As long as an organism is alive, it incorporates carbon from the environment at the ratio present in the atmosphere.

When it dies, the carbon-14 begins to decay at a known rate.

By measuring how much carbon-14 remains relative to carbon-12, laboratories can calculate how long ago the organism died.

The challenge with aquatic environments — as researchers working on the White Sands footprints in New Mexico also confronted — is that water can contain carbon that is older than the water itself, derived from ancient geological sources that dissolved into the water over long periods.

If an organism incorporates some of this old carbon during its life, its radiocarbon date will appear older than it actually is — a problem called the reservoir effect.