LIMITATIONS ON LIMITATION

In the low-oxygen, low-sulfate environments of the Proterozoic, efficient burial of organic carbon would have slowed the recycling of nutrients to the photic zone, limiting global rates of photosynthesis. A series of steady-state mass balance calculations suggests that the rate of primary production in the ocean was no more than 10% of its modern value during the Proterozoic eon, and possibly less than 1%. The supply of nutrients in such a world is dominated by river inputs, rather than upwelling, leading to a small marine biosphere focused in estuarine environments. The resulting demand for nutrients is so low that nitrogen could not have limited the rate of primary production following the evolution of extant nitrogenases. Molybdenum and iron supplies are more than large enough to support the required rate of biological nitrogen fixation, even if abiotic fixation was slow and denitrification was extremely rapid. Phosphorus demand was approximately equal to the modern riverine flux, making P the most likely candidate for the limiting nutrient during the Proterozoic, and possibly during the Archean.