Abundant Porewater Mn(III) Is a Major Component of the Sedimentary Redox System

Mn at Work

Biogeochemical cycling involves oxidation-reduction reactions with common metals like iron and manganese. Depending on the oxygen concentration and pH of the surrounding water, these metals—which exist in multiple oxidation states—can either accept or donate electrons; however, detecting which chemical species is dominant in these reactions often proves difficult. Using a spectrophotometric method sensitive to Mn oxidation state, Madison et al. (p. [Related article:]875 ) show that up to 90% of the total Mn in porewater collected from sediment cores in the St. Lawrence Estuary is soluble Mn(III)—a phase traditionally assumed to be unimportant in aqueous redox reactions except in a few niche environments.

Abstract

Soluble manganese(III) accounts for up to 90% of the total manganese in the near-surface porewaters of hemipelagic sediments.

Abstract

Soluble manganese(III) [Mn(III)] can potentially serve as both oxidant and reductant in one-electron-transfer reactions with other redox species. In near-surface sediment porewater, it is often overlooked as a major component of Mn cycling. Applying a spectrophotometric kinetic method to hemipelagic sediments from the Laurentian Trough (Quebec, Canada), we found that soluble Mn(III), likely stabilized by organic or inorganic ligands, accounts for up to 90% of the total dissolved Mn pool. Vertical profiles of dissolved oxygen and dissolved and solid Mn suggest that soluble Mn(III) is primarily produced via oxidation of Mn(II) diffusing upwards from anoxic sediments with lesser contributions from biotic and abiotic reductive dissolution of MnO _2. The conceptual model of the sedimentary redox cycle should therefore explicitly include dissolved Mn(III).