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On the seventh day, god created uranium (lemmy.zip)

submitted 18 hours ago by favoredponcho@lemmy.zip to c/science_memes@mander.xyz

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[-] Thalfon@sh.itjust.works 7 points 9 hours ago

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.

[-] Allero@lemmy.today 4 points 8 hours ago* (last edited 8 hours ago)

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.

[-] Thalfon@sh.itjust.works 5 points 8 hours ago

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.

[-] Allero@lemmy.today 1 points 7 hours ago* (last edited 7 hours ago)

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

[-] maxwellfire@lemmy.world 2 points 6 hours ago* (last edited 6 hours 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.