- Radiometric or Radiocarbon dating
- Relative Dating
- Radiometric Dating
- Radiometric dating relies on measuring the amount of carbon present in a living organ-
ism. The carbon atom exists as 3 different isotopes (atoms with the same proton num-
ber but different molecular weight numbers). These are Carbon-12 (C-12), Carbon-13
(C-13) and Carbon-14 (C-14). All living organisms maintain an equilibrium of Carbon-
14 with the atmospheric C-14. As organisms die,the Carbon-14 present within them
begins a process known as radioactive decay. - Thehalf-lifeis the time taken for half the amount of an isotope to decay. The half-life
of C-14 is 5730 years. This means that if we start out with say 10 units of C-14, in
5730 years’ time only 5 units will remain. In another 5730 years’ time only 2.5 units
will remain and so on. - Thus, using radiometric techniques, scientists can determine how much C-14 is present
within a particular fossil and by so doing infer the age of the fossil.
Figure 11.4: Radiometric dating: Graph showing the half-life of C14. The amount of Carbon-14
halves every 5730 years.
- Relative Dating
- Relative dating is the method of determining the order of events from the fossil record.
- By studying the order in which fossils occur in the fossil record, geologists can deter-
mine the order of events as they occurred but not when exactly they occurred. - Fossils found at the lowest layer of rock would be the oldest, as these would have been
buried for the longest time, whereas fossils found closer to the surface would be buried
more recently and therefore be younger. - The geological time-scale you studied earlier was almost entirely developed by relative
dating methods. It is a useful method of dating when fossil materials lack radioactive
isotopes.
Chapter 11. History of Life on Earth 321