Sedimentation and Stratigraphy Geology 5142 Dr' Thieme

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Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 24 Radiocarbon Wrapup and Radiation
  Effect Techniques

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Radiocarbon Dating Problems

• sample collection errors (contamination)
• fractionation effects
• deVries ("reservoir") effects
• Suess ("fossil fuel") and "bomb spike" effects
• use of the old Libby half-life
• laboratory errors

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Contamination by modern material will
dramatically decrease the age estimated by a
radiocarbon date.
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Contamination by material that is older than the
sample dated will increase the age estimated by a
radiocarbon date.
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Carbon isotopes are fractionated by plant
metabolic pathways and by surface and subsurface
geochemical processes.
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Shells and other carbonate material that have
precipitated in equilibrium with sea water have
to be corrected for "old carbon" unevenly mixed
in the oceans.
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The abundance of 14C has fluctuated through the
Holocene. The deVries effects are probably driven
primarily by climate change, with abrupt cooling
episodes creating "plateaus" in the calibration
curve. The Suess effect is an abrupt decline in
14C due to fossil fuel combustion.
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Solutions

• identify material dated to species, even molecule
• measure all three C isotopes
• concentrate 14C and extend counting time
• measure individual atoms (AMS)
• precisely identify exchange reservoirs
• calibrate conventional dates to calendar years
  (tree rings, corals, varves)
• combine 14C with other dating methods
• understand stratigraphic context

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Accelerator Mass Spectrometry
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Calibration of "conventional" radiocarbon ages to
calendar years is based upon measurements of tree
rings, mostly from the western United States.
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Radiocarbon and other carbon isotopes have turned
out to be extremely powerful clues to the
mechanisms and fluxes of the Earth's carbon cycle.
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Several dating methods have been developed which
make use of the decay of Uranium to Lead but go
beyond the standard "decay constant" approach.
The most complex methods are those involving
"equilibrium" in the entire isotope system. Other
methods work instead with effects of the decay.
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Uranium series "disequilibrium"

• In a system containing uranium, which has been
  undisturbed for 106 yr
• each daughter product will be decaying at the
  same rate that it is produced by its parent
• the ratio of one isotope to another will be
  nearly constant
• Disturbance of the decay products will alter
  those proportions and ratios
• restoration of full equilibrium takes 106 yr

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Intermediary decay products in the chains that
culminate in 206Pb, 207Pb, and 208Pb are all
measured. Ratios between intermediary products
and their parents typically decrease as the
system approaches equilibrium.
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Uranium Series Nuclides
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Most of the ratios of daughter to parent isotope
in the Uranium series decrease as the system
approaches equilibrium. One exception shown here,
however, is the ratio of 234U/236U. All but the
ratio of 236Ra/230Th decline to zero before
equilibrium is reached in 106 yr.
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The isotope 210Pb decays to 206Pb with a
half-life of 22 years. It can be used to date
extremely young events and measure modern
sedimentation rates.
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Luminescence dating methods are all based on the
light energy that is given off by trapped
electrons when the mineral containing them is
exposed either to heat or to light. Decay events
due to minor amounts of Uranium result in
electrons becoming trapped above their "valence"
band in most minerals.
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Thermo-luminescence (TL) occurs when trapped
electrons are released with heat . Optically
stimulated luminescence (OSL) or infrared
stimulated luminescence (IRSL) occur when the
trapped electrons are released with light.
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A thermoluminescence "glow curve" is produced by
heating a sample gradually in the range from 0 to
500 celsius and measuring the light that is given
off. Some "residual" luminescence will remain in
the sample, however, no matter how high a final
temperature is reached.
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To calculate a sample age the sample must then be
irradiated in the laboratory and heated again
after each successive irradiation.
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The "shine down" curves of optically stimulated
and infrared stimulated luminescence are similar
in principle to the TL "glow curve" but have
peaks at very specific light wavelengths.
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Sampling a sediment column for luminescence
dating is surprisingly simple if you keep in mind
the need to keep the samples from exposure to
light.
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An IRSL laboratory.
To obtain the most accurate results it is really
necessary to measure the radiation dose of
sediment in the field.
A "reader" for luminescence dating
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Other "radiation effect" methods

• Fission Tracks (produced directly by decay
  events). Track density "saturates" at 2 Ma.
• Electron Spin Resonance (used primarily for bone
  and teeth, also for stone that has been heated).
  Good from 30 ka to 300 ka.