Science for Non-Scientists: Carbon Dating
It’s time to get down and dirty with science and uncover one of the coolest tricks of chemistry, used by scientists and non-scientists alike, to understand the history of our planet, our species, and even the sun: carbon dating. The main use of carbon dating, as you may already know, is to determine roughly how old something is. It has been used to date the Dead Sea Scrolls, the Pyramids of Egypt, mummies, bones, and pretty much anything 50,000 years and younger. Along with that, carbon dating is used to study climate change and show that humans have increased the concentration of carbon dioxide in the atmosphere by burning fossil fuels. Carbon dating is a window into the past that has revolutionized our understanding of earth science, human anthropology, and archaeology, and is arguably the most exciting science technique used by non-scientists. So put on your science goggles and jump into the world of carbon dating!
I’m assuming that you are not a scientist and don’t remember much from high school biology or chemistry, so you can skip the first paragraph if you already know what a proton/neutron/atom is. For all you non-scientists, though, bear with me for a short (and painless!) review of chemistry. This is important. Consider the following as an adult picture book, since I’ve drawn some cute doodles to go along with the explanations. There are a couple asterisks (*) in the article, which indicate that you should scroll to the bottom if you want to get more detail on the subject.
Let’s start from the basics: atoms. An atom is made of protons and neutrons (and electrons, but that doesn’t really matter right now). One type of atom is nitrogen, which makes up almost 80% of our atmosphere. Another type of atom is carbon, infamously known for its role in climate change, but carbon is also present in literally everything that has ever been alive at some point, from trees to giraffes to you. By definition, carbon always has 6 protons. Nitrogen always has 7. They can have varying numbers of neutrons, but normally have the same number of neutrons as they do protons.
Now onto the fun stuff. Picture the earth from outer space. Planet earth is constantly being bombarded by cosmic rays, which is really just science jargon for a bunch of neutrons* traveling super fast in a big pack, coming from somewhere in outer space. Neutrons are tiny and invisible, so don’t go pulling out your telescope to look for them, but I’ve drawn a picture of what they might look like if you could see them. Upon impact with the earth, cosmic rays send showers of neutrons into our atmosphere.
Remember when I told you our atmosphere was made of mostly nitrogen? Neutrons from cosmic rays bump into nitrogen atoms as they shower down through the atmosphere. Occasionally, a neutron will hit a nitrogen atom just right, and do something cool. The neutron will hop on into the nitrogen atom upon impact, forcing a sad little proton out. In other words, the nitrogen atom will gain a neutron and lose a proton.
I’ll remind you, however, that nitrogen always has 7 protons, and when it no longer has 7 protons, it’s actually not considered nitrogen anymore. Our atom now has 6 protons, and by definition any atom with 6 protons is a type of carbon atom. This carbon atom, however, is not your average carbon atom. It’s unique in that it has a miss-matched number of protons and neutrons, which gives it some special properties that allow scientists to do the magic of carbon dating. This type of carbon is called carbon-14 (6 protons+ 8 neutrons =14), drawn above. Keep in mind that our basic run-of-the-mill carbon atom is called carbon-12 (6 protons + 6 neutrons = 12), which I drew in the very beginning of the article. The main difference between carbon-14 and carbon-12 is that carbon-14 is radioactive, and carbon-12 is not. This fact is crucial for carbon dating!
Let’s move on to the fun stuff. So we have all these carbon-14 atoms floating around in the atmosphere, along with a bunch of nitrogen and oxygen atoms. Carbon and oxygen, though, are happiest and most stable when they are together, so if they get close enough they will bond and form the world’s most famous molecule: carbon dioxide (CO2), which is composed of one carbon atom and two oxygen atoms.
Carbon dioxide is both a friend and an enemy of humanity. It is probably most famous for its role in climate change, since it is a product of the combustion of fossil fuels. On the other hand, carbon dioxide is also a critical component of photosynthesis, allowing plants to produce both their own food and oxygen for us to breathe. All plants except the gross ones that eat bugs (I’m talking to you, Venus Fly Trap) take in atmospheric carbon dioxide for photosynthesis.
Most of the carbon dioxide plants take in, though, is actually not made of carbon-14 , because the vast majority of carbon in our atmosphere is just garden variety carbon-12. Plants take in a mix of these two different types of carbon dioxide, most of which is made of carbon-12, with a small amount made of carbon-14 from cosmic rays.
Because plants absorb all this carbon in the form of carbon dioxide, and incorporate the carbon into their body chemistry, they are composed of the same ratio of carbon-12 to carbon-14 as the atmosphere! Because everything animals eat is either a plant or something that eats plants, every living organism, from a tree to a giraffe to a human, is also made of this same carbon-12 to carbon-14 ratio.
