Radiometric dating is the method for establishing the age of objects by measuring the levels of radioisotopes in the sample. It decays to nitrogen 14 with a half life of 5730 years.
Sediment columns giving an unbroken history for more than 25,000 years have been identified in about 30 locations around the world.
Coral growth patterns are also seasonal and provide a long independent date history. The dates obtained by different radiometric isotope pairs cross-check each other.
Age estimates can be cross-tested by using different isotope pairs.
Results from different techniques, often measured in rival labs, continually confirm each other.
Measuring the ratio of C14 to C12 and C13 therefore dates the organic matter for periods back to about eight half-lives of the isotope, 45,000 years.