Absorbed Mn<SUP>2+</SUP> and Mn redox cycling in Iberian continental margin sediments (northeast Atlantic Ocean)

Abstract

Although Mn 2+ sorption has been investigated extensively in the laboratory, the role of Mn 2+ sorption in natural marine sediments remains speculative. Our objectives were to study (1) the role of Mn 2+ 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 Mn 2+ , adsorbed Mn 2+ 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 Nazere Canyon. In the profiles, peaks of adsorbed Mn 2 were observed in-between those of solid phase Mn and pore water Mn 2+ , We propose that upon Mn reduction, the produced Mn 2+ is adsorbed onto adjacent Mn oxide or oxyhydroxide surfaces. Available adsorption-sites diminish and/or saturate as Mn reduction continues, upon which Mn 2+ 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 Mn 2+ 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 Mn 2+ desorption and oxidation rates range between 1 and 35 pmoles 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 Mn 2+ is an important redox intermediate between Mn oxide and pore water Mn 2+ in the reduced sediment layer. Adsorption of Mn 2+ restrains the efflux of dissolved Mn 2+ into the water column, by lowering the pore water gradient at stations with a thin oxidation zone. There, adsorbed Mn 2+ enhances the retention of Mn 2+ in the sediment column.