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Dating Techniques

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:

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Dating Techniques

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  1. Dating Techniques • Four Categories • Radio-isotope methods • Paleomagnetic methods • Organic/inorganic chemical methods • Biological methods

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

  3. Radio-isotopic Method • Based on disintegration of unstable nuclei • Negatron decay (n p+ + b- + energy) • Positron decay (p+ n + b+ + energy) • Alpha decay (AX A-4Y + He)

  4. Radioactivity-Concepts • Half-life (t1/2 ): N= N0/2 • Mean life: t=1/l • Activity: # radioactive disintegrations/sec (dps) • Specific activity: dps/wt. or dps/vol • Units: Becquerel (Bq) =1 dps

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

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

  7. Radioactivity-based Dating • Quantity 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).

  8. Radiocarbon Dating • 12C: 42*1012; 13C: 47*1010; 14C: 62 tons • t1/2 = 5730 yr • l= 1.0209*10-4/yr • Formed in the atmosphere: 14N + 1n 14C + 1H • Decay: 14C 14N + b-

  9. W.F. Libby’s discovery of radiocarbon • S. Korff’s discovery: cosmic rays generate ~2 neutrons/cm2sec • 14C formed through nuclear reaction. • 14C readily oxidizes with O2 to form 14CO2 • Libby’s t1/2= 5568 yr.

  10. Conventional Radiocarbon Dating • Current t1/2= 5730±40 yr • t=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/g • Astandard in 2000 = 0.225 Bq/g

  11. Conventional Radiocarbon dating • Activity of 14C needs to be “normalized” to the abundance of carbon: • D14C: “normalized value” • D14C(‰) = d14C –2(d13C+25)(1+d13C/103) • d14C(‰) = (1-Asample/Astandard)*103 • Radiocarbon age = 8033*ln(1+ D14C/103)

  12. Conventional Radiocarbon dating • Precision has increased • Radiocarbon disintegration is a random process. • If date is 5000±100: • 68% chance is 4900-5100 • 99% chance is 4700-5300

  13. Radiocarbon dating-Problems

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

  15. Radiocarbon dating-Problems • Two 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.

  16. Deviation of the initial radiocarbon activity.

  17. Bomb-radiocarbon Nuclear testing significantly increased D14C

  18. Bomb 14C can be used as a tracer

  19. Radiocarbon dating-conclusion • Precise 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.

  20. Other Radio-isotopes • K-Ar • 40K simultaneously decays to 40Ca and 40Ar(gas) • t1/2=1.3*109 yr (useful for rocks >500 kyr • Amount of 40Ar is time-dependent • Problems: • Assumes that no 40Ar enters or leaves the system • Limited to samples containing K • U-series

  21. Other radio-isotopes • Uranium series • 236U and 238U decay to 226Ra and 230Th • U is included in carbonate lattice (e.g., corals) • Age determined on the abundance of decay products • Problems: • Assumes a closed system • Assumes known initial conditions.

  22. 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

  23. TL-Applications • Archaeological artifacts • Heating (>500oC) re-sets TL signal to zero • Used for dating pottery and baked sediments • Sediments • Exposure to sunlight re-sets the “clock” • Used for dating loess, sand dunes, river sand.

  24. TL-Problems • Different response to ionization • # lattice defects • saturation • Incomplete re-setting • Water can absorb radiation • Unknown amount of ionization

  25. Fission-Track Dating • 238U can decay by spontaneous fission • Small “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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