Signals of the paleo‐Kuroshio Current: The Pleistocene Chinen Formation, Okinawa Island, south‐west Japan

The Kuroshio Current of the north‐western Pacific is recognised as one of the strongest oceanic currents globally. Spatiotemporal fluctuations of the current are closely linked to the oceanic environment, climate and the physiographic features of both land and sea; therefore, the historical development of the current over geological time is essential for interpreting the region's palaeoceanographic, palaeoclimatic and palaeogeographical evolution. However, the Kuroshio Current is less well‐understood for the Early Pleistocene time interval than for the Late Pleistocene and Holocene. The lithofacies architecture of the Lower Pleistocene Chinen Formation on Okinawa Island, south‐west Japan, which was potentially influenced by the paleo‐Kuroshio Current, was investigated. The succession of the formation is characterised by three distinct lithofacies. Lithofacies Fb, the main component of the succession, consists of bioturbated calcareous lithic sand with well‐stratified structures associated with back‐set structures, forming a continuous succession. Shell fragments of shallow‐water species tend to be oriented parallel to the bedding plane. The Chinen Formation, including lithofacies Fb, is interpreted to have mainly been formed by sediments supplied from shallow water under the influence of persistent and weak currents on a slope environment. The palaeogeography of the time suggests that the inferred depositional process was influenced by downslope transfer within a spillover branch of the Kuroshio Current. Sheet‐like sandy sediments occur widely from shallow to deep water around present‐day Nansei Islands; these sediments are deposited under the influence of bottom currents associated with the Kuroshio Current. This modern example is a likely analogue of the Chinen Formation. Thus, the Chinen Formation is an important record for understanding the early history of the Kuroshio Current and the contemporaneous palaeoceanography, palaeoclimate and palaeogeography of the north‐western Pacific region.