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2. Paleozoic

History of Earth's Climate 1. - Precambrian

Introduction - Hadean Eon - Archean Eon - Proterozoic Eon - Links og literature -

Introduction

Visible tangible life in the form of trilobites and other animals appeared for the first time in Cambrian. That is why the traditional boundary between the distant geological past and the period of life on Earth, is defined at start of Cambrian. Cambrian is the first part of Phanerozoic, which traditionally denotes the era, where life has existed on Earth. This article is about the climate in the whole period before Phanerozoic that often is referred to as Precambrian.
The geological periods in pre-Cambrian
The geological eons in Precambrian - Hadean was the burning inferno right after Earth's creation. The first rocks, that we know about, were formed in Archean, water vapor condensed, and an atmosphere of nitrogen and methane was created. Proterozoic was the eon when cyanobacteria produced oxygen, iron and methane were oxidized, and life emerged in the late of the period on the bottom of the sea. Several very severe ice ages occured in Proterozoic that is the Huronian, Sturtian, Marinoan and Gaskiers ice ages.

Hadean Eon

Earth's earliest geological period has been named the Hadean after the underworld in the Greek mythology, Hades. The period started with the formation of Earth 4,56 billion years ago and ended 3,8 billion years ago. There have not been found any rocks from this period on Earth.

The astronomical Sun cloud
An artist's representation of the astronomical cloud that would become the Solar System.

Earth and probably most of the other planets were formed prior to the Hadean period out of a giant cloud of matter. Thanks to its mass and the resulting gravitational force it grew constantly by attracting still more matter from outer space. This huge mass of rock and other materials rotated around its center of gravity as a giant disk.

Within this disk matter collected into larger clumps, which again attracted even more matter due to its mass. The sun was formed first in the center of the disc, and later the Earth and other planets were created.

In the beginning everything was a glowing inferno. The pressure created by the matter, that was brought together, was together with radioactive decay causing an intense heat. In the beginning the Earth's mass was a glowing liquid.

Liquid iron and other heavy materials sank to the "bottom", that is towards the center of Earth, and came to form the earth's magnetic core. The descent of the heavy elements converted gravitational energy into heat, which further raised the liquid Earth's temperature some thousand degrees.

The collision between Theia and Earth
An artistic reconstruction of the clash between Theia and Earth.

It is assumed, that while Earth was still in this hot liquid state, it was hit by a planet, by some called Theia, that was of the size of the planet Mars. Caused by the collision most of Theias mass was hurled out into orbit around Earth together with some initial mass of Earth. Earth got a net mass gain of about 10% by the collision. Planet Theias' kinetic energy was converted to heat that further raised the temperature of Earth's liquid mass.

During the following 10 to 15 million years the escaped matter in orbit around Earth slowly collected itself to the Moon. It was originally about 16,000 kilometers closer to Earth, than it is now, and it must have appeared much larger in the sky than we presently see it.

Also other planets show evidence of dramatic collisions with large celestial bodies. Mercury's crust is peeled away, so mainly just the core is left. Venus rotates in the opposite direction of all other planets.

Earth seen from space in the early Hadean periode
Artistic reconstruction of Earth seen from Space in the early Hadean period.

After the first fiery inferno the Earth's surface began to solidify. However none of Earth's early crust has survived to present day. The planet's first thin and crispy pieces of solidified crust were constantly pushed around by the flow of liquid magma in which they were floating around. Again and again they sank into the molten magma and were remelted as a homogeneous mass. At the same time magma solidified elsewhere and formed new pieces of crispy crust. The initial thin crust was constantly broken and remelted by volcanoes and new meteoroid impacts.

But however many of the early surfaces have been preserved on the Moon. Moon rocks have namely not been eroded by wind and water, and there has been much less volcanism on the Moon than on Earth. The Apollo expeditions to the Moon brought rocks back to Earth that were dated to be just 4.5 billion years old.

The sun's brightness reached a low around mid Hadean with only 75% of its current luminosity. However, the lack of heat input from sunlight was amply compensated by geothermal heat from the liquid magma just beneath the thin crust and from radioactive decay.

Artistic reconstruction of a Hadean landscape
Artistic reconstruction of a landscape in the Hadean period.

