ARCHAEOLOGICAL DATING TECHNIQUES

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ARCHAEOLOGICAL DATING TECHNIQUES
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SIR MORTIMER WHEELERnew archaeology excavated
urban centres layer by layer

• The important thing is that the archaeologist
  must know his dates and how to use themrecorded
  dates where they are valid and unwritten dates
  where geological or physical or chemical or
  botanical science can win them from the earth.

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RELATIVE /ABSOLUTE

• RELATIVE TECHNIQUES
• Stratigraphy and Seriation
• Typology and Cross Dating
• Fluorine
• Obsidian Hydration
• Pollen Analyses
• ABSOLUTE TECHNIQUES
• Radio Carbon Dating
• Potassium Argon Dating
• Thermoluminescence
• Electron Spin Resonance
• Dendrochronology

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RELATIVE DATING TECHNIQUES

• One of the most fundamental principles of
  archaeology is the Law of Superposition.
• The law states that strata that are younger will
  be deposited on top of strata that are older,
  given normal conditions of deposition.
• This law is the guiding principle of
  stratigraphy, or the study of geological or soil
  layers.
• Stratigraphy is still the single best method
  that archaeologists have for determining the
  relative ages of archaeological materials.

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RELATIVE DATING

• Relative dating is an invaluable tool, but does
  not tell us WHEN an event occurred, just the
  ORDER in which events occurred. The oldest
  technique for establishing the actual ages of
  deposits is to use artifacts of a known age.
• These can be coins with minting dates stamped on
  them, writings with dates included, or objects
  that we know were only manufactured during a
  certain time.

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SUPERPOSITION
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STRATIGRAPHY

• Stratigraphy is the study of strata, or layers.
• Specifically, stratigraphy refers to the
  application of the Law of Superposition to soil
  and geological strata containing archaeological
  materials in order to determine the relative ages
  of layers.
• In addition, stratigraphy can tell us much about
  the processes affecting the deposition of soils,
  and the condition of sites and artifacts

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THE LAYERS OF TROY
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SERIATION

• This technique places assemblages of artefacts
  into relative order. Petrie used sequence dating
  to work back from the earliest historical phases
  of Egypt into pre-dynastic Neolithic times, using
  groups of contemporary artefacts deposited
  together at a single time in graves. Seriation
  was developed in the USA to place in order finds
  from strata or other kinds of assemblages such as
  potsherds collected from the surface of sites.

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 Petries system of Seriation or Sequence Dating
was developed in 1899. The system emphasises the
relation of one find to another rather than an
exact date of manufacture. Nine hundred graves
were selected from  almost 4000 excavated
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CROSS DATING

• Cross-dating is a technique used to take
  advantage of consistencies in stratigraphy
  between parts of a site or different sites, and
  objects or strata with a known relative
  chronology.
• A specialized form of cross-dating, using animal
  and plant fossils, is known as biostratigraphy.

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FLUORINE DATING

• Fluorine is an element that is found in most
  ground water around the world. It can be used as
  a relative dating technique.
• Skeletal remains buried in the earth are subject
  to a wide range of chemical changes. One of these
  changes can occur when percolating ground water
  comes into contact with the remains. The ground
  water inundates the bone remains with a solution
  of minerals drawn from local soils. This can
  cause a change in the mineral composition of the
  bone. Hydroxyl ions are displaced with a form of
  soluble fluorides. These ions form fluorapatite
  which is markedly less soluble. Over time, more
  and more fluorides are accumulated. The rate
  varies depending on the specific condition in the
  soil of the area and increases with age.
• Fluorine dating is chiefly of value in
  determining whether bone implements or human
  skeletal remains found in association with other
  bones were buried at the same time. It was
  fluorine dating that was instrumental in the
  debunking of Piltdown Man.

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Potassium Argon Dating

• The Potassium-Argon (K-Ar) dating method is the
  measurement of the accumulation of argon in a
  mineral. In contrast to a dating method such as
  C14 dating which measures the disappearance of a
  substance, K-Ar dating measure the accumulation
  of argon in a substance from the decomposition of
  potassium.
• This is relatively easy because argon, being an
  inert gas, usually does not leech out of a
  mineral and is easy to measure in small samples.
  The actual date is comprised of the time it has
  been formed from molten/heated minerals.
• This method, therefore, is not very useful when
  dating the time a human bone has been in the
  ground, but it does help in giving the time of
  many of the artifacts that are often times found
  alongside burials.
• If you were to take a piece of everyday rock, the
  K-Ar method would give you the date that piece of
  rock was "reset" by the changing of it's chemical
  structure. Many things can and do change the
  structure of rocks. Heating, weathering and many
  kinds of alterations will reset this time.
• Therefore, archaeologists can determine
  relatively accurately how long ago a heat treated
  projectile point was made, or a piece of pottery
  was last used to cook food.

