Pelagic Sedimentary Records And Its Palaeoenvironmental Implication In Ninetyeast Ridge Of The NE Indian Ocean Since Middle Miocene

The equatorial seamounts of the NE Indian Ocean are 1000m higher than the sea floor. Pelagic deposit, as the dominant sedimentation, has a low sedimentation rate, continuous record and it contains both long- and short- scale palaeoenvironmental signals. It's an ideal place for the study on palaeoceanography, mirroring the lift of the Qing-Tibet plateau and its environmental effect.Through the study on the foraminifera, sampled from the core ODP758 and MD81349, combining the data of stable isotopes, carbonate abundances and magnetism stratigraphy, this paper discusses the sedimentation's response to the Qing-Tibet Plateau rising, the relation between the sedimentry records and regional or even global climate changes and the palaeoceanography and palaeoclimate characters of the equator seamounts in the NE India Ocean since 0.3 Ma years.According to the studied records from the Ninetyeast Ridge, four turns under influence of the episodic Qing-Tibet rising can be recognized, which take place at 3.7-3.IMa, 1.65Ma, 0.8-0.6Ma and 0.17-0.16Ma. They respectively correspond to the Episode A(3.6Ma),Episode C(1.7Ma) of Qing-Tibet Movement, Kunlun-Huanghe Movement(0.6-1.2Ma) and Gonghe Movement(0.15Ma).It seems that there was a big lag between the winter wind and summer wind in India. The monsoon intensity is the key mark of tectonic movement. In the late Cenozoic, the most important environment change took place between 3.7Ma and 3.2Ma. Due to the rising of Qing-Tibet Plateau, the India winter monsoon started and prevailed and it promoted the oceanic productivity. Globigerina bulloides, which was regarded as the mark species for the SW monsoon prevailing in the Arab Sea, while in this area, it's more likely to be a mark species for the strong NE monsoon.The climatical variation mirrored by the sedimentary records in Ninetyeast Ridge can well correspond to the global climate changes. Main replace indexes reflect the change of orbital forcing. The rhythm cycle of the obvious climate change , which took place in the period of 0.8 Ma, transferred from 40ka to 100ka.The typical monsoon in this area has a great influence on the oceanic productivity and the salinity of the sea water. During the periods of 12ka , 60ka, 118-130ka, the decreasement of summer monsoon resulted in the decreasement of SW monsoon, then correspondingly, the less high salinity water mass make this area low salinity. The productivity in episode 2, 4, 5, 6, 7 is higher.