Proterozoic Eon

The Proterozoic Eon is the interval between 2.5 billion and 542 million years ago. Many of the geological processes presently active on Earth first appeared during this period in the geologic record. This includes the first clear evidence for plate tectonics. Rocks of the Proterozoic Eon also document changes in conditions on Earth, particularly an apparent increase in atmospheric oxygen.

Earth's Four Eons

The largest subdivision of geologic time is the eon. Four eons constitute the known history of Earth: Phanerozoic, Proterozoic, Archean, and Hadean. The Phanerozoic, whose abundant fossil record permits fine subdivisions of geologic time and the intercontinental correlation of strata, is the most known interval. The Proterozoic, by contrast, is generally characterized by a sparse fossil record of simple life-forms that do not permit the sort of stratigraphic subdivision possible in the Phanerozoic. The ages of rocks in the Proterozoic can be established only by radiometric dating. For rocks as old as the Proterozoic, the inherent uncertainty in even the best dating is on the order of 10 million to 20 million years, comparable in length to some Phanerozoic periods.

The Proterozoic has now been subdivided into eras and periods comparable to those in the Phanerozoic. The Hadean is not divided, but the Archean now comprises the Eoarchean (3.6 to 3.2 billion years ago [bya]), the Mesoarchean (3.2 to 2.6 bya), and the Neoarchean (2.8 to 2.5 bya). This is followed by the Proterozoic, which is divided into three eras. The Paleoproterozoic (2.5 to 1.6 bya) consists of the Siderian, Rhyacian, Orosirian, and Statherian periods. It is followed by the Mesoproterozoic (1.6 to 1.0 bya), comprising the Calymmian, Ectasian, and Stenian periods, and the Neoproterozoic (1.0 to 0.542 bya), containing the Tonian, Cryogenian, and Ediacaran periods.

Much of the significance of the Proterozoic derives from the first appearance of processes still operating on Earth. The Archean, in contrast, may have experienced a quite different set of processes. Therefore, it is impossible to discuss the Proterozoic without discussing the Archean to some extent. There are many unanswered questions about major Earth processes in the Archean. In particular, the Archean lacks structures that closely resemble Phanerozoic orogenic belts. Whether plate tectonics operated in its present form during the Archean is still unresolved. Dynamic processes within the Earth may have undergone a significant change about 2.5 billion years ago.

The contrast between Archean and Proterozoic geology is striking, if still imperfectly understood. The Archean is dominated by two principal types of regional structure: greenstone belts and gneiss-migmatite terrains. Greenstone belts are troughs of volcanic rocks and deep-water sedimentary rocks intruded by elliptical granite bodies. Gneiss-migmatite terrains are bands of very highly metamorphosed and deformed rocks. Structures similar to both the greenstone belts and the gneiss-migmatite terrains have formed throughout the history of the Earth. They can form through conventional plate tectonic processes, but the almost complete lack of other types of structures in Archean time is perplexing. It is widely (though not universally) believed that the Archean crust was more mobile than at later times and that large masses of continental crust did not exist then. The first extensive shallow-water rocks deposited in a continental-shelf or stable continent setting occur in the latest Archean rocks of South Africa. They are abundant throughout the Proterozoic and Phanerozoic. The appearance of such rocks may mark the first appearance of stable continental crust.

Crustal and Atmospheric Events

During the Proterozoic, there is the first widespread evidence for plate tectonics and orogeny (mountain building) comparable with the Phanerozoic. There seem to have been two major periods of plate collision during which large continents were assembled from small continental plates. North America was assembled about 1.9 and 1.6 billion years ago by the collision of several smaller Archean blocks and the accretion of many smaller terranes. This episode is known as the Trans-Hudson Event. The overall process was very similar to the accretion known to have added large areas to western North America during the Phanerozoic. Between about 900 and 600 million years ago, South America and Africa also appear to have been assembled in much the same way to form part of the early Phanerozoic supercontinent of Gondwanaland. The sequence of events that assembled South America and Africa is called the Pan-African Event.