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POTASSIUM ARGON DATING

• Potassium-Argon Dating Potassium-Argon dating is
  the only viable technique for dating very old
  archaeological materials. Geologists have used
  this method to date rocks as much as 4 billion
  years old.
• It is based on the fact that some of the
  radioactive isotope of Potassium, Potassium-40
  (K-40) ,decays to the gas Argon as Argon-40
  (Ar-40).
• By comparing the proportion of K-40 to Ar-40 in
  a sample of volcanic rock, and knowing the decay
  rate of K-40, the date that the rock formed can
  be determined.

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POLLEN DATING PALYNOLOGY

• Each year seed-bearing plants release large
  numbers of pollen grains. This process results in
  a "rain" of pollen that falls over many types of
  environments. Pollen that ends up in lake beds or
  peat bogs is the most likely to be preserved, but
  pollen may also become fossilized in arid
  conditions if the soil is acidic or cool.
  Scientists can develop a pollen chronology, or
  calendar, by noting which species of pollen were
  deposited earlier in time, that is, residue in
  deeper sediment or rock layers, than others

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Value of Pollen Dating

• pollen dating provides relative dates beyond the
  limits of radiocarbon (40,000 years), and can be
  used in some places where radiocarbon dates are
  unobtainable.
• Pollen dating was crucial in the dating of Ice
  Man and the Shroud of Turin

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Weakness of Relative Dating

• The potential flaws in relative dating are
  obvious. Simply assuming that an object is older
  because it was found at a lower depth in the
  record is only subjective science.
• There are many instances of deep holes being dug
  for rubbish pits or to locate well water that
  protrude into the record of older strata
  injecting more modern material as they are filled
  in over time.
• Landslides and slips can completely change the
  topography of an entire site burying what was
  once on top by that which is much older, hence
  reversing the strata layers

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ABSOLUTE DATING

• A more precise and accurate system is known as
  absolute dating and can in most circumstances
  provide a calendar year to the object. Since 1950
  there has been a transformation in the dating
  techniques of archaeologists.
• Absolute dating is highly dependant on
  laboratory analysis. There are a number of
  techniques that have come to archaeology through
  the nuclear research efforts during WW2.

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A Watershed

• 'The application of the 14C method to
  archaeological materials is generally considered
  to be a watershed event in the history of
  archaeology and, in particular, in prehistoric
  studies
• R E Taylor

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RADIO CARBON DATING

• Radiocarbon dating uses the biological assumption
  that all living things absorb carbon, both
  ordinary carbon, C12, and radioactive carbon,
  C14, into their living tissue.
• At the moment of death the C14 begins to decay
  at a rate that scientists already know from other
  experiments. The missing amount can then
  determine how long it took to be lost and
  therefore date the object to a precise period.
• C14 dating can only be used on organic matter.

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LIMITATIONS OF C14 DATING

• First, the size of the archaeological sample is
  important. Larger samples are better, because
  purification and distillation remove some matter.
  Although new techniques for working with very
  small samples have been developed, like
  accelerator dating, these are very expensive and
  still somewhat experimental.
• Second, great care must be taken in collecting
  and packing samples to avoid contamination by
  more recent carbon. For each sample, clean
  trowels should be used, to avoid cross
  contamination between samples..
• Third, because the decay rate is logarithmic,
  radiocarbon dating has significant upper and
  lower limits. It is not very accurate for fairly
  recent deposits

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AMS radiocarbon dating is a form of radiocarbon
dating that is more precise and requires less
carbon than conventional radiocarbon methods
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DENDROCHRONOLOGY

• Dendrochronology is another traditional technique
  for establishing the abolute date of events. This
  is also called Tree-Ring Dating.
• Tree-Ring dating is based on the principle that
  the growth rings on certain species of trees
  reflect variations in seasonal and annual
  rainfall. Trees from the same species, growing in
  the same area or environment will be exposed to
  the same conditions, and hence their growth rings
  will match at the point where their lifecycles
  overlap.

