A paleoatmosphere (or ''palaeoatmosphere'') is an
atmosphere
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
, particularly
that of Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
, at some unspecified time in the geological past.
When regarding
geological history of Earth, the paleoatmosphere can be chronologically divided into the
Hadean ''first atmosphere'', which resembled the compositions of the
solar nebula; the
Archean
The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...
''second atmosphere'' (also known as the
prebiotic atmosphere), which became
nitrogen-abundant due to
volcanic outgassing and
meteoric injections during the
Late Heavy Bombardment; and the
Proterozoic and
Phanerozoic ''third atmosphere'', which started to contain free
oxygen due to
biotic photosynthesis.
Composition
The composition of Earth's paleoatmosphere can be inferred today from the study of the abundance of
proxy materials such as
iron oxide
An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust.
Iron ...
s 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
reducing atmosphere, having virtually no free oxygen, virtually no argon, which is generated by the
radioactive decay of
40 K, and to have been dominated by nitrogen,
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
and
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
.
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
The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
, when atmospheric carbon dioxide was drawn down below current concentrations (about 400 ppm) 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
The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...
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.
Oxygen content
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
Neoproterozoic oxygenation event may have happened much earlier than previously thought.
See also
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References
{{reflist
Geological history of Earth
Paleoclimatology