Some Proterozoic crustal events are still poorly understood. In particular, a widespread heating event resulted in the intrusion and eruption of silica-rich igneous rocks (granite and rhyolite) between about 1.5 and 1.3 billion years ago across much of North America. This interval also resulted in the intrusion of large bodies of anorthosite, an igneous rock made mostly of feldspar minerals, on many continents. Anorthosite is otherwise uncommon, and the reason it was formed so extensively in the Proterozoic is not known. Several significant developments took place on the surface of the Proterozoic Earth. There is evidence for increasing oxygen content in the atmosphere, as indicated by the appearance of the first “red beds”— red sandstone and conglomerate colored by iron oxide. Also, there were several major ice ages during the Proterozoic. One occurred about 2.3 billion years ago, and there is widespread evidence for a series of ice ages between about 900 and 600 million years ago.

The early Earth probably lacked free oxygen, except in minor amounts. Free oxygen is not a component of the raw materials that formed the planets, and oxygen is a highly reactive gas that would rapidly have combined with other substances. Oxygen was originally given off as a waste product by early organisms and is toxic to some organisms even today. How and when the present oxygen level of the atmosphere was attained is controversial. Many scientists consider that increasing oxygen was related to the abrupt expansion of life at the start of the Phanerozoic. There is also evidence that a significant threshold in oxygen level was crossed about 2.0 to 1.8 billion years ago.

Sedimentary Deposits

Several types of sedimentary deposit, all closely related to the availability of free oxygen, either ceased or began to be deposited about 1.8 billion years ago. Detrital uranium deposits and banded iron formations, both of which are believed to indicate low oxygen levels, become very uncommon in the geologic record after that time. At the same time, red beds appear. Detrital uranium deposits are sandstone deposits in which dense minerals are concentrated by current action. This process is common today, but the Proterozoic deposits include uranium minerals, which are highly susceptible to weathering, and even pyrite (iron sulfide), which oxidizes extremely quickly and is rarely found in recent sedimentary deposits.

A detrital uranium deposit at Okolo, in the African nation of Gabon, is remarkable for another reason. The uranium ore from the deposit was found to be greatly depleted in uranium-235, the isotope used in nuclear reactors. Further investigation showed that the ore deposit had been a natural nuclear reactor. Present-day uranium is too poor in uranium-235 to sustain a nuclear chain reaction except under very controlled conditions. Yet, 2.2 billion years ago, when the Okolo deposit formed, uranium-235 was far more abundant—abundant enough for a sustained chain reaction to begin when sedimentary processes collected enough uranium minerals in one place. So far, the remarkable deposit at Okolo is unique; although, other detrital uranium deposits are being studied for the effects of natural chain reactions.

Another type of ore deposit common in early Proterozoic rocks is banded iron formations. Banded iron formations consist of layers of iron oxides and silicates interbedded with fine-grained silica, or chert. The significance of these ore deposits, apart from their great importance as sources of iron, lies in the degree of oxidation of the iron. Iron in nature has two oxidation states. Ferrous iron, the less oxidized state, consists of iron atoms that have lost two electrons and thus have a positive electric charge of +2. Ferric iron atoms have lost three electrons and have a +3 charge. Common rust is mostly ferric iron oxide, as would be expected in the Earth's presently oxygen-rich atmosphere. The iron in banded iron formations, however, is mostly ferrous, even though it was deposited on the surface of the Earth. The ferrous iron in banded iron formations presents one of the strongest arguments for an oxygen-poor atmosphere on the early Earth. The iron was originally dissolved in seawater and was probably precipitated by microorganisms. Indeed, some of the best-preserved Proterozoic fossils are those of microorganisms preserved in the chert of banded iron formations. Ferrous iron is far more soluble in water than ferric iron, so the Proterozoic seas may have been richer in dissolved iron than are the present oceans. Iron deposits precipitated by microorganisms have also formed in the Phanerozoic, but the iron is mostly ferric.

Both detrital uranium deposits and banded iron formations become rare in the geologic record after 1.8 billion years ago. At about the same time, red sandstones appear in abundance. Red sandstones owe their color to ferric (highly oxidized) iron and are most commonly deposited on land or in shallow water. One of the earliest extensive red bed deposits is in northwestern Canada. These rocks were deposited shortly after the region's crust had experienced a major orogeny about 1.8 billion years ago and represent debris eroded from the newly formed mountain range. A deposit of this sort is termed molasse. These red beds furnish not only evidence for the oxygen level of the atmosphere but also some of the first evidence for topographically high mountains.

