Chronology

An important aspect of palaeoenvironmental research is the ability to accurately determine the timing of climatic or environmental change in a sediment record or the age of an artefact or archaeological feature. The CHERISH team will be routinely employing two different techniques to provide the dating framework for sedimentary sequences and archaeological investigations: radiocarbon dating and luminescence dating. The combination of these approaches means that we can date a greater variety of sample types and contexts. Where both techniques can be used, this provides a valuable opportunity for cross-validation of age estimates.

Radiocarbon Dating

Radiocarbon dating is based on the principle that all living organisms absorb carbon dioxide during their lifetime, and that after death, a proportion of that carbon which is radioactive (radiocarbon or 14C), decays at a constant rate. By measuring the amount of 14C remaining in plant material, shells or bone, an estimate how long ago that organism was alive can be made. Radiocarbon dating is generally suitable for samples ranging from a few hundred years old up to 50,000 years old, ideally suited to the timeframe under consideration by the CHERISH Project. Advances in analytical techniques mean that very small samples containing as little as 2 mg of carbon can be dated, and it is the most widely used chronological method in palaeoenvironmental studies. Variation in the temporal production of 14C, variance of the proportion of 14C in natural systems, the recycling of “old carbon” by organisms and contamination with younger or older material are all possible sources of error. Therefore it is preferable to date organic material that can be identified such as charcoal, wood and terrestrial plant material, which represent the best choice for 14C analysis of sediments.

Sample of Alder prepared for dating using radiocarbon AnalysisSample of Alder prepared for dating using radiocarbon Analysis

Optically Stimulated Luminescence (OSL) Dating

OSL dating utilises the ability of naturally occurring radiation (uranium, thorium and potassium) can get trapped within the crystalline structure of minerals such as quartz and feldspar. The radiation builds up whist the mineral grains are buried in the ground, but released when exposed to sunlight. Samples protected from exposure to daylight are stimulated in the laboratory with light of a particular wavelength to release the stored radiation in the form of light, and by measuring the brightness an estimate can be made as to time the grains were last exposed to sunlight prior to burial. The Aberystwyth Luminescence Research Laboratory (ALRL) in the Department of Geography and Earth Sciences at Aberystwyth University is a world leader in the development and application of luminescence dating methods in environmental and archaeological research. Professor Geoff Duller, ALRL co-director and CHERISH team member has devised guidelines for the use of luminescence dating in archaeology, which provides a more detailed introduction to the technique here

Processing Luminescence Samples in Red LightProcessing Luminescence Samples in Red Light