Week 2

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Week 2

• Dating Methods
• Stratigraphy and Correlation

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Types of methods
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AGE DETERMINATION IN QUATERNARY RESEARCH
TYPE OF RESULT
NUMERICAL AGE
RELATIVE AGE
CALIBRATED AGE
CORRELATED AGE
TYPE OF METHOD
Sidereal (calendaric)
Geomorphic and Pedogenic
Chemical and Biological
Radiogenic
Isotopic
Correlation
TECHNIQUES
Fission Track Potassium-Argon Thermo- luminesce
nse Optically-stimulated Luminescence Infra-red
stimulated Luminescence Electron-spin Resonance
Historical Records Dates on Artifacts and
Features Dendochronology Varve Chronology
Radiocarbon (conventional and AMS) Potassium-Arg
on Uranium Series Uranium-trend Cosmogenic
isotopes
Geomorphic Position Progressive
Landscape Modification Rate of Deposition Rate
of Deformation Stratigraphic Sequence Rock and
Mineral Weathering Soil Profile Development
Lithostratigraphy Biostratigraphy (fossils) Teph
rochronology Paleomagnetism Archaeomagnetism St
able Isotopes Seriated artifacts
Aminoacid Racemization Obsidian and Tephra
Hydration Lichenometry Soil Chemistry Rock
Varnish Chemistry Oxidizable Carbon Ratio
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Time ranges for different dating methods
Lowe, J.J. and Walker, M.J.C. 1997.
Reconstructing Quaternary Environments. 2nd.
Ed. Prentice Hall.
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Radiocarbon dating

• Type of method Isotopic
• Time-dependant basis half-life of carbon
  radioisotopes
• Time span A.D. 1950 to 45 ka B.P.
• Methods scintillation liquid, gas counting, AMS

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Isotopes and time

• Carbon isotopes
• Isotope decay
• Half lives

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ISOTOPIC METHODS
12C, 13C, 14C
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Principles of 14C dating

• 1. Radiocarbon (14C) is produced in the upper
  atmosphere by neutron bombardment of atmospheric
  nitrogen atoms

14N 1n 14C 1H
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0
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1
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• 2. Plants and animals absorb 14C from the
  atmosphere

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• 3. Plants and animals die, and once dead they no
  longer absorb 14C . Therefore, the process of
  decay begins

14C 14N ß neutrino
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• 3. Plants and animals die, and once dead they no
  longer absorb 14C . Therefore, the process of
  decay begins

14C 14N ß neutrino
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ß particles (neutrons) are released
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• 4 The original radiocarbon method actually
  determines age by the amount of ß particles
  released

The older the sample, the lower the levels of ß
particles produced
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• 4. The original radiocarbon method actually
  determines age by the amount of ß particles
  released

The older the sample, the lower the levels of ß
particles produced
Thats why there is a time limit in radiocarbon
dating!
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14C Half life

• 5730 yr (before 1950)

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Methods

• 1950 to ca. 1980 Conventional methods
  scintillation liquid and proportional gas
  counters
• Early 1980s ---- AMS dating (Accelerator Mass
  Spectrometry)

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• 1950 to ca. 1980 Conventional methods
  scintillation liquid and proportional gas
  counters
• Early 1980s ---- mass spectrometer (AMS dating)

Instead of counting ß-particle emissions AMS
counts the concentration of individual ions
(12C, 13C, 14C)
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Accuracy of radiocarbon dating

• 14C assays are not a gospel truth. They are
  statements of probability.
• For example
• 5000100

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Accuracy of radiocarbon dating

• 14C assays are not a gospel truth. They are
  statements of probability.
• For example
• 5000100 yr B.P.

