Radiocarbon and other dating methods

Radiocarbon Dating

Archaeology
DATING METHODS
Archaeologists use a variety of dating methods:

•RADIOCARBON DATING
•DENDROCHRONOLOGY (more next week)
•POTASSIUM ARGON DATING
•LUMINESCENCE DATING
•OBSIDIAN HYDRATIAN DATING
•ARCHEOMAGNETIC DATING


RADIOCARBON DATING
Carbon has two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C).
In addition, there are trace amounts of the unstable isotope carbon-14(14C) on Earth.
Carbon-14 has a half-life of 5730 years and would have long ago vanished from Earth were it not for the unremitting cosmic ray impacts on nitrogen in the Earth's atmosphere, which create more of the isotope.

DENDROCHRONOLOGY
POTASSIUM ARGON DATING

Thermoluminescence
OBSIDIAN HYDRATIAN DATING
The obsidian hydration dating method was introduced to the archaeological community in 1960 by Irving Friedman and Robert Smith of the U. S. Geological Survey (Friedman and Smith 1960). The potential of the method in archaeological chronologic studies was quickly recognized and research concerning the effect of different variables on the rate of hydration has continued to the present day by Friedman and others.
Archaeomagnetic dating
•The position of magnetic North wanders around the North Pole, and even reverses completely to the South Pole for extended periods on a geological time-scale.
•From any reference point its position is measurable in terms of two components: movement up or down (inclination or 'dip') and from side to side (declination).
•The earth's magnetic field is indeed dynamic and does shift. At present the declination for London changes by approximately 1 degree every decade.

How does it work?
•Magnetic information is recorded in iron elements in baked clay which have kept their position on cooling from the last firing of the clay.
•This means that baked clay, used for thousands of years in the construction of hearths, ovens and kilns, contains a weak but permanent magnetization which can be measured to determine the magnetic intensity and declination at the time of its last cooling.
•Before clay is baked these properties are orientated in random directions. If the temperature is raised to over several hundred degrees Celsius, the thermal agitation of the crystals allows some of the domains to be aligned by the earth's magnetic field.
•When the clay cools their directions remain fixed, and there is a weak permanent magnetization in the same direction as the earth's field.
Archaeomagnetic Example
Radiocarbon dating
•Radiocarbon dating is a method that uses the naturally occurring isotope carbon-14 (14C) to determine the age of carbonaceous (contains carbon) materials up to about 60,000 years.
•Benefits are that it provides an independent method of dating any organic material.
•Pitfalls are that it needs to be used properly.
N-14 is bombarded by cosmic radiation and loses a proton and C-14 is formed in the atmosphere
C-14 combines with oxygen in the atmosphere to form CO2
CO2 is absorbed by plants for photosynthesis, plants are eaten by herbivores
Herbivores are eaten by carnivores, etc. etc.
Herb/Carnivores store carbon for energy
One the carbon-absorbing organism has died, those parts that remain can be “radiocarbon-dated”.
What is measured?
•Radiocarbon dating measures the rate of decay in 14C after the organism stopped absorbing atmospheric carbon.
•The amount of 14C is measured relative to 12C.
Conception
•Radiocarbon dating was begun by Willard Libby

•In 1946, he suggested that 14C exists in living matter

•Confirmed a year later

•1949 found that several trees contained roughly the same amount of activity due to 14C

•1960 won Nobel prize
Assumptions
•14C production is constant

•The biosphere and atmosphere have roughly the same 14C concentration

•After death there is no 14C exchange and it is only affected by radioactive decay, except where particular factors are involved such as
–Glacial effects
–Human activity
–Variations in natural production rate

C-14 decreases over the years

Glacial effects
CO2 solubility is temperature dependant


Human activity
Fossil fuel (Suess) effect and bomb effect

Variations in production rate

Major cause of Suess wiggles
Pitfalls
•The cumulative effect of the pitfalls were to demonstrate that the radiocarbon decay was variable.
•Thus dates could be expressed as years before present, often written as ‘BP’, so measurement of the decay in the sample might produce a result of 2500 years BP.
•As there is a potential error margin, this is usually added to the date range.
•Thus dates would be written as 2500 BP +/- 50.

Pitfalls
•But how do years BP equate to dates like 500 BC or AD 785?
•The only way of checking this was to produce control samples from a tree-ring chronology from which samples of known date could be extracted, by counting back to tree rings that grew in 500 BC and AD 785.
•From this use of the tree-ring chronologies it was possible to relate radiocarbon decay to actual calendar dates.
•It is possible to use a graph or a computer programme to translate a ‘BP’ date to a real calendar date.

Sample Preparation
•Unearth sample
•Physical separation
•Treat with acid
•Convert carbon to CO2 via combustion
•Remove impurities (ie nitrogen oxides, sulfur, products of incomplete combustion, and radon)
•Isolate carbon: 2 Mg + CO2 à MgO + C
•Limit exposure to air

•Accelerator Mass Spectrometer
Accelerator Mass Spectrometry
What C-14 and AMS date