Optically Stimulated Luminescence (OSL) dating

Optically Stimulated Luminescence is a relatively new technique which we have successfully used in dating sediments in the Nile Delta, The Caspian and offshore West Africa. Theoretically, it should be possible to date sediments as old as 500Ka but experience has shown that in many cases results can be wildly inaccurate probably because incorrect calibration factors have been applied.

Method

All sediments contain low concentrations of uranium, thorium and potassium which produce a constant flux of ionising radiation.  Energy absorbed from this source frees electrons to move through the lattice of crystals such as quartz, and some are trapped at imperfections in the lattice.  Subsequent stimulation by the absorption of light releases some of the trapped electrons, which is followed by the emission of light as a proportion of the electrons return to their original state.  This stimulated emission therefore records the total radiation exposure, which can in turn be related to the residence time in the sediment once the annual dose rate is known. Optically Stimulated Luminescence (OSL) is a technique which measures the elapsed time since a sample was last exposed to light, i.e. since it was deposited. Undisturbed borehole samples therefore provide ideal material for this method.  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).

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 and in such cases only a minimum age can be determined.

Material used

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. The total absorbed dose (termed De, measured in units of Gy) is measured using standard luminescence dating procedures. 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.

Together with Fugro GeoConsultants we have developed a method for converting the measured water content to a time-averaged water content for each sample.

The method requires knowledge of several soil parameters and assumes the following:

*         The soil is normally consolidated

*         The soil has been deposited at a constant rate

*         There is no excess pore pressure

*         The soil is fully saturated (i.e. without gas)

*         The liquid limit has remained constant throughout deposition

*         Pore water salinity has remained constant

*         Water content has been derived according to BS1377

Since many of these factors are unknown, consequential errors in calibration of the OSL values are almost inevitable.

It should be noted that the geotechnical definition of "water content" (mass of fluids/mass of solids) differs from the "moisture content" value required to calculate OSL ages (mass of water/mass of water+ mass of solids).