Chapter 8 Precambrian Earth and Life History—The Archean Eon

Chapter 8 Precambrian Earth and Life HistoryThe Archean Eon

The Beartooth Mountains on the Wyoming and Montana border consists of Archaean-age gneisses, some of the oldest rocks in the US. Archean Rocks

The Precambrian lasted for more than 4 billion years! This large time span is difficult for humans to comprehend Suppose that a 24-hour clock represented all 4.6 billion years of geologic time then the Precambrian would be slightly more than 21 hours long, constituting about 88% of all geologic time Precambrian

88% of geologic time Precambrian Time Span

The term Precambrian is informal but widely used, referring to both time and rocks The Precambrian includes time from Earths origin 4.6 billion years ago to the beginning of the Phanerozoic Eon 542 million years ago It encompasses all rocks below the Cambrian system No rocks are known for the first 600 million years of geologic time The oldest known rocks on Earth are 4.0 billion years old Precambrian

The earliest record of geologic time preserved in rocks is difficult to interpret because many Precambrian rocks have been altered by metamorphism complexly deformed buried deep beneath younger rocks fossils are rare, and the few fossils present are not of any use in biostratigraphy Subdivisions of the Precambrian have been difficult to establish Two eons for the Precambrian are the Archean and Proterozoic which are based on absolute ages Rocks Difficult to Interpret

Eoarchean refers to all time from Earths origin to the Paleoarchean 3.6 billion years ago Earths oldest body of rocks the Acasta Gneiss in Canada is about 4.0 billion years old We have no geologic record for much of the Archaen Precambrian eons have no stratotypes unlike the Cambrian Period, for example Eons of the Precambrian

Although no rocks of Eoarchean age are present on Earth, except for meteorites, we do know some events that took place then Earth accreted from planetesimals and differentiated into a core and mantle and at least some crust was present Earth was bombarded by meteorites Volcanic activity was ubiquitous An atmosphere formed, quite different from todays Oceans began to accumulate What Happened During the Eoarchean?

Shortly after accretion, Earth was a rapidly rotating, hot, barren, waterless planet bombarded by meteorites and comets with no continents, intense cosmic radiation and widespread volcanism Hot, Barren, Waterless Early Earth about 4.6 billion years ago

Continental crust was present by 4.0 billion years ago Sedimentary rocks in Australia contain detrital zircons (ZrSiO 4 ) dated at 4.4 billion years old so source rocks at least that old existed The Eoarchean Earth probably rotated in as little as 10 hours and the Earth was closer to the Moon By 4.4 billion years ago, the Earth cooled sufficiently for surface waters to accumulate Oldest Rocks

Early crust formed as upwelling mantle currents of mafic magma, and numerous subduction zones developed to form the first island arcs Eoarchean continental crust may have formed by collisions between island arcs as silica-rich materials were metamorphosed. Larger groups of merged island arcs protocontinents grew faster by accretion along their margins Eoarchean Crust

Origin of Continental Crust Andesitic island arcs form by subduction and partial melting of oceanic crust The island arc collides with another

Continents consist of rocks with composition similar to that of granite Continental crust is thicker and less dense than oceanic crust which is made up of basalt and gabbro Precambrian shields consist of vast areas of exposed ancient rocks and are found on all continents Outward from the shields are broad platforms of buried Precambrian rocks that underlie much of each continent Continental Foundations

A shield and its platform make up a craton, a continents ancient nucleus Along the margins of cratons, more continental crust was added as the continents took their present sizes and shapes Both Archean and Proterozoic rocks are present in cratons and show evidence of episodes of deformation accompanied by igneous activity, metamorphism, and mountain building Cratons have experienced little deformation since the Precambrian Cratons

Distribution of Precambrian Rocks Areas of exposed Precam- brian rocks constitute the shields Platforms consist of buried Pre- cambrian rocks Shields and adjoining platforms make up cratons

The exposed part of the craton in North America is the Canadian shield which occupies most of northeastern Canada a large part of Greenland parts of the Lake Superior region in Minnesota, Wisconsin, and Michigan and the Adirondack Mountains of New York Its topography is subdued, with numerous lakes and exposed Archean and Proterozoic rocks thinly covered in places by Pleistocene glacial deposits Canadian Shield

