Applications and limitations of thermoluminescence to date quaternary sediments

Quaternary International, Vol. 1, pp. 47-59.1989.

1046-6182/89 $0.00 + .50 ~) 1989 Pergamon Press plc

Printed in Great Britain. All rights reserved.

APPLICATIONS AND LIMITATIONS OF THERMOLUMINESCENCE TO DATE QUATERNARY SEDIMENTS

S t e v e n L. F o r m a n

Center for Geochronological Research, Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309--0450, U.S.A.

The thermoluminescence(TL) method dates sediment by quantifying time-dependent dosimetric properties of mineral grains. Sunlight exposure of the sediment during transportation and deposition or heating resets the TL clock to time 'zero'. After burial of the mineral grains, ionizing radiation from the decay of radioisotopes imparts a TL signal. This accumulated TL signal is proportional to the equivalent dose (ED), which is a measure of radiation exposure during the burial period. A TL age estimate is determined from the ratio of the ED to the environmental radiation (dose rate). The calculationof an ED is influencedby the susceptibilityof sediments to optical bleaching, the emissionspectra of minerals, and possible long and short term fading. There are three basic methods for determining an ED: regeneration, total and partial bleach. The regeneration technique may yield under-estimates because of sensitivitychanges with laboratory bleaching. The latter two techniques are most useful for well and partially light bleached sediments, respectively. A number of sediments can be dated by the TL method. Eolian and a variety of terrestrial sediments yield apparently accurate TL age estimates, though there are possible complicationsassociatedwith pedogenesis. It appears that some fluvialsediment can be accurately dated by TL, but little is known about the efficiencyof optical bleaching for fluvial depositional systems. Till and glacier-proximaldeposits are inadequatelylight bleached to be dated by TL. Sedimentsdeposited distal from the glacier margin, which are at least partially bleached, are preferred for dating. Future research is needed to understand better the relationship between sedimentary environments and the extent of light bleaching. Also, studies should concentrate on dating feldspar extracts from well-dated sequences to more fully ascertain the temporal limitations of the method. Only by understanding the interplay of sedimentology,TL dosimetry and environmental radiation can the TL method be effectively applied to the myriad of depositional environments spanning the late to middle Pleistocene. 1985; Singhvi and Mejdahl, 1985; Berger, 1986, 1988; Mejdahl, 1986). A recent book by Aitken (1985b) provides detailed coverage of the theory and the techniques of T L dating and a book by McKeever (1985) gives a thorough discussion of the physical basis of TL. This contribution departs from previous reviews by emphasizing the geological aspects of T L dating which have been mostly overlooked until recently. It is through the combined understanding of geological constraints and physical limitations that we gain greater insight into the promises and problems of T L dating.

INTRODUCTION

A persistent problem in Quaternary studies is determining the timing of climatic, tectonic, or anthropogenic events as recorded in terrestrial or marine sedimentary records. The ability to understand regional mechanisms of environmental change is severely limited if an independent chronology cannot be established for disjunct areas. Without a secure geochronology it is not possible to address the causal relationship between changes in the ocean, terrestrial, glacier and anthropogenic systems. The thermoluminescence (TL) technique, which directly dates sediments, has potentially broad spatial and temporal applications; thus it has aroused the interest of Quaternary scientists. Because it is one of the few techniques that can be applied in a variety of terrestrial stratigraphic settings as well as to deep-sea sediments, T L is an important tool to temporally link paleoclimate records. However, an incomplete understanding of the sedimentologic, diagenetic, and physical mechanisms that control the acquisition and retention of the thermoluminescence signal has impeded development and application of the technique. During the past decade T L dating has been the focus of many reviews (Dreimanis et al., 1978; Wintle, 1980; Wintle and Huntley, 1982; Mejdahl and Wintle, 1984; Lamothe et al., 1984; Aitken, 1985a; Dreimanis et al.,

HISTORY Daniels et al. (1953) first proposed the T L dating of fired archaeological materials, which is now a well established technique. During the 1960s and 1970s the Oxford University Research Laboratory for Archaeology and History of Art led in the development of T L to date archaeological materials. This laboratory was instrumental in developing two fundamental dating procedures, the quartz inclusion (Fleming, 1970) and the fine grained techniques (Zimmerman, 1971). Other research defined dosimetric parameters for T L dating (i.e. Aitken and Bowman, 1975) and documented the effects of anomalous fading (Wintle, 1974). The potential of dating unheated sediment by T L was first recognized by Shelkoplyas and Morozov (1965) in 47

48

S.L. Forman

Kiev, U.S.S.R. Shelkoplyas (1971) reported TL dates for loess and other sediments from the Soviet Union. Though these dates appear to be in correct stratigraphic order, Wintle and Huntley (1982) concluded that the reported TL ages may be in error by a factor of 2 to 10. Chinese workers (Li et al., 1977; Li and Sun, 1982) applied TL to date the thick loess sequences of central China. The TL dates generally agree with other age indicators, though there are methodological problems. This work by the Russians and Chinese went mostly unrecognized by western scientists. Early research in North America by Bothner and Johnson (1969) documented an increase in TL down a marine core, which was attributed to foraminiferal calcite. Subsequently, Huntley and Johnson (1976) documented a similar increase in TL and ascribed the TL signal to radiolaria. Only in 1979 did Wintle and Huntley report that the TL signal was most likely not from the nannofossils, but from the adhering detrital grains, and that the TL signal was probably reduced by exposure to natural sunlight. This rediscovery of the age dependent TL signal of mineral grains in unheated sediments by Wintle and Huntley (1979, 1980) ushered in an expansion in the research and application of TL to date Quaternary sediment.

radioactive decay of uranium, thorium, and potassium40. Radiation causes electrons to be displaced by orbitals, which are subsequently trapped within latticecharge disequilibrium sites called electron traps. Heating or light exposure of sediments causes vibration of the mineral lattice and eviction of electrons from traps. Some of these electrons may be conducted to luminescence centers and be emitted as light. This light signal is actually three dimensional, defined by temperature, and the light intensity and wavelength of light emission (Fig. 2A). In most TL dating procedures the ultraviolet or blue regions are selected for analysis (Fig. 2B). Sediment grains are long-term radiation dosimeters, with the TL signal being a measure of accumulated radiation exposure. The radiation level responsible for producing the TL signal is termed the equivalent dose [ED; measured in grays (100 r a d s = I gray)] and determining this value solves half of the TL age equation (Eq. 1). TL age estimate =

Dose rate (Grays/year)

!

Natural

OF THERMOLUMINESCENCE DATING

i

+

laboratory ktadiation

7=

PRINCIPLES

Equivalent dose (Grays)

i

(1)

i

B

O c

The basic principles of the technique for dating Quaternary sediments are similar to those used in dating archaeological materials. However, for pottery the exposure to heat during firing removes any TL signal that has accumulated in the minerals, whereas in sediments the inherited TL signal in quartz and feldspar grains must be reduced (bleached) by exposure to sunlight prior to deposition (Fig. 1). The TL signal of feldspar is reduced by visible and ultraviolet wavelengths; the TL of quartz is most sensitive to tiitraviolet light (Kronborg, 1983). The TL signal emitted by mineral grains is acquired by exposure to ionizing radiation mostly from the 1.0 ~,

I

}

I

Natural "

Z ¢/) .,J I--

o

1.2 = 1.0 =

I

"i

~ 0.4

F-

~ 0.2

|

0.6 -

~ o.o

Q 0.4

1.0 0.8

~ 0.6

r~\

1.2

0.8

.J 0.8 j

i

3OO 4O0 500 lOO 2o0 0 TEMPERATURE.'C. (during progressive heating}

0.6 0.4 0.2 0.0

-

IJJ

re 0.2 or~/., -- 0

Polossium F e l d s p o r ~ I I 3 6 EXPOSURE

I 9

I 12 TO SUN (h)

15

FIG. I. The reduction in natural TL for quartz and potassium feldspar from the exposure to sunlight. The natural dose was about 100 Gy. (From Mejdahl, 1986.)

