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| Titel |
Intra-annual variability of carbon and nitrogen stable isotopes in suspended organic matter in waters of the western continental shelf of India |
| VerfasserIn |
M. V. Maya, S. G. Karapurkar, H. Naik, R. Roy, D. M. Shenoy, S. W. A. Naqvi |
| 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 (2011-11-28), S.3441-3456 |
| Datensatznummer |
250006210
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| Publikation (Nr.) |
 copernicus.org/bg-8-3441-2011.pdf |
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| Zusammenfassung |
| Intra-annual variations of δ13C and δ15N of
water-column suspended particulate organic matter (SPOM) have been
investigated to understand the biogeochemical cycling of C and N in the
Western Continental Shelf of India (WCSI). The key issues being addressed
are: how the δ15N of SPOM is affected by seasonally varying
processes of organic matter production and respiration and how it relates to
the δ15N of sedimentary organic matter that appears to show a
decreasing trend despite an apparent intensification of seasonal oxygen
deficiency over the past few decades? A secondary objective was to evaluate
the sources of organic carbon. Elemental carbon and nitrogen concentrations,
C/N ratios in SPOM, along with ancillary chemical and biological variables
including phytoplankton pigment abundance were also determined on a seasonal
basis (from March 2007 to September 2008), with the partial exception of the
southwest (SW) monsoon period. The results reveal significant shifts in
isotopic signatures, especially δ15N, of SPOM before and after
the onset of SW monsoon. Very low δ15N values, reaching a
minimum of −4.17 ‰, are found during the pre-monsoon period. Our results
provide the first direct evidence for the addition of substantial amounts of
isotopically light nitrogen by the diazotrophs, especially Trichodesmium, in the region.
The δ15N of SPOM is generally lower than the mean value (7.38 ‰)
for surficial sediments, presumably because of diagenetic enrichment. The
results support the view that sedimentary δ15N may not
necessarily reflect denitrification intensity in the overlying waters due to
diverse sources of nitrogen and variability of its isotopic composition. The
observed intra-annual variability of δ13C of SPOM during the
pre-monsoon and post-monsoon periods is generally small. Phytoplankton
production and probably species composition could drive some of the observed
changes. The largest changes (depletion of δ13C and increase in
C/N) appear to occur during the pre- and post-monsoon seasons, presumably
through episodic deposition of terrestrial organic matter from the
atmosphere. During the SW monsoon, when a large input of terrestrial organic
matter is expected through runoff from land, the C/N ratio remains low, but
significant difference is observed between δ13C data in 2007
and 2008. Inputs of soil organic matter that may have elemental and isotopic
signatures different from those of the conventional (C3 plant derived)
organic matter could explain the constancy of the C/N ratio. |
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