Absolute Dating Techniques

Steve Jones briefly, when discussing the geological record, writes about why “the answer to time lies not with deposition, but with decay (195).” This is evident by studying chemical breakdowns which can be used to determine how old matter is on earth. One such technique is known as Carbon-14 Dating.

In 1949, Williard F. Libby, a physical chemist discovered radiocarbon dating. He was able to explain that in the atmosphere, nitrogen particles change into a new form of carbon when atomic nuclei are hit by cosmic rays, carbon-14 rather than the usual carbon-12 due to the amount of 14 particles. Because it is naturally unstable, it begins to break down at once, and continues to decay for thousands of years. Every living organism is made up of this carbon which is constantly replaced while the creature is alive. However, upon death, no new carbon is added and begins to decay at a constant rate. Scientists were able to determine that there exists a half-life in the rate of carbon-14 decay. Every 5,730 (plus or minus a few) years the carbon material is cut in half. By knowing this, researchers are able to calculate how old an organic material is based on how much carbon is left.

The value of radiocarbon dating is in the vast amount of objects it can date, anything from buildings made of wood, seashell jewelry, leather clothes, hearth deposits, and coprolite remains. Unfortunately, contamination is an easy and common threat by the way of modern particles easily transferred onto older ones. Scientists also need large amounts of the remains in order to get a sizable amount of carbon for testing, as carbon-14 only accounts for 0.0000000001% of the carbon isotopes in the body, which may be difficult to come by.

Another method for dating is the analysis of other isotope compositions. Argon/Potassium (Ar/K) dating, for example, is similar to radiocarbon dating described above with argon as the parent material. A sample is instead studied to find how much potassium is present. Because its half-life is much longer, this technique can be used to date sites millions of years old and early hominid remains.

Of importance to the topic of this course is that “time can be measured only by change (194).” The alteration from nitrogen to carbon, and potassium to argon, gives us a constant time frame for how old the earth really is. By analyzing these changes can we discern how much time has elapsed allowing for the feasibility of natural selection.

Additional sources:
Balme, Jane and Paterson, Alistair, ed., Archaeology in Practice: A Student Guide to Archaeological Analyses, Blackwell Publishing, 2006

Thomas, David Hurst; Archaeology: Down to Earth, 2nd ed.; Harcourt Brace College Publishers, 1999