Reindeer Bone at Mammoth Cave
Radiocarbon years are not calendar years. A laboratory measures the remaining radioactive carbon in a sample such as bone collagen, then estimates when the animal died. The result must be calibrated against changing atmospheric carbon levels, using curves assembled from independently dated archives including tree rings, cave deposits, lake sediments and corals. At more than 30,000 years ago, those conversions have appreciable uncertainties. A result may translate into a range of calendar ages rather than a single point, and revised pretreatment of old bone can alter earlier determinations. Modern dating conventions also count “before present” from AD 1950, not from the present day.