Proterozoic

From Palaeos

Precambrian 4,567-542			Phanerozoic 542-0
Hadean 4,567-3,800	Archean 3,800-2,500	Proterozoic 2,500-542	Phanerozoic 542-0

The Proterozoic Eon of Precambrian Time:

2500 to 542 million years ago

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Previous: Archean | Next: Phanerozoic

The Proterozoic, which lasted even longer than the Archean Era, saw the atmosphere changes from reducing to oxygenated, driving the original anaerobic inhabitants of the Earth into a few restricted anoxic refuges and enabling the rise of aerobic life (both prokaryote and the more complex eukaryotic cell, which requires the high octane boost that oxygen enables.) Stromatolites (colonial cyanobacteria), which had appeared during the Archean, were common. The modern regime of continental drift began, and saw the formation of supercontinent of Rodinia, and several extensive ice ages. Late in the Proterozoic a runaway icehouse effect meant that the preceding warm conditions were replaced by a "Snowball Earth" with ice several kilometers deep covering the globe. Warming conditions saw the short-lived Edicarian biota and finally the appearance of first metazoa.

The Proterozoic or "First Life" was originally the third and youngest era of the Precambrian Eon, but has more recently been reclassified as an eon. It includes the period from about 2500 to a little over 540 million years ago. During this very long span of Earth's history, the atmosphere switched from reducing to oxygenated, and new types of microorganisms, and the first multicelluar life, evolved. The Proterozoic also includes several periods of severe glaciation, including one period - Snowball Earth - when it is thought that the Earth was covered in a layer of ice several kilometers thick. It was in or following these harsh conditions that the first animal life appeared.

Proterozoic eon 2,500-542
Paleoproterozoic era 2,500-1,600				Mesoproterozoic era 1,600-1,000			Neoproterozoic era 1,000-542
Siderian 2,500-2,300	Rhyacian 2,300-2,050	Orosirian 2,050-1,800	Statherian 1,800-1,600	Calymmian 1,600-1,400	Ectasian 1,400-1,200	Stenian 1,200-1,000	Tonian 1,000-850	Cryogenian 850-630	Ediacaran 630-542

1 Plate Tectonics
2 Palaeogeography
3 Life
4 Reference

[edit] Plate Tectonics

The Proterozoic is, roughly speaking, the time when plate tectonics began to govern over other processes in determining the form of the Earth's crust. Although continents were small, they consisted of stable cratons. Mid-ocean spreading ridges did a good deal of the moving, just as they do today. However, everything happened a good deal faster. The magma on which the continents floated was hotter, less viscous, and closer to the surface. Hot spots were probably hotter. The continents moved more swiftly, collided more often and tended to fracture or suture with greater frequency.

[edit] Palaeogeography

We know very little of the geography of the Proterozoic. As Dr. Christopher Scotese notes on his paleomap site: "With available data, 650 million years is about as far back as we can go." Of course, this has not stopped him (or many others) from pushing the paleomap envelope to about 750 Mya. However, beyond that there are only educated guesses.

One fact now does seem reasonably clear. About 1100 or 1200 Mya, most of the landmass of the Earth was locked up in a continent called Rodinia. How Rodinia assembled is speculative at this point.

About 900 Mya, Rodinia started to fracture; and the pattern of fragmentation has become fairly well known. This map sets up a very nice structure around the Amazonia - Baltica - Laurentia corner which is known as a three-armed graben. This is not some sort of con game or high tech slot machine. It is a true continent-busting confluence of two faults of exactly the type which tore Pangea apart 800 My later and which is trying (but probably failing) to tear up East Africa in the present day. In fact, the Kalahari - East Antarctica - Laurentia corner may be the site of a second graben, just as there were really two such structures involved in the demise of Pangea. Scotese, seems to place most emphasis on the northwest to southeast rift splitting Rodinia apart. The huge bulk of Laurentia and Siberia then rotates clockwise about 120° as it moves down almost to the South Pole and back up again, hitting West Africa from the South just as East Gondwana (i.e. North Rodinia), having rotated a bit clockwise, hits it from the Northeast.

One could imagine a more peaceful scenario in which Laurentia and Baltica simply cut through the "strait" between Kalahari and East Antarctica, while rotating (either way) to reach their Cambrian positions and orientations. There are excellent reasons why this cannot be the case, not the least of which is the far southern paleolatitude of North America (Laurentia) during the latest Neoproterozoic. Nevertheless, in the spirit of being obnoxious, we offer this additional map, from Steven Dutch of the University of Wisconsin. The large, dark green thing in the west on Dutch's map is the Niger block -- actually pieces of both Africa and South America. See Johnson & Rivers (2004) which supports this this placement. If this version -- not so very different from the map above -- is correct, Laurentia would have a tough time rotating all the way south around these obstacles to reach a Cambrian position roughly where the Niger block is shown, all in only 230 My. Not impossible, mind you, but not really the most parsimonious interpretation from a geometrical point of view.

The progressive collision and accretion of these minor continents is encouraged by sea floor spreading, and the next supercontinent, Pangea, comes together between 350 and 300 Mya. Supercontinents, however, contain the seeds of their own destruction. 'Heat-capping' sees collossal geothermal power concentrated in the centre sees the supercontinent fracture and spread again. This process has been called 'supercontinent supercycling' [1].

[edit] Life

Life developed from the infant stage of single celled organisms to an adolescence of Eukarya and early plants, fungi, animals. Perhaps other forms developed as well which we know less about because they failed to explode in the Cambrian Explosion. Like all other adolescents, Life grew much larger, discovered sex, and changed its mind frequently about what it was going to be when it grew up. Undoubtedly it tried out many forms and lifestyles which, had we learned of them at the time, we would have sternly disapproved. Life engaged in risky behaviors, such as carelessly spewing so much oxygen into the atmosphere that it nearly poisoned itself until it learned to adapt. It moved out from the warm geothermal vents where, perhaps, it was raised, and nearly froze to death once or twice by wandering into very serious Ice Ages without its mittens. Somehow, in spite of a number of these very close calls, it grew up into the sort of grown-up Life we know today.