Before Present
Encyclopedia : B : BE : BEF : Before Present
Before Present (BP) years are the units of time (counted backwards to the past) used to report raw radiocarbon ages and dates referenced to the BP scale origin in the year AD 1950 (identical to 1950 CE). For example, 12,000 BP means a raw radiocarbon age equal to 12,000 years, or a raw radiocarbon date equal to 12,000 radiocarbon years before AD 1950 [link]
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Why \"raw radiocarbon age\"?
A raw BP date cannot be used directly as a calendar date, because the level of atmospheric 14C has not been strictly constant during the span of time that can be radiocarbon dated. It is also unlikely to point exactly to a single year. These issues are discussed in the main article on radiocarbon dating - measurements, scales, and calibration. Radiocarbon dating of specimens may be used alongside other dating techniques.Why 1950?
Scientists established 1950 as the origin year for the BP scale because it is the year in which calibration curves for carbon-14 dating, or radiocarbon dating, were established. The year 1950 also predates large scale atmospheric testing of nuclear weapons, which altered the global balance of carbon-14 to carbon-12.\"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 14C age computations. The choice of AD 1950 was, to a degree, arbitrary; it was adopted to honor the publication of the first 14C dates in December 1949 (Arnold and Libby, 1949). The agreement to use AD 1950 as the origin or zero (reference) year for 14C 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.See also
- radiocarbon dating scales and measurements
- bya
- mya (unit)
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]
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