FIG. 2(A). Isometric plot of the thermoluminescence emission spectrum for an irradiated sediment (from McKeever, 1985, p. 46). The T L signal of mineral grains is three-dimensional, with the intensity of light emission dependent on temperature and wavelength. (B) T L emission in the ultraviolet (300-400 nm) range for polymineral fine-grains as.a function of heating temperature. This sample was preheated at 150°C for 16 hr prior to analysis.

Applications and Limitationsof Thermoluminescence

49

Mineralogy and Emission Spectra of Sediments Laboratory procedures have been developed to isolate the feldspar signal from polymineral sediments. Feldspars are often the preferred mineral for dating because these materials exhibit higher TL output, are more easily light bleached, and usually have higher radiation saturation values than quartz. Mejdahi (1985) FACTORS THAT CONTROL EQUIVALENT DOSE outlined procedures to separate coarse grain (100-300 DETERMINATION /zm) feldspars from polymineral samples using heavy liquids. The mineral-specific TL signal in fine grain (411 /xm) sediments is isolated either by dissolving Light Bleaching of Sediments To determine an accurate equivalent dose (ED) the feldspars with hydrofluorosilicic acid, thus concentratsediment must have been exposed to light for a ing quartz (Berger et al., 1980), or by the use of glass sufficient period of time during transportation and filters during analysis, which enhances emissions of deposition to reduce (bleach) previously acquired TL selected mineralogies. The Schott UG-11 filter which to some low, definable level. This level, called the allows transmission of ultraviolet light (300-400 nm) is residual level, is the point from which the geological TL thought to enhance the emissions of feldspar for a accumulated after burial. The extent to which sediment polymineral sample (Debenham and Walton, 1983). is bleached by light is controlled primarily by the mode However, Berger (1985b) disputes the selectivity of this of sedimentation which dictates the spectral character- filter for glacially-derived silts from British Columbia, istics, the duration of light exposure, and the light citing the complex TL properties of quartz. intensity to which the sediment is subjected. In addiAnother filter often used by TL researchers is a tion, the residual level attained is influenced by the 'blue' filter (i.e. Corning 5-58), which transmits blue susceptibility of minerals to light bleaching. spectrum (400-500 nm) and is believed to enhance the Light in the ultraviolet to the blue part of the emissions of quartz. Recent studies of natural quartz of spectrum is most effective in evicting electrons from plutonic and volcanic origin indicate that blue (400-500 traps, but for at least some alkali feldspars limited nm) as well as red (600-700 nm) wavelengths may bleaching occurs with exposure to red light (Kronborg, dominate the TL emissions (Hashimoto et al., 1986, 1987). A recent contribution by Rendell et al. (1988) 1983). Loess that has accumulated in thick terrestrial sequences is usually borne high in the atmosphere and outlines the spectral properties of loess from Pakistan exposed to ultraviolet and visible spectrum for many in the ultraviolet, blue, green and yellow regions and days; thus loess is usually well light-bleached (cf. possible geochronologic potential. Also Huntley et al. Wintle, 1982). Water-lain sediments are not as well (1988) provide valuable information on the spectral bleached, and have a higher residual level (cf. Berger et properties of feldspars and quartz from a variety of al., 1984; Forman, 1988), because water severely Quaternary sediments. attenuates ultraviolet radiation (Jerlov, 1976). In addition turbidity shifts the penetrating spectrum to the Short Term Instability of the TL Signal: Anomalous blue-green region, thus further reducing the efficiency Fading Wintle (1973) showed volcanic feldspars stored after of bleaching. A number of laboratory procedures have been used to compensate for the partial bleaching of irradiation had lost TL compared with sediment measwater-lain sediments (cf. Berger et al., 1984; Forman et ured immediately after irradiation. This loss of TL in the laboratory is termed 'anomalous fading' because al., 1987; Forman, 1988). Little is actually known about the effect of light the observed stability of the TL signal is much less than intensity on the bleaching of TL. Kronborg (1983) that predicted from kinetic studies (Fig. 3). Possible documented a 50-70% reduction in the TL signal of mechanisms of fading have been examined by Templer feldspar exposed to diffuse sunlight for 96 hr at the (1985). If corrections are not made for anomalous bottom of a 7 m deep lake. Forman (1988) found that fading, then both the ED and thus the TL age will be both low light intensity and blockage of ultraviolet to under-estimated. When irradiated samples are stored the blue part of the spectrum resulted in partial at room temperature, the fading, usually 5-10% but occasionally 25% or more, is normally completed bleaching of high Arctic near-shore sediments. Quartz is generally less susceptible than feldspar to within a few weeks (cf. Lamothe, 1984; Berger, 1984). Anomalous fading is present in many minerals. light bleaching (Bailiff et al., 1977; Debenham and Walton, 1983; Mejdahl, 1985). Quartz grains from Akber and Prescott (in press) showed that calcic Denmark reached a residual level after approximately 9 plagioclase faded while sodic plagioclase did not fade hr whereas feldspars from the same deposit attained a after 2 hr storage. However, Hassan et al. (1986) similar level after approximately 3 hr of sunlight document fading in a low temperature sodic feldspar exposure (Fig. 1). Quartz grains from Chinese loess from meteorites. Tyler and McKeever (1988) report exhibit resistance to light exposure with only partial fading in both high and low temperature forms of bleaching after 36 hr or sunlight exposure (Forman, sodic feldspar. Quartz may also exhibit anomalous fading of ca. 10% (Mejdahl, 1983; Readhead, 1988; unpub, data).

The other half of this equation is the dose rate, which is a measure of the environmental radioactivity of the sediment for the time of burial. This over-simplified equation encompasses the assumptions and analyses that are used to calculate a TL age estimate.

50

S.L. Forman CTLH3 /~

/

\

/

/

\

~

/

~

~oo O~ I

~'t~stored . . . .

\

'

20o '

Teml~CI 3oo ' '

40o '

~---;~ J'~

T g

/ I

._J 1.-

/ I00

~ \ ~ \

L

!

200

300

I~'e- I~lat

lunstoted",,.,

O0

Tlm9{'C ) ~ "'

~i

400

Temp~'C )

• FIG.3. Anomalousfadingcharacteristicsfor a polymineralfinegrainswithan additionallaboratorydoseof 447 Grays. (A) Thd TL signalis comparedfor irradiated samplesstored for 22 daysand glowedwithin 10 min after irradiation.Thissampleexhibits 31 _ 3% fading(reduction)of its TL signal over 22 days. (B) The TL signal is compared for irradiatedsamplesstored for 22 days, then preheated at 150"Cfor 16 hr and samples that were preheated within 10 rain of irradiation. Preheating reduces anomalous fadingto within detectabilitylimits (<10%). (From Forman et al., 1987.)

Bowman, 1988). Potassium feldspars from a variety of ka. In this study, TL age estimates on the fine-grained fraction are in approximate agreement with known ages deposits from northwest Europe usually exhibit negligible fading (<10%) (Mejdahl and Kolstrup, 1986; up to 50 ka, but beyond, ages are consistently too young; samples with known ages of ca. 700 ka, have TL Lundqvist and Mejdahl, 1987). Laborious mineral ages of 100 ka (Fig. 4). Wintle (1985b) documented a separations have been attempted to isolate the low fade similar effect for the Saint Roman loess section, France components (Berger, 1985a). Anomalous fading in a variety of minerals has been with regeneration age estimates not exceeding ca. 100 ka, presumably younger than their known ages. Debenobviated by preheating samples prior to analysis betham (1985) explains this underestimate in TL ages by ween 75°C to 150°C for various lengths of time the time-dependent decay in luminescence centers, (Templer, 1985; Berger, 1987; Forman et al., 1987). Preheating removes a meta-stable TL component which is approximated by the equation: responsible for fading, isolating the time sensitive TL (2) T = L [1-exp(-t/L)] signal (Fig. 3). where T = TL age estimates, t = 'known' geological Long Term Stability of the TL Signal: Temporal age, and L = mean life of luminescence centers (100 Limitations ka). Using the above equation Debenham (1985) Kinetics studies indicate that the high temperature (> 250°C) TL signal of feldspar and quartz is theoretic- concluded that the upper limit of TL dating is approxially stable over periods of 1-10 million years (cf. mately 200 ka. However, a recent study (Rendell and Townsend, Strickertsson, 1985). Though quartz exhibits long term stability, it reaches TL saturation after 75-150 ka ,1988) indicates that the age limitations outlined by (depending on the environmental dose rate; e.g. Lu et Debenham (1985) may be the effect of the regeneration al., 1987a,b). Most potassium feldspars have a TL technique, and thus may not be an accurate assessment saturation level that is 10 times that of quartz and thus of the temporal limitation of the TL method. Rendell are approaching the theoretical range of TL stability and Townsend (1988) document a maximum regeneration age of 72 ka for a continuous loess sequence in (cf. Mejdahl, 1986). One of the most important and presently perplexing Pakistan whereas TL age estimates by the total bleach questions in TL dating research is the long-term method (see equivalent dose section) are ca. 135 ka stability of the TL signal. An obvious way to assess the (Fig. 5). They explain the plateau in ages by the stability of the TL signal in the geological record is by regeneration technique as a result of sensitivity changes dating a series of feldspar-rich deposits that span the from the exposure to laboratory ultraviolet light. Other studies of known age sediments provide Quaternary. Debenham (1985) used the regeneration technique (see Procedures for Determining Equivalent additional information on the temporal limitations of Dose section) to date a variety of Ioessic sediments the TL method. Pr6szyfiska-Bordas (1985) determined from Europe that were deposited during the past 700 the equivalent dose for two eastern European sites by

