Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23What are the two categories of datingtechniques?relative dating and absolute datingWhich one tells you a year or range of years?absoluteWhich one tells you only that something isolder than or younger than something else?relativeSummary of selectedRelative Dating techniquesStratigraphyIndex fossil concept and faunal successionSeriation (stylistic or frequency)Absolute Dating TechniquesRadiometric techniques:Radiocarbon (C14) and AMS datingPotassium/Argon (K/Ar) or Argon/Argon (Ar/Ar)Other techniques:Dendrochronology (tree ring dating)ArchaeomagnetismLuminescence (OSL, ESR, TL)What absolute dating technique didI talk about in the last class?C14 or Radiocarbon datingWhat materials can be dated with C14?something organic (that once was living)What time range can it date?historic up to 50,000 yaPotassium-Argon (K/Ar) or Argon-Argon (Ar/Ar)A radiometric technique:Dating is based on the fact that an isotopedecays at a known rate (that’s why it isradiometric)You can measure how much of that isotoperemains in an object, versus how much shouldbe there, and figure out when the isotopebegan to degradeAbsolute Dating TechniquesIn K/Ar, Potassium (K) degrades to Argon gas (Ar)Must have a rock created byvolcanic action. Tuff is good. Dates when rock formed:in the case of tuff, when the volcano eruptedHalf-life is 1.3 billion yearsCan date rock 100,000 years or olderGives you a range of years of when volcanoerupted and formed the tuffhttp://0.tqn.com/d/geology/1/0/R/X/weldtuff.jpgLayer of volcanic ash (tuff) ASo how can K/Ar help us date fossils?Fossils are mineralized bone: they are not madewhen a volcano eruptsWe actually combine the Law of Superpositionwith the absolute date from K/ArFossil humanLayer of volcanic ash (tuff) BGiven the Law of Superposition, the fossil humanmust be younger than the dated layer A, but olderthan the dated layer B: gives us an age bracketAbsolute dating technique covered today:Radiometric technique Potassium/Argon (K/Ar) or Argon/Argon (Ar/Ar)100,000 ya or older, VOLCANIC TUFFRadiometric techniques covered so far: Radiocarbon (C14) or AMS Potassium/Argon (K/Ar) or Argon/Argon (Ar/Ar)Historic - 50,000 ya. ORGANIC100,000 ya or older, VOLCANIC TUFFArchaeomagnetismBased on the idea thatas something was magnetizedin the past, it pointed towhere the magnetic northpole was at the timeFor example, at A.D. 800the particles would havepointed to hereBut at A.D. 1150 they would have pointed to hereCan date magnetized objects up to 10,000 yaWhen sample is collected, must record the orientationand dip and present magnetic northThe sample is enclosedin plaster and sent tothe laboratoryAbsolute Dating TechniquesDendrochronology (Tree-ring dating)(Photo by H. D. Grissino-Mayer)Red oakDendrochronology is based on the idea thatthe amount of tree growth (and therefore the widthof each tree ring) differs each year according toclimatic factors1 yr 1 yr 1 year(H. D. Grissino-Mayer)However, climaticpatterns & thusgrowth patternsare localized toregions:Must build a MasterKey for your regionUniv. of ArizonaLaboratory of Tree-Ring Researchhtml://tree.ltrr.arizona.eduHow build aMaster Key:Cross-datingDendrochronology is the most accurateabsolute dating technique known:Tells us not only the exact year, but the seasonIt has been used to calibrate C14 dates(since wood is organic, you can C14 datea particular ring to check whether the C14dating is accurate)LuminescenceDating is based on measurement of the amountof light released upon thermal or opticalstimulation by minerals such as quartz and feldsparThe light signal indicates the radiation dose thataccumulated in these minerals through time(http://users.ugent.be/~pvdhaute/lumin.htm)When exposed to sunlight, or in the case ofpottery, when heated hot enough, the clock is set to zero.After burial or heating, minerals absorb naturalionizing radiation from the surrounding sedimentsAbsolute Dating TechniquesThree techniques are based on the measurementof trapped electronic charges that accumulate incrystalline materials as a result of low-level radioactivity:Electron Spin Resonance (ESR)Optically Stimulated Luminescence (OSL)Thermoluminescence (TL)To act as a clock, the electronic charges must be zeroed by heating (TL), exposure to daylight (OSL), or growth (ESR on fossilized teeth).What’s the take-away information?Recognize which are absolute and which arerelative dating techniquesEspecially important in relative dating:Stratigraphic analysis/Law of SuperpositionIndex fossil conceptEspecially important in absolute dating:Radiocarbon (C14) datingPotassium-Argon (K/Ar) datingFor the important absolute dating techniques,know what materials can be dated, and howfar back in time the method can date somethingLater, when we talk about the fossil record,you’ll want to know which dating techniqueis used for which areas where fossils occur in AfricaRecognize which absolute dating techniques areradiometric, and understand how radiometricdating works (knowing the half-life of somethingand measuring the amount left of that something)Summary ofDating TechniquesWhich dating techniques are used where and when?Stratigraphic analysis is used at EVERY archaeological site and EVERY fossil locality, on a daily basis by the excavatorsIndex fossil concept is useful for dating early humanfossil localities in South Africa, where there has notbeen any volcanic activity but where we do have animal fossils along with the early human fossilsWhich dating techniques are used where and when?The premier dating technique for early human fossillocalities in East Africa, where there was volcanic activity, is K/Ar or Ar/ArRadiocarbon or AMS dating cannot be used to date early human fossil localities: it can date back only to 50,000 yrs ago at the very mostHowever, when we start talking about modern humans,we will rely on C14 dating of sitesSummary ofAbsolute Dating TechniquesRadiocarbon (C14) and AMS dating can’t date far enough back to help us with early human fossil localities. However, used frequently to date archaeological sites less than 50,000 years oldPotassium/Argon (K/Ar) & Argon/Argon (Ar/Ar) major technique to date fossil localities in areas like east Africa where there has been volcanic activityDendrochronology (tree-ring dating) the most accurate absolute
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