Precambrian History

1
Precambrian History

• The Archean
• Chapter 8
• Part 2

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Precambrian

• Archean eon 4.6-2.5 Ga
• Eoarchean 4.6-3.6 Ga
• Proterozoic eon 2.5-.545 Ga

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Last time

• Eoarchean
• Oldest mineral
• Oldest rocks
• Archean
• Craton and shield
• Continental accretion
• Granite/greenstone belts

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Today

• Atmosphere
• Hydrosphere
• Life

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Atmosphere

• 1st atmosphere H, He
• Why?
• Where did it go?
• Why?
• Why do we have an atmosphere today?
• Magnetic field deflects solar wind

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Atmosphere

• Where did 2nd atmosphere come from?
• Outgassing release of gas from volcanoes

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Atmosphere

• Where does oxygen come from?
• Breaking up water (photochemical dissociation)
• Accounts for 2 of current oxygen
• Forms ozone layer

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Atmosphere

• Major source of oxygen
• Photosynthesis
• End of Archean only 1 of current oxygen

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Hydrosphere

• Where does water come from?
• Outgassing
• Water most abundant gas coming from volcanoes

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Hydrosphere

• Water vapor condenses when Earth cool enough
• Perhaps 4.4 Ga

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Archean life

• What is life?
• Anything that reproduces and metabolizes
• What do you need for life?
• Source of elements for organic molecules
• Atmosphere, water, rocks
• Energy source for chemical reactions
• Make simple molecules, monomers
• Amino acids

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Archean life

• Millers experiment to make amino acids
• Simulate early atmosphere, add lightning, make
  amino acids

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Archean life

• Started on ocean floor?
• Elements from ocean and rocks
• Problem No sunlight
• Energy from black smokers

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• Hydrothermal vents hot water entering ocean
• Black Smokers Black with minerals (sulfur, iron)

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Black SmokerChimney
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Tube worms found near black smokers
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Life at the black smokers

• Heat loving microbes (up to 250) that breathe
  iron
• Animals dont need sunlight to live
• Not photosynthetic - chemosynthetic

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Life starts

• Primordial soup
• Shallow puddle/lightning
• Ocean floor
• Black smokers/chemosynthetic critters

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First life

• No known fossils
• Anaerobic
• Need no oxygen
• Heterotrophic
• Dependent on external nutrients
• Prokaryote cells
• No cell nucleus/internal structure

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Oldest evidence of life

• Chemical evidence in 3.7-3.8 billion year old
  sedimentary rocks in Greenland

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Oldest evidence of life

• Possible cells 3.4 billion from South Africa

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Oldest known organisms

• Stromatolites reefs built by algae
• Phylum cyanobacteria

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Oldest known organisms

• Stromatolites reefs built by algae
• Oldest undisputed 3.0 billion years old

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2 billion year old stromatolites
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Stromatolites

• Colonies of algae (prokaryotes)
• Still anaerobic
• Photosynthetic

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Archean fossils

• Anaerobic
• Prokaryotes (algae)
• Autotrophic (make own nutrients since
  photosynthetic)

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Archean achievements

• Origin of life (anaerobic prokaryotes)
• Development of photosynthesis (3.0 billion)
• 1st stable crust (4.0 billion)
• Large cratons by 2.5 billion
• 1st and 2nd atmospheres