Section IV: How to Collect Samples for TL and OSL Dating

In general, try to avoid sediments that show any post-depositional disturbances such as root penetration, krotavinia (bioturbation), carbonates, ground-water leaching, certain soil formations or large stones that may hamper sampling. If the only exposure that is available is one in which cracks, roots or krotovina may be encountered at depth, or the sediment is so hard that it will be difficult to remove a tube driven in at depth, then take a cube block sample at the site. Mark the sample as to the side exposed to the surface. Wrap it in aluminum foil, a black bag (several layers if being shipped) and tape tightly to prevent disintegration.

Ideally, a 0.5 meter (about a one-foot hole) must be augured into the soil sample above or below the boundary of the layer so that the luminescence and bulk samples are taken or counted on a homogenous layer. If this is not possible, take the luminescence sample and dig sideways for adequate bulk sample sediment. Sampling tubes of either 0.15" wall PVC (white or black) or steel should be driven into the back of the hole to collect the luminescence sample. PVC tubes often benefit from a sharpened or beveled edge on the back of the tube to be driven into the sediment. Tubes are driven in by sledgehammer, however, a hard metal plate should be used against the end being hammered to prevent shattered or collapse.

If the sample is being collected during the daylight hours, a black or opaque cloth MUST be used to shield the sample collection and the immediate site from light upon removal of the sample. If the sample is collected at dusk or night, a red-filter light can be employed at a minimum for collector's visual aid. The lab will need about 50 grams of sediment if the sample is fine-grained, about 75 to 100 grams if the sample is sand-size for adequate replicate analyses.

The sample tube should then be carefully pried or dug out from the hole by pushing a knife or chisel alongside the tube and levering it sideways. Cover the sample hole with a black cloth (this may require the aid of others) while freeing the tube. Once the tube is free, DO NOT remove it from the hole or black cloth until the open end is closed with a black cap or duct tape. Both ends of the tube can then be sealed with black electrical tape if the caps look loose or if they might pop off during sample shipment. Place the tubes in a black polyethylene black bag (the kind used in photography supply stores and not black trash bags). Write the sample name on both tube and bag.

A second sample must be collected from the back or sides of the same hole. This sample, referred to as the "bulk sample", does not need protection from light. The bulk sample should be collected in a double Ziploc 1 quart size bag to inhibit the loss of moisture, so that gross moisture content can be measured in the lab. This sample can also be used to obtain a DR if field analysis is not possible. In that case, at least 600-grams of sample is needed for the gamma ray counting.

Photographs or trench logs showing the locations of samples should be included if possible. Usually one photograph is required to locate the sample hole in stratigraphic context and one close-up to show texture of the sediment sample. The augured holes often show up well in distant photos of the site. Location of the site using GPS systems is appreciated and depth of sample below the original surface of the ground should be noted, as these are needed for a calculation of cosmic ray component of the DR.

Other materials that can be collected to date archeological finds using luminescence:

Artificial Glass: There have been few successful attempts, caused principally by glass' non-crystalline state. TL and OSL can only used if glass has not been heated above "glass transition" temperature (? ° C). (Mueller, P. and Schvoerer, M., 1993, Archaeometry 35: pp. 299-304, Factors Affecting the Viability of TL Dating of Glass).

Burned Flint and Stones: Well suited for TL, especially in dating Middle to Lower Paleolithic periods. Zeroing requires 450°C and this assumption can be tested for. Oldest age obtained is 453 ±39 Ka. (Richter, D.G., 1997, Dissertation for University Tubingen, in German, no translation).

Potboilers (heated stones) have also been successfully dated. (Huxtable, J., Aitken, M. J., Hedges, J. W., Renfrew, A. C., 1976, Archaeometry 18: pp. 5-17, Dating a Settlement Pattern by TL: The Burnt Mounds of the Orkneys).

Ceramics and Burnt Clayware: Important concepts of TL methods were established on ceramic shards, bricks and kilns during the 1960's and 1970's. The usual method is to date both polymineralic fine-grains (4 to 11 ±) and quartz coarse-grains (100-200 µ), thus combining TL ages. Typical error is 6-10%. (Wagner, G.A. and Lorenz, I. B., 1997, another untranslated German paper).

Medieval kiln structures or the burned surface lining (containing quartz) have been successfully dated (Wagner, G. and Wagner, I., 1994, another untranslated German paper).

