Hypoxic Effects on the Radiocarbon in DIC of the ECS Subsurface Water

Abstract

AbstractDissolved inorganic carbon (DIC) in the East China Sea (ECS) is physically and chemically affected by the northward intrusion of the Kuroshio Branch Current (KBC) and the Changjiang River (CR) outflow. We trace these processes using dual DIC isotopic compositions (Δ14C and δ13C) along transects that begin in the CR and estuary, and extend out into middle shelf waters of the ECS. We especially focus on frequently occurred nearshore hypoxia, as they appeared in the summer of 2017. The Δ14C values (28.8‰–35.3‰) from the ECS mid‐shelf water were comparable to the reported value from the Kuroshio Current (KC) at the continental slope (34.4‰ ± 7.4‰), but Δ14C values in the CR (−173.6‰ & −162.3‰) were substantially lower than observations made in 2014. We also found that the Δ14C of DIC in the bottom waters along the inner shelf, including hypoxic area, decreased from 25.0‰ to 18.5‰, from south to north. Water masses mixing between the CR and the ECS mid‐shelf waters with and without organic carbon degradation can be identified in plots of Δ14C versus DIC‐1. With simple mixing calculations, we show that the northward decreasing Δ14C of inner shelf bottom water is caused by downward transport of DIC with low Δ14C value mediated by biological process from the surface, as opposed to simple physical mixing.