NON-STEADY STATE CARBONATE RECYCLING AND IMPLICATIONS FOR THE EVOLUTION OF ATMOSPHERIC PCO2

Abstract

Most treatments of the Phanerozoic evolution of the carbon dioxide content of the atmosphere (PCO2) assume a steady state closed system. Release of CO2 by mantle degassing and by biogenic precipitation of carbonates and their metamorphism in subduction zones balances the consumption by continental aluminosilicate weathering. Small perturbations in this balance bring about changes in PCO2, but given the small size of the atmospheric CO2 reservoir relative to the rate of fixation by weathering, mechanisms that maintain this apparently precarious balance dominate current thinking. At present, the Atlantic and Indian oceans are major depocenters of CaCO3, but subduction of ocean floor and the deposits on it is minimal in these basins. The locus of metamorphic regeneration of CO2 is restricted to the trenches off Central America. This is due to global asymmetries in the age of crust being subducted, in the distribution of oceanic carbonate productivity, and in the carbonate compensation depth, coupled with the poor preservation of old carbonate sediments. There is no causal relationship between the metamorphic release and weathering uptake of CO2 and subsequent deposition of carbonate on timescales shorter than a complete cycle of opening and closure of a basin. We hypothesize that the low present-day PCO2 is maintained by a time lag between: (1) mantle outgassing and metamorphic regeneration related to orogenic events in the geologic past, and (2) consumption driven by recent mountain building in the Tethyan zone and in the Western Americas. If this is true, then at the present ‘kinetic minimum’ both the terrestrial biosphere and the weathering rates are CO2 limited. Atmospheric PCO2 levels are controlled by weathering reactions only at this limit. In epochs of tectonic stability, outgassed CO2 can accumulate in the atmosphere to very high concentrations with no obvious limit. Thus, as in the past, the current ice age will persist for tens of millions of years, possibly until the closure of the Atlantic recycles the first deep carbonate depocenter since the destruction of the Tethys. A greater understanding of all these processes is required for the geochemical evolution of the Earth surface environment to be simulated.