Proterozoic Eon §

• 2.5 billion years ago to 541 million years ago (42.5% of geologic time)
• Proterozoic rocks
  • Many more rocks exposed than Archean
    • Ultramafic mantle-derived volcanic rocks became rare
  • Appearence of shallow marine strata
  • Banded iron formations
  • Growth of (super)continents
    • ~43% of modern continental crust came from this time period
• Proterozoic events
  • Great Oxidation Event
  • Snowball Earth glaciation (at least 3x)
  • Boring billion (1.8-0.8 billion years ago)
  • Ediacaran Fauna
• Proterozoic Supercontinents
  • Columbia/Nuna
    • 1.6 billion years ago
    • Created by collisions and suturing of Archean proto-continents
    • Made up of proto-cratons that make up cores of most modern continents
  • Rodinia
    • 900 million years ago
    • Mostly located along equator
    • Began to fragment 750 million years ago
  • Pannotia
    • 600 million years ago
    • Short-lived (existence is debated)
    • Centered on south pole
• Poterozoic History - Laurentia
  • Basement rock = craton
    • Crystalline igneous or metamorphic rocks that lie beneath sedimentary rock
    • Can be any age, but most is Archean and Proterozoic
  • Orogeny
    • Mountain-building events
    • Deformation, igneous intrusions
  • Laurentia
    • Craton of North America and Greenland
    • Major continent of Proterozoic
    • Lon, complex history of orogenic events and sutering
    • 
• Wilson Cycle: Stages of evolution of continents and oceans
  • 
  • (A and B) Continental stretching and rifting
  • (C) seafloor spreading begins, forming a new ocean basin
  • (D) the ocean widens and is flanked by sedimented passive margins
  • (E) subduction of oceanic lithosphere begins on one of the passive margins
  • (F) so the ocean basin gets smaller
  • Eventually, the ocean basin is all subducted away and the continents collide, building a mountain range (G)
  • This helps to explain the evolution of Laurentia
• The Great Oxidation Event
  • ~2.3-2.0 billion years ago, rise in atmospheric oxygen and ocean sulfate
  • Result of cyanobacteria producing oxygen through photosynthesis
  • Important implications for life evolution (free oxygen, ozone to block UV radiation, respiration)
  • Evidenced by BIFs, red beds, fossils, isotopic evidence
• Snowball Earth
  • Rises in Oxygen -> reductions in CH4?
  • Clustering of continents at low latitudes
    • Allows extensive weathering and CO2 drawdown?
  • Accumulating snow and ice
    • Increases Earth’s albedo (reflectance) leading to further cooling and ice accumulation
  • Two main Neoproterozoic glaciatiations
    • Sturtian (720-660 million years ago)
    • Marinoan (650-630 million years ago)
  • Ending of Snowball Earth
    • Volcanic and metamorphic CO2 emissions accumulate in atmosphere

Proterozoic Life §

• Stromatolites
  • Increase in size of continents made more continental shelf available
  • Greatest diversity 1.2 billion years ago
• Early eukaryotes
  • Unicellular organisms - Acritarchs and others
  • Algae - cyanobacteria became major producers in marine environments after 2 billion years ago
• Beginnings of animal life
  • Neoproterozoic evolutionary radiation
  • Evidence of multicellular life as much as 780 million years ago in trace fossils
  • 650-600 million years ago have sponge body fossils and soft-bodied bilaterally symmetrical organisms
• 
• The Ediacaran Environment
  • 635-539 million years ago (end of Proterozoic)
    • coincides with rapid retreat of ice sheets and glaciers
  • Some of first evidence of multicellular life
  • Likely fed on dissolved organic material on seafloor
    • Microbial mats - sediment + colonies of microbes
  • Evidence of high salinity, low surface productivity oceans with increasing oxygen levels
• Ediacaran Fauna
  • Originally identified in Ediacara Hills, Australia
  • Latest Neoprotroerozic
    • 570-541 million years ago
  • No hard parts
    • Uncommon fossils
    • Just impressions (trace and body)
  • Found on 6 of 7 continents
  • Three phyla may be present
    • Cnidaria - jellyfish and sea pens
    • Annelida - segmented worms
    • Arthropoda - jointed legs
    • Possible early echinoderms
  • Burrowing
    • After 600 million years ago, clear evidence of burrowing disrupts layered sediments
    • mm-scale, simple, mostly horizontal/parallel and near sediment surfaces
    • segmented worms?
  • Skeletal fossils?
    • nested cone structures made of calcium carbonate (Cloudina)
    • Spicules and fossizlied sponges