How Accurate is Carbon Dating?

Table of Contents

Introduction

When browsing the internet for the next cool thing that scientists are getting up to, you must have stumbled across an article or two about the Earth’s past. Maybe it was a story about a   fossil that tells you about how animals have evolved into the ones we see today! Or the discovery of an ancient species that doesn’t even exist anymore! Surely, you must have found yourself wondering: how can scientists tell how old these fossils really are?

 

Much of the practice of science is built on using the right kind of tools to answer your questions, and one very useful tool for archeologists and geologists is radiocarbon dating.

So, what is radiocarbon dating?

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Scientists use a method called radiocarbon dating to learn about the world’s past. Credit: Wikimedia/Nils Knötschke

With some organisms it’s quite easy to tell how old they are. If you’re curious about the age of a tree, you can count their rings. What about the rest?

 

A fossil that has been buried under the earth’s surface has its own internal clock. In order to read this internal clock, scientists use a method called radiocarbon dating, or carbon dating for short.

A bit about carbon

Do you know what you have in common with every other living thing that ever was and ever will be? You’re made up of carbon! Reaching into the atmosphere, plants take up carbon dioxide and use it to form complex organic molecules. In search of nutrients, animals seek out plants to eat and in turn take up carbon too! But here’s the thing; there’s more than one form, or isotope , of carbon.
The carbon-12 isotope. Credit: Wikimedia/Inkspace

It helps to remember that atoms are made up of things like protons, neutrons and electrons. The majority of carbon atoms have 6 neutrons with 6 protons. This is the carbon-12 isotope, named for the total of neutrons and protons. It makes up over 98% of all carbon atoms.

The carbon-14 isotope. Credit: Wikimedia/Inkspace

Another important kind of carbon that’s formed naturally is the carbon-14 isotope. High in the earth’s atmosphere, when high energy cosmic rays hit nitrogen atoms the cause the atoms to gain a neutron. Yes, you read that right: nitrogen. Now, this newly-formed nitrogen atom isn’t very stable with all this new energy, so it quickly drops one of its protons. Now it’s given rise to a whole new, more stable atom: the carbon-14 isotope, with 6 protons and 8 neutrons. (Nitrogen to carbon? Isn’t science amazing?)

 

Since we aren’t all too fussy about what kind of carbon we get, plants generally use a bit of carbon-14 isotopes as well as carbon-12 to make their compounds. As we move up the food chain, so do these carbon isotopes. So, all living things contain a lot of carbon-12 atoms, but also a significant amount of carbon-14. Now, why is this important, you ask? It’s all about the half-life

Did you say half-life?

When we spoke about the high energy nitrogen atoms that give rise to carbon-14 isotopes, we said that these nitrogen atoms aren’t particularly stable. Well, it turns out the carbon-14 isotope isn’t completely stable either!
The decay of the carbon-14 isotope. Credit: Wikimedia/Kurt Rosenkrantz

Carbon-14 atoms undergo a process called a beta-decay

 

Since these carbon-14 isotopes aren’t fully stable, over time they lose one of their electrons, and their extra neutron turns into a proton…now giving rise to the stable nitrogen 14 atom! (Now we’ve gone back to nitrogen from carbon; way to go, Science!)

 

Carbon-14 decay happens all the time. When this decay happens within living beings, the amount undergoing decay is offset by how much new carbon-14 is taken in. Something different happens after an organism dies. Since it isn’t possible to take in new carbon, the amount of carbon-14 isotope decreases and, most importantly, it happens at a steady rate! We know that it takes around 5,700 years for half of a sample of carbon-14 to decay into nitrogen-14. This duration is what we know as its half-life.

 

Back in 1946, Dr. William Libby and his lab proposed a creative method of looking at organic matter by measuring the ratio of two different types, or isotopes, of carbon atoms. Since carbon-12 is stable and doesn’t experience decay, and we know the rate of decay of carbon-14 atoms, all we need to do to predict the age of our fossil is to measure the ratio between the amount of carbon-12 isotopes and carbon-14 isotopes. This method helps us piece together an objective and more precise story of the lives lived thousands of years ago. In fact, Dr. William Libby went on to win the Nobel Prize, since he knew exactly how to make sense of this molecule called carbon!

 

Radiocarbon dating is very cool to learn about, but how accurate is it?

