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 mu m) 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 delta Fe-56(dis) across the ferruginous layer with delta Fe-56(dis) =-0.4% in the euxinic deep basin and delta Fe-56(dis) =+0.3 parts per thousand 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 delta Fe-56(SPM) values than delta Fe-56(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, Delta Fe-56(Fe IOH)-Fe(II)(aq) < 0 parts per thousand, is in contrast to similar, mostly non-marine redox environments, where Delta Fe-56(Fe-IOH)-Fe(II)(aq) > 0 parts per thousand. 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 delta(56)Fedis 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 delta(FeSPM)-Fe-56 trend with depth and a generally low delta(56)Fedis are comparable to trends generally observed in marine anoxic sediment profiles where microbial reductive Fe dissolution occurs. The isotope composition of the redoxcycled Fe inventory in anoxic marine basins mainly reflects the balance between external fluxes, driving the composition towards crustal delta Fe-56 values, and intensity of internal recycling, driving delta Fe-56 towards negative values.