The Precambrian Record

1
The Precambrian Record
2
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
3

• Precambrian
• 4.6 billion years to, say, 548 or 544 million
  years (depending on method).
• Represents 88 of all of the history of the
  earth.
• Referred to as the Cryptozoic Eon.
• hidden life

Proterozoic
(no more BIFs)
Archean
(prokaryotes)
Hadean (oldest)
4
Early Hadean Highlights 1

• Earth formed about 4.6 billion years ago from
  coalescing interstellar dust.
• Earth was bombarded by large planetesimals 
  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 sink to center forms core.
• silicate material floats up, forms mantle

5
Early Hadean Highlights 2

• Huge impact from a Mars-sized planetessimal
  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

6
Moon Origin hypotheses -1
Speed and approach angle unlikely.
7
Moon Origin hypotheses - 2
Does not explain the depletion of metallic iron
in the Moon
8
Moon Origin hypotheses - 3
9

• Precambrian Early Atmosphere
• First earth atmosphere H He. Lost to solar wind.
  No magnetic field.
• Post-differentiation start of liquid core induced
  magnetic field
• Early permanent earth atmosphere mostly N2 CO2
  H2O
• gasses from volcanic outgassing. Not
  lost-protected by magnetic field
• 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 was converted
  to O2 by photosynthesis.
• Enough by 2.0 bya
• CO2 is locked up in life, limestones, dolomites!

Mars
Earth
Venus
10
Early Permanent Atmosphere

• Gasses from cooling magmas formed early
  atmosphere mostly N2, CO2, with CH4, H2O
• Earth not conducive to modern oxygen breathing
  organisms too much UV.
• Little oxygen O2 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
11

• 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 has chemistry of seawater.
• Part of the earths water probably came from
  comets.
• Comets are literally large dirty snowballs.
• Provide fresh water.

OCEANS
12

• Archean To Proterozoic Sedimentary Rocks
• Archean
• 3.8 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

13

• 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. Floating Volcanic
  Islands of less dense higher silica magmas.

14

• Precambrian Hadean and Archean
• Formation of Felsic Islands
• Convection fast due high temperatures
  ultramafic melts.
• Partial Melting of base makes new melt,
  fractionates, melt higher Silica SiO2. Lava piles
  up, stack thickens. Base deeper, melts,
  fractionation leaves melt richer in silica.
  Silica-rich melts have a lower density, float up.
• Increasing amounts of Felsic continental
  material, form protocontinents.
• Once rocks with different densities exist,
  subduction of low silica rocks under higher
  silica protocontinents is possible.
• Water squeezed from subducted ocean materials
  partially melts mantle, magma rises,
  fractionates and assimilates. Continents build
  up, they are too bouyant to be subducted.

15
First continental crust
1.At high temperatures, only Olivine and
Ca-Plagioclase crystallize Komatiite
Then
First
Water out
2. Komatiite partially melts, Basalt gets to
surface, piles up. The stack sinks, base
partially melts when pressure high enough.
Fractionation makes increasingly silica-rich
magmas
3.Density differences allow subduction of mafic
rocks. Further partial melting and fractionation
makes higher silica melt that wont subduct
16
Archean Growth of the early continents
Magmatism from Subduction Zones causes thickening
17
Growth of the early continents
Island Arcs and other terranes accrete to edge of
small continents as intervening ocean crust is
subducted. Temps so high that convection is
intense, divergence breaks up protocontinents. Lit
tle Archean ocean crust survives most was
subducted
18
Growth of the early continents
Sediments extend continental materials seaward
19
Growth of the early continents

• Continent-Continent collisions result in larger
  continents
• Again, not very big in Archean convection cells
  too small

20
Archean-Age Surface Rocks
21

• PrecambrianEarly Continents (Cratons) Archean
• 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.

Greenstone Belts Felsic Islands
40km
22
Archean Crustal Provinces were once
separatedCanadian Shield assembled from small
cratons
Intensely folded rocks, now planed off flat,
where cratons were later sutured
together in Early Proterozoic Longest
Trans-Hudson Orogen
23
(No Transcript)
24
Granulite gneiss and greenstone
Canadian Shield Exposed by Pleistocene glaciers
25
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
DEMO Banded Iron Sample
Note similarity to modern Ophiolite
26
Archean Formation of greenstone belts

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

• Protocontinents small, rapid convection breaks
  them up

27
(No Transcript)
28
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

29
Crustal provinces Proterozoic Tectonics
Slave Craton Rift and Drift Followed by Wopmay
Orogen remnants of old collisional mountains
Intensely folded rocks where cratons
were sutured together in Early Proterozoic
30
Wilson Cycle 12 Rift DriftCoronation
Supergroup
2. Passive Margin sediments
Much later stuff
1. Rift Valley
Proterozoic 2 bya as Slave craton pulled apart
31
Near-collision phase of the Wilson Cycle in the
Wopmay Orogen
32
3. End of Wilson cycle in the Wopmay Orogeny
Coronation Supergroup thrust faulted eastward
over Slave Craton Note the vertical exaggeration
33
Key Events of Precambrian time
34
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

