A few weeks ago some friends called to chat. They were on their way to see Mount Rushmore in South Dakota and were wondering about a place nearby that we had mentioned as being very interesting. The place is called The Mammoth Site and is in Hot Springs, South Dakota, not far from Rushmore. This place is absolutely fascinating, and we recommend it to anyone who travels in that direction.

Our friends were pretty impressed with the site and its many intact fossils from approximately 190,000-140,000 years ago to the late Ice Age. With over 60 Columbian mammoths (the largest of the mammoths) and the remains of many other mammals, and with active excavation occurring, this completely enclosed facility is one not to be missed.

However, my friend had a question—  how do they know how old these fossils are, and how accurate is their estimate? I quickly replied to his text that radiocarbon-14 dating is the technique used and that, while you can’t set your watch by it, it is a very accurate technique. As I thought about it though, I realized I didn’t know all that much about the process and decided to delve a little deeper.

Radiocarbon-14, often just called carbon-14 or carbon dating, is a method devised to determine the age of organic material. It isn’t of use for determining the age of things like rocks, and you will see why shortly.

Carbon-14 is one of three naturally occurring carbon isotopes: carbon-12, carbon-13 and carbon-14. The first two are stable and not radioactive. However, carbon-14 is radioactive, and because it is, it decays (the process by which an unstable atomic nucleus loses energy by radiation) at a specific rate, with a half-life, the time it takes half of a sample to decay, of about 5,730 years. With that in mind, you might think that carbon-14 would eventually fade from the scene. However, it is always being created way over our heads in the stratosphere as galactic and solar cosmic rays bombard nitrogen-14 atoms and change them into carbon-14. Once changed, these atoms combine with oxygen atoms to form first carbon monoxide, then carbon dioxide.

This radioactive carbon dioxide is quickly assimilated into the atmosphere and oceans and is readily picked up by plants. Before you panic and wonder why we aren’t all glowing with radioactivity, the ratio is 1.25 atoms of carbon-14 to 1,012 atoms of carbon-12, with another 1% as carbon-13. In a normal environment, all animals and plants have internal ratios of carbon-14 in balance with the environment. We constantly exchange carbon with the atmosphere. Rock, on the other hand, does not absorb or eat carbon-14, making this a technique only usable on organic material.

However, when an organism dies, this balanced exchange no longer occurs. The body has what it has, and the carbon-14 begins to decay at a steady rate. Let’s say that a tree died 5,730 years ago. If sampled today, it would have half the carbon-14 that is in the environment, and the ratio of carbon-14 to carbon-12 in its remains will diminish. In another 5,730 years, it would have half of the carbon-14 of what was left when it died, or 25% And again, in another 5,730 years, half of that and so on. It is this steady rate of decay that makes this an accurate technique for aging. The proportion of carbon-14 can be used to determine how long it has been since a given sample stopped exchanging carbon — the older the sample, the less carbon-14.

This technique was discovered by Willard Libby in about 1945. In his first attempts with carbon-14 dating, Libby wanted to test against something known. He and James Arnold tested two samples taken from the tombs of two Egyptian kings, Zoser and Sneferu, which had already been independently dated to 2625 BC ± 75 years. Libby-Arnold were able to carbon date the samples to an average of 2800 BC ± 250 years. Later tests using tree rings also verified the technique.

Modern times have created a few challenges that need to be factored in when dating a sample. One of the largest is that it assumes a steady rate of carbon in the atmosphere over several thousand years. Burning of fossil fuels and nuclear testing in the 1950s and 1960s have both monkeyed with that and need to be factored in.

One last thing: carbon-14 can only accurately date organic materials as old as about 60,000 years. For instance, petroleum oil takes much longer to form from organics than 60,000 years and therefore has almost no carbon-14. So, I was wrong to tell my friend that carbon dating was used on fossils because they are rock (no longer organic) and they are much older than 60,000 years. How do scientists date much older remains? More on that next time.