Today's heat radiation from the Earth's interior is very low that is 0.06 Watt/m2, to be compared with the present insolation from the Sun, which is 240 Watt/m2. In the Hadean period heat received from the Sun was significantly less, while the radiation of heat from Earth's interior was much larger.

Since the moon's tidal forces have slowly reduced the Earth's rotation speed, Earth must have been rotating much faster in Hadean than today, and so were days and nights much shorter.

One can only speculate on the atmospheric composition of Hadean, as there are no rocks preserved from this period. One can imagine that during the first 100 million years the high temperatures led to degassing from the rocks that together with widespread volcanism created an atmosphere consisting of methane, hydrogen, nitrogen, water vapor and small amounts of CO2 and inert gases. The sky was constantly dark and cloudy because of sulphurous clouds and dust that was thrown up by the numerous meteoroid impacts.

The atmospheric pressure was most likely very high, about 250 atm. The atmosphere was probably highly electrically charged and ravaged by violent storms. The light hydrogen molecules gradually escaped to space. In the early part of Hadean water vapor did not condense, as it was still too hot.

Gradually radioactivity decreased, the original meteor storm calmed down, and Earth's surface cooled slowly.

Analysis of zirconium silicate found in Western Australia show that it has been exposed to liquid water as early as 4.3 to 4.4 billion years ago. Perhaps some of the water condensed in low-lying areas due to the high pressure.
Artistic reproduction of Late Heavy Bombartment The moon sea Mare Imbrium with crater Plato
Left: Artistic reproduction of Late Heavy Bombartment
Right: The moon sea Mare Imbrium with crater Plato.

As we can see with a small telescope on a clear night with full moon, there are on the Moon large flat areas, called "moon seas". It is generally assumed that they have been created by super-sized meteors that smashed holes in the newly solidified moon's surface, thereby allowing lava to flow out through the holes and in this way creating the typical flat moon-surface for example, Mare Imbrium.

This indicates that the meteors were simply gigantic. "Lunar seas" are dotted with huge craters caused by impacts of later large meteors. Moon craters may be up to 360 kilometers in diameter.

It has been stated that between 4.1 to 3.8 billion years ago, the Moon was subject to an intense bombardment of large and small meteors. It is called LHB, which means "Late Heavy Bombardment".

But it is logical that Earth must have been subjected to exactly the same devastating bombardment, only the craters no longer exist, because they have been destroyed by new volcanic eruptions and erosion by wind and water. Moreover, as mentioned before, there are no longer rocks on Earth from the Hadean period. It is the traditional and still prevailing view that the LHB was all-destructive. All, that existed of rocks and possibly life on the surface of the planet, were destroyed and devastated, and everything started all over again, perhaps several times.

Archean Eon

Examples of Archean stromatolites Examples of fossils of stromatolites from Archean: (a-c) Stratiform and conical stromatolites from the 2.985 million old Insuzi Group of South Africa (Beukes and Lowe, 1989), (b) photo by N.J. Beukes. (d) layered cut-sheet and conical stromatolites from 2.985 million years ago from Insuzi Group. (d) Laterally linked, low relief of stratiform to domical stromatolitic mats from the 3.245 million years old Fig Tree Group of South Africa (Byerly et al., 1986) - photo by D.R. Lowe. (e) Stratiform microbial mats from the 3.320 Million years old Kromberg Formation of South Africa (Walsh and Lowe, 1985). (f-h) Conical stromatolites from the 3.388 million years old Strelley Pool Chert of Western Australia (Hofmann et al., 1999), (i) Domical and (j) stratiform stromatolites from the 3.496 million years old Dresser Formation of Western Australia (Walter et al., 1980; Buick et al., 1981).

The name Archean comes from Greek and means "beginning" or "origin". In later years it is commonly believed that life on Earth began in this period.

"Late Heavy Bombardment" ended the Hadean period, and in the Archean Eon the newly formed crust continued to stabilize, a process that eventually led to formation of small stable continental plates, which are called cratons. Original rocks from this period can today be found as smaller parts of greater continental plates. But continents, as we know them today, with continental plates and plate tectonics, did not show up before in the very last part of the Archean.

Start of Archean about 3.8 billion years ago gives the age of the oldest rocks that have been found on Earth. The oldest dated rocks are in Isua Green Stone belt on some islands off the South West Greenland coast.