35
Proterozoic Oxygen - Rich Atmosphere

• Eubacteria are photosynthetic
• 2 bya formed stromatolites along shores
• Free oxygen O2 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

36
Redbeds (also our campus)
37
Key Events of Precambrian time
38
Final Assembly of RodiniaGrenville Orogeny 1.3
1.0 BYA

• Eastern US Grenville collided with Grenville
  Craton, possibly west coast of S.America
• Southwest US collided w/ Antarctica
• Grenville Orogeny continues in Antarctica
• South collided with Africa
• Rifted apart by 700 600 mya, about the Time of
  Snowball Earth at 635 mya

39
Growth of Laurentia
Grenville Shallow Water sandstones (lots of
graywacke), mudstones and carbonates subjected to
high-grade metamorphism and igneous intrusion
40
Grenville Collider was Western S. America?
41
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?

42
Midcontinent rift
1500 km long, exposed near L. Superior
43
Key Events of Precambrian time
44
Plenty of highlands, equator to poles
What Plate Tectonic conditions favor glaciation?
Grenville Orogen
45
Snowball Earth

• Rodinia abundant basalts with easily weathered
  Ca feldspars. Ocean gets Ca . CO2 tied up in
  extensive limestones. Less greenhouse effect.
  Atmosphere cant trap heat Earth gets colder
• Grenville Orogeny left extensive highlands
• From high latitudes to equator
• About 635 mya glacial deposits found in low
  latitudes and elevations
• Huge Ice sheet reflects solar radiation Albedo
• Some workers believe oceans froze

46
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
47
Break up of Rodinia

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

48
Now, Precambrian Life

• Return to the Archean

49

• Origin of Archean Life
• The origin of life required the 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).

50

• 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 theory life started at
• deep sea vents near Black smokers
• 2 bya atmosphere has oxygen O2
• and ozone O3 which blocks UV

Stanley L. Miller, working in the laboratory of
Harold C. Urey at the University of Chicago.
51

• DNA gt mRNA, TRNAaa bound to mRNA in Ribosomes
• Makes chain of amino acids (protein)

The DNA sequence in genes is copied into a
messenger RNA (mRNA). Ribosomes then read the
information in this RNA and use it to produce
proteins. Ribosomes do this by binding to a
messenger RNA and using it as a template for the
correct sequence of amino acids in a particular
protein. The amino acids are attached to transfer
RNA (tRNA) molecules, which enter one part of the
ribosome and bind to the messenger RNA sequence.
The attached amino acids are then joined together
by another part of the ribosome. The ribosome
moves along the mRNA, "reading" its sequence and
producing a chain of amino acids.
http//en.wikipedia.org/wiki/Ribosome
http//en.wikipedia.org/wiki/Archaea
52
Key Events of Precambrian time
Ca and CO2 abundant during Rodinia Rifting Ended
Snowball Earth
53

• 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 environment volcanic vents that
  would have formed near Mid-Ocean Ridges
• Vents provide
• chemical and heat energy,
• abundant chemical and mineral compounds,
  including sulfur
• deep water protection from oxygen and
  ultraviolet radiation.

54
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
55

• Fossil Bacteria
• . About 2 bya Eubacteria (prokaryotes lack
  membrane bound nucleus)
• 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 Late
  Archean and Proterozoic cherts.

Palaeolyngbya 1. bya
Grypania 2.1 bya
850 million years old Chroococcalean 0.85 bya
56
2 bya Photosynthesis Modern Stromatolites Shark
Bay Australia Formed in 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
57
Endosymbiosis origin energy conversion plastids
in Eukaryotes
Food
Energy transfer from sunlight
oxidative reactions
58
Evolution of Eukaryotes

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

59
Evolution of Metazoans

• Multi-cellular organisms appear in the Late
  Neoproterozoic (570 million years ago).
• Trace fossils (burrows, etc.) indicate motion of
  early multicellular forms.
• Ediacaran (Vendian 580-542 mya) fauna consist of
  simple organisms.
• Although originally believed to be related to
  Cnidarians or sponges, a closer look reveals they
  may represent several unknown early phyla.
• Idea Early life forms had no competitors and
  were highly experimental in form?

60
Proterozoic Life

• First metazoans evolve 580-542 mya.

Ediacara Fauna
An arthropod?
Jellyfish, Sea Pens? Not really.
61
Earliest hard parts Late Ediacaran to base of
Cambrian
http//en.wikipedia.org/wiki/Cloudinid
62
Next week, the Paleozoic