Carbon 14 dating seal
It may conjure up a very odd mental picture, but every living thing on Earth has its own internal clock that's ready to start ticking the moment it dies.
This 45 years of setting off nuclear weapons above ground generated a massive spike in carbon-14 that doubled the normal amount, but this has been dropping as time goes on and the atmospheric isotope is absorbed into the biosphere.
This has made the atmosphere appear much younger just as the fossil carbon makes it appear much older.
Industrial carbon has been throwing off dating since about 1880 and it hasn't been restricted to atmospheric emissions.
Many fertilizers, pesticides, and other agricultural chemicals use petrochemicals, which are routinely sprayed on crops and have to be taken into account as a dating contaminant.
It's based on the very simple principle that radioactive isotopes decay at a steady, predictable rate.