The Kuroshio Current plays a major role in the paleoceanographic characteristics of the Pacific Ocean south of Japan due to of the large quantities of heat and tropical oligotrophic warm water that it transports. However, there is currently no data describing Pleistocene radiolarian biostratigraphy in this region. This study examined Pleistocene radiolarian biostratigraphy in southwestern Japan using cores from the Nankai Trough region that were collected as part of Expedition 315, Integrated Ocean Drilling Program (IODP). We identified 70 radiolarian species (> 63 μm) that were useful for biostratigraphic studies. Oxygen isotope analyses were performed to establish the chemostratigraphy and to construct an age model for this site. The age model, which was based on biostratigraphy, oxygen isotope stratigraphy, and magnetostratigraphy, revealed that the geological age of the study site ranged from 0.30 Ma (Middle Pleistocene) to 1.80 Ma (Early Pleistocene). Sedimentation rate of 39.43 cm/kyr was observed from 83 m logging depth below seafloor (LSF) to 106 m LSF. Six bioevents, including two new first occurrence datums for the collodarian Si pho no-sphaera abyssi (1.44 Ma) and the spumellarian Actinomma archadophorum (0.64 Ma) were identified. For the first time, five radiolarian zones were identified in the southwestern Japan. In these five radiolarian zones, three new Lower Pleistocene radiolarian zones were defined: Sphaeropyle robusta Zone (>1.80 Ma); Polyselonia spinosa Zone (1.75-1.44 Ma), Siphonosphaera abyssi Zone (1.44-0.94 Ma). In addition, the Stylatractus universus Zone and Botryostrobus acquilonaris Zone were placed in the Middle Pleistocene.
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