North America evolved by the amalgamation of Archean cratons that served as a nucleus around which younger continental crust was added. Evolution of North America

Drilling and geophysical evidence indicate that Precambrian rocks underlie much of North America, exposed only in places by deep erosion or uplift North American Craton

Only 22% of Earths exposed Precambrian crust is Archean The most common Archean rock associations are granite-gneiss complexes Other rocks range from peridotite to various sedimentary rocks all of which have been metamorphosed Greenstone belts are subordinate in quantity, account for only 10% of Archean rocks but are important in unraveling Archean tectonic events Archean Rocks

Outcrop of Archean gneiss cut by a granite dike from a granite-gneiss complex in Ontario, Canada Archean Rocks

Shell Creek in the Bighorn Mountains of Wyoming has cut a gorge into this 2.9 billion year old granite Archean Rocks

Greenstone Belts A greenstone belt has 3 major rock units volcanic rocks are most common in the lower and middle units the upper units are mostly sedimentary The belts typically have synclinal structure Most were intruded by granitic magma and cut by thrust faults Low-grade metamorphism makes many of the igneous rocks green Because they contain chlorite, actinolite, and epidote

Two adjacent greenstone belts showing synclinal structure Greenstone Belts and Granite- Gneiss Complexes They are underlain by granite-gneiss complexes and intruded by granite

Pillow lavas in greenstone belts indicate that much of the volcanism was subaqueous Greenstone Belt Volcanics Pyroclastic materials probably erupted where large volcanic centers built above sea level Pillow lavas in Ispheming greenstone belt at Marquette, Michigan

The most interesting rocks in greenstone belts are komatiites, cooled from ultramafic lava flows Ultramafic magma (< 45% silica) requires near surface magma temperatures of more than 1600C 250C hotter than any recent flows During Earths early history, radiogenic heating was greater and the mantle was as much as 300 C hotter than it is now This allowed ultramafic magma to reach the surface Ultramafic Lava Flows

As Earths production of radiogenic heat decreased, the mantle cooled and ultramafic flows no longer occurred They are rare in rocks younger than Archean and none occur now Ultramafic Lava Flows

Sedimentary rocks are found throughout the greenstone belts although they predominate in the upper unit Many of these rocks are successions of graywacke sandstone with abundant clay and rock fragments and argillite slightly metamorphosed mudrock Sedimentary Rocks of Greenstone Belts

Small-scale cross-bedding and graded bedding indicate an origin as turbidity current deposits Sedimentary Rocks of Greenstone Belts Other sedimentary rocks are present, but not abundant sandstone, conglomerate, chert, carbonates Iron-rich rocks, banded iron formations, are more typical of Proterozoic deposits

In North America, most greenstone belts (dark green) occur in the Superior and Slave cratons of the Canadian shield Canadian Greenstone Belts

Greenstone belts formed in several tectonic settings Models for the formation of greenstone belts involve Archean plate movement In one model, greenstone belts formed Evolution of Greenstone Belts in back-arc marginal basins

According to this model, There was an early stage of extension as the back- arc marginal basin formed volcanism and sediment deposition followed Evolution of Greenstone Belts

Then during closure, the rocks were compressed, metamorphosed, and intruded by granitic magma The Sea of Japan is a modern example of a back-arc basin

In another model accepted by some geologists, greenstone belts formed over rising mantle plumes in intracontinental rifts As the plume rises beneath sialic crust it spreads and generates tensional forces The mantle plume is the source of the volcanic rocks in the lower and middle units of the greenstone belt and erosion of volcanic rocks and flanks for the rift supply the sediment to the upper unit An episode of subsidence, deformation, metamorphism and plutonism followed Another Model

Greenstone BeltsIntracontinental Rift Model Ascending mantle plume causes rifting and volcanism

Greenstone BeltsIntracontinental Rift Model Erosion of the rift flanks accounts for sediments

Greenstone BeltsIntracontinental Rift Model Closure of rift causes compression and deformation

