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On the seventh day, god created uranium (lemmy.zip)
submitted 5 months ago by to c/science_memes@mander.xyz
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[–] 8 points 5 months ago (10 children)

I actually have a question on a similar topic

How does carbon dating work? Sure, things made in the past have some carbon-14 broken down, but don't the raw materials used in making today's products do just the same?

What makes carbon-14 break down differently in things made before and raw materials we use today?

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  • [–] 13 points 5 months ago (4 children)

    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.

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  • [–] 2 points 5 months ago (3 children)

    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?

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  • [–] 5 points 5 months ago

    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.

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  • [–] 5 points 5 months ago* (1 child)

    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.

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  • [–] 7 points 5 months ago (4 children)

    It's not that it breaks down differently (in fact, we rely on it being consistent), it's how much it has broken down. Carbon-14 has a half-life of 5700 +/- 30 years (per Wikipedia), so if you see 1/2 the expected amount of carbon-14 then something would be around 5700 years old, with 1/4 the expected amount you'd predict 11400 years old, etc.

    This relies on the amount of carbon-14 originally being predictable. This worked well in the past for living things (which from what I understand tended to maintain a consistent ratio of C-14 to C-12) or objects made from their organic material, but stopped being true around the industrial revolution when we started pumping the atmosphere full of carbon.

    We use other isotopes, or other techniques in general, for very recent objects, or for things more than 50-60 thousand years old.

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  • [–] 4 points 5 months ago* (last edited 5 months ago) (3 children)

    So, living objects can regulate the amount of carbon-14 specifically, not just carbon in general? And then, knowing the ratio of carbon-14 in a living creature, we know how much time passed by? Or is it that it breaks into nitrogen that reduces the overall amount of carbon, and this is what we detect?

    Because otherwise it shouldn't matter whether they died and carbon-14 broke down for a thousand years or carbon-14 broke down for a thousand years and then the recent creature consumed it. The ratio of carbon isotopes must be the same, as carbon-14 would decay anyway.

    Here I assume that whatever happens with carbon-14 in fossils also happens with any carbon-14 around us. It's not that it breaks down in fossils specifically, but not in everything else. So the order shouldn't matter, unless the ratio is different in a living organism. As a matter of assumption, that is.

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  • [–] 5 points 5 months ago (2 children)

    So, from what I understand, living things maintain (or at least prior to the industrial revolution did maintain) a predictable ratio of C-14 to C-12. I'm not super familiar with the mechanics of this, I imagine it's a case of the amount of C-14 lost matching the rate it was replaced via respiration.

    Once the organism dies, it stops controlling that ratio and we can measure the decay using a sample of the material.

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  • [–] 1 point 5 months ago* (1 child)

    I see! If so, that makes sense, but the mechanics of C14 accumulation would be curious to see.

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  • [–] 2 points 5 months ago*

    I don't think it's that the plants are controlling the ratio. I think it's that more C14 is being made all the time. And it only gets mixed into plants when they are living. Specifically it looks like C14 based CO2 is made in the atmosphere and then consumed by plants.

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