They have the same ratio of carbon-14 to carbon-12 as the atmosphere, and this same ratio is then carried up the food chain all the way to apex predators, like sharks.
If you ever wondered why nuclear tests are now performed underground, this is why.
Most radiocarbon dating today is done using an accelerator mass spectrometer, an instrument that directly counts the numbers of carbon 14 and carbon12 in a sample.
Follow the links in this post to learn more about radiocarbon dating.
Rachel Wood does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
The unstable carbon-14 gradually decays to carbon-12 at a steady rate. Scientists measure the ratio of carbon isotopes to be able to estimate how far back in time a biological sample was active or alive.