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Carbon 14 Dating 2
相關課程
- In the last video, we talked about the idea that
- If I dug up a bone someplace and if I were to measure its
- carbon 14 I found that it had half the carbon 14 that I would
- expect to find in a living animal or plant. Then I would say hey, maybe 1 half life is gone by
- or roughly for Carbon 14 one half life is 5,730 years. So I'd say maybe
- its 5,730 years since this bone was part of a living animal, or it is roughly that old.
- Now when I did that I made a pretty big assumption
- Some of y'all have touched on this on you tube in the last video
- is how do I know that, this this estimate I made is based on the
- assumption that the amount of carbon 14 in the atmosphere
- is roughly constant from when this bone was living to now
- so the question 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.
- The way that you can make that calibration, because it turns
- out that it isn't perfectly constant, the way that you can
- make that calibration, there are two ways, and I have
- pictures here of both of them. One is to look at tree rings
- and I am told that this will work up to about 10 thousand
- years, up to ten thousand years old, I don't know of any
- ten thousand year old trees, I don't think anyone does
- but maybe there are some remains of old trees and you can
- look at there 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, adjust 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 is kind of a record of the atmosphere up to
- ten thousand 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 the 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 ten thousand years
- the water in the cave deposits that calcium carbonate
- so it is a record of the fraction of carbon 14 in
- some of those years. And you can go down to resolutions
- of up to as small as ten years.
- So this will give us pretty good estimates of over tens
- of thousands of years (up to 50 thousand years) and
- frankly carbon 14 isn't even useful beyond really 50
- or 60 thousand years. So this gives us a pretty good,
- pretty good, this gives us a good record of carbon 14
- in the atmosphere. Assuming that it is fairly uniform
- throughout the atmosphere
- and all evidence suggests that and that uniformity
- through the atmosphere 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 that because we are spewing so much fossil fuel
- right now, we are changing the amount or proportion
- of carbon 14 much much faster than has happened
- in other time periods. So just to answer the question
- in probably really the last 50 years when the fossil
- fuel use has really exploded 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 assumptions 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
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