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Before Present

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Before Present (BP) years, also known as "time before present" or "years before present (YBP)", is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date (epoch) of the age scale. The abbreviation "BP" has been interpreted retrospectively as "Before Physics",[1] which refers to the time before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, which scientists must account for.[2][3]

In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating; the alternative notation RCYBP stands for the explicit "radio carbon years before present".


The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy.[4][5] This usage differs from the 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 "annum", Latin 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.[9]

The Centre for Ice and Climate at the University of Copenhagen instead uses the unambiguous "b2k", for "years before 2000 AD", not necessarily, however, often in combination with the Greenland Ice Core Chronology 2005 (GICC05) time scale.[10]

Some authors who use the YBP dating format also use YAP (years after present) to denote years after 1950.[11]

SI prefixes[edit]

SI prefix multipliers may be used to express larger periods of time, e.g. ka BP (thousand years BP), Ma BP (million years BP) and many others.[12]

Radiocarbon dating[edit]

Radiocarbon dating was first used in 1949.[13][14] 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 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.[15]

The year 1950 was chosen because it was the standard astronomical epoch at that time.[citation needed] It also marked[2] the publication of the first radiocarbon dates in December 1949,[16] and 1950 also antedates large-scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12.[17]

Radiocarbon calibration[edit]

Dates determined using radiocarbon dating come as two kinds: uncalibrated (also called Libby or raw) and calibrated (also called Cambridge) dates.[18] 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 calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (dating based on tree growth-rings) and stratigraphy (dating based on 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 and 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.


Conversion from Gregorian calendar years to Before Present years is by starting with the 1950-01-01 epoch of the Gregorian calendar and increasing the BP year count with each year into the past from that Gregorian date.

For example, 1000 BP corresponds to 950 AD, 1949 BP corresponds to 1 AD, 1950 BP corresponds to 1 BC, 2000 BP corresponds to 51 BC.

Example milestone years in the BP time scale
Gregorian year BP year Event
9701 BC 11650 BP End of the Pleistocene and beginning of the Holocene epoch[19]
4714 BC 6663 BP Epoch of the Julian day system: Julian day 0 starts at Greenwich noon on January 1, 4713 BC of the proleptic Julian calendar, which is November 24, 4714 BC in the proleptic Gregorian calendar[20]: 10 
2251 BC 4200 BP Beginning of the Meghalayan age, the current and latest of the three stages in the Holocene era.[21][22]
45 BC 1994 BP Introduction of the Julian calendar
1 BC 1950 BP Year zero at ISO 8601
AD 1 1949 BP Beginning of the Common Era and Anno Domini, from the estimate by Dionysius of the Incarnation of Jesus
1582 368 BP Introduction of the Gregorian calendar[20]: 47 
1950 Epoch of the Before Present dating scheme[23]: 190 
2024 74 YAP Current year

See also[edit]


