Before Present
From Wikipedia, the free encyclopedia
Before Present (BP) years are the units of time used to report raw, uncalibrated ages and dates determined by radiocarbon dating. The years are counted backwards to the past from the year AD 1950. For example, 12,000 BP means a raw radiocarbon age of 12,000 years, or a raw radiocarbon date equal to 12,000 radiocarbon years before AD 1950.
Raw BP dates are not identical to calendar dates because the level of atmospheric radiocarbon (carbon-14 or 14C) has not been strictly constant during the span of time that can be radiocarbon-dated. Also, the earliest radiocarbon dates were calculated on basis of a carbon-14 half-life of 5568 years, later determined to be 5730 years.
Raw BP dates can be converted to calendar dates by their calibration based on comparison of raw radiocarbon dates of samples independently dated by other methods such as dendrochronology, i.e. the examination of tree growth-rings. Calibrated dates are expressed as cal BP, cal BC or cal AD.
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[edit] Why 1950?
Scientists established 1950 as the origin year for the BP scale because it is the year in which calibration curves for radiocarbon dating were established. The year 1950 also predates large scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12.
[edit] "Before Present" or "Before Physics"
In this forum it is explained that an aspect of the introduction of the original NBS [U.S. National Bureau of Standards] radiocarbon standard implied that AD 1950 constitutes 0 BP in carbon-14 age computations. The choice of AD 1950 was, to a degree, arbitrary; it was adopted to honor the publication of the first carbon-14 dates in December 1949 (Arnold and Libby, 1949). The agreement was to use AD 1950 as the origin or zero (reference) year for carbon-14 dating. The meaning of BP was redefined from 'Before Present' to 'Before Physics' (Flint and Devey, 1962) (Taylor 1987:97–98), however "before present" is used more frequently.
[edit] See also
[edit] References
- AD or BC? Questions at California State University, Los Angeles
- 14C-Age Definition
- Mook, Willem G and van der Plicht, Johannes, Reporting 14C Activities and Concentrations, Radiocarbon, Vol. 41, Nr. 3, 1999, p. 227-239. pdf html
