Tectonics and climate of the Precambrian

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Tectonics and climate of the Precambrian

• Geology 103

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Evolution of the atmosphere

• Hadean Lots of carbon dioxide, water vapor and
  methane
• Archean Water vapor forms oceans, oxygen starts
  to be made by photosynthetic organisms
• Proterozoic Significant oxygen in atmosphere,
  massive drop in carbon dioxide

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Graphically
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The Witwatersrand (South Africa) goldfields
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Evidence against the theory

• Not all gold deposits are the same age
• Clearly, some other mechanism deposits gold in
  this fashion anoxic inland seas?

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More evidence for atmospheric change in Archean

• Banded iron formations (BIFs) are interlayered
  alternating chert (jasper) and iron oxide
• Mostly found in Archean, some in Proterozoic,
  almost none in the Phanerozoic

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Mechanism for generating BIFs
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Meanwhile, plate tectonics settles down

• Archean rocks worldwide are of only two types
  granite/gneiss complexes (a high-grade
  metamorphic rock) and intervening greenstones
  (metamorphosed basalt and some sedimentary rock)
• Superior province in North America is among the
  biggest in the world

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What does a greenstone belt remind you of?
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But still different than todays plate tectonics

• Komatiites are ultramafic igneous rocks that are
  common in the Precambrian but unknown today
• Hotter mantle?
• Wetter mantle?
• Diamonds!

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First continents form and stick around
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So, by the Proterozoic

• Division between Archean and Proterozoic is based
  on oxidizing conditions found in surface waters
  (1.8 by)
• Tectonics is more similar to todays evidence
  for rifting and subduction and terrane accretion

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What evidence exists for Rodinia?

• Grenville orogeny rocks (sometimes called mobile
  belts), originally defined to explain Canadian
  shield rocks, were found to exist on many other
  continents
• All this mountain-building implies some
  large-scale tectonic event, like a supercontinent
  (name was suggested in the 1990s)
• Rodinia is constructed at 1.1 by, rifts apart by
  0.85 by

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The Grenville orogeny rocks

• Primarily marine sandstones and carbonates
  (limestones)
• No bioturbation
• Since then, these rocks have been metamorphosed,
  but the original rock is easily inferred

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Conventional reconstruction

• Line up all the Grenville orogenic belts and
  create the supercontinent
• Note that Antarctica and the US (Laurentia) are
  quite separated

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The SWEAT hypothesis

• Rodinia joined the southwest (SW) US (West Texas,
  specifically) with eastern Antarctica (EAT)
• Shown through lead isotope measurements of
  similar age rocks that were part of a rift in
  both areas
• Key point there was not just one zone of orogeny
  as in the conventional theory

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Precambrian climate

• Positions of continents, especially existence of
  polar continents, determines when ice ages occur

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Positive feedback

• If glaciers can build extensively to within 30
  of the equator, the extensive ice will reflect a
  large portion of the Suns energy back into
  space, cooling the surface and allowing more
  glaciers to grow
• Icehouse Earth or Snowball Earth hypothesis
  (W. Brian Harland, Cambridge, 1964)

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How to get out of the Icehouse

• Joe Kirschvink (Caltech, 1992) argued that
  volcanic activity and carbon dioxide production
  would not cease even during an Icehouse event,
  and nothing would scrub the carbon dioxide out
  of the atmosphere, enhancing the greenhouse
  effect

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More detail about CO2 scrubbing
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Life alters as Rodinia breaks up

• Ediacaran fauna appears first evidence of
  multicellular life
• No hard parts, preserved as molds
• Unclear if they are truly related to modern
  phyla, or represent extinct phyla
• Ediacaran period is a recognized division of the
  Proterozoic eon (630 542 my)