Saturday , January 16 2021

The two Stalagmites found in the Chinese cave are the "holy grail" for precise dicurban dating



The sentries from the Hollow cave, with signs of the specimen seal.
Photo: Chai Cheng et al., 2018 / Science

Since its founding in the 1950s, it has been shown that carbon dioxide is essential for archaeologists and climate scientists who rely on the technique to accurately date organic compounds. But a good thing only improved, because of the discovery of two sentinels in the Chinese cave containing a seamless chronological atmospheric record dating back to the last ice age.

The unremitting record, high resolution of atmospheric carbon 12 and 14 carbon is found in a pair of Sentinels located inside the Hollow Cave near Nanjing, China, according to a new study published today in Science. Because this peak extends to the last ice age, up to 54,000 years ago, scientists are now equipped with a more accurate standard for use in radioactive calibration.

There is no doubt that the introduction of radioactivity has revolutionized archeology. Armed with this technique, scientists can date organic compounds, such as bone, hair, wood, seeds, and shells. However, as the use of the technique relies on accurate historical measurements of atmospheric carbon, especially the ratio between carbon-12 and carbon-14.

Carbon 14, or C14, is a rare type of carbon that, unlike carbon-12 (called "normal" carbon), is radioactive. C14 is an isotope consisting of six protons and eight neutrons, and is in a constant state of decay, with a generous half-life of 5,370 years. As a standard carbon, C14 combines with oxygen to form carbon dioxide, which is absorbed by all living organisms, whether they are animals or plants. As a result, the ratio between C12 and C14 in all living organisms is always the ratio between the atmosphere.

Because the atmospheric levels of C12 and C14 vary over time, the specific ratio in an organic sample (eg, bones, wood) is used as a time stamp for the death of a living creature. When an organism dies, it ceases to acquire new carbon. As time passes, the C14 breaks down like a ticking clock, but it is not replaced. By measuring the amount of radioactive decay, scientists can determine when an organism has lived in the past.

But there are limits to this dating approach, and this is related to half-life C14. Organic objects can be dated only up to 55,000 to 60,000 years, after which the amount of C14 in the sample diminishes to negligible dimensions. Furthermore, calibration is critical to this technique; Changes in the amount of radioactive atmosphere over time means that radioactive dates should be calibrated against a chronological, or calendar timeline.

Building these calendars is easier said than done. Ideally, scientists would like to have accurate and clear chronological documentation of changing atmospheric concentrations in C12 and C14 over time. This can be done, for example, by counting wooden rings (also known as dendrochronology), which, like any 8 year old happily telling you, is a reliable way to determine the age of the tree. Unfortunately, some calibrated datasets that directly demonstrate carbon in the atmosphere are even more present in time than the Holocene tree, about 12,600 to 14,000 years ago (trees are obviously not tens of thousands of years old, but fossils can be dated by other methods). Thus, the representation of a radiocarbon is limited by the ability of a given material to provide absolute age, while maintaining a record of changing atmospheric conditions.

But now, with the discovery and analysis of two special stalagmites at the Hollow Cave, scientists have encountered a continuous record of atmospheric carbon returning about 54,000 years ago. Instead of counting tree rings or learning coral reefs (another technique used to conclude absolute dates), the researchers, led by Hi Cheng of the Global Environmental Change Institute at Xi'an University of Yutong, analyzed the mineral composition within the stalactites. By dating hundreds of layers within these structures, which is done by using a very reliable isotopic dating technique known as thorium-230 dating, the researchers were able to establish an unprecedented chronological basis that can now be used for radioactive dating.

"So far, different approaches for C14 calibration have their own constraints," said Chai Gizmodo. "For example, it's still difficult [to use] The wooden rings calibrate atmosphere C14 beyond the present limit of about 14,000 years before the present. Corals do not accumulate continuously over thousands of years and are difficult to collect since those in the time span of interest are now largely immersed. Stalagmites, which can be an excellent choice for thorium-230 dating, usually contain a significant portion of carbon ultimately derived from limestone rock.

The UC Berkeley geologist, Larry Edwards, one of the authors of the new study, helped develop the thorium-230 method back in the late 1980s, but he could not find the ideal cave deposits for conducting such research.

"In addition to carbon from the atmosphere, cave deposits contain carbon limestone around the cave," Edwards said. "We have discovered that Hulu Caves samples contain very little carbon derived from limestone, so they are almost ideal for this type of research – hence our ability to complete an accurate calibration of C-14 timescale, a goal of the scientific community For the last nearly seven decades. "

In the study, the Island and its colleagues exhibit around 300 pairs of carbon 14 and thorium-230 extracted from the thin calcite layers within the Hulu Cave. The average temporal resolution between each pair is about 170 years. These particular stalactites, said Hai, are very special, containing "dead carbon" that is remarkably stable and reliable.

"As such, C14 in Hulu samples are derived mainly from atmospheric sources, which enable us to contribute a milestone towards the refinement of the C14 calibration curve using the combined measurements of C12 / C14 and Thorium 230," he said. Hai, adding: "The new record Hulu has less uncertainty solves previously unknown scale scale."

As the researchers write in their paper, the new calendar record represents a "sacred cup" for scientists, offering a high resolution and sustained record of atmospheric C14 covering the full range of radio-carbon export method. For archaeologists, it also means that they can now date organic compounds from 14,000 to 54,000 years with greater confidence, especially older samples.

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

Excitingly, this research will also be of interest to climate scientists, who can use this data to study atmospheric changes over time.

This is a very cool result from a very cold and improbable source-the slow drizzle, dripping, dripping in a dark cave in eastern China.

[Science]

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