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# 碳十四定年法（二）

碳十四定年法（二）. 由 Sal Khan 创建

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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 five thousand seven hundred and thirty years so I said maybe it's five thousand seven hundred thirty 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 of y'all have touched on this in the comments on on YouTube on the last video it's how do I know that the this 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 is it perfectly constant the way that you can make that calibration there's two ways and I've have pictures here of both of them one is to look at tree rings tree rings and I'm told that this will work up to about ten 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 their tree rings and I think most of us 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 it just for the half-life of of carbon-14 and then figure out how much carbon-14 was there in the atmosphere at that period in time and so you'll it's 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 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 as 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 up to of as small as 10 years and so this will give us pretty good estimates over tens of thousands years up to 50,000 years and frankly carbon-14 isn't even useful beyond really 50 or 60,000 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's fairly uniform throughout the atmosphere and all evidence suggests that and that that 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 it's then then has happened in in other time periods so just to answer the question it's actually it probably in the last 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 that rests some some of your worries about the assumption that I made in the last video about carbon-14 being relatively constant there are there are ways to look back at specific years and figure out the relative amounts of carbon-14 so it is a it is a pretty pretty good way of estimating how old living things are especially things that are less than 50,000 years old