The Archean Eon lasted 2.3 billion years and ended 2.5 billion years ago.
Present stromatolites at Shark Bay in Western Australia Comparison of modern cyano-bacteria with fossils from Apex Chert
Left: Present stromatolites at Shark Bay in Western Australia.
Right: Comparison of modern cyano-bacteria with fossils from Archean. The two green ones at the top are micro photos of living cyano-bacteria; the below shown fossils have been found in the Apex Chert rock formation in Western Australia.

Some researchers believe that the Archean atmosphere was mainly composed of nitrogen and methane, as it is the case on Titan, Saturn's largest moon. Maybe it contained minor amounts of ammonia and CO2, but however little or no oxygen, and can thus be regarded as a chemically reducing atmosphere.

CO2 was largely dissolved in the oceans. This is in contrast to, for example, the thin atmosphere on Mars, which consists mainly of CO2, precisely because Mars have no oceans that it can be dissolved in. The sky on the Achean Earth was orange caused by the high concentration of methane.

Throughout Archean the general cooling, that started in the end of the Hadean period, continued due to the decrease in the radioactive decay and the reduced meteor bombardment. The initial drop in temperature caused a further decrease in the atmospheric content of important greenhouse gases that were water vapor, CO2 and methane. This in turn lowered the temperature even more, making the absolute humidity in the atmosphere to decrease as water condensed and formed the Earth's oceans.

A piece of frozen 
methane hydrates brought up from the seabed off the American state of Oregon
A piece of frozen methane hydrates brought up from the seabed off the American state of Oregon. - One of the theories, explaining how the Proterozoic ice ages were concluded, is that the climates' initial warming melted large quantities methanehydrat on the seabed thus triggering the strong greenhouse gas methane to escape to the atmosphere in large quantity.

The new cyano-bacteria took up carbon from CO2 in the atmosphere using their photosynthesis, and some of the newly created organic materials sank to the bottom of the oceans, and thereby decreased the concentration of CO2 in the atmosphere.

Today methane's survival time in the atmosphere is only about ten years, as it becomes oxidized by oxygen. It was also oxidized in Archean, but it lasted probably longer time. Part of the methane was deposited on the seabed as frozen methane hydrates. Many believes that also a large proportion of methane disappeared into space.

Earth was mostly covered by water, with volcanoes and volcanic islands sticking up here and there. The seas were acidic and green because of dissolved iron compounds.

The sun is a star in the Hertzsprung-Russell diagram's main sequence, in which it will stay around 11 billion years, during that time it will increase its luminosity three times in total. Start of Archean happened 3.8 billion years ago, which means that the Sun then had a brightness of 77% of today's. At the end of Archean the brightness might had come up around 83% of present value.

Suns luminosity, radius and temperature
Suns luminosity, radius and temperature as a function of time in billion of years - after Ignasi Ribas: "The Sun and stars as the primary energy input in planetary atmospheres - Proceedings of the International Astronomical Union, IAU Symposium."

The sun's brightness in Archean was only about 80% of today's value, but the Earth produced much more heat from its interior, than it does today, probably making the climate temperate. There was a significant geothermal activity.

Most likely most of the very first organisms did not use photosynthesis, but could have used perhaps methane, ammonia or sulfates to their energy needs.

Photosynthesis began with cyano-bacteria, also called blue-green algae, perhaps 3.5 billion years ago. These bacteria are a sort of cross between plants and living organisms; They can grow up to 0.5 mm. long. They have chlorophyllic and photosynthetic pigment, which they use to exploit the rays of the Sun and atmospheric CO2, in this process they emit oxygen just as today's plants.

At shallow muddy shores cyano-bacteria can live in symbiosis with other "organisms and together form moss-like cushions with a few cm. thick calcareous crust; such pads are called stromatolites. These ancient organisms can still be found at the coasts of the Bahamas, Australia and Mexico.

However, the oxygen that the stromatolites produced, immediately reacted chemically with oxidizing rocks on land and iron compounds in the oceans, and therefore there was no increase in atmospheric oxygen for a very long time. The oxygen content in the atmosphere did not begin to rise significantly until billion of years after the beginning of photosynthesis.

Sun's life cycle
Sun's development as a star in the Hertzsprung-Russell diagram's main sequence.