Authenticity dating of ceramic objects uses both TL and OSL. For TL, the lab needs 200 mg of powder and only really looks for baseline signal, indicating recent age or a natural TL glowcurve, indicating some elapsed time greater than 500 years. (Aitken, M. J., 1985, Thermoluminescence Dating, Academic Press, 359 p.). For OSL, 100 mg or less is needed and higher precision of measurement gives 1-2% error if the single-aliquot method is used. (Mejdahl, V. and Botter-Jensen, L., 1997, Radiation Measurements 27: pp. 291-294, Experience with the SARA OSL Method).

Success in using the quartz extracted from fired bricks has been obtained, recently in using Chernobyl exposed bricks to the increased radiation during the accident as "dosimeters" for reconstruction of dose rates (Banerjee, D., Botter-Jensen, L. and Murray, A.S., 2000, Applied Radiation and Isotopes 52: pp. 831-844, Retrospective Dosimetry: Estimation of the Dose to Quartz Using the Single-aliquot Regenerative-dose Protocol).

Slags: Attempts have been made to date archeometallurgic slag by TL, but the glass phase is problematic. Since slags consist essentially of silicates, heated to high degrees, in theory even OSL should work. Uncertainties about DR, however, render errors at 20%. (Elitzsch, C., Pernicka, E., and Wagner, G. A., 1983, PACT 9: p. 271- 286, TL Dating of Archeometallurgical Slags).

Vitrified Forts or Earth Mounds: Prehistoric walls consisting of vitrified quartz and feldspar bearing rocks are of widespread occurrence in western Europe and TL ages have been obtained from Scotland. (Sanderson, D. C. W., Placido, F. and Tate, J. O., 1988, Nuclear Tracks and Radiation Measurements 14: pp. 307-316, Scottish Vitrified Forts: TL Results for Six Study Sites).

In northeastern Louisiana Paleo-Indian earthworks have been dated, presuming exposure to sunlight during construction or occupation of the mound. Dating was by means of OSL using 90-120 µ quartz. Insufficient bleaching proved to be a problem, even with OSL, but minimum ages were obtained (Feathers, J. K., 1997, Quaternary Geochronology 16: pp. 333-340, Luminescence Dating of Early Mounds in Northeast Louisiana).

Wasp Nests: Using OSL to date quartz grains imbedded in petrified mud-wasps nests is a new way to give dates to rock petroglyphs and paintings. Nest thickness need to be at least 5mm and in some cases, an experimental method of single-aliquot analyses (due to lack of material) gave excellent results. Ages up to 16.4 Ka were obtained with about 10% error. (Roberts, R., et. al., 1997, Nature: vol. 387, pp. 696-699, Luminescence Dating of Rock Art and Past Environments using Mud-wasps Nests in Northern Australia).

Other materials that can be collected to date geological sediment using luminescence:

Carbonates: TL has interesting potential in dating calcitic sinter, and an extensive series of stalagmite samples were taken from a French cave which yielded agreement with U-series and U-U ages. TL age underestimation was found for samples whose dose rate is dominated by the internal alpha and beta components, however. (Debenham, N. C. and Aitken, M. J., 1984, Archaeometry 26: pp. 155-170, TL dating of Stalagmitic Calcite).

Another form of TL has been proposed which dates "dirty" pedogenic carbonates. The method uses changes in the dose rate to quartz grains when the sediment becomes carbonated. (Singhvi, A., et. al., 1996, Earth and Planetary Science Letters 139: pp. 321-322, A Luminescence Method for Dating "Dirty" Pedogenic Carbonates for Paleoenvironmental Reconstruction).

Because the samples of mollusk shells suffer structural alteration at usual TL glow-curve temperatures only sporadic studies exist for these. General consensus seems to be that it works in a limited number of cases, if it can be proven that the shells exhibit a glow-curve in the 240°C red spectral region or are of the Pectinidae or Ostreidae family. (Ninagawa, K., et. al., 1994, Quaternary Science Reviews 13: pp. 589-593, TL dating of Calcite Shell Crassostrea gigas (Thunberg) in the Ostreidea Family).

A recent literature research reveals no OSL studies being done on carbonates, possibly because ESR use is more prevalent.

