Radiometric dating seafloor
Carbon dating measures the decay of the carbon-14 (14C) isotope in the coral skeleton.
The 14C isotope has a known decay rate that can be measured relative to the atmosphere to establish the age of the coral.
Second, one must be certain that there were no daughter isotopes in the beginning.
Third, one must be certain that neither parent nor daughter isotopes have ever been added or removed from the specimen.
These vast ages for the sediments are generally not obtained by radioisotopic dating.