Keywords:
Biostratigraphy of calcareous nannofossils and inferred sea surface
conditions around the Brunhes–Matuyama Boundary of the Kokumoto
Formation, Kazusa Group, distributed in the Boso Peninsula, central
*Koji Kameo1, Yuki Haneda2, Yoshimi Kubota3, Yusuke Suganuma4, Makoto Okada2
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 
A Pleistocene formation, the Kokumoto Formation of the Kazusa Group, widely distributes in the middle
part of the Boso Peninsula, central Japan. The formation exposed in the Chiba composite section is one of
the GSSP (Global Boundary Stratotype Section and Point) candidate for the lower/middle Pleistocene
boundary as same as two other candidates, Montalbano Jonico and Valle di Manche sections (e.g., Ogg et
al., 2016). The Kokumoto Formation in the section includes a clear boundary of the Brunhes-Matsuyama
geomagnetic boundary (MBB) and thus, a lot of chronologic and/or chronostratigraphic studies in the
formation have been done (e.g., Suganuma et al., 2015). Recently, paleoenvironmental and
paleoceanographic studies have also performed in order to clarify past sea surface conditions, land
vegetation, and related paleoclimatic changes during this time around the Northwestern Pacific region
(e.g., Haneda et al., 2016). This study added some new calcareous nannofossil data to Kameo et al.
(2016) and discusses nannofossil events around the MBB and sea surface conditions, especially temporal
and spatial changes of the Kuroshio Current, one of the major sea surface currents around Japanese
Islands. Approximately 60 mudstone samples were examined in the middle part of the Kokumoto
Formation and well-preserved, abundant calcareous nannofossils were obtained. At least fifteen genera
and 16 species are observed throughout the examined section but no characteristic changes of
nannofossil occurrences are observed. Even though any last and/or first occurrences of specific species
were not detected, larger Gephyrocapsa specimens are characteristically found in the upper part of the
examined formation. Their occurrences might correspond to the presence of larger specimens of
Gephyrocapsa sp. C (Matsuoka and Okada, 1990), and/or Gephyrocapsa sp.3 (Rio et al., 1990). It means
that the base of occurrence of larger forms of Gephyrocapsa sp. 3. can be a possible biohorizon near the
MBB. At the same time, some environmental nannofossil indicators near the Japanese islands, were
characteristically found. Florisphaera profunda, a lower photic taxon that preferred stable sea surface
conditions (Ahagon et al., 1993), and Umbilicosphaera sibogae, a Kuroshio water taxon (Tanaka, 1990)
became abundant after the boundary of the MIS 20/19. It suggests that the northward penetration of the
Kuroshio Current occurred after 790 ka. Moreover, some local upwelling events in the basin might be
estimated because an upwelling indicator, Coccolithus pelagicus braarudii, was occasionally observed
after the boundary of MIS 20/19. 
References 
Ahagon et al., 1993, Marine Micropaleontol., 22, 255-273., Haneda, Y., 2016, Abst., Paleosciences Soc.,
2nd annual meeting, 24., Ogg et al., 2016, A Concise Geologic Time Scale 2016, 236 pp., Kameo et al.,
2016, Abst. 166th Regular Meeting, Palaeotological Soc., Japan, 39., Rio et al., 1982, Int. Repts. DSDP,
68, 325–343., Suganuma et al., 2015, Geology, 43, 491-494., Tanaka, Y.,1991, Sci. Rep., Tohoku Univ.,
2nd ser. (Geol.), 61, 127-198. 
Calcareous nannofossil biostratigraphy, Chiba composite section
SGL38-P04JpGU-AGU Joint Meeting 2017
©2017. Japan Geoscience Union. All Right Reserved.- SGL38-P04 -
SGL38-P04JpGU-AGU Joint Meeting 2017
©2017. Japan Geoscience Union. All Right Reserved.- SGL38-P04 -