Sampling for luminescence dating – Part III

When sampling for luminescence dating, it is important to collect samples for water content measurement, as well as for optical and dose rate measurements. Water content (typically reported as a percentage of a mass of dry sediment) is determined for the sediments collected at the sampling site to help “guestimate” how moist a sample was over its entire burial history. (And yes, we do mean “guestimate”…)

So, why do we care?

Water absorbs radiation emitted from sediments at sample site. This means that if sediments were wet during their entire burial history, they will have lower environmental dose rates than those that were dry (all other factors being equal). So if we assume a sample has been dry, when in fact it was wet, our calculated age will be an underestimate.

Ironically, the moisture history of a sample is not only one of the largest controlling factors on its environmental dose rate, it is also the most difficult to determine. It is usually derived from measured sample water contents in the laboratory, as well as the climatic and geomorphic context of the site. Was the sample taken from a river terrace, alluvial fan, or some other landform that was inundated by a river in the past? Was the sample taken at a depth close to, or below the water table? Has the water table risen or dropped through time? All of these factors should be considered, and appropriate (i.e. large) error should be applied to any water content value to account for these uncertainties.

(LEFT) Calculated age vs estimated water content for two dune samples from Tasmania (Neudorf et al., 2019). These sediments become saturated at a water content of ~20% (blue shaded region), and measured water contents of collected samples were 15 ± 3% (grey shaded region). The age estimate of the young sample (W4014) ranges from ~16 ka to ~19 ka, while the age estimate of the older sample (SB7) ranges from ~175 ka to ~210 ka – a difference of ~35 ka! (RIGHT) The change in absolute age with water content decreases as the age of a sample decreases. For instance, if we recalculate the ages of sample W4014 assuming this sample De value is only ¼ of its true value, then a 20% increase in assumed water content will increase the age by less than 1000 years.

The influence of water content on alpha, beta, gamma and total environmental dose rates at the site of sample W4014 (Neudorf et al., 2019). Water can absorb all three forms of radiation (alpha, beta and gamma), but it has the largest impact on alpha dose rates. The alpha dose rate error is an order of magnitude larger than the errors of all other dose rates, and have been removed from the graph for clarity.

The influence that water content has on sample age, not only depends on the true age of the sample, but also the mineral and grain size fraction that was dated. Below are some examples.

The influence of water content on a sample age can depend on the grain size used for dating. Thompson et al. (2018) dated sample LED11-210 using both the 4-11 µm (FQ) and 90-180 µm (SA) quartz fractions. Water content has a slightly smaller influence on the larger grain size because i) the coarse quartz grains were treated with HF acid to remove the outer alpha-penetrated rind as is commonly done in coarse grain luminescence dating, and ii) radiation is not attenuated in fine grains as much as it is in coarser grains. The measured water contents include an error of 15% that is not plotted for clarity.

Dose rate (parsed by radiation type) vs water content for the fine grain (LEFT) and coarse grain (RIGHT) samples of Thompson et al. (2018) in the previous figure. The measured water contents include an error of 15% that is not plotted for clarity.

The influence of water content on quartz and feldspar ages measured from the same sample (T4BATT03) (Smedley et al., 2017; 2019). Water content has a slightly larger influence on age estimates from quartz than from feldspar because a proportion of the total dose rate absorbed by feldspar grains comes from K-40 inside the grains. The grey shading covers ±5% error on the water content measurement.

Dose rate (parsed by radiation type) vs water content is plotted for both quartz (LEFT) and feldspar (RIGHT) grains from sample T4BATT03 (previous figure). Again, the alpha contribution can be excluded from the dose rate for the quartz sample, because this sample has been treated with HF acid to remove the alpha-penetrating outer rind. The grey shading covers ±5% error on the water content measurement.

Sampling for luminescence dating – Part II

Luminescence dating techniques determine the length of time a mineral has been buried by measuring the total radiation dose that mineral has acquired from surrounding sediments and cosmic rays from outer space. So when we collect samples in the field, it’s important to consider where that radiation is coming from, and whether nearby features in the sampling environment may deliver radiation at different rates.

The sources of radiation and their measured dose rates for alluvial sediments (sample DDU011) collected 0.7 m below the surface in the southern Great Basin, USA. Because the grains were etched in HF acid, we can ignore the external alpha contribution to the total dose rate.

Buried minerals absorb radiation from cosmic rays, gamma rays, beta particles and alpha particles, each penetrating the lithosphere to different extents.  Cosmic rays can penetrate sediments to several meters and attenuate by ~14% per meter of ~2 g/cm3 sediment.  Gamma rays can travel up to 30 cm, beta particles ~3 mm, and alpha particles, the most destructive particles of all, travel only ~25 µm (Aitken, 1998). 

A schematic illustrating the sources of radiation in the sampling environment and their approximate travel distances.

Because the sphere of influence of gamma rays is ~30 cm, we prefer to extract luminescence samples from areas where the composition and granulometry of the surrounding sediments is consistent (or homogeneous) within 30 cm of the sample.  If it is not, the dose rate that we measure from the sample may not accurately reflect the dose rate of the surrounding sediments. Examples of ideal and non-ideal sampling sites are shown below.

