Precambrian

The Precambrian (Pre-Cambrian) is an informal name for the span of time before the current Phanerozoic Eon, and is divided into several eons of the geologic timescale. It spans from the formation of Earth around 4500 Ma (million years ago) to the beginning of the Cambrian Period, when macroscopic hard-shelled animals first appeared in abundance about 542 Ma. The Precambrian is so named because it precedes the Cambrian, the first period of the Phanerozoic Eon, which is named after the Roman name for Wales, Cambria, where rocks from this age were first studied. The Precambrian period accounts for 87% of geologic time.

Overview

Very little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history, and what little is known has largely been discovered in the past four or five decades. The Precambrian fossil record is poor, and those fossils present (e.g. stromatolites) are of limited biostratigraphic use.^[1] Many Precambrian rocks are heavily metamorphosed, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.^[2][3]

It is thought that the Earth itself coalesced from material in orbit around the Sun roughly 4500 Ma and may have been struck by a very large (Mars-sized) planetesimal shortly after it formed, splitting off material that came together to form the Moon (see Giant impact theory). A stable crust was apparently in place by 4400 Ma, since zircon crystals from Western Australia have been dated at 4404 Ma.^[4]

The term Precambrian is somewhat out-moded, but is still in common use among geologists and paleontologists. It was briefly also called the Cryptozoic eon. It seems likely that it will eventually be replaced by the preferred terms Proterozoic, Archaean, and Hadean, and become a deprecated term. (See geologic time scale.)

Life before the Cambrian

It is not known when life originated, but carbon in 3.8 billion year old rocks from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3.46 billion years have been found in Western Australia.^[5] Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian.

Excepting a few contested reports of much older forms from USA and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600 Ma. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that timespan.

A very diverse collection of forms appeared around 544 Ma, starting in the latest Precambrian with a poorly understood small shelly fauna and ending in the very early Cambrian with a very diverse, and quite modern Burgess fauna, the rapid radiation of forms called the Cambrian explosion of life.

Planetary environment and the oxygen catastrophe

Weathered Precambrian pillow lava in the Temagami greenstone belt of the Canadian Shield

Details of plate motions and other tectonic functions are only hazily known in the Precambrian. It is generally believed that small proto-continents existed prior to 3000 Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1000 Ma. The supercontinent, known as Rodinia, broke up around 600 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch, roughly 2200 Ma. The best studied is the Sturtian-Varangian glaciation, around 600 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth".

The atmosphere of the early Earth is poorly known, but it is thought to have been smothered in reducing gases, containing very little free oxygen.

When evolving life forms developed photosynthesis, molecular oxygen began to be produced in large quantities, causing an ecological crisis sometimes called the oxygen catastrophe. The oxygen was immediately tied up in chemical reactions, primarily with iron, until the supply of oxidizable surfaces ran out. After that the modern high-oxygen atmosphere developed. Older rocks contain massive banded iron formations that were apparently laid down as iron and oxygen first combined.

Subdivisions

An established terminology has evolved covering the early years of the Earth's existence, as radiometric dating allows plausible real dates to be assigned to specific formations and features.^[6] The Precambrian Supereon is divided into three Precambrian eons: the Hadean (4500-3950 Ma), Archean (3950-2500 Ma), and Proterozoic (2500-542 Ma). See Timetable of the Precambrian.

• Proterozoic: this eon refers to the time from the lower Cambrian boundary, 542 Ma, back through 2500 Ma. The boundary has been placed at various times by various authors, but has now been settled at 542 Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary. The Proterozoic eon is divided into three eras: the Neoproterozoic, Mesoproterozoic, and Paleoproterozoic.
  • Neoproterozoic: The upper (i.e., youngest) geologic era of the Proterozoic eon, from the Cambrian period lower boundary (542 Ma) back to 1000 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
    • Ediacaran: The upper (youngest) geologic period within the Neoproerozoic Era. The "2009 GSA Geologic Time Scale" dates it from 630-542 Ma. (542 Ma is the beginning of the Cambrian period, the earliest period of the Paleozoic Era.) In this period the Ediacaran fauna appeared.
    • Cryogenian: the middle period in the Neoproterozoic Era: 950-630 Ma.
    • Tonian: the earliest period of the Neoproterozoic Era: 1000-950 Ma.
  • Mesoproterozoic: the middle era of the Proterozoic Eon, 1000-1600 Ma. Corresponds to "Precambrian Y" rocks of older North American geology.
  • Paleoproterozoic: oldest era of the Proterozoic Eon, 1600-2500 Ma. Corresponds to "Precambrian X" rocks of older North American geology.
• Archaean Eon: 2500-3800 Ma.
• Hadean Eon: 3950-4500 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 meteorites. ^[7]}

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.^[8]

1. Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.
2. Hadean: dominated by heavy bombardment from about 4.51, (possibly including a Cool Early Earth period) to the end of the Late Heavy Bombardment period.
3. 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.
4. Transition: a period of continued iron banded formation until the first continental red beds.
5. Proterozoic: a period of modern plate tectonics until the first animals.

References

Further reading

• 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
• Wilde, S. A.; Valley, J. W.; Peck, W. H.; Graham, C. M. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago". Nature 409 (6817): 175–178. doi:10.1038/35051550. 
• Wyche, S.; Nelson, D. R.; Riganti, A. (2004). "4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton". Australian Journal of Earth Sciences Volume 51 (1): 31–45. doi:10.1046/j.1400-0952.2003.01042.x. 

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External links

• Late Precambrian Supercontinent and Ice House World from the Paleomap Project