Calcareous nannofossil biochronology from the upper Pliocene to lower Pleistocene in the southernmost Boso Peninsula, central part of the Pacific side of Japan

Abstract

The calcareous nannofossil biostratigraphy in the southernmost Boso Peninsula, central part of the Pacific side of Japan, was examined to calibrate nannofossil biohorizons to an existing oxygen isotope stratigraphy for the northwestern Pacific. Five nannofossil biohorizons were detected in the uppermost Pliocene and the lower Pleistocene Mera and Minamiasai Formations. Each biohorizon can be correlated with a previously presented magnetic reversal and a corresponding marine isotope stage (MIS) in both formations. They are from bottom to top: last occurrence (LO) of Discoaster asymmetricus (MIS G8–G9 transition), LO of Reticulofenestra minutula var. B (MIS G8–G9 transition), LO of Discoaster tamalis (MIS G7–G8 transition), LO of Discoaster surculus (MIS 100), and LO of Discoaster pentaradiatus (MIS 95). Based on a comparison with previous studies, most of these biohorizons are traceable events, although the degree of synchrony varies depending on the biohorizons. Moreover, the LO of R. minutula var. B, an abrupt disappearance event of larger Reticulofenestra specimens in the lower part of the Minamiasai Formation, is also traceable to other regions.