Before Present - Wikipedia

Before Present (BP) years, or "years before present" is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when ... BeforePresent FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Timescaleusedmainlyinarchaeology,geologyandotherscientificdisciplines "YBP"and"ybp"redirecthere.FortheairportwiththeIATAcodeYBP,seeYibinWuliangyeAirport. BeforePresent(BP)years,or"yearsbeforepresent"isatimescaleusedmainlyinarchaeology,geologyandotherscientificdisciplinestospecifywheneventsoccurredbeforetheoriginofpracticalradiocarbondatinginthe1950s.Becausethe"present"timechanges,standardpracticeistouse1January1950asthecommencementdate(epoch)oftheagescale.[a]Theabbreviation"BP"hasbeeninterpretedretrospectivelyas"BeforePhysics",[1]whichreferstothetimebeforenuclearweaponstestingartificiallyalteredtheproportionofthecarbonisotopesintheatmosphere,makingcarbondatingafterthattimelikelytobeunreliable.[2][3] Inaconventionthatisnotalwaysobserved,manysourcesrestricttheuseofBPdatestothoseproducedwithradiocarbondating;thealternativenotationRCYBPisexplicitlyRadioCarbonYearsBeforePresent. Contents 1Usage 2Radiocarbondating 3Radiocarboncalibration 4Seealso 5Footnotes 6Citations Usage[edit] TheBPscaleissometimesusedfordatesestablishedbymeansotherthanradiocarbondating,suchasstratigraphy.[4][5]ThisusagediffersfromtherecommendationbyvanderPlicht&Hogg,[6]followedbytheQuaternaryScienceReviews,[7][8]bothofwhichrequestedthatpublicationsshouldusetheunit"a"(for“annum”,Latinfor“year”)andreservetheterm"BP"forradiocarbonestimations. Somearchaeologistsusethelowercaselettersbp,bcandadasterminologyforuncalibrateddatesfortheseeras.[9] TheCentreforIceandClimateattheUniversityofCopenhagenhasproposed"b2k"asyearsbefore2000CE,basedontheGreenlandIceCoreChronology2005(GICC05)timescale.[10] Radiocarbondating[edit] Radiocarbondatingwasfirstusedin1940.Beginningin1954,metrologistsestablished1950astheoriginyearfortheBPscaleforusewithradiocarbondating,usinga1950-basedreferencesampleofoxalicacid.AccordingtoscientistA.CurrieLloyd: Theproblemwastackledbytheinternationalradiocarboncommunityinthelate1950s,incooperationwiththeU.S.NationalBureauofStandards.AlargequantityofcontemporaryoxalicaciddihydratewaspreparedasNBSStandardReferenceMaterial(SRM)4990B.Its14Cconcentrationwasabout5%abovewhatwasbelievedtobethenaturallevel,sothestandardforradiocarbondatingwasdefinedas0.95timesthe14Cconcentrationofthismaterial,adjustedtoa13Creferencevalueof−19permil(PDB).Thisvalueisdefinedas"moderncarbon"referencedtoAD1950.Radiocarbonmeasurementsarecomparedtothismoderncarbonvalue,andexpressedas"fractionofmodern"(fM)."Radiocarbonages"arecalculatedfromfMusingtheexponentialdecayrelationandthe"Libbyhalf-life"5568a.Theagesareexpressedinyearsbeforepresent(BP)where"present"isdefinedasAD1950.[11] Theyear1950waschosenbecauseitwasthestandardastronomicalepochatthattime.[citationneeded]Italsomarked[2]thepublicationofthefirstradiocarbondatesinDecember1949,[12]and1950alsoantedateslargescaleatmospherictestingofnuclearweapons,whichalteredtheglobalratioofcarbon-14tocarbon-12.[13] Radiocarboncalibration[edit] Mainarticle:Radiocarbondating§ Calibration Datesdeterminedusingradiocarbondatingcomeastwokinds:uncalibrated(alsocalledLibbyorraw)andcalibrated(alsocalledCambridge)dates.