EVOLUTION OF TERRESTRIAL PROTO-CO2 ATMOSPHERE COUPLED WITH THERMAL HISTORY OF THE EARTH

The effect of volatile exchange between surface reservoirs and the mantle on the evolution of proto-CO2 atmosphere on the Earth is investigated using a global carbon cycle model coupled with thermal evolution of the mantle. Carbon is assumed to circulate among five reservoirs (atmosphere, ocean, continents, seafloor and mantle) and the carbon flux of each reservoir is calculated under varying conditions, such as an increase in solar luminosity, continental growth, and a decrease in tectonic activity with time. We consider processes such as continental weathering, carbonate precipitation in the ocean, carbonate accretion to the continents, metamorphism of carbonates followed by CO2 degassing through arc volcanism, carbon regassing into the mantle, and CO2 degassing from the mantle. The degassing rate of volatiles from the mantle is assumed to be proportional to the volatile concentration in the mantle multiplied by the mantle degassing volume. The mantle degassing volume is determined by the seafloor spreading rate and the melt generation depth in the mantle. We use a parameterized convection model to calculate the thermal evolution of the mantle, from which we estimate the seafloor spreading rate and the melt generation depth in the mantle. Numerical simulations suggest that the amount of surface carbon at the present time would be in a steady state. This is because the response time of the carbon cycle system against the perturbation for surface carbon is short, being estimated at about 900 million years under the present conditions. Thus the present amount of surface carbon would not be affected by the initial amount of surface carbon. The seafloor spreading rate would have been almost constant throughout the Earth's history to explain apparent constancy of the carbon isotope ratio in the mantle after about 3.5 billion years ago. The surface carbon might probably have been circulated between surface reservoirs and the mantle once or twice after the Archean period. In any event, CO2 in the proto-atmosphere on the Earth is suggested to have decreased with the growth of continents, resulting in stabilization of the terrestrial environment against the increase in solar luminosity.