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Fe isotope ratios and concentrations of dissolved Fe (Fedis, < 0.45µm) and of suspended particulate Fe (FeSPM) from a depth profile in the Eastern Gotland Basin, Baltic Sea, show a sharp gradient in δ56Fedis across the suboxic interface with δ56Fedis=-0.4‰ in the euxinic deep basin and δ56Fedis=+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 into suspended iron-oxyhydroxides (FeIOH) during typical oxidative precipitation across the redox interface in the marine environment. The sign of the fractionation, δ56FeFe IOH-Fe(II)(aq) < 0 ‰, is in contrast to similar, mostly non-marine redox environments, where δ56FeFe 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 δ56Fedis immediately above the suboxic interface and throughout the oxic water column suggest that any potential Fe export from reducing waters or sediments into the open water column is enriched in the heavy isotopes. Within the suboxic to euxinic deep basin the decreasing δ56FeSPM trend with depth and a generally low δ56Fedis are comparable to trends in marine anoxic sediment profiles indicative of microbial reductive Fe dissolution. This confirms supply of dissolved Fe to the euxinic water column 20 mainly from reductive dissolution of settling particles. [ABSTRACT FROM AUTHOR]

Abstract: Measurements of the concentrations of total CO2 (CT), O2, H2S, NO2 −, NO3 −, and NH4 + were performed between 2003 and 2007 during intervals of 2–3months in the deep water of the Gotland Basin (Baltic Sea). The data were used to determine mineralization and denitrification rates in four sublayers below 150m during a period of stagnation. To account for the vertical mixing at the boundaries of the sublayers, mixing coefficients were determined based on the temporal changes of the salinity profiles. Mass balances were established for each measured variable and yielded the sink/source rates in the individual sublayers for each time interval. The seasonality of mineralization rates was confined to water layers that were in close contact with the sediment surface, indicating that the mineralization took place mainly at the sediment surface. Moreover, it was shown that the mineralization rates did not depend on the redox conditions in the water column. The total mineralization rate in the basin was found to be 2.0molm–2 yr–1, thereby implying that about 80% of the particulate organic carbon input occurred by lateral transport. A redox balance indicated that manganese dioxide and possibly iron oxides contributed to the oxidation of organic matter by about 13%. Intense denitrification was observed during the transition from oxic to anoxic conditions. A value of 456mmol–Nm–2 yr–1 was found, which included a possible anammox contribution of about 5%. [Copyright &y& Elsevier]