precambrian life. earth’s atmosphere today’s atmosphere and hydrosphere is different than...
TRANSCRIPT
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Precambrian
Life
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Earth’s Atmosphere
• Today’s atmosphere and hydrosphere is different than Precambrian
• Today’s atmosphere: – Nitrogen (N2)– Abundant free oxygen
(O2)– Water vapor (H2O)– Ozone (O3)
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Earth’s Early Atmosphere• Primitive atmosphere
– He, H in H2O vapor• Blown away (no magnetosphere) or
lost to space (not enough gravity)
– O2 in H2O & CO2– C in CO2– But deficient in O2 & rich in CO2
• Gases from cooling magma– Simple gases – methane (CH4) &
ammonia (NH3)• Atmosphere not conducive to
O2-breathing organisms• Little free O2 in atmosphere
until evolution of photosynthetic organisms – Some oxygen by photochemical
disassociation– Reducing environment changed to
oxygenation one
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Precambrian Atmosphere• Evidence for oxygen production and
accumulation in Earth’s atmosphere– Banded Iron Formations (BIF’s)– Red Beds
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Banded Iron Formations (BIF’s)
• Occur in rock record about 3.2 Ga—most at2.0-2.5 Ga
• Formed in oceans
• Consist of chert (SiO2) & red bands– Red Bands rich in
iron oxides Fe2O3, Fe3O4
• Record major oxygenation event
PreCambrian BIFs
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Origin of BIF’s
• Photosynthesis produced oxygen
– Combined with Fe to produce “rusty rain” in ocean
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Red Beds• Similar to BIF’s,
but . .– Terrestrial formations– Lower in Fe
concentration
• Occur in rock record about 2 Ga– Atmosphere at this
time only had 1-2% O2
• Indicate O2 present in atmosphere to “rust” sediments – O & O3 more
effective oxidizing agents
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Origin of Red Beds
ferric iron oxides: red beds
• Red beds formed after all reduced iron in ocean had been oxidized
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Where did the O2 come from?
• Prokaryotes• Eukaryotes• Ediacaran Fauna
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Protein Synthesis• S. Miller, chemist (1953)• Reconstructed “early atmosphere”
– Mixed methane, ammonia, H2 and H2O vapor– Applied electrical charges produced amino acids
Heat, UV radiation, sunlight, radioactivity can do same• Process called abiotic synthesis• Today, only organisms produce amino acids
– Amino acids + organic molecules = protein
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Earliest Organisms• Must have had
anaerobic (no O2) heterotrophs– Used organic soup for
food• Free O2 lethal to
anaerobic heterotrophs– Need to adjust to ↑
O2• Cherts important
– Silica gel (volcanism) trapped organisms
• Fig tree chert– S. Africa = 3.1 Ga
• Stromatolite– NW Australia = 3.5 Ga
• 3.85 in Greenland
3.5 Ga Stromatolite
Modern Stromatolite
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Archean - Prokaryotes• Cyanobacteria
– Blue-green algae– Single celled, lack nucleus
• Contain DNA, but no membrane-bound organelles• Undergo photosynthesis
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Prokaryotes
3.3-3.5 Ga ProkaryoteWarrawoona Group, W. Australia
3.3-3.5 Ga ProkaryoteWarrawoona Group, W. Australia
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Archean Eukaryotes• Contain nucleus, DNA and are larger• Membrane-bound organelles• Fig tree has chemical indicators of life
– Pristane/phytanes Chlorophyll products
– C-12 & C-13 Used by photosynthesizing organisms
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Eukaryotes
Microfossils, Gunflint Fm, Canada
700-800 Ma Microfossils, Beckspring Dolomite, California
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Common Proterozoic Eukaryotes
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Metaphytes and Metazoans• 3.5-0.9 Ga small organism
– Single cell or few cells attached
• Next important evolutionary step– Combination of cells to form
macroscopic organism
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Metaphytes and Metazoans• Metaphytes
(plants)
• Metazoans (animals)
– First plants = algae– May have multi-cellular algae
– First evidence is trace fossils– Found in late Precambrian – Montana, Canada– Made by large organism
Possible multi-cellular algae, Little Belt Mtns., Montana
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Ediacaran Fauna• Soft-bodied fossils in SS, S. Australia
– 1.0”-2.0”, some a few feet– Heterotrophs; previously all autotrophs
Depend on outside food source
– Multi-celled organisms Led to specialized cells Led to organs
– Evidence of systems in organisms
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Reconstruction Ediacaran Environment