J Bret Bennington
The Archean Eon
The rock record of the Archean, or ╬ancient eonÔ begins with the deposition and metamorphism of some of the oldest known crustal rocks, including the Acosta Gneiss in northwestern Canada and the Isua Supergroup located at the southern tip of Greenland. These rocks were metamorphosed from 4.0 to 3.8 billion years ago and thus must have been deposited as sedimentary rocks even earlier than that.
These regions of ancient continental crust are found forming the interior cores or cratons of our modern continents. Through time the continents have grown by the accretion of new continental crust around the margins of the cratons.
Archean Plate Tectonics
Many small regions and a few large areas of Archean continental rock are found in cratonic areas on all continents. These Archean formations have several elements in common that give us important clues to the nature of Archean geology. In all cases they show that Archean continents and collisions were somewhat different from those of younger ages.
The interior of the Archean Earth was probably about 3X hotter than it was today because of the greater concentration of radioactive isotopes and the residual heat from the Earth's accretion. The mantle was probably much more fluid and the crust much thinner, resulting in rapid formation of oceanic crust at ridges and hot spots and rapid recycling of oceanic crust at subduction zones. The Earth's surface was probably broken up into many small plates with volcanic islands and arcs in great abundance. Small protocontinents formed as crustal rock was melted and remelted by hot spots and recycled in subduction zones. Evidence that this is true comes from the nature of Archean rocks.
Felsic Protocontinents and Greenstone belts
Viewed from above, Archean cratons are seen to consist of regions of light-colored felsic rock (gneisses) surrounded by pods of dark-colored greenstone (chlorite rich metamorphic rocks). The regions of felsic rock are highly metamorphosed protocontinents and the greenstone belts between them represent volcanic rocks and sediments that accumulated along subduction zones and then were sutured to the protocontinents during collisions.
Most early Archean rocks appear to have originated as deep water clastic
deposits such as mudstones and greywackes, often with a high concentration
of eroded volcanic minerals. Conspicuously absent are shallow water, shelf
deposits such as carbonates and sandstones. The reason for this absence
is that there were no large shelf areas in the Archean. Most sedimentary
rocks appear to have been deposited in subduction zones and tectonic basins,
rather than in shallow water along the margins of continents.
The first large continents
The first large cratonic landmass appears to have formed about 3 billion years ago toward the end of the Middle Archean and is now preserved in southern Africa. Overlying a sequence of greenstone is a thick accumulation of shallow water sediments called the Pongola Supergroup that were deposited over a broad shelf. These ancient sedimentary rocks include tidal flat sandstones with ripple marks and mudstones with mudcracks. Overlying these rocks are younger (2.5 - 2.8 Ga) terrestrial deposits of the Witwatersrand sequence. These deposits are world famous for their detrital accumulations of gold that developed in broad stream channels. All of these deposits are evidence for an extensive continental landmass.
By the end of the Archean there were significant episodes of metamorphism occurring leading to the aggregation of protocontinents and the production of large amounts of continental crust. This interval of extensive cratonization is unparalleled in Earth history - it has never been repeated - and geologists are not sure why it occurred.
Banded Iron Formations
The Archean also includes several times during which extensive banded
iron formations were deposited. These unusual rocks consist of thin
layers of alternating chert (silica) and iron - sometimes oxidized, but
often in almost metallic form (pure iron). It is not at all clear what
caused the formation of the BIFs. Traditionally, geologists have argued
that they formed in deep water near hydrothermal vents, which supplied
abundant silica and iron in an oxygen-free, deep water environment. However,
some geologists have recently argued that they are shallow-water deposits
caused by the chemical replacement of limestone by iron and silica after
the sediments were buried.
The Origin of Life on Earth
Perhaps the most significant event to have occurred in the Archean (or earlier Hadean) was the origin of life. We know very little about how life came to be, however, recent studies of living bacteria have radically changed our ideas about where life evolved and what the first organisms were like.
