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| Titel |
Interaction between hydrocarbon seepage, chemosynthetic communities, and bottom water redox at cold seeps of the Makran accretionary prism: insights from habitat-specific pore water sampling and modeling |
| VerfasserIn |
D. Fischer, H. Sahling, K. Nöthen, G. Bohrmann, M. Zabel, S. Kasten |
| 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 ; 9, no. 6 ; Nr. 9, no. 6 (2012-06-07), S.2013-2031 |
| Datensatznummer |
250007115
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| Publikation (Nr.) |
 copernicus.org/bg-9-2013-2012.pdf |
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| Zusammenfassung |
The interaction between fluid seepage, bottom water redox, and
chemosynthetic communities was studied at cold seeps across one of the
world's largest oxygen minimum zones (OMZ) located at the Makran convergent
continental margin. Push cores were obtained from seeps within and below the
core-OMZ with a remotely operated vehicle. Extracted sediment pore water was
analyzed for sulfide and sulfate concentrations. Depending on oxygen availability
in the bottom water, seeps were either colonized by microbial mats or by
mats and macrofauna. The latter, including ampharetid polychaetes and
vesicomyid clams, occurred in distinct benthic habitats, which were arranged
in a concentric fashion around gas orifices. At most sites colonized by
microbial mats, hydrogen sulfide was exported into the bottom water. Where
macrofauna was widely abundant, hydrogen sulfide was retained within the
sediment.
Numerical modeling of pore water profiles was performed in order to assess
rates of fluid advection and bioirrigation. While the magnitude of upward
fluid flow decreased from 11 cm yr−1 to <1 cm yr−1 and the
sulfate/methane transition (SMT) deepened with increasing distance from the
central gas orifice, the fluxes of sulfate into the SMT did not
significantly differ (6.6–9.3 mol m−2 yr−1). Depth-integrated
rates of bioirrigation increased from 120 cm yr−1 in the central
habitat, characterized by microbial mats and sparse macrofauna, to 297 cm yr−1
in the habitat of large and few small vesicomyid clams. These
results reveal that chemosynthetic macrofauna inhabiting the outer seep
habitats below the core-OMZ efficiently bioirrigate and thus transport
sulfate down into the upper 10 to 15 cm of the sediment. In this way the animals
deal with the lower upward flux of methane in outer habitats by stimulating
rates of anaerobic oxidation of methane (AOM) with sulfate high enough to
provide hydrogen sulfide for chemosynthesis. Through bioirrigation,
macrofauna engineer their geochemical environment and fuel upward sulfide
flux via AOM. Furthermore, due to the introduction of oxygenated bottom
water into the sediment via bioirrigation, the depth of the sulfide sink
gradually deepens towards outer habitats. We therefore suggest that – in
addition to the oxygen levels in the water column, which determine whether
macrofaunal communities can develop or not – it is the depth of the SMT and
thus of sulfide production that determines which chemosynthetic communities
are able to exploit the sulfide at depth. We hypothesize that large
vesicomyid clams, by efficiently expanding the sulfate zone down into the sediment,
could cut off smaller or less mobile organisms, as e.g. small clams and
sulfur bacteria, from the sulfide source. |
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