Abstract

Although Mn2+ sorption has been investigated extensively in the laboratory, the role of Mn2+ sorption in natural marine sediments remains speculative. Our objectives were to study (1) the role of Mn2+ sorption in the redox cycling of Mn, (2) to quantify Mn cycling and (3) to identify its rate-determining factors at the Iberian margin. Profiles of pore water Mn2+, adsorbed Mn2+ and solid phase Mn were measured together with benthic oxygen fluxes along three transects across the margin from the shelf to the deep sea as well as in the Nazeré Canyon. In the profiles, peaks of adsorbed Mn2+ were observed in-between those of solid phase Mn and pore water Mn2+. We propose that upon Mn reduction, the produced Mn2+ is adsorbed onto adjacent Mn oxide or oxyhydroxide surfaces. Available adsorption-sites diminish and/or saturate as Mn reduction continues, upon which Mn2+ is released into the pore water. Mn redox chemistry is controlled by the organic carbon flux to the sediment. A simple steady state model was formulated that includes Mn2+ sorption as a combination of an instantaneous reversible equilibrium process and a first-order kinetic reaction. Model derived, depth integrated rates of Mn reduction as well as Mn2+ desorption and oxidation rates range between 1 and 35 µmoles m-2 d-1. Mn cycling is most intense at moderate carbon fluxes. Moreover, Mn cycling is enhanced at higher deposition fluxes of Mn oxide in the canyon. Budgets based on the model indicate that adsorbed Mn2+ is an important redox intermediate between Mn oxide and pore water Mn2+ in the reduced sediment layer. Adsorption of Mn2+ restrains the efflux of dissolved Mn2+ into the water column, by lowering the pore water gradient at stations with a thin oxidation zone. There, adsorbed Mn2+ enhances the retention of Mn2+ in the sediment column.

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