While there is no proof that the rates were different in the past than they are today, there is also no proof that they were the same. Thus radioactive dating relies purely on assumptions. We could put forward the following counter arguments to the constancy of these assumptions:
a) The constancy of cosmic ray bombardment might be questioned. The current high rate of entry might be a consequence of a disturbed post-Flood environment that altered the carbon-14 to carbon-12 ratio. Pre-Flood dates would thus have to be discarded.
b) An increase in the magnetic field of the earth would have shielded the earth from cosmic rays. Some scientists argue that the magnetic field of the earth has declined over time.
c) Atmospheric carbon forms just 0.0005% of the current carbon reservoir—99.66% of the earth's carbon exists in limestone, 0.31% in oil and gas, and 0.02% in coal. Carbon-14 comes from nitrogen and is independent of the carbon-12 reservoir. If even a small percentage of the limestone deposits were still in the form of living marine organisms at the time of the Flood, then the small amount of carbon-14 would have mixed with a much larger carbon-12 reservoir, thus resulting in a drastically reduced ratio. Specimens would then look much older than they actually are.
d) Even if the rate of decay is constant, without knowledge of the exact ratio of carbon-12 to carbon-14in the initial sample, the dating technique is subject to question." AmazingDiscoveries
Declare his glory among the heathen, his wonders among all people.
Psalm 96:3