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Carbon-13 dating does indeed need corrections for the level of solar activity, but that's a bit of an exception. The corrections don't have *anything* to do with how fast the Carbon-13 decays, though: they relate to how much C-13 is in the atmosphere. The way it works is like this: Carbon-13 decays in about 5000 years. Why do we still have some around then? That's because it's constantly being made as cosmic rays hit the upper atmosphere of the earth. Now, as a tree grows, it incorporates carbon from the environment, including C-13. When the tree dies, the incorporation pretty much stops. So, once the tree dies, no more C-13 comes in, and whatever is there continues to decay. Thus, some wood with a lot of C-13 is new because the C-13 hasn't had time to decay. Wood with very little C-13 is old, because it has been dead (i.e. disconnected from the environment) for long enough for most of the C-13 to decay. So, how does solar activity come into the picture? If the sun is inactive, more cosmic rays come in from outer space and hit the Earth's atmosphere. Thus more C-13 is made. Thus, a tree growing in a time with less solar activity incorporates more C-13, and thus when you do C-13 dating of it, it seems to be newer than it really is. The C-13 always decays at the same rate, but if you start with more, you end up with more. Vice versa, if the sun is particularly active some decade. Now, this explanation doesn't apply to most kinds of radioactive dating. Uranium dating, for instance, looks at U-238 that has been there since the beginning of the solar system, and cosmic rays just don't matter.
[ Papers | | Phonetics Lab | Oxford ] Last Modified Sun Dec 3 18:38:29 2006 Greg Kochanski: [ ]