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| Titel |
Quantifying drivers of chemical disequilibrium: theory and application to methane in the Earth's atmosphere |
| VerfasserIn |
E. Simoncini, N. Virgo, A. Kleidon |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
2190-4979
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| Digitales Dokument |
URL |
| Erschienen |
In: Earth System Dynamics ; 4, no. 2 ; Nr. 4, no. 2 (2013-09-11), S.317-331 |
| Datensatznummer |
250084954
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| Publikation (Nr.) |
 copernicus.org/esd-4-317-2013.pdf |
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| Zusammenfassung |
| It has long been observed that Earth's atmosphere is uniquely far from its
thermochemical equilibrium state in terms of its chemical composition.
Studying this state of disequilibrium is important both for understanding the
role that life plays in the Earth system, and for its potential role in the
detection of life on exoplanets. Here we present a methodology for assessing
the strength of the biogeochemical cycling processes that drive
disequilibrium in planetary atmospheres. We apply it to the simultaneous
presence of CH4 and O2 in Earth's atmosphere, which has long been
suggested as a sign of life that could be detected remotely. Using a
simplified model, we identify that the most important property to quantify is
not the distance from equilibrium, but the power required to drive it. A weak
driving force can maintain a high degree of disequilibrium if the residence
times of the compounds involved are long; but if the disequilibrium is high
and the kinetics fast, we can conclude that the disequilibrium must be driven
by a substantial source of energy. Applying this to Earth's atmosphere, we
show that the biotically generated portion of the power required to maintain
the methane–oxygen disequilibrium is around 0.67 TW, although the
uncertainty in this figure is about 10% due to uncertainty in the global
CH4 production. Compared to the chemical energy generated by the biota
by photosynthesis, 0.67 TW represents only a very small fraction and,
perhaps surprisingly, is of a comparable magnitude to abiotically driven
geochemical processes at the Earth's surface. We discuss the implications of
this new approach, both in terms of enhancing our understanding of the Earth
system, and in terms of its impact on the possible detection of distant
photosynthetic biospheres. |
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