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Precambrian Earth and Life History

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Title: Precambrian Earth and Life History


1
Chapter 8
Precambrian Earth and Life HistoryThe Archean Eon
2
Archean Rocks
  • The Beartooth Mountains
  • on the Wyoming and Montana border
  • consists of Archaean-age gneisses,
  • some of the oldest rocks in the US.

3
Precambrian
  • 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

4
Precambrian Time Span
  • 88 of geologic time

5
Precambrian
  • 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

6
Rocks Difficult to Interpret
  • 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

7
Eons of the Precambrian
  • 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

8
What Happened During the Eoarchean?
  • 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

9
Hot, Barren, Waterless Early Earth
  • about 4.6 billion years ago
  • 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

10
Oldest Rocks
  • Continental crust was present by 4.0 billion
    years ago
  • Sedimentary rocks in Australia contain detrital
    zircons (ZrSiO4) 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

11
Eoarchean Crust
  • 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

12
Origin of Continental Crust
  • Andesitic island arcs
  • form by subduction
  • and partial melting of oceanic crust
  • The island arc collides with another

13
Continental Foundations
  • 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

14
Cratons
  • 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

15
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

16
Canadian Shield
  • 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

17
Evolution of North America
  • North America evolved by the amalgamation of
    Archean cratons that served as a nucleus around
    which younger continental crust was added.

18
North American Craton
  • Drilling and geophysical evidence indicate
  • that Precambrian rocks underlie much
  • of North America,
  • exposed only in places by deep erosion or uplift

19
Archean Rocks
  • 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

20
Archean Rocks
  • Outcrop of Archean gneiss cut by a granite dike
    from a granite-gneiss complex in Ontario, Canada

21
Archean Rocks
  • Shell Creek in the Bighorn Mountains of Wyoming
    has cut a gorge into this 2.9 billion year old
    granite

22
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

23
Greenstone Belts and Granite-Gneiss Complexes
  • Two adjacent greenstone belts showing synclinal
    structure
  • They are underlain by granite-gneiss complexes
  • and intruded by granite

24
Greenstone Belt Volcanics
  • Pillow lavas in greenstone belts
  • indicate that much of the volcanism was
  • subaqueous
  • Pyroclastic materials probably erupted
  • where large volcanic centers built above sea
    level

Pillow lavas in Ispheming greenstone belt at
Marquette, Michigan
25
Ultramafic Lava Flows
  • The most interesting rocks
  • in greenstone belts are komatiites,
  • cooled from ultramafic lava flows
  • Ultramafic magma (lt 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

26
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

27
Sedimentary Rocks of Greenstone Belts
  • 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

28
Sedimentary Rocks of Greenstone Belts
  • Small-scale cross-bedding and graded bedding
  • indicate an origin as turbidity current deposits
  • Other sedimentary rocks are present, but not
    abundant
  • sandstone, conglomerate, chert, carbonates
  • Iron-rich rocks, banded iron formations, are more
    typical of Proterozoic deposits

29
Canadian Greenstone Belts
  • In North America,
  • most greenstone belts
  • (dark green)
  • occur in the Superior and Slave cratons
  • of the Canadian shield

30
Evolution of 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
  • in back-arc marginal basins

31
Evolution of Greenstone Belts
  • According to this model,
  • There was an early stage of extension as the
    back-arc marginal basin formed
  • volcanism and
    sediment
    deposition

    followed

32
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

33
Another Model
  • 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

34
Greenstone BeltsIntracontinental Rift Model
  • Ascending mantle plume
  • causes rifting
  • and volcanism

35
Greenstone BeltsIntracontinental Rift Model
  • Erosion of the rift flanks
  • accounts for sediments

36
Greenstone BeltsIntracontinental Rift Model
  • Closure of rift
  • causes compression
  • and deformation

37
Archean Plate Tectonics
  • 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

38
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

39
Archean World Differences
  • The Archean world was markedly different than
    later
  • but associations of passive continental margin
    sediments
  • are widespread in Proterozoic terrains
  • We have little evidence of Archean rocks
  • deposited on broad, passive continental margins
  • but the ophiolites so typical of younger
    convergent plate boundaries are rare,
  • although Neoarchean ophiolites are known
  • Deformation belts between colliding cratons
  • indicate that Archean plate tectonics was active

40
The Origin of Cratons
  • 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

41
Southern Superior Craton Evolution
  • Greenstone belts (dark green)
  • Granite-gneiss complexes (light green
  • Geologic map
  • Plate tectonic model for evolution of the
    southern Superior craton
  • North-south cross section

42
Canadian Shield
  • 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

43
Atmosphere and Hydrosphere
  • 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 (N2)
  • abundant free oxygen (O2),
  • or oxygen not combined with other elements
  • such as in carbon dioxide (CO2)
  • water vapor (H2O)
  • small amounts of other gases, like ozone (O3)
  • which is common enough in the upper atmosphere
  • to block most of the Suns ultraviolet radiation

