The two stalagmites found in the Chinese cave are "Holy Grail" for accurate data on radiocarbons


Stalagmites from the Hulu cave, with sample labels.
Image: Hai Cheng et al., 2018 / Science

Since its inception in the 1950s, radio-carbon associations have proved to be indispensable to archaeologists and climatic scientists who rely on precision organic compounds. But the good thing has just improved because of the discovery of two stalagmites in the Chinese cave, which contained a flawless chronological atmospheric record from the last ice age.

According to new research published today in Science, they found a continuous record with atmospheric carbon-12 and carbon-14 in a pair of stalagmites in the Hulu jama near Nanjing, China. Since this record stretches back to the last glacial period, about 54,000 years ago, scientists are now equipped with a more accurate standard for use in the calibration of radiocarbons.

There is no doubt that radiocarbon dating revolutionized archeology. Armed with this technique, scientists can put organic compounds, such as bones, hair, wood, seeds and shells. The next time we go, however, it becomes a less reliable carbon step because the technique is based on precise historical measurements of atmospheric carbon, in particular the ratio between carbon-12 and carbon-14.

Carbon-14 or C14 is a rare form of carbon that is, in contrast to carbon-12 (so-called "normal" carbon), radioactive. C14 is an isotope consisting of six protons and eight neutrons, and is in constant explosion with a generous half-life of 5,370 years. As a conventional carbon, C14 combines with oxygen to produce carbon dioxide absorbed by all living beings, regardless of whether they are animals or plants. Therefore, the ratio of C12 to C14 in all living organisms is always the same as the ratio in the atmosphere.

Since the C12 and C14 concentrations in the atmosphere change over time, the specific ratio in the organic sample (eg Bones, wood) is used as a time stamp for the death of a living being. When the organism dies, it ceases to produce new carbon. With time, the C14 decays as a tick, but it does not change. By measuring the amount of radioactive decay, scientists can determine when a former living organism has died.

But there are limits to this approach for them, and this is associated with the half-life of C14. Organic objects can only be dated to about 55,000 to 60,000 years, after which the amount of C14 in the sample is reduced to negligible proportions. What's more, calibration is the key to this technique; changes in the amount of atmospheric radiocarbonate over time mean that the radiocarbon dates must be calibrated relative to the chronological or calendar period.

Building these calendars is easier said than done. Ideally, scientists would like a precise and continuous chronological record of changing concentrations of C12 and C14 atmospheric concentrations over a longer period of time. This can be done, for example, by counting tree rings (also known as dendrochronology), which will, as you consider an 8-year-old, happy to say, a reliable way of determining the age of the tree. Unfortunately, a few calibrated data sets that directly sample atmospheric carbon remain at a time like the Holocene Tree Ring record about 12,600 to 14,000 years ago (it is evident that the trees do not live tens of thousands of years, but the ancient, fossilized trees can be dated with other methods). Radiocarbon dating thus limits the ability of a particular material to provide an absolute age, while maintaining a record of changing atmospheric conditions.

But now, with the discovery and analysis of two special stalagmites in the Hulu jama, scientists have encountered a continuous record of atmospheric carbon that has taken as many as 54,000 years. Researchers led by Hai Cheng of the Institute of Global Environmental Change at Xi'an Jiaotong University, instead of counting tree rings or studying coral reefs (another technique used to make absolute dates), analyzed the mineral composition inside stalagmites. By encountering hundreds of layers in these structures, which was done using a very reliable isotope dating dating technology, known as thorium-230, researchers were able to set up an unprecedented chronological basis that can now be used for radiocarbon use .

"So far, different approaches to C14 calibration have their limitations," he told Hai Gizmodu. "For example, it remains difficult [to use] tree rings to calibrate the atmospheric C14 above the present limit of about 14,000 years before the present. The corals are not constantly collected over thousands of years, and are difficult to collect, since they are now largely submerged in the time zone of interest. Stalagmites, which can be excellent choices for the administration of thorium-230, usually contain an important part of the carbon that is eventually obtained from limestone. "

UC Berkeley, UC Berkeley Larry Edwards, co-author of the new study, helped develop the thorium-230 method in the late 1980s, but could not find ideal cave caves to carry out a study such as this.

"In addition to carbon from the atmosphere, the cave landfill contains carbon from the limestone around the cave," said Edwards for Gizmodo. "We therefore had to fix limestone carbon. We found that Hulu Jama samples contain very little limestone carbon and are therefore almost ideal for this type of study, so we are able to accurately calibrate the C-14 timetable, the goal of the scientific community for the last nearly seventy years . "

In the Hai study and his colleagues, there are approximately 300 steam carbon 14 and toorium-230 dates obtained from thin calcite layers inside the stalagmites of the Hulu jama. The average time resolution between each pair is about 170 years. These special stalagmites, said Hai, are very special, containing "dead carbon", which is extremely stable and reliable.

"Thus, the C14 in Hulu samples is derived primarily from atmospheric sources, which enables us to contribute to improving the calibration curve C14 by means of well-known measurements of the age of C12 / C14 and thorium-230," said Hai, adding: "The new Hulu record has less uncertainty and solves the previously unknown structure of small scale. "

As the researchers write in their article, the new calendar record represents the scientific council of "holy grail" offering high resolution and continuous record of the atmospheric C14, which covers the entire range of the method of radiocarbon dating. For archaeologists, this also means that they can now give organic compounds between 14,000 and 54,000 years of age, especially older samples, with greater confidence.

"For a sample that is actually 40,000 years old, the C14 nominal age would be approximately 35,000 years old, the age that would have been calculated from previous calibration data would be approximately 38,000 years, with great uncertainty," explained Edwards. "So the difference is from 2,000 to 5,000 years, depending on how you decided to calibrate your age before our work."

It will be delightful that these studies will also be of interest to climate scientists who can analyze weather conditions in the atmosphere with these data.

This is a very cool result from a very cool and unlikely source – slow droplet, dripping, dripping in a dark cave in eastern China.



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