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JOURNAL ARTICLE Radiocarbon Dating by Accelerator MassSpectrometry Robert E. M. Hedges and John A.J. Gowlett Scientific American Vol. 254, No. 1 (January 1986), pp. 100-107 Published by: Scientific American, adivision of Nature America, Inc. https://www.jstor.org/stable/24975875 Page Count: 8
In general, conventional radiocarbon dating has effectively mapped out the past 30,000 years in areas where organic preservation has been favorable. Yet beyond a threshold of about 20,000 years the number of objects whose age can be determined with certainty diminishes rapidly. Accelerator mass spectrometry can push the threshold further back in time, however.
At a horizon of 40,000 years the amount of carbon 14 in a bone or a piece of charcoal can be truly mi nute: such a specimen may contain only a few thousand C-14 atoms. Consequently equally small quantities of modern carbon can severely skew the measurements. Contamination of this kind amounting to 1 percent of the carbon in a sample 25,000 years old would make it appear to be about 1,500 years younger than its actual age. Such contamination would, however, reduce the apparent age of a 60,000-year-old object by almost 50 percent. Clearly proper sample-decontamination procedures are of particular importance in the dating of very old artifacts. Unfortunately stringent specimen handling and treatment procedures ultimately result in relatively small sample sizes, which conventional radiocarbon dating is poorly equipped to handle. Moreover, conventional radiocarbon dating would still face the insurmountable problem of discriminating the radioactivity of the sample from ambient background radiation.
Neither sample size nor background radiation present problems to radiocarbon accelerator dating, and so rel atively minor improvements in sample chemistry can lead to sharper and more extensive chronologies. For example, accelerator dating of purified amino acids from bones more than 25,000 years old showed that their age had previously been consistently underestimated by 1,000 or more years. The earlier, conventional measurements had been based on whole collagen, which cannot be guaranteed to be free of modern carbon.
One controversy of long standing on which the new dating technique has already had a major impact concerns the first human migrations to the New World. An accurate time scale for the colonization of America is crucial in order to assess how quickly the first Paleo-Indian hunters and gatherers dispersed, settled and developed their ethnic and linguistic diversity. Most observers agree that the earliest human inhabitants of America came from northeastern Asia probably between 25,000 and 12,000 years ago, crossing over a land bridge that then connected Siberia with Alaska. Nevertheless, human skeletons were found in the New World at disparate locations, such as Canada and Peru, that seemed to be considerably older than expected. Accelerator mass spectrometry has refuted these claims: no skeletal remains yet found in America appear to be more than 12,000 years old. If other such finds are made in the future, accelerator mass spectrometry will quite probably be the dating technique called on to determine the skeleton's age directly.
Accelerator dating may also prove to be invaluable in establishing an uthoritative chronology of Neanderthal man. The archaeological evidence available indicates a rather abrupt disappearance of this human subspecies at the beginning of the Upper Paleolithic, about 35,000 years ago. Although it is too early to be sure, dating by accelerator mass spectrometry may reveal that the Upper Paleolithic and the period preceding it, the Middle Paleolithic, were unduly compressed by the limitations of conventional dating methods. If this was the case, there would have been much more time for the Neanderthal's disappearance than has commonly been supposed.
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