Characteristics Of Environmental Magnetism Of ODP Site 1148 In South China Sea Since Neogene Andits Response For Structures And Climate

The South China Sea (SCS) is the largest western Pacific marginal sea, and it is a natural laboratory for the studying of the Cenozoic Asian monsoonal climate dynamics and the evolution of marginal seas in general. ODP Site 1148 is located on the lower continental slope of China near the continent/ocean crust boundary.It contains 853 m thick sedimentary sequence to the Lower Oligocene, representing the longest record so far to study regional paleoceanographic changes and the impacts on global climate and the tectonic evolution of the SCS.Rock magnetism and environmental magnetism were conducted on the core samples of the ODP Sicte 1148 in order to explore magnetic characteristics of the sediment. Environmental magnetism study showsmagnetic minerals are mainly low coercivity PSD magnetite with a small amount of high coercivity goethite in the middle and low intervals with small amount of iron sulfide in the upper intervals.The magnetic parameters aredivided into three parts named A (457-412 mcd)、B (412-260 mcd) and C (260-0 mcd) separately from bottom to up. Magnetism of mineral from part A is weak. The value of magnetic parameters is very low indicating very low amout of magnetic minerals. For the part B, the value of x and SIRM slowly increase and areconsistent with xARM/x、xARM/SIRM, which can be interpreted as the variations of the grain size of magnetic mineral of magnetite. The change of magnetic parameters of part C is dominated by the concentration of magnetic mineral.Combining with the existing age model, the fluctuations of magnetic parameters may respond the following climate events and tectonic events:(1) the low value of magnetic parameters from 360 to 330 mcd may respond to the Middle Miocene Climate Optimum during 17-15 Ma; (2) the value of xARM、xARM/x and XARM/SIRM largely increased at 330 mcd may respond to the global climate cooling at 15 Ma; (3) the increase of xARM、SIRM and x and the decrease of xARM/x and xARM/SIRM at 220 mcd may respond to the exposure of Taiwan accretionary prism during 6.5 Ma; (4) the decrease of xARM、x and SIRM at 170 mcd may respond the global climate cooling at 3.5 Ma.Besides, environmental magnetism may also reveal the evolution of East Asian Monsoon:(1) changes of XARM/x and XARM/SIRM arefluctant and also showconsistant with Hematite/Geothite of ODP Site 1148 during 20-10 Ma, wnich may respond to an extended period of summer monsoon maximum; (2) trend of S-300 showed decrease on long time scales since 23 Ma, and it iscorrespond to chemical weathering proxies of north SCS and palynomorph proxy of the eastern Qaidam Basin, which may suggest enhanced East Asian Winter Monsoon since Neogene.