why would a radioactive element need to have a known rate of decay in order for it to be useful as a dating tool?

This process by which an unstable atomic nucleus loses energy by releasing radiation is called radioactive decay. The thing that makes this decay process so valuable for determining the age of an object is that each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life.

Explanation:

i think the miocene epoch spans the time from 23 million to 12 million years ago.

nucleus

would be most reasonable answer tbh