I got into a debate the other day with a friend, and she brought up carbon dating. I thought to myself, "How does carbon dating even work?" So I really did not go into it with her because I have always heard about carbon dating but have been oblivious to what carbon dating is all about.

So I ask the great people of TA. What is carbon dating? How does it work? How do they determine this rock or bone is over 5,000 years old vs a rock or bone that is 5,000,000 years old? How precise is this science? Can there be false positives? What other questions am I missing? 

Sure, I could do a google search but I thought other people might have the same exact questions as me about this topic. 


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Great job Nelson!

Nelson has to be a bot.  :)

Great answer!  One criticism: Include a simple 'in a nutshell' explaination for those of us who aren't on that level of intelligence (I understand most of it, but still a little fuzzy).

Well, I'm trying to wrap my head around the half-life of C-14 being 5730 years and that allowing carbon dating to date things back 60,000 years and I'm not much of a math wiz.  I guess another question that goes back to the original post about the accuracy of carbon dating would be more specific.  I've heard that the supposed Shroud of Turin (sp?) was misdated due to a fire in the building where it was being kept.  How would that effect carbon dating if it effects it at all?

Indeed, I lol'd.  Thanks for the explanation!  I guess I did have a pretty good understanding of isotope decay, but I wasn't entirely sure how far (in amount mearurable) measurements could go. 

I thought the Shroud of Turin scenario was a bit flimsy especially considering the difference in dates and the accuracy of carbon dating.

Again, thanks!

Let me follow this example further, to illustrate the complications that arise when using this one-year-half-life radioisotope to date something in the real world.  I said that these were complications, and they do make it more complicated.  But they do not invalidate the dating.

First off you are never going to be dating something that used to be a pure block of the isotope in question.   It will be a mixture of chemicals.  Also, it's more than likely that the isotope we are talking about is only one of several isotopes of that element.  Let's say it's potassium; there's a stable isotope and then there's the one that decays with a half-life of a year.  The two isotopes behave identically, when it comes to chemistry; they form the same compounds, the same minerals.  Or it could be carbon.  It will turn out that there are special considerations for each case,

Potassium really does have a stable and an unstable isotope, and it exists in a lot of rocks.  The rocks form as minerals crystallize, usually underground but sometimes in lava flows.  At the time the rock forms, there's no difference between the stable potassium and the unstable potassium.  But the unstable isotope turns into argon.  Now argon and potassium are about as unalike as possible; potassium is a solid metal that is so reactive that even water oxidizes it.  Argon is a gas that doesn't react with anything; in fact it forms 1% of the air that's in your lungs right now, with no ill effect.  So when the unstable potassium in the rock decays, it becomes argon, and you can assume that any argon in the rock today used to be potassium, since surely argon would not have combined with the other elements in the rock to begin with.  (I suppose it's possible some argon could be dissolved in the lava, but I have no doubt they know how to account for this.)  So count the atoms of unstable potassium and argon in the rock, and you have your ratio.  (The example I gave elsewhere (at the end of this thread) of the uranium-lead decay is a bit more straightforward, because you simply won't have original lead dissolved in zircon crystals to potentially mess things up)

Carbon-14 is a lot trickier.  It is in organic tissue and decays into perfectly ordinary nitrogen, which is also in organic tissue.  It's very hard to figure out a ratio when you simply don't know which nitrogen used to be carbon-14 and which did not.  What they do instead, as I understand it, is to calculate the ratio of stable carbon to carbon-14 and then compare that to what we have in the atmosphere today.  For example, if five atoms in every billion carbon atoms in the sample is carbon 14, but ten atoms in one billion carbon in the atmosphere is carbon 14, then since carbon 14 is only half as common in the sample, we figure half of what was originally there when the tree or critter died, has since decayed away, and we date the object as having died 5760 years ago.

But that relies on an assumption that we know isn't quite true; that carbon 14 ratios in the atmosphere are constant.  In fact we know that they tend to vary a tiny bit.  Maybe back when the thing died there were 12 atoms per billion, in which case it took a little longer to decay down to the five we measure today.  Maybe back when it died, there were only nine atoms per billion, in which case it took less time to get down to five.  That's where the calibration comes in.  We can, in some special circumstances, date things by other methods, for example we can often date based on patterns of thick and thin tree rings.  This particularly thick ring may be from 1738 which was really rainy.  Sometimes they can build a tree ring sequence that goes back thousands of years, back before "creation" according to the creationists.

