Darkover
Illuminated
- Jul 29, 2021
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Living cells continuously rebuild themselves, by taking in food and putting out waste. A lot of the atoms in that cycle are carbon atoms, which is the main component of sugars and pretty much all proteins that make living cells work.
Now, the interesting thing is there are several different kinds of carbon. An atom consists of a cloud of electrons orbiting a nucleus made up of protons and neutrons. The number of protons determines what element it is - carbon has 6 protons, hydrogen has 1 proton, oxygen has 8 protons, etc. The number of electrons is usually the same as the number of protons. But the number of neutrons can vary, and this affects how stable the atom is. Most carbon atoms have 6 protons and 6 neutrons, and they're called "carbon-12", or C-12, because they have 12 protons and neutrons total. But there are also small amounts of C-14 in nature - carbon with 6 protons and 8 neutrons, 14 total. C-14 is also carbon and behaves like normal C-12 as far as life chemistry is concerned, but those extra neutrons don't like to be there. Because of that, in every second there's a small chance that a C-14 nucleus will spontaneously decay into a nitrogen-14 nucleus by transforming one of the neutrons into a proton. Even though the decay happens at a random point in time for each individual atom, for large numbers of atoms the decay becomes very predictable: in about 5730 years, about half of the C-14 atoms in any given sample will have decayed.
Now remember that living organisms continuously replace their atoms. So in any living organism, the fraction of its carbon atoms that are C-14 will be about the same as the amount of C-14 in nature, which is about 1 in 1 trillion atoms. That might not sound like much, but 12 grams of carbon is about 6000 trillion trillion atoms, so there'd still be about 6000 trillion C-14 atoms in those 12 grams.
But once an organism dies it no longer keeps replacing its atoms, so the composition of atoms it had when it died is preserved. Combine this with the above decay phenomenon, and you'll see that over thousands of years the fraction of C-14 in the organism will decrease because the C-14 decays. So if you find a fossil where about 1 in 2 trillion carbon atoms are C-14, that organism probably died about 5700 years ago. If it's 1 in 4 trillion that's about 12,000 years, and if it's 1 in 256 trillion that's about 46,000 years.
That's also why carbon dating only works on (previously) living things, and only on somewhat long but not too long time scales. You can't measure the C-14 fraction with absolute precision, and that measurement error gets magnified the farther back you go. But you also can't use carbon dating on an organism that died last week or a few decades ago, because that time is too short to make out a difference in the C-14 fraction with any certainty.
Now, the interesting thing is there are several different kinds of carbon. An atom consists of a cloud of electrons orbiting a nucleus made up of protons and neutrons. The number of protons determines what element it is - carbon has 6 protons, hydrogen has 1 proton, oxygen has 8 protons, etc. The number of electrons is usually the same as the number of protons. But the number of neutrons can vary, and this affects how stable the atom is. Most carbon atoms have 6 protons and 6 neutrons, and they're called "carbon-12", or C-12, because they have 12 protons and neutrons total. But there are also small amounts of C-14 in nature - carbon with 6 protons and 8 neutrons, 14 total. C-14 is also carbon and behaves like normal C-12 as far as life chemistry is concerned, but those extra neutrons don't like to be there. Because of that, in every second there's a small chance that a C-14 nucleus will spontaneously decay into a nitrogen-14 nucleus by transforming one of the neutrons into a proton. Even though the decay happens at a random point in time for each individual atom, for large numbers of atoms the decay becomes very predictable: in about 5730 years, about half of the C-14 atoms in any given sample will have decayed.
Now remember that living organisms continuously replace their atoms. So in any living organism, the fraction of its carbon atoms that are C-14 will be about the same as the amount of C-14 in nature, which is about 1 in 1 trillion atoms. That might not sound like much, but 12 grams of carbon is about 6000 trillion trillion atoms, so there'd still be about 6000 trillion C-14 atoms in those 12 grams.
But once an organism dies it no longer keeps replacing its atoms, so the composition of atoms it had when it died is preserved. Combine this with the above decay phenomenon, and you'll see that over thousands of years the fraction of C-14 in the organism will decrease because the C-14 decays. So if you find a fossil where about 1 in 2 trillion carbon atoms are C-14, that organism probably died about 5700 years ago. If it's 1 in 4 trillion that's about 12,000 years, and if it's 1 in 256 trillion that's about 46,000 years.
That's also why carbon dating only works on (previously) living things, and only on somewhat long but not too long time scales. You can't measure the C-14 fraction with absolute precision, and that measurement error gets magnified the farther back you go. But you also can't use carbon dating on an organism that died last week or a few decades ago, because that time is too short to make out a difference in the C-14 fraction with any certainty.
