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| Titel |
Comparative organic geochemistry of Indian margin (Arabian Sea) sediments: estuary to continental slope |
| VerfasserIn |
G. Cowie, S. Mowbray, S. Kurian, A. Sarkar, C. White, A. Anderson, B. Vergnaud, G. Johnstone, S. Brear, C. Woulds, S. W. A. Naqvi, H. Kitazato |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 11, no. 23 ; Nr. 11, no. 23 (2014-12-04), S.6683-6696 |
| Datensatznummer |
250117710
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| Publikation (Nr.) |
 copernicus.org/bg-11-6683-2014.pdf |
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| Zusammenfassung |
| 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. |
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