Someone asked me a question about carbon dating, and since most of us have probably come across the term at some point and since the idea of carbon dating has fascinated me from the first time I read about it when I was a kid, it seems like a dandy topic for an article. So, there are these two carbon atoms, and they sort of like each other. One of the atoms works up the courage to ask the other one to a movie. Voila! Carbon dating! Next week, we will talk about carbon break-up. Bah-dum-bum!
Just kidding. Carbon dating is a very precise method for determining the age of anything that contains carbon from a source that was once living. This can be anything from a leather belt that is found in a Viking burial to a Neanderthal skeleton to a frozen flower in a glacier. Like many things that seem so commonplace and obvious today, the idea behind carbon dating was really remarkable when Willard Libby, an American chemist, developed it in the 1940’s. The idea (now) seems really simple—that you can measure the amount of Carbon-14 in any sample of once-living material and use that to estimate the age of the sample. Yeah, it seems obvious and simple now that we’ve all been hearing about it for 75 years, but when it was a new idea in Willard Libby’s head, it wasn’t that obvious. It was, in fact, amazing enough of an idea that Libby won the Nobel Prize for his work.
So, everything that is alive has to grow and use energy—those are two of the characteristics of every living thing. In order to do that, all organisms have to take in carbon, which is a major component of practically all the molecules in living things, like carbohydrates, fats, proteins and nucleic acids (like DNA). Animals get their carbon from eating other things that contain carbon, like plants. Plants, on the other hand, get their carbon from carbon dioxide in the air, through the process of photosynthesis, using the energy from sunlight. Photosynthesis is an AMAZING process, and certainly too big a deal to cover right now, but the point is that plants take carbon from the air and use it to make sugars to supply their energy needs. Animals either eat plants to get the energy they need from the sugars contained in the plants, or they eat other animals that ate the plants. In any event, plants and animals are constantly taking in carbon while they are alive. When they die, obviously, they stop taking in carbon. Now that is only part of the story of carbon dating. This is the really cool part of how carbon dating works: carbon atoms are almost all what are called carbon-12. You might need to think back to one of my earlier columns to help you with this next part. Atoms of carbon-12 are composed of 6 protons and 6 neutrons (6+6=12) in the nucleus, with 6 electrons in orbit around that nucleus. There are, however, some carbon atoms that are different. All carbon atoms have 6 protons. They have to have six protons or they wouldn’t be carbon. If, for instance, an atom has seven protons, it isn’t carbon. It’s nitrogen, because anything with seven protons is nitrogen. So all carbon atoms have 6 protons. However, some of them have 7 neutrons, which makes them carbon-13 and some even have 8 neutrons, which makes them carbon-14. Atoms of an element with different numbers of neutrons are called isotopes of that element. In any sample of carbon, the carbon atoms can be found as one of three isotopes, carbon-12, which is the VAST majority of carbon at 99%; carbon-13, which is about 1%; and carbon-14 which is found in trace amounts.
Carbon-12 and carbon-13 are referred to as stable isotopes, meaning they don’t undergo any changes with time. Carbon-14, however, is an unstable isotope. It changes, or decays over time. In this change, which is very slow, one of the neutrons in the nucleus of the carbon-14 atoms breaks down, releasing an electron and becoming, in the process, a proton. When this happens, the atom goes from having 6 protons to having 7, which means the atom changes from carbon-14 to nitrogen-14, since anything with 7 protons is nitrogen.
This decay process, which is called beta decay, by the way, happens very slowly and, more importantly for our story today, at a very precise and predictable rate. Half of the carbon-14 atoms in any sample of carbon will turn into nitrogen-14 every 5,700 years. This means that half of the carbon-14 in a piece of wood will be gone is 5,700 years. After another 5,700 years, half of what remains will be gone (one quarter of the original amount). After another 5,700 years, half of that amount will be gone (leaving one-eighth). Then 1/16 after another 5,700 years, then 1/32, then 1/64 and so on. 5,700 years is called the half-life of carbon-14, and it is defined as the amount of time necessary for one half of the amount of an unstable isotope present in a sample to decay. Different isotopes have different half-lives. The half-life of uranium-235 (the isotope of uranium used in atomic bomb and nuclear reactors), for instance, is about 700 million years. That is also a topic for another story.
So, given that carbon-14 decays at a predictable rate, the amount of carbon-14 present in any sample of carbon can be used to measure age. The ratio of carbon-12 to carbon-14 in the environment is known, and constant (within known limits). Since all living things are constantly incorporating carbon from the environment into themselves when they are alive, as long as the organism is alive, that ratio will stay constant, since the organism is always taking in more carbon. However, when the organism dies, it stops taking in new carbon. Over time, the carbon-14 atoms decay, so the ration of carbon-12 to carbon-14 gets higher. After 5,700 years, only half of the carbon-14 that was there when the organism died remains. We have instruments that can measure these amounts, and by determining how much of the carbon-14 has decayed, we can determine how long ago the organism died. Due to the limits of detection, carbon dating is considered pretty accurate to measure the age of previously living samples up to about 50,000 years old. In samples older than that, there is so little carbon-14 left that the measurements become unreliable. Carbon dating is good, but not perfect. Due to lots of variations and limitations in measuring, dates derived from carbon dating are usually expressed as ranges, like “this leather shoe is from 8,000-10,000 years old”. That is usually more than precise enough for scientists to determine how things relate to each other and where they fit on a timeline. However, as our measuring instruments are constantly being improved, the accuracy of carbon dating is always getting better, too.
Source: Australian National University
Variations of the idea behind carbon dating, using other isotopes with different half-lives can also be used to estimate age, and since they don’t use carbon, these dating processes can be used on non—organic (“organic” in this sense means “carbon-containing”) materials, like rocks. The decay of uranium-235, for instance, can be used to determine the age of some geological samples that are billions of years old.
Our ability to explore and investigate the universe we live in has always been limited by our ability to measure and experiment. As our tools get better, our ability to explore and learn get better. Carbon dating was one of those revolutionary ideas that opened the door on our understanding of our past.
Good for Willard Libby!