Colluvial and Alluvial Silts: Most recent studies have found that different degrees of bleaching depend on depositional conditions and that all too often only distal wash facies or the upper part of the colluvial wedge are sufficiently bleached and suitable for TL dating. (Forman, S. L., et. al., 1991, Journal of Geophysical Research 96 B1: pp. 595-605, TL Dating of Fault-scarp Derived Colluvium: Deciphering the Timing of Paleoearthquakes on the Weber Segment of the Wasatch Fault Zone, North Central, Utah). Careful application of the partial bleach method however, can give ages for some alluvium.

Combinations of TL and IRSL have been done with excellent agreement in many arid climates, notably Israel (Porat, N., et. al., 1996, Quaternary Research 46: pp. 107-117, Late Quaternary Earthquake Chronology from Luminescence Dating of Colluvial and Alluvial deposits of the Arava Valley, Israel). Typical minerals used in IRSL are K -feldspars of coarse-grained size, although with judicious application the polymineralic fine-grained portion is also acceptable. The emerging opinion seems to be that modern colluvial sediments in arid regions are bleached better and far more uniformly than modern alluvium, thus better suited to luminescence age control and reflect past sediment conditions.

Dune Sand: Along with loess, considered routine for both TL and OSL. It has an advantage over loess, in that dune sand's monomineralic, coarse-grained fractions can be separated. Any feldspar found in these sands has unusually high TL sensitivity (important for dating Holocene sands). Many studies have been done with respect to sea levels over time, interglacial sea maximas (responsible for dune formation), rock shelter occupational periods and human migration patterns. (Roberts, R. G., et. al., 1994, Quaternary Science Reviews 13; pp. 575-583, The Human Colonisation of Australia: OSL dates of 53,000 and 60,000 years Bracket Human Arrival at Deaf Adder Gorge, Northern Territory).

On the other hand, OSL has been extremely useful for dating dunes too young to be assessable by TL. Ages as young as 100 years have been obtained with the use of quartz, sanidine and potassium feldspars (Stokes, S., et. al., 1997, Nature pp. 154-158, Multiple Episodes of Aridity in Southern Africa Since the last Interglacial Period).

Fluvial Systems: Due to its better "bleachability", feldspar is generally preferable to quartz for TL dating in fluvial systems. However, if sands have been transported in shallow, clear rivers, even quartz grains may be fully bleached. More systematic studies on the suitability of fluvial, glaciofluvial, limnic and coastal marine sands are needed. Light conditions are crucial in each case, since the water column tends to absorb light, particurlary short-wave spectral regions.

Most sands of fluvial origin can be dated, provided some other technique (such as U-series or Electron Spin resonance) is used as a check. Recent cases also combine TL with PPTL (phototransfer thermoluminescence) and results are encouraging. (Murray, A., S., 1996, Geochimica Cosmochima Acta 60: pp. 565-576, Developments in OSL and PTTL Dating of Young Sediments: Application to a 20000-year Sequence of Floods). Green light luminescence as well as blue light luminescence (OSL) on coarse quartz grains is encouraging and flood deposits using OSL have also been successfully dated (Lepper, K., Larson, N.A. and McKeever, S.W.S, 2002, Radiation Measurements 32: pp. 603-608, Equivalent Dose Distribution Analysis of Holocene Eolian and Fluvial Quartz Sands from Central Oklahoma). Blue-light OSL has also been shown to date coarse-grained quartz in river environments with even more success than IRSL (Srivastava, P., Juyal, N., Singhvi, A., Wasson, R.J. and Bateman, M. D., 2001, Geomorphology 36: pp. 217-229, Luminescence Chronology of River Adjustment and Incision of Quaternary Sediments in the Alluvial Plain of the Sabarmati River, North Gujarat, India).

Fulgurite: Basically, no one has ever tried it and reported on it in the literature. In theory, if lightening hits a sandy surface the heat effect is sufficient to cause fritting and melting of quartz grains, therefore TL signal would be reset.

Glacial Deposits (including glaciofluvial): Not much is known about the suitability of glacio-fluvial sediments for TL dating on a general basis. The degree of bleaching strongly depends on type and distance of transport; thus every site must be individually assessed. Most sites just don't possess enough "reset" sediments, some showing a bleaching equivalent of only 10 minutes (on a sunny day)!! TL is a bust on most tills and debris flows, although several studies have been done on lakebed cores of mainly glacigenic origin with much success, using infrared stimulation (IRSL) on the fine-grained portion and as well as ground wedges (associated with permafrost). (Hardy, F. and Lamothe, M., 1997, Quaternary Science reviews 16: pp. 417-426, Quaternary Basin Analysis using IRSL on Borehole Cores and Cuttings; Porter, S.C., Singhvi, A., Zhisheng, A. and Zhongping, L, 2001, Permafrost and Periglcial Processes 12: pp. 203-210, Luminescence Age and Palaeoenvironmental Implications of a Late Pleistocene Ground Wedge on the Northern Tibetan Plateau).

