Precambrian Earth and Life HistoryThe Eoarchean and Archean

1
Chapter 8
Precambrian Earth and Life HistoryThe Eoarchean
and Archean
2
Time check

• The Precambrian lasted for more than 4 billion
  years!
• Such a time span is almost impossible for us
  comprehend
• If a 24-hour clock represented all 4.6 billion
  years of geologic time
• the Precambrian would be slightly more than 21
  hours long,
• It constitutes about 88 of all geologic time

3
Precambrian Time Span
4
Precambrian

• The term Precambrian is informal term referring
  to both time and rocks
• It includes time from Earths origin 4.6 billion
  years ago to the beginning of the Phanerozoic Eon
  545 million years ago
• No rocks are known for the first 640 million
  years of geologic time
• The oldest known rocks on Earth are 3.96 billion
  years old

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Rocks of the 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
• the few fossils present are of little use in
  stratigraphy
• Because of this subdivisions of the Precambrian
  have been difficult to establish
• Two eons for the Precambrian
• the Archean and Proterozoic

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Eons of the Precambrian

• The onset of the Archean Eon coincides with the
  age of Earths oldest known rocks
• approximately 4 billion years old
• lasted until 2.5 billion years ago (the beginning
  of the Proterozoic Eon)
• The Eoarchean is an informal designation for the
  time preceding the Archean Eon
• Precambrian eons have no stratotypes
• the Cambrian Period, for example, which is based
  on the Cambrian System, a time-stratigraphic unit
  with a stratotype in Wales
• Precambrian eons are strictly terms denoting time

7
US Geologic Survey Terms

• Archean and Proterozoic are used in our
  discussions of Precambrian history, but the U.S.
  Geological Survey (USGS) uses different terms
• Precambrian W begins within the Early Archean and
  ends at the end of the Archean
• Precambrian X corresponds to the Early
  Proterozoic, 2500 to 1600 million years ago
• Precambrian Y, from 1600 to 800 million years
  ago, overlaps with the Middle and part of the
  Late Proterozoic
• Precambrian Z is from 800 million years to the
  end of the Precambrian, within the Late
  Proterozoic

8
The Hadean?

• Except for meteorites no rocks of Eoarchean age
  are present on Earth, however we do know some
  events that took place during this period
• Earth was accreted
• Differentiation occurred, creating a core and
  mantle and at least some crust

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Earth beautiful Earth.
about 4.6 billion years ago

• Shortly after accretion, Earth was a rapidly
  rotating, hot, barren, waterless planet
• bombarded by comets and meteorites
• There were no continents,
• intense cosmic radiation
• widespread volcanism

10
Oldest Rocks

• Judging from the oldest known rocks on Earth, the
  3.96-billion-year-old Acasta Gneiss in Canada
  some continental crust had evolved by 4 billion
  years ago
• Sedimentary rocks in Australia contain detrital
  zircons (ZrSiO4) dated at 4.2 billion years old
• so source rocks at least that old existed
• These rocks indicted that some kind of Hadean
  crust was certainly present, but its distribution
  is unknown

11
Hadean Crust

• Early Hadean crust was probably thin, unstable
  and made up of ultramafic rock
• rock with comparatively little silica
• This ultramafic crust was disrupted by upwelling
  basaltic magma at ridges and consumed at
  subduction zones
• Hadean continental crust may have formed by
  evolution of sialic material
• Sialic crust contains considerable silicon,
  oxygen and aluminum as in present day continental
  crust
• Only sialic-rich crust, because of its lower
  density, is immune to destruction by subduction

12
Crustal Evolution

• A second stage in crustal evolution began as
  Earths production of radiogenic heat decreased
• Subduction and partial melting of earlier-formed
  basaltic crust resulted in the origin of
  andesitic island arcs
• Partial melting of lower crustal andesites, in
  turn, yielded silica-rich granitic magmas that
  were emplaced in the andesitic arcs

13
Crustal Evolution

• Several sialic continental nuclei had formed by
  the beginning of Archean time by subduction and
  collisions between island arcs

14
Dynamic Processes

• During the Hadean, various dynamic systems
  similar to ones we see today, became operative,
• not all at the same time nor in their present
  forms
• Once Earth differentiated into core, mantle and
  crust,
• internal heat caused interactions among plates
• they diverged, converged and slid past each other
  
• Continents began to grow by accretion along
  convergent plate boundaries

15
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

16
Cratons

• A shield and platform make up a craton
• a continents ancient nucleus and its foundations
• 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
• Metamorphism
• igneous activity
• and mountain building
• Cratons have experienced little deformation since
  the Precambrian

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Distribution of Precambrian Rocks

