Optically Stimulated Luminescence Dating

StrataData offers a dating service using the Optically Stimulated Luminescence (OSL) technique. This is done in collaboration with the University of Oxford Luminescence Dating Laboratory. Although a relatively new technique, particularly in subaqueous sediments, StrataData have pioneered its industrial application in dating superficial seabed deposits for geohazard risk assessment.

Application

Suitable for samples up to about 150Ka containing quartz. The quartz can be very fine grained (c.5um). Ideal for young sediments with no biogenic material present or where the age of the sediments exceeds the range of 14C dating (c. 40Ka). Requires precise measurement of sample water content and salinity.

Cost and turnaround time

The basic cost is £550/sample plus reporting time based on your requirements. The report will contain OSL results calibrated for sample water content and salinity. Turnaround time is several weeks to several months and very dependent on the time taken to prepare the samples, with organic rich samples taking the longest. Please contact us for more information.

Technical Information

Method

All sediments contain trace minerals including uranium, thorium and potassium. These slowly decay over time and the ionising radiation they produce is absorbed by other constituents of the sediments such as quartz and feldspar. Stimulating samples using infrared light causes luminescence, the intensity of which varies depending on the amount of radiation absorbed. Exposure to sunlight resets the luminescent signature and so the time period since the sediment was buried can be calculated. The method is routinely applied to sediments as old as 250Ka and, consequently, is ideal for dating material beyond the range of 14C dating (>40Ka).

A short length of undisturbed core is sub sampled in a dark room to extract a few grams of sediment. A residue of pure quartz is extracted by chemical digestion in hydrochloric acid, hydrogen peroxide and fluorosilicic acid, in a process which may take several weeks. The luminescence of each sample is measured using industry-standard Luminescence Readers (manufactured by Risoe National Laboratories, Denmark) which incorporate 90Sr beta-sources, and 470nm LED optical stimulation. The total absorbed dose (termed De, measured in units of Gy) is measured using standard luminescence dating procedures (Murray and Wintle, 2000). Quartz purity is monitored using infra-red (830nm) stimulation within the standard dating procedure.

Calculation of the annual dose rate is based on the measured quantities of Uranium, Thorium and Potassium from the sample. This is done using standard ICP-MS and ICP_AES techniques.

Water Content Calibration

Water within the soil has an attenuating effect on the ambient radiation. Consequently, samples analysed without consideration of their water content or using a low estimate of water content will return ages younger than samples corrected for this effect. For instance, a sample with an actual water content of 44% giving an age of 40Ka dated using a water content value of 30% may give a false age of 31Ka (the precise values depend on the details of the sediment chemistry). Similarly, inaccurate estimates of pore water salinity will dramatically affect the results.

With the assistance of Fugro UK we have developed a method for converting the measured water content to a time-averaged water content for each sample.

Limitations

The limiting factor in the age range for luminescence dating is the 'saturation' of the signal at large dose rates (i.e. the crystal lattice becomes 'full up'). Accurate age determination therefore becomes increasingly difficult for older samples and there is a loss in dating precision (an increase in statistical uncertainty). The point at which a sample becomes saturated depends on the dose rate of the sample. Samples subjected to a high dose rate will become saturated more quickly, and fully saturated samples will not record the full duration of their burial history. In these cases only a minimum age can be determined. However, it is possible to identify saturated samples through measurement.

A measure is made of a natural signal which is then correlated with the saturating exponential growth curve. The corresponding Equivalent dose is calculated by reading it down to the x-axis. For relatively young samples (i.e. where there is still scope for much more charge accumulation) the interpolation is simple as illustrated in Figure 1a.

If the sample is relatively old then the interpolation of the natural value is at relatively high positions on the OSL axis because it is near a state of saturation as illustrated in Figure 1b.

A small change in the value of the natural value can result in a very significant change in the resulting dose interpolation and for some samples all that can be inferred is that they are in saturation and the resultant age is regarded as a minimum age.

osl1

Figure 1a: Interpolation for a relatively young sample

osl2

Figure 1b: Interpolation for a relatively old sample