Drastic change in the late Pliocene subarctic Pacific diatom community associated with the onset of the Northern Hemisphere Glaciation

To date there is scarcely clear understanding of the detailed responses of the subarctic Pacific phytoplankton communities to the significant climate transition across the initiation of the Northern Hemisphere Glaciation (NHG) approximately 2.7 million years ago. We present more highly resolved evidence of the rapid and drastic change in the phytoplankton biosphere, using diatom as a tracer of environmental changes and based on quantitative assemblage analysis of two marine cores from oceanic regions of the North Pacific subarctic gyre.In the late Pliocene subarctic Pacific, Coscinodiscus marginatus and Neodenticula kamtschatica were widely dominant, but the flora rapidly and drastically shifted to one rich in Neodenticula complex (N. koizumii +N. seminae) at the onset of NHG perhaps due to intense changes in paleoceanographic conditions such as cooling and/or a collapse of the previously nutrient regime in the region.We also suggest that vertical pumping of biogenic matter by diatoms may have been more efficient in this region earlier than the onset of NHG. After the NHG intensification, the major primary productivity role in the subarctic Pacific shifted from the C. marginatus-rich flora to the Neodenticula complex. The drastic decrease of biogenic opaline accumulation reportedly associated with the initiation of NHG would have resulted from not only a decrease of primary production, but also from the change in diatom assemblages and consequent effects on their principal role in export flux.