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 fifty years. The Precambrian
fossil record is poor, and those fossils present (e.g.
stromatolites) are of limited
biostratigraphic use. Many Precambrian
rocks are heavily metamorphosed,
obscuring their origins, while others have either been destroyed by erosion,
or remain deeply buried beneath
Phanerozoic strata.
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.
The term Precambrian is somewhat out-moded, but is still in common use among
geologists and
paleontologists. In order to study the Precambrian, we will break it
into its currently recognized divisions.
Hadeon Eon
The Hadean is the earth's first geologic eon. It started at Earth's formation about 4.6 billion years ago (4,600 Ma), and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the conditions on Earth at the time. Since few geological traces of this period remain on Earth there are no official subdivisions.
Proto-Earth
The Proto-Earth grew by accretion, until the inner part of the protoplanet was hot enough to melt the heavy, siderophile metals. Due to their larger densities such (now liquid) metals began to sink to the Earth's center of mass. This so called iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form, producing the layered structure of Earth and setting up the formation of Earth's magnetic field.
During the accretion of material to the protoplanet, a cloud of gaseous silica must have surrounded the Earth, to condense afterwards as solid rocks on the surface. What was left surrounding the planet was an early atmosphere of light (atmophile) elements from the solar nebula, mainly hydrogen and helium, but the solar wind and Earth's heat would have driven off this atmosphere.
This changed when Earth was about 40% its present radius, and gravitational attraction retained an atmosphere which included water.
A rare characteristic of our planet is its large natural satellite, the Moon. During the Apollo program, rocks from the Moon's surface were brought to Earth. Radiometric dating of these rocks has shown the Moon to be 4527 ± 10 million years old, about 30 to 55 million years younger than other bodies in the solar system. Another special feature is the relatively low density of the Moon, which must mean it does not have a large metallic core, like all other terrestrial bodies in the solar system. The Moon has a bulk composition closely resembling the Earth's mantle and crust together, without the Earth's core. This has led to the giant impact hypothesis, the idea that the Moon was formed during a giant impact of the proto-Earth with another protoplanet, by accretion of the material blown off the mantles of the proto-Earth and impactor.
The impactor, sometimes named Theia, is thought to have been a little smaller than the current planet Mars. It could have formed by accretion of matter about 150 million kilometres from both the Sun and Earth, at their fourth or fifth Lagrangian point. Its orbit may have been stable at first, but destabilized as Theia's mass increased due to the accretion of matter. Theia oscillated in larger and larger orbits around the Lagrangian point until it finally collided with Earth about 4.533 Ga.
Models show that when an impactor this size struck the proto-Earth at a low angle and relatively low speed (8–20 km/sec), much material from the mantles (and proto-crusts) of the proto-Earth and the impactor was ejected into space, where much of it stayed in orbit around the Earth. This material would eventually form the Moon. However, the metallic cores of the impactor would have sunk through the Earth's mantle to fuse with the Earth's core, depleting the Moon of metallic material. The giant impact hypothesis thus explains the Moon's abnormal composition. The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.
The radiometric ages show the Earth existed already for at least 10 million years before the impact, enough time to allow for differentiation of the Earth's primitive mantle and core. Then, when the impact occurred, only material from the mantle was ejected, leaving the Earth's core of heavy siderophile elements untouched.
The impact had some important consequences for the young Earth. It released a gigantic amount of energy, causing both the Earth and Moon to be completely molten. Immediately after the impact, the Earth's mantle was vigorously convecting, the surface was a large magma ocean. Due to the enormous amount of energy released, the planet's first atmosphere must have been completely blown off. The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons (a simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons). It may also have sped up Earth’s rotation.
Origin of the oceans and atmosphere
Because the Earth lacked an atmosphere immediately after the giant impact, cooling must have been fast. Within 150 million years a solid crust with a basaltic composition must have formed. The felsic continental crust of today did not yet exist. Within the Earth, further differentiation could only begin when the mantle had at least partly solidified again. Nevertheless, during the early Archaean (about 3.0 Ga) the mantle was still much hotter than today, probably around 1600°C. This means the fraction of partially molten material was still much larger than today.
