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摘要
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 similar to 2000m on the continental slope, thus spanning nearshore muds and sands on the shelf and both the oxygen minimum zone (OMZ) on the upper slope (similar to 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 similar to 4 wt% for nearshore muds, while upper slope sites within the OMZ showed a wide range (similar to 2 to 7 + wt %), progressively decreasing below the OMZ to <= 1 wt% at 2000 m. Thus, major variability (similar to 5 wt %) was found at slope sites within the OMZ of similar depth and near-identical bottom-water O-2 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 O-2 availability. This inference is supported by multiple regression analysis. Hydrodynamic processes (expressed as % Silt) followed by O-2 availability, can explain the large majority of % C-org variability when the shelf and slope are considered as a whole. However, while O-2 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 O-2 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.
- 出版日期2014
