1. CARBON DATING Determining the actual age of fossils

2. Radiometric dating Radioactive isotopes decay at a predictable rate The amount of time it takes for half of a sample of the isotope to decay is called its half-life The half life is a function of the nuclear stability of the atoms A more stable atom will have a longer half-life, while one that is particularly unstable will have a shorter half-life

3. Half-lives of some important isotopes Parent Isotope Stable Daughter Product Currently Accepted Half- Life Values Uranium-238Lead-2064.5 billion years Uranium-235Lead-207704 million years Thorium-232Lead-20814.0 billion years Rubidium-87Strontium-8748.8 billion years Potassium-40Argon-401.25 billion years Samarium-147Neodymium-143106 billion years

4. How to determine age As time passes, the number of atoms of the radioactive isotope will diminish, while the number of atoms of the stable product of the decay will increase The proportion of the two is a mathematical function of the age In simple terms, a rock that contains both Uranium-238 and Lead-206 probably once had a higher content of U238, which then decayed into the Pb206 The more Pb206 in proportion to U238, the older the rock

5. The actual formula for calculating age

6. I was told that there would be no math The equation from the previous slide isn’t really all that difficult, but it’s difficult enough. There are also difficulties to be overcome that have nothing to do with the math. For example, the equation is useless if you don’t have rocks that still contain both the original isotope and the stable product of decay Maybe your rock never had Uranium-238 Maybe the stable product of decay was a gas that escaped (as it does in the decay of Carbon-14)

7. Choosing an Isotope for Dating No, not that kind of dating You need an isotope that is actually part of the rock or the fossil you are trying to date You need an isotope that decays at a rate that is appropriate for the age of rock that you have Older rocks can only be dated using isotopes with long half-lives, where younger fossils need to be dated with shorter half-life isotopes. U238 has a half-live of 4.5 billion years. It can’t give you an accurate age of a fossil a few thousand years old

8. Why Carbon? Carbon-14 is a radioactive isotope of carbon. It comprises about 1.1% of all carbon atoms on earth Carbon-14 has a half-life of 5770 years Carbon-14 decays into Nitrogen-14 (which will escape) All living things contain carbon, so rocks containing fossils will contain carbon (of which 1.1% will be C-14) Through the process of feeding and metabolism, organisms will reach equilibrium with their environment. In other words, living organisms will maintain a content of about 1.1% carbon-14 Once they die, they will no longer maintain that equilibrium, and C-14 will start turning into N-14

9. Carbon Dating So, if an organism was 1.1% carbon-14 at the moment of death, and the half-life of carbon-14 is 5770 years, a fossil 5770 years old would be.55% carbon 14 After another 5770 years the C-14 content would be half of that, and so on... http://www.youtube.com/watch?v=31-P9pcPStg Using the data table on the next slide, construct a graph of Carbon-14 content vs. Time

10. Radioactive Decay rate of Carbon 14 half livesTime (years)% Carbon 14 001.1 157700.55 2115000.275 3173000.1375 4231000.0688 5289000.0344 6346000.0172 7404000.0086 8462000.0043 9519000.0021 10577000.0011 11635000.0005 12692000.0003 13750000.0001

11. Video http://www.youtube.com/watch?v=2io5opwhQMQ http://www.youtube.com/watch?v=ErgdpG_N9vQ http://www.youtube.com/watch?v=udkQwW6aLik http://www.youtube.com/watch?v=w5369-OobM4

12. Resources: http://pubs.usgs.gov/gip/geotime/radiometric.html