Kinetic and thermodynamic factors controlling manganese concentrations in oceanic waters

Abstract Data are presented describing the changes in the distribution of dissolved and particulate Mn observed over a 16-month period in the periodically anoxic waters of Saanich Inlet, a fjord located on the coast of Vancouver Island, British Columbia. During the spring and summer when the bottom waters were anoxic, a dense cloud of particulate Mn was found at mid-depths where Mn 2+ enriched anoxic bottom waters were mixing with oxygenated waters; then, during the autumn aand winter following an intrusion which reoxygenated the bottom water, an intense precipitation of Mn was observed throughout the entire water column. During this latter period, dissolved Mn concentrations in the bottom water, which exceeded 1000 nmol/l under anoxic conditions, decreased towards a lower limit of 1.6 nmole/l, a value comparable to that observed in Pacific Ocean waters of similar pH and dissolved oxygen content. Mn in the particulate matter collected just above the oxic-anoxic interface was found to have an average oxidation number of +3.05; and, on this basis, it is proposed that dissolved Mn concentrations in oceanic waters are controlled by the precipitation of the metastable oxide mineral manganite (γ-MnOOH), a hypothesis consistent with the fact that dissolved Mn values in subsurface Pacific Ocean waters closely approach the equilibrium solubility of this phase. Temporal and spatial gradients in the particulate Mn distribution were used to calculate the in situ rate of Mn precipitation, and the results of these calculations then were fitted to theoretical rate equations which suggest that the precipitation of Mn is controlled by 2 parallel processes: bacterial oxidation and an inorganic autocatalytic oxidation reaction.