3:45 PM - 4:00 PM
[SGL32-02] A high-resolution biostratigraphy of calcareous nannofossils around the Matuyama-Brunhes boundary in the Kokumoto Formation, Kazusa Group, in the Boso Peninsula, central Japan
Keywords:Chiba composite section, calcareous nannofossils
Based on biostratigraphic results of the Kokumoto Formation in the Chiba composite section, it is slightly difficult to find clear bioevents because of the lack of apparent disappearances and/or appearances of any nannofossil species. However, a possible biohorizon composed of the re-appearance of larger Gephyrocapsa specimens was recognized below the MBB (Kameo et al., 2017b). This event may correspond to the base of occurrence of Gephyrocapsa sp. C (Matsuoka and Okada, 1990) and/or of Gephyrocapsa sp. 3 (Rio et al., 1982) but these specimens in the section are slightly larger than those of Gephyrocapsa sp. C and/or Gephyrocapsa sp. 3.
Floral changes of calcareous nannofossils as surface water indicators are characteristically observed in the Chiba composite section. Florisphaera profunda, indicating warm, pelagic and stratified surface water conditions (Ahagon et al., 1993), gradually increased above the boundary of MIS 20/19. It suggests that warm and open ocean waters may strongly affect during MIS 19. The northward shift of the Kuroshio front, however, was possibly earlier because Umbilicosphaera sibogae, a warm Kuroshio water indicator (Tanaka, 1991), became suddenly abundant after 790 ka, prior to increasing of F. profunda. Other environmental indicators, a possible cool and upwelling indicator Coccolithus pelagicus braarudii (e.g., Parente et al., 2004) and an offshore dweller Calcidicus leptoporus (Tanaka, 1991), are also found characteristically and their occurrences may relate to sea-surface environmental changes.
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