Wednesday, October 19, 2011

Ageing maple leaves

a maple leaf in the sun
Yesterday, in the parking lot at work, a maple leaf rested on the pavement. The golden-hued morning light caught the leaf highlighting the red-tending-to-maroon tones. The leaf sharply contrasted the cold grey of the pavement, its vividness catching my eye. What if I picked up the leaf and saved it? Could archeologists in the far future figure out when the leaf fell from the tree?

Currently, we can estimate how old plant-based objects are using radiocarbon dating - often just called carbon dating. In 1949, Willard Libby and his team accurately estimated the age of the wood in an ancient Egyptian barge – a barge with a recorded age. This process works through knowing the ratio of carbon-12 (the ordinary stuff) to carbon-14 (a radioactive isotope) in the atmosphere.

Carbon-14 isn't particularly stable and decays quickly. It has a half-life of about 5,730 years - only a moment of time compared to the approximately 4.5 billion year half-life of uranium-238 (which is roughly the age of Earth). Continuously formed in the atmosphere by cosmic rays, carbon-14 reacts with oxygen becoming carbon dioxide. Plants take up some of this carbon dioxide along with carbon dioxide formed from the more abundant carbon-12. When the plant dies, no more carbon dioxide is taken in and the existing carbon-14 begins to decay.

If we assume the carbon-12 to carbon-14 ratio was the same when the plant died to now, using the decay rate of carbon-14 will give us the item's age (back to about 60,000 years). But, we know this ratio has fluctuated over time. To compensate, the age results are calibrated to something known like written records or tree rings. The biggest change to the carbon-12 to carbon-14 ratio has occurred in modern times through nuclear testing. Carbon-14 levels in the atmosphere were boosted around 1950 and peaked in the 1960's (at which time, testing bans were agreed to).

So, could a future archeologist figure out the are of my leaf using carbon dating? Probably not accurately because we've messed with the carbon-12 to carbon-14 ratio in our atmosphere. It would be more accurate for that archeologist to look at the date of this article.

As a tangent: At the end of the day when I returned to my car, the leaf was still there. Without the sunlight shining on it, the leaf looked brown and uninteresting.

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