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In the last video, we talked about the idea that

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If I dug up a bone someplace and if I were to measure its

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carbon 14 I found that it had half the carbon 14 that I would

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expect to find in a living animal or plant. Then I would say hey, maybe 1 half life is gone by

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or roughly for Carbon 14 one half life is 5,730 years. So I'd say maybe

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its 5,730 years since this bone was part of a living animal, or it is roughly that old.

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Now when I did that I made a pretty big assumption

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Some of y'all have touched on this on you tube in the last video

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is how do I know that, this this estimate I made is based on the

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assumption that the amount of carbon 14 in the atmosphere

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is roughly constant from when this bone was living to now

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so the question is the amount of carbon 14 in the atmosphere

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and in the water and in living plants and animals is it

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constant and if it isn't constant how do you calibrate

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your measurement so you can actually figure out how

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much carbon 14 there is relative to living plants and

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animals at that time.

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The way that you can make that calibration, because it turns

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out that it isn't perfectly constant, the way that you can

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make that calibration, there are two ways, and I have

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pictures here of both of them. One is to look at tree rings

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and I am told that this will work up to about 10 thousand

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years, up to ten thousand years old, I don't know of any

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ten thousand year old trees, I don't think anyone does

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but maybe there are some remains of old trees and you can

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look at there tree rings. And I think most of us are

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familiar with this idea that every year that a tree

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grows it forms another layer of bark. And so you can look back

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to that layer of bark, adjust for the half life

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of carbon 14, and then figure out how much carbon 14

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was there in the atmosphere at that period in time

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and so it is kind of a record of the atmosphere up to

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ten thousand years. If you want to go even further

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back you can look at cave deposits. And the fancy word

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for these cave deposits are speleothems, speleothems,

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speleothems, you might be familiar with stalagmites,

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those are the speleothems that are kind of coming out

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of the bottom of the cave, or stalactites, those are the

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speleothems, that are coming from the top of the cave

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but the reason why these are useful is

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these are formed by calcium carbonate, so they have carbon

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in them and slowly over really ten thousand years

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the water in the cave deposits that calcium carbonate

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so it is a record of the fraction of carbon 14 in

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some of those years. And you can go down to resolutions

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of up to as small as ten years.

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So this will give us pretty good estimates of over tens

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of thousands of years (up to 50 thousand years) and

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frankly carbon 14 isn't even useful beyond really 50

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or 60 thousand years. So this gives us a pretty good,

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pretty good, this gives us a good record of carbon 14

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in the atmosphere. Assuming that it is fairly uniform

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throughout the atmosphere

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and all evidence suggests that and that uniformity

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through the atmosphere goes into the water supply

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and into living plants and animals. Now the other thing

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and I looked into this a little bit, it actually turns

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out that because we are spewing so much fossil fuel

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right now, we are changing the amount or proportion

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of carbon 14 much much faster than has happened

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in other time periods. So just to answer the question

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in probably really the last 50 years when the fossil

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fuel use has really exploded we've really been changing

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the proportion of carbon 14 relative to the other

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isotopes of carbon. But anyway, hopefully that

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rests some of your worries about the assumptions I

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made in the last video about carbon 14 being relatively

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constant. There are ways to look back at specific years

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and figure out the relative amounts of carbon 14 so

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it is a pretty good way of estimating how old living things

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are especially things that are less than 50,000 years old