Carbon dating is used to work out the age of organic material. But the technique assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock. Since the 1960s, scientists have started accounting for the variations by calibrating the clock against the known ages of tree rings. The problem is that tree rings provide a direct record that only goes as far back as about 14,000 years.
Now, using sediment from bed of Lake Suigetsu, west of Tokyo, researchers have pushed the calibration limit back much further. Two distinct sediment layers have formed in the lake every summer and winter over tens of thousands of years. The researchers collected roughly 70-metre core samples from the lake and painstakingly counted the layers to come up with a direct record stretching back 52,000 years.
The recalibrated clock could help to narrow the window of key events in human history. Take the extinction of Neanderthals, which occurred in western Europe less than 30,000 years ago. Archaeologists disagree over the effects changing climate and competition from recently arriving humans had on the Neanderthals' demise. The more accurate carbon clock should yield better dates for any overlap of humans and Neanderthals, as well as for determining how climate changes influenced the extinction of Neanderthals."