Coastal Sources, Sinks and Strong Organic Complexation of Dissolved Cobalt within the US North Atlantic GEOTRACES Transect GA03
Authors: Noble Abigail E. , Ohnemus Daniel C. , Hawco Nicholas J. , Lam Phoebe J. , Saito Mak A. 
Published: December 2016

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 U.S. 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 by increased abundances of dissolved cobalt relative to surrounding waters. 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 a confluence of processes including reductive dissolution, biouptake and remineralization, and aeolian dust deposition. This 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 higher cobalt concentrations due to the interaction of this water mass with resuspended sediment at the western margin or from transport even 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 with the regional high dust deposition. However, labile cobalt was found below the euphotic zone, demonstrating that strong cobalt binding ligands were not present in excess of the total cobalt concentration there and implying mesopelagic labile cobalt was sourced from the remineralization of sinking organic matter. Significant correlations were observed in the upper water column between total cobalt and phosphate, and between labile cobalt and phosphate, demonstrating a strong biological influence on cobalt cycling across much of the North Atlantic transect. Along the western margin off the North American coast, this linear relationship 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 were lower in concentration than at intermediate depths, providing evidence 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.