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

doi:10.1016/j.marchem.2017.11.006

Marine Chemistry

Volume 198, 20 January 2018, Pages 64-69

https://doi.org/10.1016/j.marchem.2017.11.006 Get rights and content

Highlights

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The Labrador Sea Water formed during the 2014 winter convection contains very low Hg.
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The Labrador Current waters, from the Canadian Arctic Archipelago, are Hg-enriched.
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Hg exhibits a nutrient-like distribution in the central Labrador Sea.

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 (HgT_UNF) 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 (HgT_F, < 0.45 μm) represented from 62 to 92% of the HgT_UNF and exhibited a similar distribution. High HgT_UNF 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, HgT_UNF 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. HgT_F 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.

Introduction

Mercury (Hg) is a volatile trace element of great environmental concern, which is delivered to the Oceans mostly as dissolved and particulate oxidized species (Hg^II) in rivers and precipitations, but also as elemental Hg (Hg⁰) during ocean-atmosphere gas exchange (e.g., Fitzgerald et al., 2007, Pirrone and Mason, 2009, Obrist et al., 2017). After its advection/deposition onto sea surface, Hg can be reinjected into the atmosphere via photoreduction in surface waters, transported with currents, taken up by bio-pumping (i.e., sorbed into/onto plankton), and vertically transferred with settling material and potentially released at depth (e.g., Mason et al., 2012, Lamborg et al., 2014). Mercury is incorporated this way into the intermediate and deep waters of the oceans, following a “regenerative scavenging” recycling process, akin to the nutrient-like behavior described for many other trace elements (e.g., Lamborg et al., 2016).

The Labrador Sea (LS, Fig. 1) is the main site in the North Atlantic Ocean where intermediate and deep waters are produced during wintertime ocean convection (e.g., Talley and McCartney, 1982, Dickson et al., 2008). This convection mixes Arctic outflow and Atlantic waters with underlying layers and produces a particular intermediate water mass called the Labrador Sea Water (LSW). Irminger and Nordic seas are the other basins of the North Atlantic Ocean where convection leads to the formation of intermediate and deep waters, namely the Denmark Strait Overflow Water (DSOW) and the Iceland-Scotland Overflow Water (ISOW). Underlying LSW, the cold and fresh West Greenland Current flows northwestwardly along the Greenland coasts, whereas the Labrador Current flows southeastwardly along the Labrador shelf (Fig. 1). Below LSW, Northeast Atlantic Deep water (NEADW), ISOW and DSOW circulate cyclonically in the LS basin (Azetsu-Scott et al., 2003). NEADW is a cluster of higher salinity and colder waters than LSW; ISOW and DSOW protrude from the NEADW, with DSOW being fresher and more oxygenated than ISOW (Talley and Pickard, 2011), while ISOW evolves “en route” to become NEADW (Yashayaev and Dickson, 2008). These intermediate and deep waters supply the lower limb of the Atlantic Meridional Overturning Circulation, which redistributes heat and substances between polar and equatorial regions in the North Atlantic (e.g., Talley and McCartney, 1982, Lherminier et al., 2010). During the formation of LSW, gases and atmospheric deposition are sequestered and incorporated into the deepening water. From chlorofluorocarbons concentrations, Azetsu-Scott et al. (2005) described the structure and ventilation age (time since a water mass last had contact with the atmosphere) of the LS water column. Deep convection in the LS varies from 200 to 2000 m from one year to the next, depending on atmospheric conditions and stratification of the water column (Azetsu-Scott et al., 2003). In the upper part, the newly ventilated LSW overlays LSW produced in the previous winters. Below, NEADW and DSOW have ventilation ages estimated to be 11–13, and 5–8 years (Azetsu-Scott et al., 2005). More generally, according to Yashayaev et al. (2015a), DSOW and NEADW show quasi-pentadal and multi-decadal oscillations, respectively, while LSW changes at higher frequencies. Therefore, the LS is the best location where full water column observations allow the discrimination and identification of LSW “vintages” (LSW_Year vs LSW_Deep), which overlay the older NEADW/ISOW and DSOW. Year after year, every winter convection results in a variable renewal of LSW entraining substances, such as mercury (Hg), from the atmosphere into the deep layers, and the transfer of the atmospheric signal to the intermediate depths of the North Atlantic Ocean. In addition, since it is now well established that the oceanic Hg cycle is heavily impacted by human atmospheric emissions (e.g., Sunderland and Mason, 2007, Lamborg et al., 2014), it is important to appraise the anthropogenic Hg impregnation of the water column of the North Atlantic Ocean. Here, we report the first high-resolution Hg distribution pattern along a transect, which closes off the Labrador Sea from Greenland to Labrador coasts held in June 2014 during the Geotraces-Geovide cruise, i.e., after the 2014-winter convection (Fig. 1).

