Some middle Miocene planktonic microfossil datum planes in Northern Honshu, Japan: Their paleoceanographic implications

Abstract

In the middle Miocene sequence of northern Honshu, Japan, a radiolarian biohorizon called “the Rapid Decrease Datum of Cyrtocapsella tetrapera” was found useful as a datum level for inter-basinal correlation. Subsequent studies have proven that several biohorizons of various planktonic taxa occur in close association with this datum level. They are: the last appearance datum (LAD) of Cyclicargolithus floridanus, first appearance datum (FAD) of Denticulopsis praedimorphia, FAD of Globigerina nepenthes, FAD of Catinaster coalitus, LAD of Eucyrtidium inflatum, and LAD of Denticulopsis praedimorpha in upward sequence. Further investigation of the spatial distribution of these datum levels based on a number of Miocene sections revealed that a set of these biohorizons, at least of siliceous fossils, occurs both on the Pacific and Japan Sea sides of Honshu. In order to understand the paleoenvironmental conditions which gave rise to the development of these consecutive bio-events, quantitative analyses of benthic foraminifera were first carried out. Foraminiferal taxa from sections such as Sendai, Ichinoseki, Sannohe, Takasaki and DSDP Site 438A can all be grouped into four assemblages by means of a Q-mode cluster analysis. Based on the ecology of modern counterparts and/or species related to those constituting these assemblages, the depth range is estimated for each assemblage. The paleobathymetric history of several basins in northern Honshu reconstructed from these foraminiferal data do not indicate that these planktonic bio-events were either influenced by a local environmental change or controlled by geographic differences of the transgressive phase. To elucidate the distribution pattern of these biohorizons in space and time, paleoceanographic conditions in the region were reconstructed using nannofloral composition. The geographic distribution of the warm Discoaster-Sphenolithus group and cold Coccolithus pelagicus group suggests that the paleo-Kuroshio (warm-water) and Oyashio (cold-water) Currents converged in this region. A relative paleotemperature curve deduced from the pattern of quantitative variation in the composition of nannofloras appears to agree with the oxygen isotopic curve reconstructed for the western equatorial Pacific bottom water.