Vertical distribution of Methylmercury in the Central Arctic Ocean explained by In Situ Methylation and Demethylation

High levels of neurotoxic monomethylmercury (MMHg) have been reported in the Arctic food web with tissue burdens across various biota estimated to be higher than pre-industrial levels. This enrichment is largely attributed to anthropogenic releases of inorganic mercury, which is assumed to convert partially to methylmercury (MeHg: MMHg + volatile dimethylmercury (DMHg)) in marine systems. While in situ production of MeHg in the Central Arctic Ocean has been hypothesized, direct links between methylation and demethylation rates, and ambient MeHg concentrations have not been established. Here, we report full oceanic water column (3700 m) high-resolution profiles of mercury (Hg) methylation and MeHg demethylation rates. These profiles were obtained during the 2015 German GEOTRACES expedition, using changes in isotopically labeled MeHg and divalent Hg (Hg ^II ) during 24-hour incubations. Total Hg, MMHg, DMHg, and ancillary parameters were measured at the same station. We show that both methylation and demethylation rates vary with depth, with methylation rates ranging from 2.0*10 ^-5 to 1.4*10 ^-3 day ^-1 and demethylation rates varying from 1.0*10 ^-4 to 0.12 day ^-1 . Using measured rates, along with evasion and photodecomposition algorithms, we can simulate MMHg and DMHg distribution with depth. We find that DMHg does not build up in the surface water because of rapid dark demethylation, rather than evasion or photodecomposition, explaining the low surface often DMHg found under sea ice. We show that the peak MeHg is primarily explained by formation of DMHg in the halocline and the location of the peak is controlled by either mixed layer depth and/or the mixing of water with high methylation potential but low MMHg/Hg ^II levels from the Eastern Siberian Sea with Atlantic water carrying substrates for methylation.