Organic matter preservation and sulfur uptake in sediments from the continental margin off Pakistan

The bulk and molecular geochemical characteristics of a suite of kerogens isolated from organic matter-rich sediments from the Pakistan margin has been characterised by elemental analysis, Rock-Eval pyrolysis and pyrolysis-gas chromatography (Py-GC). Special attention was given to the geochemistry of alkylthiophenes which are thought to reflect the incorporation of sulfur into organic matter (OM) in the context of early diagenetic sulfate reduction. Organically bound sulfur comprises up to 60% of total sedimentary sulfur. The weight ratios for organic sulfur (S org) over organic carbon (C org) varies from 0.01 in some of the recently deposited sediments to 0.11 in older samples. The results show that sulfur incorporation into organic matter occurs primarily within the upper few meters of the sediments. The amount of sulfur which can be incorporated into the organic matter clearly depends on the quality of OM. The hydrogen index values [HI; which characterise the total amount of generated hydrocarbon equivalents of the pyrolysate (in mg HC equiv./g C org)] are related to the extent of reworking of organic matter either in the water column or at the sediment-water interface or to remineralization processes by sulfate reduction itself which is strongly controlled by the availability of reactive iron. High quality organic matter characterised by elevated HI values is capable of sequestering higher amounts of sulfur [as expressed by the thiophene content, which represent the organically bound sulfur of the pyrolysate (in mg/g C org)]. As indicated by the ratio of thiophenes versus HI, which characterises the proportion of thiophenes in the total pyrolysate, sulfurisation of organic matter seems to be terminated at a ratio of 0.01.