Castlepook Cave Assemblage
For this period, precision is necessarily limited. Radiocarbon laboratories measure the remaining radioactive carbon in once-living material, such as bone collagen. The result is expressed in radiocarbon years before present, not in BCE. Atmospheric carbon levels changed over millennia, so those results must be compared with calibration curves assembled from independently dated records, including tree rings, corals, cave deposits and lake sediments. At nearly 34,000 years ago, calibrated ranges can be broad, and revisions to methods or samples can alter previous conclusions.