What are the two categories of dating techniques relative dating and absolute dating Which one tells you a year or range of years absolute Which one tells you only that something is older than or younger than something else relative Summary of selected Relative Dating techniques Stratigraphy Index fossil concept and faunal succession Seriation stylistic or frequency Absolute Dating Techniques Radiometric techniques Radiocarbon C14 and AMS dating Potassium Argon K Ar or Argon Argon Ar Ar Other techniques Dendrochronology tree ring dating Archaeomagnetism Luminescence OSL ESR TL What absolute dating technique did I talk about in the last class C14 or Radiocarbon dating What materials can be dated with C14 something organic that once was living What time range can it date historic up to 50 000 ya Absolute Dating Techniques Potassium Argon K Ar or Argon Argon Ar Ar A radiometric technique Dating is based on the fact that an isotope decays at a known rate that s why it is radiometric You can measure how much of that isotope remains in an object versus how much should be there and figure out when the isotope began to degrade In K Ar Potassium K degrades to Argon gas Ar Must have a rock created by volcanic action Tuff is good Dates when rock formed in the case of tuff when the volcano erupted Half life is 1 3 billion years Can date rock 100 000 years or older Gives you a range of years of when volcano erupted and formed the tuff http 0 tqn com d geology 1 0 R X weldtuff jpg So how can K Ar help us date fossils Fossils are mineralized bone they are not made when a volcano erupts We actually combine the Law of Superposition with the absolute date from K Ar Layer of volcanic ash tuff B Fossil human Layer of volcanic ash tuff A Given the Law of Superposition the fossil human must be younger than the dated layer A but older than the dated layer B gives us an age bracket Absolute dating technique covered today Radiometric technique Potassium Argon K Ar or Argon Argon Ar Ar 100 000 ya or older VOLCANIC TUFF Radiometric techniques covered so far Radiocarbon C14 or AMS Historic 50 000 ya ORGANIC Potassium Argon K Ar or Argon Argon Ar Ar 100 000 ya or older VOLCANIC TUFF Archaeomagnetism Based on the idea that as something was magnetized in the past it pointed to where the magnetic north pole was at the time For example at A D 800 the particles would have pointed to here But at A D 1150 they would have pointed to here Can date magnetized objects up to 10 000 ya When sample is collected must record the orientation and dip and present magnetic north The sample is enclosed in plaster and sent to the laboratory Absolute Dating Techniques Dendrochronology Tree ring dating Red oak Photo by H D Grissino Mayer Dendrochronology is based on the idea that the amount of tree growth and therefore the width of each tree ring differs each year according to climatic factors H D Grissino Mayer However climatic patterns thus growth patterns are localized to regions Must build a Master Key for your region 1 yr 1 yr 1 year How build a Master Key Cross dating Univ of Arizona Laboratory of TreeRing Research html tree ltrr arizona edu Dendrochronology is the most accurate absolute dating technique known Tells us not only the exact year but the season It has been used to calibrate C14 dates since wood is organic you can C14 date a particular ring to check whether the C14 dating is accurate Luminescence Dating is based on measurement of the amount of light released upon thermal or optical stimulation by minerals such as quartz and feldspar The light signal indicates the radiation dose that accumulated in these minerals through time When exposed to sunlight or in the case of pottery when heated hot enough the clock is set to zero After burial or heating minerals absorb natural ionizing radiation from the surrounding sediments http users ugent be pvdhaute lumin htm Absolute Dating Techniques Three techniques are based on the measurement of trapped electronic charges that accumulate in crystalline 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 are relative dating techniques Especially important in relative dating Stratigraphic analysis Law of Superposition Index fossil concept Especially important in absolute dating Radiocarbon C14 dating Potassium Argon K Ar dating For the important absolute dating techniques know what materials can be dated and how far back in time the method can date something Later when we talk about the fossil record you ll want to know which dating technique is used for which areas where fossils occur in Africa Recognize which absolute dating techniques are radiometric and understand how radiometric dating works knowing the half life of something and measuring the amount left of that something Summary of Dating Techniques Which 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 excavators Index fossil concept is useful for dating early human fossil localities in South Africa where there has not been any volcanic activity but where we do have animal fossils along with the early human fossils Which dating techniques are used where and when The premier dating technique for early human fossil localities in East Africa where there was volcanic activity is K Ar or Ar Ar Radiocarbon 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 most However when we start talking about modern humans we will rely on C14 dating of sites Summary of Absolute Dating Techniques Radiocarbon 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 old Potassium Argon K Ar Argon Argon Ar Ar major technique to date fossil localities in areas like east Africa where there has been volcanic activity Dendrochronology tree ring dating the most accurate absolute dating technique can tell us not only the year but also the season But can only be used in areas where a Master Key has been constructed and so far the oldest goes back only 9 500 years Has been used to calibrate C14
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