New Scientist said:
Mud is not renowned for its clarity, but the murky gloop at the bottom of a Japanese lake could provide the clearest, most accurate way yet of calibrating radiocarbon dates. The finding promises to help us pin down exactly when important events in prehistory occurred, including periods of climate change and the date of the Neanderthals' extinction.
We gauge the age of artefacts containing organic matter – like bones, plant remains and wooden tools – by comparing the amounts of the radioactive isotope carbon-14 and the stable carbon-12 that they contain. While it is alive, an organism's tissues contain a ratio of the two isotopes that matches the ratio in the atmosphere. Since carbon-14 decays at a steady rate, you can work backwards to figure out when the carbon isotope ratios would have matched that of the atmosphere, and so figure out how long ago the organism died.
The trouble is that the atmospheric ratio of carbon-14 to carbon-12 changes over time, making it difficult to establish exactly how much was in the organism when it was alive, and making the time-of-death calculation less accurate. That's where Lake Suigetsu comes in.