Keys to the Past


Carbon 14; Carbon 14 dating

A method of dating archaeological artefacts or layers based on the amount of a radioactive particle. Carbon-14 is a natural type of Carbon (C) produced in the atmosphere by the interaction of cosmic rays and Nitrogen (N2) gas. It is only produced in small amounts. This is dispersed through the atmosphere as Carbon Dioxide (CO2), and then into other parts of the environment through time as it is taken up by animals, marine organisms and plants.

When an organism dies it ceases taking up this radioactive particle. Therefore, over the time the amount an organism of this radioactively unstable compound will go down in proportion to the time that the organism has been unable to replenish their Carbon-14. Collection of a sample from an archaeological artefact or layer of material which contains Carbon will also contain Carbon-14 in small amounts. Measurement of the amount of Carbon-14 to a normal amount held will therefore produce, via complicated mathematics, a Carbon date with + and - values. The date is subtracted from 1950AD for convenience and a date is produced.

For the prehistoric periods and with marine-derived samples a form of calibration is required. This is due to the amount of Carbon-14 being produced in total and it's time in distribution varying. The amount of Carbon-14 is known for specific years in the past through precise dendrochronological samples of a single years growth. This can allow for a calibration procedure. In the case of marine samples a offset is needed - since it takes time for any Carbon-14 to enter the marine environment from being a gas.

In referring to a Carbon date the experimental date is given, the sample code of the laboratory where the measurement was taken, the calibrated date at one and two levels of experimental uncertainty. A Carbon date strictly does not exist - date ranges are produced, with a strong statistical probability that the archaeological artefact or event is within those ranges. Calibration means that the limits of the date ranges cannot be used to make a statistical mean date for a single year - however, tempting that might be.

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