The proposal by Quay et al.  that the time histories of C-13 in atmospheric CO2 and oceanic SIGMACO2 provide a constraint on the magnitude of uptake of fossil fuel CO2 by the ocean is examined. Our analysis suggests that, while the potential is there, the data base is too inaccurate to permit a distinction to be made among the carbon budgets currently on the table. Examples are given to demonstrate that the twenty or so percent uncertainties in the size of the effective, exchange reservoir and-in the magnitudes of the temporal changes in the C-13/C-12 ratio in atmospheric CO2 and ocean SIGMACO2 are just too large to permit a reliable estimate of oceanic uptake of fossil fuel CO2. We conclude that tracer-verified ocean general circulation models offer much better estimates than that based on the C-13 budget.
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