Paleoproterozoic Era

This is the earliest and longest part of the proterozoic, from about 2.5 billion to 1.6 billion years ago. This was the period when the Earth's crust first stabilized. Volcanism in the past (Archean) had produced mainly basaltic magmas, but volcanoes in the paleoproterozoic first started producing lighter, siliceous magmas. This was much lighter rock, which tended to remain on the surface, and led to the buildup of large continental masses by accretion.

Early proto continents had started forming even in the Archean, with the first evidence of crustal rock and liquid water going back to 4.4 billion years (Cryptic Era in the Hadean), and the first continents in the middle Archean. At the beginning of the Paleoproterozoic, many small landmasses existed, some of which had aggregated into the first continent of Ur (~ 3 billion years ago, or half a billion years before the beginning of the Paleoproterozoic). Also at the beginning of the Paleoproterozoic, another continent appeared -- Arctica, which contained cratons from the Canadian Shield (Kenorland), parts of Siberia, and the Wyoming craton. Another half billion years later, around 2 billion years ago, the next large continent (Atlantica) appeared in the region that is now the southern Atlantic ocean. This continent contained parts of West/Central Africa (Guyana, Congo, Kasai), and North-Eastern parts of South America (Brazil, the Sao Francisco and Rio Plato cratons). The third continent to appear was Nena (derived from North Europe - North America), around 1.8 billion years ago. This was an agglomeration of earlier cratons from the Baltic and Ukraine, Greenland, and the much older cratons of Arctica.

Cross section of stromatolite showing the layered structure. From the University of California Museum of Paleontology.

It has been speculated that the first super continent, named Columbia, started to form during the Paleoproterozoic. Around 2 billion years ago, the older continents of Atlantica and Ur started moving towards each other. Around 1.8 billion years ago, the newly formed Nena started moving towards them as well, forming a huge landmass that grew further through volcanism. Columbia lasted for a few hundred million years towards the end of the Paleoproterozoic, probably between 1.8 and 1.5 billion years ago.

Life had appeared in the early Archean, possibly even as early as the Hadean. The oldest cyanobacteria are inferred from stromatolites dating back 3.5 billion years. There is some debate whether these earliest cyanobacteria performed photosynthesis. The earliest conclusive evidence of photosynthetic cyanobacteria is from the Paleoproterozoic, roughly 2.45 - 2.32 billion years ago. This coincides with the earliest dates for the oxygenation of the surface layers of the ocean. Deeper parts of the ocean, and the atmosphere for the most part, still remained poorly oxygenated for a very long time afterwards. Note that the first appearance of photosynthesis is somewhat earlier in the record, about 2.7 billion years ago. It is unknown if these earliest photosynthetic bacteria were cyanobacteria or perhaps rhodobacteria.

During the Paleoproterozoic, cyanobacteria flourished, and coastlines were littered profusely with stromatolites. Stromatolites are the remains of colonies of cyanobacteria, which form outcrops in shallow water along the coastlines. Photosynthesis by cyanobacteria depletes the surrounding water of carbon dioxide, which causes precipitation of calcium carbonate. The calcium carbonate is trapped by the mucilagenous material secreted by the colonies, and gradually hardens into rock. As the colony continues to grow, layers of calcium carbonate continue to be added seasonally, much like tree rings. Stromatolites can still be found in some rare spots, such as Shark Bay in Australia.

The Paleoproterozoic is divided into four periods, described below.

Siderian

This is the earliest period of the Paleoproterozoic, lasting from about 2.5 billion to 2.3 billion years ago. This period is known for the abundance of banded iron formations, which are the source of much of today's commercial iron ore. Banded iron formations are layered rocks, rich in iron oxides. Iron was initially dissolved in the oceans, and was gradually oxidized by photosynthetic bacteria into insoluble magnetite (Fe3O4) and hematite (Fe2O3), which precipitated and formed deposits on the ocean floor. These deposits are layered, showing that the deposition was cyclic. It is presently unknown whether the cycles were seasonal or related to something else.

There was a vast amount of iron in the early oceans, and for a very long time any oxygen produced by photosynthetic bacteria raised oxygen concentrations only locally. The rest of the oxygen was used up in oxidizing the early rocks and the iron in the oceans. It has been estimated that roughly 20x as much oxygen as the atmosphere holds today is locked up in the banded iron formations alone.

