Early diagenetic alteration of organic matter by sulfate reduction in Quaternary sediments from the northeastern Arabian Sea

Abstract

Hemipelagic sediments from the northeastern Arabian Sea off Pakistan were analyzed by geochemical and microscopical methods to study the importance of bacterial degradation processes on the quantity and composition of sedimentary organic matter (OM). On the basis of elemental analysis of total organic carbon (TOC) and total sulfur (TS), the decay of OM by sulfate-reducing bacteria were calculated. Concentrations of total iron, reactive iron, total nitrogen (TN), total phosphorus (TP) and hydrogen indices (HI) were used to assess organic matter reactivity and redox conditions. Stable sulfur isotopic compositions provided further information about pathway, timing and mechanisms of sulfate reduction. Microscopical observations indicate that throughout the study area, OM is amorphous (75–95%) in almost all samples with only a small contribution of terrigenous OM. Bacterially mediated sulfate reduction leads to continuously decreasing TOC/TS ratios from 0.4 m to a burial depth of 2–3 m. At this depth, the anaerobic oxidation of particulate organic matter ceases. Sulfate reduction in this narrow interval accounts for the decay of up to 70% of the OM which has primarily entered the anoxic domain. During the bacterially mediated diagenesis preferential consumption of nitrogen and phosphorus containing organic compounds can be observed and quantified. The regeneration of up to 60% of TN and more than 50% of TP can lead to a feedback of nutrients to the seawater. This recycling process may contribute to the high productivity in the northeastern Arabian Sea.