Rolling Back the Years

Radiocarbon dating gets a remake

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.

MIGHTY OLD. Researcher Jonathan Palmer of the Gondwana Tree-Ring Laboratory in Auckland, New Zealand, stands by the recently excavated trunk of a Kauri tree. Counting tree rings can yield accurate dates by which to calibrate radiocarbon dating tens of thousands of years into the past. Turney

Scientists dispute how long Neandertals overlapped with modern 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; SN: 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 60,000-year barrier, long considered radiocarbon’s 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 vacuum-extraction 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.

Pigati of the USGS has recently built on the ABOX method, devising an extraction system to further isolate and purify carbon dioxide samples while preventing contamination by modern carbon. That approach is “especially important for very old samples,” he says. “Even very small amounts of modern contamination can be fatal for old samples.”

The technique, detailed in the May Quaternary International, promises to be more reliable and “beyond the limit of other systems,” he says, extending reliability to perhaps 55,000 or 60,000 years ago. Pigati says unpublished research shows that his system produces “ages that are either older, in the case of archaeological charcoal, or less variable, in the case of [cave deposits], than ages obtained using standard techniques.”

While the new contamination-reduction systems look promising, their length and cost and the dearth of appropriate samples to date mean that they might have only a small impact on pinning down a precise radiocarbon timeline.

Curves and numbers

Even in the early days, Libby suspected that the carbon-12 to carbon-14 ratio had not remained constant through time. Work on solar cycles and the Earth’s magnetic field proved him right. Both phenomena are known to influence radiocarbon amounts by altering the level of cosmic radiation entering the atmosphere. And radiocarbon traveling through the Earth’s carbon cycle can do so inconsistently. As cold, dense water sinks to the depths of the ocean, it drags down radiocarbon that might get trapped for hundreds of years before resurfacing. Scientists call this the marine offset, and it has important implications for inferring carbon dioxide levels in past climates.

These fluctuations in the historic radiocarbon clock mean that to find the calendar dates of artifacts, scientists need methods and samples that can independently verify the amount of carbon-14 in the atmosphere at a given time. Such independent data serve as a measuring stick against which scientists calibrate radiocarbon dates.

Tree rings are the gold standard for calibration because they can pinpoint individual years, but the tree ring dates reliable enough to validate absolute radiocarbon dates go back only about 12,500 years. Decades of work piecing together data from lake sediments, mineral deposits, ice cores, and especially corals, have pushed the date of confidence back to 26,000 years. In the September 2004 Radiocarbon, an international working group called IntCal published an official calibration curve that represented the consensus of the field. “If everyone agrees to use the same curve, then carbon-14 data will be directly comparable between labs, researchers, and locations,” says Pigati.

“Although 26,000 [years ago] is pretty well nailed down now, there’s a sort of best guess for what comes after that,” says Gillespie. Scientists can’t rely on any one method because of the inherent assumptions and limitations in each method and technique, he says.

Improvements in the particle-counting technique called accelerated mass spectroscopy (AMS) have greatly contributed to filling in the gaps of uncertainty in the time line, says John Southon of the University of California, Irvine. Scientists began using AMS in the 1970s, but recent advances mean that it can be used to measure radiocarbon in much smaller samples. Other advances have cut the time it takes to count the radiocarbon emission. “If you can do measurements in 10 minutes as opposed to an hour, you can measure more samples, and repeatedly,” says Southon. “The more counts you get, the lower the uncertainty becomes. People are certainly knocking on the door of 60,000.”

Getting a pure sample for AMS to count is critical to obtaining a precise absolute date, the prize target for radiocarbon scientists. Instead of trying to strip away contaminants to get to the valuable pure carbon source, as techniques like ABOX do, some researchers are turning the process on its head by isolating the carbon in specific organic compounds such as leaf waxes, lipid membranes, and other organic material trapped in marine sediments. This would provide another independent—and highly reliable and precise—way of dating organisms.

There’s been only a smattering of studies since the method was first reported by Timothy Eglington at the Woods Hole Oceanographic Institution in Massachusetts in the late 1990s. In the February Chemical Reviews, Ann McNichol, Eglington’s colleague at Woods Hole, reported that compound-specific dating could not only help refine radiocarbon dates but also contribute to pinning down past carbon cycles. The method’s potential is enormous, McNichol says.

Others in the carbon dating field are equally enthused. “It’s exciting stuff,” says Exeter’s Turney. “Ideally, when dating samples, you want to use material that fixed their carbon directly from the atmosphere. By using compound-specific material [like] leaf waxes, it’s possible to isolate components that reflect what was going on in the atmosphere and therefore give a more precise age.”

Rings of time

What scientists are really holding out for is tree ring data that calibrate absolute radiocarbon dates back to 60,000 years. “That would be the ultimate calibration curve,” says Timothy Jull of the University of Arizona in Tucson, editor of Radiocarbon.

New Zealand Kauri trees (Agathis australis) seem to be the most likely candidates. Growing up to 50 meters tall with diameters of about 5 m, the giant flora can live for at least 2,200 years, says Alan Hogg of the University of Waikato in Hamilton, New Zealand. Hogg, Turney, and colleagues have been dating the fossil Kauri trees from swamps in the country’s North Island.

Given the trees’ long lives, scientists would in theory be able to piece together a chronology by overlapping data from trees of different ages, all the way back to 60,000-odd years ago. This hinges, of course, on whether they can find sufficiently old trees and samples that represent a continuum of ages throughout the past. In the May Radiocarbon, Hogg, Turney, and colleagues reported that they had unearthed tree ring samples that would span significant periods of time, and importantly, extend back more than 60,000 years.

“At the moment we have a floating chronology,” Hogg says. “It’s not connected.” The team has dated some trees, rings from different periods of time, but where exactly the rings fit in can be determined only by finding the rest of the overlapping puzzle pieces. The researchers have so far calculated the Kauri chronology back 5,000 years, but plenty of work remains—perhaps several decades’ worth, Hogg says. “A continuous chronology is a long way off.”

Such constant refinement and updating of the calibration curve might have already made the 2004 IntCal curve obsolete, says University of Arizona’s Jull. He suggests that an updated and more accurate official calibration curve dating back to 50,000 years might be just a few years away. The next radiocarbon conference in 2009 would be the impetus, but “to get a consensus might take another year or two,” he says. The Neandertal archaeologists might have to put the debate on hold just a little longer.

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