1. Part 4: Radiometric Dating
Unit 4: Geophysics
Part 4: Radiometric Dating

2. Dating with Radioactivity
Radioactivity: The breakdown of an unstable element into another
“Parent” into a “daughter” product
Some atoms of elements exist as (radioactive) isotopes
Example: Carbon-14, a radioactive isotope of regular carbon (carbon-12)
Carbon-14 decays into nitrogen

3. Dating with Radioactivity
Elements decay at a constant rate
Decay rate is not effected by any known force
Radioactive elements create a very precise and unchanging clock
Decay is used figure the absolute age of earth material

4. Radioactivity and radiometric dating
Half-life – the time for one-half of the radioactive nuclei to decay
Requires a closed system
Cross-checks are used for accuracy
Complex procedure
Yields numerical dates

5. The radioactive decay curve
Figure 11.17

6. Useful Isotopes for Radiometric Dating
Radioactivity offers clocks ranging from milliseconds to billions of years;
These methods support and check others
– Rubidium-87, half life 47 billion years
– Fermium-244, 3.3 milliseconds
– Carbon-14, half life of 5730 years, just right for dating in archaeology
– Potassium-40, half-life of 1.26 billion years,
just right for dating earth history, found in volcanic rocks and lava flows

7. Potassium-Argon Dating (K-Ar)
Radioactive isotope of potassium is commonly found in earth rocks
Potassium decays into argon, an inert gas, with a half-life of 1.3 billion years; Amount of trapped argon used for absolute dating (rocks over 100,000 years)
Liquid magma allows argon to escape; in crystalline rocks (igneous), argon remains
trapped as it forms from the decay of potassium

8. Carbon-14 and radiometric dating
Carbon-14 dating
Half-life of only 5,730 years
Used to date very recent events
Carbon-14 produced in upper atmosphere
Incorporated into carbon dioxide
Absorbed by living matter
Useful tool for anthropologists, archaeologists, historians, and geologists who study very recent Earth history

10. Carbon-14 Dating Used to date once-living organic materials
All living things absorb carbon-14, a radioactive isotope of carbon; it is produced in earth’s atmosphere by cosmic rays
While living, carbon-14 is continuously replaced as it decays
Living matter possesses a constant ratio of carbon-14 to carbon-12 (regular carbon)

11. Generation of C-14
In the atmosphere, Nitrogen-14 is changed to Carbon-14
Carbon-14 is then taken in by living things
When dead, carbon-14 changes slowly back into nitrogen-14
Ratio of C-14/C-12 determines age

13. First Humans in America?
As an example, radiocarbon dating was used to measure the age of human coprolites (ancient poop) found in an Oregon cave
14,300 years old, this is the oldest known evidence for humans in North America
Extracted DNA shows them to be the likely
ancestors of today’s Native Americans