One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.So, we have a “clock” which starts ticking the moment something dies.
Accordingly, carbon dating carefully applied to items from historical times can be useful.
However, even with such historical calibration, archaeologists do not regard C produced and therefore dating the system.
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.
Unless this effect (which is additional to the magnetic field issue just discussed) were corrected for, carbon dating of fossils formed in the flood would give ages much older than the true ages.
A stronger magnetic field deflects more cosmic rays away from the Earth.