Geochemical behavior of dissolved manganese in the East China Sea: Seasonal variation, estuarine removal, and regeneration under suboxic conditions

Abstract

AbstractTo better understand the geochemical cycle of dissolved manganese (Mn) in the East China Sea (ECS), the distribution of dissolved Mn across the ECS was investigated during three field studies in 2011 (May, August, and November). The concentration of dissolved Mn decreased across the ECS with distance from the coast. Mn‐rich ECS shelf waters could export to the Kuroshio Waters, and had the potential to influence the northwest Pacific Ocean as well as the Japan Sea. The Kuroshio Waters were devoid of dissolved Mn, so its incursion could be tracked as it entered the ECS continental shelf region (approximately 50 m isobath). Seasonal variations of dissolved Mn in the ECS were significant, with the highest concentrations occurring in summer. Dissolved Mn in the Changjiang Estuary was nonconservative, and significant quantities were removed by net sorption onto suspended particulate matter. A model describing the sorption processes was applied to data for the Changjiang Estuary. Regeneration of dissolved Mn took place in near‐bottom waters of the suboxic zone in August 2011, following extensive consumption of oxygen. The benthic flux of dissolved Mn was estimated based on Mn concentrations in the overlying waters and the near‐bottom waters. A preliminary box model was established to develop a dissolved Mn budget for the ECS. Based on the dissolved Mn content in the ECS and the total input flux, a residence time of 76–350 days for dissolved Mn in the ECS was inferred.