The reactivity of marine organic carbon toward oxidation has been described as a simple power-law function that decreases with carbon age and is applicable from fresh phytoplankton to nearly kerogen. We demonstrate that a correction to this law that accounts for a slowing in carbon mineralization under anoxic conditions makes the law consistent with observed carbon burial efficiencies, which are known to be higher in anoxic environments. Our analyses of data from freshwater lakes reveal that the reactivity of autochthonous organic carbon follows similar trends in freshwater and marine environments. The similarity suggests that the power law may be applicable to environments with diverse water chemistries, such as the anoxic or low-sulfate oceans of the geologic past. Using this power law, we calculate the theoretical burial efficiencies for organic carbon under a range of conditions, including those in anoxic ancient oceans. The calculations suggest that in abyssal sediments the buried fraction of deposited organic carbon may have been 6–40 times higher in Precambrian anoxic oceans than today, whereas for coastal areas the difference was much smaller. Oxygenation of the deep ocean then would have caused a large-scale decrease in organic carbon burial with a corresponding negative feedback on rates of atmospheric oxygen accumulation.