Lake mud offers secret of more precise carbon dating

Posted by Kate Taylor

A muddy Japanese lake has yielded data that could be used to make radiocarbon dating more accurate, especially for older objects.

It could be used to refine estimates of the ages of organic material by hundreds of years, potentially providing far more accurate dating of the extinction of Neanderthals, or more detailed data on the advance and retreat of the ice sheets during the last glacial period.

In Lake Suigetsu, a layer of tiny, relatively light-colored diatoms blankets the floor each year, followed by a layer of darker sediments. The lake bottom is so still and anoxic that these layers have remained undisturbed over tens of thousands of years - and can now give a well-preserved record of the past 52,800 years.

Standard radiocarbon dating has certain drawbacks, in that the amounts of radiocarbon in the environment - and incorporated into growing organisms - can vary from year to year and between different parts of the global carbon cycle.

Adjusting for this requires long, known-age records with associated radiocarbon data. Some of the best come from marine sediments or cave formations, but need to be corrected on the basis of assumptions about how radiocarbon levels change in ocean water and groundwater.

However, the new lake record requires no such correction. The radiocarbon in the leaf fossils preserved in the sediment comes directly from the atmosphere, and isn't subject to the same processes that affect radiocarbon in marine sediments or cave formations.

The only other direct record of atmospheric carbon comes from tree rings and extends to 12,593 years ago. The Lake Suigetsu record stretches back 52,800 years.

"In most cases the radiocarbon levels deduced from marine and other records have not been too far wrong. However, having a truly terrestrial record gives us better resolution and confidence in radiocarbon dating," says Bronk Ramsey of the University of Oxford.

"It also allows us to look at the differences between the atmosphere and oceans and study the implications for our understanding of the marine environment as part of the global carbon cycle."

The team's lined up segments of their record with those of other records from the same time periods and generally found good agreement.

"Although this record will not result in major revisions of dates, for example in archaeology, there will be changes in detail that are of the order of hundreds of years," says Ramsey.

"Such changes can be very significant when you are trying to look at human responses to climate, often dated by other methods, for example through the Greenland ice cores. So, a more accurate calibrated time-scale will allow us to answer questions in archaeology, which previously we have not had the resolution to address."