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%), oxygen (about 21%), and argon (about 1%), the pre-biological atmosphere is thought to have been a highly reducing atmosphere, having 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 about 2,500 million years ago (Ma). After the Great Oxygenation Event, quantities of oxygen produced as a by-product of photosynthesis by cyanobacteria or blue-green algae began to exceed the quantities of chemically reducing materials, notably dissolved iron. By the beginning of the Cambrian period 541 Ma, free oxygen concentrations had increased sufficiently to enable the evolution of multicellular organisms. Following the subsequent appearance, rapid evolution and radiation of land plants, which covered much of the Earth's land surface, beginning about 450 Ma, oxygen concentrations reached and later exceeded current values during the early Carboniferous, when atmospheric carbon dioxide was drawn down below current concentrations. This may have contributed to the Carboniferous Rainforest Collapse during the Moscovian and Kasimovian ages of the Pennsylvanian subperiod.
Geological studies of ancient rock formations can give information on paleoatmospheric composition, pressure, density, etc. at specific points in Earth's history.
A 2012 study looked at the imprints made by falling raindrops onto freshly deposited volcanic ash, laid down in the Archean Eon 2,700 Ma in the Ventersdorp Supergroup, South Africa. They linked the terminal velocity of the raindrops directly to the air density of the paleoatmosphere and showed that it had less than twice the density of the modern atmosphere, and likely had similar if not lower density.
A similar study in 2016 looked at the size distribution of gas bubbles in basaltic lava flows that solidified at sea level also during the Archean (~2,700 Ma). They found an atmospheric pressure of only 0.23 ± 0.23 bar (23 kPa).
Both results contradict theories that suggest the Archean was kept warm during the Faint Young Sun period by extremely high levels of carbon dioxide or nitrogen.
A 2016 study performed mass spectrometry on air bubbles trapped inside rock salt deposited 813 Myr ago. They detected an oxygen content of 10.9%, much higher than had been expected from indirect measures. This suggested the Great Oxygenation Event may have happened much earlier than previously thought.