The Fossil Record

How old is that fossil? There are two major ways of dating objects or fossils: Relative Dating Absolute Dating

How old is that fossil? Relative Dating Uses the relative position of fossils for age comparison

Relative Dating Example: Cannot be used to determine the actual age of a fossil Can only be used to compare fossil age Example: When archaeologists measure the fluorine content in bones. Fluorine is from underground water. Fluorine eventually replaces other things that are in bones, so the more fluorine the bones contain the older they are.

Radiometric Dating A technique used to date materials A comparison between the amount of a naturally occurring radioactive isotope and its decay products Each radioactive isotope has a unique, known half-life (time for ½ of the substance to decay) Example: Radioactive Carbon-14 has a half-life of 5730 years The choice of dating material is based on estimated age of the artifact or fossil to be dated.

Radiometric Dating Radioactive samples decay at a predictable rate Estimates of the original composition of a radioactive sample are made Then the amounts of the radioisotopes present can provide a measurement of time elapsed Example: Carbon dating, limited to organic materials Longer lived radioisotopes in minerals are used for longer time scales “clocks in the rocks”

Table 1. Some Naturally Occurring Radioactive Isotopes and their half-lives (Parent) Product (Daughter) Half-Life (Years) Samarium-147 Neodymium-143 106 billion Rubidium-87 Strontium-87 48.8 billion Rhenium-187 Osmium-187 42 billion Lutetium-176 Hafnium-176 38 billion Thorium-232 Lead-208 14 billion Uranium-238 Lead-206 4.5 billion Potassium-40 Argon-40 1.26 billion Uranium-235 Lead-207 0.7 billion Beryllium-10 Boron-10 1.52 million Chlorine-36 Argon-36 300,000 Carbon-14 Nitrogen-14 5715 Uranium-234 Thorium-230 248,000 Radium-226 75,400

Radioactive Decay The amount of decay in one half-life is used to measure the time