Applications and Limitations of Thermoluminescence

!

I

I

~

I

I

51

I

!

!

2OO ka

/

UJ 0

/

I ...-------r-"--. ~

.~ IOC I-

/ I .

1 . - - ! ~ ~ -

I

~ : 120 ka tO0 ka

,I

80 ka

S{

$.~,.~.:. . I

I

50

I00

I

I

I

I

200

,

I

//

,

/.00

300

700 ka

AGE ESTIMATES (various sources)

FIG. 4. TL age estimate for polymineral fine grains from European loess as a function of geological age estimates. The TL age estimates were determined by the regeneration method. (From Debenham, 1985.)

TL age (ka) o

~o

20

3o

40

so

so

70

8o

90

120

130

140

100

IlO

~

TotalBlench

melhod

- ' * - Reqenerotion

method

1.0 2.0

--o--.-~.~ ~

3.0

_g

4.0

u I o 'o .1=

6.0

C~

1.0

T

&O o

9.0

,I !

T

,,,,

\"

10.0

FIG. 5. Variation of TL age estimates by the regeneration and total bleach method for a 10 m loess section in Pakistan. (From Rendell and Townsend, 1988.)

the regeneration method for loess from Brunhes/ Matayama (0.73 Ma) and Matayama/Gauss (2.5 Ma) magnetic boundaries. Mejdahl (1986), using Pr6szyfiska-Bordas' (1985) equivalent dose determinations and assuming a dose rate of 4 mGy/year, calculated the corresponding TL age estimates of 190 and 250 ka. Kronborg (1983) did not encounter the same limitations on coarse-grain feldspars using the total bleach method, assuming linear growth in TL. He obtained ages of 325 ka and 500 ka which are in approximate agreement with the known geologic age. Mejdahl (1986) reported TL ages of 300-500 ka by the total bleach technique on potassium feldspars from the Kap Kobenhavn Formation (0.7-2 Ma) of northern Green-

land. Additional studies by Mejdahi (1986) suggest that the upper age limit for potassium feldspar by the total bleach technique is approximately 500 ka. PROCEDURES FOR DETERMINING EQUIVALENT DOSE There are three often used procedures for determining an equivalent dose (ED): regeneration (Wintle and Pr6szyfiska, 1983); total bleach (Singhvi et al., 1982); and partial bleach (Wintle and Huntley, 1980) (Fig. 6). These methods define an ED from functions based on the TL response of the sample to additive beta/gamma doses from a calibrated radioactive source and the

52

S.L. Forman

reduction of the TL signal by laboratory light exposure. The regeneration and total bleach methods assume full reduction of the TL signal of mineral grains by sunlight (or heat) prior to deposition. The partial bleach technique does not require full light bleaching of the TL signal. An exciting recent development in luminescence dating research is the use of an argon ion laser to selectively release the most light-sensitive traps in mineral grains (Huntley et al., 1985; Godfrey-Smith et al., 1988; Rhodes, 1988). This technique is particularly attractive for partially bleached sediments.

PARTIALBLEACH [3I,

(a)

12O0O

.d

:

4ooo

5O

5O ED

100

~

Dose (Gy)

(b)

TOTALB y

12OOO

,,.1 [.., I 5O

~-o5o

I 100

.,~.- ED --.~.

Dose (Gy)

(c)

[REGENERATION

0

"4"-

50 ~

/

100

Regeneration Method In this method an equivalent dose is determined by comparing the natural TL signal to the TL signal that has been 'regenerated' by laboratory dose after extended light exposure (Fig. 6). This method is mostly used on well light-bleached eolian material in which the residual level was reduced to a low level prior to deposition. This residual level is redefined in the laboratory by exposure to ultraviolet light or natural sunlight for more than 7 hr. Wintle and Pr6szyriska (1983, p. 550) state: "It is currently the most problematical but also has the greatest potential since it would allow one to obtain the EDs for samples which exhibit non-linear growth characteristics". This method requires fewer analyses than the partial bleach method and often yields higher precision in dating. The greatest uncertainty of this method is with the laboratory bleaching. Laboratory light exposure may overbleach the sediment resulting in an overestimate of age. In addition, laboratory light, especially from high wattage ultraviolet sources, may cause changes in mineral grains' sensitivity to acquire TL with a subsequent dose (Murray et al., 1983; Wintle, 1985c; Rendell and Townsend, 1988). As discussed earlier, these sensitivity changes have resulted in underestimates in age, particularly for sediments >50 ka old (Wintle, 1985b; Rendell and Townsend, 1988). Total Bleach Method In this method the ED is calculated from two data sets: (1) the residual TL level reached after a long (>7 hr) laboratory light exposure, which is presumed similar to the level attained prior to depositon; (2) a TL growth curve, which is the response of the sample to additive beta or gamma radiation. This curve has been described by linear (Singhvi et al., 1982) and non-linear functions (Singhvi et al., 1987). The ED is defined by the intercept of the additive dose curve with residual TL level measured in the laboratory (Fig. 6). The greatest potential error is determining the residual level. Similar to the regeneration technique, over-bleaching of sediments results in over-estimates of age. However, many terrestrial sediments (e.g. loess, buried A horizons) which receive long sunlight exposure have been accurately dated by this technique (e.g. Wintle and Catt, 1985a; Wintle, 1987; Forman et al., 1988b, in press). A possible additional problem for some sediments ca. >50 ka, is the onset of non-linearity which may make modeling of the TL growth function problematic, though recent growth curve functions of Singhvi et ai. (1987), Berger et al. (1987) and Berger (1987) have yielded apparently reliable dates.

ED

(Gy)

Partial Bleach Method The partial bleach method is similar to the total FIG. 6. Three often used methods for determining an equivalent dose. The total bleach and partial bleach methods involveextrapola- bleach method except the residual level is extrapolated.~ tion of the (N + beta) growth curve to intersect with the inferred In this method two sets of samples are irradiated with residual level. The regeneration method determines an equivalent beta (or gamma) radiation to produce TL growth dose by comparing the natural T L to a laboratory reproduced T L curves. Before the TL measurement one of these sets is signal. (From Wintle and Pr6szyaska, 1983.)

