To reconstruct the Cenozoic paleoceanographic evolution, in particular, temporal changes in nutrient levels, we studied calcareous nannofossil assemblages from Ocean Drilling Program holes 1210A, located in the northwestern Pacific Ocean, and 762B, situated in the eastern Indian Ocean. At each site we focused on the relationships among the nannofossil accumulation rate (NAR), relative abundance of Discoaster, and coccolith size of the genus Reticulofenestra. The co-occurrence of a low NAR, high Discoaster abundance and presence of large Reticulofenestra suggest a deep thermocline and nutricline, typical of oligotrophic conditions. Conversely, the co-occurrence of a high NAR, low Discoaster abundance and presence of small Reticulofenestra indicate a shallow thermocline and nutricline, typical of eutrophic conditions. The combination of these three parameters suggests that the gradual eutrophication and collapse of sea surface stratification occurred in the northwestern Pacific Ocean at 7.9, 6.4, and 5.0. Ma. In contrast, abrupt eutrophication is indicated at 8.8. Ma in the eastern Indian Ocean. Although the timing of eutrophication differs between the Pacific and Indian Ocean sites, it roughly coincides with tectonic and/or climatic events around the two oceans. This finding suggests that tectonic and climatic events are capable of causing an increase in the terrestrial input and/or coastal upwelling of nutrients that subsequently leads to the eutrophication of sea surface waters.
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