• Areas of exposed Precambrian rocks constitute the
  shields
• Platforms consist of buried Precambrian rocks

Shields and adjoining platforms make up cratons
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Canadian Shield

• The craton in North America is the Canadian
  shield
• Occupies most of northeastern Canada, a large
  part of Greenland, parts of the Lake Superior
  region in Minnesota, Wisconsin, 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

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Canadian Shield Rocks

• Gneiss, a metamorphic rock, Georgian Bay Ontario,
  Canada

20
Canadian Shield Rocks

• Basalt (dark, volcanic) and granite (light,
  plutonic) on the Chippewa River, Ontario

21
Amalgamated Cratons

• The Canadian shield and adjacent platform
  consists of numerous units or smaller cratons
  that were welded together along deformation belts
  during the Early Proterozoic
• Absolute ages and structural trends help
  geologists differentiate among these various
  smaller cratons

22
Archean Rocks

• The most common Archean Rock associations are
  granite-gneiss complexes
• The rocks vary from granite to peridotite to
  various sedimentary rocks all of which have been
  metamorphosed
• Greenstone belts are subordinate in quantity
• but are important in unraveling Archean tectonism

23
Greenstone Belts

• An ideal 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 greenish
  (chlorite)

24
Greenstone Belt Volcanics

• Abundant pillow lavas in greenstone belts
  indicate that much of the volcanism was under
  water

• Pyroclastic materials probably erupted where
  large volcanic centers built above sea level

Pillow lavas in Ispheming greenstone at
Marquette, Michigan
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Ultramafic Lava Flows

• The most interesting rocks in greenstone belts
  cooled from ultramafic lava flows
• Ultramafic magma has less than 40 silica
• requires near surface magma temperatures of more
  than 1600C250C
• hotter than any recent flows
• During Earths early history, radiogenic heating
  was higher and the mantle was as much as 300 C
  hotter than it is now
• This allowed ultramafic magma to reach the surface

26
Sedimentary Rocks of Greenstone Belts

• Sedimentary rocks are found throughout the
  greenstone belts
• Mostly found in the upper unit
• Many of these rocks are successions of
• graywacke
• a sandstone with abundant clay and rock fragments
• and argillite
• a slightly metamorphosed mudrock

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Sedimentary Rocks of Greenstone Belts

• Small-scale cross-bedding and graded bedding
  indicate an origin as turbidity current deposits

• Quartz sandstone and shale, indicate delta,
  tidal-flat, barrier-island and shallow marine
  deposition

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Relationship of Greenstone Belts to
Granite-Gneiss Complexes

• Two adjacent greenstone belts showing synclinal
  structure

• They are underlain by granite-gneiss complexes

• and intruded by granite

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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

• Models for the formation of greenstone belts
  involve Archean plate movement
• In one model, plates formed volcanic arcs by
  subduction

• the greenstone belts formed in back-arc marginal
  basins

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Evolution of Greenstone Belts

• According to this model,
• volcanism and sediment deposition took place as
  the basins opened

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Evolution of Greenstone Belts

• Then during closure, the rocks were compressed,
  deformed, cut by faults, and intruded by rising
  magma

• The Sea of Japan is a modern example of a
  back-arc basin

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Archean Plate Tectonics

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

34
Archean Plate Tectonics

• As a result of the rapid movement of plates,
  continents, no doubt, 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 were
  more common due to the higher temperatures

35
Archean World Differences

• 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

• 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 Late Archean ophiolites are known

36
The Origin of Cratons

• Certainly several small cratons existed by the
  beginning of the Archean
• During the rest of that eon they amalgamated into
  a larger unit
• during the Early 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

37
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

38
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)
• oxygen not combined with other elements
• such as in carbon dioxide (CO2)
• water vapor (H2O)
• ozone (O3)
• which is common enough in the upper atmosphere to
  block most of the Suns ultraviolet radiation

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Present-day Atmosphere

• 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 0.034
• Ozone O3 0.0006
• Other gases Trace
• Particulates normally trace

40
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.
• Also because Earth had no magnetic field until
  its core formed the solar wind would have swept
  away any atmospheric gases

41
Outgassing

• Once a core-generated magnetic field protected
  Earth, gases released during volcanism began to
  accumulate
• Called outgassing
• Water vapor is the most common volcanic gas today
• also emitted
• carbon dioxide
• sulfur dioxide
• Hydrogen Sulfide
• carbon monoxide
• Hydrogen
• Chlorine
• nitrogen

42
Hadean-Archean Atmosphere

• Hadean 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

43
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)
• Oxidized iron becomes increasingly common in
  Proterozoic rocks