Steam escaped from the crust, and more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity.
The large amount of water on Earth can never have been produced by volcanism and degassing alone. It is assumed the water was derived from impacting comets that contained ice. Though most comets are today in orbits farther away from the Sun than Neptune, computer simulations show they were originally far more common in the inner parts of the solar system. However, most of the water on Earth was probably derived from small impacting protoplanets, objects comparable with today's small icy moons of the outer planets. Impacts of these objects can have enriched the terrestrial planets (Mercury, Venus, the Earth and Mars) with water, carbon dioxide, methane, ammonia, nitrogen and other volatiles. If all water in the Earth's oceans was derived from comets alone, a million impacting comets are required to explain the oceans. Computer simulations show this is not an unreasonable number.
As the planet cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming by 4.2 Ga, or as early as 4.4 Ga. In any event, by the start of the Archaean eon the Earth was already covered with oceans. The new atmosphere probably contained water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. As the output of the Sun was only 70% of the current amount, the presence of significant amounts of greenhouse gas in the atmosphere most likely prevented the surface water from freezing. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.
The first continents
Mantle convection, the process that drives plate tectonics today, is a result of heat flow from the core to the Earth's surface. It involves the creation of rigid tectonic plates at mid-oceanic ridges. These plates are destroyed by subduction into the mantle at subduction zones. The inner Earth was warmer during the Hadean and Archaean eons, so convection in the mantle must have been faster. When a process similar to present day plate tectonics did occur, this will have gone faster too. Most geologists think that in the Hadean and Archaean subduction zones were more common, and therefore tectonic plates were smaller.
The initial crust, formed when the Earth's surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. It is however assumed that this crust must have been basaltic in composition like today's oceanic crust, because little crustal differentiation had yet taken place. The first larger pieces of continental crust, which is a product of differentiation of lighter elements during partial melting in the lower crust, appeared at the start of the Archaean, about 4.0 Ga. What is left of these first small continents are called cratons. These pieces of Archaean crust form the cores around which today's continents grew.
The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites and about 4.0 Ga. They show traces of metamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing rivers and seas existed at the time.
Cratons consist mostly of two alternating types of terranes. The first are so called greenstone belts, consisting of low grade metamorphosed sedimentary rocks. These "greenstones" are similar to the sediments today found in oceanic trenches, above subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction during the Archaean. The second type are complexes of felsic magmatic rocks. These rocks are mostly tonalite, trondhjemite or granodiorite, types of rock similar in composition to granite (hence such terranes are called TTG-terranes). TTG-complexes are seen as the relicts of the first continental crust, formed by partial melting in basalt. The alternation between greenstone belts and TTG-complexes is interpreted as a tectonic situation in which small proto-continents were separated by a thorough network of subduction zones.
In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada and Western Australia. The oldest known rock formations (the Isua greenstone belt) comprise sediments from Greenland dated around 3.8 billion years ago somewhat altered by a volcanic dike that penetrated the rocks after they were deposited. Individual zircon crystals redeposited in sediments in Western Canada and the Jack Hills region of Western Australia are much older. The oldest dated zircons date from about 4,400 Ma – very close to the hypothesized time of the Earth's formation.
The Greenland sediments include banded iron beds. They contain possibly organic carbon and imply some possibility that photosynthetic life had already emerged at that time. The oldest known fossils (from Australia) date from a few hundred million years later.
A sizeable quantity of water would have been in the material which formed the Earth. Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. Hydrogen and helium are expected to continually leak from the atmosphere, but the lack of denser noble gases in the modern atmosphere suggests that something disastrous happened to the early atmosphere.
Part of the young planet is theorized to have been disrupted by the impact which created the Moon, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses. However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy carbon dioxide atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230 °C because of the atmospheric pressure of the heavy CO2 atmosphere. As cooling continued, subduction and dissolving in ocean water removed most CO2 from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.
