Mercury in the North Atlantic

Reactive mercury (Hg) concentrations (0.80±0.44 pM), measured during a cruise in the sub-polar North Atlantic (50–70°N) in August 1993 were lower those of more southerly regions of this ocean. A large fraction of the Hg in the surface waters was elemental Hg (Hg°; 88% of the reactive Hg, on average; 0.65±0.50 pM). Little dimethylmercury (DMHg) or monomethylmercury (MMHg) was found in surface waters but higher concentrations were found at depth. The high surface water Hg° concentrations likely reflect production of Hg° in concert with the short season of primary production in these waters. Overall, gas exchange plays a dominant role in the cycle of Hg in the upper waters of this region. For the sub-thermocline waters, a comparison of the Hg speciation in different water masses suggests that surface waters accumulate reactive Hg and methylated Hg—due to input of reactive Hg via remineralization and conversion of reactive Hg to methylated Hg—after sinking during winter to form North Atlantic deep water. Reactive Hg concentrations varied between water masses with the lowest concentrations being found in the waters north of Iceland (the source waters for deep water formation). DMHg was found throughout the sub-thermocline water column and the limited data set for MMHg suggests that this species was also prevalent. These observations suggest that methylated Hg production in the ocean is not confined to low oxygen or anoxic regions, as has been found in lakes. Concentrations of DMHg (0.08±0.07 pM, on average) were lower than those previously measured in the equatorial Pacific. Total Hg concentrations averaged 2.4±1.6 pM but little of this was particulate Hg (<10 pg l−1, typically). There was evidence of colloidally-bound Hg in surface waters. Overall, the data strengthen our hypotheses concerning Hg biogeochemical cycling in the ocean and confirm the importance of gas exchange and Hg methylation in the fate and transport of Hg in the ocean.