Applications and Limitations of Thermoluminescence given a short laboratory light exposure. These two data sets define two curves: natural + beta and natural + beta + light exposure. The equivalent dose of the sediment is determined by the intersection of these two curves; linear (e.g. Wintle and Huntley, 1980) and nonlinear functions (e.g. Berger et al., 1987) have been used. This method is based on the assumption that laboratory light exposure is so brief that only the TL acquired since deposition is bleached. This assumption can be checked by repeat analysis at various bleaching times. If the EDs are in agreement then these exposures are not overbleaching the sediment. The partial bleaching of mineral grains in the laboratory can be accomplished by blocking the ultraviolet to green part of the spectrum (<540 nm) with a glass filter interposed between the sample and the lamp (cf. Berger et al., 1984; Huntley, 1985). This attenuated bleaching spectrum approximates the wavelengths that penetrate turbid water (Jerlov, 1976). THE DOSE RATE The dose rate (DR) is a measure of the environmental radioactivity of the sediment over a period of time. Dose rate is usually reported as mGrays/year or Grays/ ka. For most sediment, nearly all of the dose is provided by potassium-40, and the thorium and uranium decay series; the remaining few percent comes from rubidium-87 and cosmic rays. When nuclei of potassium-40 decay beta particles and gamma rays are emitted; rubidium-87 emits only beta particles. The radioactive decay of natural thorium and uranium releases alpha and beta particles and a small amount of gamma energy. This emitted radiation is responsible for imparting a TL signal in mineral grains. Alpha, beta, and gamma radiations have penetration distances in sediment of 10--20 ~m, 2 mm, and approximately 30 cm, respectively. Thus, mineraologic inhomogeneities may affect the dose rate calculation if within approximately 30 cm of the sampled site. Fine grain minerals (4-11 ~m) are often preferred for analysis because they receive exposure from all forms of radiation. The other often used grain size is 100-150 p.m with the outer alpha affected approximately 20 ~m removed with hydrofluoric acid (Carriveau, 1977). The majority of the dose for isolated coarse-grained quartz is from external beta and gamma radiation; feldspars receive a significant internal dose from potassium-40. The calculated dose rate is commonly corrected for absorption of alpha, beta, and gamma radiation caused by water in the sediment. Water absorbs more radiation than sediment on a weight for weight basis. Zimmerman (1971) calculated that the absorption coefficient of alpha particles is 50% greater for water than for pottery, and is 25% and 14% higher for.beta and ,gamma radiation, respectively. Estimating the long-term water content of the sediment is a source of error in TL-age determinations because of possible variations in pore water content since burial. This

53

uncertainty in moisture content has twice as great an effect on the standard deviation in TL age determinations for fine-grain than coarse-grain minerals. Additional concerns are possible changes in dose rate due to disequilibrium of the uranium decay series, especially if a significant amount of radon has diffused through the sediment. Disequilibrium occurs during deposition of deep-sea sediments and cave speleothems; analysis of these materials requires compensation for excess daughter isotopes (Wintle, 1978; Wintle and Huntley, 1980). Post-depositional weathering, accumulations of secondary minerals (silica, calcium carbonate and clay) and ground water movement can also alter the amounts and types of radioactive elements in the sampled material. Measurement of Dose Rate Many TL laboratories use thick source alpha-counting (Cherry, 1963) to determine U and Th content because this technique is relatively simple and inexpensive. Alpha counting directly detects alpha radiation, one of the causative particles in producing TL. Uranium and thorium contents are determined by counting the total number of alpha particles from a sample, as well as paired counts associated with and 232Th decay chain (cf. Huntley and Wintle, 1981). Sometimes this technique may 'overcount' the U and Th content. Fine grinding of the sample or fluxing the material into a glass can obviate 'overcounting' (Jensen and Prescott, 1983). Calculation of U and Th content by this technique is predicated on the assumption of secular equilibrium; radon loss and other disequilibria greatly reduce the accuracy of this technique. An alternative and favored technique, but more expensive and involved, is gamma ray spectrometry (Hofstader and Mclntyre, 1950). This technique provides a direct measurement of K, U, and Th content of sediment in the laboratory and, with a portable unit, in the field. Field detection is often preferred to compensate for granulometric and lithologic inhomogeneities at the sample location and to assess more accurately the gamma component. Radioactive nuclides are detected by their characteristic emitted gamma energies (cf. Bunker and Bush, 1966). Germanium type detectors measure individual members in the U and Th decay series and thus direct information on the degree of radioactive disequilibrium is obtained (cf. Murray and Aitken, 1982). Often, potassium-40 content is determined from bulk potassium analysis by atomic absorption spectrophotometry or X-ray fluorescence. Neutron activation analysis can provide direct determination of potassium, rubidium and parent isotopes of uranium and thorium. Multiple analysis of a sample is recommended to compensate for any inhomogeneity. Another method of assessing gamma or beta dose of sediments is by thermoluminescence dosimetry (TLD). Phosphors such as calcium fluoride or calcium sulfate activated with dysprosium are loaded into small capsules and implanted in the sampled layer for up to one

54

S.L. Forman

year (i.e. Mejdahl, 1978; Valladas and Valladas, 1983). These phosphors, like sediments, record environmental gamma or beta radiation as a potential TL signal. However, the phosphors have a much higher sensitivity to environmental radiation. The environmental beta or gamma dose is determined by calibrating the TL signal of the phosphor to a known laboratory dose. SEDIMENTOLOGICAL CONSIDERATIONS There are many different types of sediments that can be, or have the potential to be, dated by TL. The selection and sampling of sediment is the first crucial step in the TL analysis. A TL date is a measure of the time since the last sunlight-exposure (or heating) event of the sediment. For a TL date to be meaningful, the initial bleaching, usually by sunlight, must be related to a significant environmental event (i.e. climate change, tectonism). The TL dating of sediment that remains essentially unbleached by light during a specific event may yield a spurious age. In general, the preferred sediment for TL dating has had prolonged (i.e. >8 hr) exposure to sunlight prior to deposition, accumulated as a relatively homogeneous stratigraphic unit, >50 cm thick, and has not undergone significant water content variations or diagenetic changes during burial. Selection of this preferred sample for a stratigraphic setting requires a sedimentary facies analysis that considers the efficiency of sunlight bleaching with transportation and deposition. In many sedimentary settings the fine-grain (4-11 /zm) polymineral fraction is preferred for dating; it is an abundant particle size, often receiving extended light exposure during transport in air or water. Also, dosimetry calculations for this particle size are relatively simple. However, in a number of sedimentary environments fine-grain particles are rare or may be secondary detrital contaminants, and thus the coarse grain (100-150 /zm) fraction is preferred. There is concern whether such large grains during transport and depositions have had the opportunity to be adequately light bleached. However, several studies indicate that large feldspar grains from dune sands are well light bleached and yield TL age estimates that are in good agreement with radiocarbon control (e.g. Kolstrup and Mejdahl, 1986; Lundqvist and Mejdahl, 1987).

Sampling Sediment for Thermoluminescence Dating Sampling sediment for TL dating is relatively straightforward. Approximately 30 g of sediment should be collected, though more or less sediment may be adequate depending on the concentration of the chosen particle size. The geological TL signal of sediment is reduced with exposure to sunlight, thus care must be taken not to expose the sediment to light during sampling. Prior to sampling, the section should be excavated back 15-30 cm to expose a fresh face. Immediately prior to sampling, the face should be scraped free of surface light-exposed grains. It is best to take the sediment intact; though the outer mineral

grains may have been light exposed the internal grains have been shielded from light. Fine-grained (<2 mm) pliable sediment can be directly sampled by pushing the sampling container into the cleaned section face. Light- and moisture-tight containers are used in sampling. Black film canisters are inexpensive (often free) and will not allow moisture loss if the cap is sealed with electrical tape. Excellent sample containers similar to film canisters but with built-in moisture seals can be purchased from numerous chemical supply firms. If the sediment is coarse grained (>2 mm), hardened or excessively dried out, other techniques are employed. Cohesive sediment can be sampled by cutting out a block and wrapping it in aluminium foil and placing it in a plastic bag; an internal non-light-exposed sample can be analyzed. Indurated sediments can be sampled by hammering a metal pipe into the section. Alternatively, aluminium cans with their edges serrated by metal snips can be easily 'screwed' into and out of the section. The serrated end of the can is closed with aluminium foil and wrapped with 'duct' tape to insure tight closure. This is an effective method for sampling coarse grain deposits in the western U.S.A. (J. McCalpin, pers. commun. 1985). It is recommended that an additional 500-800 g sample be taken from the same layer for dose rate, mineralogic and granulometric analysis. This sample does not have to be sealed from light or moisture loss.

