- Most acurate form of chronometric dating
- early cells- early in annual growth
- late wood- late in annual growth
- produces visible rings until the bark
- the rings are wider in good conditions e.g heavy rainfall
- rings a narrower where there are bad conditions e.g. little rainfall.
- Sapwood discarded by carpenters can hold up to 16 rings, making the date an estimate.
- central slump in sea henge (april/june 2050 BC)
Radio Carbon Dating ( C-14)
- All living things absorb several types of carbon from the atmosphere
- 1% of this is unstable and decays at a known rate.
- by comparing the amount of carbon remaining with the the carbon isotopes(don't decay) when in organic sample we can work out how much C14 has decayed.
- this indicates how long it has been since decaying began.
- amount of carbon in the atmosphere has varied over time,leading to many dates being underestimated by up to 1000 years
- due to this results have to also be calibrated
- carbon in sea is 400 years older than that on land.
- bone, shell, plant remains, charred bone, wood samples, twigs, nuts from trees
200-10000 years most reliable
40,000 less reliable.
80,000 pushing it
- radiocarbon decay in quartz crystals found in clay builds up an electric charge at a known rate which is then released as light when crystals are heated .
- when crystals are heated the 'clock' is re-set.
- flash of light energy released by a given weight of a ceramic sample can be measured in a lab to calculate the number of years since pottery was fired.
- less acurate than C14 and can give faluse readings die to radiation from soil or if firing was at a low temperature
- glass and burn flint or stone
400,000 years to present day
Optically Stimulated Luminescence (OSL)
- electrons from decaying radio active elemnts are trapped in crystals/ quartz.
- when stimulated with light the electrons free themselves giving off luminescence(light energy) in the process.
- amount released is used to date the last 'clock setting event'
- measures the time since sediment was last exposed to sunlight/ heated.
- calculated with more accuracy than TL
50- 1 million years
Potassium Argon dating
- as potasium in rock crystals decay it produces argon gas at a known rate
- measuring amount in a lab provides the date that crystals are formed
- used in volcanic regions to date layers of rock which sandwich human remains.
Amino acid racemization
- chemical structures of amino acids found in all living things change slowly over time at a known rate
- can vary with climate
- ostrich eggs on Paleolithic sites in Africa
1000-1 million years
- buried bones take up fluorine from surrounding soils.
- the amount of fluorine that it absorbs is proportional to ammount in the sorrounding deposit and the length of time that the bone has been burried
- pitdown man
- earths magnetic field changes over time. when iron oxide is heated to 600c and then cools it record the magnetic field at that time.
- variations in earth field have been calculated which enables the data to be established.
site mustn't be disturbed.
- kiln- containing iron oxide
up to 50,000 years
Electron Spin resonance (ESR)
- electric charge build up at a known rate in some crystal structures. time since process begin can be calculated by measuring the charge.
- works best in dry environments
- teeth enamel
- calcite depostis in caves
50,000- 1 million years
Fission track dating
- uranium decays regularly though fission (SPLITTING) which releases energy and damages crystalline structures leaving a TRACK.
- tracks and holes then counted to estimate the time the process of decay has taken
- 10% error margin
- obsidian (burnt)
- heated stones(containing uranium)
- sites sandwiched between volcanic layers
10,000 - several million years
- stems flowing into still bodies commonly deposit layers (varves) or summer silt and winter clay through the year.
- autumn/winter- dark colour due to dead vegetation
- reast of year- light colour
- this creates annual layers -like tree rings
- changing climate will lead to changing deposits, which can be cross referenced over large areas
up to 12,000 years