Isotope fractionation between dissolved and suspended particulate Fe in the oxic and anoxic water column of the Baltic Sea
Fe isotope ratios and concentrations of dissolved Fe (Fe dis, < 0.45 μm) and of suspended particulate Fe (Fe SPM) were analyzed from a depth profile through the anoxic Eastern Gotland Basin water column, Baltic Sea. Results show a sharp gradient in δ 56Fe dis across the ferruginous layer with δ 56Fe dis = −0.4‰ in the euxinic deep basin and δ 56Fe dis = +0.3‰ in the oxic upper water column. The isotopic gradient overlaps with a strong concentration gradient of Fe dis, a concentration maximum in Fe SPM and lower δ 56Fe SPM values than δ 56Fe dis. These features indicate preferential loss of light Fe isotopes from solution to suspended iron-oxyhydroxides (Fe IOH) during typical oxidative precipitation across the redox interface. The sign of the overall fractionation, Δ 56Fe IOH-Fe(II)(aq) < 0‰, is in contrast to similar, mostly non-marine redox environments, where Δ 56Fe IOH-Fe(II)(aq) > 0‰. The difference appears to be the result of isotope exchange dominated by reaction kinetics in the marine water column, rather than equilibrium fractionation generally inferred for oxidative Fe precipitation elsewhere. High residual δ 56Fe dis immediately above the oxic–ferruginous interface and throughout the oxic water column suggests that any potential dissolved Fe export from marine reducing waters into the oxic open water column is enriched in the heavy isotopes. In the deep, mildly euxinic water column above the level of Fe sulfide saturation, a decreasing δ 56Fe SPM trend with depth and a generally low δ 56Fe dis are comparable to trends generally observed in marine anoxic sediment profiles where microbial reductive Fe dissolution occurs. The isotope composition of the redox-cycled Fe inventory in anoxic marine basins mainly reflects the balance between external fluxes, driving the composition towards crustal δ 56Fe values, and intensity of internal recycling, driving δ 56Fe towards negative values.