Selection of Sediments for Thermoluminescence Dating During the past decade the TL properties of a variety of sediments have been explored. However, our knowledge of what sedimentary facies are amenable to TL dating remains incomplete. Below is a survey of the current status of TL dating for a number of sedimentary settings. Deep-sea sediments Deep-sea sediments were an early focus of TL dating research (Wintle and Huntley, 1979, 1980). Sediment from the sub-polar Pacific yielded TL age estimates by the partial bleach method that increased down core to ca. 130 ka and are in general agreement with ages based on oxygen isotope stratigraphy (Wintle and Huntley, 1980). Berger et al. (1984) reported for a core from the Gulf of Mexico TL age estimates of 2 to 22 ka by the partial bleach method that were in general agreement with the radiocarbon chronology. These studies indicate that deep-sea sediments are adequately light bleached but care must be exercised in the laboratory not to overbleach the sediment. Eolian sediments During the last ten years the majority of TL dating research has concentrated on wind-borne sediments because they usually receive prolonged exposure to light prior to deposition and accumulate in a relatively homogeneous section. There have been numerous studies to date thicl~ loess sequences in North America

Applications and Limitations of Thermoluminescence

(e.g. Norton and Bradford, 1985; Pye and Johnson, 1987; Wintle and Westgate, 1986;0 Berger, 1987), Europe (e.g. Debenham, 1985; Wintle et al., 1985; Pr6szyfiska-Bordas, 1985; Wintle, 1987), China (e.g. Li and Sun, 1982; Lu et al., 1987a,b), India (e.g. Singhvi et al., 1987) and Pakistan (Rendell and Denneli, 1987; Rendeil and Townsend, 1988). In general the TL method appears to yield accurate dates for the last 75100 ka, although there are methodological complications (cf. Berger, 1987), especially associated with the regeneration technique (Debenham, 1985; Wintle, 1985b; Rendell and Townsend, 1988). Recently, TL studies on coarse grains from cover sands have yielded age estimates that are in good agreement with other chronologic indices (e.g. Jungner, 1985; Singhvi et al., 1986; Kolstrup and Mejdahl, 1986; Lundqvist and Mejdahl, 1987). Buried soils

Holocene buried A horizons have yielded TL age estimates by a variety of techniques that are in approximate agreement with radiocarbon control (Huntley et al., 1983; Wintle and Catt, 1985a; Forman et al., 1988b, in press). Wintle and Catt (1985a) reported an increase in TL ages, with some reversals possibly related to decalcification or chemical weathering, down a buried soil profile (Fig. 7). The light bleaching of soil A and B horizons probably occurs through direct sunlight exposure, enhanced by pedoturbation or possibly through chemical effects. Most likely the depth of optical bleaching in the soil is related chiefly to the depth and degree of pedoturbation. In many soils the surface A horizons commonly are bleached and TL in the upper part of B horizons may be partially bleached depending on the extent and depth of mixing (cf. Wintle and Catt, 1985a). Forman et al. (1988b) report that incipient (10 to 100 years) soil development is not sufficient to light bleach parent material but a few hundred years of pedogenesis effectively light-bleaches A horizons in the western U.S.A. Further research is needed to understand the complex effects of pedogenesis on TL properties.

55

Colluvial sediments

Though one of the most common sediments on the earth's surface, there has been little research on the feasibility of TL to date slope-derived sediments. Wintle and Catt (1985b) reported a TL date of 17.5 + 1.6 ka (a mean of three determinations by the regeneration, total and partial bleach methods) on a loessenriched solifluction deposit above a Late Weichselian till in northeast England. This TL age is in good agreement with radiocarbon dates on organic material associated with the till. Late Holocene hillwash near Kent, England yielded erroneously old TL age estimates of ca. 9-10 ka (Wintle and Catt, 1985a), presumably because of incomplete light bleaching with slope transport. Recently, Forman et al. (1988a) report TL age estimates (by the three previously mentioned methods) for fine-grained distal colluvium associated with late Holocene movement of the Wasatch Fault, Utah that are in agreement with radiocarbon control. This type of coiluvium yields accurate TL dates because most particles are derived from well light-exposed A horizons upslope and these particles are further exposed to light during the passage downslope with seasonal rains. In contrast, adjacent coarse proximal colluvium and debris-fan material are inconsistently and poorly light-bleached and thus yield spurious TL age estimates. Fluvial sediments

Little is known about the TL properties of fluvial sediments especially in reference to variations in the efficiency of optical bleaching with sedimentary environment. Huntley et al. (1983) analyzed a variety of stratigraphic and modern river silts from British Columbia, of unknown facies association by the partial bleach method, with wavelengths below 550 nm blocked. A majority of sediments yielded TL age estimates in approximate agreement with their known age, though a number of samples yielded TL age estimates that were more than four times their known age. In a related study, Huntley (1985) analyzed suspended silt from a number of rivers in British Columbia. Four silt i Z a m

mmq art

I0 - -hillwash

I L~orgonlc~

ge ,o,,.

"7 o

II

..,~ ~,

.---"-"

*

m 9

•

L..-- ir

4~:k0.4

2

mS

7.4:1=0.7

mR t I2"ZilA

Thane! Oeds

~ . ;~. ^.,9a~~"" ~oy.;~"

"" -

iu ~.v

"..

4

I

I

80

90

!

I00

.

11.7=1=1.1

8DtO.7

3J 4I OtQ5

I

~.mw 9,8*0.9

0,=61

max 10.5:1=0.9

8t971

"-.

metres east of concrete steps FIG. 7. Cliff section at Pegwell Bay (Kent) showing location of samples and resultant T L dates. The organic horizon of the buried soil is dated by radiocarbon to 6120 _.+ 250 BP. (From Wintle and Catt, 19~5a~)

56

S.L. Forman

samples yielded equivalent doses of <4 Gy (ca. 2 ka) in general agreement with their modern age. However, one sample gave an elevated ED of 25 + 10 Gy (ca. 10 ka) which Huntley (1985) explained as a result of sampling during the winter, when solar insolation is reduced. Nanson and Young (1987) report TL age estimates (by the regeneration and total and partial bleach methods) of 41-47 ka on sandy channel fills from near Sydney, Australia, that are in general agreement with radiocarbon control. Finer grain units in the fluvial sequence yielded overestimates in age presumably because of insufficient light bleaching with deposition. In a preliminary study Forman et al. (1988) report that the growth of natural TL signal of the flood-plain silts from a fluvial terrace sequence in Colorado is nonlinear, reaching a saturation value at approximately 130 ka. Incipient soil development associated with the facies is not sufficient to completely light bleach the sediment. These studies indicate some of the limitations and promise of TL to date fluvial deposits. It remains unclear what facies in the various river systems (i.e. braided, meandering) is most amenable for TL dating. Additional research is needed on the sedimentary controls of optical bleaching in a variety of modern fluvial settings before extending the method into the stratigraphic record.

