Allochthonous Diatoms in DSDP Site 436 on the Abyssal Floor off Northeast Japan

During the interval from 4.9 to 4.1-3.7 Ma in the early Pliocene, extinct fossil freshwater diatom species Aulacoseira praeislandica and near-shore marine fossil diatom species Koizumia tatsunokuchiensis are abundant at DSDP Site 436 on the abyssal floor far east over the Japan Trench as sediment trap. The close correspondence of abundances of two characteristic diatom groups with occurrence of coarse volcanic debris, increase of eolian material, and large numbers of fecal pellets in the early Pliocene sediment at Site 436 suggests deposition by the settled water column after being transported to the area through the Kuroshio-Kuroshio Extension system driven by winds and the atmosphere by the typhoons during a warming climate interval. The early Pliocene is considered to be the warmest interval and a period of reduced latitudinal thermal gradients. Tropical cyclones (hurricanes and typhoons) and ocean wind-driven circulations are simulated by modeling studies to have increased during the early Pliocene because of a vast pole-ward expansion of the tropical warm pool that associated with expanded El Nino-like conditions. And the Pliocene coral records support that stronger winds in the tropical Pacific act as a possible driving force of the western Pacific warm pool. Tropical storms in the early Pliocene increased ocean vertical mixing and transported heat pole-wards. Atmosphere-ocean circulations are considered as efficient media for the upwelling and transportation. The Pliocene extinct freshwater and near-shore marine diatoms at Site 436 may have been derived from the continent or/and sea bottom by atmosphere-ocean dynamics along frontal boundaries moved northward off Japan during the warm early Pliocene.

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