DATING SEDIMENTS USING
LUMINESCENCE SIGNALS
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efore siting a nuclear power station or a nuclear waste repository, it is necessary to establish that the area has been free of earthquake activity for a sufficient period of time. Evidence of past earthquake activity is often provided by faults i n surface sediments, where parts of the land surface are o f f s e t relative to t h e adjacent surface by several tens or hundreds of centimeters. As time passes, the slip face erodes and eventually a smooth land surface is reestablished, making the presence of the fault invisible to the passerby. With time, new sediment may be blown in and further cover the fault. Below the land surface, evidence of the fault can still be seen, particularly when a soil layer covers the original land surface. Faults can thus be located by excavation of trenches across the area of interest. Age limits for fault formation can be set by obtaining the depositional ages of the sediment unit in which the fault was formed and the overlying sediment. The age
B Y ANN WINTLE closest to that of fault formation would be urovided bv the deposit formed by erosion of the fault scarp. Soil-forming processes may have left sufficient organic material in the surface soil to allow the application of radiocarbon dating. However, this is not always the case, particularly in the remote desert areas often considered most suitable sites for nuclear waste repositori (e.&, Yucca Mountain, NV]. Also, because the half-life of [the radioactive isotope used in the radiocarbon dating method) is 5730 years, it can he applied only to soils less than 30,000 years old. A more useful technique would be one that could be applied to the mineral grains that make up the sediments and that would give the time that has passed since the grains were blown or washed into position. Experimental details Luminescence dating techniques, ofwhich the most well known
Views are insightful commentaries on timely environmental topics, represent an outhor's opinion, and do not neces-
tion. Thermoluminescence is the name given to the light emitted from crystarline mineral; when they are heat?-' -q+-* -----? to ionizing rarlilti ,ha, beta, and being pro;mall pro5 will bet s i n the rapped for thermal or Ked. of heat, as in heating the grains mom room temperature to 500 "C at rates of about 5 "CISon a hot plate, releases electrons from their traps and allows them to recombine at luminescence centers. Deexcitation of a luminescence center results in the release of a photon whose energy is typical of the type of luminescence center. Through use of a W-visible (200800 nm) spectrometer ( I ) , it has been shown that the natural TL signal from a 5-mg sample of potassium feldspar separated from a beach dating from the last interglacial had several emission peaks. Such information is used to select a band pass filter centered on a particular emission band for use with a nhotomultinlier tube for datine nrocedures. Dating is based on the use of TL Y
0013-936w93/0927-803$04.00/0 @ 1993 American Chemical Society
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inviron. Sci. Technol.. Vol. 27, No. 5, 1993 803
Thermoluminescence(TL) and infrared stimulated luminescence (IRSL) growth with added laboratory dosea TL (cps x 1000)
LVV
Luminescence
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Temperature ('C)
TL age (years) =
DE (GY) dose rate (Gy/year)
where the dose rate is assessed from a totally different set of measurements which involve determination of the gamma dose rate from the uranium (z38Uand T J ) and thorium (Z3ZTh)decay chains and from the naturally occurring radioactive isotope of potassium ('OK). The lifetimes of the parent isotopes are very long and, provided that there is no disequilibrium in the decay chains, the dose rate during the Quaternary period (the last 2.4 million years) may be considered as constant. Similarly, the dose delivered to the 804 Environ. Sci. Technol.. Vol. 27, No. 5, 1993
a"
,i)
Time (seconds)
dose enablt lent dose) that the grains have received since deposition. At the time of d e p o s i t i o n a n y previously trapped electrons would have been emptied out by exposure to sunlight. A particular mineral type (e.g., potassium feldspar or quartz) and grain size (typically in the range of 100-300 pm) are selected by use of heavy liquids and sieving. With about 60 five-mg aliquots for TL measurements, the natural TL signal is compared with those for identical aliquots that have received additional radiation doses from a laboratory source (s0Sr/80Y or E°Co). This enables a TL growth curve to be constructed (Figure 1).DE can be obtained by extrapolating the curve (using an appropriate fitting function) back to the base level given by optically bleached aliquots. This value of DE (in S.I. units of absorbed radiation dose, grays, abbreviated Gy [I Gy = 100 rads]) is used in the age equation
IRSL (cps x 1000)