Single-aliquot analysis may be necessary because glacio-fluvial sediments contain such a mix of bleached and unbleached grains. IRSL ages have been obtained on coarse-grains (100-200 µ) with limited success. (Hutt, G. and Junger, H., 1992, Quaternary Science Reviews 11: pp. 161-163, Optical and TL dating of Glaciofluvial Sediments). Blue-light OSL holds the most promise to date any glacial deposit, but even OSL is not "reset" in most of glacial terrain.

Impactites: This potential of the TL method was tried on the Arizona Meteor Crater and it was determined that 680°C and a shock-wave intensity of 10 Gpa was enough to reset the Coconino Ss. quartz. An age of 49 ± 3 Ka was determined by TL. (Sutton, S. R., 1985, Journal of Geophysical Research 90: pp. 3690-3700, TL Measurements on Shock-metamorphosed Sandstone and Dolomite from Meteor Crater, Arizona).

Loess: Now considered to be "cookbook" for TL dating, particularly the fine-grained portion of quartz and feldspar. For particularly sandy loess coarse-grained fractions are also used. Most reliable ages are under 120 Ka (last glacial-interglacial), although ages up to 800 Ka have been reported. (Berger, G., W., et. al., 1992, Geology 20: pp. 403-406, Dating Loess Up to 800 Ka by TL). If the site has a low dose rate (i.e. 4Gy/Ka) ages of 100 to 200 Ka can be obtained routinely and reliably. (Waters, M. R., Forman, S. F., and Pierson, J. M., 1997, Science 275: pp. 1281-1284, Diring Yuriakh: a Lower Paleolithic site in Central Siberia).

If the TL signal has been reset in loess, OSL is expected to work just as well. In fact, there are several problems, if the fine-grained portion is used. Ages past 120 Ka are frequently underestimated using green light OSL and Infrared. It is now suggested that medium grains (43 to 54 µ) and coarse grains are used, especially for infrared. (Li, S.H., and Wintle, A. G., 1992, Quaternary Science Reviews 11: pp. 133-137, A Global View of the Stability of Luminescence Signals from Loess).

Marine sands: Sands are commonly able to be dated by TL due to the fact that they are well exposed to light by repeated displacement in shallow coastal regions. Both quartz and feldspars are used to date littoral deposits and high sea levels. (Mauz, B., et. al., 1997, Palaeogeogr Palaeoclimatol Palaeoecol 128: pp. 269-285, Middle to Upper Pleistocene Morpho-structural Evolution of the NW coast of Sicily: TL dating and Palaeontological-stratigraphical Evaluations of Littoral Sediments). Other marine facies such as terraces, sublittoral, and intertidal sediments all show age overestimations with TL.

Very recent studies in sublittoral sediments indicates that Red Light OSL holds promise for dating modern and Holocene sediments, but that IRSL is the more robust geochronometer. IRSL still shows considerable variability in solar resetting, which erodes precision for ages <50 Ka. The fine-grained fraction of the sediment is used most frequently as it receives the greatest light exposure during deposition. (Forman, S. L., 1998, Arctic and Alpine Research, in press, Infrared and Red Stimulated Luminescence Dating of late Quaternary Near Shore Sediments from Spitsbergen, Svalbard).

Coarse-grain K-feldspar grains using IRSL has been used to date tsunami deposits (mainly in tidal-flat and tidal-channel sands) that had probably been reworked by currents, waves and burrowing organisms prior to the tsunami (thus increasing light exposure). (Huntley, D. J. and Clague, J. J., 1996, Quaternary Research 46: pp. 127-140, Optical Dating of Tsunami-laid Sands).

Mars (soil simulates at present): This is a whole new field developing around trying to use blue light and blue-green light OSL to date the existence of young, fluvial landforms at latitudes above 32° in both Martian hemispheres. The first to propose dating these sediments were Ken Lepper and Steve McKeever (Lepper, K. and McKeever, S.W.S., 2000, Icarus 144: pp. 295-301, Characterization of Fundamental Luminescence Properties of the Mars Soil Stimulant JSC Mars-1 and Their Relevance to Absolute Dating of Martian Eolian Sediments). Other papers are sure to follow as various Mars simulates are tried, perhaps more, if the proposed instrumentation actually goes to sample in-situ on Mars.

