VOA常速英语:Lake Sediment Could Better Date Ancient Finds

Core samples found at the bottom of a Japanese lake could provide much more precise timelines for important archeological finds and climate-history questions.

Radiocarbon dating is the best-known and most widely used method to determine the age of organic material, such as bone or wood or plant matter. All such material contains radioactive carbon atoms, known as carbon-14, that decay at an understood and measurable rate.

Oxford University radiocarbon dating expert Christopher Ramsey and his colleagues were looking for organic material preserved for long periods of time in a still and airless environment, where radiocarbon levels would not have been affected by interactions with ocean water or groundwater.

They found what they were looking for in core samples taken from deep sediment at the bottom of Japan’s Lake Suigetsu. Ramsey, who reports on the find in Science, says the alternating layers of fossilized leaves and algae that settled to the lake-bottom offer a perfectly preserved record of more than 50,000 thousand years.

“What’s special about Lake Suigetsu is that we have an alternative dating technique that we can use there, which is essentially counting the annual layers, which are deposited in the sediment of Lake Suigetsu," Ramsey says. "And so putting that together with the radiocarbon dates, it gives us a sort of tool, if you like, for saying if we’ve got a radiocarbon measurement for a particular value, then we can now put that into an accurate, absolute date.”

Ramsey says radiocarbon as a dating tool must be anchored in time with some other technique to compare for accuracy. The only other physical record of atmospheric carbon comes from tree rings, which don’t go back nearly as far as the Japanese lake sediment.

“The step forward here is that for the first time we have a complete record that actually covers the whole last 50,000 years or so of radiocarbon in the atmosphere with known age samples," he says, "whereas before we only had that for the last 12,000 or 13,000 years.

The radiocarbon in the core-sample leaf fossils, like the carbon in tree rings, comes directly from the atmosphere. It is not subject to the chemical changes that can affect radiocarbon in ocean-floor sediment or in cave environments, where dripping groundwater and minerals form structures known as stalagmites and stalactites.

Ramsey says dating using the lake-sediment record is more accurate.

“People have tried to do this before, but they have been limited to using marine sediments and using stalagmites and stalactites in caves. And all of those have the problem that the carbon in the oceans and the stalagmites and stalactites isn’t exactly the same as that in the atmosphere. So it’s a bit more difficult to work out what’s going on with those.”

According to Ramsey, the new record will help refine the dating of organic material by centuries. It can also help clarify the chain of events that led to the advance and retreat of ice sheets during the last ice age.

“It will enable us to pinpoint much more precisely, exactly when changes take place in the environment and when we get changes in the archeological record, exactly how those relate to other changes that are taking place in the world which are recorded in things like the Greenland ice cores where we have a very good record of the climate.

Ramsey adds the improved radiocarbon reference will also provide more precise dates for when Neanderthals died out and modern humans spread across Europe.