Reconstructing and Dating the Past

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Reconstructing and Dating the Past

• Preservation and Dating Methods

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Preservation

• Not everything made by past peoples survive and
  make it into the archaeological record.
• Softer more perishable subsistence, such as
  cloth, often disintegrates very quickly,
  especially in soils that are particularly acidic.
• Best conditions for preservation
• Very wet
• Very cold
• Worst conditions (those that change)
• Hot ? cold
• Wet? dry
• Tropics

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Dating Methods

• Relative dating techniquesrefers simply to
  evaluating the age of one item of data relative
  to other items. EX determining if artifact A is
  older than artifact B.
• Absolute dating techniques provides an absolute
  date in the form of a calendar date.

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Relative Dating

• Seriationpatterns of human behavior change
  continually, and as behavior changes, so do its
  material products.
• We observe how changes through time in design and
  style are after familiar objects in our own
  society.
• The artifacts and features of past societies also
  exhibit changes through time, and by observing
  and studying their attributes, archaeologists can
  usually discover the trends.

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Sequence Comparison

• If seriation cannot be used for the artifacts
  being studied, the archaeologist has another
  recourse.
• Sequence comparison-- If other well-documents
  artifact sequence exists in the geographical area
  being investigated, the artifact classes in
  question may be compared to those already defined
  form nearby sites and placed into a temporal
  order corresponding to those already established.

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Stratigraphy

• Stratigraphy refers to the archaeological
  interpretation of the significance of
  stratification.
• The age of archaeological materials can sometimes
  be assessed by their association with geologic
  deposits or formations.
• Often these assessments are relative, as in cases
  based on superposition, where materials in lower
  strata were deposited earlier that those in
  higher strata.
• As long as that context and, therefore, the
  temporal order of a stratified deposit is clear,
  the archaeologists can use stratigraphy to
  determine the relative age of the deposit of
  artifacts and other materials in the deposit.

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Absolute dating techniques

• Chronometric, or time-measured techniques
• Radiometric method
• Potassium/Argon (40K40Ar)
• Optically stimulated luminescence (OSL) and
  Thermoluminescence (TL)
• Electron Spin Resonance (ESR)
• Fission track dating

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Carbon 14

• This method was developed in the late 1940s by
  Willard F. Libby, who was measuring the amount of
  14C detected by Geiger counters, which is used to
  measure the rate of decay emissions from a
  sample.
• Half-life 5730 40
• Several age-determination techniques exploit the
  principle of radioactive decay, the
  transformation of unstable radioactive isotopes
  into stable elements.

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• Radiocarbon dating is the most important
  radiometric technique for archaeologists.
• Carbon dioxide enters plants through
  photosynthesis, and the plants are in turn eaten
  by animals.
• Thus all living things constantly take in both
  ordinary carbon (12C) and radioactive carbon
  (14C) throughout their lifetimes.
• The proportion of 14C to 12C in an organism
  remains constant until its death.

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• At that point, no further 14C is taken in, and
  the amount of radioactive carbon present at that
  time begins to decrease through radioactive
  decay.
• Thus, measurement of the amount of 14C still
  present (and emitting radiation) in plant and
  animal remains enables the determination of the
  amount of time elapsed since death.

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Potassium/Argon (40K/40Ar)

• Based on the radioactive decay of a rare isotope
  of potassium (40K )to form argon (40Ar )gas.
• The half-live of 40K is 1.31 billion years, but
  the method can be used to date materials as
  recent as 100,000 years old.
• The technique is used principally to determine
  ages for geological formations that contain
  potassium.
• The 40K/40Ar technique has been particularly
  helpful in dating geological formations
  associated with the remains of fossil hominids
  and lower Paleolithic tools.

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Optically stimulated luminescence (OSL) and
Thermoluminescence (TL)

• These are related dating techniques which attempt
  to measure the amount of energy trapped in
  sediments (OSL) or heated clay and stones (TL) by
  reheating the material and then measuring the
  amount of energy releases in the process.
• The common principle uniting the approaches is
  the exposure to radiation causes electrons to
  separate from atoms.
• Some of these electrons are caught in what are
  called traps, or defects in the material being
  heated.
• This process continues until the traps are
  fullsaturation.

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• By exposing the traps to radiation in the lab,
  the traps can be opened and the energy that is
  emitted in the form of light can be accurately
  measured.
• This can be measured and produces a dated based
  on the total amount of radiation received by the
  sample.
• However, OSL and TL can be problematic.
• The annual dose of radiation received by the
  sample must be accurately estimated.
• They change over time.
• Moisture history of the sample is also crucial,
  because water attenuates (reduces the strength or
  value) radiation effects.
• This process destroys the sample.

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Electron Spin Resonance (ESR)

• A technique best suited for the analysis of tooth
  enamel, shells, and burnt stone, it uses a
  spectrometer to measure the amount of energy
  released from an object when bombarded with
  microwaves.
• Has an effective range between a few thousand to
  500,000 ya.
• This method used microwaves to bombard the sample
  and then used an ESR spectrometer to measure the
  trapped electrons.

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Fission track dating

• This method functions differently from the 3 just
  mentioned.
• When atoms of 238U (uranium 238) fission, they do
  so at a constant rate (8.2 1015 years).
• In the process of fissioning, the nucleus bursts
  apart into two parts that are violently repelled
  from on another.
• These atomic bodies create fission tracks in
  crystalline material and obsidian.
• By counting the frequency of fission tracks and
  comparing these to the know rate of fissioning,
  the age of the sample can be calculated.