Also known as chronometric dating

  1. Until c.1900, all dating depended on historical methods

    i.e. connections with chronologies and calendars established by people in written documents

Chronologies

    • Romans dated in terms of Consuls and Emperors and AUC
    • Greeks reckoned from Olympic Games, started in 776BC
    • Egyptians reckoned in terms of successive Kings and Dynasties

Calendars

    • E.g. Mayan calendar, giving great precision for Classics Period (AD 300 - 900)

2. Historical chronology is easy to use, when lots of artefacts are found related to it, often carrying the date on them e.g. coins

3. To date a coin or any artefact is not the same thing as to date the context in which it is found

    • Date of coin = year it was made
    • (sealed) deposit can be no earlier than date on the coin but could be later
    • coin gives a TERMINUS POST QUEM = a date after which the deposit may have been lain down

4. Cross Dating

    • The use of exports and imports to extend chronological linkages
    • E.g. Aegean pottery turning up in a well-dated Egyptian context
    • Gives a TERMINUS ANTE QUEM = a date before which the deposit may have been lain down
    • Or Egyptian objects turning up on Aegean sites
    • Still only a rough guide to absolute dating; nowadays superseded by scientifically based methods
Tree Ring Dating - Dendrochronology
    • Used in Europe since 1930s
    • Computerised since 1960s

Uses

    1. To calibrate or correct radiocarbon dates
    2. An independent dating method in its own right
    3. Its precision can document the founding, expansion and usage of a settlement

Method

    1. New ring of wood produced each year
    2. Thickness varies with fluctuations in climate age of the tree (rings become narrower with increasing age)
    3. Trees of same species, from same area show same pattern of rings, \ growth sequence can be matched between successively older timbers to build up a chronology for an area
    4. Not necessary to cut tree down - a sample can be bored
    5. Match sequence of growth rings in living and old timbers to give chronology for several thousands of years
    6. A tree ring sequence of e.g. 100 years is matched with 100 year sequence in master sequence - to give felling date of that timber to within one year
    7. Long Master Sequences have been established, against which to check and calibrate radiocarbon dates

Limitations of Dendrochronology

      • Not a worldwide method of dating because it cannot be applied to the tropics where there are no clearly defined annual rings
      • It is restricted to wood for which there is a master sequence - some local chronologies remain "floating" i.e. cannot be tied into a master sequence
      • The date found is the date of felling , not the date when it became part of any structure, so it may be earlier or later than the structure of which it is a part
Radiocarbon Dating
1.
    • The work of Willard Libby, in 1949
    • Single most important method of dating for last 50,000 years
    • Has inaccuracies, sometimes caused by archaeologist

Theory:

Cosmic radiation bombards atmosphere
Creates high energy neutrons
neutrons react with nitrogen in the atmosphere
Creates radiocarbon i.e. carbon14 isotope which is unstable (has 8 neutrons in nucleus, not regular 12 of carbon12)
radioactive decay at known rate: 5568 years = 1/2 the carbon14 in any sample to decay
    • So, compare the "constant" background radiation level (flawed because it is variable over time) with sample to be dated
    • Background radiation: 75 disintegrations per minute
    • Sample: 371/2 disintegrations gives age of 5730 years
    • Calculate the age of dead plant or animal by how much radiocarbon is left in the sample
    • Samples may be of charcoal, wood, seeds, human or animal bones

Disintegration: each atom decays by releasing beta particles, emissions are counted by a Geiger counter

2. Uncertainty of measurement

    • Counting of emissions is open to error
    • Background atmospheric radiation varies in its concentration of 14C due to change in the Earth's magnetic field

3. Accelerator Mass Spectometry

    • Since 1980s, special gas counters, take very small samples
    • Counts 14C atoms directly, disregarding their radioactivity

4. Calibration of Radiocarbon dates

    • Needed, to account for variation in concentration of 14C due to change in the Earth's magnetic field
    • Tree ring dating corrects this
    • Shows radiocarbon dates before 100BC are increasingly too young
    • Pushing back of these dates = "Second Radiocarbon Revolution"

     

 

 

Potassium - Argon Dating
  • works on the principle of radioactive decay
  • Potassium is abundant throughout minerals in the Earth's crust

    K-AR method

  • Used by geologists to date rocks hundreds/thousands/millions of years old
  • Therefore, appropriate for dating early human sites (up to 5 million years old, especially E. African)
  • Technique is restricted to volcanic rock no more recent than 100,000yrs
  • Based on radioactive decay
Radioactive isotope
decays to
inert gas

Decay rate;

        Half life = c. 1.3 billion year

      • A measure of the quantity of argon40 trapped within 10gr rock sample gives an estimate of the date of the rock's formation
      • The less of argon40 there is, the more recent was the formation of the material involved.

        Limiting factors
        • Error estimate can be as much as 30,000 years (but still only 2% of the total age…)
        • Can only be used to date sites buried by volcanic rock
        • Can also use pumice in laser-fusion argon-argon dating
                          • More sensitive and needs smaller sample
                          • Has been used on Vesuvius


Thermoluminescence - TL clock
  • One of 3 methods depending on amount of radiation received by the specimen to date, NOT the radiation emitted by it
  • Only used for dating crystalline materials (minerals)
  • Analyses behaviour of electrons within a crystal when exposed to radiation
  • They absorb energy and become trapped in lattice defects
  • Can date POTTERY (unlike radiocarbon)-the most abundant inorganic material on archaeological site of last 10,00 years
  • But poorer precision than radiocarbon

POT

  • Made of clay, containing "high" level of radiation
  • Loses it when fired
  • TL clock set to zero
  • Starts to build up again when in use and then buried
  • Artificially fired in laboratory to measure light radiation emissions
Obsidian Hydration
  • Depends on the decay of a substance
  • Based on measurement of microscopically thin layer which forms on the surface of Obsidian
  • (natural volcanic glass, popular alternative to flint for manufacture of flaked tools)


Layer

  • Formed by absorption of water
  • Starts as soon as fresh surface is exposed
  • But does not form at constant rate and is especially sensitive to changes in temperature
  • May weather at different rates in different geological sources
  • Can only be considered a relative dating technique
                • (like dating of bone)
  • can be used to check contemporaneity of artefacts from same deposit, as a check for modern forgeries.