Absolute Dating

1. Absolute Dating Shmulik Marco and Neta Wechsler

2. Relative dating

3. Relative dating

4. Biochronology (relative dating, limited to fossil record)

5. The basic principle Every parent atom of a particular type has the same probability of decaying to a specified daughter atom. Decay only depends on structure of atomic nucleus - NOT on external conditions. If N = number of parent atoms at time "t" and N0 is number of parent atoms present at start where l is the DECAY CONSTANT which is constant for a particular parent-daughter system.

6. Rb-> beta decay->Sr Sr has several isotopes: 84, 85, 86, 87, 88 Only 87Sr has a radiogenic source – 87Rb

8. 87Rb87Sr

10. How much 87Sr was at the beginning and how much came from 87Rb? For this method, t1/2 = 47 Byr

11. Rb-Sr Method An example from some of the oldest rocks on Earth. A suite of different samples from a group of granites and gneisses were analyzed. These had different concentrations of parent rubidium 87 and so generated different amounts of the daughter (strontium 87). The resulting data plot on a straight line, called an isochron. This means that all samples have seen the same amount of time. If different samples had formed at different times (a random collection of rocks on earth) then the data would not fall on a straight line (no single isochron).

12. Dating meteorites with the 87Rb 87Sr method – the age of the Solar system

13. K-> beta decay->Ar 40K – 89% -> 40Ca, 11% -> 40Ar For this method, t1/2 = 1.93Byr

14. K-> beta decay->Ar Parent against daughter concentrations. In this hypothetical example there are no daughter atoms present in the original rock but a certain amount of parent atoms. Consequently a line passing through this point to the origin has zero slope. After time a number of parent atoms have decayed and the number of daughters increase. Eventually there will be no parents and only daughters - the composition will plot on the daughter axis. Note therefore the gradient of line connecting any composition with the origin increases with time. Or - the gradient of the line is a function of time and also a function of the decay constant.

16. Carbon dating Carbon has 3 isotopes: 12C – stable 13C – stable 12C:13C = 98.89 : 1.11 14C – radioactive Abundance:

17. Radiocarbon Forms: in the upper atmosphere Decays: t½ = 5730 yr.

18. Calculated Measured ??? Constant Half life time

19. Dating Dating methods: * Conventional way * Accelerator mass spectrometry (AMS) Bowman S., 1990

20. Assumptions * Constant production rate * Constant and rapid mixing * Constant exchange rate * Constant reservoirs’ size * Same concentration in all parts of the biosphere * Decay determines the radiocarbon concentration

21. Problems concerning the radiocarbon dating: - Atmospheric 14C variations - Alteration effects - Contamination - Source or reservoir effects

22. Atmospheric 14C variations * Natural variations- Secular changes- Sun spots * Men caused variations: - Fossil fuel effect - Bomb effect Bowman S., 1990

23. Calibration curve Hans Suess calibration curve – based on dendrochronology Bowman S., 1990

24. Lake Kinneret reservoir time by Stiller M. et al., 2001 Source or reservoir effects Marine effect Hard water effect Volcanic effect