Carbon-14 dating has revolutionized a number of scientific disciplines, most notably archeology and climatology. Stalagmites in a Chinese cave have given scientists all they need to reconstruct the historical record of atmospheric radiocarbon (carbon-14), back to the carbon dating limit of around 54,000 years ago.
The stalagmites in China were found in Hulu Cave, and they offer a continuous record of atmospheric carbon ratios.
A team of researchers, with members affiliated with institutions in the U.S. and China, has found two stalagmites that offer a way to improve the accuracy of the carbon-14 dating technique. In their paper published in the journal, Science, the group describes the stalagmites and their study of them.
The researchers claim that this work is a step closer towards the ‘holy grail’ of carbon dating, precisely refining the calibration of carbon-14 against a calendar timescale so that dating of historical treasures can become more accurate.
Stalagmites are columns formed by mineral-bearing water, dripping on the floors of caves over thousands of years. As a drop evaporates, it leaves behind residual minerals over time, and they grow, forming tall columns.
Hai Cheng at Xi’an Jiaotong University in China and his colleagues in the US had previously discovered that a stalagmite in Hulu Cave showed unusually low and stable amounts of dead carbon that allowed for accurate carbon-14 calibration between around 27,000 and 10,500 years ago. Now, the team has studied two older stalagmites in the cave to help take carbon dating to its limit.
"The Hulu carbon-14 dataset provides a robust reconstruction of the atmospheric carbon-14 history beyond the current tree ring method limit of around 14,000 years before present," said Cheng. "This is a substantial contribution toward the refinement of the carbon-14 calibration curve."
Understanding Carbon-14 Dating
The carbon-14 dating technique has been the go-to for archaeologists since the 1950s. In recent years, it has become an important tool for use in measuring climate change, as well. But, the technique has suffered one fatal flaw - its accuracy depends on a strong record of atmospheric carbon ratios (carbon-12 to carbon-14) for the time period involved. Of course, in recent years, this has not been a problem. However, the method itself is less precise for dating older objects. Scientists have used tree rings (both preserved and fossilized) and coral reefs as a way to measure ratios in given time periods, but this method too started to suffer many years ago.
In this new effort, the researchers reported on the finding of two stalagmites in a Chinese cave that could offer an accurate measure of such ratios, going back approximately 54,000 years. The half-life of carbon-14 is 5,370 years.
The Thorium-230 Method
UC Berkeley geologist, Larry Edwards, a co-author of the study, helped to develop the thorium-230 method back in the late 1980s, but he wasn’t able to find ideal cave deposits to perform research like this one.
“In addition to carbon from the atmosphere, cave deposits contain carbon from the limestone around the cave,” said Edwards. “We thus needed to make a correction for the limestone-derived carbon. We discovered that the Hulu Cave samples contain very little limestone-derived carbon and are therefore nearly ideal for this kind of study—hence our ability to complete a precise calibration of the C-14 timescale, a goal of the scientific community for the last nearly seven decades."
The stalagmites from Hulu Cave with sampling etch marks. (Source: Hai Cheng et al., 2018, Science)
As part of the investigation, Hai and his colleagues had presented around 300 paired carbon-14 and thorium-230 dates extracted from the thin calcite layers within the Hulu Cave stalagmites. The average temporal resolution between each pair was found to be about 170 years. These stalagmites were "very special" containing stable and reliable "dead carbon."
Hai added, “As such, the C14 in the Hulu samples are mainly derived from atmospheric sources, which allows us to make a milestone contribution towards the refinement of the C14 calibration curve through the paired measurements of the C12/C14 and thorium-230 ages. The new Hulu record has less uncertainty and resolves previously unknown fine-scale structure.”
This is certainly an important discovery from the caves of eastern China.
Top Image: The stalagmites in the Hulu Cave have been used to calibrate radiocarbon dating with accuracy. (Source: Hai Cheng/Jiaotong University)