Carbon exchange between the mantle and the crust, and its effect upon the atmosphere - Today compared to archean time

Estimates of the midocean ridge mantle carbon flux, whose exchange of carbon with the earth's surface has over geological time influenced atmospheric CO2, can be made by estimating concentration ratios of carbon to helium in hydrothermal fluids and tholeiitic glasses, followed by their multiplication by the oceanic primordial He-3 flux. A net carbon flux from the mantle of 1 billion moles/yr would require less than 700 million years to generate the present day crustal carbon inventory. It is deduced that the atmosphere of 3 billion years ago contained at least two orders of magnitude more CO2 than it does today.