Radiocarbon dating of fossils taken from caves
If the atmosphere's ratio of carbon-14 to carbon-12 has doubled since the flood and we did not know it, radiocarbon ages of things living soon after the flood would appear to be one half-life (or 5,730 years) older than their true ages.If that ratio quadrupled, organic remains would appear 11,460 (2 x 5,730) years older, etc.Afterward, less carbon would be available to enter the atmosphere from decaying vegetation.
Half of the remainder will decay in another 5,730 years, and so on.
When granite rock hardens, it freezes radioactive elements in place.
The most common radioactive element in granite is Uranium-238.
Cosmic radiation striking the upper atmosphere converts about 21 pounds of nitrogen each year into radiocarbon (carbon-14).
Most carbon-14 quickly combines with oxygen to form radioactive carbon dioxide, which then spreads throughout the atmosphere.
They concluded that the helium in the rock was 100,000 times more plentiful than it should have been if the rocks were really 1.5 billion years old.