Mercury Export Flux in the Arctic Ocean Estimated from ²³⁴Th/²³⁸U Disequilibria

Abstract

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 234 Th/ 238 U, coupled to pHg/ 234 Th 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.

Global Ocean Sediment Composition and Burial Flux in the Deep Sea

“…When integrated across zones, our estimate provides nearly full coverage of the Arctic Ocean deeper than 1,000 m (Figure 8c and Figure S11). The integrated Arctic Hg flux is 4.4 ± 0.7 Mg/yr, which is consistent with a recent estimate in the deep Arctic (4 ± 3 Mg/yr) that derived fluxes by assuming a constant sedimentation rate (Tesán Onrubia et al., 2020). The uncertainty in our flux estimate is a minimum as it only accounts for uncertainty in the Th‐normalized flux interpolation.…”

Mercury Accumulation in Marine Sediments – A Comparison of an Upwelling Area and Two Large River Mouths

“…This is while the scavenged Hg by productivity-related particles in the water column is buried more rapidly in sediments. The scavenging of Hg has an important control on the residence time of Hg waters (Tesán Onrubia et al, 2020). This phenomenon facilitates the downward flux of Hg to the seafloor which could be slow otherwise.…”

The Effect of Particle Composition and Concentration on the Partitioning Coefficient for Mercury in Three Ocean Basins

“…At present, the global burial flux for Hg in ocean sediments is not very well-constrained but likely lies between the value of 1.2 µg m −2 y −1 in open ocean conditions and 1,200 µg m −2 y −1 in hyper-accumulating regions like the Antarctic margin (Soerensen et al, 2016;Zaferani et al, 2018). Models have suggested that the global burial flux falls between 1.7 and 7 µg m −2 y −1 (e.g., Outridge et al, 2018;Tesán Onrubia et al, 2020). Given the current oceanic inventory of Hg, these burial fluxes suggest a residence time of Hg of about 520 years (Outridge et al, 2018).…”