Carbon dioxide cycling through the mantle and implications for the climate of ancient Earth
Published:January 01, 2002
Kevin Zahnle, Norman H. Sleep, 2002. "Carbon dioxide cycling through the mantle and implications for the climate of ancient Earth", The Early Earth: Physical, Chemical and Biological Development, C. M. R. Fowler, C. J. Ebinger, C. J. Hawkesworth
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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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The Early Earth: Physical, Chemical and Biological Development
This volume contains a series of papers that cover a wide range of aspects, including geophysics, structure and tectonics, atmosphere, origin of life, biosphere, deep mantle geochemistry, early oceans, microbial ecology, on the development of the Earth in the first 2000 Ma of its history.The aim of this publication is to facilitate future discussions and understanding of this area of research.
This book is divided into three parts:
Geophysical and petrological constraints on Archaean lithosphere
Models of cratonic evolution and modification
Constraints on the Archaean environment
Subjects covered include the chemical and biological controls on the atmosphere and oceans, early controls on the carbon cycle and photosynthesis, petrologic, isotopic, tectonic and seismic evidence for the composition and structure of Archaean lithosphere.
This volume should be of interest to geologists and geophysicists who work on the Archaean, and students at all levels.