Before Present (BP) or "years before present (YBP)" is atime scale used mainly inarchaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practicalradiocarbon dating in the 1950s. Because the "present" time changes, standard practice is to use 1January 1950 as the commencement date (epoch) of the age scale, with 1950 being labelled as the "standard year". The abbreviation "BP" has been interpreted retrospectively as "Before Physics",[1][2] which refers to the time beforenuclear weapons testing artificially altered the proportion of thecarbon isotopes in the atmosphere, which scientists must account for when using radiocarbon dating for dates of origin that may fall after this year.[3][4]
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 asstratigraphy.[5][6] This usage differs from the recommendation by van der Plicht & Hogg,[7] followed by theQuaternary Science Reviews,[8][9] both of which requested that publications should use theunit "a" (for "annum", Latin for "year") and reserve the term "BP" for radiocarbon estimations.
Somearchaeologists use the lowercase lettersbp,bc andad as terminology for uncalibrated dates for these eras.[10]
The Centre for Ice and Climate at theUniversity of Copenhagen instead uses the unambiguous "b2k", for "years before 2000 AD", often in combination with theGreenlandIce Core Chronology 2005 (GICC05) time scale.[11]
Some authors who use the YBP dating format also use "YAP" ("years after present") to denote years after 1950.[12]
Radiocarbon dating was first used in 1949.[14][15] 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 ofoxalic 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 contemporaryoxalic aciddihydrate was prepared as NBS Standard Reference Material (SRM) 4990B. Its14C 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 the14C concentration of this material, adjusted to a13C 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.[16]
Dates determined using radiocarbon dating come as two kinds:uncalibrated (also calledLibby orraw) andcalibrated (also calledCambridge) dates.[19]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 or14C) has not been strictly constant during the span of time that can be radiocarbon-dated. Uncalibrated radiocarbon ages can be converted to calendar dates bycalibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such asdendrochronology (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 fromGregorian 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.
^abTaylor RE (1985). "The beginnings of radiocarbon dating inAmerican Antiquity: a historical perspective".American Antiquity.50 (2):309–325.doi:10.2307/280489.JSTOR280489.S2CID163900461.
^Dincauze, Dena (2000). "Measuring time with isotopes and magnetism".Environmental Archaeology: Principles and Practice. Cambridge, England: Cambridge University Press. p. 110.ISBN978-0-5213-1077-2.
^"The GICC05 time scale". Centre for Ice and Climate – University of Copenhagen. 3 September 2009. Archived fromthe original on 18 September 2018. RetrievedSeptember 17, 2018.