The constancy of the 13C/12C ratio in marine carbonate over the last half-billion years provides evidence that an amount of organic carbon comparable to that presantly contained in the earth's sedimentary reservoir must have existed in early Paleozoic time. Since the amount of organic carbon is a direct measure of the net amount of oxygen produced by photosynthesis, this in turn implies no great change in available oxygen over that time period. Finally, unless the distribution of oxygen between the bound and molecular form has undergone a large change, the oxygen content of the atmosphere must have been comparable to its present value in early Paleozoic time.
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