A sample of the human femur is removed for carbon dating using accelerated mass spectrometry (AMS).

Carbon dating hampered by rising fossil fuel emissions – Nature.com

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

Researchers tracking the increasing levels of carbon dioxide in the atmosphere have mapped out a historic moment. As of 2021, the combustion of fossil fuels has formally changed the carbon isotope composition of northern hemisphere air enough to cancel out a useful signal from nuclear weapons tests.1.

This can cause problems for valuable carbon dating techniques. Modern elements now look like things from the early 20th century radiocarbon dating, says Heather Graven, a chemical physicist at Imperial College London who has been mapping that effect for years. This trend “may 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 is other clues to the source of the object, “there are often stray finds without this information,” says Reimer, such as unidentified human remains that may have come from a historical burial site or from someone who has recently died.

Adjusted carbon: a line chart showing the proportion of radioactive carbon 14 in carbon dioxide in the atmosphere since 1950.

Source: reference 1.

The development also means that forensic scientists will no longer be able to use radiocarbon fingerprints to determine the ages of materials such as ivory, antiques and wine. “If you’re in forensics or you’re discovering fakes, this is a really sad moment,” says Tom Higham, an archaeologist at the University of Vienna.

Time Machine

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

Living organisms absorb both types of carbon. After their death, the relative amounts of the two isotopes begin to change as radiocarbon-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, cloth or bone, that are about 55,000 years old. Usually, the smaller the percentage of carbon-14, the older the material.

Between 1952 and 1962, nuclear weapons testing set off a spike in the amount of the “carbon bomb” that rapidly doubled the amount of carbon-14 in the air. Since then, carbon-14 has been slowly absorbed by living organisms and the ocean. At the same time, burning fossil fuels quickly releases carbon dioxide2 It does not contain carbon 14.

As of 2021, these two effects have They officially canceled each other out in the northern hemisphere (See “Modified carbon”). This means that the proportion of carbon-14 in modern materials now is the same as in pre-industrial times. And because fossil fuels are still burning, the proportion of carbon-14 in the air will be reduced further, more closely mimicking conditions in the past. By 2050, Graven predicts1the ratio of carbon to 14 will be similar to what it was in the Middle Ages (between the fifth and fifteenth centuries).

When there are vibrations and spikes in carbon-14 in the air over time, radiocarbon dating is not always indistinguishable from one date to another. That’s right for 800 B.C.E. to 400 B.C.E.For example: “You can’t literally date anything [precisely] Within 400 years,” says Higham. The phenomenon of fossil fuels eliminating bomb carbon provides another opportunity for confusion over radiocarbon.

goodbye bomb curve

For modern objects less than decades old, the decay of carbon-14 is negligible. But the rapid rise in carbon-14 released by nuclear weapons has created a diagnostic “bomb curve” for carbon-14 levels. “It’s the silver lining in the bomb test,” Higham says. This means that the amount of carbon-14 in a body can provide an accurate time stamp of organic matter that formed between 1960 and 2020. Higham used it to discover fake whiskey and the history of Chinese tea, among other things; This technology has been used in everything from groundwater to human cells2.

Researchers have known for a long time that the end of this technology was coming, but the increase in carbon dioxide2 Emissions from this process are accelerated. In the coming decades, as fossil fuel use wanes and the bomb curve flattens, the value of carbon-14 will no longer be diagnosed. “What a shame,” Higham says.

“This forensic wildlife tool; says paleontologist Kevin Ono at Columbia University in New York City, who has used the bomb curve to date ivory samples and study elephant poaching.3. “It’s kind of depressing.”

The demise of the bomb curve means researchers will increasingly have to rely on other techniques or isotopes to date them, including the third type of carbon, carbon-13. “There may be some other radionuclides we can use,” Ono says.

#Carbon #dating #hampered #rising #fossil #fuel #emissions #Naturecom

Leave a Comment

Your email address will not be published.