How does carbon dating work?

[aargh57]

I was looking for some info on the process. Any help would be appreciated.

Thanks

 

[Z]

I seem to recall it had something to do with one carbon atom picking up flowers and chocolates while the other primped in front of its mirror for several hours...

hmmm...

Maybe I should go back to bed now...

 

[Kevin_Lowe]

I'm not an expert, but I'm sure I'll be corrected if I'm way off base.

The carbon in the carbon dioxide in the atmosphere is a relatively consistent mix of carbon-12 and carbon-13, because solar radiation hitting the atmosphere converts C12 to C13 at a more or less constant rate.

All carbon-based life makes use of atmospheric CO2 to get it's carbon, so our C12/C13 ratio is about the same as that of the atmosphere.

As soon as something gets cut off from the atmosphere because it's been buried, mummified or whatever the carbon dating clock starts ticking as C13 decays and is not replaced. So until almost all the C13 is gone you can date organic material or trapped air by the proportion of C13 it contains.

Carbon dating gets unreliable and eventually stops working past a certain point - I vaguely recall that it's in the tens of thousands of years, but that's the best I can do offhand.

 

[Art Vandelay]

The CO2 can be traced back to the atmosphere, but animals don't get it directly from the atmosphere; they get it from plants which get it from the atmosphere, or they get it from other animals. So carbon dating actually gives the date from when the carbon was created. Apparently the turnover of carbon is great enough that most of the carbon at the time of an organism's death is recent. And the time scales in which carbon dating is used are so large that the lifetime of the organism isn't a large factor. I doubt that whether a dead creature is exposed to the atmosphere has much of an effect on the carbon content.

 

[Hamish]

I'm not an expert, but I'm sure I'll be corrected if I'm way off base.

The carbon in the carbon dioxide in the atmosphere is a relatively consistent mix of carbon-12 and carbon-13, because solar radiation hitting the atmosphere converts C12 to C13 at a more or less constant rate.

Reasonably correct, except it's carbon-14 not carbon-13.

Carbon-14 decays via beta emission into nitrogen-14 (that's ordinary nitrogen) with a half life of 5730 years. That makes carbon dating reasonable over tens of thousands of years. You then count the number of c-14 atoms left, either by monitoring beta decay events or using a mass spectrometer.

Here is a nice link if you want to get a more intuitive feel.

The problem is that carbon based relics don't usually survive more than a few thousand years. You can only carbon date stuff that was alive and still contains carbon so fossils can't be carbon dated, nor can ancient pottery.

There is also the issue of the ammount of atmospheric carbon. Burning oil and coal releases CO2 with a higher proportion of carbon-12 - it's been dead for a long time after all. Since the industrial revolution, things have become a bit skewed.

This is why you combine carbon dating with other forms of dating (dendrochronology for one) to calibrate a dating curve which takes into account odd events which upset the isotope balance.