Sometimes in science there is an amazing occurrence where a breakthrough in one field provides unexpected answers to questions in an entirely different field. One such breakthrough is the technique of carbon dating that is used in archaeological digs all over the world. In order to really understand carbon dating we first need to know what an isotope is.
An atom can contain three different types of basic particles: protons, neutrons, and electrons. The protons and neutrons are contained in the atom’s core (or nucleus) and the electrons orbit the core a lot like planets orbiting a star. The fundamental chemistry of an atom is determined by the number of protons in its nucleus, which we call its Atomic Number. If you have an atom with six protons in the nucleus then what you have is a carbon atom. Seven protons? That’d be nitrogen. Seventy-nine protons? You’ve got a single atom of gold. For each atom there are allowable numbers of neutrons that can accompany the protons in the nucleus. We’d have to get deep into atomic physics to understand why, but for now lets just accept that we can’t just have any old number of neutrons in a particular atom. The number of neutrons we find determines the isotope. Since the number of protons identifies the species (carbon, nitrogen, gold, etc.) we can define the isotope by adding the number of neutrons to the number of protons and tacking that onto the name of the element. Since we’re talking about carbon dating let’s look strictly at carbon isotopes. The most common isotope of carbon is carbon-12, and since we know carbon always has six protons then 12-6=6 means there are six neutrons in a carbon-12 nucleus. Carbon-12 makes up about 99% of all the carbon atoms in the earth’s atmosphere, and is a “stable” element, meaning that an individual carbon-12 atom could be billions of years old. Carbon-13 (six protons, seven neutrons) is another stable carbon isotope and makes up about 1% of the carbon in earth’s atmosphere.
Some isotopes, however, are unstable. An unstable isotope is like a toddler on a long shopping trip. There’s a good chance that at some point that kid is going to go nuclear, but you don’t know when. Some toddlers are more stable than others, so they’ll usually last longer, but some kids will flip their shit as soon as they walk into the grocery store. Unstable isotopes – alternatively called radioisotopes – release energy in the form of radiation so that they can settle into a more stable state. Exactly when they will do this is impossible to predict and is governed by statistics, but if we have a lot of atoms (and we do when dealing with anything that you can see with the naked eye) then we can use those statistics to our advantage. It’s like when you have a lot of toddlers at a birthday party: you don’t know exactly when, but at some point during the two hour party some kid is going to cry – you can practically set your watch to it.
Let’s look at carbon-14, which makes up one out of every one trillion carbon atoms in earth’s atmosphere. With six protons and eight neutrons in its core, carbon-14 is an unstable carbon isotope with a half-life of about 5,700 years. If we start with one kilogram of pure carbon-14, the half-life tells us that in approximately 5,700 years there will be a half kilogram of carbon-14 remaining. However, this does not mean that in 11,400 years there is no carbon-14 at all, because (thanks to statistics) the rate that carbon-14 disappears slows down over time. Think about the last carbon-14 atom in the block: we’re right back to trying to predict when the shopping trip toddler is going to lose it. In fact, after 11,400 years we would have one-quarter kilogram of carbon-14 remaining. After 30,000 years we would be left with about a hundredth of a kilogram of carbon-14.
What is actually happening to that carbon-14? Remember that carbon-14 has six protons and eight neutrons. Eventually one of the neutrons in the carbon-14’s core questions its life choices and spits out an electron so it can turn into a proton. Now we have an atom that has seven protons and seven neutrons – anything with exactly seven protons is a nitrogen atom! So carbon-14 emits an electron and turns into nitrogen-14. By the old language of alchemists, carbon has been transmuted into nitrogen and essentially dark magic has happened. Except it’s not dark magic, it’s atomic physics, which is almost as cool.
Now we know where carbon-14 goes when it decays, but where did it come from in the first place? Lots of atoms are formed through nuclear processes in stars and spread throughout the galaxy by supernovas, but carbon-14 pretty much disappears within 50,000 years, which means that there must be some process on earth that is producing it because there just aren’t that many supernovas. Most naturally occurring carbon-14 comes from the upper atmosphere. Cosmic rays from outer space hit the atmosphere to produce neutrons and a variety of other particles. If one of these free-flying neutrons strikes a nitrogen-14 atom then carbon-14 is formed and a free-flying proton is generated (do the proton neutron addition to prove this to yourself). Basically the edge of the earth’s atmosphere has a lot of radiation flying around that allows really amazing stuff to happen.