  1. ^ Flint, Richard Foster; Deevey, Edward S (1962). "Volume 4 – 1962". Radiocarbon. 4 (1): i.
  2. ^ a b Taylor RE (1985). "The beginnings of radiocarbon dating in American Antiquity: a historical perspective". American Antiquity. 50 (2): 309–325. doi:10.2307/280489. JSTOR 280489. S2CID 163900461.
  3. ^ Dincauze, Dena (2000). "Measuring time with isotopes and magnetism". Environmental Archaeology: Principles and Practice. Cambridge, England: Cambridge University Press. p. 110. ISBN 978-0-5213-1077-2.
  4. ^ "AGU Editorial Style Guide for Authors". American Geophysical Union. 21 September 2007. Archived from the original on 2008-07-14. Retrieved 2009-01-09.
  5. ^ North American Commission on Stratigraphic Nomenclature (November 2005). "North American Stratigraphic Code: Article 13 (c)". The American Association of Petroleum Geologists Bulletin. 89 (11): 1547–1591. doi:10.1306/07050504129. Archived from the original on 2014-02-02. Retrieved 2009-06-29.
  6. ^ van der Plicht, Johannes; Hogg, Alan (2006). "A note on reporting radiocarbon" (PDF). Quaternary Geochronology. 1 (4): 237–240. Bibcode:2006QuGeo...1..237V. doi:10.1016/j.quageo.2006.07.001. S2CID 128628228.
  7. ^ "The use of time units in Quaternary Science Reviews". Quaternary Science Reviews. 26 (9–10): 1193. May 2007. Bibcode:2007QSRv...26.1193.. doi:10.1016/j.quascirev.2007.04.002.
  8. ^ Wolff, Eric W. (December 2007). "When is the "present"?". Quaternary Science Reviews. 26 (25–28): 3023–3024. Bibcode:2007QSRv...26.3023W. doi:10.1016/j.quascirev.2007.10.008. S2CID 131227900.
  9. ^ Edward J. Huth (25 November 1994). Scientific Style and Format: The CBE Manual for Authors, Editors, and Publishers. Cambridge University Press. pp. 495–. ISBN 978-0-521-47154-1. Retrieved 4 October 2012.
  10. ^ "The GICC05 time scale". Centre for Ice and Climate – University of Copenhagen. 3 September 2009. Archived from the original on 18 September 2018. Retrieved September 17, 2018.
  11. ^ Berger, André (1988). "Milankovitch Theory and Climate". Reviews of Geophysics. 26 (4): 624–657. Bibcode:1988RvGeo..26..624B. doi:10.1029/RG026i004p00624. ISSN 8755-1209.
  12. ^ Martin Kölling (2015). "Numerous ways to say "thousand years" in a scientific paper". Universität Bremen: Marine Geochemistry - Laboratory Methods. Retrieved 2023-03-24.
  13. ^ Arnold, J.R.; Libby, W.F. (1949). "Age determinations by radiocarbon content: checks with samples of known age". Science. 110 (2869): 678–680. Bibcode:1949Sci...110..678A. doi:10.1126/science.110.2869.678. JSTOR 1677049. PMID 15407879.
  14. ^ Aitken (1990), pp. 60–61.
  15. ^ Currie, Lloyd A (March–April 2004). "The Remarkable Metrological History of Radiocarbon Dating [II]" (PDF). Journal of Research of the National Institute of Standards and Technology. 109 (2): 185–217. doi:10.6028/jres.109.013. PMC 4853109. PMID 27366605. Archived from the original (PDF) on 2010-12-06. Retrieved 30 October 2019. "The Remarkable Metrological History of Radiocarbon Dating [II]" Archived 2023-01-01 at the Wayback Machine at Google Books (accessed 30 October 2019).
  16. ^ Arnold JR, Libby WF (1949-03-04). "Age determinations by radiocarbon content: Checks with samples of known age". Science. 109 (2827): 227–228. Bibcode:1949Sci...109..227L. doi:10.1126/science.109.2827.227. PMID 17818054.
  17. ^ "Nuclear Bombs Made It Possible to Carbon Date Human Tissue". Smithsonian Magazine. 2013-02-19. Retrieved 2020-01-09.
  18. ^ Greene, Kevin (2002). Archaeology: An Introduction. Philadelphia: University of Pennsylvania Press. pp. 165–167. ISBN 0-8122-1828-0.
  19. ^ Walker, Mike; Jonsen, Sigfus; Rasmussen, Sune Olander; Popp, Trevor; Steffensen, Jørgen-Peder; Gibbard, Phil; Hoek, Wim; Lowe, John; Andrews, John; Björck, Svante; Cwynar, Les C.; Hughen, Konrad; Kershaw, Peter; Kromer, Bernd; Litt, Thomas; Lowe, David J.; Nakagawa, Takeshi; Newnham, Rewi; Schwander, Jacob (2009). "Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records" (PDF). Journal of Quaternary Science. 24 (1): 3–17. Bibcode:2009JQS....24....3W. doi:10.1002/jqs.1227. Archived (PDF) from the original on 2013-11-04.
  20. ^ a b Dershowitz, Nachum; Reingold, Edward M. (2008). Calendrical Calculations (3rd ed.). Cambridge University Press. ISBN 978-0-521-70238-6.
  21. ^ "ICS chart containing the Quaternary and Cambrian GSSPs and new stages (v 2018/07) is now released!". Retrieved February 6, 2019.
  22. ^ Conners, Deanna (September 18, 2018). "Welcome to the Meghalayan age". Retrieved February 6, 2019.
  23. ^ Currie Lloyd A (2004). "The Remarkable Metrological History of Radiocarbon Dating [II]" (PDF). Journal of Research of the National Institute of Standards and Technology. 109 (2): 185–217. doi:10.6028/jres.109.013. PMC 4853109. PMID 27366605. Archived from the original (PDF) on 2010-12-06. Retrieved 2018-06-24.