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Proterozoic

Proterozoic. Part 2 Life and atmosphere. Precambrian. Eoarchean 4.6-3.6 Ga Oldest mineral Oldest rocks Rest of the Archean 3.6-2.5 Ga Building of crust Oldest fossils Proterozoic 2.5-.545 Ga Laurentia Rodinia Passive margins. Oxygen. Archean: only ~1% of current oxygen

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Proterozoic

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  1. Proterozoic Part 2 Life and atmosphere

  2. Precambrian • Eoarchean 4.6-3.6 Ga • Oldest mineral • Oldest rocks • Rest of the Archean 3.6-2.5 Ga • Building of crust • Oldest fossils • Proterozoic 2.5-.545 Ga • Laurentia • Rodinia • Passive margins

  3. Oxygen • Archean: only ~1% of current oxygen • From photochemical dissociation and photosynthesis • Proterozoic: up to ~10% of current oxygen • How can we study ancient oxygen levels?

  4. Proterozoic oceans • Banded Iron formations (BIF’s) • Layers of chert and iron minerals • Iron oxides (hematite, magnetite)

  5. Banded iron formation • 92% of BIF are Proterozoic (2.5-2.0 Ga)

  6. More typical appearance of BIF

  7. BIF and oceans • Fe is highly reactive • If no oxygen, Fe stays in solution (oceans) • Archean atmosphere low in O • Ocean rich in Fe

  8. BIF and oceans • If oxygen around, combines to form iron oxides (rust-like minerals) • Proterozoic: more oxygen, iron oxides precipitate in oceans

  9. Model for origin of BIF • Oxygen-rich upper ocean • Oxygen-poor deep ocean • Upwelling brings Fe and Si-rich water up • Iron oxide formation occurs

  10. BIF’s are source for 90% of Fe ore

  11. Proterozoic Oceans and Atmosphere • By 2.5-2.0 Ga, oxygen in oceans (BIF’s) • Oxygen in air? • Continental red beds

  12. Continental Red beds • Red from iron oxides (hematite) • 1st appear 1.8 Ga • Free oxygen in atmosphere • Source of oxygen?

  13. Oxygen on Earth • What environment do the earliest life forms on Earth live? • Are these early forms creating or using oxygen? • Is it logical then that the oceans have oxygen first and then the atmosphere?

  14. Proterozoic life • Archean, Early Proterozoic (until 2.1 Ga): • Algae • Bacteria • Single-celled prokaryotes • Little diversity

  15. Eukaryotic cell • 2.0 Ga, eukaryotes appear • More complex • Single celled • Aerobic (need oxygen)

  16. Eukaryotic Cell • Sexually reproduce, not by dividing cells • Leads to mutation and variations

  17. Grypania spiralis, Michigan ~2 billion years old. Oldest eukaryotes

  18. Neoproteoroic eukaryotes • Larger, more complex than prokaryotes

  19. Next step: • Multicellular organisms • Cells have specific functions • 1.4 Ga multicellular algae (?) Montana • Oldest well preserved are 800 Ma

  20. Advantages of multicellular life: • Can get bigger • Live longer • Produce more offspring • Increased efficiency

  21. First animals • Ediacaran fauna • 545-600 Ma • Soft-bodied

  22. First animals • Ediacaran fauna • Related to (?): • Jellyfish • Segmented worms • Arthropods (insects, spiders etc.) • Trilobites (?)

  23. First animals

  24. Ediacaran environment

  25. Other Proterozoic animals • Worms (?) 700-900 million years old from China

  26. Proterozoic animals • Proterozoic relative of corals (?)

  27. Proterozoic animals • All soft-bodied • Early stages of skeletons may be under way, not durable

  28. Precambrian life • Oldest prokaryotes (stromatolites) • ~3 Ga (3.5 Ga?) • Oxygen in oceans • ~2.5-2.0 Ga • Oldest eukaryotes • ~2.1 Ga • Oxygen in atmosphere 1.8 Ga • Oldest multicelled life ~1.5 Ga • Snowball Earth 750-600 Ma • Oldest animals (soft bodied) • 600 Ma

  29. Questions • What evidence is there for oxygen in the Proterozoic atmosphere? • You observe some Proterozoic conglomerate that might be glacial till. How would you confirm or disprove this hypothesis? • How might you prove what latitude the till formed?

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