Chapter 20 The Precambrian Record

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Chapter 20The Precambrian Record
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Key Events of Precambrian time
Acasta Gneiss is dated at 3.96 bya. It is near
Yellowknife Lake , NWT Canada Zircons possibly a
bit older in Australia
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• Precambrian
• 4.6 billion years to 544 million years.
• Represents 88 of all of the history of the
  earth.
• Referred to as the Cryptozoic Eon.
• hidden life

(no more BIFs)
(prokaryotes)
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Early Hadean Highlights 1

• Earth formed 4.6 billion years ago from
  coalescing interstellar gasses.
• Earth is bombarded by large meteorites adding to
  earths mass (adds heat)
• Hot spinning pre-earth mass caused
  differentiation of materials according to
  density.
• Distinct earth layers begin to form
• iron and nickel migrate to center (core)
• silicate material moves out to mantle

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Early Archean Highlights 2

• Huge glancing blow from a Mars-sized impactor
  created the moon.
• Caused earth to spin faster.
• Possible Tilt change
• Moon controls earths spin and creates tidal
  forces.
• Moons orbit at an angle to planets around Sun
• Earth got most of the core - magnetic field and
  atmosphere

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Planetary capture hypothesis
Speed and approach angle unlikely
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Dual accretion hypothesis
Chemical composition of the Moon suggests that
it could not have co-formed with the earth.
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Impact hypothesis
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• Precambrian Hadean
• Formation of Continents
• Early earth surface was magma sea, gradually
  cooled to form the crust.
• Continents did not always exist but grew from the
  chemical differentiation of early, mafic magmas
  in the young hot earth. Volcanic Islands
• Young Earth probably looked like Venus, only much
  hotter.

Venus lavascape
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• Precambrian Hadean and Archean
• Formation of Felsic Islands
• Convection fast due high temperatures
  ultramafic magma
• Partial Melting of base of Ultramafic Islands,
  OR
• Fractional Crystallization of Mafic Magmas THEN
• Once both mafic and felsic rocks (with different
  densities) exist, subduction under
  protocontinents possible.
• Water squeezed from subducted ocean materials
  partially melts mantle
• Form protocontinents.
• Increasing amounts of Felsic continental material

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First continental crust
Then
First
Water out
Komatiite partially melts, Basalt gets to
surface, piles up. The stack sinks, partially
melts when pressure high Enough. Fractionation
makes increasingly silica rich magmas
Density differences allow subduction and allow
further felsic crust
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Growth of the early continents
Magmatism from Subduction Zones causes thickening
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Growth of the early continents
Island Arcs and other terranes accrete as
intervening ocean crust is subducted Little
Archean ocean crust survives, nearly all subducted
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Growth of the early continents
Sediments extend continental materials seaward
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Growth of the early continents

• Continent-Content collisions result in larger
  continents
• Again, not very big in Archean, Plate Tectonics
  too fast

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Archean-Age Surface Rocks
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• PrecambrianEarly Continents (Cratons)
• Archean cratons consist of regions of
  light-colored felsic rock (gneisses)
• surrounded by pods of dark-colored greenstone
  (chlorite rich metamorphic rocks).
• Pilbara Shield, Australia
• Canadian Shield
• South African Shield.

Mafic Greenstone Belts Felsic Islands
40km
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Archean Crustal Provinces once separated
Intensely folded rocks where cratons
were later sutured together in Early
Proterozoic Longest Trans-Hudson
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Granulite gneiss and greenstone
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Stratigraphic Sequence of a Greenstone belt
Younger lavas richer in silica
Increasingly Silica-rich extrusives, some
rhyolites with granites below them.
Komatiites form at very high temps
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Pillow lavas show underwater extrusion
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Formation of greenstone belts

• Early continents formed by collision of felsic
  proto-continents.
• Greenstone belts represent volcanic rocks and
  sediments that accumulated
• along subduction zones and then were sutured to
  the protocontinents during collisions.

• Protocontinents small, rapid convection breaks
  them up

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Proterozoic Tectonics The Wilson Cycle

• Proterozoic Convection Slows
• Rift Phase
• Coarse border, valley and lava rocks in normal
  faulted basins
• Drift Phase
• Passive margin sediments
• Collision Phase
• Subduction of ocean floor, collision with Island
  Arcs

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Crustal provinces Proterozoic Tectonics
Slave Craton Rift and Drift Followed by Wopmay
Orogen
Intensely folded rocks where cratons
were sutured together in Early Proterozoic
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Wilson Cycle 12 Rift DriftCoronation
Supergroup
2. Passive Margin sediments
Much later stuff
1. Rift Valley
Proterozoic 2 bya as Slave craton pulled apart
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Near-collision phase of the Wilson Cycle in the
Wopmay Orogen
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3. End of Wilson cycle in the Wopmay orogen
Coronation Supergroup thrust eastward over Slave
Craton Note the vertical exaggeration
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Proterozoic Assembly of Laurentia

