Radiometric dating.

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Lecture 13

• Radiometric dating.
• Earliest Evidence for life on Earth.

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Student Presentations

• Matthew Trice and Samantha . Life on Europa
  October 23
• Connie Kwong et al. Permian Triassic extinction
  October 28
• Kelsey Palmer and Lyndsey Mullens Asteroids and
  Comets and their implication to the origin of
  life October 30
• Michael Barnett et al. Life on Mars
  November 4
• Triana Henz Planetary terraforming
  November 6

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Radioactive isotopes (238U, 14C) can
spontaneously change or decay
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Radiometric dating

• Radiometric dating is a technique used to date
  materials, based on a comparison between the
  observed abundance of particular naturally
  occurring radioactive isotopes and their known
  decay rates.
• Initial radioactive isotope is called a parent
  and the product of decay is called a daughter.
• The half-life time of an isotope is the interval
  of time required for an isotope to decay to half
  of its initial amount.

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Typical Parent-Daughter pairs used in
radiometric dating
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Potassium-Argon dating

• Argon is a gas and therefore if a rock was melted
  then Ar escapes.
• Any 40Ar in the rock has to come from the decay
  of 40K after rock became solid.
• So by measuring 40K and 40Ar in the same rock we
  can find the age of that rock

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Uranium-Lead dating

• Usually performed on zircon (ZrSiO4)
• Zircons can incorporate uranium (238U) into its
  crystalline structure when they form but reject
  lead
• Any lead (206Pb) observed in zircons has to come
  from the decay of uranium
• which was already incorporated
• into zircons

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Problems with radiometric dating

• System has to remain closed no input of
  parent isotopes and no escape for daughter
  product
• Cannot perform radiometric dating on the
  sedimentary rocks

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Challenges for life detection in the ancient rocks

• Ancient rocks are rare (burial, erosion,
  subduction, ejection into space during late heavy
  bombardment)
• Survived rocks are changed by metamorphism
  (pressure and heat)
• No bones or shells! Single-celled prokaryotic
  organisms
• Not every rock can contain evidence for life (no
  life in the igneous rocks)
• Contamination

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Locations of the exposed ancient rocks

• Akilia (SW Greenland) gt 3.85 Gyr ago
• Isua (W Greenland) 3.7-3.8 Gyr ago
• Pilbara (NW Australia) 3.52 Gyr ago
• Swaziland (South Africa) 3.5 Gyr ago

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Carbon Isotopes

• On Earth 13C/12C 1/90
• Standard (Pee Dee Belemnite) 13C/12C 0.01123722
• A typical 13C/12C ratio in a living organism
  0.0109563
• ?13C 1000((13C/12C)sample -(13C/12C)standard)/(
  13C/12C)standard
• in parts per thousand (or per mil)
• During photosynthesis organic carbon becomes
  enriched in 12C

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?13C 1000((13C/12C)sample (13C/12C)standard)/
(13C/12C)standard ?13C very small in organic
matter gt organic matter has to be from the
biosphere
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Challenge!

• Organic matter in the Akilia rocks could have
  been produced without life through thermal
  disproportionation
• 6 FeCO3 ? 2Fe3O4 5 CO2 C
• Akilia rocks might not be sedimentary
• Possible presentation Fedo and Whitehouse
  (2002) Science 296 1448-1452

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Carbon isotopes in the ancient rocks

• The overall pre-1 Gyr old carbon isotope record
  is similar to the carbon isotope record of the
  modern times.
• Autotrophic organism were likely to be present
  since at least 3.5 Gyr ago.

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Microfossils

• Microfossils are the preserved remains of
  microbial organisms
• Small! Up to a few tens of microns. Either simple
  spheroids (balls) or simple filaments
  (sticks)
• See the debate between Brasier et al. (2002),
  Nature 416 p. 76-81 and Schopf et al. (2002),
  Nature 416 p. 73-76

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3.5 Gyr old microfossils Under debate!
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Stromatolites fossilized remains of the
ancient bacterial mat
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Stromatolites

• Stromatolites are laminated sedimentary
  structures
• If stromatolites are biogenic then they represent
  fossils of colonial photosynthesizing microbes
  (cyanobacteria) which built reefs similar to
  corals today
• The most ancient biological stromatolite is
  3.46 Gyr old (Warrawoona, Australia)

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Biomarkers

• Certain hydrocarbon molecules found in the
  ancient organic matter are recognizable
  derivatives of biological molecules
• When these molecules have a specific biological
  source they are called biomarkers
• Biomarkers of eukaryotes and cyanobacteria were
  found in 2.5-2.7 Gyr old rocks (Hamersly,
  Australia)

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