Mercury Export Flux in the Arctic Ocean Estimated from 234 Th/ 238 U Disequilibria
Résumé
High mercury (Hg) levels have been observed for arctic biota despite limited local
sources of anthropogenic Hg in the Arctic. Scavenging of Hg exerts an important control on
the residence time of Hg in surface waters. The downward Hg export flux, as well as Hg
burial rates in bottom sediments, are not well constrained due to the lack of particulate Hg
(pHg) observations in the Arctic Ocean. Here we estimated downward Hg export flux based
on Hg concentrations in suspended particulate matter (SPM) and by using the radionuclide
pair 234Th/238U, coupled to pHg/234Th ratios in particles. Using new observations made during
the German GEOTRACES TransArcII (GN04) and the U.S. Arctic GEOTRACES (GN01)
cruises in August - October 2015, we estimated the pHg export flux in the central Arctic
Ocean and the outer shelf. We find that 81 ± 58 Mg y-1 pHg are exported from the upper 100
m, of which 16 ± 10 Mg y-1 are ultimately buried in marine sediments. An extrapolation to
the entire Arctic Ocean, including the inner shelf, results in 156 Mg y-1 pHg export from the
surface ocean and 28 Mg y-1 Hg burial rate. Our study shows that the pHg export flux could
be higher than previously thought and this should be taken into consideration for future arctic
Hg budget estimations.
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