What is the Proterozoic Era, and what geologic divisions allow scientists to identify it?
The Precambrain Era marks the first four and a half billion years of the Earth’s formation. This era is divided into two periods: the first half is called the Archean Era and the second half is called the Proterozoic Era (sometimes called the Algokian Era). The Cambrian Era begins at the end of the Precambrian Era, approximately 570 million years ago.
In the early Proterozoic approximately 2.5 billion years ago, Eukaryotic cells (cells with nuclei) appeared for the first time. Marine algae and bacteria also evolved early in the era. Before this time, only cells without nuclei, called prokaryotic cells, existed. Later in the era, multicellular life emerged for the first time. The atmosphere underwent drastic change during the Proterozoic, going from oxygen-free to oxygen-rich, when photosynthetic bacteria and algae evolved and liberated oxygen from carbon dioxide. The new oxygen-rich world saw an explosion of multi-cellular life forms as the Cambrian Era began.
Not only did organism experience growth and change, so too did geological features of the Earth. During the later Archean Era and into the early Proterozoic Era, the continents began forming wide continental shelves, which moved around via plate-tectonics. At this point in time, plant life did not yet exist, but erosion and deposition was rapidly happening. Beds, some kilometers thick, of pure quartz and sandstone formed.
During both halves of the Precambrian Era, banded iron beds formed. The bands alternated between thin layers of quartz and iron oxide formed. This process occurred only during the Precambrian Era, and today, these beds continue to be the world’s major source of iron ore.
It should be noted that geologists have long been in dispute about the name “Proterozoic.” The word comes from the Greek: “protero” means “earlier” and “zoic” means “life.” The reason for the contention is because the rock formations that are found around the Hudson Bay and Greenland (called the “Canadian Shielf”) has rock formations that change types dependent on depth. The problem arises because this unconformity in rock formations is not found around the world in Precambrian formations. Another reason for contention is that many scientists feel that such a vast span of years, 4.5 billion, is just too long to be broken into just two periods, even if they are divided into early, middle, and late.
Source: World of Earth Science, ©2003 Gale Cengage. All Rights Reserved
The Precambrian time includes the Proterozoic eon that spreads from 2.5 billion years ago until the appearance of hard shelled animals. The Proterozoic eon is divided into 3 eras: Paleoproterozoic, Mezoproterozoic and Neoproterozoic.
The Paleoproterozoic era covers about 20% of the entire geological history of the Earth. During the Paleoproterozoic era, the small nuclei of the actual continents are formed. The Paleoproterozoic era is divided into four periods: Siderian, Rhyacian, Orosirian, Statherian. Each division was identified based on geochemical data, giving us the four divisions: oxidation state, sulfur isotope fractionation, carbon and iron geochemistry. For example, the formation of ozone layer occurs in the Siderian division, while the atmospheric free oxygen level exceeds 0.001% of PAL in Rhyacian period.
The Mesoproterozoic era is divided into 3 periods: Calymmian, Ectasian, Stenian. The division into the 3 periods is based on the techniques called radiometric dating, biostratigraphy. There are used also the biological markers such that: Anabaena cyanobacterium that indicates the presence of free oxygen.
The Neoproterozoic era is the last era of Proterozoic eon and it is divided into 3 periods: Tonian, Cryogenian and Ediacaran.