Isotope fractionation between dissolved and suspended particulate Fe in the oxic and anoxic water column of the Baltic Sea

[摘要] 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 Basinwater column, Baltic Sea. Results show a sharp gradient inδ⁵⁶Fe_dis across the ferruginous layer withδ⁵⁶Fe_dis = −0.4‰ in the euxinic deepbasin and δ⁵⁶Fe_dis = +0.3‰ in the oxicupper water column. The isotopic gradient overlaps with a strongconcentration gradient of Fe_dis, a concentration maximum inFe_SPM and lower δ⁵⁶Fe_SPM values thanδ⁵⁶Fe_dis. These features indicate preferential loss oflight Fe isotopes from solution to suspended iron-oxyhydroxides(Fe_IOH) during typical oxidative precipitation across the redoxinterface. The sign of the overall fractionation,Δ⁵⁶Fe_IOH-Fe(II)(aq) < 0‰, is incontrast to similar, mostly non-marine redox environments, whereΔ⁵⁶Fe_IOH-Fe(II)(aq) > 0‰. Thedifference appears to be the result of isotope exchange dominated by reactionkinetics in the marine water column, rather than equilibrium fractionationgenerally inferred for oxidative Fe precipitation elsewhere. High residualδ⁵⁶Fe_dis immediately above the oxic–ferruginousinterface and throughout the oxic water column suggests that any potentialdissolved Fe export from marine reducing waters into the oxic open watercolumn is enriched in the heavy isotopes. In the deep, mildly euxinic watercolumn above the level of Fe sulfide saturation, a decreasing δ⁵⁶Fe_SPM trend with depth and a generally low δ⁵⁶Fe_dis are comparable to trends generally observed in marineanoxic sediment profiles where microbial reductive Fe dissolution occurs. Theisotope composition of the redox-cycled Fe inventory in anoxic marine basinsmainly reflects the balance between external fluxes, driving the compositiontowards crustal δ⁵⁶Fe values, and intensity of internal recycling,driving δ⁵⁶Fe towards negative values.

[学科分类] 地球化学与岩石