Plate tectonic activity has operated since the Paleoproterozoic or earlier Most geologists are convinced that some kind of plate tectonic activity took place during the Archean as well but it differed in detail from today Plates must have moved faster with more residual heat from Earths origin and more radiogenic heat, and magma was generated more rapidly Archean Plate Tectonics

As a result of the rapid movement of plates, continents grew more rapidly along their margins a process called continental accretion as plates collided with island arcs and other plates Also, ultramafic extrusive igneous rocks, komatiites, were more common Archean Plate Tectonics

The Archean world was markedly different than later Archean World Differences We have little evidence of Archean rocks deposited on broad, passive continental margins but associations of passive continental margin sediments are widespread in Proterozoic terrains Deformation belts between colliding cratons indicate that Archean plate tectonics was active but the ophiolites so typical of younger convergent plate boundaries are rare, although Neoarchean ophiolites are known

Certainly several small cratons existed during the Archean and grew by accretion along their margins They amalgamated into a larger unit during the Proterozoic By the end of the Archean, 30-40% of the present volume of continental crust existed Archean crust probably evolved similarly to the evolution of the southern Superior craton of Canada The Origin of Cratons

Southern Superior Craton Evolution Geologic map Greenstone belts (dark green) Granite-gneiss complexes (light green Plate tectonic model for evolution of the southern Superior craton North-south cross section

Deformation of the southern Superior craton was part of a more extensive orogenic episode during the Mesoarchean and Neoarchean that formed the Superior and Slave cratons and some Archean rocks in Wyoming, Montana, and the Mississippi River Valley By the time this Archean event ended several cratons had formed that are found in the older parts of the Canadian shield Canadian Shield

Earths early atmosphere and hydrosphere were quite different than they are now They also played an important role in the development of the biosphere Todays atmosphere is mostly nitrogen (N 2 ) abundant free oxygen (O 2 ), or oxygen not combined with other elements such as in carbon dioxide (CO 2 ) water vapor (H 2 O) small amounts of other gases, like ozone (O 3 ) which is common enough in the upper atmosphere to block most of the Suns ultraviolet radiation Atmosphere and Hydrosphere

Nonvariable gases NitrogenN 2 78.08% OxygenO 2 20.95 ArgonAr 0.93 NeonNe 0.002 Others 0.001 in percentage by volume Present-day Atmosphere Composition Variable gases Water vaporH 2 O0.1 to 4.0 Carbon dioxide CO 2 0.038 OzoneO 3 0.000006 Other gasesTrace Particulates normally trace

Earths very early atmosphere was probably composed of hydrogen and helium, the most abundant gases in the universe If so, it would have quickly been lost into space because Earths gravity is insufficient to retain them because Earth had no magnetic field until its core formed (magnetosphere) Without a magnetic field, the solar wind would have swept away any atmospheric gases Earths Very Early Atmosphere

Once a magnetosphere was present Atmosphere began accumulating as a result of outgassing released during volcanism Water vapor is the most common volcanic gas today but volcanoes also emit carbon dioxide, sulfur dioxide, Outgassing carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen

Archean volcanoes probably emitted the same gases, and thus an atmosphere developed but one lacking free oxygen and an ozone layer It was rich in carbon dioxide, and gases reacting in this early atmosphere probably formed ammonia (NH 3 ) methane (CH 4 ) This early atmosphere persisted throughout the Archean Archean Atmosphere

The atmosphere was chemically reducing rather than an oxidizing one Some of the evidence for this conclusion comes from detrital deposits containing minerals that oxidize rapidly in the presence of oxygen pyrite (FeS 2 ) uraninite (UO 2 ) But oxidized iron becomes increasingly common in Proterozoic rocks indicating that at least some free oxygen was present then Evidence for an Oxygen-Free Atmosphere

Two processes account for introducing free oxygen into the atmosphere, one or both of which began during the Eoarchean. 1. Photochemical dissociation involves ultraviolet radiation in the upper atmosphere The radiation disrupts water molecules and releases their oxygen and hydrogen This could account for 2% of present-day oxygen but with 2% oxygen, ozone forms, creating a barrier against ultraviolet radiation 2. More important were the activities of organisms that practiced photosynthesis Introduction of Free Oxygen

