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Analytical Chemistry

Chinese cave stalagmites provide researchers with the ultimate means to calibrate radiocarbon dating

Study reports precise atmospheric 14C levels dating back to the last ice age

by Tien Nguyen
December 17, 2018

LN-hulucave.jpg
Credit: Hai Cheng
Stalagmite samples from the Hulu cave in China (shown) provide a full accounting of 14C levels for about the past 54,000 years.

Radiocarbon dating, which lets scientists determine the age of organic materials, has revolutionized archeology and climate science. In the method, researchers measure the amount of 14C in a sample and then use the isotope’s rate of radioactive decay, to calculate how much 14C was present when it was originally formed. However, this measurement provides only a relative age. Because atmospheric 14C levels fluctuate over time, rising and falling with natural changes like those of Earth’s magnetic field or human-caused events like nuclear testing, scientists need to calibrate their radiocarbon data to an independent source to figure out a material’s true age.

Now, using two rock samples from a cave in China, researchers have constructed a record of atmospheric 14C levels going back 54,000 years, essentially creating the ultimate calibration curve for the dating technique (Science 2018, DOI: 10.1126/science.aau0747). With these data, scientists will be able to accurately date materials such as fossils and organic artifacts as far back as is possible for radiocarbon dating.

LN-stalagmite.jpg
Credit: Hai Cheng
Scientists chipped and drilled 300 sample pairs from two stalagmites (pictured) to construct a complete and precise radiocarbon dating calibration curve.

Constructing a complete and precise calibration curve has been a goal of the radiodating community since the method’s invention more than 70 years ago, says R. Lawrence Edwards at the University of Minnesota, who coauthored the study along with colleagues in Minnesota, California, and China.

“It’s always been a dream of mine to do what we’ve done here,” he says.

One way scientists currently calibrate their radiocarbon data is to match their measurements with those from tree rings. But using tree ring data provides reliable information on 14C levels going back only about 15,000 years.

The new calibration curve was made possible by two stalagmite fragments, called MSL and MSD, from the Hulu cave near Nanjing, China. The rocks possessed two key features. Together, the two samples covered the entire 54,000-year range that researchers were interested in, and both had low amounts of so-called dead carbon. Dead carbon is 12C or 13C that comes from water-dissolved limestone, which is usually spread throughout cave stalagmites in high amounts and makes it difficult to accurately account for 14C.

From these stalagmites, the researchers took about 300 paired samples and analyzed them by mass spectrometry. For each pair, researchers subjected one to radiocarbon dating and the other to uranium-thorium dating. Uranium-thorium dating, which Edwards helped develop in the 1980s, gives an absolute age without needing calibration but it only works on certain types of materials like coral skeletons and cave deposits. It took the team almost a decade to complete the labor-intensive process and subsequent data analysis on the stalagmite samples, Edwards says.

Paula J. Reimer, a radiocarbon dating expert at Queen’s University Belfast, says new data will “markedly improve” existing radiocarbon calibration curves, particularly in the greater than 25,000 year range. She adds that better atmospheric 14C records will also improve the accuracy of models that investigate the exchange of carbon dioxide between the ocean and the atmosphere to predict changes in climate.

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