Much faith is now placed on carbon dating. On what is this based and how do scientists know that the age of the item is correct?

Carbon-dating is not an act of faith but a scientific technique developed in the 1940s for dating fossils and other organic specimens in the 500 to 50,000 year age bracket. Once an organism dies it ceases to absorb the radiocarbon isotope (carbon-14), half the amount present, then undergoing spontaneous disintegration during the following 5,730 years, plus or minus 40. The half left thus denotes the age of its demise.

Establishing the much longer periods of geological time relies on the decay of unstable elements such as radium into lighter, stable elements such as helium. The technique was established in the remarkably short space of a decade after Rutherford first explained radioactivity at the beginning of the 20th century. Conservative estimates based on other quantitative techniques such as rock-stratigraphy had put the Earth’s age at a few hundred million years. Overnight, as it were, the estimates leapt to several billion years. There was still much refinement to be done before the geological periods from the pre-Cambrian onwards were established at today’s level of accuracy.

Ray Long, Streatham Hill

Carbon-14 is used in radioisotope (radiocarbon) dating as a way of determining the age of relatively recent geological and archaeological artefacts of a biological origin. It is used in dating items such as woven cloth, handmade wooden tools, fossilised and mummified remains.

Elements such as carbon comprise different isotopes, each with a different atomic mass. Most isotopes are stable, but some are less so and have a tendency to decay to more stable forms. The unstable form of carbon is called carbon-14, while its two more stable, longer-lasting relatives are carbon-12 and carbon-13.

Unstable carbon-14 is “taken up” to become part of the cellular material of organic life forms — by photosynthesis in plants, the process by which plants create carbon-rich sugars and their subsequent consumption by animals.

Since the amount of carbon-14 in the atmosphere is known, and the rate of its so-called radioactive decay is also known, then measuring the remaining carbon-14 in ancient carbonaceous material gives a reasonably accurate age of the sample to be estimated.

Scientists know that the age is correct since they have accurately determined the expected amounts of airborne carbon-14 and they know the rate of breakdown of carbon-14. They can compare the ratio of the stable isotopes to the unstable isotope in the specimen.

The half-life, or the time required to convert one half of carbon-14 to a stable state, is 5,700 years; this makes the technique only reliable for dating carbonaceous objects up to about 60,000 years old — dating mankind’s past activities and life in the Ice Age, but of no use for determining the age of dinosaur bones.

In 1991 Ötzi the Iceman, a well-preserved frozen body of a man, was discovered in the Schnalstal Glacier in the Ötztal Alps, near Hauslabjoch on the border between Austria and Italy. His remains and the various weapons and tools he was carrying were dated at around 3300BC using carbon-14 methods.

Andrew G. Sharp, Aberdeen.

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