Dating: Luminescence
Luminescence (known also as thermoluminescence and
as Optically Stimulated Luminesence dating / OSL) forms an exciting new
technique
for
the absolute
dating of
archaeological
deposits.
This page is planned as a resource for luminesence dating - its techniques, best practices and application - with particular relevance to Near Eastern archaeology.
sampling for luminescence dating
Sampling sediment for luminescence dating
is relatively straightforward, though care should be taken that appropriate
sediment is sampled.
Weathered horizons exhibiting pedogenic accumulations of clay, silt, carbonate, or silica should be scrupulously avoided. Post-depositional weathering detrimentally affects the luminescence time-signal by altering the radionuclide concentration, disrupting the crystal structure of the mineral and by translocating in younger sediment particles. Ideally, at the sampling site, at least 30 cm of homogeneous sediment should surround the collected strata to maximize uniformity in the dose-rate environment during the burial period. Sampling within 30 cm of boulders or the surface should be avoided to obviate other potential radioactive inhomogeneities.
Approximately 30 g of sediment should be collected, though more or less sediment may be adequate depending on the concentration of the chosen particle size for luminescence dating. The geological luminescence signal of sediment is reduced with exposure to sunlight, thus care must be taken not to expose the sediment to light during sampling. It is particularly critical that sediments for OSL analysis are completely shielded from sunlight during sampling, because a few seconds of light exposure could significantly reduce the OSL signal. Prior to sampling, the section should be excavated back at least ca. 20 cm to expose a fresh face. Immediately prior to sampling the face should be scraped free of surface light-exposed grains. It is best to take the sediment intact; though the outer mineral grains may have been light exposed, the internal grains have been shielded from sunlight.
Fine grained (<2 mm) pliable sediment can be directly sampled by pushing the sampling container into the cleaned section face. Light and moisture tight containers are used in sampling. Black film canisters are inexpensive (often free) and will prevent moisture loss if the cap is sealed with electrical tape. Excellent sample containers similar to film canisters but with built-in moisture seals can be purchased from numerous chemical supply firms.
If the sediment is coarse grained (>2 mm), hardened or excessively dried out, other techniques are employed. Cohesive sediment can be sampled by cutting out a block and wrapping it in aluminum foil or by placing in a tin; an internal non-light-exposed sample can be analyzed. Indurated sediments can be sampled by hammering a metal pipe into the section. Alternatively, aluminum cans with their edges serrated by metal snips can be easily "screwed" into and out of the section. The serrated end of the can is closed with aluminum foil and wrapped with "duct" tape to insure tight closure. It is recommended that an additional 500 - 800 g sample be taken from the same layer for dose rate, mineralogic and granulometric analysis. This sample does not have to be sealed from light or moisture loss.
(adapted from the home page of Dr Steven L. Forman, University of Illinois)
links
Luminescence Research Group, School of Geography and
the Environment, University of Oxford
bibliography
Aitken, M.J.
An Introduction to Optical Dating: the Dating of Quaternary
Sediments by the Use of Photon-Stimulated Luminescence, London: Oxford
University Press, 1998.
[Review:
D. Sainty, Assemblage 4 (1998)]
Liritzis, I., Galloway, R., Hong, D. & Kyparissi-Apostolica, N.
"OSL
dating of three prehistoric ceramics from Theopetra cave (central Greece):
a case study", Mediterranean Archaeology and Archaeometry 2.2
(2002) 
Nicoll, K.
"Time and the shifting sands of the Sahara: records of recent environmental change and prehistoric human activity in Egypt and Northern Sudan", in Hope, C. & Bowen, G. (eds), The Oasis Papers III - Proceedings of the Third International Symposium of the Dahkhkeh Oasis Project, [Dakhleh Oasis Project , Monograph 8], Oxbow Monographs, Oxford: Oxbow Books, 2002.
