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In AMS, the material being analyzed is bombarded with cesium atoms, which sputters
off carbon atoms so the ratio of carbon-14 to carbon-12 can be measured.
The researchers tested the accuracy of carbon-14 dating in 29 animal and plant tissues killed and collected on known dates from 1905 to 2008. The samples included elephant tusks and molars, hippo tusks and canine teeth, oryx horn, hair from monkeys and elephant tails, and some grasses collected in Kenya in 1962.
Samples came from museums in Africa and elsewhere, and from Amina, an elephant that died naturally in Kenya in 2006, and from Misha, an African elephant euthanized in 2008 due to declining health at Utah’s Hogle Zoo in Salt Lake City.
The analysis revealed that various tissues that formed at the same time have the same carbon-14 levels, and that grasses and the animals eating them had the same levels. By determining carbon-14 in these samples of known dates, the researchers now can measure carbon-14 levels in other ivory to determine its age, within about a year.
The four oldest samples – from animals died between 1905 and 1953 – had minimal carbon-14 because they died before atmospheric nuclear weapons tests. So the test can identify pre-1955 ivory by its low, pre-nuclear-test levels of carbon-14.
Cerling said the method can determine the year of death for any animal killed after 1955, identifying the time of the most recent tissue formation – at the base of a tusk or tooth, for example. The method is less precise for animals killed more recently; it can tell if an animal died between 2010 and 2013, but not more precisely.
It takes about 5,700 years for half of carbon-14 to decay radioactively. But the amount in Earth’s atmosphere after the 1950s and 1960s bomb tests faded much more quickly because oceans and trees absorb carbon dioxide – including carbon-14 – from the atmosphere. So the method won’t work for tusks or other tissues that grow after about 15 years from now, when atmospheric carbon-14 returns to pre-bomb levels.
Understanding Ancient and Modern Ecosystems
While the method’s use against poaching is important, “the scientific part is the importance of understanding time in the formation of animal tissues and how diet and physiology is recorded in those tissues over time” as they grow, Cerling said.
Cerling said that will improve understanding of what prehistoric and modern animals ate over time, especially when combined with existing isotope analysis of ratios of carbon-13 to carbon-12 in teeth – data that reveal whether animals ate diets based on tree and shrub leaves and fruits, or upon grasses and grazing animals.
So as part of the new study, the scientists also analyzed another 41 samples to determine the growth rates for tusks and teeth from elephants and hippos, and elephant tail hair, Cerling said.
Extrapolating the growth rates of tusks, teeth and hair to fossil or modern elephants and other animals “will help us improve the chronology of the diet history of an individual fossil or modern animal,” Cerling said.