Quaternary Environments Dating Methods I

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Quaternary EnvironmentsDating Methods I
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Accuracy Versus Precision

• Precision means that the samples have low amount
  of error associated with the dating
• Accuracy means that the samples are dated to the
  true age of the sample
• We strive for both accuracy and precision in
  dating techniques

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Accuracy Versus Precision
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Relative Versus Absolute Dating

• Relative Dating
• Principle of Superposition
• Absolute Dating
• Provides solid chronological dates (within error
  bars) that are related to a calendar year scale

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Methods

• Radioisotopic Methods
• Based on rate of atomic disintegration
• Paleomagnetic Methods
• Relies on past reversals of the Earths magnetic
  field
• Organic and Inorganic Chemical Methods
• Based on time-dependent chemical changes in a
  sample
• Biological Methods
• Based on the growth of an organism

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Using Radioactivity in Dating

• Radiometric dating
• Useful radioactive isotopes for providing ages
• 87Rb/87Sr 47.0 billion years
• 232Th/208Pb 12.1 billion years
• 238U/206Pb 4.5 billion years
• 40K/40Ar 1.3 billion years
• 235U/207Pb 713 million years
• 14C/14N 5,730 years (5,570 Libby years)

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Sources of Error

• A closed system is required
• To avoid potential problems, only fresh,
  unweathered samples should be used

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Using Radioactivity in Dating

• Reviewing basic atomic structure
• Nucleus
• Protons positively charged particles with mass
• Neutrons neutral particles with mass
• Electrons negatively charged particles that
  orbit the nucleus

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Using Radioactivity in Dating

• Reviewing basic atomic structure
• Atomic number
• An elements identifying number
• Equal to the number of protons in the atoms
  nucleus
• Mass number
• Sum of the number of protons and neutrons in an
  atoms nucleus
• Identifies an isotope

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Using Radioactivity in Dating

• Reviewing basic atomic structure
• Isotope
• Variant of the same parent atom
• Differs in the number of neutrons
• Results in a different mass number than the
  parent atom

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Using Radioactivity in Dating

• Radioactivity
• Spontaneous changes (decay) in the structure of
  atomic nuclei
• Types of radioactive decay
• Alpha emission
• Emission of 2 protons and 2 neutrons (an alpha
  particle)
• Mass number is reduced by 4 and the atomic number
  is lowered by 2

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Using Radioactivity in Dating

• Types of radioactive decay
• Beta emission
• An electron (beta particle) is ejected from the
  nucleus
• Mass number remains unchanged and the atomic
  number increases by 1

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Using Radioactivity in Dating

• Types of radioactive decay
• Electron capture
• An electron is captured by the nucleus
• The electron combines with a proton to form a
  neutron
• Mass number remains unchanged and the atomic
  number decreases by 1

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Common Types of Radioactive Decay
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Using Radioactivity in Dating

• Parent an unstable radioactive isotope
• Daughter product the isotopes resulting from
  the decay of a parent
• Half-life the time required for one-half of the
  radioactive nuclei in a sample to decay

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Using Radioactivity in Dating

• Radiometric dating
• Principle of radioactive dating
• The percentage of radioactive atoms that decay
  during one half-life is always the same (50
  percent)
• However, the actual number of atoms that decay
  continually decreases
• Comparing the ratio of parent to daughter yields
  the age of the sample

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Radioactive decay curve
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Radiocarbon Dating

• Dating with 14C
• Half-life of 5730 years
• Used to date very recent events
• 14C is produced in the upper atmosphere
• Useful tool for anthropologists, archeologists,
  and geologists who study very recent Earth history

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Sources of Error in 14C Dating

• Problems of Sample Selection and Contamination
• Young carbon effects
• Old carbon effects
• Variation in 14C content in the ocean reservoir
• Fractionation Effects

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14C Age of Sea Water
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Radiocarbon Variation Over the Last 2000 years
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Calibration Curve Showing Departure
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Calibrated Curve
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Radiocarbon Plateaus
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14C Bomb Spike
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Potassium Argon Dating (40K/40Ar)

• Instrumental in dating sea-floor basalts and
  providing the timing of magnetic reversals
• Used in dating lava flows
• Also for dating metamorphic events

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Potassium Argon Dating (40K/40Ar)

• 39K and 41K, Stable
• 40K unstable and 0.012 of all potassium
• 40K decays to 40Ca and 40Ar
• Ca is common in rocks and is therefore not useful
  in dating
• Measure the amount of 40Ar in the lab and the
  amount of 40K is also measured from the sample

