The international GEOTRACES program has been developed to produce a global framework of key trace elements and isotopes that will describe the contemporary distributions of these properties in the ocean. This information can then be used to constrain models of the processes and fluxes of biogeochemically important elements and key tracers that frame flux processes and transformations. A scientist from Woods Hole Oceanographic Institution plans to participate in the 2010 GEOTRACES cruise to the North Atlantic and measure cobalt (Co) and Co speciation in surface samples and throughout the water column. Samples will be taken from all physical and biogeochemical processes that influence trace metals and their isotopes, namely strong meridional advection, boundary scavenging and sources, aeolian deposition, and input of intermediate waters from the Mediterranean and Labrador Seas. The resultant dataset should provide insight into the sources, sinks and chemical speciation of this element.
This research will greatly contribute to the small amount of data that exists on cobalt in the marine environment and provide a greater understanding of cobalt biogeochemistry. In regard to educational impacts, this research will provide support for a post-doctoral researcher as well as a graduate student.
The research grant enabled the study the essential micronutrient cobalt in the North Atlantic through participation and analysis of samples from the US GEOTRACES North Atlantic Zonal Transect expeditions. Cobalt is a required micronutrient for many forms of life, including marine photosynthetic organisms (e.g. cyanobacteria and many algae), yet is one of the scarcest nutrients in the oceans typically found in concentrations of 10’s of picomoles per liter. The study of nutrients and micronutrients in the oceans is critical to improving our understanding of the marine carbon cycle. The distribution, in relation to sources and sinks, as well as its chemical speciation were studied in this grant to further our understanding of cobalt’s biogeochemistry in relation to other trace elements and isotopes and to marine carbon cycling. The project involved analysis of a full depth ocean section for total dissolved cobalt and labile cobalt chemical species for the North Atlantic collected on expeditions in 2010 and 2011, with more than 700 electrochemical analyses conducted. Our measurements found that the labile cobalt chemical species was detected in much of the water column below the euphotic zone, suggesting that strong cobalt binding ligands were not present in excess of the total cobalt concentration. In contrast, near complete complexation of cobalt was observed in surface waters, and linear relationships were observed when both total and labile cobalt were compared to phosphate in surface waters, indicative of a strong microbial/biological influence on cobalt biogeochemical cycling. Decoupling of cobalt and macronutrients in the surface waters was observed approaching the North American coast, and a relationship between cobalt and salinity was observed, suggesting that coastal inputs may dominate the distributions of cobalt there. In deep waters, both total and labile cobalt were generally lower in concentration than at intermediate depths, which is evidence of scavenging processes removing cobalt from the water column. Elevated concentrations of labile and total cobalt were observed in samples taken within the TAG hydrothermal plume, and a reverse relationship between cobalt and oxygen was observed in the western basin OMZ. Together these results significantly increase our understanding of the micronutrient cobalt and will contribute to understanding its potential influence on marine carbon cycling.
