LogFAQs > #975259909

potdnewb posted...

based on the assumption that we know how much c14 is absorbed by an organism over its lifetime yes im saying the standard is still an assumption since we have learned that observed absorption rate is variable

It's not a question of how much C14 is absorbed over its lifetime. The proportion of C12 to C14 is roughly constant within the environment, and for a living organism that's constantly cycling out the carbon it contains for new stuff (whether directly from the environment by consuming CO2, or adding a trophic level or two to get it via larger macromolecules), that means the proportion of C12 to C14 in their bodies will match environmental proportions pretty closely. When they die, however, they stop replacing the carbon they contain with new carbon from the environment, and given that their corpses aren't hanging out in the upper atmosphere to be bombarded by cosmic rays, that means the C14 they contain will decay to Nitrogen and not be replaced. Over time, this means there will be less C14 relative to C12 than what we see in the environment.

This does rely on the assumption that the environmental proportion of C12 to C14 has remained roughly constant, but given that the only way it would fluctuate significantly is with a significant change in the amount of cosmic rays entering the atmosphere (such as poking a hole in the ozone layer by using too much spraypaint) and such events do leave records, that's a fairly safe assumption unless there's evidence to the contrary. It's certainly not perfect, but it gives a reasonable ballpark figure.

potdnewb posted...

still based on assumed standards

What problems do you have with those assumptions?