Changes in intermediate water conditions in the northern South China Sea using Globorotalia inflata over the last 20 ka

Abstract

ABSTRACTIntermediate circulation is one of the crucial controls of thermohaline changes in seawater. Due to the lack of data for some of the more significant indicators, there has been little research into past intermediate waters in the South China Sea (SCS). In particular, little is known of the thermohaline properties of intermediate water. Here, we present paired Mg/Ca and δ18O data for the planktonic foraminifera Globorotalia inflata from core GHE27L in the northern SCS; these data provide records of temperature and residual δ18Oseawater (δ18Oresidual, being used as a proxy for salinity) to tentatively investigate changes in the intermediate water of the SCS over the last 20 ka. Our results show that G. inflata in the SCS grew principally within the 200–450 m water depth interval. Thermohaline changes recorded by G. inflata indicate warm and relatively freshening water during the Last Glacial Maximum (LGM, 20–17.7 ka BP), changing to warm and saline waters between 17.7 and 13.6 ka BP. In general, waters during the 13.6–8.3 ka BP period were characterized by high temperatures and salinity, with obvious millennial‐scale fluctuations, after which, during the Holocene, they became cold and fresh. We suggest that the temperature and salinity of the intermediate water of the SCS recorded by G. inflata in core GHE27L were possibly associated with a switch in intermediate water masses. During the LGM, the intermediate water in the northern SCS was mainly impacted by North Pacific Intermediate Water (NPIW), but that subsequently the influence of Antarctic Intermediate Water strengthened during the last deglaciation and Pre‐Boreal period (17.7–8.3 ka BP). After this period, NPIW was the principal control of the intermediate water of the SCS again.