ATMOSPHERIC COMPOSITION ON THE HADEAN/EARLY ARCHEAN EARTH: THE IMPORTANCE OF CO

The mantle was oxidized early, and so the early atmosphere was dominated by CO₂ and N₂, not by CH₄ and NH₃. CO₂ declined from multi-bar levels during the early Hadean to perhaps a few tenths of a bar by the mid- to late Archean. Published geochemical constraints on Archean CO₂ concentrations from paleosols are highly uncertain, and those from banded iron-formations are probably invalid. Thus, CO₂ could have been sufficiently abundant during the Archean to have provided most of the greenhouse warming needed to offset the faint young Sun. Atmospheric CH₄ concentrations increased from at most tens of parts per million (ppm) on the prebiotic Earth to hundreds of ppms once methanogens evolved. CO was an important trace gas on the prebiotic Earth because of its high free energy and its ability to catalyze key reactions involved in prebiotic synthesis. Large impacts could have made the atmosphere transiently CO-rich, and this may have played a role in the origin of life and in fueling early biological metabolisms.