Absolute Dating

1. Absolute Dating

2. Earth Forms First Life Abundant Oxygen Dinos 15-25 Humans 11:20 pm Multicellular Organisms Earth Cools Plants and Animals J F M A M J J A S O N D J

3. 1 second 1/ 31,556,925.974 of the year 1900 9,192,631,770 flips of the magnetic field of a cesium atom

4. What is Absolute/Numerical Dating? • The age of an earth material or event in years Absolute Relative age 3rd 12 kya 2nd 95 mya 1st 1.7 bya 8 kya youngest

5. Estimating Earth's Age -Early (failed) Attempt Bishop Usher – Biblical Interpretation 4004 BC – before the birth of Christ October 21 9:00 in the morning

6. Estimating Earth's Age -Early (failed) Attempt Sedimentation rates - 3 my – 500 my Halley/Joly - Ocean Salinity – 100 my Lord Kelvin – Rate of Cooling – 30 my

7. Radioactive Revolutionaround 1900 • Radioactive decay - spontaneous transformation of an element to another isotope of the same element or another element.

8. Pieces of an Element • Protons - positively charged • Neutrons - no charge • Electrons - negatively charged P e N P e

9. Isotope - element with different number of neutrons in the nucleus. Hydrogen - stable Tritium - unstable P e e P N N

10. Radioactive Decay Tritium Helium 3 (parent) (daughter) P P e e nuclear N N P N decay e unstable stable

11. Radioactive Decay Alpha Decay – loss of a positively charged He ion Beta Decay – neutron splits into proton and electron

12. Alpha Decay 234 90 4 2 238 92 He ( 2protons + 2 Neutrons) Th + U  Beta Decay 234 90 234 91 Neutron splits into a proton and an electron Th  Pa + e-

14. Half-life The fixed period of time during which half the parent atoms present in a closed system decay to form daughter atoms

15. 1 2 3 4 5 Half-Life

16. Half-life Potassium 40 / Argon 40 Half-lifes 0 12 grams / 0 grams 1 6 / 6 2 3 / 9 3 1.5 / 10.5

17. Half-Life Exercise 10,000 20,000 30,000 40,000 1st 512 / 512 2nd 256 / 768 3rd 128 / 896 4th 64 / 960 Sample: 1024 grams Isotope A Isotope B (10 k) half-life Daughter Parent 960 grams 64 grams

18. Parent/Daughter Half Life (yrs) Range (yrs) 10 million to 4.5 billion 238U/206Pb 4.5 billion 14C 5730 100 to 70,000 3H/3He 12.3 1 to 50

19. Parent/Daughter Material Dated Igneous and Metamorphic rocks 238U/206Pb 14C Organic material 3H/3He Water

20. Uranium (U) - Thorium (Th) - Lead (Pb) 238U decays to 206Pb (4.5 billion) 235U decays to 207Pb (713 million) 232Th decays to 208Pb   (14.1 billion) >Rocks containing Uranium provide three possible techniques. >Because all three occur together, it allows a method to cross-check the dates. >Uses zircons, uraninite and uranium ores

21. Potassium (K) - Argon (Ar) Dating >Potassium (K) is an extremely common element. >Half-life (t1/2) is 1.3 billion years. >Range is 100,000 to 4.6 billion years. >Useful for relatively young and very old rocks. >Found in muscovite, biotite, orthoclase and glauconite. >Used to date volcanic rocks.

22. Carbon 14 / Carbon 12 >Cosmic rays hit Nitrogen 14 changing it to Carbon 14. >Carbon 14 is taken in by organisms. >When organism dies, amount of C-14 decreases.

23. Magnetic Polarity Dating

26. Dating techniques • that can be used • on this geologic • column: • absolute dating • paleomagnetism • superposition • fossil correlation

27. What is the Parent / Daughter ratio of a sample of radiogenic material if the original sample was 200g. and 3 half-lifes have passed. If the half-life is 20,000 years, how old is the sample? What is the half-life of an isotopic pair if the sample is 2 million years old and contains 625g of parent and 9375g of daughter isotope?