It is obvious that, as there was little or no oxygen in the atmosphere, there could not be created the protective ozone layer in the stratosphere, which today protects life on Earth from ultraviolet radiation. It made it extra difficult for life to get foothold on land.

In modern time it is commonly believed that life on Earth began in Archean. Among the oldest fossils of living organisms from Archean are the 3.45 billion years old stromatolites from the Strelley Pool in Western Australia, some 3.45 billion old microfossils from Swaziland in Africa and the 3.47 billion year old bacteria found in the Apex Chert rock formation in Western Australia, which is similar to modern cyano-bacteria.

Proterozoic Eon

Artistic reconstruction of coastline in Proterozoic In Proterozoic blue-green algae continued to produce ever more oxygen. An artist imagines long coastlines full of stromatolites, which are made up of blue-green algae also known as cyano-bacteria.

The name Proterozoic comes from Greek and means "former life". It was a period of intense volcanic activity, where many mineral-rich volcanic rocks were created. Proterozoic started 2.5 billion years ago and lasted until 0.542 billion years ago.

The sun was weaker than it is now. By start of Proterozoic the Sun's brightness was about 84% of today's, and at the end of the period 542 billion years ago had increased to about 95% of present luminosity.

Large continental land masses were formed for the first time around mid Proterozoic. They were composed of smaller sub-continents. When the contingents pressed against each other, it caused the buildup of mountains. As the mountains began to erode, the sediments washed into the shallow parts of the oceans, creating marine environments, where life could flourish and spread.

The Super Continent Rodinia was formed 1.1 billion before present and was again split to smaller continents around 0.75 billion years before present.

Superkontinentet Rodenia
The super continent Rodenia was composed of smaller proto-continents or cratons, as they are called. A craton is an old and stable part of a super continent, which often has survived several cycles of fusion and separation from major continents. They have a thick crust and deep roots that extend far into the Earth's mantle. Rodiana consisted of the cratons India, Congo, Amazonia, etc. That part of Earth's crust, which later would become to Denmark, is part of the Baltica craton.

At the beginning of Proterozoic, 2 billion years ago, the Earth was hit by a giant meteor near the town of Vredefort in South Africa. The meteor is estimated to have been 5 to 10 kilometers in diameter. We can compare the Vredefort crater with a diameter of 250-300 km. with the Chicxulub crater in Mexico, which is only 170 km. in diameter. The Chicxulub crater is well known as created by the dinosaur-killer meteor that struck down 65 million years ago.

Around the Vredefort crater are today important mines, which contains a very large part of the world andalusite, chromium, fluorspar, platinum and vanadium. It is uncertain whether the minerals originate from the meteor impact or whether they origin from volcanism.

Banded Iron Formations (BIF) belong to a type of iron ore deposits, consisting of layers of iron-rich rock alternating with layers of iron-poor rocks. Most instances of this type were deposited on the sea bottom about 2.5 billion years ago at the beginning of the Proterozoic, where Earth's atmosphere mainly consisted of nitrogen, CO2 and methane.

Vredefort crater in South Africa
Vredefort crater in South Africa was created in Proterozoic by a giant meteor, which was larger than the one that wiped out the dinosaurs.

The blue green algae's photosynthesis consumed CO2 and produced oxygen, just as today's plants. It is believed that this oxygen has reacted with the iron compounds, that were dissolved in the acidic seas, and thus precipitated iron ore as magnetite and hematite on the seabed. The layering shows a pattern of cyclic activity, perhaps oxygen "pulses". Glacial deposits of flint bands indicates that the layers could have been caused by recurrent ice ages interrupted by interglacial periods.

Banded iron formations (BIF) are found worldwide and represent rich iron ore deposits.

The part of the Earth's crust, that was to become Denmark, was part of the proto-continent Baltica. A proto-continent is sometimes referred to as "craton", which is a stable and original component of the Earth's crust.

Proterozoic was, especially at the end of the period, characterized by widespread ice ages. The Huron ice age occured in the beginning of Proterozoic, while the Sturtian, Marinoan and Gaskiers ice ages occurred in the end. One can point to several possible causes for these glaciations.

Cosmic radiation creates aerosols in the atmosphere
Cosmic radiation creates aerosols in the atmosphere.

Through millions of years blue-green algae produced oxygen that reduced atmospheric methane, a potent greenhouse gas, and this reduced obviously the temperature.