• Trans-Hudson Orogen mostly 2.5 - 2 bya
• Superior, Wyoming, Hearne plates sutured
• Mountain range now eroded away
• Greenland, N. Gr. Brit., Scandinavia by 1.8 bya
• Continued accretion 1.8-1.6 bya of island arcs.
  Most of S. US Mazatzal Province
• Last piece Grenville Orogeny 1.3-1 bya Exposed
  Adirondacks and Blue Ridge
• Assembly of Rodinia by about 750 mya

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• PrecambrianEarly Atmosphere
• First earth atmosphere H He lost to solar wind.
  No magnetic field
• Early permanent earth atmosphere mostly Nitrogen
  (inert) and CO2
• Post-differentiation start of liquid core dynamo
• Liquid water is required to remove CO2 from
  atmosphere.
• Mars is too cold to have liquid water.
• Venus is too hot.
• Both have CO2 atmospheres.
• On Earth, most of the worlds CO2 is locked up in
  limestones, dolomites, and life!

Mars
Earth
Venus
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Early Permanent Atmosphere

• Gasses from cooling magmas formed early
  atmosphere mostly N2, CO2, with CH4, H2O
• Atmosphere not conducive to modern oxygen
  breathing organisms.
• Little oxygen occurred in the atmosphere until
  the evolution of photosynthetic organisms
  (Eubacteria) 3.5 billion years ago. Fully
  oxygenated about 1.9 billion years ago.

Sulphur Dioxide from Kilauea http//volcanoes.usgs
.gov/Products /Pglossary/VolcGas.html
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• PrecambrianEarly Oceans from 4 bya
• Much water vapor from volcanic degassing.
• Salt in oceans is derived from weathering and
• carried to the oceans by rivers.
• Blood of most animals chemistry of seawater.
• Part of the earths water probably came from
  comets.
• Comets are literally large dirty snowballs.
• Provide fresh water.

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• Archean To Proterozoic Sedimentary Rocks
• Archean Mostly deep water clastic deposits such
  as mudstones and muddy sandstones.
• high concentration of eroded volcanic minerals.
• Absence of shallow water shelf carbonates.
• Mostly chert.
• low oxygen levels, free iron was much more common
  in the Archean.
• Free iron formed chemical sinks that consumed
  much of the early planetary oxygen.
• Formed banded ironstones, commonly with
  interbedded chert.
• Proterozoic Carbonates become important

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Key Events of Precambrian time
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Proterozoic Oxygen - Rich Atmosphere

• Eubacteria are photosynthetic
• 2 bya formed stromatolites along shores
• Free oxygen in atmosphere
• Band Iron Formations (common 3.8 2 bya) become
  rare, probably depended on disappearing
  conditions
• 2 bya Redbeds begin forming when iron in
  freshwater sediment is exposed to abundant
  atmosphere oxygen
• Oxygen in atmosphere irradiated - Ozone layer
  forms, protecting shallow water and land life
  forms from UV

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Redbeds
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Key Events of Precambrian time
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Final Assembly of RodiniaGrenville Orogeny 1.3
1.0 BYA

• Eastern US Grenville collider seems to have been
  west coast of S.America
• Southwest US and Antarctica
• Grenville Orogeny continues in Antarctica
• Temporary arrangement - rifted apart by 700 600
  mya, about the Time of Snowball Earth

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Growth of Laurentia
Grenville Shallow Water sandstones, mudstones
and carbonates subjected to high-grade
metamorphism and igneous intrusion
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Rodinia
N
Metamorphic alteration of magnetic minerals
makes arrangement uncertain.
Most of North America Boxed-In
Note orientation and
neighbors of North
America
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Same idea, rotated
NORTH
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Proterozoic Rifting

• Grenville Time Rifting 1.3 1 bya
• Kansas to Ontario to Ohio
• Rift Valley sediments and lavas 15 km
• (9 miles) thick!
• Rich in Copper, as are the rift valley sediments
  here.
• Why?

