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Mr. Andersen explains how carbon-14 dating can be used to date ancient material. The half-life of radioactive carbon into nitrogen is also discussed.
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Transcript Provided by YouTube:
00:06
Hi. This is Mr. Andersen and today I’m going to be talking about radiocarbon
00:10
dating. Radiocarbon dating is a way that we use carbon and the amount of carbon 14 left
00:17
in an object to figure out how old that object is. So if we find for example a piece of wood
00:22
that we think was used by ancient humans, we want to figure out how long ago that was,
00:27
we can use radiocarbon dating to do that. Before we talk about that we should probably
00:31
talk about what carbon 14 is. There are three types of carbon that we have on our planet.
00:36
Good old run of the mill carbon 12 has 6 protons, 6 neutrons and that’s going to be 99 percent
00:44
of the carbon that we have in the atmosphere. One percent of the carbon in the atmosphere
00:49
however is going to be carbon 13. And that means that it has 6 protons and so it’s going
00:53
to have 7 neutrons. Both of these are stable. In other words once they’ve been constructed
00:57
they’re not going to change form. But there’s another type and that’s called carbon 14.
01:02
Now carbon 14 actually comes from nitrogen. And what happens is that nitrogen is hit with
01:08
cosmic rays from space. And nitrogen is converted to carbon 14. Carbon 14 has 6 protons and
01:14
it’s going to have 8 neutrons. And it will decay. In other words it has a tendency to
01:21
give off beta particles. And as it does that it turns back into nitrogen. And so what we
01:29
can do is measure the amount of carbon 14 in something. And it tells us how old it is.
01:33
Now where does this carbon go? The carbon eventually goes into the macromolecules that
01:37
make us up. And so when you eat food you’re taking carbon that was once in a plant. But
01:42
before that it was in the atmosphere. And so before we get too into that let me kind
01:47
of talk about how food gets into you. And so if we look at the atmosphere, atmosphere
01:53
is made up of a bunch carbon dioxide. And so this has two oxygens. And it’s going to
01:59
have one carbon. And most of this carbon out here is going to be run of the mill carbon
02:05
12. And so it’s just regular carbon. But occasionally we’re going to have carbon dioxide that actually
02:11
has, it’s carbon 14. So it has those extra neutrons. And so we called this an isotope
02:19
of carbon. And it has a tendency to decay. And so how does that actually get into our
02:23
body? Well the first thing that happens is that plants are going to take in that carbon
02:28
and they do that through a process called photosynthesis. And so what photosynthesis
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is doing is it’s a way for plants to take in carbon and actually make something out
02:39
of it. And so in this case what they’re making is sugar. The sugar is used by the plant to
02:43
build itself. But it’s also used to burn sugar and to do respiration. Now we don’t care about
02:50
that in plants. What we care about is actually getting that sugar inside our body. And so
02:54
what I like to eat is something called Oatmeal Squares for breakfast. It’s my cereal of choice
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right now. And so that carbon that was in the atmosphere, carbon 14, eventually ends
03:06
up in my Oatmeal Squares and eventually ends up in my mouth and eventually ends up inside
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my body. And so the amount of carbon 14 that’s in the atmosphere is going to be equal to
03:18
the amount of carbon 14 that’s in my body. As long as I keep eating. And so as long as
03:24
I keep eating, that ratio of carbon 14 to regular carbon 12 is going to be the same
03:29
over time. But let’s say for example that I get swept up by a wind storm and I end up
03:36
dying. It’s really sad. Well at this point, I’m not eating anymore. And so since I’m not
03:43
eating anymore I’m not taking in anymore carbon 14. In other words we’ve cut off that food
03:50
channel. And so the amount of carbon 14 that I have in my body I’m stuck with. And so let’s
03:56
say that I’m swept away in a dust storm. And I’m covered up by sediment. And years later
04:03
a scientist finds a part of me and wants to figure out, well I wonder how long ago it
04:09
was that this science teacher met his doom. Well he or she can figure that out using carbon
04:15
14 dating. So how does that actually work? First of all you would have to take the bones
04:21
of Mr. Andersen into the lab. And then we’re going to measure the amount of carbon 14 inside
04:28
me. And we could put me in a chamber like that where we’re sensing the amount of beta
04:32
particles that are given off. But first let’s talk a little bit about the math. At time
04:37
0 the amount of carbon 14 that I would have would be 100 percent of the amount of carbon
04:43
14. But that carbon 14 is going to decay over time. And so carbon 14 looks like this. But
04:49
it’s going to break down into nitrogen 14. And as it does that, it’s going to give off
04:56
beta particles. And those beta particles could be measured as hitting the sides of this pretend
05:04
sensor that I have here. Now we know a little bit about the amount of time it takes. In
05:09
other words for a gram of me, and so let’s just take a little bit of me, for a gram of
05:16
me we would expect that 15 of these beta particles would be released every minute. And so if
05:26
I were to take a newly deceased or newly dead body into my theoretical lab here. And if
05:35
I was getting 15 beta particles per minute, we would know that that is 0 years old. But
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if we were to look at it 5730 years later, scientists have found that it would give off
05:47
7.5 beta particles per minute. In other words it would give off half the amount of carbon
05:53
14 particles that were in there before. And that would continue to drop off and drop off.