Photosynthesis is a metabolic process in which carbon dioxide and water to make organic molecules and oxygen is released as a waste product CO 2 + H 2 O ==> organic compounds + O 2 Even with photochemical dissociation and photosynthesis, probably no more than 1% of the free oxygen level of today was present by the end of the Archean Photosynthesis

Photochemical dissociation and photosynthesis added free oxygen to the atmosphere Oxygen Forming Processes Once free oxygen was present an ozone layer formed and blocked incoming ultraviolet radiation

Outgassing was responsible for the early atmosphere and also for some of Earths surface water the hydrosphere most of which is in the oceans more than 97% Another source of our surface water was meteorites and icy comets Numerous erupting volcanoes, and an early episode of intense meteorite and comet bombardment accounted for rapid rate of surface water accumulation Earths Surface Waters

Volcanoes still erupt and release water vapor Is the volume of ocean water still increasing? Perhaps it is, but if so, the rate has decreased considerably because the amount of heat needed to generate magma has diminished Ocean Water

Ratio of radiogenic heat production in the past to the present Decreasing Heat The width of the colored band indicates variations in ratios from different models Heat production 4 billion years ago was 3 to 6 times as great as it is now With less heat outgassing decreased

Today, Earths biosphere consists of millions of species of archea, bacteria, fungi, protists, plants, and animals, whereas only bacteria and archea are found in Archean rocks We have fossils from Archean rocks 3.5 billion years old Chemical evidence in rocks in Greenland that are 3.8 billion years old convince some investigators that organisms were present then First Organisms

Minimally, a living organism must reproduce and practice some kind of metabolism Reproduction ensures the long-term survival of a group of organisms whereas metabolism maintains the organism The distinction between living and nonliving things is not always easy Are viruses living? When in a host cell they behave like living organisms but outside they neither reproduce nor metabolize What Is Life?

Comparatively simple organic (carbon based) molecules known as microspheres What Is Life? form spontaneously can even grow and divide in a somewhat organism-like fashion but their processes are more like random chemical reactions, so they are not living

To originate by natural processes, from non-living matter (abiogenesis), life must have passed through a prebiotic stages in which it showed signs of living but was not truly living The origin of life has 2 requirements a source of appropriate elements for organic molecules energy sources to promote chemical reactions How Did Life First Originate?

All organisms are composed mostly of carbon (C) hydrogen (H) nitrogen (N) oxygen (O) all of which were present in Earths early atmosphere as carbon dioxide (CO 2 ) water vapor (H 2 O) nitrogen (N 2 ) and possibly methane (CH 4 ) and ammonia (NH 3 ) Elements of Life

Energy from Lightning, volcanism, and ultraviolet radiation probably promoted chemical reactions during which C, H, N, and O combined to form monomers such as amino acids Monomers are the basic building blocks of more complex organic molecules Basic Building Blocks of Life

Is it plausible that monomers originated in the manner postulated? Experimental evidence indicates that it is Experiment on the Origin of Life During the late 1950s Stanley Miller synthesized several amino acids by circulating gases approximating the early atmosphere in a closed glass vessel

This mixture was subjected to an electric spark to simulate lightning Experiment on the Origin of Life In a few days it became cloudy Analysis showed that several amino acids typical of organisms had formed Since then, scientists have synthesized all 20 amino acids found in organisms

The molecules of organisms are polymers such as proteins and nucleic acids RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) consisting of monomers linked together in a specific sequence How did polymerization take place? Water usually causes depolymerization, however, researchers synthesized molecules known as proteinoids or thermal proteins some of which consist of more than 200 linked amino acids when heating dehydrated concentrated amino acids Polymerization

These concentrated amino acids spontaneously polymerized to form proteinoids Perhaps similar conditions for polymerization existed on early Earth, but the proteinoids needed to be protected by an outer membrane or they would break down Experiments show that proteinoids spontaneously aggregate into microspheres which are bounded by cell-like membranes and grow and divide much as bacteria do Proteinoids

Proteinoid microspheres produced in experiments Proteinoid Microspheres Proteinoids grow and divide much as bacteria do

These proteinoid molecules can be referred to as protobionts that are intermediate between inorganic chemical compounds and living organisms Protobionts

