Paleoatmosphere
A paleoatmosphere is an atmosphere, particularly that of Earth, at some unspecified time in the geological past.
When regarding geological history of Earth, the paleoatmosphere can be chronologically divided into the following phases:
- the Hadean first atmosphere or primary atmosphere, whose composition resembled that of the solar nebula;
- the Archean second atmosphere or secondary atmosphere, which is a reducing atmosphere that became nitrogen-abundant due to volcanic outgassing and meteoric injections during the Late Heavy Bombardment, and;
- the Proterozoic and Phanerozoic third atmosphere or tertiary atmosphere, which started to contain free elemental oxygen due to ongoing biotic photosynthesis finally having released enough byproduct oxygen to overwhelm the reducing capability of the preceding second atmosphere. The appearance of free oxygen during the Neoarchean–Paleoproterozoic boundary, i.e. the Great Oxygenation Event, permanently changed the redox property of the atmosphere, and this new oxidative atmosphere can be subsequently further subdivided into several more periods of oxygenation and anoxic events associated with geological and climate catastrophes, mass extinctions and the evolution, diversification, radiations and successions of photoautotrophs before it eventually reached the current Holocene state.
Composition
Appreciable concentrations of free oxygen were probably not present until about 2,500 million years ago. After the Great Oxidation Event, quantities of oxygen produced as a by-product of photosynthesis by cyanobacteria 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 by oxygenic photosynthesis. This may have contributed to the Carboniferous rainforest collapse during the Moscovian and Kasimovian ages of the Pennsylvanian subperiod.
Indirect measurements
Geological studies of ancient rock formations can give information on paleoatmospheric composition, pressure, density, etc. at specific points in Earth's history.Density and pressure
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. They found an atmospheric pressure of only 0.23 ± 0.23 bar.
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.