Section snippets

Sampling

Water samples were collected in the Labrador Sea between 19 and 27 June 2014, during the Geotraces-Geovide cruise onboard the R/V “Pourquoi Pas?”. Seven stations (6–21 depths each) were occupied for collecting water samples for Hg determination (Fig. 1). Sampling and water treatment were performed using ultra-trace techniques following Geotraces recommendations (www.geotraces.org). During the Geovide cruise, an epoxy-coated aluminium rosette, held by a Kevlar hydrowire, was deployed with

Water masses identification

A full description of the oceanographic context of the Geovide cruise is given by García-Ibáñez et al. (2017). The distributions of salinity and potential density (σ_θ) along the section (Fig. 2a and b) allows the identification of water masses expected in the area, consistent with previous oceanographic investigations (e.g., Stramma et al., 2004, Yashayaev et al., 2015a, Yashayaev and Loder, 2016). From surface waters downward we observed: (i) a well stratified surface layer of 150 m depth in

Summary and conclusions

In summary, the HgT distribution in the LS water column is characterized by a nutrient-like behavior superimposed by Hg-enrichment originating from the Canadian Arctic Archipelago waters carried by the Labrador Current. This excess Hg is transferred southward, in surface waters with the Labrador Current, and at depth with the lower limb of the Atlantic Meridional Overturning Circulation via the Deep Western Boundary Current. This process should increase with the expected permafrost thawing over

Author information

Corresponding Authors E-mail: [email protected]. Phone: + 1-514-439-8254.

Daniel Cossa: https://www.researchgate.net/profile/Daniel_Cossa.

Acknowledgments

The first thanks are for J. Boutorh, M. Cheize, J.-L. Menzel and R. Shelley who were in charge of the ultra-trace sampling organization; they are commended for its successful achievement. Thanks are also due to other members of the Geovide team for participating to data acquisition: F. Alonso Pérez, R. Barkhouse, V. Bouvier, P. Branellec, L. Carracedo Segade, M. Castrillejo, L. Contreira, E. de Saint Léger, N. Deniault, F. Desprez de Gesincourt, L. Foliot, D. Fonseca Pereira, E. Grossteffan, P.

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

Highlights

Abstract

Introduction

Section snippets

Sampling

Water masses identification

Summary and conclusions

Author information

Acknowledgments

Mar. Chem.

Mar. Chem.

Mar. Chem.

Geochim. Cosmochim. Acta

Deep Sea Res.

Prog. Oceanogr.

Deep-Sea Res. I

Mar. Chem.

Environ. Res.

Deep Sea Res. Part I

Deep-Sea Res. I Oceanogr. Res. Pap.

Prog. Oceanogr.

Time series study of CFC concentrations in the Labrador Sea during deep and shallow convection regimes (1991–2000)

J. Geophys. Res.

Composition of freshwater in the spring of 2014 on the southern Labrador shelf and slope

J. Geophys. Res. Oceans

The distribution of dissolved and particulate mercury in three Siberian estuaries and adjacent Arctic coastal waters

Water Air Soil Pollut.

Rapid and noncontaminating sampling system for trace elements in global ocean surveys

Limnol. Oceanogr. Methods

Arctic-subarctic ocean fluxes: defining the role of the northern seas in climate