Paleoatmosphere

A paleoatmosphere (or palaeoatmosphere) is an atmosphere, particularly that 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 and charcoal and the fossil data, such as 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^{[citation needed]} 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 (Myr). After the Great Oxidation Event, quantities of oxygen produced as a by-product of photosynthesis by cyanobacteria (sometimes erroneously referred to as 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 (about 21%) during the early Carboniferous, when atmospheric carbon dioxide was drawn down below current concentrations (about 400 ppm) by oxygenic photosynthesis.^[1][2][3] This may have contributed to the Carboniferous rainforest collapse during the Moscovian and Kasimovian ages of the Pennsylvanian subperiod.