For periods beyond 14,000 years, where tree rings are not continuously available, the calibration curve incorporates data from other sources: marine sediment records, speleothems (stalactites and stalagmites from caves, which incorporate radiocarbon from the atmosphere at the time of their formation), and coral records.

For the period around 24,000 years ago — near the Last Glacial Maximum — the calibration curve is well-constrained by multiple sources, but the specific shape of the calibration curve in this period produces some complexity in the resulting calibrated age range.

The calibration curve from roughly 20,000 to 30,000 years ago includes a feature known as a “plateau” — a period where the atmospheric radiocarbon concentration was relatively flat, meaning that a range of actual calendar years corresponds to approximately the same radiocarbon age.

This plateau makes dates in this range somewhat less precise in calendar year terms than dates from periods where the calibration curve changes more steeply.