Main content

### Course: Cosmology and astronomy > Unit 4

Lesson 3: Measuring age on earth# Carbon 14 dating 2

Carbon 14 Dating 2. Created by Sal Khan.

## Want to join the conversation?

- Wait! How do we know what the C14/C12 ratio within a specific tree ring was to start with?(11 votes)
- First you measure the ratio of C14 to C12 in the ring. Then you do the reverse calculation based on the known half-life of C14 to work out what the ratio was at the time the ring formed. The age of the ring is already known thanks to dendrochronology (dating by counting the rings, basically!). Bingo, you now know what ratio of C14 to C12 was the norm at that time.(9 votes)

- How can you tell how much Carbon something has? Or is there only Carbon 14..... I'm really confused.... Can I get some help?(9 votes)
- Carbon 14 dating uses the measurement of the ratio of carbon 14, out of all carbon atoms, within something. Since carbon 14 is a radioactive isotopes of carbon, it is not stable (meaning it does not "last" forever without turning into something else). Half of all carbon 14 within a collection of carbon atoms decays into carbon 12 atoms every ~5700 years (carbon 14's half-life). The most common isotope (think of isotope as a variation of an atom consisting of different number of neutrons) of carbon is carbon 12 (12 refers to the mass of the atom), since carbon 12 is one of the only stable isotopes of carbon. Carbon 14 is produced (mostly) by cosmic rays hitting the nitrogen within our atmosphere (as said within the last video), therefore something that does not interact with it's environment (dead or buried things) will not gain more carbon 14 after it stopped interacting with the environment. So by measuring the amount of carbon 14 something has that had not yet decayed, we can determine how long it's been since that "something" had interacted with the environment (how long it's been dead or buried)(2 votes)

- What is the record for longest living tree?(2 votes)
- The world's oldest recorded tree is a 9550 year old spruce in the Dalarna province of Sweden. It was planted during the last ice age(14 votes)

- At approx.3:00, Sal makes the comment that after 50k - 60k years, carbon 14 dating isn't much help. Why is this so? Is it due to the amount of carbon 14 being minute enough to be unable to calculate a usable half-life? And how would this information be used to date fossils that are millions of years old? Thank you for your help.(4 votes)
- The half life of C14 is well known; it is 5,730±40 years. Most samples of organic material start out with a very small amount of C14. After around 54,000 years that original amount will have diminished by a factor of ~2^9 or ~500, which makes accurate measurement difficult. And for every ~6,000 years further back the remaining C14 decreases again by a factor of two.

C14 dating is not used to date fossils millions of years old. Uranium-lead or potassium-argon dating (and several other techniques as well) are used to date samples millions of years old.(9 votes)

- Sal mentioned that burning fossil fuels contribute to the amount of Carbon 14 in the atmosphere. Does nuclear testing affect it as well?(3 votes)
- Yes, I remember my teacher talking about it. There was an island used to test nuclear bombs, and the dating was messed up with too much carbon, making the test results show everything was younger than they actually are.(5 votes)

- Is carbon 14 constent?(2 votes)
- Carbon 14 is created by highly energetic cosmic rays hitting the

Earth's upper atmosphere. Since the rate of arrival of these cosmic rays has been reasonably constant, an equilibrium is reached between existing C14 decaying each year in the atmosphere versus the new C14 created by cosmic rays. Because we have organic material whose age we know independently, we can calculate the actual rate of creation of C14 from cosmic rays fairly accurately and calibrate our measurements. Then we can use our well-calibrated levels of C14 to determine the age of organic materials for which we don't have an independent way of knowing how old they are.(7 votes)

- what happens when all the carbon 14 has decayed then how do we measure the time(2 votes)
- We measure the ratio of C12 to C14 in the sample. That tells us the age, up to about 50,000 years or so. If there's no C14 left, that means the sample is too old to use this method. That's why the limit is around 50,000 years.(3 votes)

- At3:23, Sal stated ... "since in recent times, we are burning more fossil fuels (carbon) we are altering the amount of Carbon 14 being produced".

