Mercury distribution and transport in the North Atlantic Ocean along the GEOTRACES-GA01 transect

• Published in: Biogeosciences
• Main Authors: Cossa, Daniel, Heimburger, Lars-eric, Perez, Fiz F, Garcia-ibanez, Maria Isabel, Sonke, Jeroen E., Planquette, Helene, Lherminier, Pascale, Boutorh, Julia, Cheize, Marie, Barraqueta, Jan Lukas Menzel, Shelley, Rachel, Sarthou, Geraldine
• Format: Text
• Language: English
• Published: Copernicus Gesellschaft Mbh
• Subjects: envir; geo; Canada; Greenland; Labrador Sea; North Atlantic; North East Atlantic; Subarctic
• Online Access: https://doi.org/10.5194/bg-15-2309-2018; https://archimer.ifremer.fr/doc/00412/52303/53071.pdf; https://archimer.ifremer.fr/doc/00412/52303/53072.pdf; https://archimer.ifremer.fr/doc/00412/52303/56301.pdf; https://archimer.ifremer.fr/doc/00412/52303/56302.pdf; https://archimer.ifremer.fr/doc/00412/52303/

We report here the results of total mercury (HgT) determinations along the 2014 GEOTRACES GEOVIDE cruise (GA01 transect) in the North Atlantic Ocean (NA) from Lisbon (Portugal) to the Labrador coast (Canada). Ninety-seven percent of the HgT concentrations of unfiltered samples (HgTUNF) ranged between 0.16 and 1.00 pmol L−1. The geometric mean was 0.51 pmol L−1 for the 535 samples analysed. The dissolved fraction (< 0.45 μm) of HgT, determined on 141 samples, averaged 78 % of the HgTUNF for the whole water column. HgTUNF concentrations increased eastwards and with depth from Greenland to Europe and from sub-surface to bottom waters, respectively. The Hg distribution mirrored that of dissolved oxygen concentration, with highest HgTUNF levels associated with oxygen-depleted zones. The statistically significant (p < 0.01) relationship between HgTF (filtered samples) and the apparent oxygen utilization confirms the nutrient-like behavior of Hg in the NA. An extended Optimum Multiparameter Analysis allowed us to characterize HgTUNF concentrations in the different Source Water Types (SWTs) present along the transect. Mean HgTUNF concentrations ranged from 0.26 ± 0.03 pmol L−1 in the Irminger Subpolar Mode Water to 1.02 ± 0.02 pmol L−1 in the lower North East Atlantic Deep Water. Anthropogenic Hg-enriched SWTs were found in the upper oceanic layers (i.e., East North Atlantic Central Water and Subarctic Intermediate Water). The change in anthropogenic Hg concentrations in the Labrador Sea Water, during its eastward journey, suggests a continuous decrease of Hg content in this water mass over the last decades. Calculation of the water transport driven by the Atlantic Meridional Overturning Circulation across the Portugal-Greenland transect indicates a northward Hg transport within the upper limb, and a southward Hg transport within the lower limb, with a resulting net transport from low latitudes to the polar zones of about 111 kmol yr−1.