Time-capsule’ Japanese lake sediment advances radiocarbon dating for older objects

A new series of radiocarbon measurements from Japan's Lake Suigetsu will give scientists a more accurate benchmark for dating materials, especially for older objects, according to a research team that included Oxford University's Radiocarbon Accelerator Unit.

The research team extracted cores of beautifully preserved layers of sediment, containing organic material (such as tree leaf and twig fossils), from the bottom of the Japanese lake where they had lain undisturbed for tens of thousands of years. As an article in the journal Science explains, the findings are hugely significant because they provide a much more precise way to examine radiocarbon ages of organic material for the entire 11,000-53,000-year time range.

For example, archaeologists should now be able to pinpoint more accurately the timing of the extinction of Neanderthals or the spread of modern humans into Europe.

At the Oxford Radiocarbon Accelerator Unit, Professor Christopher Ramsey with his doctoral student Richard Staff and chemist Dr Fiona Brock worked with two other radiocarbon laboratories (the NERC facility at East Kilbride, Scotland, and Groningen in the Netherlands) on the radiocarbon record from the lake.  This research is part of a large international research team, led by Professor Takeshi Nakagawa of Newcastle University, studying the cores for clues about past climate and environmental change.

Radiocarbon is continuously produced in the upper atmosphere. These roughly constant levels of radiocarbon from the atmosphere are then incorporated into all living organisms.  Once the organisms die, the radioactive isotope decays at a known rate, so by measuring the radiocarbon levels remaining in samples today scientists can work out how old things are. However, the complication in the calculation is that the initial amounts of radiocarbon in the environment, which are in turn incorporated into growing organisms, vary slightly from year to year and between different parts of the global carbon cycle.

The radiocarbon in the leaf fossils preserved in the sediment of Lake Suigetsu comes directly from the atmosphere and, as such, is not affected by the processes that can slightly change the radiocarbon levels found in marine sediments or cave formations. Before the publication of this new research, the longest and most important radiocarbon dating records came from such marine sediments or cave formations, but these needed to be corrected. At last, the cores from Lake Suigetsu provide a more complete, direct record of radiocarbon from the atmosphere without the need for further correction.