50,000-year-old sediment from Japanese lake improves carbon dating

At the bottom of a lake near Japan’s Wakasa Bay, more than 50,000 years of history has been pulled out of the ground in the form of sediment and leaves.

The information contained in those samples will allow scientists to determine the age of organic materials and fossils with new clarity by improving carbon dating, according to a study published Thursday in the journal Science.

Carbon dating works by detecting the relative amounts of two varieties of carbon: carbon-14, or C-14, and carbon-12, or C-12. Animals take up both varieties during our lifetimes, but only C-14, which is also known as radiocarbon, decays over time. That means that the amount of C-14 relative to C-12 can be used to determine how much time has passed since the fossil was alive.

But the amount of C-14 in the atmosphere changes over time, which has the potential to throw off the calculations. So scientists need ways to correlate their measurements to fossils whose timelines are known. The best examples of this are tree rings, as scientists can relate the C-14 levels to the rings that build up over time. But the oldest tree ring examples are only about 13,000 years old.

The lake in Japan, called Lake Suigetsu, is ideal for improving carbon dating because a light layer and dark layer of sediment are put down on the bottom every year. Add in a few organic materials, like leaves that have sunk to the bottom, and you’ve got a perfect carbon dating scenario. Scientists can date the leaves and then correlate those dates with their location in the layers of sediment. The researchers used this approach on hundreds of samples.

The result? A near-continuous record of atmospheric C-14 over a 40,000-year span, from 53,000 years ago to 13,000 years ago. The data will allow scientists to adjust their “clocks,” changing carbon dates by hundreds of years.

That may not seem like much when we’re talking about 50,000 years. But, as the researchers note, those data may help resolve scholarly debates about when Neanderthals disappeared, or whether certain climatic events led to human expansions across the planet.

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