Before Present

Before Present (BP) years is a time scale used mainly in geology and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use 1 January 1950 as commencement date of the age scale, reflecting the fact that radiocarbon dating became practicable in the 1950s. The abbreviation "BP", with the same meaning, has also been interpreted as "Before Physics"; that is, before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable.

Usage of "BP"

The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy.

This usage differs with the recommendation by van der Plicht & Hogg, followed by the Quaternary Science Reviews, both of which requested that publications should use the unit "a" [for year] and reserve the term "BP" for radiocarbon estimations.

Some archaeologists use the lowercase letters bp, bc and ad as terminology for uncalibrated dates for these eras.

Radiocarbon dating

Radiocarbon dating was first used in 1940. 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. According to scientist A. Currie Lloyd:

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 CE 1950. Radiocarbon measurements are compared to this modern carbon value, and expressed as "fraction of modern" (fM). "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.

The year 1950 was chosen because it was the standard astronomical epoch at that time. It also marked the publication of the first radiocarbon dates in December 1949, and 1950 also antedates large scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12.

Radiocarbon calibration

Further information: Radiocarbon dating § Calibration

Dates determined using radiocarbon dating come in two kinds: uncalibrated (also called Libby or raw) and calibrated (also called Cambridge) dates. Uncalibrated radiocarbon dates should be clearly noted as such by "uncalibrated years BP", because 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 (dating on basis of tree growth-rings) and stratigraphy (dating on the basis of sediment layers in mud or sedimentary rock). Such calibrated dates are expressed as cal BP, where "cal" indicates "calibrated years." or "calendar years" before 1950. Many scholarly/scientific journals demand to add the calibrating laboratory code with year, because of differences and progress in quality.

Other dating conventions

• Ab urbe condita (A.U.C.)
• Anno Domini (A.D.)
• After the Development of Agriculture (A.D.A.)
• Anno Hegiri (A.H.)
• Anno Mundi (A.M.)
• Before Christ (B.C.)
• Before Common Era (B.C.E.)
• Common Era (C.E.)
• Holocene calendar (H.E.)
• years ago (ya)