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

IR@NIO: CSIR-National Institute Of Oceanography, Goa


Field	Value

Creator	Cowie, G. Mowbray, S. Kurian, S. Sarkar, A. White, C. Anderson, A. Vergnaud, B. Johnstone, G. Brear, S. Woulds, C. Naqvi, S.W.A. Kitazato, H.

Date	2014-12-11T06:41:15Z 2014-12-11T06:41:15Z 2014

Identifier	Biogeosciences, vol.11; 2014; 6683-6696 http://drs.nio.org/drs/handle/2264/4663

Description	Surface sediments from sites across the Indian margin of the Arabian Sea were analysed for their elemental and stable isotopic organic carbon (C<sub>org</sub>) 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 O<sub>2</sub> concentrations. A strong relationship between %C<sub>org</sub> and sediment grain size was seen for sediments within the OMZ, but lower relative C<sub>org</sub> contents were found for sites on the shelf and below the OMZ. Further, C<sub>org</sub> loadings, when related to estimated sediment surface area, indicated distinct enrichment of C<sub>org</sub> in the OMZ sediments relative to sites above and below the OMZ and to sediments from normoxic margins. Diagenetic indices confirmed that lower C<sub>org</sub> 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 O<sub>2</sub> availability. This inference is supported by multiple regression analysis. Hydrodynamic processes (expressed as %Silt) followed by O<sub>2</sub> availability, can explain the large majority of %C<sub>org</sub> variability when the shelf and slope are considered as a whole. However, while O<sub>2</sub> becomes the primary influence on %C<sub>org</sub> for sediments below the OMZ, %Silt is the primary influence across the OMZ and, apparently, the shelf. Thus, reduced O<sub>2</sub> 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.

Language	en

Publisher	European Geosciences Union

Rights	© Author(s) 2014. CC Attribution 3.0 License (CC BY 3.0)

Subject	sediment organic carbon oxygen minimum zone stable isotope

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

Type	Journal Article