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Before Present
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{{short description|Calendar based on 1950 CE (around when radiocarbon dating was developed and nuclear weapons testing artificially altered the proportion of carbon isotopes)}}{{redirect2|YBP|ybp|the airport with the IATA code YBP|Yibin Caiba Airport}}Before Present (BP) years is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred in the past. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date (epoch) of the age scale,{{efn|Unlike other eras, the sense "After Present" (AP) is never used.}} reflecting the origin of practical radiocarbon dating in the 1950s. The abbreviation "BP" has been interpreted retrospectively as "Before Physics";JOURNAL, Flint, Richard Foster, Deevey, Edward S, 1962, Volume 4 – 1962,weblink Radiocarbon (journal), Radiocarbon, 4, 1, i, that refers to the time before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable.JOURNAL
, The beginnings of radiocarbon dating in American Antiquity: a historical perspective
, Taylor RE
, American Antiquity
, 1985
, 50
, 2
, 309–325
, 10.2307/280489
, BOOK, Dincauze, Dena, Environmental Archaeology: Principles and Practice, 2000, Cambridge University Press, Cambridge, England, 978-0-5213-1077-2, 110, Measuring time with isotopes and magnetism, In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating.

Usage

The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy.WEB
,weblink
, American Geophysical Union
, AGU Editorial Style Guide for Authors
, 21 September 2007
, 2009-01-09
weblink" title="web.archive.org/web/20080714134306weblink">weblink >archivedate = 2008-07-14, JOURNAL
, North American Commission on Stratigraphic Nomenclature
,weblink
, North American Stratigraphic Code: Article 13 (c)
, The American Association of Petroleum Geologists Bulletin
, 89
, 11
, November 2005
, 1547–1591
, 10.1306/07050504129
, This usage differs with the recommendation by van der Plicht & Hogg,JOURNAL
, A note on reporting radiocarbon
, J. van der Plicht, A. Hogg
, Quaternary Geochronology
, 1
, 2006
, 4
, 237–240
, 10.1016/j.quageo.2006.07.001
, followed by the Quaternary Science Reviews,JOURNAL
, The use of time units in Quaternary Science Reviews
, Quaternary Science Reviews
, 26
, 9–10
, May 2007
, 1193
, 10.1016/j.quascirev.2007.04.002, 2007QSRv...26.1193., JOURNAL
, Wolff, Eric W.
, When is the "present"?
, Quaternary Science Reviews
, 26
, December 2007
, 25–28
, 3023–3024
, 10.1016/j.quascirev.2007.10.008, 2007QSRv...26.3023W, 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.BOOK, Edward J. Huth, Scientific Style and Format: The CBE Manual for Authors, Editors, and Publishers,weblink 4 October 2012, 25 November 1994, Cambridge University Press, 978-0-521-47154-1, 495–, The Centre for Ice and Climate at the University of Copenhagen has proposed "b2k" as "years before AD 2000", based on the Greenland Ice Core Chronology 2005 (GICC05) time scale.WEB,weblink The GICC05 time scale, Centre for Ice and Climate - University of Copenhagen, September 17, 2018,

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:{{bquote|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.JOURNAL
,weblink
, PDF
, Currie Lloyd A
, The Remarkable Metrological History of Radiocarbon Dating [II]
, Journal of Research of the National Institute of Standards and Technology
, 109
, 185–217
, 2004
, 10.6028/jres.109.013
, 2007-07-25
,weblink" title="web.archive.org/web/20101206195414weblink">weblink
, 2010-12-06
, dead
, }}
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,JOURNAL
, Arnold JR, Libby WF
, Age determinations by radiocarbon content: Checks with samples of known age
, Science
, 109
, 2827
, 1949-03-04
, 227–228
, 10.1126/science.109.2827.227
, 17818054
,
nuclear testing>atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12.AD or BC? from www.ScienceCourseware.org

Radiocarbon calibration

Dates determined using radiocarbon dating come in two kinds: uncalibrated (also called Libby or raw) and calibrated (also called Cambridge) dates.BOOK, Greene, Kevin, Kevin Greene (archaeologist), Archaeology: An Introduction, 2002, University of Pennsylvania Press, Philadelphia, 0-8122-1828-0, 165–167, 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 the 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 require that published calibrated results be accompanied by the name (standard codes are used) of the laboratory concerned, and other information such as confidence levels, because of differences between the methods used by different laboratories and changes in calibrating methods.

Other dating conventions

See also

References

{{reflist}}{{notelist}}{{Chronology}}


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