Evolution of Earth's Atmosphere

Abstract

The use of geological and geochemical proxies has enabled scientists to piece together a plausible picture of atmosphere evolution on Earth over 3.5billion years, however there is much disagreement in the detailed patterns of both O2 and CO2. The very early Earth had a reduced atmosphere (H2, He, CO) due to degassing of the planet. Active volcanism in the Archean introduced CO2, CH4 and N2, when combined with loss of H2, led to a more oxidized CO2-N2-rich atmosphere. The first O2-emitting cyanobacteria appeared between 3700 and 2700Ma, but most of the released O2 was consumed in the ocean by reductants. It was not until about 2500Ma that significant O2 built up in the atmosphere, accompanied by a rapid decline in CO2 and CH4. This O2 build up continued for about 650million years encompassing the great oxygenation event (GOE) and the Lomagundi carbon isotope excursion. Then followed a period of flat or decreasing atmosphere O2 until around 1400Ma when another significant rise in O2 took place, followed by a decrease until around 800Ma. These rises and falls in atmosphere O2 followed in a cyclical pattern through the Proterozoic with wavelengths of about 700–600million years, appearing to match the supercontinent cycles. Carbon dioxide decreased through the Proterozoic from about 3wt% to 3000ppm. Redox sensitive trace elements in black shales and sedimentary pyrite rise dramatically around 560Ma marking the position of the Neoproterozoic oxygenation event (NOE). Through the Phanerozoic (last 540million years) the oxygen cycles became more frequent on a wavelength of 60–120million years, again matching the tectonic cycles of plate collisions, mountain building and erosion. CO2 varied from around 1500–300ppm in what appears to be an antithetic series of cycles to O2. Recent modeling suggests the O2 cycles varied between <5wt% up to at least 30wt% O2. The oxygen cycle peaks match roughly with peaks in biodiversity, whereas the O2 minima match periods of nutrient deficiency and mass extinction at the end of each cycle.