Pliocene/Pleistocene boundary and paleoceanographic significance of the upper Miyazaki Group, southern Kyushu, Southwest Japan, based on calcareous nannofossil and planktic foraminiferal assemblages

Abstract

The Plio—Pleistocene succession in southwest Japan is exposed along the Nagatani River (NGT section) in the northern Miyazaki region, southern Kyushu, and on the Pacific side of southwest Japan. The NGT section is a prominent sequence that includes the upper Takanabe Formation of the uppermost part of the Miyazaki Group. In this study, we clarify the paleoceanographic conditions during deposition of the uppermost Miyazaki Group, based on faunal compositions of calcareous nannofossils and planktic foraminifera. We especially focused on two genera, Discoaster and Reticulofenestra, as well as the calcareous nannofossil species Coccolithus pelagicus and the planktic foraminifera species Globigerinoides ruber, Globigerinoides quadrilobatus, Globigerinoides sacculifer, and Neogloboquadrina incompta, as proxies for paleoceanographic conditions. Changes in the relative abundances of these flora and faunal compositions suggest that paleoceanographic conditions in the Miyazaki area shifted from that of a stratified surface-water mass (Kuroshio current) during the late Pliocene to conditions of upwelling (cold-water mass) during the early Pleistocene, via a transitional phase which occurred just after the biohorizon containing the last occurrence of the genus Discoaster.