Glaciation

The Proterozoic experienced several major glacial events about 2.3 billion years ago and again from 900 to 600 million years ago. The intervening period appears to have been largely ice-free. The glaciation 2.3 billion years ago is best documented in the Gowganda formation of Ontario, where virtually every type of glacial deposit occurs. Other deposits in Wyoming and Quebec have been interpreted as glacial deposits of the same age, but the Gowganda formation remains the clearest evidence of a 2.3-billion-year-old glaciation.

The glacial deposits 900 to 600 million years old are far more problematic. Many of these rocks are diamictites, fine-grained sedimentary rocks with scattered large pebbles. Diamictites cannot have been simply deposited by running water. Slow-moving water could not have transported the pebbles, while sediment deposited by fast-moving water would have a much higher proportion of coarse material than do diamictites. Only a few plausible ways of forming diamictites exist. One way is for floating glacial icebergs to melt and drop trapped rocks into otherwise fine-grained sediment. When the sediments are finely layered, so-called dropstones are generally taken as clear evidence of glaciation. Diamictites can also form from nonglacial processes. For example, submarine landslides can mix a small amount of coarse debris into a large amount of fine sand and silt. The glacial origin of many diamictites has been controversial, and some of the late Proterozoic diamictites may not be glacial.

Another problem with the late Proterozoic glaciation is the geographic distribution of the deposits. Glacial deposits by themselves indicate cold climates, but in many cases, there are nearby carbonate rocks (dolomite and limestone) of similar age that typically form in warm climates. Also, paleomagnetic studies indicate that some areas with glacial deposits, notably Australia, were at low latitudes during the late Proterozoic. Several possible explanations, none entirely satisfactory, have been proposed. A few geologists find flaws in the evidence, either in the climatic indicators or the paleomagnetic evidence. Others propose that the late Proterozoic Earth was abnormally cold. In contrast, others say the temperature decreased with altitude much greater than at present, allowing warm climates at sea level but glaciers at even moderate elevations.

Throughout most of the Proterozoic, simple life-forms dominated on Earth. The first life-forms, already established in the Archean, were prokaryotes, organisms without a cell nucleus, such as bacteria and blue-green algae. About 2 billion years ago, eukaryotes, or organisms with a cell nucleus, appeared. The only common Proterozoic fossils of large size are stromatolites, domelike masses of calcium carbonate or silica deposited by colonies of algae in shallow water. The first widespread evidence for large multicelled organisms appears in rocks about 800 million years old. These organisms are rare as fossils because they lacked hard body parts but seem to have resembled jellyfish and marine worms. Many are unrelated to present-day organisms and seem to represent extinct evolutionary lines.

As the twenty-first century progresses, scientists continue to make critical discoveries about many aspects of the Proterozoic Eon, including evidence of sponge-like fossils and the Great Oxygenation Event that occurred during the period. A build-up of atmospheric oxygen led to the creation of iron deposits and triggered adaptations in organisms to use oxygen. Glaciation vents, plate tectonics, and supercontinent cycles are all areas of study that involve research in the Proterozoic Eon. 

Principal Terms

Archean Eon: the period of geologic time from about 3.6 billion to 2.5 billion years ago

Hadean Eon: the period of time before the Archean and lasting from 4.5 to 3.6 billion years ago.

orogenic belt: a belt of crust that has been severely compressed, deformed, and heated, probably by convergence of crustal plates

paleomagnetism: the study of magnetism preserved in rocks, which provides evidence of the history of Earth's magnetic field and the movements of continents

Phanerozoic Eon: the period of geologic time with an abundant fossil record, extending from about 544 million years ago to the present

plate tectonics: the theory that the outer surface of the Earth consists of large moving plates that interact to produce seismic, volcanic, and orogenic activity

Precambrian: the collective term for all geologic time before the Phanerozoic—that is, before about 544 million years ago

radiometric dating: the use of radioactive elements that decay at a known rate to determine the ages of the rocks in which they occur

terrane: a structurally distinct block of crust added to a continent by plate tectonic processes

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