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 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 (>0.1 PAL). 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 100 ppmv 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.
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