05:58
And so if we look at this in a graph, this is what is going to happen to the amount of
06:02
carbon 14 inside my body. 100 percent of it would be given off at time 0. But every 5730
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years we’re going to give off half that amount. So this drops down to 25 percent. 12.5, 6.25,
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3.125. And so you can imagine that I could create, this is going to be tough to get it
06:24
good, a best fit line or a trend line of this data. And so what you could do is you could
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find, let’s say we find a bone. And that bone, we could say that it has, I don’t know, something
06:38
like 37% of the amount of carbon 14 that it should normally have. Well we could just read
06:45
down here. It’s going to tell us how old that is. And so that’s how carbon 14 dating works.
06:50
You could imagine this is kind of a cool math thing. We’ll never reach the bottom. It’s
06:55
kind of like walking half way to the wall and then half way to the wall again. But eventually
07:00
you get out like 60,000 years and the amount of carbon 14 that is left is so small that
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you can’t get accurate measurements for that. And so we have to use a different isotope
07:09
to measure it at that point. Another thing to remember is the amount carbon 14 is going
07:15
to drop off, but that’s going to be converted to nitrogen 14 as well. And so if we have
07:21
25 percent carbon 14 then we’ve got 75 percent nitrogen 14. Now you maybe asking yourself
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how do we know that this is accurate? In other words we weren’t around there 10,000 years
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ago. So how would we make sure that you’re data is actually matching up? Well what you
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would need to find is find objects that are old but we know exactly how old they are.
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And so one of the first things that they showed was they took an Egyptian, like a royal barge,
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that was made of wood. And then they handed it to scientists and scientists using radiocarbon
07:53
dating figure out how old this is. And so they figured out how old it was. They were
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able to then go back in the written record and show that those data are going to match
08:01
up. And so you don’t just use one sample. You’d use multiple samples and different isotopes
08:06
to get a real accurate kind of a measurement. One caveat to that is this. If you look right
08:11
here, this was puzzling to me the first time I saw it. And so this is 1945 through 2000.
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And this blue line represents the natural level of carbon 14 in the atmosphere. But
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what you see is that we see a peak of that coming right after the 1950s. And it’s greater
08:32
in the northern hemisphere. Let’s see. Yeah the northern hemisphere. This would be like
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in Austria then it is in the southern hemisphere. It’s almost twice the amount in the the northern
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hemisphere you know 5 years later than it normally should be. And so the reason why
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is that humans started doing nuclear weapons testing. And so as we did that we actually
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increased the amount of carbon 14 in the atmosphere. And so one thing you should remember is that
09:00
everything is measured with carbon 14 dating before 1950. And the reason why is that we
09:07
can’t get super accurate measurements within this time because it’s kind of screwed up.
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So I couldn’t for example radiocarbon date something from 1970 because it’s going to
09:17
have a different amount, or I’m going to have to somehow compensate for that amount. And
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so that’s radiocarbon dating. It’s pretty cool. It’s a neat way to figure out how old
09:27
the bones are.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.