Often times, especially in archaeology, the feature we’d like to date does not provide a homogeneous dose field for sampling. In these cases, it’s necessary to: i) subsample individual lithostratigraphic layers/features within 30 cm of the sample site individually to estimate their dose rates so that the total dose rate of the sample can be modelled, and/or ii) measure the gamma dose rate to the sample directly in the field using a portable gamma spectrometer.

Possible sample sites (circles) for luminescence dating an earthwork ditch exposed in an archaeological trench at Garden Creek (Wright, 2011, “Earthwork Update” https://gardencreekarchaeology.wordpress.com/page/3/). The yellow circle shows a sample site where the gamma dose rate field is likely to be homogeneous. All other sample sites (red) are located in heterogeneous gamma dose rate fields. The 30 cm scale is hypothetical, and may not be accurate.

Possible sample sites (circles) for luminescence dating an escavated pit hearth in western Nebraska (Photo from the lesson plan by Damita Hiemstra, “Arner Site: Out of the Prairies of Western Nebraska”, http://d1vmz9r13e2j4x.cloudfront.net/nebstudies/Lesson4c_Arner.pdf). The yellow and red circles indicate homogeneous and heterogeneous gamma ray dose fields, respectively. The 30 cm scale is hypothetical, and may not be accurate.

Heterogeneous dose fields are not only possible at the scale of a sedimentary exposure, but also at the scale of a thin section. This microscale heterogeneity is caused by spatial variations in beta dose rates, or “beta microdosimetry”. Spatial variations in K-40 concentrations (e.g., from K-feldspar grains) or U and Th in heavy minerals may lead to beta microdosimetry (e.g., Martin et al., 2015; Jankowski & Jacobs, 2018). This likely contributes to the spread (or overdispersion) of De and age distrubutions from single grains.

A) Stitched microphotograph of a thin section of beach sand from MacCauley’s Beach, NSW, Australia (sample SP5) shown in cross-polarized light. B) A beta dose rate (in Gy/ka) map created from portable XRF measurements of a separate impregnated block of the same sample. The spatial variations in beta dose rate in this sample can explain the spread in De values (overdispersion = 35%) measured from 170 individual grains (Jankowski & Jacobs, 2018).

SHEDDING SOME LIGHT ON LUMINESCENCE DATING

Few have may have heard about luminescence dating despite the fact that it is now used almost routinely in archaeological and paleoclimate studies and can surpass the upper limit of radiocarbon dating by over a hundred thousand years! Over the last 40 years, luminescence dating has become an essential tool for helping us understand the timing of early human dispersal, climate change, sea level change, landscape evolution, and the rate of retreat of the last great ice sheets, among other things. This goal of this blog is to serve as a resource for students, academics, users of luminescence data and others who want to understand luminescence dating techniques and how to interpret luminescence age data.

What is OSL?

Optically stimulated luminescence dating

Optically stimulated luminescence broadly refers to a myriad of techniques that are used to determine that last time minerals (typically quartz or feldspar) were exposed to sunlight or heat. In the case of sunlight, luminescence ages tell us the approximate time a deposit or artifact was buried. After quartz and feldspar minerals are buried, they are exposed to ionizing radiation emitted from the surrounding sediments and cosmic rays from outer space that penetrate the ground surface.  At the molecular scale, this radiation re-mobilizes electrons, which in turn accumulate within defects (so-called “traps”) inside the crystal lattice. These defects exist in the form of structural imperfections or impurities. The longer the mineral is buried, the more electrons accumulate within the traps.

How do we obtain an OSL age?

To calculate an age, we need to define:

1)     the amount of radiation absorbed by the mineral during burial (also called the paleodose), and

2)     the rate at which the mineral was irradiated during burial (called the environmental dose rate).

The age is calculated as the paleodose divided by the environmental dose rate.

To estimate the paleodose, the mineral is stimulated with a light source in the lab (blue or green light in the case of quartz), which evicts the electrons from their traps and results in the emission of photons of light (luminescence). The luminescence intensity (total photon counts) is measured by a photomultiplier tube and provides an estimate of the amount of radiation absorbed by the mineral during burial.

To measure the environmental dose rate, the quantity of radionuclides in the surrounding sediments are either measured directly, or estimated using radiation detectors in the field or in the lab.

Luminescence dating techniques are used to determine the age of artifacts, landforms and sediments that are as young as a few decades, to as old as ~ 1 Ma. This allows us to refine our understanding of Earth and human history during the Pleistocene and Holocene epochs.

Neolithic stone tools excavated from the Middle Son Valley, Madhya Pradesh, India, 2009. Photo credit: Christina Neudorf.

The following blog posts will shed some light on the physics of luminescence dating, clarify commonly misunderstood concepts, provide guidance on how interpret luminescence dating data, include some handy tips for sampling in the field, and some links for further reading.

—Christina Neudorf