I understand that it works by measuring the amount of carbon 14 which has different amounts depending how old the matter is. But how is it known how much carbon 14 there was that long ago? Is it just that is takes x years for this much to decay so 104 x yea ago they figure it out on a simple graph?
In:
Carbon Dating is measuring the **ratio** between Carbon 14 and Carbon 12.
See, when things are “alive”, they acquire Carbon 14 from the atmosphere (or from their diet!) until they’re at equilibrium. Critters and organic compounds have the same amount of C14 as their environment.
When they die, that C14 starts to degrade. However! C12 is a **stable** element, meaning it doesn’t really degrade over time. So we can compare the current C14 next to the current C12 and figure out a “maximum” C14 content (because living things have about 1.25x more C14 than C12).
We do have to use **some** guesswork regarding the total amount of C14 in an environment over time, because that can change quite a lot. That’s why carbon dating gives a large variable, “100,000 to 300,000 years old” for example.
For objects, we can only really use C14 dating on things that were once alive, like leather, sinew, bone, plant fibers, and stuff like that. Lucky for us, a lot of artifacts we find through archeology have those things in them! We might find a clay bowl alongside a bone spoon, that Bronze Age sword might have a leather grip or carrying strap!
For things that have absolutely no organic components, we use a different kind of dating. Sometimes we’ll use Uranium lead dating, which works by examining zircon crystals inside of rocks and other minerals. We know a lot about zircon crystals, and they’re **in basically all natural minerals**, so that makes it really useful for dating.
Uranium lead dating is super useful because it checks two different radioactive components, one that degrades into lead (pb206) and one that degrades to a slightly different thing from lead (pb205). They have different half lives, so when both calculations come up with the same “life estimate”, we know we’re on the money.
The amount of Carbon 14 in the atmosphere is kept stable due to some reaction in the upper atmosphere involving sunlight. The TL;DR is that we are at a balance where the same amount of Carbon 14 decays in the atmosphere as is created.
Now, plants absorb carbon from the air, and animals absorb carbon by eating plants (or other animals). So any living being constantly replenishes the ratio of Carbon 14. Once they die, however, this cycle stops, and the ratio decreases as C14 decays. This rate of decay isn’t affected by external factors, and while the rate isn’t as simple as “x per year” (its exponential), we do have mathematical formulae that can take a measured ratio and tell you how long its been since the measured thing died.
(One side note is that the amount of C14 in the atmosphere *may* have been different in the past, but we can account for this with ice samples that have air trapped in them. We can date this “old air” in a different way, and then use carbon dating “in reverse” to figure out how much C14 was in the atmosphere at that time)
Carbon-14 is constantly being generated in the upper atmosphere. When cosmic radiation hits Nitrogen it generates Carbon-14. And because this process is fairly constant and the half life of Carbon-14 is constant this means the system reaches an equalibrium where the amount of Carbon-14 in the atmosphere is fixed. There are still a lot of pitfalls when interpreting the results of the Carbon-14 dating which is why this is its own branch of archeolagy with highly trained specialists.
We know the concentration of carbon-14 in our current atmosphere, and since carbon-14 is created by cosmic rays entering our tropo-/stratosphere which, as far as we know, has not changed significantly in the timeframes we’re measuring, we can be relatively certain that the concentration was the same in those timeframes.
And since we know that there *should* be x% carbon-14, finding y% carbon-14 allows us to calculate the age since something died as carbon-14 decays, assuming that the concentration was similar as it is now.
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