Radioactive dating interactive
Since the flood was accompanied by much volcanism (see Noah's Flood…, How did animals get from the Ark to isolated places? ), fossils formed in the early post-flood period would give radiocarbon ages older than they really are.
In summary, the carbon-14 method, when corrected for the effects of the flood, can give useful results, but needs to be applied carefully.
Also, the Genesis flood would have greatly upset the carbon balance.
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.
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.
The strength of the Earth's magnetic field affects the amount of cosmic rays entering the atmosphere.
Furthermore, different types of plants discriminate differently.
However, even with such historical calibration, archaeologists do not regard C produced and therefore dating the system.