In Proterozoic were no plants on land, the continents were barren and naked. But land and bare rocks, not to mention snow, have a significantly higher albedo than today's forest and grass, and therefore a much greater part of the sparse sunlight of the time was reflected back to space, than it would have been the case today.

There also exists an astronomical explanation for the occurrence of ice ages. The Solar system travels around the galactic center, one cycle takes about 230 million years to complete; During its journey it comes through certain "dirty" areas of outer space, which are filled with cosmic dust. The dust will reduce the influx of sunlight on Earth. Some have calculated that the reduction of the influx of sunlight on the surface of Earth in this way can have been reduced by 10 Watt/m2, which represents a reduction by just over 4%. Such a marginal cooling could very likely trigger ice ages.

Additionally Danish researchers have demonstrated an excellent correlation between solar magnetic activity and temperature change on Earth, measured over the last hundred years. They have carried out experiments, which show that when the cosmic rays hit the Earth's atmosphere, they form aerosols, that are clusters of molecules that make up clouds. Thus, one can easily imagine that in periods, when Earth during its orbit is exposed to high cosmic radiation from space, more clouds will be formed leading to lower temperature.
Banded iron formations from Michigan in USA Banded iron formations from Mackenzie Mountains in Canada
Left: Banded iron ore deposits from Michigan in the U.S. The red is iron ore, and the gray-brown is a kind of flint, called chert.
Right: Banded iron formations from Mackenzie Mountains in Canada. The red is iron ore and the gray is a kind of flint. Note the stone, which is embedded in the flint layer. It has probably fallen to the ocean floor, when the iceberg, that transported it, melted. This indicates that the layered structure could be due to recurring glaciations interrupted by inter-glacials, as it also was the case during the later Pleistocene ice age.

Timeline of Earth's past and present glacial periods Timeline of Earth's past and present glacial periods. In this figure time progress from left to right. The known glacial periods in Earth's climate history are the Huronian Ice Age and the Cryogenian Ice Ages, which are Stuartian, Marinoan and Gaskiers ice ages. In Phanerozoic came first the Andean-Saharan ice age and later the Karoo ice age, the Pleistocene Ice Age is called "current".

The long-lasting Huron ice age occurred in the beginning of Proterozoic. It lasted about 300 million years, that is from 2.4 billion to 2.1 billion years before present. The reasons for this ice age are not known. Some think that the Huron Ice Age were triggered by the oxidation process that then had been going on for several billion of years.

Blue-green algae produced constantly oxygen, but atmospheric oxygen did not increase very much, as the oxygen almost immediately reacted with the iron compounds dissolved in the sea, and oxidized atmospheric methane, a powerful greenhouse gas.

Deposited and fossilized glacial silt at Ramsey Lake in the U.S.
Deposited and fossilized glacial silt at Ramsey Lake near Lake Huron in the U.S. - 2.3 billion years old. The layered structure may indicate that they were long periods of real ice ages interrupted by interglacial periods, as in the Pleistocene ice age.

Some think that the reduced greenhouse effect was the reason why the temperature dropped catastrophically. They believe that there may have been a 250 million years long break in volcanic activity and thus in CO2 emissions, resulting in a low CO2 level in the atmosphere and thus a reduced greenhouse effect and consequently a drop in temperature.

Huron Ice Age was one of the most severe and longest ice ages in Earth's history. It takes its name from geological evidence collected near Lake Huron in North America at the border between Canada and the United States.

Artistic reconstruction of the Snowball Earth theory
Artistic reconstruction of the Snowball Earth theory.

The Huron ice age was followed by a lengthy hot period. No one knows, why the ice age stopped, but most believe, it was because of extensive volcanic activity, which discharged huge amounts of CO2. In arctic Canada can be found many rocks containing kaolinit, which mineral can only be formed under tropical conditions. They were formed immediately after the great ice age, and they give evidence of the tropical climate, which replaced the Huron Ice Age.

A team of geologists led by scientists from Harvard University has studied ancient rocks in remote areas of northwestern Canada and has come to the conclusion that 716.5 million years ago the Earth was covered by ice and snow from pole to pole. At that time in the Earth's geological history the mountains, which they found in Canada, were located at the equator. "This is the first time that the Sturtian glaciation has been shown to have occurred at tropical latitudes, providing direct proof that the ice age was a "Snowball Earth event", said Francis A. Macdonald, an assistant professor in the Department of Earth and Planetary Sciences at Harvard. "Our data also suggest that the Sturtian glaciation lasted a minimum of 5 million years."

