The Pliocene Nannofossil Assemblages And Variation Of The Upper Ocean Structure From ODP1146 In The Northern South China Sea

By the quantitative investigation and analysis of the calcareous nannofossils from the 214 samples which from the ODP LEG 184 Site 1146 at 187.51- 252.51mcd (3.7-1.9Ma) in the North South China Sea, we obtained the calcareous nannofossils combination and evolved history in the Pliocene period in the north South China Sea; At the same time, by the changes F. profunda relative abundances in the nannofossils assemblage, we had discussed the nutrilcline vicissitude in the Pliocene period of the North of South China Sea ; and the absolute abundance of the calcareous nannofossils here in the same time presented different criterion changes.Between 3.7-1.9Ma, in the calcareous nannofossil assemblages of the north South China Sea Site 1146 , F. profunda, was the absolute dominant species, but P. lacunosa, R. minuta, R. minutula, R. productella was the common species in the Pliocene period in this area. The calcareous nannofossil assemblages was divided into five evolved stages. These gradual evolution possibly was due to the global environmental changes.The changes of the nutrilcline in the Pliocene period in Site 1146 were divided several stages: before 3.2Ma, the depth of the nutrilcline in the north South China Sea was shallow; between 3.2-2.4Ma the depth of the nutrilcline droped gradually, depth fill-out; after 2.4Ma the depth of the nutrilcline was stable. Between 3.2-2.4Ma the continual dropping of the depth of the nutrilcline was the response of the forms and increases of the Northern Hemisphere glaciation and the East Asian winter monsoon gradual enhancement.The absolute abundance of the calcareous nannofossils had fierce changes around 2.9M. The possible reason was the massive land source element inputing. Between 3.7-2.9Ma, the calcareous nannofossils absolute abundance was big, by the high scope and the low frequency stepped changes; between 2.9-1.9Ma, the absolute abundance of the calcareous nannofossils was continuously low, also along with glacial– interglacial change: when it was glacial, the calcareous nannofossils absolute abundance was high; when it was interglacial period, the calcareous nannofossils was low. The possible reason was the reflection to the paleoproductivity.