As the carbon 14 decays with no possibility of replenishment, the ratio decreases at a regular rate. This rate is known as half-life.
ANP264 | Spring 2013
The measurement of carbon 14 decay provides an indication of the age of any carbon-based material. Dates may be expressed as either uncalibrated or calibrated years. A raw date cannot be used directly as a calendar date, because the level of atmospheric carbon 14 has not been constant during the span of time that can be radiocarbon dated. In addition, there are substantial reservoirs of carbon in organic matter, the ocean, ocean sediments, and sedimentary rock. Finally, although radiocarbon dating is the most common and widely used chronometric technique in archaeology today, it is not unfailing.
In general, single dates should not be trusted.
Carbon dating: Science in the service of History
Whenever possible multiple samples should be collected and dated from associated sections. The trend of the samples will provide a ball park estimate of the actual date of deposition.
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- Some interesting applications of radiocarbon dating to art and archaeology;
- dating site studies!
The trade-off between radiocarbon dating and other techniques is that we exchange precision for a wider geographical and temporal range. That is the true benefit of radiocarbon dating, that it can be employed anywhere in the world, and does have about a 60, year range.
From its origins in Chicago, carbon dating spread rapidly to other centers, for example the grandly named Geochronometric Laboratory at Yale University. The best way to transfer the exacting techniques was in the heads of the scientists themselves, as they moved to a new job.
Uses of Radiocarbon Dating
Tricks also spread through visits between laboratories and at meetings, and sometimes even through publications. Any contamination of a sample by outside carbon even from the researcher's fingerprints had to be fanatically excluded, of course, but that was only the beginning. Delicate operations were needed to extract a microscopic sample and process it. To get a mass large enough to handle, you needed to embed your sample in another substance, a "carrier.
Frustrating uncertainties prevailed until workers understood that their results had to be adjusted for the room's temperature and even the barometric pressure. This was all the usual sort of laboratory problem-solving, a matter of sorting out difficulties by studying one or another detail systematically for months. More unusual was the need to collaborate with all sorts of people around the world, to gather organic materials for dating. For example, Hans Suess relied on a variety of helpers to collect fragments of century-old trees from various corners of North America.
He was looking for the carbon that human industry had been emitting by burning fossil fuels, in which all the carbon had long since decayed away. Comparing the old wood with modern samples, he showed that the fossil carbon could be detected in the modern atmosphere.
Through the s and beyond, carbon workers published detailed tables of dates painstakingly derived from samples of a wondrous variety of materials, including charcoal, peat, clamshells, antlers, pine cones, and the stomach contents of an extinct Moa found buried in New Zealand. The results were then compared with traditional time sequences derived from glacial deposits, cores of clay from the seabed, and so forth. One application was a timetable of climate changes for tens of thousands of years back.
Making the job harder still, baffling anomalies turned up.
The carbon dates published by different researchers could not be reconciled, leading to confusion and prolonged controversy. It was an anxious time for scientists whose reputation for accurate work was on the line.
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- Radiocarbon dating.
But what looks like unwelcome noise to one specialist may contain information for another. In , Hessel de Vries in the Netherlands showed there were systematic anomalies in the carbon dates of tree rings. His explanation was that the concentration of carbon in the atmosphere had varied over time by up to one percent. De Vries thought the variation might be explained by something connected with climate, such as episodes of turnover of ocean waters.
Some speculated that such irregularities might be caused by variations in the Earth's magnetic field.