2. The Carbon Cycle

3. Carbon Dating- Application of Half Life! All living things contain carbon, some of it is Carbon-14 Carbon-14 is radioactive (unstable) –It decays into Nitrogen by beta decay –C-14 half-life is 5730 years

4. Where does the C-14 come from? Cosmic Rays from space are always hitting the upper atmosphere of the earth, which is mostly Nitrogen The rays produce high energy neutrons which can blast into N-14 atoms (atomic # 7) The neutrons can knock out and replace a Nitrogen proton: p C-14N-14 n

5. Trace the path through organisms: Plants take in CO 2 from the atmosphere - including a little bit of C-14 that’s mixed in Plants build the Carbon into starch (food) -including that bit of C-14 -You know what’s next: Plants  Herbivores  Carnivores –Almost all living things ultimately get their carbon from the air!

6. Steady State Continuous cycle ensures that all living organisms have a steady ratio of C-14 to C-12 in their bodies –same ratio as in the air –until they die! Stop living, and C-14 is no longer replenished –Beta decay continually decreases C-14 no new C-14 coming in & C-12 is stable

7. Technology & Carbon Dating If we compare C-14:C-12 ratio in a fossil with ratio in a living sample  estimate how many half-lives passed. –Carbon Dating The half life of C-14 is: –5730 years After about 9 half lives C-14 gets extremely difficult to detect, even with high tech. equipment –That’s 51,570 years old So, C-14 is limited to dating formerly living artifacts that are less than 50,000 years old.

8. So how might use this fact! Bones of dead lizard have 1/16 th amount of Carbon-14 that living reptiles do now. How old is “Lizzie”??? Answer: 1/16 = (½) x = ½ x ½ x ½ x ½ = (½) 4 4 half- lives have passed since she died so bones are 5730yrs x 4 = 22,920 years old!

9. Can we date older things? Scientists can choose radioactive elements that have longer half lives than C-14 Age of some rocks can be determined if other elements/isotopes are used –But we have to know how they got there in order for this to work