To all,
If this week’s Science did not pull a sensational coup, like Nature, it nonetheless came up a very readable piece on radiocarbon dating, by Michael Balter, in which he tells us all about how archaeologists are trying to cope with matters of constantly changing calibration, and how the “calibrators” themselves seem to have difficulties reaching a consensus on whose curve is most reliable. Also particularly interesting are the few lines about how Mellars has chosen to navigate through this confusion.
Radiocarbon Dating’s Final Frontier
In a heroic and sometimes contentious effort, researchers push to extend accurate radiocarbon dating back to 50,000 years ago
Michael Balter
Science 15 September 2006:
Vol. 313. no. 5793, pp. 1560 - 1563
The 1994 discovery of France’s Grotte Chauvet revolutionized ideas about symbolic expression in early modern humans.The breathtaking drawings of horses,lions, and bears that adorned the cave walls were executed with perspective and shading and rivaled the virtuosity of all other known cave art. But when were those drawings made? Early radiocarbon dates suggested 32,000 years ago, right after a major cold spell hit Europe. This implied that modern humans blossomed under frigid conditions while their Neandertal cousins were going extinct. But improved radiocarbon dating now suggests that the oldest paintings at Chauvet could beat least 36,000 years old. That’s smack in the middle of a period of relative warmth and challenges speculation about modern humans’ adaptability to a cold climate.
Getting the dating right is“crucial,” says archaeologist Clive Gamble of the University of London’s Royal Holloway campus. “It is not just a case of winning a trophy by being the oldest. The model up to now has been that modern humans could go anywhere and do anything, and it didn’t matter what the climate was.” Thanks to more accurate dating, says Gamble, “that model is now showing signs of cracking.”
Indeed, as radiocarbon experts revise their estimates, all researchers working in the eventful period from about 50,000 to 25,000 years ago are facing an across-the-board realignment of dates. That’s when both Neandertals and modern humans lived in Europe and when wildly fluctuating temperatures culminated in the spread of glaciers across much of the Northern Hemisphere.
There’s no question about the basic principles of the radiocarbon method: Plants and animals absorb trace amounts of radioactive carbon-14 (14C) from CO2 in the atmosphere while alive but cease to do so when they die. So the steady decay of 14C in their tissues ticks away over the years. But the amount of 14C produced in the atmosphere varies with the sun’s solar activity and fluctuations in Earth’s magnetic field. This means that the radiocarbon clock can race ahead or seemingly stop for up to 5 centuries. As a result,raw radiocarbon dates sometimes diverge from real calendar years by hundreds or even thousands of years. Thus researchers must calibrate the clock to account for these fluctuations, and that can be a challenge. For example, the start of the Holocene, the period when the last ice age ended, is usually dated to 10,000 uncalibrated radiocarbon years ago.
But the radiocarbon clock stopped for several hundred years right at that point, so that the start of the Holocene—when agriculture began—can’t be pinned down any more precisely than somewhere between 11,200 and 11,800 years ago (see graph). Because the best estimate of the calibration keeps changing, many scientists avoid reporting calendar years and simply cite “radiocarbonyears” as a universal measuring stick when announcing new finds.
Yet recent progress in radiocarbon dating may finally give researchers the accuracy they seek. In 2004, after 25 years of painstaking labor, an international group of radiocarbon experts extended the calibration curve back to 26,000 years by using data from tree rings, corals, lake sediments,ice cores, and other sources to create a detailed record of 14C variations over the millennia. The final frontier, which th egroup hopes to reach by the end of this decade, will be to push calibration to th 50,000-year mark; beyond that, there is too little residual 14C to measure precisely.
Refinement of existing data, plus some promising new data sources, including ancient trees from the swamps of NewZealand, may help close the final gap.“ These are very exciting times,” says nuclear physicist Johannes van der Plicht, director of the radiocarbon laboratory at the University of Gröningen in the Netherlands. He adds that a final calibration curve “will answer so many questions in archaeology,” in large part because the 50,000-year limit coincides with a major migration of modern humans from Africa to Europe and Asia.
Earth scientists, many of whom use radiocarbon dating to study the movement of glaciers and ocean currents, are equally enthusiastic, in part because of the unprecedented climate variability that occurred between 30,000 and 50,000 years ago. Those who study sea-level fluctuations during and after the last ice age—data used to model patterns of global warming—rely“ almost entirely” on radiocarbon dating, adds geophysicist Richard Peltier of the University of Toronto in Canada.
Yet the eagerly awaited calibration is complicated by dissent in the ranks. One U.S. scientist has bypassed the international working group and published his own calibration curve, to the annoyance of many colleagues, while a British archaeologist is using provisional calibration data—prematurely, in the view of some radiocarbon experts—as evidence that Homo sapiens spread across Europe more rapidly than previously thought. Both researchers argue that science can’t wait for an internationally agreed-upon calibration curve. The question at issue, says archaeologist Sturt Manning of Cornell University, is “who actually owns time”: the experts working to calibrate radiocarbon, or the research community at large.
For the full text and a few good graphics, click
HERE.
Jacques
