Re: Age of fossil: Y10G

I will start by pointing out that there are a number of very good answers about radiometric data in the MAD Scientist Archives. You can find them by entering "carbon dating" or "uranium dating" or similar strings into the search engine. But I will refer you particularly to an answer by David Morgan in October 99 (Answer number 939419224.Es) about carbon dating, and an answer by Andrew Karam in October 99 (Answer number 939332877.Es) about other radiometric dating methods. Both are in the October 99 Earth Sciences archives.

The important point is that any of the radiometric dating methods measure the date at which some sort of interchange of material with the environment stopped, and the sample became a closed system which only underwent internal radioactive decay with no external exchange.

In radiocarbon dating, the radioactive isotope of carbon that is used has only a relatively short half life of several thousand years. It is not the sort of isotope you would expect to still be around if the earth is many millions of years old. But radioactive carbon atoms are produced in nuclear reactions when energetic particles from the sun collide with nitrogen atoms in the earth's outer atmosphere. The radioactive carbon atoms make their way downward through the atmosphere, and enter the oceans and the surface environment more generally. Living plants take up some of the radioactive carbon along with their general carbon uptake in photosyntehsis. But when they die, plants stop taking in new carbon from their surroundings. So while a plant remains alive, it maintains a small but steady equilibrium level of carbon-14 by taking in new radioactive carbon atoms from the environment at a rate that just matches the rate it loses them by radioactive decay. As soon as the plant dies, this uptake stops, and carbon-14 levels begin to fall.

For a slightly different case, let's look at potassium-argon dating. Potassium is a metallic element that is abundant in minerals in the form of many different compounds. A very small proportion of that potassium is radioactive potassium-40 that decays with a half life of a little over a billion (American billion = 10^9) years. Mostly it decays to calcium-40, and calcium is another element abundantly present as compounds in minerals. But about 10% of the potassium-40 decays to argon-40, and argon is an inert gas that does not form compounds.

Suppose there is a reservoir of underground molten magma. It will contain plenty of potassium. And because that potassium has been around for some time, it will also contain some argon-40. Now argon does not dissolve to any extent in molten rock, but with the high pressures underground, it really has no choice. But if there is a volcanic eruption, that pressure is removed, and argon will be one of the gases that escape from the molten rock (along with such things as steam and hydrochloric acid). When the lava sets solid, any of it that is away from the frothy part will contain no argon. And that makes the starting date for the radiometric clock, because from that moment onward, any argon formed from the radioactive potassium still contained in the rock will be trapped in the solid rock. We can therefore measure the age of the rock (since it solidified) by measuring the amount of argon it contains relative to the amount of potassium.

It is an interesting fact that about 1% of our atmosphere is argon, and over 99% of that argon is argon-40. On the sun, most of the argon is argon-36. Scientists believe, with good reason, that the argon in our atmosphere comes almost entirely from the radioactive decay of potassium-40 in minerals, and subsequent escape to the atmosphere by outgassing in volcanic eruptions and similar events.