Now, when I did that, I made a pretty big assumption, and some you all have touched on this in the comments on You Tube 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?
Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks.This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things.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.We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 19)].
The person who wrote these words lived in the 1800s, many years before archaeologists could accurately date materials from archaeological sites using scientific methods.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. And I'm told this will work up to about 10,000 years. 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.A commonly used radiometric dating technique relies on the breakdown of potassium (Ar in an igneous rock can tell us the amount of time that has passed since the rock crystallized.If an igneous or other rock is metamorphosed, its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism.Radioactive elements were incorporated into the Earth when the Solar System formed.