Contrasting distributions of dissolved gaseous mercury concentration and evasion in the North Pacific Subarctic Gyre and the Subarctic Front

Abstract

Dissolved gaseous mercury (DGM) and methylmercury (MeHg) produced in the oceanic water column is an important inorganic Hg(II) transformation process involving atmosphere-water interactions and food web bioaccumulations, respectively. Although oxygen utilization has been reported to contribute significantly to MeHg variation, its contribution to the DGM variation is largely unknown. Here, we investigated DGM and MeHg distributions in relation to oxygen utilization in the Western Subarctic Gyre (WSG). We observed subsurface peaks of DGM (203？878 fM) and MeHg (274？750 fM) at a water depth of 200 or 300 m with a minimum oxygen concentration of 60？86 μmol kg？1. The apparent oxygen utilization of WSG water was positively and linearly associated with DGM (r2=0.83) and MeHg (r2=0.93), which might be a consequence of particulate organic carbon remineralization over a period of years (< 25 yrs). The DGM concentration was typically lower in the Subarctic Front than the other sites in relation to the dissolved oxygen and nutrient concentrations, while MeHg concentration was not. It might be attributable to increases in kox/kred, associated with isopycnal mixing between Kuroshio and Oyashio waters. The DGM and MeHg input from suboxic water should be placed in context with in situ production at the surface to better understand the biogeochemical cycles of Hg in ocean water.