This is also the period of the Oxygen Catastrophe. Photosynthesis had evolved much earlier, in the late Archean, about 2.7 billion years ago. Over time, the accumulation of oxygen was sufficient to raise atmospheric oxygen somewhat (a very small amount by today's standards), sufficient to prove toxic to many of the anaerobic life forms that existed at the time. It is hypothesized that such life forms either died, or found anaerobic habitats in the deep ocean and subsurface regions of the early land masses.

A severe glaciation known as the Huronian glaciation occurred during the Siderian, roughly 2.4 billion to 2.1 billion years ago. Some have speculated that it was severe enough to lead to a snowball earth scenario.

Rhyacian

The second period of the Paleoproterozoic, from about 2.3 billion years ago to 2.05 billion years ago. This period has been defined chronometrically rather than based on stratigraphy.

The name derives from a Greek word for "lava flow", indicating that this was a period of high volcanism. A lot of mineral-rich igneous rock was deposited. This region is known from the Bushveld Complex in South Africa, which is still a source of rich mines, containing much of the world's andalusite, chromium, fluorspar, platinum and vanadium. The Merensky Reef in the Bushveld contains the richest deposits of platinum in the world. It is uncertain if the source of the platinum is magma or if it is extra-terrestrial, since it is near the Vredefort crater, which is the largest known impact crater on Earth.

The volcanism of this period produced large quantities of carbon dioxide and other greenhouse gases, which helped end the Huranian glaciation, around 2.1 billion years ago.

It is commonly believed that eukaryotes first evolved during the Rhyacian (though there is some evidence from stearanes - markers of eukaryotic chemistry - found in Australian shales, that eukaryotes may have evolved earlier, as much as 2.7 billion years ago).

The Vredefort Crater in South Africa. Note the rippled edges, which are very rare on Earth, but are commonly seen on craters on other planets and moons. Public domain image from NASA.

Orosirian

This period lasted from about 2.05 billion years to 1.8 billion years ago. It is also dated chronometrically rather than stratigraphically. The period is named for the extensive orogeny, or mountain forming, that occurred across virtually every continent that existed at the time. Orogeny implies the active movement of tectonic plates, since one mechanism of mountain formation is through lateral compression of the crust when two landmasses are pushed against each other. This coincides with the movement of the early continents, as Ur, Nena and Atlantica moved towards each other, forming the first super continent Columbia, around 1.8 billion years ago.

The Orosirian is also known for two of the largest impact events of which evidence still survives. These include the largest verified impact crater near Vredefort in the Free Province of South Africa. The crater, which was formed at the beginning of the Orosirian (2.023 billion years ago), is about 300 km wide (compare to the dino-killer crater event at Chicxulub, which is only 170 km). The other large impact event produced the Sudbury Basin, in Ontario, Canada. This is a 250 km wide crater, formed roughly 1.85 billion years ago, towards the end of the Orosirian. The impactor was an asteroid at least 10 km in size. This is a mineral rich area today, with large scale mining operations for copper and nickel, which may have arrived in the asteroid.

Statherian

This is the last period of the Paleoproterozoic, from about 1.8 billion to 1.6 billion years ago. This period is also dated chronometrically. The name is derived from the Greek word meaning "stable" or "firm". The previous eras had been characterized by violent events, including intense volcanism during the Rhyacian, a prolonged ice age and possible snowball Earth during the Huronian glaciation, and extensive orogeny and massive asteroid impacts during the Orosirian. The Statherian provided 200 million years of relative calm for life towards the end of the Paleoproterozoic.

The super continent Columbia formed during this period, about 1.8-1.6 billion years ago. As the continents of Ur, Nena and Atlantica drifted together, they expanded through accretion and formed a huge super continent. Continental shelves grew in extent, forming stable bases for the continents. Fold belts produced by movements of continents as they approached each other became cratonized, leading to significant thickening of the crust in these areas.

Although the origin of eukaryotes is unclear (perhaps 2.7 billion years ago according to some Australian shales, or 2.1 billion years ago during the Rhyacian, which is commonly believed), it first became established during the Statherian. This is therefore the period to which "complex life" (as opposed to bacteria) is first dated.

 

Continue reading about the Mesoproterozoic, or return to the index.