Carbon and nitrogen elemental and stable isotopic compositions of surficial sediments from the Pakistan margin of the Arabian Sea

The factors controlling sedimentary organic matter distributions are the subject of ongoing debate. We report here the carbon and nitrogen contents and stable isotopic compositions for surface sediments and downcore profiles from sites ranging in water depth from 140 to 3200 m, located across the Indus margin of the Arabian Sea. The sites spanned the intense midwater oxygen minimum zone (OMZ) and showed dramatic differences in bottom-water redox conditions, benthic communities, and degrees of bioturbation. Five primary stations were studied, at 140, 300, 940, 1200 and 1850 m water depth, with sampling in March–May (“intermonsoon”) and August–October (“late-to-postmonsoon”) 2003. High-resolution sampling was also conducted across the upper (140–275 m) and lower OMZ (600–1200 m) boundaries. The sampling strategy was selected to help clarify the roles of oxygen availability and other potential environmental factors in determining sediment organic matter content and composition, the extent of organic matter cycling within the sediments under contrasting depositional conditions, and the fate of organic matter deposited during the southwest monsoon. As previously noted on this and other OMZ margins, there was a roughly inverse spatial relationship between bottom-water oxygen and sedimentary organic matter concentrations. However, variability in organic carbon content within the OMZ that was unrelated to oxygen, and the fact that maximal organic carbon contents occurred under relatively oxidizing conditions towards the lower OMZ boundary, rather than at the OMZ core, indicate that factors other than oxygen contribute significantly as controls on carbon distributions. Despite a 3-fold range in organic carbon content, carbon-to-nitrogen ratios showed no systematic trends across the OMZ, with values reflecting uniform (predominantly marine) organic matter inputs. Nitrogen isotopic signatures (δ15N values) in the range of 6.5–8.5‰ also did not show clear cross-OMZ trends, and are attributed to a 14N-depleted nitrate pool created by pelagic denitrification. There was little downcore change in organic matter content at any site, suggesting either that organic matter decay occurs largely in the water column, or that decay within the sediments (at oxygenated sites) is masked by bioturbation. Seasonal variability in organic matter content and composition was restricted to the most surficial sediments, and changes were ephemeral, suggesting rapid turnover of organic matter deposited during the southwest monsoon, on timescales of weeks. Stable C isotopic compositions trended towards more negative values in surficial sediments at hypoxic sites within the OMZ core. This is attributed to a chemosynthetic bacterial imprint.