Manganese(II) oxidation in the suboxic zone of the Black Sea

Abstract

Measurements of manganese(II) removal from solution and oxidation were made in the suboxic zone of the Black Sea during Leg 3 of the U.S.-Turkey Black Sea Expedition in June 1988. Two types of rate measurements were conducted. First, Mn(II) removal rate measurements were made under air saturation conditions and in the presence and absence of a biological poison to determine the potential (maximal) rates of Mn(II) binding and oxidation as a result of microbial activity. Second, rates of Mn(II) removal from solution were measured under pH, O 2 and temperature conditions simulating those found in situ. By subtracting results from experiments conducted in the absence of oxygen from the oxygen-containing experiments, estimates of Mn(II) oxidation rates could be derived. Both the potential Mn(ll) removal rates and the Mn(II) oxidation rates were 1–2 orders of magnitude higher at the nearshore station (BS3-3) than at a station in the central Black Sea (BS3-6). Residence times of dissolved Mn(II) with respect to oxidation were about 0.6 days at BS3-3 and greater than 9 days at BS3-6. The absolute rates of Mn(II) oxidation in the Black Sea were 1–4 orders of magnitude faster than observed in any other marine environment. The average calculated rate constant for Mn(II) oxidation ( k Mn) was 1.4 × 10 24 M −4 day −1, 5–6 orders of magnitude faster than that calculated for autocatalytic Mn(II) oxidation on the surface of colloidal MnO 2. This extremely high rate constant coupled with the inhibition of Mn(II) removal by azide, glutaraldehyde and formaldehyde indicate that Mn(II) oxidation is biologically catalysed. Measurements conducted at BS3-3 1 week apart also demonstrated significant temporal variability in Mn(II) removal rates. Because the removal and oxidation of Mn(II) is so pronounced along the coastal margins it is possible that horizontal advective processes involving Mn(II) oxidation may have a profound influence on the biogeochemistry of other redox sensitive elements within the suboxic zone of the Black Sea and may help explain the depth distribution of particulate Mn observed in the central basin.