Cosmology and astronomy
Carbon 14 dating 1
Carbon 14 Dating 1. Created by Sal Khan.
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- If the carbon14 used to date an organism, let's say an elephant is derived mainly from the plants ingested, and the elephant is dated by comparing it's ratio of carbon atoms to another elephant's ratio of carbon atoms, how do you account for the amount of plants the elephant actually ate? For illustrative purposes, let's assume elephant A ate mostly peanuts and a little grass and elephant B ate mostly grass. If they were both born at the same time, wouldn't Elephant A be dated older?(53 votes)
- The ratio between C14 and C12 is important not the absolute value of C14 atoms. And that ratio is reflected everywhere. It will be the same in the grass as it is in the peanuts.(35 votes)
- How accurately in time can one measure the decay of C-14? The video speak in terms of years, but can one get the time down to the month, day, hour?(14 votes)
- The best precision is probably on the order of several years. There are several sources of error. For example, the production rate of 14C varies over time with changes in the Earth's orbit. We can only estimate this factor by back-calculating orbital parameters. There is also a little bit of fractionation of C by organisms such that either 14C or 12C is preferentially incorporated into tissue. And then there are the analytical uncertainties, which include measurement errors and the uncertainty of the decay constant.(19 votes)
- Interesting video... but I don't understand how one could know the exact "amount of Carbon 14" that has accumulated in one given bone.
"Every living thing has a certain proportion of Carbon 14 in their tissue" ... how do we know what that proportion is?
Is it a fraction of another element that's not decaying?!(7 votes)
- All elements can have different numbers of neutrons in their nucleuses. We call them different isotopes, and name them by the sum of the number of protons and neutrons. All carbon has six protons -- that is what makes it carbon -- and most carbon has six neutrons, making carbon-12 the normal isotope of carbon. About 99% of natural carbon is carbon-12. There is also another stable, non-radioactive isotope of carbon with seven neutrons: carbon-13. It makes up about 1% of natural carbon. The type of carbon that we are interested in, because it is radioactive, is carbon with eight neutrons: carbon-14. It makes up about one part in a trillion of natural carbon.
Since natural carbon contains all these isotopes mixed together, anything that takes up carbon from natural sources will have the same mix of isotopes in its carbon. Now the key (and really cool) thing here is that carbon-14 is continuously produced in the atmosphere by cosmic rays. Neutrons produced by the cosmic rays hit nitrogen-14 (which makes up about 78% of the atmosphere) and knocks out a proton, turning it into carbon-14. This carbon-14 then reacts with oxygen to form CO2, and that is absorbed by plants and algae in photosynthesis. This is how it is mixed into the other natural carbon. Since living things continuously replace their atoms while they live, but stop when they die, the ratio of carbon-14 will be the same as the natural ratio as long as they live, but will start dropping when they die due to radioactive decay. This is the basis for radiocarbon dating.
The production of carbon-14 has been pretty constant for a long time, so the proportion of carbon-14 in natural carbon was pretty similar in the past. There have been changes however, which we account for with several calibration methods which you can read about here: https://en.wikipedia.org/wiki/Radiocarbon_dating#Calibration
Other more recent changes in the proportion of carbon-14 have been caused by nuclear tests (which produced a lot of carbon-14) and burning of fossil fuels (which releases a lot of old carbon, without any carbon-14, thereby reducing the proportion of carbon-14 in natural carbon).(16 votes)
- At2:20"Neutron hits a N14 in just the right way, the neutron replaces the proton."
This sounds like a traumatic event, but having precise results at a constant rate.
1. Are the neutrons travelling at speed of light?
2. Is this demonstrated in particle accelerators?
3. Why aren't the electrons messed up?(8 votes)
- No, the neutrons are not travelling at the speed of light, since only photons do.(8 votes)
- Is it possible for a bone to have no Carbon 14 at all? Then how would they date it.(5 votes)
- Up to about 58,000 to 62,000 years (wikipedia "carbon dating") is the effective maximum of Carbon dating. Other methods to date the bone would include other isotopes, either in the bone or in surrounding material, or inference from the geological layer the bone is present in (usually via nearby volcanic layers, which tend to have nice datable isotopes in them).(15 votes)
- At12:13. To get accurate dates we must be assuming that the neutron bombardment has always been constant in the past. After all we weren't recording any data on this just one half life ago. What if the neutron bombardment were greater or lesser in the past?(6 votes)
- Yes, this is an important point. The intensity of cosmic ray bombardment depends on the strength of the magnetic field of the Earth, which has varied in the past. The atmospheric 14C concentration is therefore not constant, and past atmospheric 14C concentrations must be calibrated using other methods, including dendrochronology and K/Ar dating.(2 votes)
- What is an electron anti-neutrino Sal talks about at6:30?(3 votes)
- There are particles called neutrinos, there are 3 types of neutrinos one associated with the electron, muon and tau. The neutrino associated with the electron is called an electron neutrino. Every article has an anti-particle so the electron anti-neutrino is the anti-particle of the electron neutrino.(5 votes)
- Is it possible to perform a dating of a sample from Mars?(3 votes)
- Not carbon-14 dating. It only works for living organisms on Earth where we know the historical atmospheric production rate and the exchange of carbon cuts off at their death. Also, it is restricted to only around 50,000 years max. So, it is pretty much useless in geological dating.
