The isotope called carbon-14 has a half-life of 5730 years. All living things on our planet contain this form of carbon, but they stop taking it in when they die. Scientists can examine ancient substances to see how much the carbon in it has decayed. They can then give a fairly accurate date for when the substance was alive. This is particularly useful for archaeologists and historians, who can date objects they find, helping to build up a picture of the past.
Radiocarbon dating is a method of what is known as “Absolute Dating”. Despite the name, it does not give an absolute date of organic material – but an approximate age, usually within a range of a few years either way. The other method is “Relative Dating” which gives an order of events without giving an exact age: typically artifact typology or the study of the sequence of the evolution of fossils.
There are three carbon isotopes that occur as part of the Earth’s natural processes; these are carbon-12, carbon-13 and carbon-14. The unstable nature of carbon 14 (with a precise half-life that makes it easy to measure) means it is ideal as an absolute dating method. The other two isotopes in comparison are more common than carbon-14 in the atmosphere but increase with the burning of fossil fuels making them less reliable for study; carbon-14 also increases, but its relative rarity means its increase is negligible. The half-life of the 14C isotope is 5,730 years, adjusted from 5,568 years originally calculated in the 1940s; the upper limit of dating is in the region of 55-60,000 years, after which the amount of 14C is negligible. After this point, other Absolute Dating methods may be used.
Today, the amount of carbon dioxide humans are pumping into Earth’s atmosphere is threatening to skew the accuracy of this technique for future archaeologists looking at our own time. That’s because fossil fuels can shift the radiocarbon age of new organic materials today, making them hard to distinguish from ancient ones. Thankfully, research published yesterday in the journal Environmental Research Letters offers a way to save Libby’s work and revitalize this crucial dating technique: simply look at another isotope of carbon.
Carbon-12 is a stable isotope, meaning its amount in any material remains the same year-after-year, century-after-century. Libby’s groundbreaking radiocarbon dating technique instead looked at a much rarer isotope of carbon: Carbon-14. Unlike Carbon-12, this isotope of carbon is unstable, and its atoms decay into an isotope of nitrogen over a period of thousands of years. New Carbon-14 is produced at a steady rate in Earth’s upper atmosphere, however, as the Sun’s rays strike nitrogen atoms.
Radiocarbon dating exploits this contrast between a stable and unstable carbon isotope. During its lifetime, a plant is constantly taking in carbon from the atmosphere through photosynthesis. Animals, in turn, consume this carbon when they eat plants, and the carbon spreads through the food cycle. This carbon comprises a steady ratio of Carbon-12 and Carbon-14.
When these plants and animals die, they cease taking in carbon. From that point forward, the amount of Carbon-14 in materials left over from the plant or animal will decrease over time, while the amount of Carbon-12 will remain unchanged. To radiocarbon date an organic material, a scientist can measure the ratio of remaining Carbon-14 to the unchanged Carbon-12 to see how long it has been since the material’s source died. Advancing technology has allowed radiocarbon dating to become accurate to within just a few decades in many cases.
Carbon dating is a brilliant way for archaeologists to take advantage of the natural ways that atoms decay. Unfortunately, humans are on the verge of messing things up. The slow, steady process of Carbon-14 creation in the upper atmosphere has been dwarfed in the past centuries by humans spewing carbon from fossil fuels into the air. Since fossil fuels are millions of years old, they no longer contain any measurable amount of Carbon-14. Thus, as millions of tons of Carbon-12 are pushed into the atmosphere, the steady ratio of these two isotopes is being disrupted. In a study published last year, Imperial College London physicist Heather Graven pointed out how these extra carbon emissions will skew radiocarbon dating.
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