Mantle Redox Evolution and the Oxidation State of the Archean Atmosphere

Current models predict that the early atmosphere consisted mostly of$CO_{2}, N_{2}$, and$H_{2}O$, along with traces of$H_{2}$and CO. Such models are based on the assumption that the redox state of the upper mantle has not changed, so that volcanic gas composition has remained approximately constant...

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Bibliographic Details
Published in:The Journal of geology 1993-03, Vol.101 (2), p.245-257
Main Authors: Kasting, James F., Eggler, David H., Raeburn, Stuart P.
Format: Article
Language:English
Subjects:
Atmosphere
Biological Evolution
Carbon Dioxide - analysis
Carbon Monoxide - analysis
Earth
Earth (Planet)
Exobiology
Gases
Geological Phenomena
Geology
Hydrogen
Hydrogen - analysis
Iron
Iron - analysis
Mantle
Models, Theoretical
Nitrogen - analysis
Origin of Life
Outgassing
Oxidation
Oxidation-Reduction
Oxygen
Oxygen - analysis
Prehistoric era
Sea water
Space life sciences
Upper mantle
Water
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Summary:Current models predict that the early atmosphere consisted mostly of$CO_{2}, N_{2}$, and$H_{2}O$, along with traces of$H_{2}$and CO. Such models are based on the assumption that the redox state of the upper mantle has not changed, so that volcanic gas composition has remained approximately constant with time. We argue here that this assumption is probably incorrect: the upper mantle was originally more reduced than today, although not as reduced as the metal arrest level, and has become progressively more oxidized as a consequence of the release of reduced volcanic gases and the subduction of hydrated, oxidized seafloor. Data on the redox state of sulfide and chromite inclusions in diamonds imply that the process of mantle oxidation was slow, so that reduced conditions could have prevailed for as much as half of the earth's history. To be sure, other oxybarometers of ancient rocks give different results, so the question of when the mantle redox state has changed remains unresolved. Mantle redox evolution is intimately linked to the oxidation state of the primitive atmosphere: A reduced Archean atmosphere would have had a high hydrogen escape rate and should correspond to a changing mantle redox state; an oxidized Archean atmosphere should be associated with a constant mantle redox state. The converses of these statements are also true. Finally, our theory of mantle redox evolution may explain why the Archean atmosphere remained oxygen-deficient until ~2.0 billion years ago (Ga) despite a probable early origin for photosynthesis.
ISSN:0022-1376
1537-5269
DOI:10.1086/648219