Sulfidization in a shallow coastal depositional setting: Diagenetic and palaeoclimatic implications

The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, silty-clay dominant sediments. High TOC content and sedimentation rates result in high sulfate reduction rates in this region (Mazumdar et al., 2009). In the present study, we have investigated the nature of pyrite and C–Fe–S geochemistry in a sediment core (covering the last 378 years) at a water depth of 17 m off Goa, west coast, India, to understand the diagenetic and palaeoclimatic/oceanographic processes. The chromium reducible sulfur (CRS) and highly reactive iron (FeHR) profiles show significant fluctuations and a negative correlation with δ34SCRS, which is attributed to relative availability of highly reactive and less reactive iron bearing minerals close to the sediment water interface. Low δ34SCRS corresponding to high CRS content characterizes early diagenetic pyritization near the sediment-water interface, whereas high δ34SCRS and low CRS content indicate late diagenetic pyritization of less reactive iron during burial. High OBS/CRS ratios suggest the important role played by the labile organic compound in binding sulfur in the sediment. Partitioning of stable sulfur isotopes into organic and iron bound phases is apparently linked to FeHR and labile organic matter availability. We have proposed FeHR profile and FeHR/FeT ratios as a potential tool to understand runoff/monsoonal fluctuations in a shallow marine depositional setting. To develop this tool into a potential paleoclimatic/oceanographic proxy and to link it to other proxies within a chronological framework, needs high resolution sampling and age dating.