Absolute dating

• Isotopes


  Atoms of an element that have different numbers of neutrons (but same number of protons) and thus have different atomic masses
• Isotopes


  • Carbon-12
  • Carbon-13
  • Carbon-14
• Carbon-14 is radioactive but Carbon-12 and Carbon-13 are not
• Absolute Dating


  Dating method that gives actual age of rocks and fossils
• Relative dating identifies younger or older fossils, not actual age of fossils
• Radioactive Dating


  Absolute dating method that uses the natural decay (breakdown) of radioactive isotopes (radioisotopes)
• Radioactive dating


  1. Comparing ratio of radioisotopes found inside fossil to mostly stable amount found in atmosphere

  2. Breakdown is constant and can be calculated using half-life of radioisotope
• Half-life


  Time it takes for half of a radioactive sample to decay
• Half-life


  • Carbon-14 half-life is 5730 years
• Decay curve


  Graph used to determine age of fossil based on radioactive decay
• Every time the amount of radioactive C-14 is halved (1 half-life)


  The fossil would have aged by 5730 years
• Ratio of 0.5 Carbon-14 to Nitrogen-14

  1 half-life = 5730 years
• Ratio of 0.25 Carbon-14 to Nitrogen-14

  2 half-lives = 5730 x 2 = 11,460 years
• Calculating Fossil Age using Radioactive Decay Curve for Potassium-40


  1. Potassium-40 (40K) decays to Argon-40 (40Ar)

  2. Half-life = 1251 million years
• If a fossil has 1/2 the amount of 40K left
• If a fossil has 1/8 the amount of 40K left
• If a fossil has 1/4 the amount of 40K left