Trees maintain carbon 14 equilibrium in their growth rings—and trees produce a ring for every year they are alive.Although we don't have any 50,000-year-old trees, we do have overlapping tree ring sets back to 12,594 years.Reliable estimates are possible, but with large /- factors.As you might imagine, scientists have been attempting to discover other organic objects that can be dated securely steadily since Libby's discovery.
So, if you measure the amount of C14 in a dead organism, you can figure out how long ago it stopped exchanging carbon with its atmosphere.What you need is a ruler, a reliable map to the reservoir: in other words, an organic set of objects that you can securely pin a date on, measure its C14 content and thus establish the baseline reservoir in a given year.Fortunately, we do have an organic object that tracks carbon in the atmosphere on a yearly basis: tree rings.Beginning in the 1990s, a coalition of researchers led by Paula J.Reimer of the CHRONO Centre for Climate, the Environment and Chronology, at Queen's University Belfast, began building an extensive dataset and calibration tool that they first called CALIB.