Pseudotachylite and Fault Breccia: The glass phase of the pseudotachylite offers a potential for TL dating. Linear growth is usually observed and high temperatures associated with the formation of friction melts during the intense fault movements are usually enough to reset the TL signal in the quartz and feldspar relics. Both coarse- and fine-grained fractions of gouge sample have been used. (Singhvi, A., 1994, Quaternary Science reviews 13: pp. 595-600, Luminescence Studies on Neotectonic Events in South-Central Kumaun Himalaya-A Feasibility Study).

Soil Horizons: A horizon-This soil horizon forms first but is often eroded away. If present or formed in an arid to semi-arid environment, the top of this horizon is acceptable for TL, OSL and IRSL dating unless contaminated by younger clay and silt filtering down from above. The presence of clay films is evidence of such contamination. The mineral grains are repeatedly exposed to sunlight through bioturbation and therefore, TL age corresponds to the time of the formation of the soil. These soils often have humified organic matter mixed in with mineral material. (Buol, S. W., et. al., 1997, Soil Genesis and Classification, 527 pp., Iowa State University Press).

B horizon-Forms after the A horizon and is less suitable. Dominant features include evidence of removal of carbonates; residual concentrations of sesquioxides or silicate clays; coatings of sesquioxides that give a darker or stronger or redder color to the horizon; illuvial concentration of silicate clay, iron aluminum, carbonates, gypsum or humus. If it is high in clay, it can swell and shrink and thus, is often contaminated with younger silt filtering down via roots and cracks. The radiation history is less certain because water moving through this horizon creates oxidizing or reducing conditions. Weathered horizons, such as B soils, should be avoided as they often exhibit pedogenic accumulations of clay, silt, carbonate or silica.

C horizon-This horizon usually provides the most reliable TL, OSL and IRSL ages. Those samples collected at the bottom of the horizon date the start of deposition. This layer is altered very little by soil forming process. Also used as a designation for coprogenous earth, diatomaceous earth, saprolite and other sediments that are not hard enough to qualify as rock. (Millard, H. T. and Maat, P. B., 1994, USGS Open File Report 94-249, TL Dating Procedures in Use at the U.S. Geological Survey, Denver, Colorado).

Paleosols-In general the extreme weathering and mineralization that makes up these soils by definition make them a poor candidate for luminescence. However, in some cases the weathering or bioturbadation that occurs in loessic paleosols has given rise to mineral sediments that can give reasonable luminescence ages, whereas the parent material might not. Blue-light OSL and TL were used with success. (Lian, O. B. and Shane, P.A., 2000, Quaternary Science reviews 19: pp. 1649-1662, Optical Dating of Paleosols Bracketing the Widespread Rotowhu Tephra, North Island, New Zealand).

Tephra (Volcanic Ash): The fine-grained glass phase of volcanic ash bears considerable potential for TL dating. Distal ash deposits are workable as well, due to their fine-grained nature and ages up to 400 Ka have been obtained. Ages as young as a few 100 years old may be dated this way as well. The grain size used is 4-11 µ and special attention should be paid to preheating techniques and TL growth (Berger, G. W., 1991, Journal for Geophysical Research 96: pp. 19705-19720, The use of Glass for Dating Volcanic Ash by TL). The IRSL (infrared) procedure (880 nm wavelength), the green light OSL (514 nm wavelength) and red light OSL (633 nm wavelength) techniques were examined, but only the IRSL was seen as a favorable signal for Mazama Ash. (Berger, G. W. and Huntley D. J., 1994, Quaternary Science Reviews 13: pp. 509-511, Tests for OSL from Tephra Glass).

Volcanites: Direct dating (using TL) of lava streams causes difficulties owing to the rarity of quartz and problems with anomalous fading in the feldspars. Tholeiitic basalts, in particular, exhibit unfavorable TL behavior. Using plagioclase for TL analyses has been successfully done with an accuracy of 10%, (Guerin, G. and Valladas, G., 1980, Nature vol. 286: pp. 697-699, TL Dating of Volcanic Plagioclases) but the lab work is very time consuming and takes a year or more to get results back. Baked argillaceous shales in volcanic slag have also been shown to be suitable candidates for TL, with careful selection of sampling sites (Zoeller, L., 1989, untranslated German paper).


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