Marine glacier deposits in the north eastern Canada
Marine glacier deposits in the north western Canada Yokon Territory helped to prove that these rocks had once been covered by ice, while this part of the Earth's crust was located at the equator. The top grey shaded part of the mountain is a ferrous glacial deposition, and the brown bottom is an older calcareous deposition, which took place in tropical climate.

Even on a snowball Earth, Macdonald explained, there would have been temperature differences on Earth, and it is likely that the ice was dynamic: liquefied, thin or thick, forming local patches of open water that gave possibility of life. "The fossil evidence suggests that all the major eukaryotes (cells with a nucleus) groups, with the possible exception of animals, existed before the Sturtian glaciation", MacDonald continued. "Questions raised by this are: If a snowball Earth existed, how did these eukaryotes survive. In additionally one may ask: Did a Sturtian snowball Earth stimulate evolution and creation of animals?"

"From an evolutionary standpoint," he added, "it is not always a bad thing for life on Earth to come face to face with severe stress."

"Because of the high albedo of ice, climate models have long predicted that if sea ice ever develop south of 30 degrees latitude, the whole sea will quickly freeze," said MacDonald. "Our result suggests quite strongly that ice has been present at all latitudes during the Sturtian glaciation."

A rock in Namibia 
containing fossilized glacier deposition overlayed with calcareous deposited
rock
A rock in Namibia, which contains fossilized glacier deposit which are stones and the likes that sank to the ocean floor after the icebergs that had carried them were melted. This glacier layer is overlaid with calcareous marine deposited rock, which typically appears at the end of a glacial period. - This is a photo that contributed to spark the discussion of "Snowball Earth" - Photo: P. Hoffman.

Scientists do not know exactly, what caused the Sturtian glaciation, or what ended it, but Macdonald believes that its dating 716.5 million years before present corresponds closely to the dating of a large area of volcanic rocks, which extends more than 1,500 miles from Alaska to Ellesmere Island in the north eastern Canada. This coincidence could mean that end of the glacial were either caused or accelerated by volcanic activity that sent large amounts of CO2 into the atmosphere.

Some, however, have questioned that the increased CO2 content in the atmosphere was the decisive factor in the end of ice ages.

It has been assumed that the Proterozoic ice ages ended by huge volcanic eruptions, which increased atmospheric CO2 content, for example to 350 times today's level. But the Basque professor Anton Uiarte points out that ten times as much CO2 concentration does not give ten times as large greenhouse effect. The greenhouse effect is not a linear function of the CO2 concentration. Each marginal increase in atmospheric CO2 content provides an ever-smaller increase in the greenhouse effect. He believes that even such a large content of CO2 would not have generated greenhouse effect enough to melt Earth's massive glaciers.

volcanic ash deposits in the Nashville area in central Tennessee
Many believe that volcanoes have played crucial roles at the end of ice ages. Here are volcanic ash deposits in the Nashville area in central Tennessee - Photo by Matthew Saltzman.

The dating of the Marinoan glaciation, which is also called the Varanger glaciation after the Varanger Peninsula in northern Norway, appears to be associated with considerable uncertainty. Many datings have been proposed. But a fairly trustworthy one obtained from the Ghaub rock formation in Namibia in 2004 gave the result 635.5 million years before present plus/minus 1.2 million years. An analysis of glacier deposits in the Sete Lagoas rock formation in the east central Brazil gave a dating of around 635 to 610 million years before present. It is estimated that the Marinoan glaciation lasted at least three million years with a likely duration of 12 million years.

However, in Newfoundland and Massachusetts have been found evidence of an even younger glaciation in late Proterozoic, which has been named Gaskiers Ice Age after the rock formation Gaskiers in eastern Newfoundland. It is dated to have occurred around 580-582 million years before present. Probably it has lasted only about a milion years, and therefore it is not likely that it evolved into a "Snowball Earth".

One of the weaknesses of the "snowball" theory is that it can be difficult to determine the position of a particular continent at a particular time in Earth's geological history.