Now we know where carbon-14 comes from, and because cosmic rays come in a pretty steady and constant stream, the rate of carbon-14 production in the atmosphere is somewhat constant. This carbon-14 (along with all the other carbon isotopes) can combine with oxygen to create carbon dioxide. We previously discussed how carbon from carbon dioxide is incorporated into plants, and from there into animals and humans by them eating the plants. Absolutely every living thing on earth has carbon-14 in it, and the ratio of carbon-14 compared to carbon-12 and carbon-13 in a human is about the same as the ratio in the atmosphere; our carbon isotopes are in equilibrium with our environment. When we die we stop taking in new carbon in the form of food, and we also stop eliminating carbon in the form of exhaled carbon dioxide, so our carbon levels freeze. Since carbon-14 decays over time the amount of that isotope in our dead bodies will decrease. This is how carbon dating works. Archaeologists measure the amount of radiation coming from carbon-14 in a dead body or in the burned wood from a prehistoric campfire and by determining the amount of all types of carbon present they get a ratio. Thanks to the known half life of carbon-14 they can calculate the age of the sample and date one prehistoric camp fire in Montana to a similar fire in South Africa, helping to create a global timeline over the 50,000 years that carbon dating is accurate to!
Dating a sample of 7,000 year old carbon assumes that you know how much carbon-14 was in it 7,000 years ago. Unfortunately, the production of carbon-14 is not exactly constant. High levels of cosmic rays in one year (perhaps due to a supernova explosion) can increase the amount of carbon-14 in the atmosphere. A lot of research goes into establishing a baseline level for the carbon ratio throughout history because the more accurately this is known then the more accurate carbon dating is. Keep in mind that all the fluctuations in the carbon ratio are naturally occurring, at least they were until about a hundred years ago. There are two causes of a significant change in the levels of carbon-14 in the atmosphere that occurred in the last century, and it shouldn’t surprise you to learn that they were both man-made.
Nuclear bombs create a lot of neutrons. High altitude testing of nuclear weapons in the 1950’s and 1960’s caused a massive increase in the conversion of nitrogen-14 to carbon-14. In fact, the atmospheric carbon-14 level between 1950 and 1965 doubled. The realization that atmospheric nuclear weapons testing increased the levels of radioactive carbon was one of the many pieces of data that led to the ratification of the Partial Nuclear Test Ban Treaty in 1963, which by international agreement prohibits the testing of nuclear weapons in the air, under water, or in outer space.
One thing about scientists is that they work with what they have, so given the existence of a well documented spike in atmospheric carbon-14 levels they set out to learn something about biology. Years after atmospheric testing stopped, it became possible to test human tissue and through “Bomb Effect carbon dating” figure out exactly when those tissues formed. Studies using this technique challenged long standing theories about how often brain cells were replaced, and how old typical muscle tissue is in the human body. You can’t make an omelet without breaking a few eggs, and when it comes to Bomb Effect carbon dating, scientists will both break the eggs and cook you one amazing omelet.
When a creature dies it stops taking in carbon from its environment, and as a result all its carbon-14 is free to decay away. Since fossil fuels come from plants and animals that died millions of years ago all the carbon-14 has long since decayed away. This carbon was cut off from the atmosphere and trapped deep within the earth for millions of years. When we as a society burn fossil fuels the ratio of carbon isotopes shifts because we are flooding the air with carbon-12 and carbon-13; the overall amount of carbon-14 doesn’t change, but now there is less of it in relation to the other forms of carbon. In a few decades it might be possible to perform a Bomb Effect style carbon dating on all of us, using the lack of carbon-14 rather than the abundance of it.
Carbon-14 is an amazing isotope. Its natural presence allows us to connect events across the world with a single calendar. The way it is naturally produced shows us that outer space affects our world in a variety of ways. Sadly, the effect of nuclear testing and the burning of fossil fuels makes it clear that human actions alter our environment in ways that we never account for. If our actions can affect literal tons of atoms, then we have a level of power that we must take very seriously.
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