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On the seventh day, god created uranium
(lemmy.zip)
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this post was submitted on 27 May 2026
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As Thalfon said, the Carbon 14 decays at a predictable rate.
So where does the starting quantity come from?
Radiation from space (probably cosmic rays) interacting with the atmosphere causes some of the atoms to become a different isotope. Plants integrate C14 into their sugars and cellulose, and when they are dead and buried by sediment, no more C14 is integrated. While buried, the plant matter is sheilded from further radiation, the C14 decays at a predictable rate, and we measure that. Meanwhile, underground carbon like coal and oil is also sheilded from radiation and doesn't get converted to C14; instead any C14 decays into C12. Thus the problem with Carbon dating after the industrial revolution.
So, the reason the ratio changes in fossils is that they get underground and are shielded from radiation? But then we can only tell how long has it been sitting underground?
Carbon-14 dating only works back to about 50,000 years, most fossils are older than that and they use radiometric dating.
(Not a scientist, I'm sure my wording will make experts cringe, but I think my gist is good)
Fossils are basically rocks that form around hard things like bones. They look at radioactive elements in the rock and figure out how much has decayed and they know when the rock was formed.
Uranium lead dating is one of the most used methods. In it, they look at zircon crystals and measure the amount of uranium and lead. Uranium decays into lead, so that tells them how long the decay has been happening.
That always seemed sketchy to me, how do they know it didn't just have a bunch of lead in it to start with? Then I learned something...
When zircon forms, lead can't bind with it and it gets pushed out of the zircon. Uranium doesn't get pushed out, so there are small pockets of uranium in fresh zircon and no lead. A million years later, we just look at how much lead and uranium there is and get a very good idea at when it was formed.
I don't think the burial matters. It's not random atoms that are turned into C14. It's specifically nitrogen. And I think those interactions mainly happen in the mid atmosphere with cosmic rays. So it's the atmosphere providing the shielding.
The Wikipedia page for this explains it pretty well, especially the Physical and Chemical Details section.
Thank you for the clarification