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Abstract. Dissimilatory iron reduction is probably one of the oldest types of metabolisms that still participates in important biogeochemical cycles, such as those of carbon and sulfur. It is one of the more energetically favorable anaerobic microbial respiration processes and is usually coupled to the oxidation of organic matter. Traditionally this process is thought to be limited to the shallow part of the sedimentary column in most aquatic systems. However, iron reduction has also been observed in the methanic zone of many marine and freshwater sediments, well below its expected zone and occasionally accompanied by decreases in methane, suggesting a link between the iron and the methane cycles. Nevertheless, the mechanistic nature of this link (competition, redox or other) has yet to be established and has not been studied in oligotrophic shallow marine sediments. In this study we present combined geochemical and molecular evidences for microbial iron reduction in the methanic zone of the oligotrophic southeastern (SE) Mediterranean continental shelf. Geochemical porewater profiles indicate iron reduction in two zones, the uppermost part of the sediment, and the deeper zone, in the layer of high methane concentration. Results from a slurry incubation experiment indicate that the deep methanic iron reduction is microbially mediated. The sedimentary profiles of microbial abundance and quantitative PCR (qPCR) of the mcrA gene, together with Spearman correlation between the microbial data and Fe(II) concentrations in the porewater, suggest types of potential microorganisms that may be involved in the iron reduction via several potential pathways: H2 or organic matter oxidation, an active sulfur cycle, or iron-driven anaerobic oxidation of methane. We suggest that significant upward migration of methane in the sedimentary column and its oxidation by sulfate may fuel the microbial activity in the sulfate methane transition zone (SMTZ). The biomass created by this microbial activity can be used by the iron reducers below, in the methanic zone of the sediments of the SE Mediterranean.

The 17 O excess ( 17 ) of dissolved O 2 has been used, for over a decade, to estimate gross O 2 production (G 17 OP) rates in the mixed layer (ML) in many regions of the ocean. This estimate relies on a steady-state balance of O 2 fluxes, which include air-sea gas exchange, photosynthesis and respiration but notably, not turbulent mixing with O 2 from the thermocline. In light of recent publications, which showed that neglecting the turbulent flux of O 2 from the thermocline may lead to inaccurate G 17 OP estimations, we present a simple correction for the effect of this flux on ML G 17 OP. The correction is based on a turbulent-flux term between the thermocline and the ML, and use the difference between the ML 17 and that of a single data-point below the ML base. Using a numerical model and measured data we compared turbulence-corrected G 17 OP rates to those calculated without it, and tested the sensitivity of the GOP correction for turbulent flux of O 2 from the thermocline to several parameters. The main source of uncertainty on the correction is the eddy-diffusivity coefficient, which induces an uncertainty of ∼ 50 %. The corrected G 17 OP rates were 10-90 % lower than the previously published uncorrected rates, which implies that a large fraction of the photosynthetic O 2 in the ML is actually produced in the thermocline.Published by Copernicus Publications on behalf of the European Geosciences Union.