Early diagenesis and sulphur speciation in sediments of the Oman Margin, northwestern Arabian Sea

The pore-waters from two boxcores from the Oman Margin, from sites NIOP484 and NIOP487 (respectively in and below the oxygen minimum zone in the water column), were analysed for concentrations of iodide, sulphide, thiosulphate, phosphate, silica, ammonia, nitrate, nitrite, dissolved iron, dissolved manganese and dissolved sulphur. In addition, the solid phase of samples from the same cores was analysed for organic carbon, pyritic sulphur, acid volatile sulphide, elemental sulphur, organic polysulphides, humic sulphur, reactive iron phases and reactive manganese phases. Pore-water profiles show that post-oxic diagenesis dominates in these Holocene sediments. Only minor amounts of reduced sulphur species were found in pore-waters, despite the low oxygen concentration of the bottom-water and the relatively high organic-matter content of the sediments. Significant amounts of solid-phase reduced sulphur were only found in the lower half of the boxcore at station NIOP484. Consequently, sulphate reduction is not as significant in these sediments relative to sediments from other upwelling areas, and thus more sedimentary organic matter is preserved.