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| Titel |
Reply to Nicholson's comment on "Consistent calculation of aquatic gross production from oxygen triple isotope measurements" by Kaiser (2011) |
| VerfasserIn |
J. Kaiser, O. Abe |
| 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. 8 ; Nr. 9, no. 8 (2012-08-03), S.2921-2933 |
| Datensatznummer |
250007226
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| Publikation (Nr.) |
 copernicus.org/bg-9-2921-2012.pdf |
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| Zusammenfassung |
| The comment by Nicholson (2011a) questions the "consistency"
of the "definition" of the "biological end-member" used by Kaiser (2011a)
in the calculation of oxygen gross production. "Biological
end-member" refers to the relative oxygen isotope ratio difference between
photosynthetic oxygen and Air-O2 (abbreviated 17δP
and 18δP for 17O/16O and 18O/16O,
respectively). The comment claims that this leads to an overestimate of the
discrepancy between previous studies and that the resulting gross production
rates are "30% too high".
Nicholson recognises the improved accuracy of Kaiser's direct calculation
("dual-delta") method compared to previous approximate approaches based on
17O excess (17Δ) and its simplicity compared to previous iterative
calculation methods. Although he correctly points out that differences in
the normalised gross production rate (g) are largely due to different input
parameters used in Kaiser's "base case" and previous studies, he does not
acknowledge Kaiser's observation that iterative and dual-delta calculation
methods give exactly the same g for the same input parameters (disregarding
kinetic isotope fractionation during air-sea exchange). The comment is based
on misunderstandings with respect to the "base case" 17δP
and 18δP values. Since direct measurements of
17δP and 18δPdo not exist or have been
lost, Kaiser constructed the "base case" in a way that was consistent and
compatible with literature data. Nicholson showed that an alternative
reconstruction of 17δP gives g values closer to previous
studies. However, unlike Nicholson, we refrain from interpreting
either reconstruction as a benchmark for the accuracy of g.
A number of publications over the last 12 months have tried to establish
which of these two reconstructions is more accurate. Nicholson draws on
recently revised measurements of the relative 17O/16O difference
between VSMOW and Air-O2 (17δVSMOW; Barkan and Luz,
2011), together with new measurements of photosynthetic isotope
fractionation, to support his comment. However, our own measurements
disagree with these revised 17δVSMOW values. If scaled for
differences in 18δVSMOW, they are actually in good
agreement with the original data (Barkan and Luz, 2005) and support
Kaiser's "base case" g values. The statement that Kaiser's g values are "30% too high" can therefore not be accepted, pending future work to
reconcile different 17δVSMOW measurements.
Nicholson also suggests that approximated calculations of gross production
should be performed with a triple isotope excess defined as 17Δ#≡ ln (1+17δ)–λ
ln(1+18δ), with λ = θR =
ln(1+17ϵR ) / ln(1+18ϵR).
However, this only improves the approximation for certain 17δP
and 18δP values, for certain net to gross
production ratios (f) and for certain ratios of gross production to gross
Air-O2 invasion (g). In other cases, the approximated calculation based
on 17Δ† ≡17δ – κ 18δ with κ = γR =
17ϵR/18ϵR (Kaiser, 2011a)
gives more accurate results. |
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