Proterozoic

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Proterozoic

The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth

Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
. The Proterozoic Eon extended from 2500 Ma

Annum

Annum is one form of the Latin noun meaning year, not a form normally used for derivatives in modern languages: the accusative case Grammatical number of the second declension grammatical gender noun annus , anni ....
to 542.0 ± 1.0 Ma (million years ago), and is the most recent part of the old, informally named ‘Precambrian

Precambrian

The Precambrian is an informal name for the supereon comprising the eon of the geologic timescale that came before the current Phanerozoic eon....
’ time.

The Proterozoic consists of 3 geologic eras, from oldest to youngest:

The well-identified events were:

geoloic record of the Proterozoic is much better than that for the preceding Archean

Archean

The Archean is a geology eon before the Proterozoic and Paleoproterozoic, before 2.5 Ga . Instead of being based on stratigraphy, this date is defined chronometrically....
.

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The Proterozoic record

The geoloic record of the Proterozoic is much better than that for the preceding Archean

Archean

The Archean is a geology eon before the Proterozoic and Paleoproterozoic, before 2.5 Ga . Instead of being based on stratigraphy, this date is defined chronometrically....
. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata

Stratum

In geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers....
that were laid down in extensive shallow epicontinental seas

Epeiric Sea

An epeiric sea is a large shallow sea that either extends far into a continent, such as the Persian Gulf, or overlies a large part of a continent....
; furthermore, many of these rocks are less metamorphosed

Metamorphic rock

Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form"....
than Archean-age ones, and plenty are unaltered. Study of these rocks show that the eon featured massive, rapid continent

Continent

A continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents ? they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia ....
al accretion (unique to the Proterozoic), supercontinent cycle

Supercontinent cycle

The supercontinent cycle describes the quasi-periodic aggregration and dispersal of Earth's continental crust. There are varying opinions as to whether Earth's budget of continental crust is increasing, decreasing, or remaining about constant, but it is agreed that this inventory is constantly being reconfigured....
s, and wholly-modern orogenic

Orogeny

Orogeny refers to natural mountain building, and may be studied as a tectonic structural event, as a geographical event, and a chronological event: orogenic events cause distinctive structural phenomena and related tectonic activity, affect certain regions of rocks and crust, and happen within a specific period of time....
activity.

The first known glaciations occurred during the Proterozoic, one began shortly after the beginning of the eon, while there were at least four during the Neoproterozoic, climaxing with the Snowball Earth

Snowball Earth

Snowball Earth refers to hypotheses regarding paleoclimate global-scale glaciation, claiming that the Earth's surface was nearly or entirely frozen at some points in its past....
of the Varangian glaciation.

The build-up of oxygen

One of the most important events of the Proterozoic was the gathering up of oxygen

Oxygen Catastrophe

The Oxygen Catastrophe was a massive environmental change believed to have happened during the Siderian geologic period at the beginning of the Paleoproterozoic era of the Precambrian, about 2.4 billion years ago....
in the Earth's atmosphere. Though oxygen was undoubtedly released by photosynthesis

Photosynthesis

File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
well back in Archean times, it could not build up to any significant degree until chemical sinks — unoxidized sulfur

Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
and iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
— had been filled; until roughly 2.3 billion years ago, oxygen was probably only 1% to 2% of its current level. Banded iron formation

Banded iron formation

Banded iron formations are a distinctive type of rock often found in primordial sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert....
s, which provide most of the world's iron ore, were also a prominent chemical sink; most accumulation ceased after 1.9 billion years ago, either due to an increase in oxygen or a more thorough mixing of the oceanic water column.

Red beds, which are colored by hematite

Hematite

Hematite, Spelling differences#Simplification of ae .28.C3.A6.29 and oe .28.C5.93.29 h?matite, is the mineral form of Iron oxide , one of several iron oxides....
, indicate an increase in atmospheric oxygen after 2 billion years ago; they are not found in older rocks. The oxygen build-up was probably due to two factors: a filling of the chemical sinks, and an increase in carbon

Carbon

Carbon is a chemical element with chemical symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalence?making four electrons available to form covalent bond chemical bonds....
burial, which sequestered organic compound

Organic compound

An organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered Inorganic compound....
s that would have otherwise been oxidized by the atmosphere.

Paleogeography

The Mackenzie dike swarm

Mackenzie dike swarm

The Mackenzie dike swarm, also called the Mackenzie dikes, form a large igneous province in the western Canadian Shield of Canada. It is one of more than three dozen dike swarms in various parts of the Canadian Shield and is the largest dike swarm known on Earth, more than 500 kilometers wide and 3,000 kilometers long, exte...
in Canada's Canadian Shield

Canadian Shield

The Canadian Shield — also called the Laurentian Plateau, or Bouclier Canadien — is a massive shield covered by a thin layer of soil that forms the nucleus of the North American craton....
is the largest known dike swarm

Dike swarm

A dike swarm or dyke swarm in geology is a major group of parallel, linear, or radially oriented dike s Intrusion within continental crust....
on Earth, and was a source for significant massive flood basalt

Flood basalt

A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava....
eruptions throughout the Proterozoic period. The source for the Mackenzie dike swarm is thought to have been a mantle plume

Mantle plume

A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle . As the heads of mantle plumes can partly melt when they reach shallow depths, they are thought to be the cause of volcano centers known as Hotspot and probably also to have caused flood basalts....
center called the Mackenzie hotspot

Mackenzie hotspot

The Mackenzie hotspot was a hotspot that existed about 1,267 million years ago across Canada from the Northwest Territories and Nunavut. It is responsible for the Mackenzie dike swarm, which is the largest dike swarm on Earth....
.

Proterozoic life

The first advanced single-celled and multi-cellular life roughly coincides with the start of the accumulation of free oxygen; this may have been due to an increase in the oxidized nitrate

Nitrate

In inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms . In organic chemistry the esters of nitric acid and various alcohols are called nitrates....
s that eukaryotes use, as opposed to cyanobacteria

Cyanobacteria

Cyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis....
. It was also during the Proterozoic that the first symbiotic

Symbiosis

The term symbiosis commonly describes close and often long-term interactions between different biological species. The term was first used in 1879 by the Germany mycology Heinrich Anton de Bary, who defined it as "the living together of unlike organisms"....
relationships between mitochondria (for nearly all eukaryotes) and chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
s (for plant

Plant

Plants are Life organisms belonging to the Kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae....
s and some protist

Protist

Protists ; eukaryote microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy....
s only) and their hosts evolved.

The blossoming of eukaryotes such as acritarch

Acritarch

Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion....
s did not preclude the expansion of cyanobacteria; in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago.

Classically, the boundary between the Proterozoic and the Phanerozoic

Phanerozoic

The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared....
eons was set at the base of the Cambrian

Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
period when the first fossils of animals known as trilobite

Trilobite

Trilobites are extinction marine arthropods that form the class Trilobita. They appeared in the Early Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Late Devonian extinction, all trilobite orders, with the sole exception of Proetida, died out....
s and archeocyathids

Archaeocyatha

The Archaeocyatha or archaeocyathids were Sessility , reef-building Marine organisms of warm tropical and subtropical waters that lived during the early Cambrian period....
appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian

Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
, which is currently placed at 542 Ma.

External links

- Palaeos

Proterozoic

The Proterozoic record

The build-up of oxygen

Paleogeography

Proterozoic life

See also

External links