Absolute dating

• 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.

limitations

• Sapwood discarded by carpenters can hold up to 16 rings, making the date an estimate.

site example

• central slump in sea henge (april/june 2050 BC)

• 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.

limitations

• 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.

dating

• 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.

pottery

• 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.

limitations

• less acurate than C14 and can give faluse readings die to radiation from soil or if firing was at a low temperature

dating

• glass and burn flint or stone

400,000 years to present day

• 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

• 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.

• chemical structures of amino acids found in all living things change slowly over time at a known rate

limitations

• can vary with climate

dating

• bones
• teeth
• shells

Sites

• 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

sites

• 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.

limitations

site mustn't be disturbed.

dating

• ceramics
• lava
• hearth
• kiln- containing iron oxide

up to 50,000 years

• electric charge build up at a known rate in some crystal structures. time since process begin can be calculated by measuring the charge.

limitations

• works best in dry environments

dating

• teeth enamel
• shells
• calcite depostis in caves

50,000- 1 million years

• 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

limitations

• 10% error margin

Dating

• glass
• 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