"Every year the trees in our forests show the swing of Time's pendulum and put down a mark.They are chronographs, recording clocks, by which the succeeding seasons are set down through definite imprints," he wrote in the pages of National Geographic.
The unstable nature of carbon 14 (with a precise half-life that makes it easy to measure) means it is ideal as an absolute dating method.
When plants are alive the carbon-14 to carbon-12 ratio remains constant, but after they die the ratio begins to decline.
A measurement of the ratio, therefore, provides a way to estimate how much time has elapsed since the plant was alive.
“The discovery of past spikes in atmospheric radiocarbon activity, caused by major solar energetic particle events, has opened up new possibilities for high-precision chronometry,” the paper said.
Academics believe that powerful solar storms caused bursts of radiation that showered down on Earth in 775 to 994 CE.