Preliminary Testing of the Model

Abstract

We raise the hypothesis that in the glacial ocean the concentration of Mn increased to values up to 10 µMot/liter, approximately 5000 to 104 times larger than it is at present. Geochemists may find this assumption outrageous, because it is generally assumed that concentrations of Mn2+ are high only in anoxic waters, such as in the Black Sea. However, there are numerous examples of aquatic systems where a significant Mn2+ concentration was measured in the water column, oxygen concentrations being low (but 0≤ O2 ≤ 100 µM/1). In Lake Constance, for example, O2 concentrations in the water column never drop below 2 mg/l, even during periods of the summer stratification. In this lake release of manganese to the water column was shown to start at a O2 concentration of about 4 mg/1 (Ostendorp and Frevert, 1979). This was shown by direct measurement of Mn and 02 in the water column, as well as in laboratory experiments with lake sediments in regulated chambers. Bell jar experiments performed in the Kiel Bight (a fine muddy sand with Corg ≈ 0.8%) by Balzer (1982) showed release of Mn immediately after enclosure of the water mass. The Mn concentration rose at O2 concentrations larger than 6 m1/1 and without a decrease of the EH. Benthic flux chamber experiments in a Swedish fjord by Sundby et al., 1986, corroborated Balzer’s observation. Release of manganese was observed in regulated chambers even at high O2 concentrations when stirring was interrupted, suggesting that it is the supply of oxygen to the sediments (controlled by the diffusion through the diffusive boundary layer) that determines when Mn is being released. When the redoxcline in the sediments (the depth level at which all of O2 is used up for oxidation of organic matter) lies close to the water sediment interface and high concentrations of dissolved Mn occur, diffusive loss of Mn to the water column is possible. The slow velocity for the oxidation of Mn2+ to MnO2, could explain the fact that significant amounts of Mn2+ are observed in oxygenated water. As shown in laboratory experiments the oxidation rates of Mn predict a residence time in the water column on the order of hundreds of years in the absence of particulate material, weeks to months in the presence of iron oxides, and a few days by bacterial catalysis (Emerson et al. 1982).