Controls on dissolved cobalt in surface waters of the Sargasso Sea: Comparisons with iron and aluminum

• Published in: Global biogeochemical cycles 2012-06, Vol.26 (2), p.n/a
• Main Authors: Shelley, R. U., Sedwick, P. N., Bibby, T. S., Cabedo-Sanz, P., Church, T. M., Johnson, R. J., Macey, A. I., Marsay, C. M., Sholkovitz, E. R., Ussher, S. J., Worsfold, P. J., Lohan, M. C.
• Format: Article
• Language: English
• Subjects: Aerosols; Aluminum; Animal and plant ecology; Animal, plant and microbial ecology; biogeochemical cylces; Biological and medical sciences; Biological effects; Biological oceanography; Chemical oceanography; Cobalt; Earth sciences; Earth, ocean, space; Eddies; Euphotic zone; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; General aspects; Geochemistry; iron; Nutrients; Solubility; Surface water; Synecology; trace metals; Vertical distribution; Water column; Wet deposition
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2011GB004155

Dissolved cobalt (dCo), iron (dFe) and aluminum (dAl) were determined in water column samples along a meridional transect (∼31°N to 24°N) south of Bermuda in June 2008. A general north‐to‐south increase in surface concentrations of dFe (0.3–1.6 nM) and dAl (14–42 nM) was observed, suggesting that aerosol deposition is a significant source of dFe and dAl, whereas no clear trend was observed for near‐surface dCo concentrations. Shipboard aerosol samples indicate fractional solubility values of 8–100% for aerosol Co, which are significantly higher than corresponding estimates of the solubility of aerosol Fe (0.44–45%). Hydrographic observations and analysis of time series rain samples from Bermuda indicate that wet deposition accounts for most (>80%) of the total aeolian flux of Co, and hence a significant proportion of the atmospheric input of dCo to our study region. Our aerosol data imply that the atmospheric input of dCo to the Sargasso Sea is modest, although this flux may be more significant in late summer. The water column dCo profiles reveal a vertical distribution that predominantly reflects ‘nutrient‐type’ behavior, versus scavenged‐type behavior for dAl, and a hybrid of nutrient‐ and scavenged‐type behavior for dFe. Mesoscale eddies also appear to impact on the vertical distribution of dCo. The effects of biological removal of dCo from the upper water column were apparent as pronounced sub‐surface minima (21 ± 4 pM dCo), coincident with maxima in Prochlorococcus abundance. These observations imply that Prochlorococcus plays a major role in removing dCo from the euphotic zone, and that the availability of dCo may regulate Prochlorococcus growth in the Sargasso Sea. Key Points Aerosol cobalt has a significantly higher fractional solubility than iron Dissolved cobalt did not correlate with aerosol deposition (dissolved aluminum) Dissolved cobalt inversely correlated with Prochlorococcus abundance