Science Sunday: Radiocarbon Dating

By: Meradeth Houston on 10:14 AM

Welcome back to Science Sunday. It's been heck of a week. Unfortunately not in a good way. My heart goes out to those in Connecticut that lost love ones. The events there are incomprehensible to me. As a teacher, even of adults, I just can't see how anyone would enter a classroom with a gun and open fire. It hurts my heart.

I do want to write about science today, and I have a great topic (I think) to share. Have you ever heard of radiocarbon dating? And by dating, I don't mean dinner and a movie. I mean, estimating the age of something :) (ooooh, bad joke, but I can't resist telling it!) Anyhow, this is a form of radiometric dating, or estimating the age of an artifact, layer of soil, or something else that uses the principle of radioactive decay and half-lives. Oh no, duck and cover, we are entering chemistry and physics territory!

I promise, I'll make this simple :)

Say you have some archaeologists excavating a site and they want to know how old things are, or your character finds some artifact and they have the money and means (some connection with the type of lab that does this work--it is pricey) to get an age estimate. Whatever the reason, they're able to get a sample of something that was once alive (this part is vital--it must have once been living!). Anyhow, if they're estimating the age of an archaeological site, picking the right thing to be dated is also vital, because you want to chose something that was associated with when people were actually there--not something that was dragged in later. Charcoal from a fireplace always works well :)

Okay, once you've got a sample, usually it's handled as little as possible. Otherwise it can be contaminated. And by that, I mean introducing carbon (only one of the most common elements on earth) into the sample. If this isn't possible, getting it tested will often mean attempting to get at a spot within the artifact that hasn't been touched a whole lot (this same kind of thing goes for ancient DNA samples too, FYI).

Okay, sample obtained, and usually it's then sent to a lab. The process for testing the radiocarbon age isn't something that can be done at home. I have a friend that used to work at one of these labs and it's highly tricky. What they're basically going to do is measure the ratio of the unstable isotope of carbon (carbon 14) and the normal carbon isotope (carbon 12). It's this ratio that's important, and allows for estimating age.

A word about Carbon 14: it's made by high energy particles (like cosmic rays) entering the atmosphere from space, messing up carbon atoms and introducing two extra neutrons (you know, the neutral bits of matter found in the nucleus of the atom with the protons--there's some extra complexity here that I'm going to skip over...). Carbon 13 can also be made, but it's a little more rare. This Carbon 14 is found at a fairly stable rate in the world (though fluctuations for the last few thousand years or more have been measured), and it is continuously degrading (turning into) carbon 12 by losing those extra neutrons.

During life, a person is constantly consuming carbon, both the 12 and 14 varieties, through the food we eat, water we drink, etc. So all living things have about the same ratio of carbon 12 to 14 that is in the atmosphere. But once something dies, it is no longer absorbing Carbon 12 or 14, and the amount of the unstable Carbon 14 starts to decay in to Carbon 12. This will happen at a pretty predictable rate, based on the half-life of the isotope (by half-life I mean how long, on average, it takes for half the unstable isotope to decay into its more stable form--Carbon 12). This is about 5000 years for carbon, which is actually pretty quick :) The trick is to measure how much of each of these there are in the sample.

So, the ratio of how much C14 and C12 there is left in a once-living sample allows for telling how old something is! Pretty cool, right?

The thing about this short half-life is that it only allows for dating stuff that's less than 50k years old. Any older than that, and all the carbon 14 is decayed and gone. There are other methods, based on the same principle (like Potassium-Argon, or Uranium-Lead dating) that have much longer half-lives and allow for dating stuff that's much older.

Anyhow, that's it in a nutshell. Questions? Possible writing applications?