13O andδ13C), bulk carbonate content and calcareous nannofossil assemblages from IMAGES XIV MD155-Marco Polo 2 cruise.Based on theδ18O of the planktonic foraminifer G.ruber, a 2.36 Ma timescale for the West Phillipine Sea was established through visual alignment to LR04. The age of several calcareous nannofossil bioevents and one foraminiferal bioevent agreed with the age model.Two sediment cores MD06-3050 (15°57.0943′N, 124°46.7747′E, 2967 m) and MD06-3047 (17°00.44′N, 124°47.93′E) recovered in the Benham Rise, east off Luzon Island in the Western Philippine Sea, were analyzed to study the Quaternary calcareous nannofossil records based on oxygen isotope stratigrapy. 7 calcareous nannofossil bioevents were identified over the past 2.36 Ma: LAD of Discoaster Brouweri, LAD of Calcidiscus macintyrei, FAD of Large Gephyrocapsa, LAD of Large Gephyrocapsa, LAD of Pseudoemiliania lacunosa, abrupt increase in the abundance of Gephyrocapsa Caribbeanica, and FAD of Emiliania huxleyi. The results suggested that calcareous nannofossil stratigraphy was reliable in the Western Philippine Sea, hence provided the proper age control points in building Quaternary oxygen isotope stratigraphy in the Western Pacific. In addition to, we observed that Australasian microtektites were distributed in the core MD06-3050, with maximum abundance in the layer 1364-1366 cm. Calibrated with the oxygen isotope stratigraphy, the average age for layer of 1360-1366 cm was 794±2 ka, which was deemed to microtektites occurred time. It was concurrent with Australiasian microtektites events dated in other regions.Oxygen isotopic differences between G. ruber and P. obliquiloculata demonstrate the upper ocean thermocline variations of the Western Philippine Sea during the Pleistocene. And the relative abundance of lower-photic layer coccolith species, Florisphaera profunda, were be used to construct the nutricline changes since 1040 ka.The variations ofΔδ18O indicated that thermocline changed in long term cycles over the past 2.36Ma. Nutricline showed two evolution patterns before and after MBE. The notable feature in the upper water structure was not changed in phase since MIS13. The filter analysis of both oxygen and carbon isotope in shells of planktonic foraminifera demonstrates the existence of the mid-Pleistocene transition (MPT). The event appears to start at about 1.0 Ma suggested by theδ18OG.ruber,δ18OP.o,δ13CG.ruber,δ13CP.o, bulk carbonate content, indicating its impacts on these climatic variables as a system. Accordingly, the transition of the dominant climatic cycles at ~1Ma can be interpreted as a stepwise but not an abrupt process. Futhermore, changes ofδ13C, CaCO3% lead the shift of upper water structure during the MPT.Carbonate content in bulk show high values during glacial period and low values in the interglacial. Carbon isotope differences between G. ruber and P. obliquiloculata demonstrate was used as paleoproductivity (PP) proxies in the study area, which suggested that the productivity was high in glacial and low in interglacial. Both the planktonic foraminifera fragments ratio and >63μm coarse fraction indicated the dissolution was more intense during interglacial periods than in glacial interval. So, we considered that the preservation of carbonate as Pacific Pattern, which was controled by dissolution and PP.δ13CP.O peaks in different periods suggest that shift is independent from G-IG clycle, so show the long term cycles since the Pliocene. Comparison of the foraminiferalδ13C of core MD06-3050 with other cores records in different basins, we recognized sixδ13Cmax events in the study core. It's worthy noting that theδ13Cmax event coincided with the abrupt decreasing carbonate content and the intensely dissolution condition, so we infered that global carbon reservoir was changed obviously and linked to large climate changes.