Evolution of terrestrial proto -CO 2 atmosphere coupled with thermal history of the earth

The effect of volatde exchange between surface reservoirs and the mantle on the evolution of proto-CO 2 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 m solar luminosity, continental growth, and a decrease m tectomc activity w~th time. We consider processes such as continental weathering, carbonate precxp~tation in the ocean, carbonate accretion to the continents, metamorphism of carbonates followed by CO 2 degassmg through arc volcanism, carbon regassmg into the mantle, and CO 2 degasslng from the mantle. The degasslng rate of volatiles from the mantle is assumed to be proportional to the volatile concentration m the mantle multlphed by the mantle degassing volume The mantle degassmg volume is determmed by the seafloor spreading rate and the melt generation depth in the mantle. We use a parametenzed convection model to calculate the thermal evolution of the mantle, from which we estimate the seafloor spreading rate and the melt generation depth m the mantle. Numerical simulations suggest that the amount of surface carbon at the present time would be m a steady state This is because the response t~me of the carbon cycle system against the perturbation for surface carbon is short, being estimated at about 900 mdhon years under the present conditions Thus the present amount of surface carbon would not be affected by the mitml 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 m the mantle after about 3.5 bilhon years ago. The surface carbon m~ght probably have been circulated between surface reservoirs and the mantle once or twice after the Archean period In any event, CO 2 m the proto-atmosphere on the Earth ~s suggested to have decreased w~th the growth of continents, resulting m stablhzat~on of the terrestrial environment against the mcrease m solar luminosity