15:45 〜 16:00
[SGL32-02] 上総層群国本層のマツヤマ—ブリュンヌ境界付近の高分解能石灰質ナノ化石層序
キーワード：千葉セクション、石灰質ナノ化石
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.
References
Ahagon et al., 1993, Marine Micropaleontol., 22, 255-273., Haneda et al., 2016, Abst., Paleosciences Soc., 2nd annual meeting, 24., Haneda, et al., 2017, Abst., JpGU-AGU Joint Meeting 2017., Matsuoka and Okada, 1990, Proc. ODP, Sci. Res., 115, 255–270., Kameo et al., 2017a, Abst. 166th Regular Meeting, Palaeotological Soc., Japan, 39., Kameo et al., 2017b, Abst., JpGU-AGU Joint Meeting 2017., Okada, et al., 2017, Earth, Planets Space, 69:45, 2017. doi:10.1186/s40623-017-0627-1, Parente et al., 2004, Micropaleontology, 50, 107-120., Rio et al., 1982, Int. Repts. DSDP, 68, 325–343., Sato, et al., 1988, Jour. JAPT, 53, 474–491., Suganuma et al., 2015, Geology, 43, 491-494., Tanaka, 1991, Sci. Rep., Tohoku Univ., 2nd ser. (Geol.), 61, 127-198., Tsuji et al., 2005, Jour. Geol. Soc. Japan, 111, 1–20.
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.
References
Ahagon et al., 1993, Marine Micropaleontol., 22, 255-273., Haneda et al., 2016, Abst., Paleosciences Soc., 2nd annual meeting, 24., Haneda, et al., 2017, Abst., JpGU-AGU Joint Meeting 2017., Matsuoka and Okada, 1990, Proc. ODP, Sci. Res., 115, 255–270., Kameo et al., 2017a, Abst. 166th Regular Meeting, Palaeotological Soc., Japan, 39., Kameo et al., 2017b, Abst., JpGU-AGU Joint Meeting 2017., Okada, et al., 2017, Earth, Planets Space, 69:45, 2017. doi:10.1186/s40623-017-0627-1, Parente et al., 2004, Micropaleontology, 50, 107-120., Rio et al., 1982, Int. Repts. DSDP, 68, 325–343., Sato, et al., 1988, Jour. JAPT, 53, 474–491., Suganuma et al., 2015, Geology, 43, 491-494., Tanaka, 1991, Sci. Rep., Tohoku Univ., 2nd ser. (Geol.), 61, 127-198., Tsuji et al., 2005, Jour. Geol. Soc. Japan, 111, 1–20.