Sunday, 27 January 2008

Radiometric dating techniques

A major problem for archaeologists and palaeontologists is the reliable determination of the ages of artefacts and fossils.

As far back as the 17th Century the Danish geologist Nicolas Steno proposed the Law of Superimposition for sedimentary rocks, noting that sedimentary layers are deposited in a time sequence, with the oldest at the bottom. Over a hundred years later, the British geologist William Smith noticed that sedimentary rock strata contain fossilised flora and fauna, and that these fossils succeed each other from top to bottom in a consistent order that can be identified over long distances. Thus strata can be identified and dated by their fossil content. This is known as the Principle of Faunal succession. Archaeologists apply a similar principal, artefacts and remains that are buried deeper are usually older.

Such techniques can provide reliably relative dating along the lines of “x is older than y”, but to provide reliable absolute values for the ages of x and y is harder. Before the introduction of radiometric dating in the 1950s dating was a rather haphazard affair involving assumptions about the diffusion of ideas and artefacts from centres of civilization where written records were kept and reasonably accurate dates were known. For example, it was assumed – quite incorrectly as it later turned out - that Stonehenge was more recent than the great civilization of Mycenaean Greece.

The idea behind radiometric dating is fairly straightforward. The atoms of which ordinary matter is composed each comprise a positively charged nucleus surrounded by a cloud of negatively charged electrons. The nucleus itself is made up of a mixture of positively charged protons and neutral neutrons. The atomic weight is total number of protons plus neutrons in the nucleus and the atomic number is the number of protons only. The atom as a whole has the same number of electrons as it does protons, and is thus electrically neutral. It is the number of electrons (and hence the atomic number) that dictate the chemical properties of an atom and all atoms of a particular chemical element have the same atomic number, thus for example all carbon atom have an atomic number of six. However the atomic weight is not fixed for atoms of a particular element, i.e. the number of neutrons they have can vary. For example carbon can have 6, 7 or 8 neutrons and carbon atoms with atomic weights of 12, 13 and 14 can exist. Such “varieties” are known as isotopes.

The physical and chemical properties of various isotopes of a given element vary only very slightly but the nuclear properties can vary dramatically. For example naturally-occurring uranium is comprised largely of U-238 with only a very small proportion of U-235. It is only the latter type that can be used as a nuclear fuel – or to make bombs. Many elements have some unstable or radioactive isotopes. Atoms of an unstable isotope will over time decay into “daughter products” by internal nuclear change, usually involving the emission of charged particles. For a given radioisotope, this decay takes place at a consistent rate which means that the time taken for half the atoms in a sample to decay – the so called half-life – is fixed for that radioisotope. If an initial sample is 100 grams, then after one half-life there will only be 50 grams left, after two half-lives have elapsed only 25 grams will remain, and so on.

It is upon this principle that radiometric dating is based. Suppose a particular mineral contains an element x which has a number of isotopes, one of which is radioactive and decays to element y with a half-life of t. The mineral when formed does not contain any element y, but as time goes by more and more y will be formed by decay of the radioisotope of x. Analysis of a sample of the mineral for the amount of y contained will enable its age to be determined provided the half-life t and isotopic abundance of the radioisotope is known.

The best-known form of radiometric dating is that involving radiocarbon, or C-14. Carbon – as noted above – has three isotopes. C-12 (the most common form) and C-13 are stable, but C-14 is radioactive, with a half-life of 5730 years, decaying to N-14 (an isotope of nitrogen) and releasing an electron in the process (a process known as beta decay). This is an infinitesimal length of time in comparison to the age of the Earth and one might have expected all the C-14 to have long since decayed. In fact the terrestrial supply is constantly being replenished from the action of interstellar cosmic rays upon the upper atmosphere where moderately energetic neutrons interact with atmospheric nitrogen to produce C-14 and hydrogen. Consequently all atmospheric carbon dioxide (CO2) contains a very small but measurable percentage of C-14 atoms.

The significance of this is that all living organisms absorb this carbon either directly (as plants photosynthesising) or indirectly (as animals feeding on the plants). The percentage of C-14 out of all the carbon atoms in a living organism will be the same as that in the Earth’s atmosphere. The C-14 atoms it contains are decaying all the time, but these are replenished for as long as the organism lives and continues to absorb carbon. But when it dies it stops absorbing carbon, the replenishment ceases and the percentage of C-14 it contains begins to fall. By determining the percentage of C-14 in human or animal remains or indeed anything containing once-living material, such as wood, and comparing this to the atmospheric percentage, the time since death occurred can be established.

This technique was developed by Willard Libby in 1949 and revolutionised archaeology, earning Libby the Nobel Prize for Chemistry in 1960. The technique does however have its limitations. Firstly it can only be used for human, animal or plant remains – the ages of tools and other artefacts can only be inferred from datable remains, if any, in the same context. The second is that it only has a limited “range”. Beyond 60,000 years (10 half-lives) the percentage of C-14 remaining is too small to be measured, so the technique cannot be used much further back than the late Middle Palaeolithic. Another problem is the cosmic ray flux that produces C-14 in the upper atmosphere is not constant as was once believed. Variations have to be compensated for by calibration curves, based on samples that have an age that can be attested by independent means such as dendochronology (counting tree-rings). Finally great care must be taken to avoid any contamination of the sample in question with later material as this will introduce errors.

The conventions for quoting dates obtained by radiocarbon dating are a source of considerable confusion. They are generally quoted as Before Present (BP) but “present” in this case is taken to be 1950. Calibrated dates can be quoted, but quite often a quoted date will be left uncalibrated. Uncalibrated dates are given in “radiocarbon years” BP. Calibrated dates are usually suffixed (cal), but “present” is still taken to be 1950. To add to the confusion, Libby’s original value for the half-life of C-14 was later found to be out by 162 years. Libby’s value of 5568 years, now known as the “Libby half-life”, is rather lower than the currently-accepted value of 5730 years, which is known as the Cambridge half-life. Laboratories, however, continue to use the Libby half-life! In fact this does make sense because by quoting all raw uncalibrated data to a consistent standard means any uncalibrated radiocarbon date in the literature can be converted to a calibrated date by applying the same set of calculations. Furthermore the quoted dates are “futureproofed” against any further revision of the C-14 half-life or refinement of the calibration curves.

If one needs to go back further than 60,000 years other techniques must be used. One is Potassium-Argon dating, which relies on the decay of radioactive potassium (K-40) to Ar-40. Due to the long half-life of K-40, the technique is only useful for dating minerals and rocks that are over than 100,000 years old. It has been used to bracket the age of archaeological deposits at Olduvai Gorge and other east African sites with a history of volcanic activity by dating lava flows above and below the deposits.

© Christopher Seddon 2008

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