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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.

BP may also be considered to be an abbreviation of Before Physics.^[1]^[2]^[3]

[edit] Usage of "BP"

The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy.^[4]^[5]

This would force us to invent a new name for the normal concept of "present" or "today". It contrasts with a recommendation by van der Plicht & Hogg^[6], followed by the Quaternary Science Reviews,^[7]^[8] both of which requested that publications should use the unit "a" [for year] and reserve the term "BP" for radiocarbon estimations.

[edit] 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 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). "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.^[9]

The year 1950 was simply the year in which calibration curves for radiocarbon dating were established. It also marked^[1] the publication of the first radiocarbon dates in December 1949.^[10] The year 1950 is convenient because it predates large scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12.^[11]

[edit] 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.^[12] 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 "calendar years" or "calibrated years".

[edit] See also

[edit] References

^ ^a ^b Taylor RE (1985). "The beginnings of radiocarbon dating in American Antiquity: a historical perspective". American Antiquity: Journal of the Society for American Archaeology 50 (2): 309–325. doi:10.2307/280489. http://www.jstor.org/stable/280489.
^ CalPal software documentation, CalPal.de
^ Dena Ferran Dincauze (2000). Environmental Archaeology: Principles and Practice. Cambridge: Cambridge University Press. p. 110. ISBN 0521310776. http://books.google.ca/books?id=yS_t_4SHbpcC.
^ . "AGU Editorial Style Guide for Authors". American Geophysical Union. 21 September 2007. http://www.agu.org/pubs/style_guide_intro.html. Retrieved 2009-01-09.
^ North American Commission on Stratigraphic Nomenclature (November 2005). "North American Stratigraphic Code". The American Association of Petroleum Geologists Bulletin 89 (11): 1547-1591. http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13.
^ J. van der Plicht, A. Hogg (2006). "A note on reporting radiocarbon". Quaternary Geochronology 1: 237-240. doi:10.1016/j.quageo.2006.07.001.
^ "The use of time units in Quaternary Science Reviews". Quaternary Science Reviews 26: 1193. May 2007. doi:10.1016/j.quascirev.2007.04.002.
^ Wolff, Eric W. (December 2007). "When is the "present"?". Quaternary Science Reviews 26: 3023-3024. doi:10.1016/j.quascirev.2007.10.008.
^ Currie Lloyd A (2004), "The Remarkable Metrological History of Radiocarbon Dating [II]" (PDF), Journal of Research of the National Institute of Standards Technology 109: 185–217, http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf
^ Arnold JR, Libby WF (1949-03-04), "Age determinations by radiocarbon content: Checks with samples of known age", Science 109 (2827): 227–228, doi:10.1126/science.109.2827.227
^ AD or BC? from www.ScienceCourseware.org
^ Greene, Kevin (2002). Archaeology: An Introduction. Philadelphia: University of Pennsylvania Press. pp. 165–167. ISBN 0-8122-1828-0.

[Taylor-0] Taylor RE (1985). "The beginnings of radiocarbon dating in American Antiquity: a historical perspective". American Antiquity: Journal of the Society for American Archaeology 50 (2): 309–325. doi:10.2307/280489. http://www.jstor.org/stable/280489.

[1] CalPal software documentation, CalPal.de

[2] Dena Ferran Dincauze (2000). Environmental Archaeology: Principles and Practice. Cambridge: Cambridge University Press. p. 110. ISBN 0521310776. http://books.google.ca/books?id=yS_t_4SHbpcC.

[AGUStyle-3] . "AGU Editorial Style Guide for Authors". American Geophysical Union. 21 September 2007. http://www.agu.org/pubs/style_guide_intro.html. Retrieved 2009-01-09.

[Strat-4] North American Commission on Stratigraphic Nomenclature (November 2005). "North American Stratigraphic Code". The American Association of Petroleum Geologists Bulletin 89 (11): 1547-1591. http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13.

[5] J. van der Plicht, A. Hogg (2006). "A note on reporting radiocarbon". Quaternary Geochronology 1: 237-240. doi:10.1016/j.quageo.2006.07.001.

[6] "The use of time units in Quaternary Science Reviews". Quaternary Science Reviews 26: 1193. May 2007. doi:10.1016/j.quascirev.2007.04.002.

[7] Wolff, Eric W. (December 2007). "When is the "present"?". Quaternary Science Reviews 26: 3023-3024. doi:10.1016/j.quascirev.2007.10.008.

[Currie-8] Currie Lloyd A (2004), "The Remarkable Metrological History of Radiocarbon Dating [II]" (PDF), Journal of Research of the National Institute of Standards Technology 109: 185–217, http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf

[Arnold-9] Arnold JR, Libby WF (1949-03-04), "Age determinations by radiocarbon content: Checks with samples of known age", Science 109 (2827): 227–228, doi:10.1126/science.109.2827.227

[10] AD or BC? from www.ScienceCourseware.org

[Archaeology-11] Greene, Kevin (2002). Archaeology: An Introduction. Philadelphia: University of Pennsylvania Press. pp. 165–167. ISBN 0-8122-1828-0.

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Contents

[edit] Usage of "BP"

[edit] Radiocarbon dating

[edit] Radiocarbon calibration

[edit] See also

[edit] References