Study of zircons has found that liquid water must have existed as long ago as 4400 Ma, very soon after the formation of the Earth. This requires the presence of an atmosphere. The Cool Early Earth theory covers a range from about 4400 Ma to 4000 Ma.
Recent (September 2008) studies of zircons found in Australian Hadean rock hold minerals that point to the existence of plate tectonics as early as 4 billion years ago. If this holds true, the previous beliefs about the Hadean period are far from correct. That is, rather than a hot, molten surface and atmosphere full of carbon dioxide, the earth's surface would be very much like it is today. The action of plate tectonics traps vast amounts of carbon dioxide, thereby eliminating the greenhouse effects and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life
End of Monday's Reading
Archean Eon
The Archean is the geologic eon
that begins at the end of the Hadeon, approximately 3.8 billion years ago,
and it ends about 2.5 billion years ago. At the
beginning of the Archean, the
Earth's
heat flow was nearly three times higher than it is today, and was still
twice the current level by the beginning of the
Proterozoic (2,500 Ma). The extra heat may
have been remnant heat from the planetary accretion, partly heat of
formation of the iron core, and partially caused by greater radiogenic heat
production from short-lived radionuclides such as
uranium-235.
The majority of Archean rocks which still survive are
metamorphic and
igneous rocks.
Volcanic
activity was considerably higher than today, with numerous
hot spots,
rift
valleys, and eruption of lavas including
unusual types such as
komatiite. Nevertheless, intrusive igneous
rocks predominate throughout the crystalline
cratonic
remnants of the Archean crust which survive today. These are magmas which
infiltrated into host rocks, but solidified before they could erupt at the
Earth's surface. Examples include great melt sheets and voluminous plutonic
masses of
granite,
diorite,
layered intrusions,
anorthosites and
monzonites known as
sanukitoids.
The Earth of the early Archean may have had a different
tectonic style. Some scientists think that because the Earth was hotter,
plate tectonic activity
was more vigorous than it is today, resulting in a much greater rate of
recycling of crustal material. This may have prevented
cratonisation and continent formation until
the mantle cooled and convection slowed down. Others argue that the
subcontinental lithospheric mantle was too buoyant to
subduct,
and that the lack of Archean rocks is a function of erosion by subsequent
tectonic events. The question of whether or not plate tectonic activity
existed in the Archean is an active area of modern geoscientific research.
There were no large continents until late in the
Archean: small protocontinents were the norm, prevented from coalescing into
larger units by the high rate of geologic activity. These
felsic
protocontinents probably formed at
hotspots rather than
subduction zones, from
a variety of sources:
igneous differentiation
of mafic rocks to produce intermediate and felsic rocks,
mafic
magma melting more felsic rocks and forcing
granitization of intermediate rocks,
partial melting of mafic rock, and from the
metamorphic alteration
of felsic sedimentary rocks. Such continental fragments may not have been
preserved unless they were buoyant enough or fortunate enough to avoid
energetic subduction zones.
An explanation for the general lack of Hadean rocks
(older than 3800 Ma) is the amount of extrasolar debris present within the
early solar system. Even after planetary formation, considerable volumes of
large
asteroids and
meteorites still existed, and bombarded the
early Earth until approximately 3800 Ma. A barrage of particularly large
impactors known as the
late heavy bombardment
may have prevented any large crustal fragments from forming by literally
shattering the early protocontinents.
Archean palaeoenvironment
The Archean
atmosphere is thought to have lacked free oxygen.
Temperatures appear to have been near modern levels even within 500 Ma of
Earth's formation, with liquid water present, as evidenced by certain highly
deformed
gneisses produced by
metamorphism of
sedimentary
protoliths. Astronomers think that the sun
was about one-third dimmer than at present, which may have contributed to
lower global temperatures than otherwise expected. This is thought to
reflect larger amounts of greenhouse gases than later in the
Earth's history.
