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| Titel |
Isotopic constraints on the pre-industrial oceanic nitrogen budget |
| VerfasserIn |
C. J. Somes, A. Oschlies, A. Schmittner |
| 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 ; 10, no. 9 ; Nr. 10, no. 9 (2013-09-06), S.5889-5910 |
| Datensatznummer |
250085325
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| Publikation (Nr.) |
 copernicus.org/bg-10-5889-2013.pdf |
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| Zusammenfassung |
| The size of the bioavailable (i.e., "fixed") nitrogen inventory in the ocean
influences global marine productivity and the biological carbon pump.
Despite its importance, the pre-industrial rates for the major source and
sink terms of the oceanic fixed nitrogen budget, N2 fixation and
denitrification, respectively, are not well known. These processes leave
distinguishable imprints on the ratio of stable nitrogen isotopes, δ15N, which can therefore help to infer their patterns and rates. Here
we use δ15N observations from the water column and a new
database of seafloor measurements to constrain rates of N2 fixation and
denitrification predicted by a global three-dimensional Model of Ocean
Biogeochemistry and Isotopes (MOBI). Sensitivity experiments were performed
to quantify uncertainties associated with the isotope effect of
denitrification in the water column and sediments. They show that the level
of nitrate utilization in suboxic zones, that is the balance between nitrate
consumption by denitrification and nitrate replenishment by circulation and
mixing (dilution effect), significantly affects the isotope effect of water
column denitrification and thus global mean δ15NO3–.
Experiments with lower levels of nitrate utilization within the suboxic zone
(i.e., higher residual water column nitrate concentrations, ranging from
20 to 32 μM) require higher ratios of benthic to water column
denitrification, BD : WCD = 0.75–1.4, to satisfy the global mean
NO3– and δ15NO3– constraints in the modern
ocean. This suggests that nitrate utilization in suboxic zones plays an
important role in global nitrogen isotope cycling. Increasing the net
fractionation factor ϵBD for benthic denitrification
(ϵBD = 0–4‰) requires even higher
ratios, BD : WCD = 1.4–3.5. The model experiments that best
reproduce observed seafloor δ15N support the middle to high-end
estimates for the net fractionation factor of benthic denitrification
(ϵBD = 2–4‰). Assuming a balanced
fixed nitrogen budget, we estimate that pre-industrial rates of N2
fixation, water column denitrification, and benthic denitrification were
between 195–350 (225), 65–80 (76), and 130–270 (149) Tg N yr−1,
respectively, with our best model estimate (ϵBD = 2‰) in parentheses. Although uncertainties still exist,
these results suggest that marine N2 fixation is occurring at much
greater rates than previously estimated and the residence time for oceanic
fixed nitrogen is between ~ 1500 and 3000 yr. |
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