We can use other radiometric dating techniques though.(3 votes)
- doesn't the atom have the same amount of electrons and protons, so when it bumps of a proton, doesn't a electron fall off(4 votes)
- If an atom undergoes nuclear decay that causes it to have fewer protons in the nucleus, then it will become a negative ion (more electrons than protons) until it gets rid of the excess electrons, which will generally happen pretty easily(1 vote)
- what if we found a bone with 0% carbon 14? we could expect that it has an age above 11,460 years (which is 5,730 years * 2) , but how could we specify its age exactly? it may has a million years old(2 votes)
- Half life is how long it takes for half of the material to decay. Take for example C-14. If we started with 10 grams of C-14, after 5730 years, we would expect to find half of that amount to remain, or 5 grams. Then, after another 5730 years (11,460 years total) half of that 5 grams would decay, leaving us with 2.5 grams, a quarter of the original 10 grams. This method works only so far back though as C-14 is rare to begin with so, as it decays, it becomes more difficult to detect and measure accurately. But for older objects where C-14 isn't reliable, we have other radiometric dating methods using other isotopes.(3 votes)
What I want to do in this video is kind of introduce you to the idea of, one, how carbon-14 comes about, and how it gets into all living things. And then either later in this video or in future videos we'll talk about how it's actually used to date things, how we use it actually figure out that that bone is 12,000 years old, or that person died 18,000 years ago, whatever it might be. So let me draw the Earth. So let me just draw the surface of the Earth like that. It's just a little section of the surface of the Earth. And then we have the atmosphere of the Earth. I'll draw that in yellow. So then you have the Earth's atmosphere right over here. Let me write that down, atmosphere. And 78%, the most abundant element in our atmosphere is nitrogen. It is 78% nitrogen. And I'll write nitrogen. Its symbol is just N. And it has seven protons, and it also has seven neutrons. So it has an atomic mass of roughly 14. Then this is the most typical isotope of nitrogen. And we talk about the word isotope in the chemistry playlist. An isotope, the protons define what element it is. But this number up here can change depending on the number of neutrons you have. So the different versions of a given element, those are each called isotopes. I just view in my head as versions of an element. So anyway, we have our atmosphere, and then coming from our sun, we have what's commonly called cosmic rays, but they're actually not rays. They're cosmic particles. You can view them as just single protons, which is the same thing as a hydrogen nucleus. They can also be alpha particles, which is the same thing as a helium nucleus. And there's even a few electrons. And they're going to come in, and they're going to bump into things in our atmosphere, and they're actually going to form neutrons. So they're actually going to form neutrons. And we'll show a neutron with a lowercase n, and a 1 for its mass number. And we don't write anything, because it has no protons down here. Like we had for nitrogen, we had seven protons. So it's not really an element. It is a subatomic particle. But you have these neutrons form. And every now and then-- and let's just be clear-- this isn't like a typical reaction. But every now and then one of those neutrons will bump into one of the nitrogen-14's in just the right way so that it bumps off one of the protons in the nitrogen and essentially replaces that proton with itself. So let me make it clear. So it bumps off one of the protons. So instead of seven protons we now have six protons. But this number 14 doesn't go down to 13 because it replaces it with itself. So this still stays at 14. And now since it only has six protons, this is no longer nitrogen, by definition. This is now carbon. And that proton that was bumped off just kind of gets emitted. So then let me just do that in another color. So plus. And a proton that's just flying around, you could call that hydrogen 1. And it can gain an electron some ways. 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-14 forms. So this right here is carbon-14. You can essentially view it as a nitrogen-14 where one of the protons is replaced with a neutron. 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. Constant formation. 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-12. So carbon-12 is the most common. So most of the carbon in your body is carbon-12. But what's interesting is that a small fraction of carbon-14 forms, and then this carbon-14 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-14, 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. 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-12 is the most common isotope, some of our body, while we're living, gets made up of this carbon-14 thing. Well, the interesting thing is the only time you can take in this carbon-14 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-14 to become part of your tissue anymore because you're not eating anything with new carbon-14. And what's interesting here is once you die, you're not going to get any new carbon-14. And that carbon-14 that you did have at you're death is going to decay via beta decay-- and we learned about this-- back into nitrogen-14. So kind of this process reverses. So it'll decay back into nitrogen-14, 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-14. And carbon-14 is constantly doing this decay thing. 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. The carbon-14 in that tissue gets frozen. And this carbon-14 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-14 decay, is essentially half disappears, half gone, in roughly 5,730 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-14's on the left are going to decay and all the carbon-14's on the right aren't going to decay in that 5,730 years. What it's essentially saying is any given carbon-14 atom has a 50% chance of decaying into nitrogen-14 in 5,730 years. So over the course of 5,730 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-14 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 you say, hey, that bone has one half the carbon-14 of all the living things that you see right now. It would be a pretty reasonable estimate to say, well, that thing must be 5,730 years old. Even better, maybe you dig a little deeper, and you find another bone. Maybe a couple of feet even deeper. And you say, wow, you know this thing right over here has 1/4 the carbon-14 that I would expect to find in something living. So how old is this? Well, if it only has 1/4 the carbon-14 it must have gone through two half lives. After one half life, it would have had 1/2 the carbon. And then after another half life, half of that also turns into a nitrogen-14. And so this would involve two half lives, which is the same thing as 2 times 5,730 years. Or you would say that this thing is what? You'd say this thing is 11,460 years old, give or take.