times the TL age estimates (Miller, 1982). Recently, Drozdowski and Fedorowicz (1987) summarized TL dating results on Weichselian tills from the Vistula region in Poland. TL dates were computed by the regeneration method by analysis on quartz grains (88102/~m). The dates are stratigraphically consistent and basically occur in groups at 59-51 ka and 17-15 ka. It is difficult to assess the accuracy of these dates because none of the sites have independent calibration. Jungner (1983, 1987) examined the TL properties of coarse grains (100-300 /zm) from late Pleistocene glacial-fluvial sands from western Finland. TL age determinations exhibited significant scatter presumably because of inconsistent bleaching of individual mineral grains. When finer grain layers were analyzed the TL data exhibited less scatter with age estimates (by the partial bleach method) clustering at 90-100 ka and 115130 ka ago. Jungner (1987) believes that the finer grains yield more consistent TL ages because these layers were deposited during periods of relatively slow sedimentation, enhancing the light exposure of the sediment. A study of the TL in modern sediments from a glaciated fjord environment on Svalbard documents the effect of the sedimentary environment on light bleaching (Forman, 1988). Till and ice-proximal glacialmarine sediments exhibit the highest natural TL, icedistal glacial-marine sediment shows intermediate TL levels, and sublittorai and littoral sediments yield the Glacial-genic sediments lowest TL signal (Fig. 8) because they are deposited During the past decade the TL dating of till and other above wave base and thus are resuspended high in the glacially derived sediments has been the focus of many water column. Sediment in the sublittoral and littoral studies. TL dating of tills is problematic because of environments receives extended light exposure (albeit their highly variable genesis and resultant uncertainty attenuated by water) and is the preferred sediment for in the reduction of the TL signal with deposition. It has TL dating in this region. been suggested that the crushing of grains during glacial A number of studies have attempted to date, with transport releases trapped electrons within mineral some success, glacially-derived water-lain sediments defects, thus reducing the TL signal to a low residual (Berger, 1984, 1985b; Lamothe, 1984; Berger et al., level (cf. Morozov and Shelkoplyas, 1980). Glaciologi| i i ! cal studies (i.e. Blankenship et al., 1986) indicate that 400. Till (9) subglacial pressure varies substantially, thus subglacial grinding is not a consistent mechanism for the reduction of TL. Most likely the majority of the TL signal of till is inherited from glacially eroded sediments (i.e. 300Berger, 1984; Wintle and Catt, 1986a; Forman, 1988). t. Rapidly-deposited sediments (deltas, proximal glacialC :) marine sediments) associated with a glacier margin 0 u 200. probably receive brief and non-uniform light exposure, C 0 and thus are not the desired sediment for TL dating (cf. 0 Berger, 1985a,b; Forman, 1988). O. Despite the inherent difficulties in dating glacial~ I(X), genic sediments by TL, Troitsky et al. (1979) attempted _1 I.s,.,.,,,,o..,,. to date quartz grains from till and glacial-marine units "oo,,q.," II/ \\\\\ from western Spitsbergen. The TL age estimates are in stratigraphic order spanning ca. 25-120 ka but may be |1 underestimates because of non-linearity in TL growth characteristics (cf. Troitsky et al., 1979, p. 406). Amino Temperature {°C) acid, radiocarbon and biostratigraphic analyses of fauna from the same site of Troitsky et al. (1979) FIG. 8. Natural TL level for Late Weichselian/Early Holocene indicate that a more realistic age of the unit may be 2-3 glacial-genic sediments. (From Forman, 1988.)

II!2

Applications and Limitations of Thermoluminescence

1987; Forman et al., 1987). All of these studies have dated sediment <85 ka by the partial bleach method with wavelengths blocked below 550 nm to simulate the light penetrating a turbid water column (cf. Berger, 1984). Most sediments that yield TL age estimates in agreement with independent chronologic control were collected from sedimentary environments distal to the glacial source. For example, Berger et al. (1987) identified the clay laminae of a glacial varve sequence as adequately light-bleached to be dated and Forman et al. (1987) accurately dated estuarine muds, the topmost sediment of a deglacial-emergence sequence. In distal sedimentary environments deposition rates are slow enough to allow at least partial bleaching of the sediment. CONCLUSION

Since Wintle and Huntley's (1979, 1980) rediscovery of the geochronologic potential of the thermoluminescence properties of sediment there has been a rapid expansion of the technique, with over 50 TL dating laboratories operating worldwide. Over the past decade there have been significant strides in understanding the short- and long-term stability of the TL signal as well as the establishment of new laboratory procedures to effectively date sediments. However, the greatest and most persistent variables in TL dating are the natural spatial and temporal variations in sedimentoiogy, mineralogy, light bleaching conditions, and environmental radiation. Each sample may exhibit unique TL and dosimetric characteristics; thus the technique cannot simply be routinely applied. However, detailed study of specific sites focusing on the linkages between sedimentology, TL properties and radiation may prove that many sequences are datable by TL. There are two fundamental frontiers of TL dating research; sedimentologic controls on the light reduction of the TL signal and temporal limitations of the TL method. Additional research is needed to better define the TL variations of modern sedimentary environments as a key to the effective application of the technique in the stratigraphic record. Such studies would provide needed information on the extent of optical bleaching in different sedimentary environments and may also provide insight into the dominant operative sedimentary processes. In addition these studies provide sound criteria for sample selection and laboratory dating strategies which are particularly needed for the fluvial, near-shore marine, and lacustrine environments as well as in the pedologic context. Presently TL can provide, in many stratigraphic situations, accurate dates on sediment for at least the last 75-100 ka. The precision of TL age estimates is usually 10-15%, though in a few situations errors may be <10%. To better define the temporal limitations of the TL method, future studies should concentrate on dating feldspar extracts by a variety of procedures from well dated sites that span the last 500 ka. Laboratory research should focus on developing a better under-

57

standing of the spectral properties of feldspars in order to isolate the most time-sensitive TL signal. In addition, the dating of identical samples by different TL dating laboratories would provide an internal check on the precision of the technique. ACKNOWLEDGEMENTS Thermoluminescence studies are supported by United States' Geological Survey Contract #14--08-0001--G1396, Office of Naval Research Grant 1488K0017 and a University of Colorado Enrichment Award to the Center for Geochronological Research. Sincere appreciation is extended to G.H. Miller for providing comments on this work and his strong commitment to TL dating research. Also I proffer many thanks to A.G. Wintle for sharing her enthusiasm and extensive knowledge o n thermoluminescence dating, This manuscript benefited from the constructive comments of A.G. Wintle and D.L. Carter. All opinions in this contribution are solely those of the author.

REFERENCES Aitken, M.J. (1985a). Thermoluminescence dating: past progress and future trends. Nuclear Tracks and Radiation Measurements, 10, 3-6. Aitken, M.J. (1985b) Thermoluminescence Dating. Academic Press, New York, 291 pp. Aitken, M.J. and Bowman, S.G.E. (1975). Thermoluminescent dating: assessment of alpha particle contribution. Archaeometry, 17, 132-138. Akber, R.A. and Prescott, J.R. (1988). Thermoluminescence dating: can we date all types of feldspars? Nuclear Tracks and Radiation Measurements (in press). Bailiff, I.K., Bowman, S.G.E., Mobbs,S.F. and Aitken, M.J. (1977). The phototransfer technique and its use in thermoluminescence dating. Journal of Electrostatics, 3, 269-280. Berger, G.W. (1984). Thermoluminescence dating studies of glacial silts from Ontario. Canadian Journal of Earth Sciences, 21, 1393-1399. Berger, G.W. (1985a). Thermoluminescence dating studies of rapidly deposited silts from south-central British Columbia. Canadian Journal of Earth Sciences, 22, 704-710. Berger, G.W. (1985b). Thermoluminescence dating applied to a thin winter varve of the late glacial South Thompson silt, south-central British Columbia. Canadian Journal of Earth Sciences, 22, 1736-1739. Berger, G.W. (1986). Dating Quaternary deposits by luminescence-recent advances. Geoscience Canada. 13, 15--21. Berger, G.W. (1987). Thermoluminescence dating of the Pleistocene Old Crow tephra and adjacent loess, near Fairbanks, Alaska. Canadian Journal of Earth Sciences, 24, 1975-1984. Berger, G.W. (1988). Dating Quaternary events by luminescence. In: Easterbrook, D.J. (ed.), Dating Quaternary Sediments, pp. 13-50. Geological Society of America Special Paper 227. Berger, G.W., Brown, T.A. and Huntley, D.J. (1980). Isolation of silt-sized quartz grains. Ancient TL, 1I, 8-9. Berger, G.W., Huntley, D.J. and Stipp, J.J. (1984). Thermoluminescence studies on a 14C-dated marine core. Canadian Journal of Earth Sciences, 21, 1145-1150. Berger, G.W., Clague, J.J. and Huntley, D.J. (1987). Thermoluminescence dating applied to glaciolacustrine sediments from central British Columbia. Canadian Journal of Earth Sciences, 24, 425434. Blankenship, D.D., Bentley, C.R., Rooney, S.T. and Alley, R.B. 0986). Seismic measurements revel a saturated porous layer beneath an active Antarctic ice stream. Nature. 322, 54-57. Bothner, M.H. and Johnson, N.M. (1969). Natural thermoluminescent dosimetry in late Pleistocene pelagic sediment. Journal of Geophysical Research. 74, 5331-5338. Bowman, S.G. (1988). Observations of anomalous fading in maiolica. Nuclear Tracks and Radiation Measurements, 14, 131-137. Bunker, C.M. and Bush, C.A. (1966). Uranium, thorium and radium analyses by gamma-ray spectrometry (0.184-4).352) million electron volts. United States Geological Survey Research, 550-B, 176181.