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Midcontinent rift
1500 km long, exposed near L. Superior
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Key Events of Precambrian time
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Plenty of highlands, equator to poles
Grenville Orogen
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Snowball Earth

• Rodinia abundant basalts with easily weathered
  Ca feldspars. Ocean gets Ca and CO3--.
  Atmospheres CO2 tied up in extensive
  limestones. No greenhouse effect. Atmosphere
  cant trap heat gets colder
• Grenville Orogeny left extensive highlands
• From high latitudes to equator
• About 600 mya glacial deposits found in low
  latitudes
• Huge Ice sheet reflects solar radiation Albedo
• Oceans froze

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Stable isotopes of C and O
d13C and d18O 3 - 4 Proterozoic
Glaciations Earth surface became cold enough to
produce glaciations and ice ages
G - Glaciation BIF - Banded Iron Formation
Cambrian
Snow-ball Earth
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Break up of Rodinia

• Hypothesis Ice an insulator, heat builds up
• Heavy volcanic activity poured CO2 into
  atmosphere greenhouse effect
• Warming melted snowball earth

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Now, PreCambrian Life

• Return to the Archean

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• Origin of Archean Life
• The origin of life required the spontaneous
• organization of self-replicating organic
  molecules.
• The basic minimum requirements
• A membrane-enclosed capsule to contain
• the bioactive chemicals.
• Energy-capturing chemical reactions
• capable of promoting other reactions.
• Some chemical system for replication (RNA-DNA).

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• Formation of Enzymes
• 1950's and 1960's experiments produced amino
  acids by combining atmospheric gases, electrical
  sparks and heat.
• Further experiments demonstrated that drying and
  re-wetting of these organic compounds could
  produce
• cell-like membranes and simple proteins.
• Led to shallow water primordial soup theory.
• But organic compounds in shallow pools would have
  been instantly destroyed by ultraviolet
  radiation. Need an Oxygen-rich atmosphere to make
  an Ozone-Layer
• 2 bya

Stanley L. Miller, working in the laboratory of
Harold C. Urey at the University of Chicago.
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• RNA and DNA
• RNA can replicate and act as a catalyst
• that drives other nucleic acid reactions.
• Evolution of DNA (Deoxyribonucleic acid)
• easier once RNA was formed
• in early oceans.

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Key Events of Precambrian time
Ca and CO2 abundant during Rodinia Rifting Ended
Snowball Earth
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• Origin of Life Origin of Archaebacteria 3.5 bya
• Archaebacteria are the most primitive fossil life
  forms
• Likely ancestors of all life.
• Primitive Archaebacteria are hyperthermophiles
  that thrive in boiling point of water.
• Modern Archaebacteria live in deep-sea volcanic
  vents.
• Some Archaebacteria feed directly on sulfur
  (chemoautotrophs).
• Archean life probably arose in deep oceans
  hydrothermal, volcanic vents that would have
  dotted the ocean floor near rifting zones.
• Vents provide
• chemical and heat energy,
• abundant chemical and mineral compounds,
  including sulfur
• protection from oxygen and ultraviolet radiation.
  

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Archaebacteria

• They differ from other bacteria (called
  Eubacteria) because
• they are mostly anaerobic
• the RNA of their ribosomes is different to that
  of Eubacteria.
• They include the methane forming, the salt loving
  and the heat loving bacteria.
• Example Methane Forming
• The methanogenic bacteria create ATP by reducing
  carbon dioxide from the atmosphere using
  hydrogen, formate, or methanol. As a result
  methane is liberated. This can only be done in
  the absence of free oxygen.

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• Fossil Bacteria
• Prokaryotic archaebacteria and eubacteria are
  dominant. 2 bya
• Eubacteria form stromatolites (photosynthetic).
• More common in upper Archean as shallow water
  shelves began to form along margins of early
  continents.
• Archean is the age of pond-scum.
• Molds of individual bacterial cells found in
  Precambrian cherts.

Palaeolyngbya 1.8
Grypania 2.1 bya
850 million years old Chroococcalean 1.8
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2 bya Photosynthesis Modern Stromatolites Shark
Bay Australia Formed in hypersaline areas where
grazing gastropods can not thrive. Used to
dominate the landscape in Pre-Cambrian and Early
Cambrian.
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Evolution of Eukaryotes

• Probably began as a symbiotic relationship
  between different prokaryotes.
• Early eukaryotes ate but could not digest a
  cell which became a mitochondria. energy
• Plant-like eukaryotic ancestors ate
  chloroplast-bearing cyanobacteria. photosynthesis
• Once eukaryotes evolved, multi-cellular and
  colonial forms proliferated.

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Endosymbiosis
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Evolution of Metazoans

• Multi-cellular organisms appear in the Late
  Neoproterozoic (570 million years ago).
• Trace fossils indicate motion of early
  multicellular forms.
• Ediacaran (Vendian) fauna consist of simple flat,
  platelike organisms.
• Although originally believed to be homologous to
  Cnidarians or sponges, they may represent several
  unknown early phyla.
• Idea Early life forms had no competitors and
  were highly experimental in form?

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Proterozoic Life

• First metazoans evolve about 570 million years
  ago.

Ediacara Fauna
An arthropod?
Jellyfish, Sea Pens?
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Earliest hard parts
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End of Chapter 20