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| Titel |
The transformation of macromolecular organic matter within and below the oxygen minimum zone in the Arabian Sea. |
| VerfasserIn |
Klaas G. J. Nierop, Gert-Jan Reichart, Jaap S. Sinninghe Damsté |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053626
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| Zusammenfassung |
| The Arabian Sea exhibits an extensive oxygen minimum zone (OMZ) which results among
others in a high accumulation of organic carbon in sediments that intersect the OMZ. Below
the OMZ, the decrease of organic carbon concentrations occurs already to a large extent,
while bottom water oxygen concentrations are still fairly small (15 micromoles O2/kg). To
what extent the organic matter composition, and hence it degradability, arriving at the surface
of the sediments affects the organic carbon concentrations within the sediments is
still a major question in our understanding of organic carbon biogeochemistry and
preservation.
Here we investigate relative changes in the composition of the macromolecular (residue
after solvent extraction) organic matter in three sediment cores. Cores were recovered from
(1) the heart of the OMZ, (2) from the deeper part of the OMZ where bottom and pore
water oxygen availability is slightly higher than in the upper part of the OMZ, and
(3) from sediments below the OMZ where normal oxygen concentrations prevail.
The early stages (upper 20 cm) of organic matter diagenesis and preservation was
studied by analytical pyrolysis. Six different groups of main pyrolysis products
were distinguished: 1) polysaccharide-derived compounds, 2) alkylbenzenes, 3)
alkylphenols, 4) alkylpyrroles, 5) n-alkenes/n-alkanes, and 6) pristene/phytadienes.
Surface sediments showed relatively similar, highly degraded organic compositions
irrespective of bottom water O2 concentrations, implying that major decomposition took
place within the water column. With depth, the relative amounts of alkylbenzenes
increased, being more pronounced in the sediment below the OMZ. All other groups of
pyrolysis products decreased relatively, apart from the alkylpyrroles within the
OMZ. In this sediment alkylpyrroles consisted largely of products derived from
tetraalkylpyrrole pigments. Below the OMZ, markers of tetraalkylpyrrole-containing
compounds were relatively less abundant and decreased even more with depth, while
alkylpyrroles derived from proteins were still present. These data suggest that the bulk
organic matter, although degraded already, is diagenetically altered, at the one hand
in the same direction, but on the other hand at different rates (more pronounced
in the sediment below the OMZ where oxygen penetration extends deeper into
the sediment) and divergent with respect to tetraalkylpyrrole-containing pigments. |
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