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  • Hey there, and welcome to Brain Stuff.

  • I'm Josh Clark and this is the Brain Stuff where I explain to you how carbon-14 dating works.

  • Carbon-14 dating, which we also just call carbon dating, is a form of radiometric dating.

  • And all it is, is measuring the decay of a certain type of atom found in a once living organism to determine when it was last alive.

  • And all of this starts in the up, up, upper atmosphere of Earth, where it's constantly bombarded by cosmic rays.

  • And one of the things these cosmic rays do is knock neutrons off of some atoms and protons off of others and before you know it, the nice, stable, family-man nitrogen-14 atom is all wrapped up and gone crazy, and becomes what's known as a carbon-14 atom, which is radioactive.

  • Now, carbon-14 atoms aren't the only ones in the upper atmosphere.

  • There's also carbon-12 atoms.

  • Carbon-12 atoms are in much more abundance and they're pretty stable.

  • Carbon-14 atoms are, again, radioactive and unstable.

  • But, they're formed at a reliable, steady rate, so at any point in time, we have a pretty good idea of the ratio of carbon-12 atoms to carbon-14 atoms.

  • Got that?

  • It's important.

  • Now, carbon dioxide is essential to life here on Earth.

  • Plants breathe it in, animals eat the plants, we eat the animals and the plants, and these carbon molecules, carbon-14 and 12 that make up the carbon dioxide, get in everything.

  • What's neat is that the ratio of carbon-12 to carbon-14 found in all these living things here on Earth is pretty much the same as what's in the atmosphere.

  • Which means it's predictable.

  • And some very, very smart scientists have figured out that carbon-14 actually decays at a predictable rate.

  • Carbon-14, like all radioactive particles, has what's called a half-life.

  • Now the half-life is the amount of time it takes for the number of radioactive particles in a sample to decrease by half.

  • Carbon-14 has a half-life of 5,730 years.

  • That means that after 5,730 years, the amount of carbon-14 atoms in a plant that you found fossilized will be half of what it was the last time that plant took a breath of life.

  • After 11,460 years, which is two half-lives, there'll be a quarter of the amount of carbon-14 that was originally present.

  • And then after 17,190 years, you'll have just an eighth of the number of carbon-14.

  • And so on, and so on, until there's none left.

  • And this is actually one of the limitations of carbon-14 dating, that eventually you're going to go far enough back in time that all of the carbon-14 atoms have decayed.

  • And you don't know whether this took place a day before or 100,000 years before.

  • Which means the time limit that you can date things using carbon-14 is roughly 50,000 years.

  • But as long as you are trying to date something that lived on Earth within the last 50,000 years, you can figure out roughly when it was last alive.

  • The way that you do that is by measuring the rate of decay of carbon-14 atoms compared to the slow and steady carbon-12 atoms that are also present in there.

  • Presto, chango, you've got some carbon-14 dating, and all of a sudden you say, "Oh my God, this wooden axe handle is 12,000 years old."

  • Pretty sweet stuff.

  • And this whole thing gets even more interesting when you realize that future archaeologists are going to have a lot of trouble using carbon-14 dating thanks to us, humans of the present time.

  • Our industrial activities have been pumping CO2 into the atmosphere and really messing with the ratio of carbon-12 to carbon-14 and, even more astounding, all of the nuclear bombs that we set off in the mid-20th century, well those messed with the atmospheric ratios, too.

  • So, good luck with all of that, future anthropologists and archaeologists.

  • Sorry.

Hey there, and welcome to Brain Stuff.

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How Carbon Dating Works

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