Rolling back the years: radiocarbon dating gets a remake

Science News,  Dec 1, 2007  by Carolyn Barry

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Archaeologists agree that Neandertals lost their evolutionary fight with Homo sapiens to become the Earth's dominant humanoid life form. But controversy continues over how long that fight lasted, and whether it was modern humans or changing climate that played the primary role in orchestrating the Neandertals' demise.

Scientists dispute how long Neandertals overlapped with modem humans. Some experts say that Neandertals hung around until as recently as 24,000 years ago. Others insist that Neandertals vanished closer to 30,000 years ago (SN: 5/13/06, p. 302; 9/23/06, p. 205). Resolving that discrepancy, as well as many other archaeological mysteries, requires precise knowledge of the ages of artifacts from those times. "Without firm chronologic control, it is nearly impossible to determine ... relationships between populations and locations" says Jeff Pigati of the U. S. Geological Survey (USGS) in Tucson, Ariz.

From archaeologists to climate-change researchers, scientists covet a reliable time line for piecing together ancient events. Because most archaeological remains contain carbon, the method of choice for determining age is carbon dating in which scientists compare the relative amounts of a stable carbon isotope to one that radioactively decays. Radiocarbon ages are reliable--to a point. Corroborating data from ice cores, corals, and tree rings have pushed dependable carbon dates back to about 26,000 years, but in preceding millennia the dates become increasingly less certain. By 50,000 to 60,000 years ago, a radiocarbon date might be off as much as 2,000 years from the true date.

Carbon dating specialists are working toward constructing a precise time line back to the 60,O00-year barrier, long considered radiocarbons outer limit. Advances in reducing sample contamination, improved techniques to extract specific compounds out of samples, and a new source for ancient tree ring data have offered hope that the 60,000-year benchmark is within reach.

REVOLUTIONARY SEWERS The most widely used dating method had quite unsophisticated origins: the sewers of the Patapsco Sewage Plant in Baltimore. In the May 30, 1947 Science, Willard Libby first reported finding trace amounts of radiocarbon (carbon-14) in the methane collected in the sewers that wasn't evident in older petroleum deposits. He received a Nobel Prize in 1960 for the discovery that organisms possess radioactive carbon that can be used to compute how long ago they lived.

Libby proved that minuscule amounts of radiocarbon, formed from cosmic rays in the upper atmosphere, turn up in plants as they absorb carbon dioxide during photosynthesis. That trace amount of radiocarbon makes its way up the food chain as animals eat the plants or other animals that have consumed the plants.

In living organisms, radiocarbon lost by decay is constantly replenished. But after an organism dies, the radiocarbon clock starts ticking as carbon-14 steadily dwindles in comparison with stable carbon-12. Scientists measure radiocarbon by detecting the energy or particles it emits as it decays.

Radiocarbon's half-life--the time it takes for half of any quantity to decay--is roughly 5,730 years. That makes it a good clock for dating remains of organisms that lived tens of thousands of years ago. But objects dating back 40,000 to 60,000 years retain around 0.1 percent or less of their original radio carbon. That level becomes indistinguishable from present background levels, says Chris Turney of the University of Exeter in England. The older an artifact is, the less certain scientists can be about its age, he adds. "With radiocarbon, it's not possible to obtain absolute dates--there's always a bit of an unknown."

GARBAGE IN, GARBAGE OUT Many factors can interfere with radiocarbon dates. One of the biggest issues, says Turney, is contamination, particularly by "modern carbon" acquired later than 1950. Nuclear testing in the 1950s and 1960s blasted out radiation that scientists see clearly as a spike in the radiocarbon record. Scientists now refer to radiocarbon years as "before present" (BP), where present means 1950. If any modern carbon mixes with a sample, it will seem younger than it really is. "Charcoal is notorious" Turney says. "It soaks up anything in the ground."

Because charcoal is so commonly carbon dated, Michael Bird, now at the University of St. Andrews in Fife, Scotland, developed a specialized cleaning protocol that rids samples of almost all modern contamination and makes the dating much more precise. Called ABOX, the technique, which Bird reported in 1999 when he was at the Australian National University in

Canberra, uses acid-base wet oxidation to dissolve virtually everything but the pure charcoal. Samples are treated in a vacuumextraction system that sucks away by-products while making sure no modern carbon from the air enters the chamber.

"ABOX showed some dates are seriously wrong" says Richard Gillespie of the Australian National University. Gillespie used the method to date remains of Australian megafauna and reported in the January 2006 Archaeology of Oceania that humans were likely to have caused the great megafauna extinction about 45,000 years ago, since both groups existed around the same time. That report helped quell the argument that climate effects caused the mass die-off (SN: 3/15/03, p. 173). "Something like a hundred dates were wrong and we ended up chucking them all out,' he says. "Some of the dates were 10,000 years out," he adds.