CARBON DIOXIDE CYCLING THROUGH THE MANTLE AND IMPLICATIONS FOR THE CLIMATE OF ANCIENT EARTH

Abstract

The continental cycle of silicate weathering and metamorphism dynamically buffers atmospheric CO 2 and climate. Feedback is provided by the temperature dependence of silicate weathering. Here we argue that hydrothermal alteration of oceanic basalts also dynamically buffers CO 2. The oceanic cycle is linked to the mantle via subduction of carbonatized basalts and degassing of CO 2 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 CO 2 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 CO 2 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 CO 2. Fast weathering and probable subduction of abundant impact ejecta would have reduced CO 2 levels still further in Hadean time. Despite its name, the Hadean Eon might have been the coldest era in the history of the Earth.