The origin of life would have required the spontaneous organization of self-replicating organic molecules. The basic minimum requirements would have been the following:
The crucible of creation
In the 1950's and 1960's experiments showed that organic molecules such as amino acids could form from the reaction of atmospheric gases, electricity and heat. Further experiments demonstrated that drying and re-wetting of these simple organic compounds could produce cell-like membranes and simple proteins. Because of this it was widely held that life must have arisen in shallow pools of surface water (an idea suggested by Charles Darwin himself). However, organic compounds in shallow pools would have been exposed to ultraviolet radiation and whatever oxygen might have existed in the Archean atmosphere - both of which would have instantly destroyed any organic compounds that might have formed.
Archaebacteria and deep sea vents
Recently, biologists have discovered a variety of extremely simple bacteria
(the Archaebacteria) that appear to be the most primitive life forms on
Earth and the closest to the ancestors of all life. Amazingly, almost all
of these primitive bacteria are hyperthermophiles meaning that they
thrive in extremely hot temperatures, up to the boiling point of water.
These bacteria are found living today in hot springs and in volcanic vents
deep in the ocean. Also interesting is the fact that many of them can feed
directly on elemental sulfur and sulfur compounds for their metabolic energy
- they are chemoautotrophs. The characteristics of these bacteria
strongly argue that life arose deep in the oceans of the Archean, at the
many hydrothermal, volcanic vents that would have dotted the ocean floor
near rifting zones. These vents would have provided chemical and heat energy,
abundant chemical and mineral compounds, including sulfur, and protection
from oxygen and ultraviolet radiation.
The Fossil Record of Early Life
The Earliest Indirect Evidence of Life
There is reason to believe that life may have already evolved by the time the oldest known rocks were deposited. Banded iron and chert deposits of the Isua Supergroup contain graphite (carbon) with an isotopic ratio of C12 to C13 that is identical to that produced by living systems.
The Earliest Fossil Evidence of Life
3.5 Ga old rocks of the Warrawoona Group found in the Pilbara Shield of Western Australia and slightly younger rocks of the Onverwacht Group in S. Africa contain sedimentary structures that may be stromatolites.
Stromatolites - These are finely layered, mound-shaped accumulations
of mud trapped by growing mats of cyanobacteria (blue green algae).
Cyanobacteria are very primitive photosythetic procaryotes (bacteria) that
grow in marine environments, producing mats of cell filiments that can
mound-up with marine mud to produce structures several meters high. Stromatolites
are rare in the early Archean but become increasingly common in rocks from
the middle and later Archean.
Fossil cells - Ultra fine-grained cherts in Archean rocks have been found that contain what appear to be preserved organic matter from cells. Filimentous structures resembling modern cyanobacteria have been found in 3.5 Ga rocks of Australia and cells apparently in various stages of division have been found in the Fig Tree Group of South Africa in rocks that are 3.0 Ga old.
Archean Oxygen Levels
Once cyanobacteria were present in the oceans they would have been producing photosynthetic oxygen. Currently, geologists are debating how rapidly the Archean atmosphere filled with oxygen. Most geologists argue that atmospheric oxygen levels were very low in the Archean - probably far less than one thousandth of their present levels. Evidence for this comes from the abundance of detrital pyrite and uraninite in Archean sediments. Pyrite and uraninite are rapidly oxidized and do not survive long enough to be reworked into sedimentary grains in an oxygenated atmosphere. Furthermore, common oxidized sedimentary minerals such as hematite are completely absent in Archean rocks.
However, some geologists point out that there are instances of oxidized Archean sediments and that the level of oxygen in the Archean atmosphere may have increased rapidly once cyanobacteria were widespread in the world's oceans.
The Age of Pond Scum
What is perhaps most remarkable about Archean life is that it does not appear to change much for about 1.5 billion years. The fossils record is one of stromatolites and cyanobacteria from the beginning of the Archean until well into the Proterozoic.