44
Present-day Atmosphere Composition
  • Nonvariable gases
  • Nitrogen N2 78.08
  • Oxygen O2 20.95
  • Argon Ar 0.93
  • Neon Ne 0.002
  • Others 0.001
  • in percentage by volume
  • Variable gases
  • Water vapor H2O 0.1 to 4.0
  • Carbon dioxide CO2 0.038
  • Ozone O3 0.000006
  • Other gases Trace
  • Particulates normally trace

45
Earths Very Early Atmosphere
  • 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

46
Outgassing
  • 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,
  • carbon monoxide, sulfur,
  • hydrogen, chlorine, and nitrogen

47
Archean Atmosphere
  • 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 (NH3)
  • methane (CH4)
  • This early atmosphere persisted
  • throughout the Archean

48
Evidence for an Oxygen-Free 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 (FeS2)
  • uraninite (UO2)
  • But oxidized iron becomes
  • increasingly common in Proterozoic rocks
  • indicating that at least some free oxygen
  • was present then

49
Introduction of Free Oxygen
  • 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

50
Photosynthesis
  • Photosynthesis is a metabolic process
  • in which carbon dioxide and water
  • to make organic molecules
  • and oxygen is released as a waste product
  • CO2 H2O gt organic compounds O2
  • 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

51
Oxygen Forming Processes
  • Photochemical dissociation and photosynthesis
  • added free oxygen to the atmosphere
  • Once free oxygen was present
  • an ozone layer formed
  • and blocked incoming ultraviolet radiation

52
Earths Surface Waters
  • 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

53
Ocean Water
  • 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

54
Decreasing Heat
  • Ratio of radiogenic heat production in the past
    to the present
  • 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

55
First Organisms
  • 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

56
What Is Life?
  • 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

57
What Is Life?
  • Comparatively simple organic (carbon based)
    molecules known as microspheres
  • 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

58
How Did Life First Originate?
  • 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

59
Elements of Life
  • 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 (CO2)
  • water vapor (H2O)
  • nitrogen (N2)
  • and possibly methane (CH4)
  • and ammonia (NH3)

60
Basic Building Blocks 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

61
Experiment on the Origin of Life
  • Is it plausible that monomers
  • originated in the manner postulated?
  • Experimental evidence indicates that it is
  • During the late 1950s
  • Stanley Miller
  • synthesized several amino acids
  • by circulating gases approximating
  • the early atmosphere
  • in a closed glass vessel

62
Experiment on the Origin of Life
  • This mixture was subjected to an electric spark
  • to simulate lightning
  • 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

63
Polymerization
  • 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

64
Proteinoids
  • 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

65
Proteinoid Microspheres
  • Proteinoid microspheres produced in experiments
  • Proteinoids grow and divide much as bacteria do

66
Protobionts
  • These proteinoid molecules can be referred to as
    protobionts
  • that are intermediate between
  • inorganic chemical compounds
  • and living organisms

67
Monomer and Proteinoid Soup
  • 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

68
Next Critical Step
  • 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

69
RNA World?
  • 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

70
Much Remains to Be Learned
  • 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

71
Submarine Hydrothermal Vents
  • Seawater seeps into the crust near spreading
    ridges, becomes heated, rises and discharges
  • Black smokers
  • Discharge water saturated with dissolved minerals
  • Life may have formed near these in the past

72
Submarine Hydrothermal Vents
  • Several minerals containing zinc, copper, and
    iron precipitate around them
  • Communities of organisms
  • 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

73
Oldest Known Organisms
  • 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

74
Oldest Know Organisms
  • Charles Walcott (early 1900s) described
    structures
  • from the Paleoproterozoic Gunflint Iron Formation
    of Ontario, Canada
  • that he proposed represented reefs constructed by
    algae
  • Now called stromatolites,
  • not until 1954 were they shown
  • to be products of organic activity

Present-day stromatolites (Shark Bay, Australia)
75
Stromatolites
  • Different types of stromatolites include
  • irregular mats, columns, and columns linked by
    mats

76
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

77
Other Evidence of Early Life
  • 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

78
Earliest Organisms
  • 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

79
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

80
Fermentation
  • 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

81
Photosynthesis
  • 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

82
Fossil Prokaryotes
  • Photomicrographs from western Australias
  • 3.3- to 3.5-billion-year-old Warrawoona Group,
  • with schematic restoration shown at the right of
    each

83
Archean Mineral Resources
  • 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

84
Archean Sulfide Deposits
  • 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

85
Chrome
  • 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

86
Chrome and Platinum
  • 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

87
Iron
  • 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

88
Pegmatites
  • 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

89
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

90
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

91
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

92
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

93
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

94
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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