Measuring carbon in a sample
Carbon-14 forms in the upper atmosphere and enters living organisms through carbon dioxide, photosynthesis and food webs. After carbon exchange ceases, its abundance decreases through radioactive decay. Comparison with standards estimates the elapsed time.
Accelerator mass spectrometry measures isotope proportions. Pretreatment remains necessary to remove exogenous carbon introduced during burial, packaging or conservation. Obtaining an instrument result does not by itself eliminate contamination.
Radiocarbon ages and calendar dates
Atmospheric carbon-14 has varied, so a conventional age is not converted to a calendar date by simple subtraction. BP uses the conventional reference year 1950; calibration requires a curve appropriate to the carbon source.
Calibrated results are probability ranges, sometimes separated into several intervals. Laboratory identifiers, original measurements and uncertainties, curves and methods support reassessment. Marine or freshwater reservoir effects can also offset contemporary samples.
References: [1]
Relating the sample to an event
Inner tree rings predate felling, while timber can be stored and reused. A charcoal date need not be the construction or burning date. Short-lived samples reduce some offsets but do not remove the need to assess context.
Stratigraphy, preservation and associated finds help distinguish fresh deposition from residual or intrusive material. A laboratory determination alone is not a single definitive date for an entire site.