Samples for luminescence dating can be collected in a myriad of ways, and can include different types of material. Materials most commonly dated are grains of sand or silt, however pottery, rock surfaces, rock art, and even archaeological constructions, such as walls and buildings have also been sampled.
Because luminescence dating methods determine the last time a mineral has been exposed to light or heat, it is imperative that light or heat is not introduced to the sample during the sampling process. This can be tricky! Below we list some common, and not so common ways of collecting luminescence samples. Stay tuned for upcoming blogs that delve into more details about the sampling process.
1) Sampling tubes. This is the most common method of collecting a luminescence sample from sediments. Sampling tubes are typically hammered into a sedimentary exposure, or the side of an archaeological trench, to collect sediments from the sedimentary unit or archaeological layer of interest.
Before hammering, the tube may be stuffed with foam, plastic or paper. After the tube is fully inserted, it is then excavated back out, and the light-exposed ends are sealed with opaque plastic, tape or a cap. The size of the tube you use, will depend on the thickness of the unit/layer you are sampling, and how much time you think it took for the sediment to accumulate. To avoid obtaining an imprecise age, tubes of smaller diameter should be used for sedimentary units thought to have accumulated over long periods of time.
2) A hand auger. Luminescence samples can be collected using an auger equipped with a light-tight core sampler, allowing us to target sediments several meters below the surface.
3) Coring. Luminescence samples can be extracted from sediment cores that have been collected by hand, by percussion coring, or by vibracoring. If cores are collected from loose or saturated sediments, a core catcher maybe placed at the penetrating end of the core to prevent sediment from falling back out during extraction.
In a light-safe lab, multiple luminescence samples can be extracted from a single core to construct “age vs depth” profiles.
4) Block samples. When luminescence sample tubes cannot penetrate extremely cohesive sands or silts, it may be necessary to extract a block sample. This can be done using a rock hammer or saw. After sampling, the block is wrapped tightly in opaque plastic, then shipped in containers with sufficient cushioning (e.g., bubble wrap) to prevent the block from cracking during transport. Once in the light-safe lab, the outer-most sediments are carefully carved away from the block before the inner, non-light-exposed sediments are processed for dating.
5) Under a tarp, or at night. If light contamination cannot be prevented using sampling tubes or cores, a sample may be collected under an opaque tarp and/or at night. Finding tarp material that is truly light-safe can be tricky, and multiple layers may be necessary. As always, the sample must be packaged in such a way that any surface sediments that were exposed to the sun during the day do not contaminate the non-light-exposed sediments.
6) Rock surface sampling. The surfaces of boulders or cobbles can be sampled for luminescence dating by coring and slicing (e.g., Freiesleben, et al., 2015; Jenkins et al., 2018). Rock surface dating has also been applied to rock art (e.g., Liritzis et al., 2018). Cores can be extracted using a hand drill with a diamond-tipped drill core, then sliced into sub-centimeter thick slices using a microsaw or wafering blade. A water cooling system for both the core and saw may be necessary to prevent the sample from heating up due to friction. Rock slices may be crushed and prepared as sediment samples prior to measurement, or measured directly.
In most cases, it is desirable to collect a “modern” sample to check how well the luminescence signal is re-set during sun-exposure. This allows us to evaluate ages from our ancient samples, by determining the likelihood that their signals were also fully re-set prior to burial. The modern sample should be collected from rocks or sediments that have experienced the same mode of transport and deposition as the ancient samples. Unfortunately, even when this is the case, there is always the possibility that the bleaching history of the modern sample will not be representative of the ancient samples collected at the same site.