Carbon dating disrupted by rising fossil-fuel emissions

Samples are being extracted from human femur bone for carbon dating using Accelerator Mass Spectrometry (AMS).

A small sample is removed from a piece of bone for carbon dating.credit: James King-Holmes/SPL

Researchers tracking ever-increasing levels of atmospheric carbon dioxide have charted a historic moment. As of 2021, the burning of fossil fuels has officially shifted the composition of carbon isotopes in Northern Hemisphere air to rule out a useful signal from nuclear-weapon testing.1,

This could pose problems for valuable carbon-dating techniques. Modern objects now look like objects from the early twentieth century in terms of radiocarbon dating, says Heather Graven, a chemical physicist at Imperial College London who has been charting the effect for years. This trend “can soon make it difficult to tell whether something is 1,000 years old or modern”, says Paula Reimer, a radiocarbon-dating specialist at Queen’s University Belfast, UK. Although there are usually other clues to the origin of an object. There are “frequent discoveries without that information”, Reimer says, such as unidentified human remains that may have come from a historic burial site or from a person who died recently.

Altered carbon: Line chart showing the ratio of radioactive carbon-14 to atmospheric carbon dioxide since 1950.

Source: Reference 1.

The development also means that forensic scientists will no longer be able to use radiocarbon fingerprints to pinpoint the age of materials such as ivory, antiques and wine. “If you’re working in forensics or detecting counterfeits, it’s a really sad moment,” says archaeologist Tom Higham of the University of Vienna.

time machine

Carbon-dating techniques rely on the fact that there are many isotopes of carbon in the air. Stabilized carbon-12 is the most common. But there is also a small amount of radioactive carbon-14, which is mainly produced when cosmic rays interact with the atmosphere. The carbon-14 ratio naturally varies over time.

Living things absorb both types of carbon. After they die, the relative amounts of the two isotopes begin to change as radioactive carbon-14 decays with a half-life of 5,700 years. By measuring how much carbon-14 is left in an object, researchers can date organic materials, such as wood, fabric or bone, that are about 55,000 years old. Typically, the smaller the carbon-14 ratio, the older the material.

Between 1952 and 1962, nuclear weapons testing released a spike of ‘bomb carbon’ that quickly doubled the amount of carbon-14 in the air. Since then, that carbon-14 has been slowly absorbed by living things and the oceans. Also, the burning of fossil fuels releases CO. emissions are increasing rapidly2 which does not contain carbon-14.

By 2021, these two effects have officially canceled each other in the Northern Hemisphere (see ‘Converted carbon’). This means that the carbon-14 ratio in modern materials is now about the same as in pre-industrial times. And because fossil fuels are still being burned, the proportion of carbon-14 in the air will drop even further, mimicking even greater conditions in the past. By 2050, Graven predicts1The carbon-14 ratio would be similar to that of the Middle Ages (between the fifth and fifteenth centuries).

When there are fluctuations and spikes in the carbon-14 ratio in the air over time, radiocarbon dating may not always distinguish one date from another. It’s 800. true for the duration of BC 400. until BCFor example: “You literally can’t date anything [precisely] Within that 400 years,” Higham says. The phenomenon of fossil fuels canceling bomb carbon provides another avenue for the radiocarbon confusion.

goodbye bomb curve

The loss of carbon-14 is negligible only for modern objects that are decades old. But the rapid spike in carbon-14 released by nuclear weapons has created a clinical ‘bomb curve’ of carbon-14 levels. “This is the silver lining of the bomb test,” Higham says. This means that the amount of carbon-14 in an object can provide an accurate time stamp for organic matter that formed between 1960 and 2020. Highham has used it to detect, among other things, forged whiskey and date tea; This technology has been used on everything from groundwater to human cells.2,

Researchers have long known that this technology was coming to an end, but CO. was growing2 Emissions have accelerated the process. In the coming decades, as fossil-fuel use diminishes and the bomb curve flattens, the carbon-14 value will no longer be a diagnosis of a date. “It’s such a shame,” Higham says.

“This wildlife forensic tool; The window is closing on its effectiveness,” says palaeontologist Kevin Uno at Columbia University in New York City, who has used the bomb curve to date ivory specimens and study elephant hunting.3, “It’s kind of disappointing.”

The demise of the bomb curve means that researchers will have to rely on other techniques or isotopes to do their dating, including a third type of carbon, carbon-13. “There may be some other radionuclides that we can use,” Uno says.

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