Evolution of current circulation and bottom redox conditions in the central Okinawa Trough over the last 19 ka

The foraminifera and geochemical element records from the Core CS2 in the center of the Okinawa Trough (OT) revealed a history of current circulation and bottom redox conditions over the last 19 Ka. During the last glaciation, the few Kuroshio Current (KC) and tropical/subtropical species components, the shoaled depth of thermocline (DOT) and high infaunal foraminifera, together suggest the hypoxia in the isolated and semiclosed OT might be related to an absence/shift of the KC. Furthermore, the hypoxia variations in three episodes coincided with changes in the North Pacific Intermediate Water (NPIW), as stronger hypoxia and enhanced NPIW occurred in the Heinrich Stadia 1 and Younger Dryas periods, and rapidly decreasing hypoxia corresponded with weaker NPIW ventilation in the Bølling/Allerød period. Previously recorded hypoxia in the OT occurred broadly at a water depth of between ~1,300 and 1,800 m. Meanwhile, the lowest productivity precluded the possibility of hypoxia caused by organic matter consumption. We thereby infer the enhanced glacial NPIW might be another reason for the hypoxia. After the postglaciation, the OT transitioned into a fully oceanic environment and was reoccupied by main pathway of the KC. This reoccupation greatly alleviated the hypoxia via weakening stratification and increasing vertical mixing. Evolution research of current circulation and bottom redox conditions can help in understanding the changes in the storage/release of carbon as the paleoclimate changed. Moreover, the variations in bottom redox conditions in the OT offer a possible informative window for understanding the influence of glacial NPIW in the mid-latitude zone