Proterozoic Eon Summary

Proterozoic Eon Overview

  • Characteristics: Many rocks exposed; fossils uncommon; includes komatiites, greenstone belts, granite-gneiss complexes; passive continental margins; banded iron formations (BIFs).

Geologic Timeframe

  • Duration: Encompasses >2 billion years (42.5% of Earth's history).

  • Divisions: Paleoproterozoic (2.5-1.6 Ga), Mesoproterozoic (1.6-1.0 Ga), Neoproterozoic (1.0-0.541 Ga).

Laurentia's Growth

  • Formation: Ancient continent formed from Archean cratons.

  • Major Events: Several orogenies (Wopmay, Thelon), significant accretion events from 1.8 to 1.0 Ga.

Midcontinent Rift

  • Opening: Formed around 1.1 Ga, a failed spreading zone, active for ~20 million years.

  • Deposits: Sandstones and volcanics recorded in the area.

Supercontinents

  • Rodinia: Assembled 1.3-1.0 Ga, fragmented ~750 Ma; possible earlier supercontinent Nuna around 1.8 Ga.

  • Ophiolites: Indicators of oceanic-continental convergence, containing some of the oldest known sequences.

Glaciation Events

  • Paleoproterozoic & Neoproterozoic: Two major glacial episodes; Neoproterozoic glaciers extensive even in equatorial regions, leading to 'Snowball Earth' conditions.

Atmospheric Changes

  • Oxygen Levels: Increased from <1% to <10% current levels by end of Proterozoic; Great Oxidation Event occurred.

  • Banded Iron Formations: Mounted during periods of low atmospheric oxygen;

    • Formed in shallow-water environments 2.5-2.0 Ga, indicating photosynthetic activity.

Life in the Proterozoic

  • Fossil Record: Dominated by prokaryotic bacteria; stromatolites were common.

  • Eukaryote Evolution: Eukaryotes evolved through endosymbiosis; significant microfossil discoveries track early life forms.

  • Ediacaran Fauna: First complex multicellular life forms appeared around 575 Ma, includes diverse organisms yet to be classified.

Microfossil Discovery

  • Acritarchs: Acid-resistant microfossils of variable origins; first index fossils.

  • Early Multicellularity: Cells developed specialization; limited information on origins from fossils, insights gained from modern analogs.