That is, they take up less than would be expected and so they test older than they really are.
Furthermore, different types of plants discriminate differently.
It cannot be used to date volcanic rocks, for example.
The rate of decay of N in 5,730 years (plus or minus 40 years).
Overall, the energy of the Earth's magnetic field has been decreasing, so more C is being produced now than in the past.
This will make old things look older than they really are.
So, we have a “clock” which starts ticking the moment something dies.
Obviously, this works only for things which were once living.
The flood buried a huge amount of carbon, which became coal, oil, etc., lowering the total C ratio in plants/animals/the atmosphere before the flood had to be lower than what it is now.
Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes.
One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.
These techniques are applied to igneous rocks, and are normally seen as giving the time since solidification.
The isotope concentrations can be measured very accurately, but isotope concentrations are not dates.