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The Mesoarchean ( , also spelled Mesoarchaean) is a
geologic Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...
era An era is a span of time. Era or ERA may also refer to: * Era (geology), a subdivision of geologic time * Calendar era Education * Academy of European Law (German: '), an international law school * ERA School, in Melbourne, Australia * E ...
in the Archean Eon, spanning , which contains the first evidence of modern-style
plate subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second plat ...
and expansion of microbial life. The era is defined chronometrically and is not referenced to a specific level in a rock section on Earth.


Tectonics

The Mesoarchean era is thought to be the birthplace of modern-style plate subduction, based on geologic evidence from the Pilbara Craton in western
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
. A convergent margin with a modern-style oceanic arc existed at the boundary between West and East Pilbara approximately 3.12 Ga. By 2.97 Ga, the West Pilbara Terrane converged with and accreted onto the East Pilbara Terrane. A
supercontinent In geology, a supercontinent is the assembly of most or all of Earth's continent, continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", ...
,
Vaalbara Vaalbara is a hypothetical Archean supercontinent consisting of the Kaapvaal craton (now in eastern South Africa) and the Pilbara Craton (now in north-western Western Australia). E. S. Cheney derived the name from the last four letters of each ...
, may have existed in the Mesoarchean.


Environmental conditions

Analysis of
oxygen isotopes There are three known stable isotopes of oxygen (8O): , , and . Radioactive isotopes ranging from to have also been characterized, all short-lived. The longest-lived radioisotope is with a half-life of , while the shortest-lived isotope is ...
in Mesoarchean cherts has been helpful in reconstructing Mesoarchean surface temperatures. These cherts led researchers to draw an estimate of an oceanic temperature around 55-85°C (131-185 Fahrenheit), while other studies of
weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...
rates postulate average temperatures below 50°C (122 Fahrenheit). The Mesoarchean atmosphere contained high levels of atmospheric
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 ...
and
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 ...
, which could be an explanation for the high temperatures during this era. Atmospheric
dinitrogen Nitrogen is a chemical element; it has symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seventh ...
content in the Mesoarchean is thought to have been similar to today, suggesting that nitrogen did not play an integral role in the thermal budget of ancient Earth. The
Pongola glaciation The Pongola glaciation is a glacial episode that occurred in the Mesoarchean, 2.9 Ga for about 150 Ma. It is the oldest known glaciation on the planet. Geology The oldest known traces of glaciation date from the Mesoarchean. They correspond t ...
occurred around 2.9 Ga, from which there is evidence of ice extending to a palaeolatitude (latitude based on the magnetic field recorded in the rock) of 48 degrees. This glaciation was likely not triggered by the evolution of photosynthetic cyanobacteria, which likely occurred in the interval between the Huronian glaciations and the Makganyene glaciation.


Early microbial life

Microbial life with diverse metabolisms expanded during the Mesoarchean era and produced gases that influenced
early Earth Early Earth also known as Proto-Earth is loosely defined as Earth in its first one billion years, or gigayear (Ga, 109 y), from its initial formation in the young Solar System at about 4.55 Ga to some time in the Archean eon in approximately 3.5 Ga ...
's
atmospheric composition Atmospheric chemistry is a branch of atmospheric science that studies the chemistry of the Earth's atmosphere and that of other planets. This multidisciplinary approach of research draws on environmental chemistry, physics, meteorology, compute ...
.
Cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
produced oxygen gas, but oxygen did not begin to accumulate in the atmosphere until later 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 ...
. Small oases of relatively oxygenated water did exist in some nearshore shallow marine environments by this era, however.


See also

* * * *


References


External links


Mesoarchean (chronostratigraphy scale)
{{portal bar, Geology, Paleontology *03 Geological eras Precambrian geochronology