44
Introduction of Free Oxygen

• Two processes account for introducing free oxygen
  into the atmosphere,
• 1. Photochemical dissociation involves
  ultraviolet radiation in the upper atmosphere
• The radiation breaks up water molecules and
  releases 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 organism
  that practiced photosynthesis

45
Photosynthesis

• Photosynthesis is a metabolic process in which
  carbon dioxide and water combine into organic
  molecules and oxygen is released as a waste
  product
• CO2 H2O 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

46
Earths Surface Waters

• Outgassing was responsible for the early
  atmosphere and also for Earths surface water
• the hydrosphere
• Some but probably not much of our surface water
  was derived from icy comets
• At some point during the Hadean, the Earth had
  cooled sufficiently so that the abundant volcanic
  water vapor condensed and began to accumulate in
  oceans
• Oceans were present by Early Archean times

47
Ocean water

• The volume and geographic extent of the Early
  Archean oceans cannot be determined
• Nevertheless, we can envision an early Earth with
  considerable volcanism and a rapid accumulation
  of surface waters
• Volcanoes still erupt and release water vapor
• Is the volume of ocean water still increasing?
• Much of volcanic water vapor today is recycled
  surface water

48
First Organisms

• Today, Earths biosphere consists of millions of
  species of bacteria, fungi, protistans, plants,
  and animals,
• only bacteria are found in Archean rocks
• We have fossils from Archean rocks
• 3.3 to 3.5 billion years old
• Carbon isotope ratios in rocks in Greenland that
  are 3.85 billion years old convince some
  investigators that life was present then

49
What Is Life?

• Minimally, a living organism must reproduce and
  practice some kind of metabolism
• Reproduction insures the long-term survival of a
  group of organisms
• whereas metabolism such as photosynthesis, for
  instance insures the short-term survival of an
  individual
• 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

50
What Is Life?

• Comparatively simple organic (carbon based)
  molecules known as microspheres

• form spontaneously
• show greater organizational complexity than
  inorganic objects such as rocks
• 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

51
How Did Life First Originate?

• To originate by natural processes, life must have
  passed through a prebiotic stage
• it showed signs of living organisms but was not
  truly living
• In 1924 A.I. Oparin postulated that life
  originated when Earths atmosphere had little or
  no free oxygen
• Oxygen is damaging to Earths most primitive
  living organisms
• Some types of bacteria must live where free
  oxygen is not present

52
How Did Life First Originate?

• With little or no oxygen in the early atmosphere
  and no ozone layer to block ultraviolet
  radiation, life could have come into existence
  from nonliving matter
• The origin of life has 2 requirements
• a source of appropriate elements for organic
  molecules
• energy sources to promote chemical reactions

53
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)

54
Basic Building Blocks of Life

• Energy from
• lightning
• ultraviolet radiation
• probably promoted chemical reactions
• during which C, H, N and O combined
• to form monomers
• comparatively simple organic molecules
• such as amino acids
• Monomers are the basic building blocks of more
  complex organic molecules

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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

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Polymerization

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

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Proteinoids

• The heated dehydrated 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
• are bounded by cell-like membranes
• grow and divide much as bacteria do

58
Proteinoid Microspheres

• Proteinoid microspheres produced in experiments

• Proteinoids grow and divide much as bacteria do

59
Protobionts

• Protobionts are intermediate between inorganic
  chemical compounds and living organisms
• Because of their life-like properties the
  proteinoid molecules can be referred to as
  protobionts

60
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 (J.B.S. Haldane, British
  biochemist)
• The amino acids in the soup might have washed
  up onto a beach or perhaps cinder cones where
  they were concentrated by evaporationand
  polymerized by heat
• The polymers then washed back into the ocean
  where they reacted further

61
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

62
Azoic (without life)

• Prior to the mid-1950s, scientists had little
  knowledge of Precambrian life
• They assumed that life of the Cambrian must have
  had a long early history but the fossil record
  offered little to support this idea
• A few enigmatic Precambrian fossils had been
  reported but most were dismissed as inorganic
  structures of one kind or another
• The Precambrian, once called Azoic (without
  life), seemed devoid of life

63
Oldest Know Organisms

• Charles Walcott (early 1900s) described
  structures
• from the Early Proterozoic 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
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Stromatolites

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

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Stromatolites

• Present-day stromatolites form and grow as
  sediment grains are trapped on sticky mats of
  photosynthesizing blue-green algae
  (cyanobacteria)
• 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

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Other Evidence of Early Life

• Carbon isotopes 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

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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
• meaning they lacked a cell nucleus
• and lacked other internal cell structures typical
  of eukaryotic cells (to be discussed later in the
  term)

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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