1-d 68 probability that the true value
is between 4900 and 5100 yr B.P.
2-d 99 probability that the true value is
between 4800 and 5200 yr B.P.
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Variations in 14C in the atmosphere through time
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Correction

• E.g., carbon reservoir
• Correction using delta 13C

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Calibration to calendaric (sidereal) years
Non-calibrated bp
291050 yr bp
Calibrated dates BP
3320-2890 BP (2-sigma)
Calibrated dates BC/AD
1270-940 BC
1105 BC
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Correction and calibration
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Calibration curves
Name Data Format Reference
1986      Groningen Stuiver and Kra 1986
1993      Groningen Stuiver and Kra 1993
IntCal98      Calib Stuiver et al 1998
Marine98      Calib Stuiver et al 1998
IntCal04      IntCal Reimer et al. 2004
Marine04      IntCal Hughen et al. 2004
ShCal04      IntCal McCormac et al. 2004
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Calibration methods

• Intercept method
• Probability method

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Intercept method
1000/-75
Cal AD 1025 (Cal 975 BP)
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Probability method
1000/-75
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(No Transcript)
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Calibration programs
http//www.radiocarbon.org/Info/index.html
http//radiocarbon.ldeo.columbia.edu/research/radc
arbcal.htm
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Sources of error in 14C dating

1. Problems of sample selection and contamination
2. Variations in 14C content of the oceanic
   reservoir
3. Carbon fractionation effects

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Uranium-Thorium dating

• It is also a isotopic dating technique
• It calculates and age from the degree to which
  equilibrium has been restored between the
  radioactive isotope thorium-230 and its
  radioactive parent uranium-234 within a sample

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• Uranium is soluble in water thorium isnt.
• Any material that precipitates near the surface
  contains traces of uranium
• Over time uranium-234 decays into thorium-230
  with a half-life of 245,000
• Instead of accumulating indefinitely, thorium-230
  tends to reach an equilibrium with its parent
  isotope (uranium 234)
• The upper age limit of U/Th is around 500,000
  years.
• It is used on carbonates (pedogenic carbonates,
  travertine, calcretes, or any product of
  precipitation)

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Luminescence dating

• TL (Thermoluminescence)
• OSL (Optically stimulated luminescence)
• IRSL (Infrared)

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What is luminescence?

• Luminescence is the light emitted from a mineral
  crystal (mainly quartz and feldspars) when
  subjected to heating or when exposed to light.

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What is luminescence?

• Luminescence is the light emitted from a mineral
  crystal (mainly quartz and feldspars) when
  subjected to heating or when exposed to light.

Light emitted in response to Heating Visible
radiation Infrared radiation
TL OSL IRSL
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• Luminescence is a measure of accumulated dose of
  ionizing radiation (expressed in units of Gray,
  Gy) which is a function of the sample exposure
  age.

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• Luminescence is a measure of accumulated dose of
  ionizing radiation (expressed in units of Gray,
  Gy) which is a function of the sample exposure
  age.

Age (t) equivalent dose (Gy)/dose rate (Gy/t).
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Deposits dated with luminescence

• Eolian (most successful)
• Alluvial
• Cultural (where there was fire or rapid burial
  that blocked light)
• Heated lithics

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Advantages and specific uses of each technique

• TL
• OSL
• IRSL

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Advantages and specific uses of each technique

• TL
• OSL
• IRSL

Heated materials, but not good for young ages
Wide variety of sediments good for young ages
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Advantages of Luminescence Dating over 14C dating

• Useful in deposits poor in organics
• Not dependant on existence of paleosols and/or
  organics
• Can be used for older periods of time
• Can also be used for recent periods of time (e.g.
  20th century).
• It provides date of sediment accumulation, not
  date of organic matter. Therefore, it is good for
  dating processes.

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Disadvantages of Luminescence dating

• It cannot be used for lacustrine deposits
• It may represent problems in some alluvial
  environments, especially where sediments have
  been transported in turbid waters
• Contamination, in this case by light and heat,
  can create problems
• Groundwater can bleach sediment radioactivity

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List of luminescence dating labs

• http//physics.okstate.edu/mckeever/lab/optical.ht
  m (Ginnis Lab)
• http//www.anthro.washington.edu/archy/TLlab/tlind
  ex.htm
• http//www.aber.ac.uk/qecwww/links/labs.htm