Dating Techniques

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Dating Techniques. Four Categories Radio-isotope methods Paleomagnetic methods Organic/inorganic chemical methods Biological methods. Relative dating: Chronological succession (e.g., dendrochronology). Synchronous events ( e.g. volcanic ash ). Absolute dating: - PowerPoint PPT Presentation

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  • Dating TechniquesFour CategoriesRadio-isotope methodsPaleomagnetic methodsOrganic/inorganic chemical methodsBiological methods

  • Relative dating:Chronological succession (e.g., dendrochronology).Synchronous events (e.g. volcanic ash).Absolute dating:Recognition of time-dependent processes (e.g., radioactivity).

  • Radio-isotopic MethodBased on disintegration of unstable nucleiNegatron decay (n p+ + b- + energy)

    Positron decay (p+ n + b+ + energy)

    Alpha decay (AX A-4Y + He)

  • Radioactivity-ConceptsHalf-life (t1/2 ): N= N0/2Mean life: t=1/lActivity: # radioactive disintegrations/sec (dps)Specific activity: dps/wt. or dps/volUnits: Becquerel (Bq)=1 dps

  • Decay Rates: Ln (No/N) = lt t = t*Ln (No/N)

  • To be a useful for dating, radio-isotopes must:be measurable have known rate of decayhave appropriate t1/2 have known initial concentrationsbe a connection between event and radioisotope

  • Radioactivity-based DatingQuantity of the radio-isotope relative to its initial level (e.g., 14C).Equilibrium /non-equilibrium chain of radioactive decay (e.g., U-series).Physical changes on sample materials caused by local radioactive process (e.g., fission track).

  • Radiocarbon Dating12C: 42*1012; 13C: 47*1010; 14C: 62 tonst1/2 = 5730 yr l= 1.0209*10-4/yrFormed in the atmosphere:14N + 1n 14C + 1HDecay: 14C 14N + b-

  • W.F. Libbys discovery of radiocarbonS. Korffs discovery: cosmic rays generate ~2 neutrons/cm2sec14C formed through nuclear reaction.14C readily oxidizes with O2 to form 14CO2Libbys t1/2 = 5568 yr.

  • Conventional Radiocarbon DatingCurrent t1/2 = 573040 yrt=8033*Ln(Asample/Astandard), where A:activity.Oxalic acid is the standard (prepared in 1950).Dates reported back in time relative to 1950 (radiocarbon yr BP).Astandard in 1950 = 0.227 Bq/gAstandard in 2000 = 0.225 Bq/g

  • Conventional Radiocarbon datingActivity of 14C needs to be normalized to the abundance of carbon: D14C: normalized valueD14C() = d14C 2(d13C+25)(1+d13C/103)d14C() = (1-Asample/Astandard)*103Radiocarbon age = 8033*ln(1+ D14C/103)

  • Conventional Radiocarbon datingPrecision has increasedRadiocarbon disintegration is a random process. If date is 5000100:68% chance is 4900-510099% chance is 4700-5300

  • Radiocarbon dating-Problems

  • Radiocarbon dating-CorrectionsRadiocarbon can be corrected by using tree-ring chronology.Radiocarbon dates can then be converted into Calendar years (cal yr).

  • Radiocarbon dating-ProblemsTwo assumptions:Constant cosmic ray intensity.Constant size of exchangeable carbon reservoir. Deviation relative to dendrochronology due to:Variable 14C production rates.Changes in the radiocarbon reservoirs and rates of carbon transfer between them.Changes in total amount of CO2 in atmosphere, hydrosphere, and atmosphere.

  • Deviation of the initial radiocarbon activity.

  • Bomb-radiocarbonNuclear testing significantly increased D14C

  • Bomb 14C can be used as a tracer

  • Radiocarbon dating-conclusionPrecise and fairly accurate (with adequate corrections).Useful for the past ~50,000 yr.Widespread presence of C-bearing substrates.Relatively small sample size (specially for AMS dates).Contamination needs to be negligible.

  • Other Radio-isotopesK-Ar40K simultaneously decays to 40Ca and 40Ar(gas)t1/2=1.3*109 yr (useful for rocks >500 kyrAmount of 40Ar is time-dependentProblems: Assumes that no 40Ar enters or leaves the systemLimited to samples containing K

    U-series

  • Other radio-isotopesUranium series236U and 238U decay to 226Ra and 230ThU is included in carbonate lattice (e.g., corals)Age determined on the abundance of decay products Problems: Assumes a closed systemAssumes known initial conditions.

  • Thermo-luminescence (TL)TL is light emitted from a crystal when it is heated.TL signal depends on # e- trapped in the crystal.Trapped e- originate from radioactive decay of surrounding minerals.TL signal is proportional to time and intensity.Useful between 100 yr and 106 yr

  • TL-ApplicationsArchaeological artifactsHeating (>500oC) re-sets TL signal to zeroUsed for dating pottery and baked sediments SedimentsExposure to sunlight re-sets the clockUsed for dating loess, sand dunes, river sand.

  • TL-ProblemsDifferent response to ionization# lattice defectssaturationIncomplete re-setting Water can absorb radiationUnknown amount of ionization

  • Fission-Track Dating238U can decay by spontaneous fissionSmall tracks are created on crystals (zircon, apatite, titanite) and volcanic glass.Track density is proportional to U-content and to time since the crystal formed.Useful for dating volcanic rocks (>200 kyr)Problem: tracks can heal over time

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