carbon-14, carbon-dating

carbon-14: noun

carbon dating: noun

carbon-14 test: adjective

carbon-date: verb

Carbon-14 may be abbreviated after first use: C-14 or 14C.

Radiocarbon dates are normally given as years before present (years BP), with 1950 as the base year because after that date testing of nuclear weapons added carbon 14 to the atmosphere. Years BP are not the same as calendar years. For instance, a radiocarbon date of 5,000 years before present (or 5000 BP) is the same as 5,750 years ago or 3750 B.C. When giving dates derived from radiocarbon content, explain whether the dates are given in radiocarbon years or in calendar years and be consistent within one article or book.

The difference occurs because the amount of carbon in the atmosphere has not been constant over geologic time. Carbon-14 dates can be adjusted using one of two calibration scales: Calib (developed by the University of Washington and Queen's University Belfast) and OxCal (developed by the Oxford Radiocarbon Accelerator Unit). The biggest difference between them is that OxCal rounds to the nearest 10 years and Calib gives exact dates. Both calibration curves use tree-ring ages up to around 11,400 years ago and coral ages between 11,400 and 20,000 years ago to match radiocarbon years with calendar years.

For more information see the September 2000 edition of Scientific American or the December 1, 2007, issue of Science News Online ("Rolling Back the Years, Radiocarbon Dating Gets a Remake").