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Weaknesses in Dendrochronolgy

• In some areas of the world, particularly in the
  tropics, the species available do not have
  sufficiently distinct seasonal patterns that they
  can be used.
• Where the right species are available, the wood
  must be well enough preserved that the rings are
  readable. In addition, there must be at least 30
  intact rings on any one sample.
• There also must be an existing master strip for
  that area and species. There is an absolute limit
  on how far back in the past we can date things
  with tree rings. Although bristle cone pine trees
  can live to 9,000 years, this is a very rare
  phenomenon. As we try to push our matching of
  archaeological specimens beyond the range for
  which we have good control data, our confidence
  in the derived dates diminishes

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LIMITATIONS

• Fourth, the ratio of C-14 to C-12 in the
  atmosphere is not constant. Although it was
  originally thought that there has always been
  about the same ratio, radiocarbon samples taken
  and cross dated using other techniques like
  dendrochronology have shown that the ratio of
  C-14 to C-12 has varied significantly during the
  history of the Earth.
• Finally, although radiocarbon dating is the most
  common and widely used chronometric technique in
  archaeology today, it is not infallable. In
  general, single dates should not be trusted.
  Whenever possible multiple samples should be
  collected and dated from associated strata

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Obsidian Hydration

• Developed in 1960, Obsidian Hydration Analysis
  (OHA) is an inexpensive technique archaeologists
  and geoarchaeologists use to find the age of a
  site they have excavated. This method is most
  often used as a means of relative dating , but an
  absolute date may also be estimated in some
  circumstances
• Obsidian was a common rock used in stone tool
  making. Obsidian is used mainly because of its
  availability in prehistoric tool making and its
  unique quality of rehydrating itself after a
  fracture
• When obsidian is newly exposed to the atmosphere,
  its surface begins to absorb water from the air,
  which gradually seeps into the interior of the
  stone. Several factors can affect the obsidian's
  water absorption, including soil type, climate,
  time and geochemistry.
• When viewed under a microscope, the layer
  permeated by moisture (known as a "rind") becomes
  visible as a rim when the rind reaches a width of
  0.5 microns (a micron being one millionth of a
  meter). The greater the rind thickness, the
  greater the age of the exposed obsidian.

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The actual measurement of hydration involves
using light transmission to determine the amount
of hydration, and therefore the relative age of
the sample. The prepared slides are observed by
means of a microscope to determine the amount of
light transmission.
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Electron Spin Resonance

• Electron Spin Resonance Dating falls into the
  group of dating methods that uses radiation
  exposure to date many materials found at
  archaeological sites. It is also known as a
  Radiometric Dating Method.
• This technique is mostly used to date minerals.It
  has been used to date such things as sedimentary
  quartz, fossilized teeth, flint, and calcium
  carbonate in limestone, coral and egg shells.
• This method works by using radiation to cause
  electrons to separate from the atoms. These
  electrons then become trapped in the crystal
  lattice of minerals. This changes the magnetic
  field of the material at a rate that is
  predictable, allowing it to be used to date an
  item. It can be used to date when mineralization,
  sedimentation, or the last heating of minerals
  took place. It is often used to date quartz from
  meteorite strikes, and places where earthquake
  activity has taken place

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THERMOLUMINESCENCE

• Artefacts that are made from crystalline
  materials can be dated using luminescence
  analysis. Crystalline minerals when subjected to
  intense heat will burn with differing colours of
  flame.
• Mostly used to date pottery the method is very
  effective but costly.
• The greatest problem with dating an object from
  antiquity is that nearly every absolute dating
  process requires the destruction of at least a
  piece of the object in conducting the analysis.
  There are relatively few dating laboratories and
  having an artefact dated can be an expensive
  exercise especially if the artefact is not of
  great value itself.

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Limitations to Thermoluminescence

• Thermoluminescence dating is in its developmental
  stages. Except for doing simple authenticity
  tests of art objects, thermoluminescence dating
  is not generally accurate enough for
  archaeological standards.
• There are many factors which have to be taken
  into account and each of these factors has its
  own random error. This, combined with poorly
  understood measurement errors, make the accuracy
  of thermoluminescence dating only about 15
  accurate for a single sample and 7 to 10
  accurate for a suite of samples in a single
  context.
• Thermoluminescence dating is used for rocks,
  minerals and pottery. It dates items between the
  years 300-10,000B.P.

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Uranium Disequilibrium

• Uranium-Thorium dating is an absolute dating
  technique which uses the properties of the
  radio-active half-life of Uranium-238 and
  Thorium-230.
• Uranium-Thorium dating was first used on fossil
  bones in 1956, however, it had been used for
  dating wood before this. This dating technique
  has been used effectively on marine sediment,
  bone, wood, coral, stone and soil. One of the
  benefits of uranium-thorium dating is that the
  sample sizes can be less than 20 grams, in fact
  bone samples can be 3-5 grams for an accurate
  date

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