The term Paleoatmosphere (or palaeoatmosphere) refers to the atmosphere, particularly of earth, at some unspecified time in the geological past.
The composition of Earth's paleoatmosphere can be inferred today from the study of the abundance of proxy materials such as iron oxides, charcoal and the stomatal density of fossil leaves in geological deposits. Although today's atmosphere is dominated by nitrogen (about 78%) and oxygen (about 21%), the pre-biological atmosphere is thought to have been highly reducing, to have contained virtually no free oxygen, virtually no argon, which is generated by the radioactive decay of 40K, and to have been dominated by nitrogen, carbon dioxide and methane.
Appreciable concentrations of free oxygen were probably not present until after the evolution of photosynthetic bacteria, such as the cyanobacteria, or blue-green algae about 2Ga before present. By about 500 M years before present, oxygen concentrations had increased sufficiently to enable the evolution of multicellular animal life. Following the appearance, rapid evolution and radiation of land plants to cover much of the earth's land surface, beginning about 450M years ago, oxygen concentrations reached and later exceeded current values during the early Carboniferous, when atmospheric was drawn down below current concentrations.