Carbon 14 dating 2 (video) | Khan Academy
Dr Fiona Petchey is using carbon (C) to date artefacts of historical importance excavated from the Wairau Bar archaeological site in. In , Willard Libby proposed an innovative method for dating organic materials by measuring their content of carbon, a newly discovered radioactive.
If it doesn't gain an electron, it's just a hydrogen ion, a positive ion, either way, or a hydrogen nucleus. But this process-- and once again, it's not a typical process, but it happens every now and then-- this is how carbon forms. So this right here is carbon You can essentially view it as a nitrogen where one of the protons is replaced with a neutron.
How Does Radiocarbon Dating Work?- Instant Egghead
And what's interesting about this is this is constantly being formed in our atmosphere, not in huge quantities, but in reasonable quantities. So let me write this down. And let me be very clear. Let's look at the periodic table over here.
So carbon by definition has six protons, but the typical isotope, the most common isotope of carbon is carbon So carbon is the most common. So most of the carbon in your body is carbon But what's interesting is that a small fraction of carbon forms, and then this carbon can then also combine with oxygen to form carbon dioxide. And then that carbon dioxide gets absorbed into the rest of the atmosphere, into our oceans.
It can be fixed by plants. When people talk about carbon fixation, they're really talking about using mainly light energy from the sun to take gaseous carbon and turn it into actual kind of organic tissue. And so this carbon, it's constantly being formed. It makes its way into oceans-- it's already in the air, but it completely mixes through the whole atmosphere-- and the air.
Carbon 14 dating 1 (video) | Khan Academy
And then it makes its way into plants. And plants are really just made out of that fixed carbon, that carbon that was taken in gaseous form and put into, I guess you could say, into kind of a solid form, put it into a living form. That's what wood pretty much is. It gets put into plants, and then it gets put into the things that eat the plants. So that could be us. Now why is this even interesting? I've just explained a mechanism where some of our body, even though carbon is the most common isotope, some of our body, while we're living, gets made up of this carbon thing.
Well, the interesting thing is the only time you can take in this carbon is while you're alive, while you're eating new things. Because as soon as you die and you get buried under the ground, there's no way for the carbon to become part of your tissue anymore because you're not eating anything with new carbon And what's interesting here is once you die, you're not going to get any new carbon And that carbon that you did have at you're death is going to decay via beta decay-- and we learned about this-- back into nitrogen So kind of this process reverses.
So it'll decay back into nitrogen, and in beta decay you emit an electron and an electron anti-neutrino. I won't go into the details of that.
But essentially what you have happening here is you have one of the neutrons is turning into a proton and emitting this stuff in the process. Now why is this interesting? So I just said while you're living you have kind of straight-up carbon And carbon is constantly doing this decay thing.
Carbon 14 dating 2
But what's interesting is as soon as you die and you're not ingesting anymore plants, or breathing from the atmosphere if you are a plant, or fixing from the atmosphere.
And this even applies to plants. Once a plant dies, it's no longer taking in carbon dioxide from the atmosphere and turning it into new tissue.
- Carbon 14 dating 1
The carbon in that tissue gets frozen. And this carbon does this decay at a specific rate. And then you can use that rate to actually determine how long ago that thing must've died. So the rate at which this happens, so the rate of carbon decay, is essentially half disappears, half gone, in roughly 5, years. And this is actually called a half life.
And we talk about in other videos. This is called a half life. And I want to be clear here. You don't know which half of it's gone. It's a probabilistic thing. You can't just say all the carbon's on the left are going to decay and all the carbon's on the right aren't going to decay in that 5, years.
So over the course of 5, years, roughly half of them will have decayed. Now why is that interesting? Well, if you know that all living things have a certain proportion of carbon in their tissue, as kind of part of what makes them up, and then if you were to find some bone-- let's just say find some bone right here that you dig it up on some type of archaeology dig.
And I'm told this will work up to about 10, years. Up to 10, years old. I don't know of any 10, year old trees, I don't think anyone does, but maybe there's some remains of old trees. And you can look at their tree rings, and I think most of us are familiar with this idea that every year that a tree grows, it forms another layer of bark.
And so you can look back to that layer of bark just for the half life of carbon, and then figure out how much carbon was there in the atmosphere at that period in time. And so it's kind of a record of the atmosphere up to 10, years. If you want to go even further back, you can look at cave deposits, and the fancy word for these cave deposits are speleothems. You might be familiar with stalagmites. Those are those speleothems that are kind of coming out of the bottom of the cave, or stalactites.
Those are the speleothems that are coming from the top of the cave.
But the reason why these are useful is these are formed by calcium carbonate, so they have carbon in them, and slowly over, really, tens of thousands of years, the water in the cave deposits that calcium carbonate.
So it's a record of the fraction of carbon in some of those years. And you can go down to resolutions of as small as 10 years.