Before Present - chemeurope.com

Before Present (BP) years are a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Mywatchlist my.chemeurope.com  Withanaccoutformy.chemeurope.comyoucanalwaysseeeverythingataglance–andyoucanconfigureyourownwebsiteandindividualnewsletter. Mywatchlist Mysavedsearches Mysavedtopics Mynewsletter Registerfreeofcharge Login  Keeploggedin Forgotyourpassword? Touseallfunctionsofthispage,pleaseactivatecookiesinyourbrowser. Login Register   DeutschEnglishFrançaisEspañol HomeEncyclopediaBefore_Present BeforePresent BeforePresent(BP)yearsareatimescaleusedinarchaeology,geology,andotherscientificdisciplinestospecifywheneventsinthepastoccurred.Becausethe"present"timechanges,standardpracticeistouse1950asthearbitrarybenchmarkofwhat'sconsidered"present"."BP"mayalsobeconsideredtobeanabbreviationofBeforePhysics.[1]Forexample,1500BPmeans1500yearsbefore1950,thatis,intheyear450. Beginningin1954,metrologistsestablished1950astheoriginyearfortheBPscaleforusewithradiocarbondatingusinga1950-basedreferencesampleofoxalicacid:Theproblemwastackledbytheinternationalradiocarboncommunityinthelate1950s,incooperationwiththeU.S.NationalBureauofStandards.Alargequantityofcontemporaryoxalicaciddi-hydratewaspreparedasNBSStandardReferenceMaterial(SRM)4990B.Its14Cconcentrationwasca.5%abovewhatwasbelievedtobethenaturallevel,sothestandardforradiocarbondatingwasdefinedas0.95timesthe14Cconcentrationofthismaterial,adjustedtoa13Creferencevalueof–19permil(PDB).Thisvalueisdefinedas“moderncarbon”referencedtoAD1950.Radiocarbonmeasurementsarecomparedtothismoderncarbonvalue,andexpressedas“fractionofmodern”(fM);and“radiocarbonages”arecalculatedfromfMusingtheexponentialdecayrelationandthe“Libbyhalf-life”5568a.Theagesareexpressedinyearsbeforepresent(BP)where“present”isdefinedasAD1950.[2]Theyear1950waschosenbecauseitistheyearinwhichcalibrationcurvesforradiocarbondatingwereestablished,andalsotohonor[citationneeded]thepublicationofthefirstradiocarbondatesinDecember1949[3].Theyear1950isalsoconvenientbecauseitpredateslargescaleatmospherictestingofnuclearweapons,whichalteredtheglobalratioofcarbon-14tocarbon-12.[4] Additionalrecommendedknowledge EssentialLaboratorySkillsGuide Betterweighingperformancein6easysteps DailyVisualBalanceCheck TheBPscaleisnowincommonusefordatesestablishedbymeansotherthanradiocarbondating.Thepracticeofanchoring"thepresent"at1950isgenerallyfollowed,althoughtimesinthedistantpast(e.g.,500,000BP)typicallyhaveuncertaintieshighenoughthatthedifferencebetween1950andtheactualpresentyearisinsignificant. Radiocarbondatingandcalibration Datesdeterminedusingradiocarbondatingcomeintwokinds:uncalibrated(or"raw")andcalibrated.UncalibratedradiocarbondatesmaybeexpressedusingBPyears;however,theyarenotidenticaltocalendardates.Thishastodowiththefactthatthelevelofatmosphericradiocarbon(carbon-14or14C)hasnotbeenstrictlyconstantduringthespanoftimethatcanberadiocarbon-dated.Uncalibratedradiocarbonagescanbeconvertedtocalendardatesbymeansofcalibrationcurvesbasedoncomparisonofrawradiocarbondatesofsamplesindependentlydatedbyothermethods,suchasdendrochronology(theexaminationoftreegrowth-rings).SuchcalibrateddatesareexpressedascalBP,where"cal"indicates"calendaryears"or"calibratedyears". Furtherinformation:Radiocarbondating#Calibration Seealso Megaannum References ^CalPalsoftwaredocumentation,fromCalPal.de ^LloydA.Currie,TheRemarkableMetrologicalHistoryofRadiocarbonDating[II]inJ.Res.Natl.Inst.Stand.Technol.vol.109,pp.185-217(2004) ^J.R.ArnoldandW.F.Libby,Agedeterminationsbyradiocarboncontent:Checkswithsamplesofknownage,Sciencevol.109(1949),pp.227-228 ^ADorBC?fromwww.ScienceCourseware.org v • d • eChronologyMajorsubjectsTime ·Astronomy ·Geology ·Paleontology ·Archaeology ·HistoryErasCalendarEras:Aburbecondita ·AnnoDomini ·AnnoMundi ·BeforePresent ·CommonEra ·Hijri ·Hindutimecycles ·Vedictime-cycles Epoch ·Regnalyear:CanonofKings ·Chinese ·Japanese · ·KoreanCalendarsPre-JulianRoman ·OriginalJulian ·ProlepticJulian ·RevisedJulian Gregorian ·ProlepticGregorian ·OldStyleandNewStyle Astronomicalyearnumbering ·Islamic ·ISOweekdate Lunisolar ·Solar ·LunarAstronomicTimeandTechniquesAstronomicalchronology ·CosmicCalendar ·Galacticyear ·Metoniccycle ·MilankovitchcyclesGeologicTimeandTechniquesGeologicalhistory ·Eon ·Eras ·Periods ·Faunalstage Chronostratigraphy ·Geochronology ·Isotopegeochemistry ·Lawofsuperposition ·Opticaldating ·Samarium-neodymiumdatingArchaeologicalTechniquesDatingmethodology Absolutedating ·Incrementaldating ·Archaeomagneticdating ·Dendrochronology ·Glottochronology ·Ice_core ·Lichenometry ·Paleomagnetism ·Radiocarbondating ·Radiometricdating ·Tephrochronology ·Thermoluminescencedating ·Uranium-leaddating Relativedating ·Seriation ·StratificationGeneticTechniquesAminoaciddating ·Molecular_clockRelatedtopicsChronicle ·NewChronology ·Periodization ·Synchronopticview ·Timeline ·Yearzero Circa ·Floruit Category:Radiometricdating   ThisarticleislicensedundertheGNUFreeDocumentationLicense.ItusesmaterialfromtheWikipediaarticle"Before_Present".AlistofauthorsisavailableinWikipedia. 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