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

• Published in: Biogeosciences 2017-06, Vol.14 (11), p.2715-2739
• Main Authors: Noble, Abigail E, Ohnemus, Daniel C, Hawco, Nicholas J, Lam, Phoebe J, Saito, Mak A
• Format: Article
• Language: English
• Subjects: Anthropogenic factors; Aquatic plants; Atmospheric particulates; Biogeochemical cycle; Biogeochemical cycles; Biogeochemistry; Chemical properties; Chemical speciation; Coastal processes; Coastal waters; Coasts; Cobalt; Complexation; Correlation; Datasets; Deep water; Deposition; Dissolution; Dust; Dust deposition; Dust storms; Ecological monitoring; Environmental aspects; Eolian dust; Euphotic zone; Expeditions; Human influences; Hydrothermal fields; Hydrothermal systems; Ligands; Metals; Micronutrients; Mid-ocean ridges; Multiship expeditions; Oceans; Organic matter; Oxygen; Phosphates; Plumes; Remineralization; Resuspended sediments; Scavenging; Seawater; Sinkholes; Speciation; Stoichiometry; Studies; Surface water; Tropical climate; Water circulation; Water column; Water depth; Water masses
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-14-2715-2017

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 ecologica