Radio Carbon Dating
The most important archaeological dating method is radiocarbon dating. It is a technique that can yield absolute dates with accuracy up to approximately 5000 years before present. However its application has caused extreme confusion and misunderstanding of the archaeological record. Knowing the limitations of this dating method can help avoid colossal archaeological misinterpretations that would otherwise distort history.
Carbon EverywhereRadiocarbon or C14 dating employs complex systems of measuring the unstable isotopes in once living matter. There are three forms of carbon that naturally occur forming the building blocks of all plant and animal life. The stable C12 and C13, and the unstable or radioactive Carbon 14. C14 is found in very low quantities in nature. Only one C14 atom exists for every one trillion C12 atoms.
Creating C14 AtomsNitrogen atoms in the upper atmosphere are struck by cosmic radiation and create C14 atoms. These atoms rapidly decay into radiocarbon-dioxide and along with ordinary CO2 are absorbed by living plants. As plants enter the human and animal food chains the C14 dioxide enters their living tissue.
The Dying C14 AtomAs with any radioactive particle it decays over time. Experiments, first conducted by Dr Willard F. Libby in 1948 at the University of Chicago, showed that C14, tested in his laboratory, decayed at the rate that, projected out, would cause half of its weight to be lost in 5568 years. Hence, the term ‘half-life’ was given to radioactive substances.
The radiocarbon method measures the rate of decay in the C14 of organic matter therefore estimating how long ago death occurred. Archaeologists can use this method to date bone, teeth, plants, seeds, burned food remains, coprolites, wood, and any artefact that contains organic materials such as an iron axe head (iron cannot be tested using C14) with a wooden handle or a bronze spear with a wooden shaft.
Problems with C14 DatingFor radiocarbon dating to be reliable scientists need to make a number of vital assumptions. Firstly, Dr Libby assumed that C14 decays at a constant rate. However, experimental evidence indicates that C14 decay is slowing down and that millennia ago it decayed much faster than is observed today.
Secondly, the theory behind C14 dating demands that there is the same rate of cosmic production of radioactive isotopes throughout time. The industrial revolution has belched hundreds of thousands of tons of carbon gases into the atmosphere increasing the C12 ratio and atomic weapons testing have increased neutron levels.
Thirdly, the environment in which the artefact lies heavily impacts on the rate of decay. For example, C14 leaches at an accelerated rate from organic material saturated in water, especially saline water.
Fourthly, for C14 to test accurately the artefact must have been protected from contamination. Organic matter, being porous, can easily be contaminated by organic carbon in groundwater. This increases the C12 content and interferes with the carbon ratio.
Archaeologists are ConcernedThe unreliability of carbon 14 date testing is a great concern to honest archaeologists. They get particularly concerned when C14 testing shows obviously inaccurate results and they are left in uncertainty about the reliability of the dates that they have previously never questioned.
New or Old?Some examples of abnormal C14 results include testing of recently harvested, live mollusc shells from the Hawaiian coast that showed that they had died 2000 years ago and snail shells just killed in Nevada, USA, dated in at 27,000 years old. A freshly killed seal at McMurdo Sound, Antarctica, yielded a death age of 1300 years ago.
A petrified miner’s hat and wooden fence posts were unearthed from an abandoned 19th century gold hunter’s town in Australia’s outback. Results from radiocarbon dating said that they were 6000 years old.