How accurate is radio-carbon dating?

The carbon decay principle is an extremely useful tool and has helped scientists make some great discoveries, but that doesn’t mean it’s completely faultless.

Since the carbon-14 isotope has a half-life of only around 5,700 years, radiocarbon dating generally works well for fossils that are at most 60,000 years old. Any older than that, and the amount of carbon-14 would be so little that it’s hard to make the right estimate.

 

Another important factor that impacts how we look at carbon dating is climate change! Carbon dating remains reliable if we can expect a predictable ratio between the two main isotopes of carbon. If there’s less carbon-14 relative to carbon-12 isotopes, we know the sample has been around for a long time, and if the ratio of carbon-14 to carbon-12 is almost the same, then it’s a relatively young sample!

 

For the longest time, the amounts of these isotopes in the atmosphere were consistent, so carbon dating was quite accurate. Over the past few decades, though, this calibration of carbon isotopes in the atmosphere has changed, largely due to the effects of burning fossil fuels.

Carbon dating is impacted due to the effects of burning fossil fuels. Credit: Wikimedia

While small changes aren’t usually a big deal, as we’ve burned more fossil fuels there’s a lot of extra carbon dioxide in the atmosphere. The carbon in it is so old (since it’s from fossils!) that almost all of its carbon-14 has decayed. This means there’s way more carbon-12 than there is carbon-14! Now, carbon dating is relative, and we can only date things back if we have a reference. The extra carbon-12 can make new materials that aren’t all that old seem like they’ve been around for thousands of years!

 

But there is a solution! Scientists have looked into different possible isotopes of carbon, like carbon-13, or have even used the same principles of decay with other elements!

 

Radiocarbon dating is just one method in a scientist’s toolbox. When combined with other methods of studying the world’s past, we have a pretty good idea of what went on when we weren’t around!

Glossary

Archeologist: An expert who studies remains, artifacts, and structures to make sense of humans and their history.

 

Beta-decay: A process by which atoms with too many protons or neutrons emits particles.

 

Electrons: A particle in the atom that carries a negative charge.

 

Fossil: The remains of old living things that have been preserved.

 

Fossil fuel: Fuel found in the Earth’s crust that comes from decomposing remains of plants and animals.

 

Geologist: A scientist who researches the matter that makes up the earth.

 

Half-life: The period of time in which the quantity of something is reduced to half of what it was originally.

 

Isotope: Atoms of an element that have the same number of protons but different neutrons.

 

Neutrons: Particles in an atom that are found inside the nucleus.

 

Protons: Particles in an atom that are found inside its nucleus and those that carry a positive charge.

 

Cosmic rays: high-energy particles that travel through space at nearly the speed of light.

Flesch Kincaid Grade Level: 8

 

Flesch Kincaid Reading Ease: 63.6

Gannon, M. I. (n.d.). Long-Awaited Update Arrives for Radiocarbon Dating. Scientific American. https://doi.org/10.1038/scientificamerican0920-12

How Does Radiocarbon Dating Work?- Instant Egghead. (n.d.). Scientific American. Retrieved February 6, 2023, from https://www.scientificamerican.com/video/how-does-radiocarbon-dating-work-i2012-11-30/

Magazine, S., & Panko, B. (n.d.). Thanks to Fossil Fuels, Carbon Dating Is in Jeopardy. One Scientist May Have an Easy Fix. Smithsonian Magazine. Retrieved February 6, 2023, from https://www.smithsonianmag.com/science-nature/carbon-dating-crucial-scientific-technique-jeopardy-thanks-our-pollution-heres-easy-way-fix-it-180961345/

Radiocarbon Dating. (n.d.). American Chemical Society. Retrieved February 6, 2023, from https://www.acs.org/education/whatischemistry/landmarks/radiocarbon-dating.html

Contributors

  • Swarna Ramakrishnan
    Swarna Ramakrishnan has been fascinated by the natural world ever since she was a young girl! She graduated from Azim Premji University, India with a Bachelor’s in Biology and a minor in applied mathematics. During her research, she trekked through the beautiful forests of the Western Ghats in India to answer questions about stomata and climate change. Currently, she is pursuing her Master’s in Biophysics from Ulm University, Germany. Swarna writes for Smore magazine to spread stories of nature in hopes of inspiring the next generation of scientists!

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