Paleoatmosphere

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 studyof 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 chem|CO|2 was drawn down below current concentrations.RA Berner (1998) The carbon cycle and chem|CO|2 over Phanerozoic time: the role of land plants. Philosophical Transactions of the Royal Society 353, 75-82] RA Berner (1997) The rise of plants: their effect on weathering and atmospheric chem|CO|2. Science, 276, 544-546.] DJ Beerling and RA Berner (2005) Feedbacks and the coevolution of plants and atmospheric chem|CO|2. Proceedings of the National Academy of Sciences, USA, 102, 1302-1305.]

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