Greenhouse warming by nitrous oxide and methane in the Proterozoic Eon

An anoxic, sulfidic ocean that may have existed during the Proterozoic Eon (0.54-2.4Ga) would have had limited trace metal abundances because of the low solubility of metal sulfides. The lack of copper, in particular, could have had a significant impact on marine denitrification. Copper is needed for the enzyme that controls the final step of denitrification, from N₂O to N₂. Today, only about 5-6% of denitrification results in release of N₂O. If all denitrification stopped at N₂O during the Proterozoic, the N₂O flux could have been 15-20 times higher than today, producing N₂O concentrations of several ppmv, but only if O₂ levels were relatively high (>0.1PAL). At lower O₂ levels, N₂O is rapidly photodissociated. Methane concentrations may also have been elevated during this time, as has been previously suggested. A lack of dissolved O₂ and sulfate in the deep ocean could have produced a high methane flux from marine sediments, as much as 10-20 times today's methane flux from land. The photochemical lifetime of CH₄ increases as more CH₄ is added to the atmosphere, so CH₄ concentrations of up to 100ppmv are possible during this time. The combined greenhouse effect of CH₄ and N₂O could have provided up to 10° of warming, thereby keeping the surface warm during the Proterozoic without necessitating high CO₂ levels. A second oxygenation event near the end of the Proterozoic would have resulted in a reduction in both atmospheric N₂O and CH₄, perhaps triggering the Neoproterozoic "Snowball Earth" glaciations.