(Table 1) Silicoflagellate abundance in ODP Hole 186-1149A sediments

Silicoflagellate assemblages have been studied from Hole 1149A (northwestern Pacific, Ocean Drilling Program Leg 185) in a continuous record of pelagic clay from lithologic Unit I. The biostratigraphic framework of D. Bukry, adapted in this work, is calibrated to tropical and subtropical fossil floras. Four biozones and three subzones have been recognized (from the lower Pliocene Dictyocha fibula Zone to the Quaternary Dictyocha aculeata aculeata Zone), and their zonal boundaries have been calibrated to the magnetostratigraphic scale. In particular, the boundary between the Dictyocha perlaevis delicata and the Dictyocha perlaevis ornata Subzones approximates the Pliocene/Pleistocene boundary.
The identification of three closely spaced climatic events (the asperoid/fibuloid reversal at 75.11 meters below seafloor [mbsf], the sharp decrease in abundance of Distephanus sp. at 70.05 mbsf, and the last occurrence of Distephanus boliviensis at 68.46 mbsf) indicates the increased strength of the warm Kuroshio Current in the interval between 3.3 and 2.8 Ma, due to the final closure of the Central American Seaway and leading to the present-day hydrodynamic regime. The beginning of the acme of Mesocena quadrangula at 37.01 mbsf (~1.1 Ma) also shows a sharp warming in surface waters.