Well, measure the carbon in those tree rings, which are already of known age and you might find that the sample with 5 atoms per billion is from a tree ring 6130 years old, not 5760.  So when you have an unknown sample with the same results, you know that it's 6130 years old, not 5760.

That's what it means to "calibrate" carbon 14 dates.

@Sophie. Nelson has given a full explanation there. It's covered quite well in the Richard Dawkin's book, 'The Greatest Show on Earth.'

Incidentally, the book also explains dendrochronology. This is dating using tree rings! That allows dating to within one year and is possible now going back 11,000 years!


We know the number of particles that are present when a radioactive element is complete. It decays at a constant rate – that is, it loses half of it energy (mass) over a fixed period of time. We call this the “half-life” of the element. If it has a half-life of 10,000 years then after 5000 years it has half the number of particles left. Then after 2500 years it would have ¼ of the original left and so on. The environment – temperature, soil erosion or any other factor has no effect on the rate of decay. Some elements have very long half-lives and others very short ones. However they can all be seen as types of “atomic clocks” as their decay (loss of energy or mass) is constant and we can work back to see how old something is. Carbon dating is just one form of radioactive decay and is used to date organic material. Other elements date non organic material. Sometimes called “clocks in the rocks.” As mentioned above the Dawkins books has a very good chapter on it.

Good job, Nelson!


One thing to be aware of is that many people use "carbon dating" to mean any sort of radiometric dating, not just the dating of recent organic samples.  (I even heard them make that goof on "Universe" early in the first season.)  This is unfortunately because carbon dating has a lot of very important differences from other variants, not just in terms of how far back one can date, or what it is being dated (plant or animal matter vs. rock), but because carbon dating is the only method that has to be calibrated.  Since the carbon-14 decays to nitrogen-14 which is already butt-common in organic matter, we have to measure the ratio of c-14 to c-12, but just doing that can be misleading without calibration because the starting ratio varies over time.  Calibration is just the process of accounting for that difference. 

That's an added extra complication that creationists will try to exploit to make the whole thing sound very iffy.  Which is ironic because most of the creationist disputes are with ages we got with other methods that don't have this issue at all!  Carbon dating of course will never be used to give evidence that the earth is 4.5 billion years old, but other methods are more useful and far simpler to deal with.

Nelson mentioned potassium-argon dating but there are many other possible methods and one commonly mentioned is uranium-lead, and I am a bit more familiar with it than potassium argon.  Uranium has a 4.5 billion year half life, so it's eminently usable for any rock we will find on earth.  Take a big hunk of granite.  Find a zircon crystal somewhere in it, there probably will be some, very tiny ones.  When zircon crystallizes, it can pick up uranium impurities but not lead impurities.  Thus the crystal starts out with no lead whatsoever.  [Creationists who are smart enough to know you don't use carbon dating to date trilobites, but are still stupid ignorant enough to be creationists, will try to claim that we have no way of knowing whether or not there was any lead to start with in the rock.  But as you can see this is simply bullshit, and the ones who work at the discovery institute or the creation science institute and write all the propaganda the less educated latch onto, goddamn fucking well know enough to know this, the lying sacks of shit.  Not that I am bitter.]  So any lead found in the zircon today was formed from uranium decay, decay of uranium that got locked into the crystal when it solidified.  To avoid any possible chance of contamination, though, they remove the outer layers of the zircon crystal before they start.  The beauty of the uranium-lead test though is that it is really TWO tests; there are two different isotopes of Uranium, 238 (with a 4.5 billion year half life) which eventually decays to lead 206, and 235 (with a 700 million year half life) which eventually decays to lead 207.  You can run both tests, and see if they each give the same answer for the age of the zircon crystal.

As for precision, nowadays they don't weigh the samples, they run them through a mass spectrometer (a lot of old cyclotrons that aren't powerful enough to do any new work in nuclear or particle physics are doing this work in retirement) and can count individual atoms.  When I was a kid, the Cambrian was thought to have begun 560 million years ago.  Now with more accurate measurement, they say it was 542 million years ago; imagine having it down to two tenths of a percent uncertainty!  [Of course, what this really means is the evil-lutionary scientists can't make up their minds what lie they want to tell!  NOT!]


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