Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Journal topic
Volume 14, issue 11
Biogeosciences, 14, 2715–2739, 2017
https://doi.org/10.5194/bg-14-2715-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 14, 2715–2739, 2017
https://doi.org/10.5194/bg-14-2715-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 02 Jun 2017

Research article | 02 Jun 2017

Coastal sources, sinks and strong organic complexation of dissolved cobalt within the US North Atlantic GEOTRACES transect GA03

Abigail E. Noble1,a, Daniel C. Ohnemus1,b, Nicholas J. Hawco1, Phoebe J. Lam1,c, and Mak A. Saito1 Abigail E. Noble et al.
  • 1Woods Hole Oceanographic Institution, Woods Hole, MA, USA
  • anow at: Gradient, 20 University Road, Cambridge, MA, USA
  • bnow at: Bigelow Laboratory for Ocean Sciences, East Boothbay, ME, USA
  • cnow at: University of California Santa Cruz, Santa Cruz, CA, USA

Abstract. Cobalt is the scarcest of metallic micronutrients and displays a complex biogeochemical cycle. This study examines the distribution, chemical speciation, and biogeochemistry of dissolved cobalt during the US North Atlantic GEOTRACES transect expeditions (GA03/3_e), which took place in the fall of 2010 and 2011. Two major subsurface sources of cobalt to the North Atlantic were identified. The more prominent of the two was a large plume of cobalt emanating from the African coast off the eastern tropical North Atlantic coincident with the oxygen minimum zone (OMZ) likely due to reductive dissolution, biouptake and remineralization, and aeolian dust deposition. The occurrence of this plume in an OMZ with oxygen above suboxic levels implies a high threshold for persistence of dissolved cobalt plumes. The other major subsurface source came from Upper Labrador Seawater, which may carry high cobalt concentrations due to the interaction of this water mass with resuspended sediment at the western margin or from transport further upstream. Minor sources of cobalt came from dust, coastal surface waters and hydrothermal systems along the Mid-Atlantic Ridge. The full depth section of cobalt chemical speciation revealed near-complete complexation in surface waters, even within regions of high dust deposition. However, labile cobalt observed below the euphotic zone demonstrated that strong cobalt-binding ligands were not present in excess of the total cobalt concentration there, implying that mesopelagic labile cobalt was sourced from the remineralization of sinking organic matter. In the upper water column, correlations were observed between total cobalt and phosphate, and between labile cobalt and phosphate, demonstrating a strong biological influence on cobalt cycling. Along the western margin off the North American coast, this correlation with phosphate was no longer observed and instead a relationship between cobalt and salinity was observed, reflecting the importance of coastal input processes on cobalt distributions. In deep waters, both total and labile cobalt concentrations were lower than in intermediate depth waters, demonstrating that scavenging may remove labile cobalt from the water column. Total and labile cobalt distributions were also compared to a previously published South Atlantic GEOTRACES-compliant zonal transect (CoFeMUG, GAc01) to discern regional biogeochemical differences. Together, these Atlantic sectional studies highlight the dynamic ecological stoichiometry of total and labile cobalt. As increasing anthropogenic use and subsequent release of cobalt poses the potential to overpower natural cobalt signals in the oceans, it is more important than ever to establish a baseline understanding of cobalt distributions in the ocean.

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Short summary
This study examines sources and sinks of dissolved and labile cobalt in the North Atlantic Ocean. The North and South Atlantic are influenced differently by dust, coastal margin sources, biota, and suspended particles. Dissolved cobalt in both basins is driven by a coastal margin source, leading to large plumes emanating from the north and south African coasts. These plumes are comparable in size despite the high dust flux observed in the North Atlantic that is absent in the South Atlantic.
This study examines sources and sinks of dissolved and labile cobalt in the North Atlantic...
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