Fossil fuel emissions interfere with radiocarbon dating

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A study published in PNAS journal last month found that fossil AlfedPalmersmokestacksfuel emissions will begin to interfere with the accuracy of radiocarbon dating.

Radiocarbon dating, developed by Willard Libby in the late 1940s, is a method used to determine the age of an object containing organic material using properties of radioactive carbon (14C). This method is employed in a range of disciplines including geography, biology, physics, anthropology and archaeology. Its applications range from estimating the age of fossils to reconstructing paleoclimate to dating historic objects. The method was famously used to estimate the age of the Shroud of Turin – thought by some to be the burial shroud of Jesus of Nazareth – which dates back to the Middle Ages.

Carbon-14 (14C) is a radioactive isotope of carbon naturally produced in the upper layers of the atmosphere. Plants take up amounts of 14C along with non-radioactive (or stable) carbon from the atmosphere through photosynthesis and introduce it into the food chain. Therefore, 14C is found in all organic matter. The ratio of radioactive carbon to stable carbon in living organisms is in equilibrium with the atmospheric ratio of radioactive to stable carbon.

After an organism dies, this ratio shifts out of equilibrium as the radioactive carbon in its tissues begins to decay. Knowing the half-life of 14C (decay rate), and based on how much 14C has decayed, the time passed since the organism’s death can be quantified. The radiocarbon dating method is generally limited to samples no more than 50,000 years old, as older samples have insufficient quantities of 14C left. This date can be pushed up to 60,000 years, and in some cases 75,000 years, by using special sample preparation techniques, large samples and very long measurement times.

The burning of fossil fuels causes a rapid increase in concentrations of stable carbon isotopes (12C and 13C) in the atmosphere. Fossil fuels were formed 300-400 million years ago and therefore contain no 14C. The huge increase in concentrations of 12C and 13C is perturbing the current radioactive:stable carbon ratio of the atmosphere. As a result of this, the radioactive:stable carbon ratio formed in organisms at present is equal to that of organisms in the past due to a lesser quantity of 14C. In other words, fossil-fuel carbon is artificially ‘ageing’ objects of the present.

By 2050, a new t-shirt will have the same radiocarbon age as a 1,000-year-old piece of clothing.

If fossil fuel emissions continue at the present rate then, by 2050, a new t-shirt will have the same radiocarbon age as a 1,000-year-old piece of clothing. Fossil fuel emissions will start impacting radiocarbon dates as early as 2020. Current radiocarbon dating methods are calibrated to adjust for errors, but an interference of this type could make the technique too unreliable. This would greatly affect research in the disciplines that depend on 14C dating for data collection.

The implications of this finding are strong, yet not completely realised. However, the effect can be mitigated: if emissions are rapidly curbed, a t-shirt bought in 2050 could have the same age as an object that is one century old, instead of one millennium.

“We can see from atmospheric observations that radiocarbon levels are steadily decreasing. How low they go depends on changes in our fossil fuel emissions,” said Dr Heather Graven, from Imperial College London.

Photograph: Alfred Palmer (Wikimedia Commons)

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