Sources, cycling and transfer of mercury in the Labrador Sea (Geotraces-Geovide cruise)

Abstract

The Labrador Sea links the Arctic and North Atlantic oceans and constitutes one of the main entrances of atmospheric inputs into the Ocean's interior. We report here the first high-resolution Hg distribution pattern along a transect from Greenland to Labrador coasts sampled after the 2014 winter convection. Total Hg concentrations in unfiltered (HgTUNF) samples ranged from 0.25 pmol L− 1 to 0.67 pmol L− 1 averaging 0.44 ± 0.10 pmol L− 1 (n = 113, 1σ). Concentrations in filtered samples (HgTF, < 0.45 μm) represented from 62 to 92% of the HgTUNF and exhibited a similar distribution. High HgTUNF concentrations were found (i) in the waters of the Labrador Current, which receive desalted waters from the Canadian Arctic Archipelago, and (ii) in the waters over the Labrador shelf and rise. In the Labrador Sea Waters formed during the 2014 winter convection, HgTUNF concentrations were low (0.38 ± 0.05 pmol L− 1, n = 23), and increased gradually with depth (up to > 0.5 pmol L− 1) in the Northeast Atlantic Deep Waters. HgTF correlates with apparent oxygen utilization implying that atmospheric deposition, biological uptake, and microbial respiration control the Hg distribution in the Labrador Sea. Subtracting the amount of Hg released during organic matter remineralization allows us to identify a Hg and organic matter enriched fraction, which originates from fluvial sources in the Canadian Arctic Archipelago waters. This fraction is transferred southward, in surface waters with the Labrador Current, and at depth with the lower limb of the Atlantic Meridional Overturning Circulation. Climate warming, which will increase the mobilization of Hg from thawing permafrost, would consequently increase the Arctic export of Hg initially associated with organic matter. Highlights ► The Labrador Sea Water formed during the 2014 winter convection contains very low Hg. ► The Labrador Current waters, from the Canadian Arctic Archipelago, are Hg-enriched. ► Hg exhibits a nutrient-like distribution in the central Labrador Sea.

Subsurface seawater methylmercury maximum explains biotic mercury concentrations in the Canadian Arctic

“…These results are comparable to the limited Hg T dataset reported for Canadian Arctic waters 15 , 19 , 20 . The lowest concentrations were found in the Labrador Sea (0.65 ± 0.18 pM), slightly higher than those (0.44 ± 0.10 pM, 0.25–0.67 pM) measured in June 2014 in a similar area 21 .…”

Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

“…In contrast, the δ 202 Hg values of the dissolved THg in the Arctic are considerably more negative (δ 202 Hg average = −2.08 ± 0.61‰, 1 SD) than particles in the NPSG. This is not surprising because the Canadian Artic Archipelago has Hg inputs not only from precipitation but also from riverine discharge (Fisher et al, ; Sunderland & Mason, ; Cossa et al, ).…”

Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean

“…On an annual basis, the AO is therefore a net Hg emission source (23 Mg y −1 ) to the atmosphere. The AO not only exports Hg to the atmosphere but also to the North Atlantic basin (30 Mg y −1 ), because marine THg levels in outgoing AO currents, at all depths, are more elevated than in incoming Atlantic waters (21,31,32). Lastly, AO Hg is exported to shelf (25 Mg y −1 ) and deep AO sediments (3 Mg y −1 ) (9).…”