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[B-PT06]
[BPT06-P09]
©Japan Geoscience Union 
Japan Geoscience Union Japan Geoscience Union Meeting 2018 
[JJ] Evening  Poster | B (Biogeosciences) | B-PT Paleontology
Biotic History
convener:Isao Motoyama(Department of Earth and Environmental Sciences, Yamagata University), Takao
Ubukata(Division of Geology &Mineralogy, Department of Earth &Planetary Sciences, Kyoto University),
Kazuyoshi Moriya(早稲田大学 教育・総合科学学術院 地球科学専修)Sun. May 20, 2018 5:15 PM - 6:30 PM  Poster Hall (International Exhibition Hall7, Makuhari Messe)
The Biotic History session covers all aspects of ancient life and the history of biosphere through the geologic
time. The study of ancient life is essential for unveiling mysteries of our planet earth. It also provides evidence
for evolution of oceans, continents and island arcs. Modern progress in this field has been enhanced by
interdisciplinary collaboration with allied sciences, such as paleoceanography and evolutionary biology. Our
session intends to be a hub of communication amongst all earth scientists studying the biosphere; we
welcome biological and biogeochemical approaches toward the understanding of the history of life. 
Calcareous nannofossil biostratigraphy of the upper part of
the Kiwada Formation and estimated marine environment
change
*Daisuke Kuwano1, Koji Kameo2 (1.Division of Earth and Environmental Sciences, Graduate School of Science
and Engineering, Chiba University , 2.Department of Earth Sciences, faculty of Science, Chiba University)
Keywords:Calcareous nannofossil, Paleoceanography, Quaternary, Kazusa Group, Kiwada Formation 
The Kazusa Group, composed of the lower-middle marine Pleistocene sediments, distributes in the middle
part of the Boso Peninsula along the Pacific side of Japan. A lot of geological studies have been conducted
(e.g., Mitsunashi et al., 1959) because the Group has been considered to be one of the representative strata
for the Quaternary due to well-exposed condition with many tephra layers and various kinds of microfossils.
In particular, microfossil biostratigraphies (Oda, 1977; Sato et al., 1988) and the reconstruction of marine
environment using microfossils (e.g. Igarashi, 1994) have been studied. Sato et al. (1988) applied nannofossil
datums established at the North Atlantic Ocean (Takayama and Sato, 1987) to the formations of the Kazusa
Group in order to clarify ages of them in the Group. Recent chronologic studies of nannofossil datums,
however, enable us to discuss geological ages with higher resolution (e.g., Raffi et al., 2006) and thus, this
study aims at establishing high-resolution calcareous nannofossil biostratigraphy in the upper part of the
Kiwada Formation which were recognized a lot of nannofossil datums. Moreover, sea-surface environmental
changes are also discussed based on nannofossil assemblages. 
In this study, we studied the Kiwada formation along the Yuki River, one of the branches of the Yoro River.
Thirteen genera and 19 species of calcareous nannofossils were found in 31 samples in this formation. As a
result, the last occurrences of large Gephyrocapsa spp. and Helicosphaera sellii were recognized slightly
above the key-tephra Kd1 and the key-tephra Kd17, respectively. The last occurrence of large specimens of
Gephyrocapsa spp. may be located near MIS 35 based on Pickering et al. (1999). However, it is difficult to
determine exactly the stratigraphic horizon of the last occurrence of H. sellii due to sporadic occurrences of
this species and the presence of slump deposits in the middle part of the Formation. According to
stratigraphic changes in environmental indicators, past movements of the Kuroshio front during the
deposition of the upper part of the Kiwada Formation can be estimated. Abundant occurrences of a cool
water taxon, Coccolithus pelagicus (Tanaka, 1991), are recognized above Kd6 and just below Kd1. Thus, two
boundaries called the Kuroshio and Oyashio fronts temporarily had moved southward and comparatively
cool surface waters occupied areas around the Boso Peninsula in 1.33 Ma and 1.21 Ma. The Kuroshio front
has repeatedly moved northward and southward in short period after 1.21 Ma because repetitions of
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©Japan Geoscience Union 
Japan Geoscience Union Japan Geoscience Union Meeting 2018 
abundant occurrences of both C. pelagicus and a warm Umbilicosphaera sibogae are observed above Kd1. 
Reference 
Igarashi, 1994, Jour. Geol. Soc. Japan, 100, 348&ndash;359., Mitsunashi et al., 1959, Bull. Geol. Survey
Japan, 10, 82&ndash;98., Oda, 1977, Sci. Rep., Tohoku Univ., 2nd ser. (Geol.), 48, 1-76., Pickering et al.,
1999, Jour. Geol. Soc., 156, 125&ndash;136., Raffi et al., 2006, Quat. Sci. Rev., 25, 3113-3137., Sato et al.,
1988, Jour. JAPT, 53, 475-491., Takayama and Sato, 1987, Initial Repts,, DSDP, 94, 651-702., Tanaka, 1991,
Sci. Rep., Tohoku Univ., 2nd ser. (Geol.), 61, 127-198.