Sedimentary pyrite formation in the Arabian Sea

In this study, pyrite formation in relation to environmental conditions was investigated in sediment records from the northern Arabian Sea. Considering the low bottom-water oxygen concentrations in the oxygen minimum zone (OMZ) and the high rates of primary productivity on the Oman and Pakistan Margins, sedimentary pyritic S concentrations are unexpectedly low (on average 0.4 wt%) compared to organic contents. Sediments located deeper than the OMZ exhibit a good correlation between organic carbon and reduced sulphur. Conversely, the C versus S distribution for OMZ sediments indicates a partial decoupling between organic carbon accumulation and pyrite formation. Early post-depositional sulphidisation resulted in the diagenetic sulphur enrichment of organic-poor intervals. This is evidenced by the presence of pyrite enrichments located below organic-rich intervals. Pyrite formation in the sediments within the Arabian Sea OMZ was limited by the availability of reactive iron oxides. This was caused by the low oxygen concentrations of the OMZ, which resulted in the reduction of iron oxides in the water column or at the sediment–water interface. However, on the basis of the relatively high sedimentary C/S ratios and the presence of bioturbation it is clear that no euxinic conditions developed in the OMZ in the Arabian sea during the last 120 kyr.