58

S.L. Forman

Carriveau, G.W. (1977). Cleaning quartz grains. Ancient TL, 1, 6. Cherry, R.D. (1963). The determination of thorium and uranium in geological samples by an alpha counting technique. Geochimica et Cosmochimica Acta, 27, 183-189. Daniels, F., Boyd, C.A. and Saunders, D.F. (1953). Thermoluminescence as a research tool. Science, 117, 343-349. Debenham, N.C. (1985). Use of UV emissions in TL dating of sediments. Nuclear Tracks and Radiation Measurements, 10, 717724. Debenham, N.C. and Walton, A.J. (1983). TL properties of some wind-blown sediments. PACT, 9, 531-538. Dreimanis, A., Hiitt, G., Raukus, A. and Whippey, P.W. (1978). Dating methods of Pleistocene deposits: I. Thermoluminescence dating. Geoscience Canada, 5, 55-60. Dreimanis. A., Hiitt, G., Raukus, A. and Whippey, P.W. (1985). Dating methods of Pleistocence deposits: I. Thermoluminescence dating, in: Rutter, N. (ed.). Dating Methods of Pleistocene Deposits and their Problems, pp. 1-8. Geoscience Canada Reprint Series 2. Geological Association of Canada Publications, Ontario, Canada. Drozdowski, E. and Fedorowicz, S. (1987). Stratigraphy of Vistulian glaciogenic deposits and corresponding thermoluminescence dates in the lower Vistula region, northern Poland. Boreas, 16, 139-153. Fleming, S.J. (1970). Thermoluminescent dating: refinement of the quartz inclusion method. Archaeometry, 15, 15-30. Forman, S.L. (1988). The solar resetting of thermoluminescence of sediments in a glacier dominated fiord environment in Spitsbergen: geochronologic implications. Arctic and Alpine Research, 20, 243253. Forman, S.L., Wintle, A.G., Thorleifson, L.H. and Wyatt, P.H., (1987). Thermoluminescence properties and age estimates for Quaternary raised marine sediments, Hudson Bay Lowland, Canada. Canadian Journal of Earth Sciences, 24, 2405-2411. Forman. S.L., Maat, P. and Jackson, M.E. (1988a). Tbermoluminescence (TL) dating of fault generated slope deposits: a new tool for deciphering the timing of paleoearthquakes. Geological Society of America Abstracts with Programs, 20, A345. Forman, S.L., Jackson, M.E., McCalpin, J. and Maat, P. (1988b). The potential of using thermoluminescence to date buried soils developed on colluvial and fluvial sediments from Utah and Colorado, U.S.A.: preliminary results. Quaternary Science Reviews, 7, 287-293. Forman, S.L., Machette, M.N., Jackson, M.E. and Maat, P. (1988). An evaluation of thermoluminescence dating of paleocarthquakes on the American Fork segment, Wasatch fault zone, Utah. Jo,trnal of Geophysical Research (in press). Godfrey-Smith, D.I., Huntley, D.J. and Chen, W-H. (1988). Optical dating studies of quartz and feldspar sediment extracts. Quaternary Science Reviews, 7, 373--380. Hashimoto, T.. Koyanagi, A., Yokosaka, K., Hayashi, Y. and Sotobayashi, T. (1986). Thermoluminescence color images from quartz of beach sands. Geochemical Journal, 20, 111-118. Hashimoto, T., Yokosaka, K. and Habuki, H. (1987). Emission properties of thermoluminescence from natural quartz - - blue and red TL response to absorbed dose. Nuclear Tracks and Radiation Measurements, 13, 57-66. Hassan, F.A., Keck, B.D., Hartmetz, C. and Sears, D.W.G. (1986). Anomalous fading of thermoluminescence in meteorites. Journal of Luminescence. 34, 327-335. Hofstader, R. and Mclntyre, J.A. (1950). Gamma-ray spectrometry with crystals of NaI(TI). Nucleonics, 7, 32-37. ¢l-luntley, D.J. (1985). On the zeroing of the thermoluminescence of sediments. Physics and Chemistry of Minerals, 12, 122-127. Huntley, D.J. and Johnson, H.P. (1976). Thermoluminescence as a potential means of dating siliceous ocean sediments. Canadian Journal of Earth Sciences, 13,593-596. Huntley, D.J. and Wintle, A.G. (1981). The use of alpha scintillation counting for measuring Th-230 and Pa-231 contents of ocean sediments. Canadian Journal of Earth Sciences. 18, 419-432. Huntley, D.J., Berger, G.W., Divigalpitiya, W.M.R. and Brown, T.A. (1983). Thermoluminescence dating of sediment. PACT, 9, 607--618. Huntley, D.J., Godfrey-Smith, D.I. and Thewalt, M.L.W., (1985). Optical dating of sediments. Nature, 313, 105--107. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W. and Berger, G.W. (1988). Thermoluminescence spectra of some mineral samples relevant to thermoluminescence dating. Journal of Luminescence, 39, 123-136. Jensen, H.E. and Prescott, J.R. (1983). The thick-source alpha

particle counting technique: comparison with other techniques and solutions to the problem of overcounting. PACT, 9, 25-35. Jerlov, N.G. (1976). Marine Optics, 2nd edn. Elsevier, New York, 231 pp. Jungner, H. (1983). Preliminary, investigations on TL dating of geological sediments from Finland. PACT, 9, 565-572. Jungner, H. (1985). Some experiences from an attempt to date postglacial dunes from Finland by thermoluminescence. Nuclear Tracks and Radiation Measurements, 10, 749-756. Jungner, H. (1987). Thermoluminescence dating of sediments from Oulainen and Vimpeli, Ostrobothnia, Finland. Boreas, 16, 231235 Kolstrup, E. and Mejdahl, V. (1986). Three frost wedge casts from Jutland (Denmark) and TL dating of their infill. Boreas, 15,311321. Kronborg, C. (1983). Preliminary results of age determination by TL of interglacial and interstadial sediments. Proceedings, 3rd Seminar on TL and ESR Dating, Helsingor. Denmark. PACT, 9, 595605. Lamothe, M. (1984). Apparent thermoluminescence ages of St. Pierre sediments at Pierreville, Quebec. and the problem of anomalous fading. Canadian Journal of Earth Sciences, 21, 14061409. Lamothe, M., Dreimanis, A., Morency, M. and Raukus, A. (1984). Thermoluminescence Dating of Quaternary Sediments. In: Mahahey, W.C. (ed.), Quaternary Dating Methods, pp. 153-170. Elsevier, New York. Li, H.-H., Pei, J.-X., Wang, Z.-Z. and Lu, Y. (1977). A preliminary study of both the thermoluminescence of the quartz powder in loess and the age determination of the loess layers. Kexue Tongbau. 22,498-502. Li, H.-H. and Sun, J.-Z. (1982). Age of loess determined by Thermoluminescence (TL) dating of quartz. In: Liu, T. (ed.). Quaternary Geology and Environment of China, pp. 39--41. China Ocean Press, Bcijing. Lu, Y., Prescott, J.R., Robertson, G.B. and Hutton, J.T. (1987a). Thermoluminescence dating of the Malan loess at Zhaitang, China. Geology, 15, 603--605. Lu, Y., Mortlock, A.J., Price, D.M. and Readhead, M.L. (1987b). Thermoluminescence dating of coarse-grain quartz from the Malan Loess at Zhaitang section, China. Quaternary Research, 28, 356363. Lundqvist, J. and Mejdahl, V. (1987). Thermoluminescence dating of eolian sediments in central Sweden. Geologiska Frreningens i Stockholm FOrhandlingar, 109, 147-158. McKeever, S.W.S. (1985). Thermoluminescence of Solids. Cambridge University Press, New York, 376 pp. Mejdahl, V. (1978). Measurement of environmental radiation at archaeological excavation sites by means of TL dosimeters. PACT, 2, 70-83. Mejdahl, V. (1983). Feldspar inclusion dating of ceramics and burnt stones. PACT, 9, 351-364. Mejdahl. V. (1985). Thermoluminescence dating of partially bleached sediments. Nuclear Tracks and Radiation Measurements, 10, 711-715. Mejdahl, V. (1986). Thermoluminescence dating of sediments. Radiation Protection Dosimetry, 17, 219-227. Mejdahl, V. and Wintle, A.G. (1984). Thermoluminescence applied to age determinations in Archaeology and Geology. In: Horowitz, Y.S. (ed.), Thermoluminescence and Thermoluminescent Dosimetry, pp. 133-190. CRC Press, Boca Raton, Florida. Miller, G.H. (1982). Quaternary Depositional episodes, western Spitsbergen. Norway: aminostratigraphy and glacial history. Arctic and Alpine Research, 14, 321-340. Morozov, G.V. and Shelkoplyas, V.N. (1980). TL of Quartz from glacial deposits and determinations of age of glacial and limnoglacial formations. Abstracts of a Seminar on Fieldand Laboratory methods of Research into Glacial Sediments. Tallinn, pp. 101-102. Murray, A.S. and Aitken, M.J. (1982). The measurement and importance of radioactive disequilibrium in TL samples. PACT, 6, 155-169. Murray, A.S., Gemmel, A.M.D., Kennedy, S. and Lamb, M. (1983). The dating of inwash material derived from glacial deposits: preliminary results. PACT, 9, 573-579. Nanson, G.C. and Young, R.W. (1987). Comparison of thermoluminescence and radiocarbon age-determinations from late-Pleistocene alluvial deposits near Sydney, Australia. Quaternary Research, 27, 263-269. Norton, D.L. and Bradford, J.M. (1985). Thermoluminescence