From the Proterozoic have not been found any sign of life on land, neither animals or plants, except for the stromatolites along the coastlines. The continent's interiors were probably barren and deserted, except maybe for some cyano-bacteria.
An artistic 
reconstruction of the ediacarane fauna on the seabed A fossil of dickinsonia from the Ediacaran period
Left: An artistic reconstruction of the Ediacarane fauna on the seabed.
Right: A fossilized imprint of a primitive creature from the Ediacaran period, which has been named Dickinsonia. It is found in the Ediacara Hills of South Australia. It looks like a footprint of an elephant.

Since the moon's tidal forces slowly decreases Earth's rotation speed, it is estimated that lenth of day and night in the end of Proterozoic was 22 hours. The Sun's brightness was about 95% of its current value. The moon was about 2,000 kilometers closer to Earth than today. There are different opinions about when the atmosphere got an oxygen content about present level. Some believe that by end of Proterozoic the atmosphere contained only 1-2% oxygen, the rest consisted of methane, CO2 and inert gases; others believe that oxygen was produced earlier.

The recurrent ice ages was a serious setback for the stromatolites. After the ice ages we find instead an exotic variety of strange life forms that do not look very much like something that you know today, but some of them have similarity to present molluscs, including jellyfish. They lived all in the deep sea and in the shallow water along the coastlines.
An Ediacaran fossil An Ediacaran fossil called fractofurus A fossil of an 
Ediacaran creature
Left: An Ediacaran fossil that looks like a fan palm.
Middle: A fossil of an Ediacaran creature, which has been named fractofurus.
Right: A fossil of an Ediacaran creature that looks like a squid.

This strange new fauna has given rise to a new sub-period in Proterozoic called Ediacaran named from the Ediacara Hills of South Australia. It extends from the end of the Marioan Ice Age 635 million of years before present until start of the Cambrian 545 million years before present. The Gaskiers Ice Age was merely an interregnum in the Ediacaran period.

These creatures, that existed prior to almost any type of life that exists today, were anchored to the seabed, and took their food from the water. Some were up to 2 meters long. As far as we know, they lacked mouths and recognizable digestive systems, and their bodies are thought to have looked like "bags of mud, disks, hubcaps and mattresses". They were among the first complex life forms that appeared on the planet, but in general they had little resemblance to the life on Earth that would come in the following periods, except that they looked like seaweed and molluscs.

The Ediacarane creatures can possibly have represented eukaryotic organisms (which are made up of cells with a nucleus). Thus seaweed, jellyfish and other molluscs well may have seen the light of day in the warm water at the coasts of the super-continents in the very last period of Proterozoic called Ediacara.
The Archean Eon and the Hadean University of California Museum of Paleontology.
Easy Web Click "Geology" - then "Geological timeline" - and then possibly "overview" or "Proterozoic" or other period.
A New Picture of the Early Earth New York Times.
Evidence of Archean life: Stromatolites and microfossils (pdf) J. William Schopfa, Anatoliy B. Kudryavtsevb, Andrew D. Czajac and Abhishek B. Tripathic.
Snowball Earth Debate about Snowball Earth theory.
Snowball Earth: New Evidence Hints at Global Glaciation 716.5 Million Years Ago Science Daily.
THE LATE HEAVY BOMBARDMENT ENDS From BBC with three videoclip.
Snowball Earth, or Not? Ole Nielsen has two articles about Snowball Earth on his blog My Opera - with many links.
Proterozoikum Snowball Earth - Entstehung der Tiere - Ediacara-Fauna -pdf. Lesson by Wolfgang Kiesling - Rather scientific, but with incredible impressive pictures.
New Evidence Supports Three Major Glaciation Events in Distant Past Fra NASA Earth Observatory.
Solar Luminosity - Wikipedia
Svensmark: The Cloud Mystery -youtube Documentary by Lars Oxfeldt Mortensen. - 52 minutter. Svensmark has demonstrated the effect of the cosmic radiation on the climate: "The cosmic radiation (which hits the surface of Earth) has decreased by approx. 15% in the last 100 years. This has caused that there are now fewer low clouds over the Earth. The low clouds have a cooling effect in the atmosphere, and since there are fewer of them, we have here probably the explanations for much of the warming of Earth's atmosphere by 0.7 degrees Celsius, that has happened over the last 100 years."
Earth's Climate History (Kindle Edition) by Anton Uriarte.

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