If the creation of carbon 14 is a function of transforming Nitrogen, via cosmic radiation, into carbon 14, estimated at a very steady rate for tens of thousands of years, what possible effect would injecting greater amounts of carbon 12 into the atmosphere, by burning large amounts of fossil fuels, now and over the past 150 years?(2 votes)- I think fossil fuels in general actually have virtually no carbon-14 left in them. If you think about it, fossil fuels are the result of plants getting buried and sitting under ground for millions of years. If carbon-14 is decaying by 50% every 5730 years, then after millions of years there will only be carbon-12 left. When we burn billions of tons of fossil fuels, we send that carbon-12 up into the atmosphere, which changes the ratio of C12/C14 because the C14 production is relatively constant from one year to the next but the C12 is going up due to humans burning fossil fuels. So it won't necessarily change the production of C14 which is a function of cosmic particles interacting with our atmosphere, but it will dilute the amount of C14 in the atmosphere so the concentration will be lower than what it would have been had we not sent so much C12 into the air.(3 votes)

- if tree we can tell how old a tree is by lookin at its tree rings, then would somthing like carbon 14 kill the tree or like make a delay in the groth of the tree?(1 vote)
- All trees, and all living things, have carbon 14 in them.(2 votes)

- How does it even help to have a Carbon 14 record? If I looked at the proportion of Carbon 14 in an animal, there is still no way to link that animal to a specific point in time because the Carbon 14 fluctuates.

For example, in year 1, animals take in 100 g of element X. In year 2, animals take in 50 g of X. Now, assume that half life is 1 yr and we are in year 3. If an organism has 25 g of X, we have no way of knowing if they are from year 1 or year 2.(1 vote)

## Video transcript

In the last video, we
talked about the idea that if I dug up a bone
someplace, if I dug up a bone, and if I were to
measure its carbon-14, and I found that it had half
of the carbon-14 that I would expect to find in a living
animal or plant, that I said, hey, maybe one half life
has gone by, or roughly for carbon-14, one half
life is 5,730 years. So I said maybe it's 5,730
years since this bone was part of a living animal,
or it's roughly that old. Now, when I did that, I made
a pretty big assumption, and some you all have touched on
this in the comments on YouTube on the last video,
is how do I know that this estimate I made
is based on the assumption that the amount of
carbon-14 in the atmosphere would have been roughly constant
from when this bone was living to now? And so the question is,
is the amount of carbon-14 in the atmosphere
and in the water, and in living plants and
animals, is it constant? And if it isn't constant, how do
you calibrate your measurement so you can actually figure
out how much carbon-14 there is relative to living plants
and animals at that time? And the way that you can
make that calibration, because it turns out it
isn't perfectly constant, the way that you can make that
calibration, there's two ways, and I have pictures
here of both of them, one is to look at tree things. Tree rings. And I'm told this will work
up to about 10,000 years. Up to 10,000 years old. I don't know of any
10,000 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-14, and then figure out how
much carbon-14 was there in the atmosphere at
that period in time. And so it's kind of a
record of the atmosphere up to 10,000 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. Speleothems. 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-14 in some of those years. And you can go down to
resolutions of as small as 10 years. And so this will give
us pretty good estimates over tens of thousands of
years, up to 50,000 years. And frankly, carbon-14
isn't even useful beyond, really, 50,000
or 60,000 years. So this gives us a good
record of carbon-14 in the atmosphere,
assuming that it's fairly uniform throughout
the atmosphere, and all evidence suggests
that, and that that uniformity through the atmosphere also
goes into the water supply, and into living
plants and animals. Now, the other thing, and I
looked into this a little bit, it actually turns out because we
are spewing so much fossil fuel right now, we are
changing the amount, or the proportion of carbon-14
much, much faster than has happened in other time periods. So just to answer
the question, it's actually probably in
really, the last 50 years where the fossil
fuel use has really exploded that we've
really been changing the proportion of
carbon-14 relative to the other isotopes of carbon. But anyway, hopefully
that rests some of your worries
about the assumption that I made in the last
video about carbon-14 being relatively constant. There are ways to look
back at specific years and figure out the relative
amounts of carbon-14, so it is a pretty
good way of estimating how old living things
are, especially things that are less
than 50,000 years old.