¹⁴C decay, calibration curves, and age probability distributions
Cosmic rays produce ¹⁴C in the atmosphere by bombarding nitrogen-14; living organisms maintain a constant ¹⁴C/¹²C ratio (~1.2×10⁻¹²) by continuously exchanging carbon with the atmosphere. At death, ¹⁴C decays with a half-life of 5,730 years (Libby used 5,568 yr — the "Libby half-life" is retained by convention in raw dates). Raw radiocarbon ages must be calibrated against the IntCal calibration curve (built from dendrochronology, coral, and speleothems) because atmospheric ¹⁴C has varied significantly over time due to solar activity, geomagnetic field changes, and ocean circulation. The resulting calibrated probability distribution can be multimodal when the calibration curve has plateaus.