Mercury species export from the Arctic to the Atlantic Ocean

Abstract

The Fram Strait is the only deep connection between the Arctic and Atlantic Oceans. The main water and mercury (Hg) fluxes between these oceans occur via the Fram Strait and Barents Sea Opening. Several Hg mass balance studies indicated a net Hg export from the Arctic to the Atlantic Ocean. However, in the absence of Hg measurements in the Fram Strait and Barents Sea Opening, these estimates were based on North Atlantic and central Arctic Ocean data alone. Here, we refine the Arctic total Hg (tHg) and methylated Hg (MeHg) mass budgets using new data acquired during the 2015 GEOTRACES (section GN04) TransArcII cruise in the Barents Sea Opening and the 2016 GEOTRACES (section GN05) GRIFF cruise, which covered the Fram Strait and Northeast Greenland Shelf. Total Hg increased westward along the Fram Strait transect, reaching the highest concentrations on the Northeast Greenland Shelf. Concentrations of tHg averaged 1.29 ± 0.43 pM in the East Greenland Current, while core waters of the West Spitsbergen Current had average values of 0.80 ± 0.26 pM. Using our new data, we estimate that 43 ± 9 Mg y-1 of tHg is transported to the Arctic Ocean in the core of the West Spitsbergen Current, while 54 ± 13 Mg y-1 of tHg is exported from the Arctic Ocean in the East Greenland Current and Recirculated Atlantic Water. This results in a net tHg export of 11 ± 8 Mg y-1 via the Fram Strait. We find a shallow MeHg maximum (at 150 m depth) in the East Greenland Current, in agreement to what was reported for the central Arctic Ocean and Canadian Arctic Archipelago. The West Spitsbergen Current is characterized by lower MeHg concentrations and a deeper MeHg maximum, that is located at approximately 1000 m depth. We estimate a net MeHg export of 6 ± 2 Mg y-1 from the Arctic Ocean via the Fram Strait, which is nearly half of the exported tHg. Most of the exported MeHg is in the form of DMHg (2:1 ratio of dimethylmercury to monomethylmercury). Previous studies reported lower MeHg proportions. Our observations show that the Arctic Ocean is producing and exporting MeHg to the Atlantic Ocean. In total, the Arctic Ocean exports about 18 Mg y-1 of tHg to the Nordic Seas and North Atlantic via the Fram Strait and Davis Strait, of which 7.5 Mg y-1 is in the MeHg form.

Arctic – Atlantic Exchange of the Dissolved Micronutrients Iron, Manganese, Cobalt, Nickel, Copper and Zinc With a Focus on Fram Strait

“…Dissolved Zn showed a contrasting behavior and was neither enriched in the Transpolar Drift (1.61 ± 0.77 nM) nor in EGC Surface Water at stations 8, 9 and 25 (1.38 ± 0.54 nM). Other trace elements from the same expeditions such as mercury ("total Hg', Charette et al, 2020;Petrova et al, 2020) corroborate our conclusion of Arctic Ocean surface waters being a source of trace elements to Fram Strait and the North Atlantic Ocean by advection with the Transpolar Drift and into East Greenland Current.…”

“…The water column of the Barents Sea Opening is analogously distinguished into layers but treated as one water mass (“Barents Sea Branch”) in accordance with Petrova et al. (2020).…”

“…To establish annual dFe, dMn, dCo, dNi, dCu and dZn fluxes, we average over monthly mean net micronutrient fluxes (September–August, in Gg·a −1 ) and report uncertainty as one standard deviation (1σ) of monthly variations following Petrova et al. (2020). This is because the Arctic Ocean is a closed system where volume fluxes are not independent variables.…”

“…Water masses of Fram Strait, that is, WSC (>5°E), Middle section (2-5°E), EGC (2°E-6.5°W), and the NE Greenland Shelf (>6.5°W) are differentiated into layers ("components") of Surface Water (σ 0 < 27.1 kg/m 3 ), Upper Atlantic Water (27.10σ 0 -27.50σ 0 kg/m 3 ), Atlantic Water (27.50σ 0 -30.28σ 0.5 kg/m 3 ), Intermediate Water (30.28σ 0.5 -32.75σ 1.0 kg/m 3 ), and Deep Water (σ 1.0 > 32.75 kg/m 3 ) by definition of isopycnal surfaces and potential density relative to 0 dbar (σ 0 ), 500 dbar (σ 0.5 ) and 1000 dbar (σ 1.0 ) using GN04 and GN05 CTD data following Tsubouchi et al (2018) (Figure 2). The water column of the Barents Sea Opening is analogously distinguished into layers but treated as one water mass ("Barents Sea Branch") in accordance with Petrova et al (2020).…”

“…The uncertainty in August micronutrient transport across Fram Strait was calculated from error propagation including the uncertainty in dissolved micronutrient concentrations, and an estimate of uncertainty in volume transport derived from assuming proportionality between volume transport rates in August 2006 and mean variability in volume transport as observed between September 2005 and August 2006 (Tsubouchi et al, 2018). To establish annual dFe, dMn, dCo, dNi, dCu and dZn fluxes, we average over monthly mean net micronutrient fluxes (September-August, in Gg•a −1 ) and report uncertainty as one standard deviation (1σ) of monthly variations following Petrova et al (2020). This is because the Arctic Ocean is a closed system where volume fluxes are not independent variables.…”