Radiocarbon Dating: Nature’s Timepiece Gets a Tune-Up
By Tara Thean
It’s relatively easy for scientists to see the signature of droughts and other climate events in the prehistoric past by digging into underground or seafloor sediments, or drilling into ancient ice. In order to say exactly when these events happened, though, you need a reliable natural dating method, and even the best of these is flawed.
However, one of the most familiar of these timelines, known as radiocarbon dating, just got a lot more precise. According to a paper published in the journal Science, measurements from the bottom of Japan’s Lake Suigetsu have allowed scientists to improve the technique dramatically.
Now, thanks to those lake sediments, scientists can narrow that range down to just 10 years or less—but only if the sample is between 11,000 and 53,000 years old. Younger and there hasn't been enough breakdown in the radioactive carbon. Older, and the lake's sediments don't go back that far.
This impressive achievement comes thanks to Lake Suigetsu’s calm waters, and also from the lucky fact that the plant matter that drifts into the water and sinks to the bottom is light-colored in winter and dark in summer. The result: alternating layers under the lake bottom that make it easy to identify every year, one after the other, going well back into the last Ice Age.
Armed with this perfect record, the scientists then performed radiocarbon dating on the plant residues from a subset of the layers. Here’s how it works: While they are alive, living organisms take in small amounts of a naturally radioactive type of carbon called carbon-14 from the atmosphere.
The carbon-14 then decays at a constant rate after the plants or animals die; how much is left today tells scientists how long ago the organism died. But there’s a catch: The amount of carbon-14 in the atmosphere fluctuates, which means there can be more or less of it to start the decay clock at different points of prehistory. Carbon dates need to be checked, therefore, against other natural timepieces.
They often use tree rings to do this – but tree ring archives only go back to just more than 12,000 years. For any time past that, researchers turn to marine records like corals. These are not perfect either, however, because carbon-14 from the atmosphere has to percolate down through seawater before it gets to marine organisms, and that can take years.
“You don’t know what the ocean is doing relative to the atmosphere,” lead author and Oxford University archaeological science professor Christopher Ramsey said.
The bits of leaf in Lake Suigetsu don’t have this problem: they provide a direct measure of radiocarbon in the atmosphere, according to Ramsey – which means a much more complete radiocarbon record free of the complications that affect the carbon in marine sediments.
The scientists examined the layers of sediment using a microscope and x-rays and took measurements of radiocarbon from preserved plant fragments in sediment cores collected from Lake Suigetsu. They also compared their data to the tree-ring record and other records to account for any ambiguities. Lake Suigetsu provided them with an ideal laboratory from which to take these data because of its annual layers of sediment, uniquely well-preserved because of the still and anoxic nature of the lake’s bottom.
Scientists have hoped to utilize Suigetsu’s annual layers for carbon dating since the early 1990s, but encountered some technical obstacles. “This is a realization of a 20-year-long Japanese dream,” University of Newcastle professor of quaternary sciences Takeshi Nakagawa said in a statement.