[14] Uncalibratedradiocarbondatesshouldbeclearlynotedassuchby"uncalibratedyearsBP",becausetheyarenotidenticaltocalendardates.Thishastodowiththefactthatthelevelofatmosphericradiocarbon(carbon-14or14C)hasnotbeenstrictlyconstantduringthespanoftimethatcanberadiocarbon-dated.Uncalibratedradiocarbonagescanbeconvertedtocalendardatesbymeansofcalibrationcurvesbasedoncomparisonofrawradiocarbondatesofsamplesindependentlydatedbyothermethods,suchasdendrochronology(datingonthebasisoftreegrowth-rings)andstratigraphy(datingonthebasisofsedimentlayersinmudorsedimentaryrock).SuchcalibrateddatesareexpressedascalBP,where"cal"indicates"calibratedyears",or"calendaryears",before1950. Manyscholarly/scientificjournalsrequirethatpublishedcalibratedresultsbeaccompaniedbythename(standardcodesareused)ofthelaboratoryconcerned,andotherinformationsuchasconfidencelevels,becauseofdifferencesbetweenthemethodsusedbydifferentlaboratoriesandchangesincalibratingmethods. Seealso[edit] Anthropocene Footnotes[edit] ^Unlikeothereras,thesense"AfterPresent"(AP)isneverused.[dubious–discuss] Citations[edit] ^Flint,RichardFoster;Deevey,EdwardS(1962)."Volume4–1962".Radiocarbon.4(1):i. ^abTaylorRE(1985)."ThebeginningsofradiocarbondatinginAmericanAntiquity:ahistoricalperspective".AmericanAntiquity.50(2):309–325.doi:10.2307/280489.JSTOR 280489. ^Dincauze,Dena(2000)."Measuringtimewithisotopesandmagnetism".EnvironmentalArchaeology:PrinciplesandPractice.Cambridge,England:CambridgeUniversityPress.p. 110.ISBN 978-0-5213-1077-2. ^"AGUEditorialStyleGuideforAuthors".AmericanGeophysicalUnion.21September2007.Archivedfromtheoriginalon2008-07-14.Retrieved2009-01-09. ^ NorthAmericanCommissiononStratigraphicNomenclature(November2005)."NorthAmericanStratigraphicCode:Article13(c)".TheAmericanAssociationofPetroleumGeologistsBulletin.89(11):1547–1591.doi:10.1306/07050504129. ^ J.vanderPlicht,A.Hogg(2006)."Anoteonreportingradiocarbon"(PDF).QuaternaryGeochronology.1(4):237–240.doi:10.1016/j.quageo.2006.07.001. ^ "TheuseoftimeunitsinQuaternaryScienceReviews".QuaternaryScienceReviews.26(9–10):1193.May2007.Bibcode:2007QSRv...26.1193..doi:10.1016/j.quascirev.2007.04.002. ^Wolff,EricW.(December2007)."Whenisthe"present"?".QuaternaryScienceReviews.26(25–28):3023–3024.Bibcode:2007QSRv...26.3023W.doi:10.1016/j.quascirev.2007.10.008. ^EdwardJ.Huth(25November1994).ScientificStyleandFormat:TheCBEManualforAuthors,Editors,andPublishers.CambridgeUniversityPress.pp. 495–.ISBN 978-0-521-47154-1.Retrieved4October2012. ^"TheGICC05timescale".CentreforIceandClimate-UniversityofCopenhagen.RetrievedSeptember17,2018. ^Currie,LloydA(March–April2004)."TheRemarkableMetrologicalHistoryofRadiocarbonDating[II]"(PDF).JournalofResearchoftheNationalInstituteofStandardsandTechnology.109(2):185–217.doi:10.6028/jres.109.013.PMC 4853109.PMID 27366605.Archivedfromtheoriginal(PDF)on2010-12-06.Retrieved30October2019. "TheRemarkableMetrologicalHistoryofRadiocarbonDating[II]"atGoogleBooks(accessed30October2019). ^ ArnoldJR,LibbyWF(1949-03-04)."Agedeterminationsbyradiocarboncontent:Checkswithsamplesofknownage".Science.109(2827):227–228.Bibcode:1949Sci...109..227L.doi:10.1126/science.109.2827.227.PMID 17818054. ^"NuclearBombsMadeItPossibletoCarbonDateHumanTissue".SmithsonianMagazine.2013-02-19.Retrieved2020-01-09. ^Greene,Kevin(2002).Archaeology:AnIntroduction.Philadelphia:UniversityofPennsylvaniaPress.pp. 165–167.ISBN 0-8122-1828-0. vteCalendarsSystems Lunar Lunisolar Solar Inwideuse Astronomical Buddhist Chinese Gregorian Hindu LunarHijri SolarHijri ISO Unixtime Inmorelimiteduse Akan Armenian Assamese(Bhāshkarābda) Assyrian Baháʼí Badí‘ Balinesepawukon Balinesesaka Bengali Bangladeshi Berber Burmese Chinese EarthlyBranches HeavenlyStems EthiopianandEritrean Gaelic Germanicheathen Georgian Hebrew HinduorIndian VikramSamvat Saka Igbo Iranian Jalali medieval Zoroastrian Islamic Fasli Tabular Jain 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