Distribution patterns of dissolved trace metals (Fe, Ni, Cu, Zn, Cd, and Pb) in China marginal seas during the GEOTRACES GP06-CN cruise

Abstract

Trace metals are both important micronutrients for life in the oceans, and can be tracers of various natural and anthropogenic processes. Metals including Fe, Ni, Zn, and Cu are known to be important nutrients for phytoplankton, elements including Pb are viewed primarily as anthropogenic contaminants, Cd is a biologically cycled element though it's nutritive importance is unclear, and all of these metals are impacted by overlapping natural and anthropogenic processes in different parts of the oceans. China is bordered by four marginal seas, including the Bohai, Yellow Sea (YS), East China Sea (ECS), and South China Sea. Here we present the first systematic study of trace metals in the China marginal seas from the GEOTRACES GP06CN cruise in October 2015. The metals Fe, Ni, Cu, Zn, Cd, and Pb were sampled in the Yellow Sea and East China Sea at 69 surface locations using a towfish and with 8 depth profiles, along with auxiliary parameters including salinity, phosphate, and apparent oxygen utilization. We find that Ni, Cu and Cd are most strongly impacted by a combination of biological cycling and fluvial inputs, recent atmospheric deposition dominates surface ocean Pb concentrations, and Fe and Zn are impacted by a combination of multiple processes. Comparison with historical data shows that Cu concentrations have been increasing through time, presumably due to increasing anthropogenic inputs. Ni and Cd are also delivered in high concentrations from rivers, which can reflect the impact of anthropogenic inputs, and Pb is delivered from atmospheric deposition which may include anthropogenic sources, but these metals do not show a distinct temporal trend. The six metals studied here are thus impacted by their broad-scale oceanographic distributions, local coastal processes, and anthropogenic inputs, each in different ways and to different degrees. This study represents a unique contribution to the international GEOTRACES effort, by highlighting an ‘end-member’ region with strong coastal and anthropogenic inputs.