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| Titel |
Can the carbon isotopic composition of methane be reconstructed from multi-site firn air measurements? |
| VerfasserIn |
C. J. Sapart, P. Martinerie, E. Witrant, J. Chappellaz, R. S. W. Wal, P. Sperlich, C. Veen, S. Bernard, W. T. Sturges, T. Blunier, J. Schwander, D. Etheridge, T. Röckmann |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 14 ; Nr. 13, no. 14 (2013-07-24), S.6993-7005 |
| Datensatznummer |
250018776
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| Publikation (Nr.) |
 copernicus.org/acp-13-6993-2013.pdf |
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| Zusammenfassung |
| Methane is a strong greenhouse gas and large uncertainties exist concerning
the future evolution of its atmospheric abundance. Analyzing methane
atmospheric mixing and stable isotope ratios in air trapped in polar ice
sheets helps in reconstructing the evolution of its sources and sinks in the
past. This is important to improve predictions of atmospheric CH4 mixing
ratios in the future under the influence of a changing climate. The aim of
this study is to assess whether past atmospheric δ13C(CH4)
variations can be reliably reconstructed from firn air measurements. Isotope
reconstructions obtained with a state of the art firn model from different
individual sites show unexpectedly large discrepancies and are mutually
inconsistent. We show that small changes in the diffusivity profiles at
individual sites lead to strong differences in the firn fractionation, which
can explain a large part of these discrepancies. Using slightly modified
diffusivities for some sites, and neglecting samples for which the firn
fractionation signals are strongest, a combined multi-site inversion can be
performed, which returns an isotope reconstruction that is consistent with
firn data. However, the isotope trends are lower than what has been concluded
from Southern Hemisphere (SH) archived air samples and high-accumulation ice
core data. We conclude that with the current datasets and understanding of
firn air transport, a high precision reconstruction of δ13C of
CH4 from firn air samples is not possible, because reconstructed
atmospheric trends over the last 50 yr of 0.3–1.5 ‰ are of the
same magnitude as inherent uncertainties in the method, which are the firn
fractionation correction (up to ~2 ‰ at individual sites),
the Kr isobaric interference (up to ~0.8 ‰, system
dependent), inter-laboratory calibration offsets (~0.2 ‰) and
uncertainties in past CH4 levels (~0.5 ‰). |
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