Comparative organic geochemistry of Indian margin (Arabian Sea) sediments: estuary to continental slope

Abstract. Surface sediments from sites across the Indian margin of the Arabian Sea were analysed for their elemental and stable isotopic organic carbon (C_org) and total nitrogen compositions, grain size distributions and biochemical indices of organic matter (OM) source and/or degradation state. Site locations ranged from the estuaries of the Mandovi and Zuari rivers to depths of ~ 2000 m on the continental slope, thus spanning nearshore muds and sands on the shelf and both the oxygen minimum zone (OMZ) on the upper slope (~ 200–1300 m) and the seasonal hypoxic zone that appears on the shelf. Source indices showed mixed marine and terrigenous OM within the estuaries, but consistent predominance (80–100%) of marine OM on the shelf and slope. Thus, riverine terrigenous OM is diluted or replaced by autochthonous marine OM and/or is efficiently re-mineralised, within or immediately offshore of the estuaries. Organic C contents of surface shelf sediments varied from < 0.5 wt% in relict shelf sands to up to ~ 4 wt% for nearshore muds, while upper slope sites within the OMZ showed a wide range (~ 2 to 7 + wt%), progressively decreasing below the OMZ to ≤ 1 wt% at 2000 m. Thus, major variability (~ 5 wt%) was found at slope sites within the OMZ of similar depth and near-identical bottom-water O₂ concentrations. A strong relationship between %C_org and sediment grain size was seen for sediments within the OMZ, but lower relative C_org contents were found for sites on the shelf and below the OMZ. Further, C_org loadings, when related to estimated sediment surface area, indicated distinct enrichment of C_org in the OMZ sediments relative to sites above and below the OMZ and to sediments from normoxic margins. Diagenetic indices confirmed that lower C_org content below the OMZ is associated with more extensive OM degradation, but that shelf sediment OM is not consistently more degraded than that found within the OMZ. Together, the results indicate that OM distribution across the margin is controlled by interplay between hydrodynamic processes and varying preservation associated with O₂ availability. This inference is supported by multiple regression analysis. Hydrodynamic processes (expressed as %Silt) followed by O₂ availability, can explain the large majority of %C_org variability when the shelf and slope are considered as a whole. However, while O₂ becomes the primary influence on %C_org for sediments below the OMZ, %Silt is the primary influence across the OMZ and, apparently, the shelf. Thus, reduced O₂ exposure is responsible for OM enrichment within the OMZ, but hydrodynamic processes are the overriding control on sediment OM distributions across both the shelf and the OMZ.