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| Titel |
Technical note: Consistent calculation of aquatic gross production from oxygen triple isotope measurements |
| VerfasserIn |
J. Kaiser |
| 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. 7 ; Nr. 8, no. 7 (2011-07-11), S.1793-1811 |
| Datensatznummer |
250006043
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| Publikation (Nr.) |
 copernicus.org/bg-8-1793-2011.pdf |
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| Zusammenfassung |
Oxygen triple isotope measurements can be used to calculate aquatic gross
oxygen production rates. Past studies have emphasised the appropriate
definition of the 17O excess and often used an approximation to derive
production rates from the 17O excess. Here, I show that the calculation
can be phrased more consistently and without any approximations using the
relative 17O/16O and 18O/16O isotope ratio differences
(delta values) directly. I call this the "dual delta method".
The 17O excess is merely a mathematical construct and the
derived production rate is independent of its definition, provided all
calculations are performed with a consistent definition. I focus on the
mixed layer, but also show how time series of triple isotope measurements
below the mixed layer can be used to derive gross production.
In the calculation of mixed layer productivity, I explicitly include
isotopic fractionation during gas invasion and evasion, which requires the
oxygen supersaturation s to be measured as well. I also suggest how bubble
injection could be considered in the same mathematical framework. I
distinguish between concentration steady state and isotopic steady state and
show that only the latter needs to be assumed in the calculation. It is even
possible to derive an estimate of the net production rate in the mixed layer
that is independent of the assumption of concentration steady state.
I review measurements of the parameters required for the calculation of
gross production rates and show how their systematic uncertainties as well
as the use of different published calculation methods can cause large
variations in the production rates for the same underlying isotope ratios.
In particular, the 17O excess of dissolved O2 in equilibrium with
atmospheric O2 and the 17O excess of photosynthetic O2 need
to be re-measured. Because of these uncertainties, all calculation
parameters should always be fully documented and the measured relative isotope ratio
differences as well as the oxygen supersaturation should be permanently
archived, so that improved measurements of the calculation parameters can be
used to retrospectively improve production rates. |
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