Global cooling in marine climates and local tectonic events in Southwest Japan at the Plio–Pleistocene boundary

This study reconstructs the temporal changes in water temperature and depth on the Pacific coast of Southwest Japan from the latest Pliocene to the earliest Pleistocene (ca. 2.6 Ma) on the basis of the modern analog technique (MAT) of fossil ostracodes and factor analysis of fossil planktonic foraminifera from the Takanabe Formation. Around the Plio–Pleistocene boundary, the MAT results reveal that the mean annual, warmest month, and coldest month temperatures of bottom water were estimated at ca. 6–17, 6–17, and 6–16 °C, respectively. The bottom water temperature before 2.6 Ma was similar to modern temperatures of the study site. Since then, the temperature dropped several degrees. Moreover, the result of the analysis of fossil planktonic foraminifera suggests that the study site clearly changed from a marine climate under the influence of the axial part of the Kuroshio Current to that of transitional water at around 2.6 Ma. Thus, the results of the microfossil analysis reveal for the first time that a global cooling event at the Plio–Pleistocene boundary influenced the Pacific coast of SW Japan. The change in water depth at ca 2.6 Ma in the study site was estimated to have been extremely large in comparison with the global sea-level fluctuations during the late Pliocene, suggesting that the tectonic movement related to the rotation of the southern Kyushu Island, Southwest Japan, started at ca. 2.6 Ma and that subsidence occurred in the study area.