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Carbon dioxide cycling through the mantle and implications for the climate of ancient Earth

By
Kevin Zahnle
Kevin Zahnle
NASA Ames Research Center, Mountain View, CA 94035, USA
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Norman H. Sleep
Norman H. Sleep
Department of Geophysics, Stanford University, Stanford, CA 94305, USA
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Published:
January 01, 2002

Abstract

The continental cycle of silicate weathering and metamorphism dynamically buffers atmospheric CO2 and climate. Feedback is provided by the temperature dependence of silicate weathering. Here we argue that hydrothermal alteration of oceanic basalts also dynamically buffers CO2. The oceanic cycle is linked to the mantle via subduction of carbonatized basalts and degassing of CO2 at the mid-ocean ridges. Feedback is provided by the dependence of carbonatization on the amount of dissolved carbonate in sea water. Unlike the continental cycle, the oceanic cycle has no thermostat. Hence surface temperatures can become very low if CO2 is the only greenhouse gas apart from water. Currently the continental cycle is more important, but early in Earth’s history the oceanic cycle was probably dominant. We argue that CO2 greenhouses thick enough to defeat the faint early Sun are implausible and that, if no other greenhouse gases are invoked, very cold climates are expected for much of Proterozoic and Archaean time. We echo current fashion and favour biogenic methane as the chief supplement to CO2. Fast weathering and probable subduction of abundant impact ejecta would have reduced CO2 levels still further in Hadean time. Despite its name, the Hadean Eon might have been the coldest era in the history of the Earth.

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Contents

Geological Society, London, Special Publications

The Early Earth: Physical, Chemical and Biological Development

C. M. R. Fowler
C. M. R. Fowler
University of London, UK
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C. J. Ebinger
C. J. Ebinger
University of London, UK
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C. J. Hawkesworth
C. J. Hawkesworth
University of Bristol, UK
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Geological Society of London
Volume
199
ISBN electronic:
9781862394476
Publication date:
January 01, 2002

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