The origin-of-life experiments are interesting, but what is their relationship to early Earth? Monomers likely formed continuously and by the billions and accumulated in the early oceans into a hot, dilute soup The amino acids in the soup might have washed up onto a beach or perhaps cinder cones where they were concentrated by evaporation and polymerized by heat The polymers then washed back into the ocean where they reacted further Monomer and Proteinoid Soup

Not much is known about the next critical step in the origin of life the development of a reproductive mechanism The microspheres divide and may represent a protoliving system but in todays cells, nucleic acids, either RNA or DNA are necessary for reproduction The problem is that nucleic acids cannot replicate without protein enzymes, and the appropriate enzymes cannot be made without nucleic acids, or so it seemed until fairly recently Next Critical Step

Now we know that small RNA molecules can replicate without the aid of protein enzymes Thus, the first replicating systems may have been RNA molecules Some researchers propose an early RNA world in which these molecules were intermediate between inorganic chemical compounds and the DNA-based molecules of organisms How RNA was naturally synthesized remains an unsolved problem RNA World?

Scientists agree on some basic requirements for the origin of life, but the exact steps involved and significance of results are debated Many researchers believe that the earliest organic molecules were synthesized from atmospheric gases but some scientist suggest that life arose instead near hydrothermal vents on the seafloor Much Remains to Be Learned

Seawater seeps into the crust near spreading ridges, becomes heated, rises and discharges Black smokers Submarine Hydrothermal Vents Discharge water saturated with dissolved minerals Life may have formed near these in the past

Several minerals containing zinc, copper, and iron precipitate around them Communities of organisms Submarine Hydrothermal Vents previously unknown to science, are supported here. Necessary elements, sulfur, and phosphorus are present in seawater Polymerization can take place on surface of clay minerals Protocells were deposited on the ocean floor

The first organisms were archaea and bacteria both of which consist of prokaryotic cells, cells that lack an internal, membrane-bounded nucleus and other structures Prior to the 1950s, scientists assumed that life must have had a long early history but the fossil record offered little to support this idea The Precambrian, once called Azoic (without life), seemed devoid of life Oldest Known Organisms

Charles Walcott (early 1900s) described structures from the Paleoproterozoic Gunflint Iron Formation of Ontario, Canada that he proposed represented reefs constructed by algae Oldest Know Organisms Now called stromatolites, not until 1954 were they shown to be products of organic activity Present-day stromatolites (Shark Bay, Australia)

Different types of stromatolites include irregular mats, columns, and columns linked by mats Stromatolites

Present-day stromatolites form and grow as sediment grains are trapped on sticky mats of photosynthesizing cyanobacteria although now they are restricted to environments where snails cannot live The oldest known undisputed stromatolites are found in rocks in South Africa that are 3.0 billion years old but probable ones are also known from the Warrawoona Group in Australia which is 3.3 to 3.5 billion years old Stromatolites

Chemical evidence in rocks 3.85 billion years old in Greenland indicate life was perhaps present then The oldest known cyanobacteria were photosynthesizing organisms but photosynthesis is a complex metabolic process A simpler type of metabolism must have preceded it No fossils are known of these earliest organisms Other Evidence of Early Life

The earliest organisms must have resembled tiny anaerobic bacteria meaning they required no oxygen They must have totally depended on an external source of nutrients that is, they were heterotrophic as opposed to autotrophic organisms that make their own nutrients, as in photosynthesis They all had prokaryotic cells Earliest Organisms

The earliest organisms, then, were anaerobic, heterotrophic prokaryotes Their nutrient source was most likely adenosine triphosphate (ATP) from their environment which was used to drive the energy-requiring reactions in cells ATP can easily be synthesized from simple gases and phosphate so it was available in the early Earth environment Earliest Organisms

Obtaining ATP from the surroundings could not have persisted for long because more and more cells competed for the same resources The first organisms to develop a more sophisticated metabolism probably used fermentation to meet their energy needs Fermentation is an anaerobic process in which molecules such as sugars are split releasing carbon dioxide, alcohol, and energy Fermentation

A very important biological event occurring in the Archean was the development of the autotrophic process of photosynthesis This may have happened as much as 3.5 billion years ago These prokaryotic cells were still anaerobic, but as autotrophs they were no longer dependent on preformed organic molecules as a source of nutrients Photosynthesis

