Articles | Volume 8, issue 10
Biogeosciences, 8, 3025–3039, 2011
Biogeosciences, 8, 3025–3039, 2011

Research article 31 Oct 2011

Research article | 31 Oct 2011

Towards accounting for dissolved iron speciation in global ocean models

A. Tagliabue1,2,3 and C. Völker4 A. Tagliabue and C. Völker
  • 1Laboratoire des Sciences du Climat et de l'Environnement, IPSL-CEA-CNRS-UVSQ Orme des Merisiers, 91198, Gif sur Yvette, France
  • 2Southern Ocean Carbon and Climate Observatory, Council for Scientific and Industrial Research (CSIR), P.O. Box 320, Stellenbosch, 7599, South Africa
  • 3Department of Oceanography, University of Cape Town, 7701, Cape Town, South Africa
  • 4Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570, Bremerhaven, Germany

Abstract. The trace metal iron (Fe) is now routinely included in state-of-the-art ocean general circulation and biogeochemistry models (OGCBMs) because of its key role as a limiting nutrient in regions of the world ocean important for carbon cycling and air-sea CO2 exchange. However, the complexities of the seawater Fe cycle, which impact its speciation and bioavailability, are simplified in such OGCBMs due to gaps in understanding and to avoid high computational costs. In a similar fashion to inorganic carbon speciation, we outline a means by which the complex speciation of Fe can be included in global OGCBMs in a reasonably cost-effective manner. We construct an Fe speciation model based on hypothesised relationships between rate constants and environmental variables (temperature, light, oxygen, pH, salinity) and assumptions regarding the binding strengths of Fe complexing organic ligands and test hypotheses regarding their distributions. As a result, we find that the global distribution of different Fe species is tightly controlled by spatio-temporal environmental variability and the distribution of Fe binding ligands. Impacts on bioavailable Fe are highly sensitive to assumptions regarding which Fe species are bioavailable and how those species vary in space and time. When forced by representations of future ocean circulation and climate we find large changes to the speciation of Fe governed by pH mediated changes to redox kinetics. We speculate that these changes may exert selective pressure on phytoplankton Fe uptake strategies in the future ocean. In future work, more information on the sources and sinks of ocean Fe ligands, their bioavailability, the cycling of colloidal Fe species and kinetics of Fe-surface coordination reactions would be invaluable. We hope our modeling approach can provide a means by which new observations of Fe speciation can be tested against hypotheses of the processes present in governing the ocean Fe cycle in an integrated sense

Final-revised paper