Seasonal variability in sediment porewater Fe and potential for water column release in the Celtic Sea

Jessica Katharina Klar1, William B Homoky2, Fanny Chever3, Anna Lichtschlag4, Carolyn Alice Graves1, Amber L Annett5,6, Heather Goring-Harford1, Torben Stichel7 and Peter John Statham7, (1)University of Southampton, School of Ocean and Earth Science, Southampton, United Kingdom, (2)University of Oxford, Department of Earth Sciences, United Kingdom, (3)CEDRE (CEntre of Documentation, Research and Experimentation on accidental water pollution), Brest, France, (4)National Oceanography Centre, Geochemistry, Southampton, United Kingdom, (5)University of Edinburgh, School of GeoSciences, Edinburgh, United Kingdom, (6)Rutgers University, Marine and Coastal Sciences, New Brunswick, NJ, United States, (7)University of Southampton, School of Ocean and Earth Science, Southampton, SO14, United Kingdom
Abstract:
Iron (Fe) supplied by continental margins to the open ocean is considered to have a vital impact on marine eco-system productivity. However, Fe fluxes from many margin types are not well constrained, and the seasonal variability in Fe flux is critically unknown. Continental margins account for ~20 % of the global sea floor, and oxic shelf systems, such as the Celtic Sea, represent a large fraction of this marginal zone.

The intra-annual cycling and release of Fe from shelf sediments to the overlying water column has been studied over three seasons, before, during and after the phytoplankton bloom in the Celtic Sea, UK. Porewater dissolved Fe (dFe, < 0.15 μm) was predominantly in the reduced Fe(II) form (70 to 100 %), most of which appeared to be present in the soluble size fraction (<0.02 μm; 75 to 100 %). This indicates that iron reduction is an important mechanism of dFe formation in this region. In cohesive surface sediments (< 1 cm) dFe concentrations changed from ~ 0.1 μM during pre-bloom conditions, to 5 – 15 μM during bloom conditions, and to ~ 1 μM during the post-bloom. Higher concentrations observed during the bloom coincide with the deposition of large amounts of bloom originated organic material to the seafloor, which is decomposed and leads to the release of Fe into porewaters. These data indicate that the pool of porewater dissolved Fe that could be released into overlying waters is at a maximum over a short period of the annual cycle. We also measured up to 20 nM dFe in oxic seawater in contact with sediment, and up to 70 % was in the reduced Fe(II) form. The measured oxidation rates of Fe(II) in waters overlying sediments were also substantially slower than empirical predictions. We conclude, the largest inventory of porewater dFe with potential to be released to the water column occurs during the spring bloom, it comprises mostly Fe(II), and is likely to be stabilized by complexation to organic ligands in the overlying water.