By the end of the Archaean c. 2600 Mya, plate tectonic
activity may have been similar to that of the modern Earth. There are
well-preserved sedimentary basins, and evidence of
volcanic arcs, intracontinental
rifts,
continent-continent collisions and widespread globe-spanning
orogenic
events suggesting the assembly and
destruction of one and perhaps several
supercontinents. Liquid water was prevalent,
despite the
faint young sun paradox,
and deep oceanic basins are known to have existed by the presence of
banded iron formations,
chert beds, chemical sediments and pillow
basalts.
Archean geology
Although a
few mineral grains are known that are Hadean, the oldest rock formations
exposed on the surface of the Earth
are Archean or slightly older. Archean rocks are known from
Greenland, the
Canadian Shield, the
Baltic shield,
Scotland,
India, Brazil,
western
Australia, and southern
Africa. Although the first
continents formed during this eon, rock of
this age makes up only 7% of the world's current
cratons;
even allowing for erosion and destruction of past formations, evidence
suggests that continental crust
equivalent to only 5-40% of the present amount formed during the Archean.
In contrast to the Proterozoic, Archean rocks are often
heavily metamorphized deep-water sediments, such as
graywackes,
mudstones, volcanic sediments, and
banded iron formations.
Carbonate rocks are
rare, indicating that the oceans were more acidic due to dissolved
carbon dioxide than during the Proterozoic.
Greenstone belts are typical Archean
formations, consisting of alternating units of metamorphosed
mafic
igneous and sedimentary rocks. The meta-igneous rocks were derived from
volcanic
island arcs, while the
metasediments represent deep-sea sediments eroded from the neighboring
island arcs and deposited in a
forearc
basin. Greenstone belts represent sutures
between protocontinents.
Archean life
Fossils of
cyanobacterial mats (stromatolites,
which were instrumental in creating the free oxygen in the atmosphere) are found throughout the Archean,
becoming especially common late in the eon, while a few probable
bacterial
fossils are known from
chert beds.
In addition to the domain
Bacteria (once known as
Eubacteria), microfossils of the domain
Archaea have also been identified.
Life was probably present throughout the Archean, but
may have been limited to simple non-nucleated single-celled organisms,
called
Prokaryota (formerly
known as Monera). There are no known
eukaryotic fossils, though they might have
evolved during the Archean without leaving any fossils.
No fossil evidence yet exists for ultramicroscopic intracellular replicators
such as viruses.
Proterozoic Eon
The Proterozoic Eon began at the end
of the Archean, approximately 2.5 billion years ago, and lasts almost 2
billion years, until the beginning of the Cambrian Period.
The geologic record of the Proterozoic is much better than that for the
preceding Archean.
In contrast to the deep-water deposits of the Archean, the Proterozoic
features many
strata that were laid
down in extensive shallow
epicontinental seas; furthermore, many of
these rocks are less
metamorphosed than
Archean-age ones, and many are unaltered. Study of these rocks shows
that the eon featured massive, rapid
continental accretion (unique to the
Proterozoic),
supercontinent cycles,
and wholly-modern
orogenic activity.
The first known glaciations occurred during the Proterozoic;
one began shortly after the beginning of the eon, while there were at least
four during the Neoproterozoic, climaxing with the
Snowball Earth of the Varangian glaciation.[3]
One of the
most important events of the Proterozoic was the
buildup of oxygen
in the Earth's atmosphere. Though oxygen was undoubtedly released by
photosynthesis well back in
Archean
times, it could not build up to any significant degree until chemical sinks
—
unoxidized
sulfur
and
iron — had been filled; until roughly 2.3
billion years ago, oxygen was probably only 1% to 2% of its current level.
Banded iron formations,
which provide most of the world's iron ore, were also a prominent chemical
sink; most accumulation ceased after 1.9 billion years ago, either due to an
increase in oxygen or a more thorough mixing of the oceanic water column.
Red beds,
which are colored by
hematite, indicate an
increase in atmospheric oxygen after 2 billion years ago; they are not found
in older rocks. The oxygen buildup was probably due to two factors: a
filling of the chemical sinks, and an increase in
carbon
burial, which sequestered
organic compounds that
would have otherwise been oxidized by the atmosphere.
