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

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Early Hadean 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 outer part molten.
  Earth rotates. We have magnetic field and,
  therefore, an 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 of mafic
rocks. Further partial melting and fractionation
makes higher silica melt that wont subduct
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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 most 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-Continent collisions result in larger
  continents
• Again, not very big in Archean convection cells
  too small

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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 (granulite 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 separatedCanadian
Shield assembled from small cratons
Discussion Where to look for diamonds
Intensely folded rocks where cratons
were later sutured together in Early
Proterozoic Longest Trans-Hudson Orogen
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Granulite gneiss and greenstone
Canadian Shield Exposed by Pleistocene glaciers
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Stratigraphic Sequence of a Greenstone belt
Banded Iron Formations
Younger lavas richer in silica
Increasingly Silica-rich extrusives, some
rhyolites with granites below them.
Komatiites form at very high temps. They are
absent later as Earth cooled
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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 and above 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, island arcs form
• Then collision

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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 2. Carbonates
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 to have liquid water.
• So 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
• 4 bya mostly deep water clastic deposits such as
  mudstones and muddy sandstones.
• high concentration of eroded volcanic minerals
  (Sandstones called Graywackes).
• 3 bya absence of shallow water shelf
  carbonates.
• increasing chert.
• low oxygen levels, free iron was much more
  common in the Archean.
• Iron formed chemical sinks that consumed much
  of the early planetary oxygen.
• Formed banded ironstones, commonly with
  interbedded chert.
• Proterozoic 2 bya Carbonates become important
• - Non-marine sediments turn red iron is
  oxidized by the oxygen in AIR

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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 (also our campus)
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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 at 635
  mya

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Growth of Laurentia
Grenville Shallow Water sandstones (lots of
graywacke), mudstones and carbonates subjected to
high-grade metamorphism and igneous intrusion
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Same idea, rotated
Grenville Collider is Western S. America
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
What Plate Tectonic conditions favor glaciation?
Grenville Orogen
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Snowball Earth

• Rodinia abundant basalts with easily weathered
  Ca feldspars. Ocean gets Ca . 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 635 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
• Modern view at deep sea vents
• 2 bya atmosphere has oxygen and ozone

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 near 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 MORs
• 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 from
  that of Eubacteria.
• They include the methane forming, the salt loving
  and the heat loving bacteria.
• Example Methane Forming
• The methanogenic bacteria create Adenosine Tri
  Phosphate 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.

CS Define Eukaryote
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• Fossil Bacteria
• Prokaryotic archaebacteria and eubacteria are
  dominant. About 2 bya Eubacteria
• 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. bya
Grypania 2.1 bya
850 million years old Chroococcalean 0.85 bya
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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. Also forming today on shores of Rift
Valley Lakes in Kenya
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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
Food
Energy transfer from sunlight
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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
  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