The ratio of carbon-12 to carbon-14 in the atmosphere is pretty stable, meaning that a giraffe living 40,000 years ago ate plants with the same percentage of carbon-14 as a giraffe in Africa eats today. Why does this matter? After a living creature dies, this ratio starts to change. The amount of carbon-12 stays the same no matter how much time has passed, because carbon-12 is super stable and happy with its even number of protons and neutrons. Carbon-14, however is radioactive. This means that carbon-14 will start to break down into other elements after the organism dies, so the percentage of carbon-14 in its body will decrease over time. Interestingly, when carbon-14 decays it reverts back into nitrogen-14, which is what it started as before it was bombarded by a cosmic ray. It breaks down by converting a neutron into a proton**, reverting back to its stable state of having an even number of protons and neutrons.
We finally know enough chemistry to start talking about carbon dating! Right at the time of death, an organism still has 100% of the carbon-12 and 100% of the carbon-14 it had when it was alive. As time passes, though, the carbon-14 will start to decay. Scientists have calculated the rate of decay for carbon-14, which means we know exactly how fast carbon-14 decays into nitrogen-14. From this rate of decay, we know that after 5,730 years, only half of the original carbon-14 in the dead organism will remain. After 11,460 years, only 25% of the original carbon-14 will remain, after 17,190 years only 12.5% will remain, and after 50,000 years just about all of the carbon-14 will be gone.
So, if you’re an archeologist trying to determine the age of some cool artifact you found, you can ship it off to a lab where a scientist will take a sample of the object and test for its chemical composition. You can then date the artifact by comparing its current carbon-12 to carbon-14 ratio with that of the atmosphere!
Now that you know how carbon dating works, let’s talk a bit about how it is used to understand climate change and prove that it is human-caused.
Fossil fuels, such as coal and petroleum, are made of ancient (up to 650 million years old) plant and animal remains that were smushed together by heat and pressure over time, a process that makes them combustible. Although fossil fuels are made of once-living organisms, they are so old— much older than 50,000 years— that they no longer contain any carbon-14. We humans combust fossil fuels every time we drive a car***, fly in a plane, or produce electricity from coal. The combustion of fossil fuels, as I’m sure you already know, releases carbon dioxide into the atmosphere. This carbon dioxide, however, contains only carbon-12. This means that since we started combusting fossil fuels on a large-scale during the industrial revolution, we have been flooding the atmosphere with carbon dioxide made of only carbon-12 atoms.
Since c14 is only produced by cosmic rays or the occasional lightening bold, its concentration in the atmosphere has largely remained constant over time. The concentration of carbon-12, however, has increased significantly over the past two hundred years , altering the once stable ratio of carbon-12/carbon-14. This change in ratio actually has a scientific name— the Suess effect—named after the scientist who discovered it while studying carbon decay. A rise of carbon-12 means a rise in CO2, which results in an increase in greenhouse gasses in the atmosphere and the climate warming effect associated with it. By looking at trends in the changing concentration of carbon-12 in the atmosphere in relation to the unchanging concentration of carbon-14, scientists have yet another method of proving that humans are altering the global climate by burning fossil fuels.
In short, carbon dating is not as complicated as you thought, climate change can be proved by looking at atmospheric carbon-14 and carbon-12 concentrations, and science is really cool. I hope you enjoyed my drawings, and thanks for reading! Stay tuned for more articles in my series on Science for Non-Scientists to see more of my cute doodles and learn more about cool science stuff.
*Cosmic rays are actually made mostly of protons, not neutrons. The protons collide with oxygen and nitrogen atoms in the atmosphere, creating secondary cosmic rays that are made of neutrons, which go on to generate carbon-14 by bumping into nitrogen atoms, as discussed in the article. For the sake of simplicity, I chose to leave out these details. If this intrigues you (which it should!), you can find more info on cosmic rays in this cool powerpoint from Dr. Cristina Lazzeroni at the University of Birmingham— fair warning, though, the entire presentation is in Comic Sans font.
**To go into more detail, carbon-14 undergoes a type of decay called beta decay. Beta decay occurs when an atom releases an electron and an antineutrino (kind of like a neutron’s evil twin), the process of which converts a neutron into a proton. Beta decay happens because it makes a radioactive atom more stable (AKA not radioactive).
***Yes, even your electric car contributes to the combustion of fossil fuels, because the electricity has to come from somewhere! The US still generates the 63% of its energy from fossil fuels, mostly coal and natural gas, according to the US Energy Information Administration. California isn’t much better, with 43% percent of the state’s energy coming from fossil fuels according to the California Energy Commission.