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| Titel |
Marine hypoxia/anoxia as a source of CH4 and N2O |
| VerfasserIn |
S. W. A. Naqvi, H. W. Bange, L. Farías, P. M. S. Monteiro, M. I. Scranton, J. Zhang |
| 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 ; 7, no. 7 ; Nr. 7, no. 7 (2010-07-12), S.2159-2190 |
| Datensatznummer |
250004895
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| Publikation (Nr.) |
 copernicus.org/bg-7-2159-2010.pdf |
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| Zusammenfassung |
| We review here the available information on methane (CH4) and nitrous
oxide (N2O) from major marine, mostly coastal, oxygen
(O2)-deficient zones formed both naturally and as a result of human
activities (mainly eutrophication). Concentrations of both gases in
subsurface waters are affected by ambient O2 levels to varying degrees.
Organic matter supply to seafloor appears to be the primary factor
controlling CH4 production in sediments and its supply to (and
concentration in) overlying waters, with bottom-water O2-deficiency
exerting only a modulating effect. High (micromolar level) CH4
accumulation occurs in anoxic (sulphidic) waters of silled basins, such as
the Black Sea and Cariaco Basin, and over the highly productive Namibian
shelf. In other regions experiencing various degrees of O2-deficiency
(hypoxia to anoxia), CH4 concentrations vary from a few to hundreds of
nanomolar levels. Since coastal O2-deficient zones are generally very
productive and are sometimes located close to river mouths and submarine
hydrocarbon seeps, it is difficult to differentiate any
O2-deficiency-induced enhancement from in situ production of CH4
in the water column and its inputs through freshwater runoff or seepage from
sediments. While the role of bottom-water O2-deficiency in CH4
formation appears to be secondary, even when CH4 accumulates in
O2-deficient subsurface waters, methanotrophic activity severely
restricts its diffusive efflux to the atmosphere. As a result, an
intensification or expansion of coastal O2-deficient zones will
probably not drastically change the present status where emission from the
ocean as a whole forms an insignificant term in the atmospheric CH4
budget. The situation is different for N2O, the production of which is
greatly enhanced in low-O2 waters, and although it is lost through
denitrification in most suboxic and anoxic environments, the peripheries of
such environments offer most suitable conditions for its production, with
the exception of enclosed anoxic basins. Most O2-deficient systems
serve as strong net sources of N2O to the atmosphere. This is
especially true for coastal upwelling regions with shallow O2-deficient
zones where a dramatic increase in N2O production often occurs in
rapidly denitrifying waters. Nitrous oxide emissions from these zones are
globally significant, and so their ongoing intensification and expansion is
likely to lead to a significant increase in N2O emission from the
ocean. However, a meaningful quantitative prediction of this increase is not
possible at present because of continuing uncertainties concerning the
formative pathways to N2O as well as insufficient data from key coastal
regions. |
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