iinesct grains from beta particles released from these isotopes in their immediate environment can be measured. For potassium feldspar grains there is an additional beta dose com onent derived from the decay of fK atoms that form part of the crystal lattice. Besides these radionuclide-derived dose rates, all grains receive a small additional component from cosmic rays. Applications This approach has been successful in dating movement on the Wasatch Fault in Utah (2). A combination of TL and radiocarbon dates indicated that three faulting events had occurred within the past 5000 years. However, accurate dating of such young sediment is limited by successful laboratory determination of the residual TL signal at deposition. This level is nonzero and is dependent upon the spectral distribution of the natural light source (e.g., direct sunlight or subaqueous light) and the time of exposure (which may be as short as a few minutes or as long as several days). The reason for this is that heating the crystals releases electrons from traps that have different sensitivities to light, including some that are not emptied by sunlight. Instead it would be preferable to monitor electrons released by light exposure in the laboratory, because such a process would preferentially observe luminescence resulting from electrons released from the most light-sensitive traps. This is possible provided that the light used for stimulation is far enough removed from the wavelength of
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emission of the selected luminescence centers for there to be negligible observation of the stimulating light by the detecting photomultiplier tube. For most feldspars this can be achieved by the use of near infrared wavelengths (centered on 880 nm) for stimulation. Luminescence emitted in the green and blue range can then be observed using an infrared rejection filter. Laboratory procedures similar to those described for TL dating can be applied, with some aliquots being irradiated to produce a growth curve of infrared stimulated luminescence (IRSL) versus dose. IRSL is measured as the integrated light emitted within a time period, maybe as long as 100 s. The IR exposure (given using IR-emitting diodes) has a negligible effect on the TL signal from an aliquot, and hence an additional determination of DE can be made using TL. On the other hand, an IRSL signal can be obtained within 0.1 s, a procedure that results in a negligible depletion of the IRSL signal, for which the initial signal decay can be l%/s.This gives rise to the possibility of using a single sample to construct a growth curve, by giving additional irradiations to the same 5-mg aliquot. This procedure gives a very precise value of DE for the measured grains. Repeated measurements give the reproducibility of D E , which is affected by the microdosimetry (e.g., variation of the potassium content) and by the bleaching history of the individual grains (well-bleached samples, such as dune sands, show relatively little scatter in DE).
Looking for climate change Luminescence methods are suitable for dating deposits that contain evidence of past climatic change. In particular, IRSL measurements are being used on sand deposits in the Mojave Desert, CA, an area of considerable climatic change in the past 20,000 years. By use of the single aliquot approach, a surface sample gave an age of 40 17 years, indicating the lower age limit of the method. Different parts of a small dune field were found to have stabilized about 4500 and 1500 years ago, with remohilization occurring about 500 years ago. Dune ridges surrounding a dry lake in the area were formed about 400 to 150 years ago, dates that agree with radiocarbon ages for the temporary occupation of the lake shores by native people during periods of higher rainfall when lakes were filled. Initial measurements have concentrated on aeolian (wind-borne) sedimentary deposits, whose grains have been well exposed to light prior to deposition. Methods using an optically stimulated signal (e.g., IRSL) may be applied to sediments from other depositional environments-such as fluvial depositsbut will require changes in laboratory procedures that have not yet been fully tested. Luminescence methods, particularly IRSL, are thus able to provide ages for emplacement of sediment grains. For aeolian sands more than 500 years old, a precision of i 7% can be achieved. The single aliquot method is highly suited for automation: however, sample preparation time still results in the processing of about two samples a week. Thus costs are higher than for a radiocarbon date, but this is offset by the ease of finding relevant samples.
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References (1) Luff. B. I.: Townsend. P.D.:Meas. Sci. Technol. 1992,3,65-71. (2)
Forman. S . L.; Nelson, A. R.: McAlpin, 1. P.I. Geophys. Res. 1991. 96.595405.
Ann Wintle is a senior lecturer ot the Institute of Earth Studies, University of Wales, Aberystwyth, U K . She received her BSc. degree in physics from Sussex University and a D.Phi1. from Oxford University. She has conducted luminescence research in Vancouver (BCI, Cambridge, and London.
Published by the American Chemical Soci under the edtorial leade3ip of Donald R. Paul, University of Texas, Ausun.
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