The distribution and biogeochemical cycling of mercury in the equatorial Pacific Ocean

Abstract

The ocean is an important part of the global mercury (Hg) cycle, both as a source and sink for atmospheric Hg. This investigation was initiated to study the speciation and distribution of Hg in the equatorial Pacific Ocean. Monomethylmercury (MMHg) and dimethylmercury (DMHg) were found principally in the low oxygen sub-thermocline region, while elemental Hg (Hg°) was found throughout the upper ocean. Flux calculations and modelling studies suggest that DMHg and MMHg are formed from reactive Hg (Hg R) primarily in the low oxygen region. Particulate dissolution is the major source of Hg to the low oxygen zone. DMHg probably decomposes into MMHg, and MMHg is further demethylated to Hg°. In the mixed layer, Hg° is also formed via direct reduction of Hg R. This investigation provides evidence that the rate of supply of Hg R controls the rate of formation of methylated Hg species and Hg°, and that competition between these processes determines the speciation and distribution of Hg in the upper ocean of the equatorial Pacific.