Orbital obliquity cycles recorded in Kuroshio Current region, eastern Asia, around Plio-Pleistocene boundary

Global climate underwent a period of significant cooling at the Plio-Pleistocene Transition (∼2.6 Ma). The influence of this change on the Kuroshio Current region in the Pacific Ocean, off eastern Asia, is not well known. In this study, we clarify temporal changes in the paleoenvironment under the influence of the Kuroshio Current during the late Pliocene and early Pleistocene using high-resolution faunal proxy records of fossil Ostracoda (Crustacea). The study unit is the Ananai Formation in the southeastern region of Shikoku, southwest Japan. The modern analog technique (MAT) is employed for the quantitative estimation of paleo-bottom water temperatures (PBWTs) and paleo-water depth (PWD) during the deposition of the formation. Ostracode MAT results show PBWT fluctuations during warmest and coldest months, with values of 16°C-20 °C and 12°C-16 °C, respectively, and a PWD of 70-140 m, reflecting sea-level oscillations. Moreover, the PBWT in the coldest month is 3 °C-4 °C lower than present-day water temperatures at the same shallow water depths. Temporal changes in these paleoenvironmental variables based on MAT are in good agreement with global oxygen isotope records. Orbital obliquity cycles with 41-kyr periodicity are recorded for the first time in an onshore section in the Kuroshio Current region at the Plio-Pleistocene boundary interval.