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Abstract

Fe isotope ratios are a potentially useful tool to determine the sources of marine dissolved and particulate Fe in the present day ocean and the ocean's redox state of the past. However, to date there are only a few measurements of Fe isotope ratios available from the marine environment. Almost no data exists for dissolved Fe, yet knowledge of the mechanisms and magnitude of Fe isotope fractionation in the marine Fe cycle are essential, if Fe isotope ratios are to be applied to adress either of the above issues. We have analyzed Fe isotope ratios in dissolved and particulate Fe across a depth profile from the anoxic Gotland Basin in the Baltic Sea, sampled during two cruises in 2005. Particulate Fe was measured on MC-ICP-MS using standard bracketing and show δ56Fe values slightly depleted in the heavy isotopes with little variation down the profile. Dissolved Fe was measured with a double spike after co-precipitation with Mg(OH)2. Anoxic samples taken from bottle-casts and filtered in a clean bench may show large offsets in dissolved Fe isotope ratios in comparison to pump-CTD in-line filtered samples, suggesting the likely introduction of artifacts during filtration in contact with air. Particles generally show much higher δ56Fe values than the dissolved phase. Among pump CTD samples, the small number of dissolved Fe samples collected during the first cruise in July at the beginning of the cyanobacteria bloom season show much lower δ56Fe values than those sampled in October, after the bloom. Since neither set of pump CTD samples was exposed to the atmosphere during filtration, an artifact is not likely. Instead, the apparent seasonal difference could be the result of a growing proportion of Fe remineralized from sinking particles of relatively high δ56Fe towards the end of the surface plankton bloom. Prior to and early during the bloom, dissolved Fe at depth may predominatly originate from the basin margin sediments after suboxic early-diagenetic remineralization, for which low δ56Fe values are commonly reported. The apparent seasonal dynamics in Fe isotope ratios may provide useful information regarding the modern marine Fe cycle, but will complicate the use of Fe isotopes as a paleo-redox proxy.