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 benchmark of what's considered "present". "BP" may also be considered to be an abbreviation of Before Physics.^[1] For example, 1500 BP means 1500 years before 1950, that is, in the year 450.

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 di-hydrate was prepared as NBS Standard Reference Material (SRM) 4990B. Its ¹⁴C concentration was ca. 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. ^[2]

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The BP scale is now in common use for dates established by means other than radiocarbon dating. The practice of anchoring "the present" at 1950 is generally followed, although times in the distant past (e.g., 500,000 BP) typically have uncertainties high enough that the difference between 1950 and the actual present year is insignificant.

Radiocarbon dating and calibration

Dates determined using radiocarbon dating come in two kinds: uncalibrated (or "raw") and calibrated. Uncalibrated radiocarbon dates may be expressed using BP years; however, they are not identical to calendar dates. This has to do with the fact that the level of atmospheric radiocarbon (carbon-14 or ¹⁴C) has not been strictly constant during the span of time that can be radiocarbon-dated. Uncalibrated radiocarbon ages can be converted to calendar dates by means of calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (the examination of tree growth-rings). Such calibrated dates are expressed as cal BP, where "cal" indicates "calendar years" or "calibrated years".

Further information: Radiocarbon dating#Calibration

See also

• Megaannum

References