Also known as chronometric dating
- 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- Still only a rough guide to absolute dating; nowadays superseded by scientifically based methods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
- To calibrate or correct radiocarbon dates
- An independent dating method in its own right
- Its precision can document the founding, expansion and usage of a settlement
Method
- New ring of wood produced each year
- Thickness varies with · fluctuations in climate · age of the tree (rings become narrower with increasing age)
- 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
- Not necessary to cut tree down - a sample can be bored
- Match sequence of growth rings in living and old timbers to give chronology for several thousands of years
- 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
- 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
1.
Radiocarbon Dating
- 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 gasDecay 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.