[M-IS14] Paleoclimatology and paleoceanography

[MIS14-P16] Paleoceanographic variations around the Kuroshio region during marine isotope stage 31 based on calcareous nannofossil and alkenone analyses

★Invited Papers

*Daisuke Kuwano¹, Nakagawa Riku^2,3, Hiroto Kajita², Yuki Haneda⁴, Masayuki Utsunomiya⁴, Yoshimi Kubota⁵, Yusuke Suganuma⁶, Makoto Okada⁷ (1.Kyoto University, 2.Hirosaki Univeristy, 3.Shizuoka Prefectural Government, 4.National Institute of Advanced Industrial Science and Technology, 5.National Museum of Nature and Science, 6.National Institute of Polar Research, 7.Ibaraki University)

Keywords:super interglacial period, calcareous nannofossils, alkenone, Kuroshio Current, Pleistocene, paleoceanography

In the northwestern Pacific Ocean, the warm Kuroshio Current meets the cold Oyashio Current, forming a north-south temperature gradient. This boundary is highly sensitive to climate variations, making it crucial to reconstruct the past variations of the Kuroshio and Oyashio Currents. However, high-resolution paleoceanographic records from the northwestern Pacific are limited to several intervals, such as Marine Isotope Stages (MIS) 1–6 (Yamamoto et al., 2005; Oba et al., 2006) and MIS 20–18 (Suganuma et al., 2018; Haneda et al., 2020; Kajita et al., 2025). Hence, we conducted calcareous nannofossil and alkenone analyses focusing on MIS 31, an extreme interglacial of the Early Pleistocene, to reconstruct past paleoceanographic variations in the Kuroshio Current region.
In this study, we examined the Otadai Formation of the Kazusa Group, distributed in the Boso Peninsula. Previous studies have reported that this formation includes the MIS 31 interval with a high sedimentation rate and contains abundant microfossils and biomarkers (Tsuji et al., 2005; Kajita et al., 2021). We investigated the Shoryuji section, located along a tributary of the Isumi River, and analyzed an approximately 50 m interval from about 10 m below the tephra layer O12 to O7. The age model for this section is based on Haneda et al. (2025, JpGU).
Our results reveal surface water mass changes and sea surface temperature (SST) variations during MIS 31. Calcareous nannofossil assemblages indicate a water mass shift from the transitional water to the Kuroshio water, suggesting a northward migration of the Kuroshio Front during MIS 31. Notably, the warmest peak in our data does not coincide with the peak in the benthic foraminiferal oxygen isotopes. Additionally, millennial-scale variations were observed in the alkenone-based SST record. Future studies are expected to provide a more detailed understanding of Kuroshio variability during MIS 31.

[Reference]
Haneda et al., 2020, Earth and Planetary Science Letters, 531, 115936.
Haneda et al., 2025, Japan Geoscience Union Meeting 2025 Abstract.
Kajita et al., 2021, Communications Earth & Environment, 2, 82.
Kajita et al., 2025, Geophysical Research Letter, 52, e2024GL113197.
Oba et al., 2006, Global and Planetary Change, 53, 5–20.
Suganuma et al., 2018, Quaternary Science Reviews, 191, 406–430.
Tsuji et al., 2005, The Journal of the Geological Society of Japan, 111, 1–20.
Yamamoto et al., 2005, Geophysical Research Letter, 32, 1–4.