What is Luminescence? Optically Stimulated Luminescence (OSL) Sediment Context Collection Procedures

Luminescence tutorial (prev: Sedimentologic Context)

Sedimentologic Context for Luminescence Dating

Collection of Sediments for OSL

Sampling sediment for OSL dating is relatively straightforward though care should be exercised against inadvertent light exposure and the appropriate stratigraphic unit is sampled. We advise against sampling at night or under a tarp where uncertainty exists on what sediment is sampled.

Weathered horizons exhibiting pedogenic accumulations of iron, 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 or bioturbating 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. Sampling within 30 cm of boulders or the surface should be avoided to obviate potential inhomogeneities in radioactivity.

Approximately 10-30 g of sediment should be collected, though more or less may be adequate depending on the concentration of the chosen particle size for luminescence dating. Geological OSL of sediment is reduced with brief (<10 seconds) exposure to sunlight, thus great care must be taken to expose the analyzed aliquot to no light. It is best to take the sediment intact for OSL

Dating: though the outer mineral grains may have been light exposed (1-2 mm depth), the internal grains have been shielded from sunlight. Taking sediment cores is one common way to insure stratigraphic and geologic luminescence integrity. An affordable and flexible sampling device and container is black ABS pipe (3-5 cm diameter, 10-20 cm long), which with end caps can be hammered into the desired outcrop sediment for dating.

If the sediment is coarse grained (>2mm), hardened or excessively dried out a variety of 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. Heavily indurated sediments can be sampled by hammering a metal pipe into the section. Alternatively, a metal (soup or tomato paste) can with it edge serrated by metal snips can be easity "screwed" into and out the section. The serrated end of the can is tightly closed with aluminum foil and duct tape. This is a particularly effective method for sampling coarse-grained colluvium. It is recommended that additional 10-20 g sample be taken from the same sampled layer for dose rate, mineralogic and granulometric analyses; this sample does not have to be shielded from light exposure.

 

References and key luminescence dating publications