Proterozoic

## General

**The Proterozoic** is a geological eon spanning the time interval from 2,500 to 538.8 million years ago. It is the youngest part of the Precambrian "Super Eon". It is also the longest eon on Earth's geological time scale and is divided into three geological epochs (from oldest to youngest): Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.

The Proterozoic Era covers the time from the appearance of oxygen in the Earth's atmosphere until just before complex life (such as trilobites or corals) spread on Earth.

The well-identified events of this eon were the transition to an oxygen-rich atmosphere during the Paleoproterozoic; the evolution of eukaryotes; various glaciations that produced the Snowball Earth hypothesis during the cryogenic period at the end of the Neoproterozoic; and the Ediacaran period (635 to 538.8 Ma), marked by the evolution of numerous multicellular soft-bodied organisms and giving us the first apparent fossil evidence of life on Earth.

## Tectonic activity

The Proterozoic was a very tectonically active period in Earth's history. The Late Archean to early Proterozoic eon corresponds to a period of increased crustal recycling, suggesting subduction. Evidence for this increased subduction activity comes from the abundance of old granites, mostly post-2.6 Ga. The occurrence of eclogite (a type of metamorphic rock formed by high pressure, 1 GPa or more) is explained by a model that incorporates subduction. The absence of Archean eclogites suggests that conditions at this time did not favor the formation of high-grade metamorphism and therefore did not reach the same levels of subduction as in the Proterozoic. Due to crustal subduction, the cores of the first continents became large enough to withstand the crustal recycling processes.

The long-term tectonic stability of these cratons is why we find continental crust that is between and a few billion years old. It is believed that 43% of the modern continental crust was formed in the Proterozoic, 39% in the Archean, and only 18% in the Phanerozoic. Studies by Condie (2000) and Rino et al. (2004) suggest that production of the cortex was episodic. Isotopic dating of Proterozoic granites revealed that there were several episodes of rapid increases in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in strength after each period.

## Atmosphere & oceans

One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth's atmosphere. Although oxygen is believed to have been released by photosynthesis since the Archean eon, it could not accumulate to any appreciable extent until the ore sinks from the unoxidized material. the sulfur and the iron were exhausted. Until about 2.3 billion years ago, oxygen levels were probably only 1% to 2% of their current levels. Banded iron formations that provide most of the world's iron ore are signs of this mineral sink process. Its accumulation stopped 1.9 billion years ago after all the iron in the oceans oxidized.

The red layers colored by hematite indicate an increase in atmospheric oxygen 2 billion years ago. Oxide formations are not found in older rocks. The accumulation of oxygen was likely due to two factors: the depletion of chemical sinks and an increase in carbon sequestration, which sequestered organic compounds that would otherwise have been oxidized by the atmosphere.

The geological record of the Proterozoic is more complete than that of the previous Archean. In contrast to the Archean deep-sea deposits, the Proterozoic has many strata deposited in extensive shallow epicontinental seas; moreover, many of these rocks are less metamorphosed than those of the Archean, and many are unaltered. Studies of these rocks have shown that the eon continued the massive continental accumulation that had begun late in the Archean eon. The Proterozoic Eon also showed the first definitive cycles of supercontinents and fully modern orogeny activity. There is evidence that the earliest known ice ages occurred during the Proterozoic.The first began shortly after the start of the Proterozoic and evidence for at least four during the Neoproterozoic at the end of the Proterozoic, possibly culminating in the Snowball Earth hypothesis of the Sturtian and Marinoan glaciations.

## Life

Characterized by the evolution of numerous multicellular soft-bodied organisms, the Proterozoic provides us with the first apparent fossil evidence of life on Earth.