Early diagenesis in anaerobic lake sediments: chemical equilibria in interstitial waters

Abstract

As part of a study of early diagenesis in anaerobic lake sediments the major ions and P, Fe, Mn, pS, pϵ and pH were measured in interstitial waters of cores from eutrophic Greifensee in north-central Switzerland. A guide for the interpretation of such data is thermodynamic equilibrium between the dissolved species and mineral phases. The results show that CaCO 3, which precipitates from the lake water, dissolves rapidly in the CO 2-rich interstitial waters resulting in equilibrium with calcite in the top few centimeters of sediment. Siderite, on the other hand, is nearly ten-fold supersaturated 20–30 cm below the surface. Iron monosulfides are metastable in the surface 10–20 cm of sediment and pS and pϵ results indicate equilibrium between polysulfides and orthorhombic sulfur. Vivianite appears to be limiting the solubility of phosphate below a few centimeters of sediment. The above processes are highly interactive, especially in the case of ferrous iron, with one result being a two order of magnitude decrease in dissolved phosphate in a depth interval of 20cm in the interstitial waters.