New evidence for Kuroshio inflow and deepwater circulation in the Okinawa Trough, East China Sea: Sedimentary mercury variations over the last 20kyr

Abstract

The evolution of the Kuroshio Current (KC) in the Okinawa Trough (OT) has long been of paleoceanographic interest and has been the focus of many investigations because of its large impacts on both the ocean environment and the climate in East Asia. Here we present a new high-resolution mercury (Hg) record from the OT sediments that provides robust evidence for the glacial-interglacial variabilities of KC inflow. The Hg levels are anomalously high in the Holocene deposits, which feature a large and abrupt Hg increase at similar to 9.6ka and a significant drop at 4.5-3.2ka. Our observations show that most Hg was derived from seafloor hydrothermal plumes via active lateral transportation. Thus, the generally high Hg levels since similar to 9.6ka are associated with enhanced basin-wide deepwater circulation triggered by the KC inflow. This driving mechanism for the deposition and widespread of Hg is also evidenced by an abrupt drop in Hg levels during the late Holocene neoglacial cold period (similar to 5-3ka) when the KC inflow was temporarily interrupted and deepwater circulation was consequently weak or even absent in the OT. The changes in sedimentary Hg appear to be synchronous with those in other geochemical proxies that suggest that bottom water conditions switched from anoxic to oxic due to increased upwelling from the bottom in tandem with the strengthening KC and reduced freshwater effect since similar to 9.6ka. This study shows that the hydrothermal Hg is a sensitive and independent tool for the reconstruction of paleodynamics of KC in the OT.