Palæos:		Archean
ARCHEAN EON		The Archean Eon

 

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The Archean

The Archean Eon of Precambrian Time: 3.8 - 2.5 billion years ago

• The Origin of the Continents
• The Origin of Life
• The Geological Time-Scale for the Archean Era
• Archean Links & References

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 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. 

The Origin of Life

The appearance of life on Earth was preceded by a period of chemical evolution, whereby the relative simple organic molecules gradually aggregated together to form larger and more complex  macro-molecules, and finally the first life itself.  Scientists claim to be able to repeat all these  stages in their laboratories, but doubts have been expressed occasionally.

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.  t 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  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 chemautotrophs, organisms which obtain their carbon from carbon dioxide by oxidizing inorganic compounds.  Later came "heterotrophs," which derive their food from other organisms or organic from 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 -- wer 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 following passage is from Benchley and Harper, Palaeoecology, p.121

The Pre-Vendian fossil record is dominated by stromatolites, sheets of calcium carbonate associated with cyanobacteria. Carbonate material is trapped and precipitated on the surface of the filamentous bacteria to generate a distinctive laminated structure. Stromatolites are generally rare in Archaean rocks, becoming more common during the late Archaean and early Proterozoic. By the end of the early Proterozoic a range of stromatolite architectures had developed including stratiform. columnar, conical, domal and nodular growth forms; during the mild- and late Proterozoic the columnar growth types diversified. Many groups of stromatolite were much reduced during the early Cambrian as burrowing and grazing metazoans together with the meiofauna and the influx of coarse siliclastic sediment inhibited and often destroyed stromatolite growth. The stromatolites provide an example of the functional morphology of a pre-Phanerozoic design.

Stromatolites were built by photosynthesizing organisms occupying shallow-water environments within the photic zone. The range of growth forms shows adaptations to a range of environmental conditions controlled by current and wave energy together with sedimentation rates. These simple morphologies show a very direct relationship to environmental conditions, indicative of early modes of adaptation.

The Geological Time-Scale for the Archean Era

Links and  References

Introduction to the Archaean Era - 3.8 to 2.5 billion years ago

Archean Life: several easy pages on the Archean from Prof. Kevin Hefferan (U. Wisc. Stevens Point)

Geol 02C: lecture notes from Brof. Bret Bennington of Hofstra.

Peripatus: Archean Era: notes and quotes from our own Chris Clowes.

Archean Summary: nice short, unattributed article.

References

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.  

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