Biogeochemical controls on the mercury cycle in the Arctic Ocean

High levels of methylmercury (MeHg) have been reported in many Arctic food webs and anthropogenic enrichment of tissue burdens across a suite of biota have been estimated to exceed pre-industrial levels by 10-fold. Previous work has reported a shallow peak in MeHg concentrations in Arctic seawater and active methylation in the water column of the Canadian Arctic Archipelago and estuarine surface waters. However, the origins of MeHg in the central Arctic Ocean water column are not well understood and simultaneous data on MeHg production and degradation are extremely limited. Here, we report full water column high-resolution profiles of mercury species (mono- and dimethylmercury, and total mercury), methylation and demethylation rates for the Central Arctic Ocean in samples collected in the Makarov Basin during a 2015 GEOTRACES expedition (GN04 carried out on board of icebreaker Polarstern (PS94, ARK XXIX/3, TransArc-II). MeHg concentrations are low at the surface (0.043 pM) and increase with depth in the polar mixed layer and halocline where it reaches a maxima of 0.405 pM at 116 m. We find a coincident peak in MeHg concentration and methylation rate in the halocline where most of the MeHg production occurs. We observe net demethylation in the Atlantic Layer and below. We use these data to discuss the vertical and biogeochemical processes that drive MeHg formation in the entire Arctic water column. Understanding these processes is necessary to develop better mechanistic models and predict future trajectories of MeHg levels in Arctic waters as they undergo climate-driven changes.