Carbon14 Dating

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Carbon-14 Dating

• Carbon-14 is produced by cosmic rays
  (high-velocity chatged particles generated by
  exploding stars) when they collide with Earths
  atmosphere.
• All living organisms absorb C-14 and C-12.
• The ratio of C-14 to C-12 is determined by the
  ratio in the atmosphere.
• When an organism dies, it ceases to absorb C-14
  and the ratio of C-14 to C-12 decays with time.
• Measuring the C-14 to C-12 ratio gives the age of
  the sample.

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Carbon-14 Dating

• The most sensitive way to measure the C-14 to
  C-12 ratio is with an Accelerator Mass
  Spectrometer (AMS). This has reduced the amount
  of material needed from the kilograms required by
  techniques that counted the radioactive decays of
  C-14 to less than a gram.
• Carbon-14 in the atmosphere varies with time due
  to changes in the flux of cosmic rays

• This is primarily due to variations in the
  strength of Earths magnetic field.
• C-14 dates assuming a constant starting ratio of
  C-14 to C12 give C-14 years that are not
  exactly the same as actual years.
• C-14 years have been calibrated using samples,
  whose age can be determined independently, such
  as tree rings.

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Carbon-14 Dating

• Carbon-14 dating is limited to less than about
  100,000 years, because after 10 or so half-lives
  there is too little C-14 left to measure
• Application examples

• Dating trees killed by the eruption of Mt. Mazama
  to form Crater Lake in Oregon - 6600 years B.P.
• Determining the timing of the re-population of
  the Puget Lowland by conifers after the last ice
  age.
• The intervals between the last very large
  earthquakes in Western Washington (300 1000
  years).
• The age of charcoal used in cave paintings in
  southern France (35,000 years).

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Half-Life the Time it Takes for Half of the
Original Atoms in a Sample to Decay to a
"Daughter" Product The daughter product may also
be radioactive, and may decay into another
product.
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Basic Assumptions of Radioactive Dating

• System has remained closed since t0. No loss or
  gain of parent or daughter atoms.
• One parent atom decays to one daughter atom.
• There are no daughter atoms present at t0.

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Uranium-Lead Dating of Zircons

• Zircons (ZrSiO4) incorporate U, but not Pb when
  they form.Thus the assumption that there is no
  daughter at t0 is very good.
• Zircons are nearly indestructible and impervious
  to fluids. Thus the assumption of a closed system
  is also good as long as the temperature has
  remained below 800C.
• Zircons from meteorites have been measured with
  an age of 4,365 ? 15 My.

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Potasium-Argon Dating

• The decay involves two daughters, but the ratio
  of the daughters is constant. 10.5 of the
  daughter atoms are Argon-40. Thus

• The daughter is a gas that can leak out of the
  rock matrix.
• Atmospheric Argon can also leak into the matrix.

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Because various dating techniques have different
closure temperatures allows one to measure
cooling curves
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• After irradiation with neutrons, the 39Ar in a
  rock is proportional to the 40K.
• 39Ar and 40Ar diffuse at the same rate, while
  40K and 39Ar do not.
• Thus, if the rock is leaky, 39Ar-40Ar dates
  depend on temperature.