Before Present (
BP) years is 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 1 January 1950 as the arbitrary origin of the age scale. For example, 1500 BP means 1500 years before 1950, that is, in the year 450 AD.
BP may also be considered to be an abbreviation of
Before Physics.
Usage of "BP"
The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as
stratigraphy.
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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.
Radiocarbon dating
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 year 1950 was chosen because it was the year in which
calibration curves for radiocarbon dating were first established. It also marked
the publication of the first radiocarbon dates in December 1949.
1950 also predates large scale
atmospheric testing of nuclear weapons, which altered the global ratio of
carbon-14 to
carbon-12.
Radiocarbon 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 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 (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."
See also
