1. The Fossil Record

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

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

4. 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.

5. 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.

7. 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”

8. 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

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