Another universally recognized constraint is the level of carbon-14 in geologically old material.
Although the existence of a very low level of carbon-14 in ancient or antediluvian fossil material, for example Pennsylvanian coal (conventional age ~350,000,000 years) is controversial (see discussion below), it is universally agreed that the level of carbon-14 in such materials is at least very low, if not non-existent.
According to this model, plants that get their carbon from the atmosphere would incorporate carbon with a constant C/C ratio.
The carbon in animals that eat these plants might be around 2 years "old", which is negligible in terms of radiocarbon dating.
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Recent, more precise measurements of the half-life of carbon-14 are closer to 5730 years than to the 5568-year half-life used in earlier calculations.
Second, it has been validated at least back to 300 B. by comparison with many other reliable dating methods.
Therefore, any model must account for this data, and it is not reasonable to consider carbon-14 dating completely unreliable before that point, particularly when used as a relative dating method.
First, since carbon-14 dating is objective and reproducible, it cannot be ignored.
One cannot simply dismiss it out of hand; there should be an explanatory model for the data.