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ARCHAEOLOGICAL DATING TECHNIQUES

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Title: ARCHAEOLOGICAL DATING TECHNIQUES


1
ARCHAEOLOGICAL DATING TECHNIQUES
2
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.

3
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

4
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.

5
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.

6
SUPERPOSITION
7
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

8
THE LAYERS OF TROY
9
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.

10
 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
11
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.

12
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.

13
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.

14
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.

15
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

16
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

17
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

18
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19
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.

20
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

21
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.

22
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

23
AMS radiocarbon dating is a form of radiocarbon
dating that is more precise and requires less
carbon than conventional radiocarbon methods
24
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.

25
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26
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

27
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

28
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.

29
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.
30
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

31
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.

32
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.

33
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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