MERCURY IN THE NORTH-ATLANTIC

Reactive mercury (Hg) concentrations (0.80 +/- 0.44 pM), measured duri ng a cruise in the sub-polar North Atlantic (50-70 degrees N) in Augus t 1993 were lower those of more southerly regions of this ocean, A lar ge fraction of the Hg in the surface waters was elemental Hg (Hg degre es; 88% of the reactive Hg, on average; 0.65 +/- 0.50 pM), Little dime thylmercury (DMHg) or monomethylmercury (MMHg) was found in surface wa ters but higher concentrations were found at depth. The high surface w ater Hg degrees concentrations likely reflect production of Hg degrees in concert with the short season of primary production in these water s. Overall, gas exchange plays a dominant role in the cycle of Hg in t he upper waters of this region. For the sub-thermocline waters, a comp arison of the Hg speciation in different water masses suggests that su rface waters accumulate reactive Hg and methylated Hg-due to input of reactive Hg via remineralization and conversion of reactive Hg to meth ylated Hg-after sinking during winter to form North Atlantic deep wate r. Reactive Hg concentrations varied between water masses with the low est concentrations being found in the waters north of Iceland (the sou rce waters for deep water formation). DMHg was found throughout the su b-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 cycl ing in the ocean and confirm the importance of gas exchange and Hg met hylation in the fate and transport of Hg in the ocean. (C) 1998 Elsevi er Science B.V.