Applications and Limitations of Thermoluminescence dating of loess from western Iowa. Soil Science Socie~ of Amerka Journal, 49, 708-712. Pr6szyfiska-Bordas. H. (1985). TL dating of loess and fossil soils from the last interglacial-glacial cycle. Nuclear Tracks and Radiation Measurements, 10, 737-742 Pye, K. and Johnson. R. (1987). Stratigraphy, geochemistry and thermoluminescence ages of lower Mississippi Valley loess. Earth Surface Processes and Landforms, 13, 103--124. Readhead. M. (1988). Thermoluminescence dating study of quartz in aeolian sediments from southeastern Australia. Quaternary Science Reviews, 7, 257-264. Rendell, H.M. and Dennell, R.W. (1987). Thermoluminescence dating of an upper Pleistocene site, northern Pakistan. Geoarchaeology, 2.63-67. Rendell, H.M. and Townsend, P.D. (1988). Thermoluminescence dating of a 10 m loess profile in Pakistan. Quaternary Science Reviews, 7. 251-255. RendeU, H.M., Mann, S.J. and Townsend, P.D. (1988). Spectral measurements of loess TL. Nuclear Tracks and Radiation Measurements, 14.63--72. Rhodes, E.J. (1988). Methodological considerations in the optical dating of quartz. Quaternary Science Reviews, 7,395--400. Shelkoplyas, V.N. (1971). Dating of the Quaternary deposits by means of thermoluminescence. In: Chronology of the Glacial Age, pp. 155-160. Geographic Society of the USSR, Pleistocene Commission, Leningrad. Shelkoplyas, V.N. and Morozov, G.V. (1965). Some results of an investigation of Quaternary deposits by the thermoluminescence method. In: Materials on the Quaternary Period of the Ukraine (for the VIlth International Quaternary Association Congress), pp. 8391). Naukova, Dumka, Kiev, U.S.S.R. Singhvi, A.K. and Mejdahl, V. (1985). Thermoluminescence dating of sediments. Nuclear Tracks and Radiation Measurements, 10, 137-161. Singhvi, A.K., Sharma, Y.P. and Agrawal, D.P. (1982). Thermoluminescence dating of dune sands in Rajasthan, India. Nature, 295, 313-315. Singhvi, A.K., Deraniyagala, S.U. and Sengupta, D. (1986). Thermoluminescence dating of Quaternary red-sand beds: a case study of coastal dunes in Sri Lanka. Earth and Planetary Science Letters, 80, 139-144. Singhvi, A.K., Bronger, A., Pant, R.K. and Satier, W. (1987). Thermoluminescence dating and its implications for the chronostratigraphy of Ioess-paleosol sequences in the Kashmir Valley (India). Chemical Geology, 65, 45-56. Strickertsson, K. (1985). The thermolumineseence of potassium feldspars-glow curve characteristics and initial rise measurements. Nuclear Tracks and Radiation Measurements, 10, 613-.-617. Templet, R.H. (1985). The removal of anomalous fading in zircons. Nuclear Tracks and Radiation Measurements, 10, 531-537. Troitsky, L., Punning, J.-M., Hutt, G. and Rajamae, R. (1979).

59

Pleistocene glaciation chronology of Spitsbergen. Boreas, g, 401407. Tyler, S. and McKeever, S.W.S. (1988). Anomalous fading of thermo[uminescence in oligoclase. Nuclear Tracks and Radiation Measurements, 14, 149-154. Valladas, G. and Valladas, H. (1983). A variant of the thermoluminescence technique for beta-ray dosimetry. PACT, 9, 73-76. Vares, K. (1982). Application of quartz thermoluminescence to understanding soil processes, Esti NSV Teaduste Akadeemia Toimetised 31. Koide Geoloogia, 3, 117-118. English translation in Anc&nt TL (1983), 1, 5--6. Wintle, A.G. (1973). Anomalous fading of thermoluminescence in mineral samples. Nature, 245, 143-144. Wintle, A.G. (1978). A thermoluminescence dating study of some Quaternary calcite: potential and problems. Canadian Journal of Earth Sciences, 15. 1977-1986. Wintle, A.G. (1980). Thermoluminescence dating: a review of recent applications to non-pottery materials. Archaeometry, 22, 113-122. Wintle, A.G. (1982). Thermoluminescence properties of fine grain minerals in loess. Soil Science, 134, 164-170. Wintle, A.G. (1985a). Stability of TL signal in fine grains from loess. Nuclear Tracks and Radiation Measurements, 10, 725-730. Wintle, A.G. (1985b). Thermoluminescence dating of loess deposition in Normandy. Ancient TL, 3, 11-13. Wintle, A.G. (1985c). Sensitization of TL signal by exposure to light. Ancient TL, 3, 16-21. Wintle, A.G. (1987). Thermoluminescence dating of loess at Rocourt, Belgium. Geologie en Mi]nbouw, 66, 35--42. Wintle, A.G. and Catt, J.A. (1985a). Thermoluminescence dating of soils develped in late Devensian Loess at Pegwell Bay, Kent. Journal of Soil Science, 36, 293-298. Wintle, A.G. and Cart, J.A. (1985b). Thermoluminescence dating of Dimlington Stadial deposits in eastern England. Boreas, 14, 231234. Wintle, A.G. and Huntley, D.J. (1979). Thermoluminescence dating of a deep-sea sediment core. Nature, 279, 710-712. Wintle, A.G. and Huntley, D.J. (1980). Thermoluminescence dating of ocean sediments. Canadian Journal of Earth Sciences, 17,348360. Wintle, A.G. and Huntley, D.J. (1982). Thermoluminescence dating of sediments. Quaternary Science Reviews, I, 31-53. Wintle, A.G. and Pr6szyfiska, H. (1983). TL-dating of loess in Germany and Poland. PACT, 9, 547-554. Wintle, A.G. and Westgate, J.A. (1986). Thermoluminescence age of Old Crow tephra in Alaska. Geology, 14, 594-597. Wintle, A.G., Shackleton, N.J. and Lautridou, J.P. (1984). Thermoluminescence dating of periods of loess deposition and soil formation in Normandy. Nature, 310, 491--493. Zimmerman, D.W. (1971). Thermoluminescent dating using fine grains from pottery. Archaeometry, 13, 29--52.