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| Titel |
Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean Sea, France): a multi proxy study |
| VerfasserIn |
S. Bourgeois, A. M. Pruski, M.-Y. Sun, R. Buscail, F. Lantoine, P. Kerhervé, G. Vétion, B. Rivière, F. Charles |
| 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 ; 8, no. 11 ; Nr. 8, no. 11 (2011-11-02), S.3107-3125 |
| Datensatznummer |
250006189
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| Publikation (Nr.) |
 copernicus.org/bg-8-3107-2011.pdf |
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| Zusammenfassung |
The Gulf of Lions is a river-dominated ocean margin that receives high loads
of nutrients and particulate matter from the Rhône River but most
particulate materials settle rapidly on the nearshore seafloor. One question
is raised on the fate of these large quantities of organic carbon delivered
by the river to the coastal marine environment. Surface sediments (0–0.5 cm)
were collected in the Rhône prodelta and its adjacent shelf during a
period of low river discharge (April 2007, 16 stations). The sources,
distribution and lability of sedimentary organic matter were examined using
bulk (organic carbon, total nitrogen, stable carbon isotope ratios, and
grain size) and molecular-level (pigments, amino acids, fatty acids,
and δ13C of individual fatty acids) analyses. Our
results confirmed previous observations of a southwestward Rhodanian imprint
in the nearshore sediments, with 97% of terrigenous inputs of organic
matter near the river mouth. Isotopic values of bulk organic carbon, as well
as fatty acid biomarkers and compound-specific δ13C signatures
of most fatty acids clearly indicate that the Rhône inputs consist of a
mixture of organic matter (OM) from different origins with a strong
contribution from terrestrial sources (soil and plant debris), and a smaller
input from freshwater microalgae, mostly diatoms. The influence of the
Rhône River was prominent within the first ten kilometers, but may still
be observed on the outer shelf (~21 km) as indicated by the
occurrence of long chain fatty acids, which are derived from vascular
plants, and their δ13C signatures. In the proximal prodelta,
bacteria-specific fatty acids were abundant (1.65 mg g−1 OC at the
mouth site) and were relatively depleted in δ13C confirming
that bacteria mostly utilize land-derived OM. In the shelf area, the inputs
of marine OM and its predominant utilization by the bacteria was confirmed,
but the coupling between the pelagic and the benthic compartments appeared
limited at this period of the year.
Overall, degradation indexes based on amino acids (Dauwe's degradation
index) and pigments (ratio of intact chlorophyll-a to the sum of
chlorophyll-a + phaeopigment-a), as well as isotopic enrichment of
source-specific fatty acids reveal an offshore gradient of OM decay
reflecting the rapid deposition of the terrestrial material in the prodelta,
the low mixing with OM deriving from marine sources and the efficient
degradation of the OM. The OM delivered by the Rhône is relatively
labile based on the intermediary value of Dauwe's degradation index, the
high proportion of bio-available nitrogen and the occurrence of
polyunsaturated fatty acids. Deltaic sediments off the Rhône River
should thus be of sufficiently high nutritional quality to sustain dense
macrofaunal communities. |
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