Abstract
Abstract. Fe isotope ratios and concentrations of dissolved Fe (Fedis, < 0.45 μm) and of suspended particulate Fe (FeSPM) were analyzed from a depth profile through the anoxic Eastern Gotland Basin water column, Baltic Sea. Results show a sharp gradient in δ56Fedis across the ferruginous layer with δ56Fedis = −0.4‰ in the euxinic deep basin and δ56Fedis = &amp;plus;0.3‰ in the oxic upper water column. The isotopic gradient overlaps with a strong concentration gradient of Fedis, a concentration maximum in FeSPM and lower δ56FeSPM values than δ56Fedis. These features indicate preferential loss of light Fe isotopes from solution to suspended iron-oxyhydroxides (FeIOH) during typical oxidative precipitation across the redox interface. The sign of the overall fractionation, Δ56FeIOH-Fe(II)(aq) < 0‰, is in contrast to similar, mostly non-marine redox environments, where Δ56FeIOH-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 δ56Fedis 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 δ56FeSPM trend with depth and a generally low δ56Fedis 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 δ56Fe values, and intensity of internal recycling, driving δ56Fe towards negative values.
Highlights
The fractionation of Fe isotopes between coexisting dissolved and particulate Fe species within the environmental Fe redox cycle often imparts an isotopic signature on residual reaction components (Bullen et al, 2001; Anbar, 2004; Beard and Johnson, 2004)
The isotopic gradient overlaps with a strong concentration gradient of Fedis, a concentration maximum in FeSPM and lower δ56FeSPM values than δ56Fedis. These features indicate preferential loss of light Fe isotopes from solution to suspended iron-oxyhydroxides (FeIOH) during typical oxidative precipitation across the redox interface
One exception is a study of the low oxygen San Pedro Basin at the Californian margin (John et al, 2012), where dissolved Fe isotope ratios in the water column appear to reflect
Summary
The fractionation of Fe isotopes between coexisting dissolved and particulate Fe species within the environmental Fe redox cycle often imparts an isotopic signature on residual reaction components (Bullen et al, 2001; Anbar, 2004; Beard and Johnson, 2004). The pycnocline a substantial flux of internally redoxcycled authigenic MnOx and FeOOH-PO4 and relatively little lithogenous (detrital) flux is observed across the redox boundaries (Neretin et al, 2003; Dellwig et al, 2010) These minerals are formed by oxidative precipitation and result in suspended Mn (MnSPM) and Fe (FeSPM) maxima (Neretin et al, 2003; Pohl et al, 2004; Pohl and Hennings, 2008; Turnewitsch and Pohl, 2010). At 120–140 m depth – the top of the ferruginous zone – oxidative precipitation results in a maximum concentration of suspended matter Fe (FeSPM), predominantly with iron oxyhydroxides (FeIOH) comprising a mixed MnOx-FeOOH-PO4 phase at the top and FeOOH-PO4 inside the sulfidic zone (Dellwig et al, 2010) This FeSPM maximum broadly overlaps with a turbidity maximum sensor profile (Fig. 2b, c). FeS precipitation below 180 m, or during later years with a fully developed euxinic layer, cannot be excluded entirely
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