Abstract
Abstract. Surface sediments from sites across the Indian margin of the Arabian Sea were analysed for their elemental and stable isotopic organic carbon (Corg) 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 %Corg and sediment grain size was seen for sediments within the OMZ, but lower relative Corg contents were found for sites on the shelf and below the OMZ. Further, Corg loadings, when related to estimated sediment surface area, indicated distinct enrichment of Corg in the OMZ sediments relative to sites above and below the OMZ and to sediments from normoxic margins. Diagenetic indices confirmed that lower Corg 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 %Corg variability when the shelf and slope are considered as a whole. However, while O2 becomes the primary influence on %Corg 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.
Highlights
The Arabian Sea is exceptional both for monsoons that drive intense, seasonally variable productivity, and for its middepth oxygen minimum zone (OMZ, ∼ 200–1300 m)
Cowie et al.: Comparative organic geochemistry of Indian margin sediments led O2 availability to be proposed as a primary control on sediment organic matter (OM) distributions (e.g. Paropkari et al, 1992, 1993; van der Weijden et al, 1998; Keil and Cowie, 1999), and the Arabian Sea OMZ to be suggested as the modern equivalent of settings with OM-rich deposits found in the geological record (Demaison and Moore, 1980)
The cross-margin distributions are consistent with others reported previously for the Indian margin (e.g. Calvert et al, 1995), which have shown maximal concentrations on the upper slope, and, for some transects, major variability within the OMZ and, on occasion, maximal Corg values at or below the lower OMZ boundary
Summary
The Arabian Sea is exceptional both for monsoons that drive intense, seasonally variable productivity, and for its middepth oxygen minimum zone (OMZ, ∼ 200–1300 m). The ring of organic-matter-rich sediments that lines the Arabian Sea’s margins coincides roughly with the OMZ, a feature that. G. Cowie et al.: Comparative organic geochemistry of Indian margin sediments led O2 availability to be proposed as a primary control on sediment organic matter (OM) distributions Margins of the Arabian Sea differ markedly in productivity, and OMZ sediments experience differing degrees of O2 depletion and have different benthic communities and degrees of bioturbation (Cowie, 2005). Pedersen et al, 1992; Calvert et al, 1995; Cowie et al, 1999, 2009) This and other evidence has led to multiple other (interrelated) factors, including productivity, winnowing and cross-margin sediment transport, bottom topography and OM–mineral interactions to be invoked as contributing controls. The interactions and relative importance of these factors remain the subject of considerable research and debate (Cowie 2005, and references therein)
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