[M-IS17] New frontiers in geology

[MIS17-04] Geological evidence of the past Kuroshio Current variations from the Kazusa Group, Boso Peninsula

*Daisuke Kuwano^1,2, Koji Kameo², Shota Sugizaki^2,3, Soki Hirota^2,4, Yoshimi Kubota⁵, Kanako Mantoku⁶, Masayuki Utsunomiya⁷, Makoto Okada⁸ (1.Kyoto University, 2.Chiba University, 3.Japan Petroleum Exploration Co., Ltd., 4.Ise Chemicals corporation, 5.National Museum of Nature and Science, 6.National Institute for Environmental Studies, 7.National Institute of Advanced Industrial Science and Technology, 8.Ibaraki University)

Keywords:Kazusa Group, Calcareous nannofossil, Oxygen isotope stratigraphy, Age model, Pleistocene, Paleoceanography

The Kuroshio Current, a western boundary current of the North Pacific subtropical gyre, transports heat, salt, and moisture from low to mid-latitudes. This current significantly influences East Asia and the global climate system (Gallagher et al., 2015). Many studies have examined the past Kuroshio Current variability in various regions and periods. Although Quaternary paleoceanographic studies are typically conducted on marine sediment cores, the shallow carbonate compensation depth in the North Pacific region limits these studies. The Kazusa Group, distributed on the Boso Peninsula, consists of a marine sedimentary succession from the Early to Middle Pleistocene (e.g., Kazaoka et al., 2015). This group is well suited for paleoceanographic study due to its high sedimentation rate, good preservation of calcareous microfossils, and excellent outcrop exposure (e.g., Suganuma et al., 2018). Recently, our group has established an age model based on oxygen isotope stratigraphy (e.g., Kuwano et al., 2021) and conducted biostratigraphic and paleoceanographic studies based on calcareous nannofossils from the Kiwada Formation to the Kokumoto Formation (marine isotope stages 40–18) of the Kazusa Group (Kameo et al., 2020; Kuwano et al., 2025). In this study, we summarize our studies and discuss the variability of the Kuroshio Current in response to Quaternary climate variations.
Calcareous nannofossil assemblages from the Kiwada Formation to the Kokumoto Formation exhibit cyclic changes related to the glacial–interglacial cycles, reflecting the north-south shifts of the Kuroshio Front. In addition to orbital-scale changes, shorter-term variations were also recorded. Moreover, the eutrophic cool-water indicator species, Coccolithus pelagicus (Tanaka, 1991), showed an increasing maximum abundance of glacial periods over the studied interval. These results indicate that the southward migration of the Kuroshio Front became more pronounced as the Quaternary cooling progressed. Future studies are expected to provide more detailed long-term reconstructions of these variations.

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