Sulphate Reduction, Organic Matter Decomposition and Pyrite Formation [and Discussion]

Laboratory, field, and theoretical studies have shown that the rate of bacterial sulphate reduction during early diagenesis depends primarily on the reactivity of sedimentary organic matter whose decomposition follows first-order kinetics, with rate constants varying over six orders of magnitude. Decay rates decrease with decreasing sediment burial rate and, for a given sediment, with depth, because of the successive utilization by bacteria of less and less reactive organic compounds. High burial (and bioturbation) rates enable reactive compounds to become available for sulphate reduction, at depth, which otherwise would be destroyed by molecular oxygen at or above the sediment--water interface. An important consequence of bacterial sulphate reduction is the formation of sedimentary pyrite, FeS$_{2}$. In normal marine sediments (those deposited in oxygenated bottom waters) pyrite formation is limited by the concentration and reactivity of organic matter, whereas in euxinic (sulphidic) basins pyrite is limited by the abundance and reactivity of detrital iron minerals, and in non-saline swamp and lake sediments by the low levels of dissolved sulphate found in fresh water. Because of these differences in limiting factors, the three environments can be distinguished in both modern sediments and ancient rocks by plots of organic carbon, C against pyrite sulphur, S. Values of the C:S ratio based on theoretical calculations indicate that worldwide the bulk of organic matter burial has shifted considerably between these environments over Phanerozoic time.