The Archean Eon of Precambrian Time:
3800 to 2500 million years ago
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Lasting more than twice as long as the Phanerozoic eon, the Archean was a time when diverse microbial life flourished in the primordial oceans, and the continental shields developed from volcanic activity. The reducing (anaerobic) atmosphere enabled anaerobic microbes to develop, and plate tectonics followed a regime of continental drift different to that of the Proterozoic and later. During this era, one type of organism, the Cyanobacteria (blue-green algae) produced oxygen as a metabolic by-product; the eventual build-up of this highly reactive gas was to eventually prove fatal to many life-forms.
The Origin of the Continents
Rocks of the Lower Archean (in geology time is often referred to vertically, because younger rocks are deposited above older ones) are rare, and include the oldest known terrestrial rocks, about 3.8 billion years old. In fact, the "age of the oldest preserved rocks on Earth's surface" has been formally proposed as a definition for the base of the Archaean. Most of the oldest rocks are so altered through subsequent metamorphic processes that it is difficult to know under what conditions they were formed. The situation is rather brighter with the more numerous rocks of the Meso- and Neoarchean, from 3.2 to 2.5 billion years ago. These are mostly volcanic in nature, consisting of pillow-like structures identical to those of present-day lavas which have formed underwater. The implication is that at this time the entire Earth was covered by ocean. Perhaps the bulk of the continental masses, formed through volcanic outpourings, had yet to appear from beneath the waves.
This general period, from about 3.0 to 2.5 billion years ago, was the period of maximum continent formation. 70% of continental landmasses date from this period (Thus, most of the continents are extremely ancient). Modern Earth sciences recognize that the present continents are built around cores of extremely ancient rock, called "shields". A large part of Australia is a "shield", as is much of Canada, India, Siberia, and Scandinavia. During the beginning of this time the first known continent Ur formed (c. 3 Billion years ago). In the early period of its existence, it was probably the only continent on Earth, and is so considered by some to be a supercontinent, even though it was probably smaller than Australia is now.
The Origin of Life
The appearance of life on Earth was preceded by a period of chemical evolution, whereby the relatively simple organic molecules gradually aggregated together to form larger and more complex macro-molecules, and finally the first life itself.
We do not know when life first appeared on Earth. According to some sources, the oldest fossil microorganisms are as old as the oldest sedimentary rocks. If so, we can assume that life has been around as long as conditions have been suitable. At the time of these first organisms there was probably no free oxygen, as there is now, but rather a "reducing atmosphere" composed of methane, carbon dioxide, hydrogen and water vapor.
The microorganisms of this period may have used methane or hydrogen rather than oxygen in their metabolism. They are therefore referred to as "anaerobic" (non-oxygen-using). Fermentation is a modern example of anaerobic metabolism. This type of metabolism is 30 to 50 times less effective than oxygen-based ("aerobic") metabolism, or respiration. The first organisms may have been chemoautotrophs, organisms which obtain their carbon from carbon dioxide by oxidizing inorganic compounds. Later came "heterotrophs," which derive their food from other organisms or from organic matter which they were able to consume, and autotrophs, which create organ carbon compounds from carbon dioxide, using energy from sunlight. The first autotrophs -- the "plants" of the Archean ecosystems -- were quite similar to modern blue-green algae.
Not all of the single-celled organisms of this time were solitary. Beginning perhaps 3 billion years ago, and much more often from 2.3 billion years ago, blue-green algae would form the basic structure of large mats, called stromatolites. Modern-day stromatolites can still be found in sheltered bays in West Australia, where the water is so salty that creatures that would otherwise eat them are not able to exist. The fact that such organisms have survived to the present day gives some idea of how slow their evolution is. The transformation of the biosphere seemed to be as slow as the transformation of the geosphere.
The Geological Time-Scale for the Archean Era
| Archean eon|
Neoarchean (2800 - 2500 mya): First large continental shields
Mesoarchean (3200 - 2800 mya): First widely-accepted fossil evidence of life. First banded iron formations.
Paleoarchean (3600 - 3200 mya): First stromatolites? Formation of relatively stable crust units (possibly even earlier [Nutman et al. 2001], but see generally negative review [Sankaram, 2002]).
Eoarchean (~3800 - 3600 mya): Debatable geochemical evidence for life (no longer widely accepted)
Nutman, AP, CRL Friend & VC Bennett (2001), Review of the oldest (4400–3600 Ma) geological and mineralogical record: Glimpses of the beginning. Episodes 24: 93-101.
Ryder, G (2001), Mass flux during the ancient Lunar bombardment: the cataclysm. Lunar & Planet. Sci. 32: 1326.
Sankaram, AV (2002), The controversy over early-Archaean microfossils. Current Sci. 83: 15-17.
Palaeos com - Archean
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