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( Before Present)
Before Present (BP) years are a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use 1950 as the arbitrary origin of the age scale. For example, 1500 BP means 1500 years before 1950, that is, in the year 450. "BP" may also be considered to be an abbreviation of Before Physics.[1][2][3] Beginning in 1954, metrologists established 1950 as the origin year for the BP scale for use with radiocarbon dating using a 1950-based reference sample of oxalic acid The problem was tackled by the international radiocarbon community in the late 1950s, in cooperation with the U.S. National Bureau of Standards. A large quantity of contemporary oxalic acid dihydrate was prepared as NBS Standard Reference Material (SRM) 4990B. Its 14C concentration was about 5% above what was believed to be the natural level, so the standard for radiocarbon dating was defined as 0.95 times the 14C concentration of this material, adjusted to a 13C reference value of –19 per mil (PDB). This value is defined as “modern carbon” referenced to AD 1950. Radiocarbon measurements are compared to this modern carbon value, and expressed as “fraction of modern” (fM); and “radiocarbon ages” are calculated from fM using the exponential decay relation and the “Libby half-life” 5568 a. The ages are expressed in years before present (BP) where “present” is defined as AD 1950. [4]
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