Before Present

The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy.{{Cite web|url=http://www.agu.org/pubs/style_guide_intro.html|publisher=American Geophysical Union|title=AGU Editorial Style Guide for Authors|date=21 September 2007|access-date=2009-01-09|archive-url=https://web.archive.org/web/20080714134306/http://www.agu.org/pubs/style_guide_intro.html |archive-date=2008-07-14}}{{Cite journal|author=North American Commission on Stratigraphic Nomenclature|url=http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13|title=North American Stratigraphic Code: Article 13 (c)|journal=The American Association of Petroleum Geologists Bulletin|volume=89|issue=11|date=November 2005|pages=1547–1591|doi=10.1306/07050504129|access-date=2009-06-29|archive-date=2014-02-02|archive-url=https://web.archive.org/web/20140202155725/http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13|url-status=dead|url-access=subscription}} This usage differs from the recommendation by van der Plicht & Hogg,

{{Cite journal|title=A note on reporting radiocarbon|first1=Johannes|last1=van der Plicht|first2=Alan|last2=Hogg|journal=Quaternary Geochronology|volume=1|year=2006|issue=4|pages=237–240|doi=10.1016/j.quageo.2006.07.001|bibcode=2006QuGeo...1..237V |s2cid=128628228 |url=https://pure.rug.nl/ws/files/6706881/2006QuatGeochronolvdPlicht.pdf}} followed by the Quaternary Science Reviews,

{{Cite journal|title=The use of time units in Quaternary Science Reviews|journal=Quaternary Science Reviews|volume=26|issue=9–10|date=May 2007|pages=1193|doi=10.1016/j.quascirev.2007.04.002|bibcode=2007QSRv...26.1193.}}{{Cite journal|last=Wolff|first=Eric W.|title=When is the "present"?|journal=Quaternary Science Reviews|volume=26|date=December 2007|issue=25–28|pages=3023–3024|doi=10.1016/j.quascirev.2007.10.008|bibcode=2007QSRv...26.3023W|s2cid=131227900 }} 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.{{Cite book|author=Edward J. Huth|title=Scientific Style and Format: The CBE Manual for Authors, Editors, and Publishers|url=https://books.google.com/books?id=PoFJ-OhE63UC&pg=PA495|access-date=4 October 2012|date=25 November 1994|publisher=Cambridge University Press|isbn=978-0-521-47154-1|pages=495–}}

The Centre for Ice and Climate at the University of Copenhagen instead uses the unambiguous "b2k", for "years before 2000 AD", often in combination with the Greenland Ice Core Chronology 2005 (GICC05) time scale.{{Cite web|url=http://www.iceandclimate.nbi.ku.dk/research/strat_dating/annual_layer_count/gicc05_time_scale/|title=The GICC05 time scale|date=3 September 2009|publisher=Centre for Ice and Climate – University of Copenhagen|access-date=September 17, 2018|archive-date=18 September 2018|archive-url=https://web.archive.org/web/20180918012233/http://www.iceandclimate.nbi.ku.dk/research/strat_dating/annual_layer_count/gicc05_time_scale/|url-status=dead}}

Some authors who use the YBP dating format also use "YAP" ("years after present") to denote years after 1950.{{Cite journal |last=Berger |first=André |date=1988 |title=Milankovitch Theory and Climate |url=https://dial.uclouvain.be/pr/boreal/en/object/boreal:52561 |journal=Reviews of Geophysics |language=en |volume=26 |issue=4 |pages=624–657 |doi=10.1029/RG026i004p00624 |bibcode=1988RvGeo..26..624B |issn=8755-1209}}

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.{{Cite web|author = Martin Kölling |title = Numerous ways to say "thousand years" in a scientific paper |date = 2015 |access-date = 2023-03-24 |publisher = Marine Geochemistry - Laboratory Methods |place = Universität Bremen |url = https://www.sedgeochem.uni-bremen.de/kiloyears.html}}

Radiocarbon dating was first used in 1949.{{cite journal |last1=Arnold |first1=J.R. |last2=Libby |first2=W.F. |year=1949 |title=Age determinations by radiocarbon content: checks with samples of known age |url=http://hbar.phys.msu.ru/gorm/fomenko/libby.htm |journal=Science |volume=110 |issue=2869 |pages=678–680 |bibcode=1949Sci...110..678A |doi=10.1126/science.110.2869.678 |jstor=1677049 |pmid=15407879|url-access=subscription }}Aitken (1990), pp. 60–61. 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:

{{blockquote|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.{{Cite journal

|url=http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf

|author=Currie, Lloyd A

|title=The Remarkable Metrological History of Radiocarbon Dating [II]

|journal=Journal of Research of the National Institute of Standards and Technology

|volume=109

|issue=2

|pages=185–217

|date=March–April 2004

|doi=10.6028/jres.109.013

|pmc=4853109

|pmid=27366605

|access-date=30 October 2019

|archive-url=https://web.archive.org/web/20101206195414/http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf

|archive-date=2010-12-06

|url-status=dead

}}

[https://books.google.com/books?id=bKoL_vPa47EC&pg=PA185 "The Remarkable Metrological History of Radiocarbon Dating [II]"] {{Webarchive|url=https://web.archive.org/web/20230101102736/https://books.google.com/books?id=bKoL_vPa47EC&pg=PA185#v=twopage&q&f=false |date=2023-01-01 }} at Google Books (accessed 30 October 2019).}}

The year 1950 was chosen because it was the standard astronomical epoch at that time.{{citation needed|date=November 2021}} It also marked the publication of the first radiocarbon dates in December 1949,

{{Cite journal

|author=Arnold JR, Libby WF

|title=Age determinations by radiocarbon content: Checks with samples of known age

|journal=Science

|volume=109

|issue=2827

|date=1949-03-04

|pages=227–228

|doi=10.1126/science.109.2827.227

|pmid=17818054

|bibcode = 1949Sci...109..227L }} and 1950 also antedates large-scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12.{{Cite web |title=Nuclear Bombs Made It Possible to Carbon Date Human Tissue |url=https://www.smithsonianmag.com/smart-news/nuclear-bombs-made-it-possible-to-carbon-date-human-tissue-20074710/ |work=Smithsonian Magazine | date=2013-02-19 | access-date=2020-01-09}}

{{Main|Radiocarbon dating#Calibration}}

Dates determined using radiocarbon dating come as two kinds: uncalibrated (also called Libby or raw) and calibrated (also called Cambridge) dates.{{Cite book | last=Greene | first=Kevin | author-link=Kevin Greene (archaeologist)| title=Archaeology: An Introduction | year=2002 | publisher=University of Pennsylvania Press | location=Philadelphia | isbn=0-8122-1828-0 | pages=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 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.

{{Notelist}}

• Anthropocene

{{Reflist}}

• {{cite book |last=Aitken |first=M.J. |title=Science-based Dating in Archaeology |publisher=Longman |year=1990 |isbn=978-0-582-49309-4 |location=London |ref=Aitken1990}}

{{Calendars}}

{{Chronology}}

Category:1950 introductions

Category:Calendar eras

Category:Geochronology