Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

S (Solid Earth Sciences) » S-GL Geology

[S-GL32] Lower-Middle Pleistocene Boundary GSSP in the Kazusa Group

Thu. May 24, 2018 3:30 PM - 5:00 PM A10 (Tokyo Bay Makuhari Hall)

convener:Makoto Okada(Department of Earth Sciences, Faculty of Science, Ibaraki University), Yusuke Suganuma(National institute of Polar Research), Koji Kameo(千葉大学理学研究科地球科学, 共同), Yoshimi Kubota(National Museum of Nature and Science), Chairperson:Haneda Yuki(茨城大学理工学研究科), Okada Makoto(Ibraki University)

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

*Koji Kameo1, Yuki Haneda2, Yoshimi Kubota3, Yusuke Suganuma4, Makoto Okada5 (1.Department of Earth Sciences, Faculty of Science, Chiba University, 2.Graduate School of Science and Engineering, Ibaraki University, 3.National Museum of Nature and Science, 4.National institute of Polar Research, 5.Department of Earth Sciences, Faculty of Science, Ibaraki University)

Keywords:Chiba composite section, calcareous nannofossils

The middle part of the Kokumoto Formation in the Kazusa Group, exposed in the Chiba composite section (along the Yoro River and its nearby areas), is a candidate for the GSSP (Global Stratotype Section and Point) of the lower/middle Pleistocene boundary. This section has been studied in order to establish standard chronostratigraphy and clarify environmental changes around the Matuyama-Bruhnes geomagnetic boundary (MBB) based on microfossil biostratigraphies, magnetostratigraphy and chemostratigraphy (e.g., Suganuma et al., 2015; Okada et al., 2017; Haneda et al., 2016, 2017). Among recent investigations, marine microfossils have been utilized because the Kazusa Group, including the Kokumoto Formation, contains abundant and various kinds of calcareous and siliceous microfossils. This study summarizes calcareous nannofossil biostratigraphy of the Kazusa Group based on pioneer studies (Sato et al., 1988; Tsuji et al., 2005) and recent higher-resolution biostratigraphic data. Then, we focus on calcareous nannofossils of the Kokumoto Formation in the Chiba composite section (Kameo et al., 2017a, b) with some new additional assemblage data and discuss sea-surface environmental changes around the Matuyama-Bruhnes boundary in the Pacific side of Japan.

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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