The first
advanced single-celled and multi-cellular life roughly coincides with the
start of the accumulation of free oxygen; this may have been due to an
increase in the oxidized
nitrates
that
eukaryotes use, as
opposed to
cyanobacteria.[6]
It was also during the Proterozoic that the first
symbiotic relationships between
mitochondria (for nearly all eukaryotes) and
chloroplasts (for
plants and some
protists
only) and their hosts evolved.
The blossoming of eukaryotes such as
acritarchs did not preclude the expansion of
cyanobacteria; in fact, stromatolites reached their greatest abundance and
diversity during the Proterozoic, peaking roughly 1.2 billion years ago.
Classically, the boundary between the Proterozoic and
the
Phanerozoic eons was
set at the base of the
Cambrian
period when the first
fossils of animals including
trilobites and
archeocyathids appeared. In the second half
of the 20th century, a number of fossil forms have been found in Proterozoic
rocks, but the upper boundary of the Proterozoic has remained fixed at the
base of the
Cambrian, which is
currently placed at 542 Ma.
End of Reading
Wednesday - The Earth's Atmosphere
Earliest atmosphere
The outgassings of the Earth were stripped away by
solar
wind early in the history of the planet
until a steady state was established, the first atmosphere. Based on today's
volcanic evidence, this atmosphere would have contained 80% water vapor, 10%
carbon dioxide, 5 to 7% hydrogen sulfide, and smaller amounts of nitrogen,
carbon monoxide, hydrogen, methane and inert gases.
A major
rainfall led to the buildup of a vast ocean, enriching the other agents,
first carbon dioxide and later nitrogen and inert gases. A major part of
carbon dioxide exhalations were soon dissolved in water and built up
carbonaceous sediments.
Second atmosphere
Water related
sediments have been found dating from as early as 3.8 billion years ago.
About 3.4 billion years ago, nitrogen was the major part of the then stable
"second atmosphere." An influence of life has to be taken into account
rather soon in the history of the atmosphere, since hints of early life
forms are to be found as early as 3.5 billion years ago. The fact that
this is not perfectly in line with the - compared to today 30% lower - solar
radiance of the early Sun has been described as the "Faint
young Sun paradox".
The
geological record however shows a continually relatively warm surface during
the complete early temperature record of the Earth with the exception of one
cold glacial phase about 2.4 billion years ago. Sometime during the late
Archaean era an oxygen-containing atmosphere
began to develop, apparently from photosynthesizing algae which have been
found as stromatolite fossils from 2.7 billion years ago. The early basic
carbon
isotopy (isotope ratio
proportions) is very much in line with what is found today, suggesting that
the fundamental features of the carbon cycle were established as early as 4
billion years ago.
Third atmosphere
Oxygen content of the Atmosphere since one Billion
years
The
accretion of continents about 3.5 billion years ago[14]
added
plate tectonics,
constantly rearranging the continents and also shaping long-term climate
evolution by allowing the transfer of carbon dioxide to large land-based
carbonate storages. Free oxygen did not exist until about 1.7 billion years
ago and this can be seen with the development of the red beds and the end of
the banded iron formations. This signifies a shift from a reducing
atmosphere to an oxidising atmosphere. O2 showed major ups and downs until
reaching a steady state of more than 15%.[15]
The following time span was the
Phanerozoic era, during which
oxygen-breathing
metazoan life forms
began to appear.
Life
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. 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.
Theological Perspective
By far the biggest issue during the Precambrian has to be the beginning of life. Atheist scientists are trying to show that life can begin on its own, without any help from God. At the same time, young earth creationists are arguing that the earth is only 6,000 years old, and life began when God created it. From the old earth creationist perspective, we believe that God created life in the order that we find it in the fossil record, starting with simple life forms, and progressing to more complex ones throughout earth's history. There are two ways to view the creation of life forms:
1. Theistic Evolution: God created the first life forms, and then they evolved over the billions of years of earth's history. Some people believe that after creating the first life form, God did nothing else, and simply let the physical laws that He put in place run their course. Others believe that God actively guided the evolutionary process.
