A switch from carbonate ocean to silica ocean in the East Sea (Sea of Japan) during the mid- Pleistocene

In this study, we report on the extraordinary CaCO3 preservation in the East Sea (Sea of Japan) during the mid-Pleistocene, which provides a new perspective of carbon cycling as a carbonate ocean. Core 96EBP3 collected from the South Korea Plateau and core MD01-2407 collected from the Oki Ridge show extraordinary downcore variation of CaCO3 contents. CaCO3 contents of core 96EBP3 were measured using UIC coulometer (Model CM5014) and those of core MD01-2407 were converted from CaO measurement using high- resolution major element analysis data determined by an XRF microscanneran (Horiba XGT-2700). The correlation between these two cores was conducted with diverse approaches; radiolarian extinction, warm-water species of radiolaria, and CaCO3 variation. These two cores are in accordance with various features, although core 96EBP3 records some missing ages of MIS 9 and 10. The lower parts of both cores are characterized by extraordinarily high CaCO3 contents up to 50%. The high CaCO3 contents occurred before MIS 11 in both cores. The major constituents of these CaCO3 are coccoliths derived from calcareous nannoplankton. Therefore, these durations of high coccolith dump are quite unique in the East Sea, suggesting that the present-day silica ocean has, periodically in the past, been a carbonate ocean too.