What is the main idea of the passage?

Radiocarbon dating estimates when organic material stopped exchanging carbon, but sampling, calibration, contamination, context, and uncertainty shape the result. (The passage explains the decay basis while emphasizing the steps required to turn measurement into a historical date.)

Why might a timber date be older than a building?

Wood from an old tree or earlier structure may have been reused later. (The fourth paragraph distinguishes when wood grew from when people used it in the current structure.)

What does “calibrated” mean here?

adjusted by comparison with independently dated records (Raw radiocarbon ages are compared with tree-ring and other dated archives to relate them to calendar time.)

What independently dated archive is especially valuable?

tree rings (The third paragraph identifies countable and matchable tree rings as especially valuable.)

Reading Time in Ancient Wood: Level 4 Reading Comprehension Questions

A living tree takes in atmospheric carbon. It includes a small amount of radioactive carbon-14. After the tree dies, exchange largely stops, and carbon-14 in its wood decays at a known rate. Measuring how much remains can help estimate when the living material stopped exchanging carbon.

The measurement is not a clock with a printed calendar date. Laboratories compare carbon-14 with stable carbon and report an age with uncertainty. Contamination from newer roots, conservation glue, or older carbon can shift a result. Researchers clean samples and choose material tied closely to the event they want to date.

Atmospheric carbon-14 has varied over time. A raw radiocarbon age must therefore be calibrated against records with independently known dates. Tree rings are valuable because their calendar years can be counted and matched across overlapping wood. Other archives extend parts of the calibration record.

Calibration can turn one measured range into an irregular set of possible calendar ranges. Greater numerical precision in the laboratory does not remove every ambiguity in the curve. Archaeological context also matters. A beam cut from an old tree may be reused centuries later, so the wood's age need not equal the building's age. Radiocarbon dating is powerful because physical decay supplies a measurable connection to time. It is careful rather than magical: sampling, chemistry, calibration, context, and uncertainty all shape the final interpretation.

Comprehension questions