Fractionation of iron species and iron isotopes in the Baltic Sea euphotic zone
1Division of Applied Geology, Luleå University of Technology, Luleå, Sweden
2Department of Chemistry, University of Gothenburg, Gothenburg, Sweden
3Department of Chemistry, University of Otago, Dunedin, New Zealand
4Department of Marine Science, University of Southern Mississippi, Hattiesburg, USA
Abstract. Measurements of the physiochemical speciation of Fe in the euphotic zone were performed at three different locations, over a well defined salinity gradient, during spring and summer in the Baltic Sea. The average of total Fe changed from 114 nM in the Bothnian Sea, 44 nM at Landsort Deep and 15 nM at Gotland Deep. Particulate Fe (PFe) was the dominating phase at all stations and on average accounted for 75–85% of the total Fe pool. At all three locations, a decrease in total Fe of 80–90% from initial measurements compared to the summer was found. A strong positive correlation between PFe and chl-a was observed. Hence, primary production strongly regulates cycling of suspended Fe. However, this relation is not dominated by active uptake of Fe in phytoplankton; instead this reflects cycling of phosphorus, growth of diatoms, and removal of PFe during phytoplankton sedimentation. The average colloidal iron fraction, CFe, showed decreasing concentrations along the salinity gradient; Bothnian Sea 15 nM; Landsort Deep 1 nM and Gotland Deep 0.5 nM. Field Flow Fractionation data indicate that the main colloidal carrier phase for Fe in the Baltic Sea is a carbon-rich fulvic acid associated compound, likely of riverine origin. The Fe isotope composition (δ56Fe) of the PFe showed constant positive values in the Bothnian Sea surface waters (+0.08 to +0.20‰). Enrichment of heavy Fe in the Bothnian Sea PFe is most likely associated to input of aggregated land derived Fe-oxyhydroxides and a rapid overturn of Fe(II). At the Landsort deep, the fractionation of PFe changed between −0.08‰ to +0.28‰. The negative values, in early spring, probably indicate exchange over the oxic-anoxic boundary at ~80 m depth.