2. Progressive Creation: God created each and every life form as a unique creation. They did not evolve from previously existing life forms.
Old earth creationists are equally divided among these two beliefs. Which should you believe? That is something that you should discuss with your parents.
How is this reconciled with the events portrayed on each of the six days of creation? For example, on day three, all the plants were created, before day four, which is the creation of the seasons (which many plants depend on), and before all the animals were created on days five and six. However, the fossil record shows new plant species appearing throughout the same time that new animals were being created. The only way to reconcile this is with Creation Overlap. This simply states that God used the six days of creation to explain six events or group of events, some of which overlapped each other. For additional reading on this, see the explanation of Genesis 1 from an old earth creationist perspective (optional reading).
End of Reading
Thursday - Stromoatolites
Stromatolites (from Greek στρώμα, strōma, mattress, bed, stratum, and λιθος, lithos, rock) are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae). They include some of the most ancient records of life on Earth.
A variety of stromatolite morphologies exist including conical, stratiform, branching, domal, and columnar types. Stromatolites occur widely in the fossil record of the Precambrian, but are rare today. Very few ancient stromatolites contain fossilized microbes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic (non-organic) precipitation. Finding reliable ways to distinguish between biologically-formed and abiotic (non-biological) stromatolites is an active area of research in geology.
Fossil Record
Stromatolites were much more abundant on the planet in Precambrian times. While older, Archean fossil remains are presumed to be colonies of single-celled blue-green bacteria, younger (that is, Proterozoic) fossils may be primordial forms of the eukaryote chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic record is Collenia. The earliest stromatolite of confirmed microbial origin dates to 2,724 . A recent discovery provides strong evidence of microbial stromatolites extending as far back as 3450.
Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth, with their abundance peaking about 1,250 million years ago. They subsequently declined in abundance and diversity, which by the start of the Cambrian had fallen to 20% of their peak. The most widely-supported explanation is that stromatolite builders fell victims to grazing creatures (the Cambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago.
The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered.
While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis.
Cyanobacteria use water, carbon dioxide, and sunlight to create their food. The byproducts of this process are oxygen and calcium carbonate (lime). A layer of mucus often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis.
Modern Stromatolites
Modern stromatolites are mostly found in hypersaline lakes and marine lagoons where extreme conditions due to high saline levels exclude animal grazing. One such location is Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia where excellent specimens are observed today, and another is Lagoa Salgada, state of Rio Grande do Norte, Brazil, where modern stromatolites can be observed as bioherm (domal type) and beds. Inland stromatolites can also be found in saline waters in Cuatro Ciénegas, a unique ecosystem in the Mexican desert, and in Lake Alchichica, a maar lake in Mexico's Oriental Basin. Modern stromatolites are only known to prosper in an open marine environment in the Exuma Cays in the Bahamas.
Freshwater stromatolites are found in Lake Salda in southern Turkey. The waters are rich in magnesium and the stromatolite structures are made of hydromagnesite.
Layered spherical growth structures named oncolites are similar to stromatolites, and are also known from the fossil record.
Today you will take the end of chapter test. Please close all other browser windows, and click on the link below. During the test, do not click the Back button on your browser.
After you have completed the test, you may proceed to Chapter 2 on your next school day. Please return to the introduction page for the link to the next chapter.
Return to the Old Earth Ministries Online Earth History Curriculum homepage.
Adapted from these Wikipedia pages:
Precambrian, Timetable of the Precambrian, Evolution of Earth's Atmosphere, Paleoclimatology
Copyright issues. Wikipedia pages may be re-used on other websites. A link back to the original page is provided. Changes to the text in this curriculum were made to clarify the interpretation from a Christian perspective. Thus, all statements in the text that are religious in nature have been added. Some duplicate material has been removed, and some material not relevant for a high school class was removed.