Abstract:
Calcareous nannofossil assemblages and productivity at ODP Site 846 from the eastern equatorial Pacific and ODP Site 807 from the western equatorial Pacific were analyzed in order to reconstruct the climate evolution around the last 550,000 years. From Marine Isotope Stage (MIS) 14 to MIS 8, the nannofossil assemblage at Hole 846B exhibits a high relative abundance of small coccoliths. The assemblage in the coeval intervals at Holes 846B and 807A contains a high absolute abundance of coccoliths and a low relative abundance of warm water species. These characteristics of the tropical Pacific cores suggest an intensified trade wind and coastal upwelling system during MIS 14 through MIS 8. However, from boundary of MIS 8/7, a shift in calcareous nannofossil assemblage from a high to a low abundance of very small coccoliths is recorded at all holes, while the relative abundance of warm water species increased in the equatorial regions. From MIS 7 to MIS 1, very small coccoliths remained low in abundance, while warm water species continued to exhibit high values. These characteristics suggest that the strength of the trade winds was weakened during MIS 7 through to the present, as the upwelling system along the equator is controlled by the strength of the trade winds which linked Walker circulation during El Nino-like conditions. Thus, the modern hydrography in the equatorial Pacific Ocean formed after boundary of MIS 7/8, and the drastic shift in biological productivities and assemblages in the surface waters occurred suddenly during MIS 8, resulting from the change of Walker circulation and Hadley circulation strength.