Manganese oxide reactivity in North Sea sediments

Bulk Mn oxide dissolution rates were determined in dilute acid (HCl, pH 3, proton-assisted dissolution) and in saturating ascorbic acid (pH 3, proton-assisted plus reductive dissolution) for North Sea sediments from three sites characterised by different energy regimes, one at the Frisian Front, one in the German Bight, and one in the Skagerrak. Profiles of extractable manganese (0.1 N hydrochloric acid, 1 N hydrochloric acid, citrate-dithionite-bicarbonate buffer and hydroxylamine hydrochloride in acetic acid) are presented for the three sites. The assemblage of sedimentary Mn oxides was described as a reactive continuum with a gamma distribution of Mn oxide reactivities. Initial reduction rates were highest for the oxidised sediment samples. The reactivity of the Mn assemblages during reductive dissolution in ascorbic acid decreased by up to ~9 orders of magnitude. The largest heterogeneity in Mn reactivity of the oxidised sediment samples was found at the German Bight site, while the most homogeneous Mn assemblage was found at the Skagerrak site. The large range in sedimentary Mn oxide reactivity may explain the great variability in values for kinetic coefficients used to describe Mn reduction. Considering the broad spectrum of Mn oxide reactivities, overlap of the Mn reduction zone with that of Fe oxides is expected