Photomicrographs from western Australias 3.3- to 3.5-billion-year-old Warrawoona Group, with schematic restoration shown at the right of each Fossil Prokaryotes

A variety of mineral deposits are of Archean-age but gold is the most commonly associated, although it is also found in Proterozoic and Phanerozoic rocks This soft yellow metal is prized for jewelry, but it is or has been used as a monetary standard, in glass making, electric circuitry, and chemical industry About half the worlds gold since 1886 has come from Archean and Proterozoic rocks in South Africa Gold mines also exist in Archean rocks of the Superior craton in Canada Archean Mineral Resources

Archean sulfide deposits of zinc, copper and nickel occur in Australia, Zimbabwe, and in the Abitibi greenstone belt in Ontario, Canada Some, at least, formed as mineral deposits next to hydrothermal vents on the seafloor, much as they do now around black smokers Archean Sulfide Deposits

About 1/4 of Earths chrome reserves are in Archean rocks, especially in Zimbabwe These ore deposits are found in the volcanic units of greenstone belts where they appear to have formed when crystals settled and became concentrated in the lower parts of plutons such as mafic and ultramafic sills Chrome is needed in the steel industry The United States has very few chrome deposits so must import most of what it uses Chrome

One chrome deposit in the United States is in the Stillwater Complex in Montana Low-grade ores were mined there during war times, but they were simply stockpiled and never refined for chrome These rocks also contain platinum, a precious metal, that is used in the automotive industry in catalytic converters in the chemical industry for cancer chemotherapy Chrome and Platinum

Banded Iron formations are sedimentary rocks consisting of alternating layers of silica (chert) and iron minerals About 6% of the worlds banded iron formations were deposited during the Archean Eon Although Archean iron ores are mined in some areas they are neither as thick nor as extensive as those of the Proterozoic Eon, which constitute the worlds major source of iron Iron

Pegmatites are very coarsely crystalline igneous rocks, commonly associated with granite plutons Some Archean pegmatites, such in the Herb Lake district in Manitoba, Canada, and Rhodesian Province in Africa, contain valuable minerals In addition to minerals of gem quality, Archean pegmatites contain minerals mined for lithium, beryllium, rubidium, and cesium Pegmatites

Summary Precambrian encompasses all geologic time from Earths origin to the beginning of the Phanerozoic Eon The term also refers to all rocks that lie stratigraphically below Cambrian rocks The Precambrian is divided into two eons the Archean and the Proterozoic, which are further subdivided Rocks from the latter part of the Eoarchean indicate crust must have existed, but very little of it has been preserved

Summary All continents have an ancient stable nucleus or craton made up of an exposed shield and a buried platform The exposed part of the North American craton is the Canadian shield, and is make up of smaller units delineated by their ages and structural trends Archean greenstone belts are linear, syncline-like bodies found within much more extensive granite-gneiss complexes

Summary Greenstone belts typically consist of two lower units dominated by igneous rocks and an upper unit of mostly sedimentary rocks They probably formed in back-arc basins and in intracontinental rifts Many geologists are convinced some type of Archean plate tectonics occurred, but plates probably moved faster and igneous activity was more common because Earth had more radiogenic heat

Summary The early atmosphere and hydrosphere formed as a result of outgassing, but this atmosphere lacked free oxygen and contained abundant water vapor and carbon dioxide Models for the origin of life by natural processes require an oxygen deficient atmosphere, the necessary elements for organic molecules, and energy to promote the synthesis of organic molecules

Summary The first naturally formed organic molecules were probably monomers, such as amino acids, that linked together to form more complex polymers such as proteins RNA molecules may have been the first molecules capable of self-replication However, how a reproductive mechanism evolved is not known

Summary The only known Archean fossils are of single-celled, prokaryotic bacteria or cyanobacteria but other chemical evidence may indicate presence of archaea Stromatolites formed by photosynthesizing bacteria are found in rocks as much as 3.5 billion years old Archean mineral resources include gold, chrome, zinc, copper, and nickel

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Chapter 8 Precambrian Earth and Life History—The Archean Eon