Today’s fasctismal: The oldest age that carbon-14 can give is 114,600 years; the youngest is 570 years.
If you have ever cleaned up a teenager’s room, then you have probably discovered one of the most fundamental tools of archeology: relative dating. (You’ve also discovered what it feels like to excavate a midden pile.) As you dug down through the layers of trash, petrified food, and old homework assignments, you probably noticed that the older stuff was on the bottom and the newer stuff was on the top. But what might surprise you more than the Wall Street Journal hidden under your child’s bed is the fact that until 1949, relative dating was about the only method that archeologists had for dating really old artifacts.
That’s because very few artifacts come with a date on them. And even those that do come with an identifying mark, such as coins, only give you an approximate range of dates; for example, a coin with Julius Caesar’s face on it was probably minted sometime between 50 BCE (when he conquered Gaul) and 44 BCE (when .his political opponents put a permanent end to his ambitions). And the older something is, the less likely it is to have any sort of identifying mark; an empty, twelve hundred year old clam shell from the Spiro Mound looks an awful lot like an empty, fifteen thousand year old clam shell from Siberia.
Archeologists have come up with several methods for working around this problem (e.g., by counting tree rings), but they fail more often than not (after all, how many tree rings are there in a clay pot?). They needed something more. They needed a method that would work on almost everything and that could be easily verified. And, in 1949, a chemist by the name of Willard Libby gave it to them. He realized that by comparing the amount of carbon-14 that was in an object to the amount of carbon-12, he could tell how old something was. But, because carbon-14 had a half-life of 5,730 years, it could only be used to measure things that were between 0.1 and 20 half lives; that is, things that were no younger than 570 years old and no older than 114,600 years old.
But how does carbon dating work? In lieu of my usual citizen science link, tonight I’ll give you an experiment that you can do at home to understand this basic concept (Teachers: This works really well in a large class if you ad up everyone’s numbers.). To do the experiment, you’ll need 84 pennies, 16 nickles, and 16 dimes. We’ll pretend that the pennies are atoms of carbon-12; because carbon-12 is stable, it doesn’t decay. A carbon-12 atom today will still be a carbon-12 atom 100,000 years from now. And we’ll pretend that the dimes are carbon-14 atoms. Carbon-14 is unstable; in 5,730 years, half of the carbon-14 that is present today will decay into nitrogen-14. (We never run out of carbon-14 because it is always being created by cosmic rays hitting nitrogen-14 in the atmosphere and turning it into carbon-14.)
Now a living thing will take in carbon-12 and carbon-14, so the proportion of the two atoms will be roughly the same as is in the atmosphere. But once it dies, it stops adding new carbon. As a result, when the carbon-14 decays it changes the ratio of the carbon atoms. To see that, we need to do our experiment. Start by placing the pennies in a pile and lining up the dimes, all heads up. (If you want, you can draw a dead critter around the money.) This is what the ratio of carbon-14 to carbon-12 looked like right after the critter died. There were 84 pennies/carbon-12 atoms and 16 dimes/carbon-14 atoms. (This was a very small critter.)
Since we don’t want to wait 5,730 years for the atoms to decay naturally, we’ll flip the dimes, one by one. If the dime comes up heads, put it back in the critter because it didn’t decay. But if it comes tails, the carbon-14 atom decayed and turned into nitrogen-14 (aka, a nickle). You’ll probably have about eight of the dimes decay, so your new ratio will be 86 pennies/carbon-12 atoms to 8 dimes/carbon-14 atoms. Now flip the dimes again, once more replacing those that come up tails with nickles. Odds are that you’ll lose about 4 dimes this time and your ratio will be 86 pennies/carbon-12 atoms to 4 dimes/carbon-14 atoms. Do it again and you’ll get something close to 86 pennies/carbon-12 atoms to 2 dimes/carbon-14 atoms.
As you can see from the experiment, the ratio can tell you when a critter, such as a possum or a palm tree, died. And if that critter was then used to make something else, such as a shoe or a house, then we know about when the something else was made. So all you have to do to find out how old something is is measure the ratio of the carbon-14 to the carbon-12 in it. Pretty nifty, huh?
But I’m willing to bet that your ratios didn’t exactly match mine. That’s because we only used a very few atoms; in most living things, there are quadrillions of carbon atoms instead of just 100. But there are still some variances in the ratios because radioactive decay happens randomly. As a result, most carbon-14 ages have an error of about 3-5% (i.e., a 570-year old sample is probably somewhere between 540 and 600 years old).
So that’s our experiment on carbon dating. I hope you enjoyed it; I’ll be doing more things like this in the new year. And if you didn’t like it, remember that at least it was better than cleaning your room…