Greenhouse warming by nitrous oxide and methane in the Proterozoic Eon

Abstract

An anoxic, sulfidic ocean that may have existed during the Proterozoic Eon (0.54–2.4 Ga) 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 N_2O to N_2. Today, only about 5–6% of denitrification results in release of N_2O. If all denitrification stopped at N_2O during the Proterozoic, the N2O flux could have been 15–20 times higher than today, producing N_2O concentrations of several ppmv, but only if O_2 levels were relatively high (>0.1 PAL). At lower O_2 levels, N_2O is rapidly photodissociated. Methane concentrations may also have been elevated during this time, as has been previously suggested. A lack of dissolved O_2 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 CH_4 increases as more CH_4 is added to the atmosphere, so CH_4 concentrations of up to 100 ppmv are possible during this time. The combined greenhouse effect of CH_4 and N_2O could have provided up to 10° of warming, thereby keeping the surface warm during the Proterozoic without necessitating high CO_2 levels. A second oxygenation event near the end of the Proterozoic would have resulted in a reduction in both atmospheric N_2O and CH_4, perhaps triggering the Neoproterozoic "Snowball Earth" glaciations.

Additional details