Variation in subsurface water temperature and its link to the Kuroshio Current in the Okinawa Trough during the last 38.5 kyr

Abstract

An 820.4-cm long piston core (YK12-15 PC-01) was obtained from site OT-1B (28°07.15′N, 127°11.97′E, 1050 m water depth), located in the middle of the Okinawa Trough in the southern East China Sea (ECS). For core YK12-15 PC-01, oxygen and carbon isotopes (δ18O and δ13C) of planktic foraminifera Globigerinoides ruber sensu stricto (250–355 μm size fraction) and Globorotalia inflata (>250 μm size fraction) together with total organic carbon (TOC), nitrogen content (TN), and δ13C of organic matter were analyzed for the last 38.5 thousand years (kyr). The δ18O records of G. ruber (δ18Oruber) and G. inflata (δ18Oinflata) were used to estimate the temperature/salinity of the surface and subsurface waters, respectively. In addition, we estimated the vertical temperature gradient between the surface and the subsurface based on the difference in δ18O between G. ruber and G. inflata (Δδ18Oruber-inflata). The δ18Oinflata and Δδ18Oruber-inflata indicate millennial-scale variations in association with the oscillations in Antarctic temperatures during the Marine Isotope Stage (MIS) 3. These results suggest that variations in subsurface temperature and vertical temperature gradient in the upper water column in the ECS were closely related to the Atlantic bipolar seesaw during MIS 3. The subsurface temperature in the ECS was higher at the beginning of Greenland stadials when the Antarctic temperatures showed the lowest values. The warming of the subsurface is likely attributed to the stronger Kuroshio transport. High peaks in the TOC and TN records coincide with the low peaks in δ18Oinflata (high temperature), suggesting higher primary productivity, which is in good agreement with the interpretation of the intensified Kuroshio. However, the subsurface warming was not recognized during Herinch events, indicating different temperature responses in the upper water column from other stadials.