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Potassium Argon Dating (40K/40Ar)

• Long half-life makes this useful over long time
  scales but not really usable at less than 100,000
  years
• Optimal time range is around 30 ma and up to 1 ba
  rocks can be dated
• Dating is done on sanidine, plagioclase, biotite,
  hornblende, and olivine in volcanic rocks and
  glauconite, feldspar, and sylvite in sedimentary
  rocks

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Problems of 40K/40Ar

• Assumptions
• No Ar was left in the rock at formation
• System has remained closed since formation
• Checks
• The ratio of 36Ar to 40Ar is known in the
  atmosphere and can be measured in the rock to
  determine atmospheric contamination
• Problem
• Loss of Ar due to diffusion, recrystallization,
  solution, and chemical reactions

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40Ar/39Ar Dating

• A problem with 40K/40Ar dating is that K and Ar
  are measured at different places in the rock
• This can be solved by irradiating the samples and
  converting 39K to 39Ar
• With the known ratio of 40K to 39K, the amount of
  40K can be calculated from the same lattice
  structure as the 39Ar

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Uranium Series Dating

• 238U and 235U have a decay process that cascades
  through a series of elements
• Each decay stage can be used as a dating tool
• Thermal Ionization Mass Spectrometry (TIMS)
  allows very accurate estimates from small samples
• U series are useful in dating corals and
  speleothems
• Mollusks seem to be an open system in relation to
  U and are not generally conducive to U series
  dating

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Problems of U Series Dating

• Assumes the initial 230Th/234U, 234U/238U, and
  231Pa/235U ratios
• Likely in ocean sediments but more in flux in the
  atmosphere
• Assumes a closed system

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Luminescence Dating Principles and Applications

• Light emitted from a mineral crystal (usually
  quartz or feldspars) when exposed to heat or
  light
• The light emitted is related to the amount of
  ionizing radiation that the sample has been
  exposed to from sediment
• The clock is set to zero by heating or optical
  bleaching
• Therefore Loess and fluvial sediments make good
  candidates for luminescence dating

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Luminescence

• Thermoluminescence (TL Dating)
• When the light is emitted as a result of thermal
  hearting
• First published Wintle and Huntley 1979
• Optical and Infrared Stimulated Luminescence (OSL
  and IRSL Dating)
• Visible or infrared energy emitted in response to
  radiation

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Problems in TL Dating

• Assumes that the relationship between the
  radiation dose and the resulting luminescence is
  a linear relationship not always the case
• lt5,000 yrs the rate of electron accumulation is
  slow, possibly needing to exceed a threshold
• Some minerals may reach saturation gt300,000 yrs
• Anomalous Fading Minerals do not hold the
  electrons beyond a few weeks
• Variations in environmental dose related to
  moisture content for one example

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Optical and Infrared Stimulated Luminescence (OSL
and IRSL Dating)

• Zero in the modern sediments
• Sensitive to light bleaching setting the system
  to zero
• Multiple measurements are possible because short
  stimulation to the light source does not deplete
  the potential luminescence

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Fission Track Dating

• Uranium will decay through fission, splitting the
  nucleus and shooting the two halves into the
  mineral
• The results are fission tracks from 10-20µm in
  length
• Some glassy minerals will loose their fission
  tracks through heating, setting the clock to zero
• Different minerals have different annealing
  temperatures

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Fission Track Dating

• The samples are polished and etched with a
  chemical that brings out the tracks
• The tracks are counted, then the sample is
  heated, annealing the tracks. Then the sample is
  irradiated with a slow neutron beam and the
  tracks from the fission of 235U are counted
• The number of induced tracks is proportional to
  the amount of 238U in the sample
• The known fission rate of 238U is used to
  calculate the age of the sample

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Fission Track Dating

• Can be used in apatite, micas, sphene, and
  zircons
• Can also be used in rocks such as volcanic ash,
  obsidian, basalts, granites, tuffs, and
  carbonatites
• Ranges from 103 to 108 years
• The error associated with this technique is hard
  to determine and is seldom reported
• Repeat measures are the best, but are seldom
  undertaken because of time constraints

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Dendrochronology

• Temperate trees produce annual rings.
• The trees are recording all of the environmental
  variables that affect tree growth.
• Can be used to date objects with annual
  resolution back 10,000 years in the best
  circumstances.

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Neutron capture (A) and Beta emission (B)