The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first geologic period of the Phanerozoic, which is named after Cambria, the Latinized name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.
The Precambrian is an informal unit of geologic time,
Overview
Relatively little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history, and what is known has largely been discovered from the 1960s onwards. The Precambrian fossil record is poorer than that of the succeeding
Phanerozoic, and fossils from the Precambrian (e.g.
stromatolites) are of limited
biostratigraphy use.
This is because many Precambrian rocks have been heavily
metamorphic rock, obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.
[ Outlined in ]
It is thought that the Earth coalesced from material in orbit around the Sun at roughly 4,543 Ma, and may have been struck by another planet called Theia shortly after it formed, splitting off material that formed the Moon (see Giant-impact hypothesis). A stable crust was apparently in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma.
The term "Precambrian" is used by and paleontologists for general discussions not requiring a more specific eon name. However, both the United States Geological Survey
(from "earliest") was a synonym for pre-Cambrian and Precambrian,
Life forms
A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks (Archean Eon) from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia.
Complex multicellular organisms may have appeared as early as 2100 Ma.
While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic Algal mat that covered terrestrial areas.
Tracks from an animal with leg-like appendages have been found in what was mud 551 million years ago.[ The Independent]
Emergence of life
The RNA world hypothesis asserts that RNA evolved before coded proteins and DNA genomes.[ This primitive RNA replicative system also may have been able to undergo template strand switching during replication (genetic recombination) as is known to occur during the RNA replication of extant .]
Planetary environment and the oxygen catastrophe
Evidence of the details of plate tectonics and other tectonic activity in the Precambrian is difficult to interpret. It is generally believed that small proto-continents existed before 4280 Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia, broke up around 750 Ma. A number of glaciation have been identified going as far back as the Huronian epoch, roughly 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth".
It is believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthesis life forms evolved and began to produce it in large quantities as a byproduct of their metabolism. This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis, sometimes called the oxygen catastrophe. At first, oxygen would have quickly combined with other elements in Earth's crust, primarily iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, and the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides.
Subdivisions
A terminology has evolved covering the early years of the Earth's existence, as radiometric dating has allowed absolute dates to be assigned to specific formations and features.
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Proterozoic: this eon refers to the time from the lower Cambrian boundary, Ma, back through Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary.
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Neoproterozoic: The youngest geologic era of the Proterozoic Eon, from the Cambrian Period lower boundary ( Ma) back to Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American stratigraphy.
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Mesoproterozoic: the middle era of the Proterozoic Eon, - Ma. Corresponds to "Precambrian Y" rocks of older North American stratigraphy.
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Paleoproterozoic: oldest era of the Proterozoic Eon, - Ma. Corresponds to "Precambrian X" rocks of older North American stratigraphy.
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Archean Eon: - Ma.
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Hadean Eon: – Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although some zircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the Moon and .
It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows:[ also available at Stratigraphy.org: Precambrian subcommission]
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Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.
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Hadean: dominated by heavy bombardment from about 4.51 Ga (possibly including a cool early Earth period) to the end of the Late Heavy Bombardment period.
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Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due to increasing atmospheric oxygen content.
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Transition: a period of continued banded iron formation until the first continental red beds.
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Proterozoic: a period of modern plate tectonics until the first .
Precambrian supercontinents
The movement of Earth's Plate tectonics has caused the formation and break-up of continents over time, including occasional formation of a supercontinent containing most or all of the landmass. The earliest known supercontinent was Vaalbara. It formed from proto-continents and was a supercontinent 3.636 billion years ago. Vaalbara broke up c. 2.845–2.803 Giga-annum ago. The supercontinent Kenorland was formed c. 2.72 Giga-annum ago and then broke sometime after 2.45–2.1 Giga-annum into the proto-continent called Laurentia, Baltica, Yilgarn craton and Kalahari craton. The supercontinent Columbia, or Nuna, formed 2.1–1.8 billion years ago and broke up about 1.3–1.2 billion years ago. supercontinent 2.5 billion years ago]]
See also
Further reading
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Valley, John W., William H. Peck, Elizabeth M. King (1999) Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-Madison Wgeology.wisc.edu – Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Accessed Jan. 10, 2006
External links
