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Methodological aspects of thermoluminescence dating of late glacial and holocene dune sands from Brandenburg, Germany
()IIIIII'IIlllI~V (ic¢*(/IrOllO/Qk~ { (._)lltl/{'llltll ~ '~'~i{'lll I' ]{l'l Ii'l* ~ ), \ t!] ] ~. pp ,~.-7 Pergamon
( ' o p \ righl ~
4,~11 ] t)v~ I~ltJ4 t:Nc ~.icr Science [_ld
Ptmtcd in (ircal la,rilaiu +\ll tlght,, tc',crxcd 0 2 7 - 37t)1 t14 $2[~ DD
0277-3791 (94)E0{142-9
METHODOLOGICAL ASPECTS OF THERMOLUMINESCENCE DATING OF LATE GLACIAL AND HOLOCENE DUNE SANDS FROM BRANDENBURG, GERMANY M. Musa B a r a v and L u d w i g Z611er
f"orscl;tmg.s.stellc Archiiometrie der Heidelberger Akademie der Wi.s.sensclu(ften am Ma.v-Phmck-lnstitut fiTr Kernl?hysik, Pos(fach lO 39 80, 69029 Heidelberg, Germany
Scvcral prolilcs of Late Glacial-Holoccnc dune sands in the glacial outwash plains of Brandenburg were sampled to systematically investigate and compare thermoluminesccnce ages from the 9(1-2()() #m quartz and K-feldspar fractions. An ultraviolet pcak in the emission spectrum of thc H6nlc SOL2 sunlamp, which is not normally abundant in natt, ral sunlight after passing through the atmosphere, is thought to be responsible for a stronger phololransfcr of charges in quartz compared with daylight bleaching. To eliminatc these errors as far as possible, a two day blcaching under natural sunlight is suggested.
INTRODUCTION
Thc Brandenburg profile "post-dune" was selected becausc both independent age control is available and the gcomorphological rcsponsc to significant palaeoecological change and to human activities is preserved in the sedimentary record. The purpose of the sampling was a systematic investigation of the thermoluminescence (TL) agcs of quartz and K-feldspar to find out whether their TL ages arc consistent with each other and with independent age controls. Another aim of the study was to investigate whether TL age underestimates, such as those reported from K-feldspars extractcd from l)t, tch and Danish cover sands, can be circt, mvcntcd by the use of appropriate laboratory techniques. Thc TL blcaching of quartz under natural stmlight and artificial laboratory light (Dr H6nlc SOL2 sunlamp) werc compared to evaluate their influence on the TL itgcs.
SAMPLING AND E X P E R I M E N T A l . CONDITIONS
Samples werc collected using lightproof tins pushed into the freshh cleaned profilc. AI lhe same time samples were collcctcd in moiSlUl-C-tigh! bags for the determination of the moisture content and radioactivity measurements of the samples in the laboratory. The gamma dose rate was also nlCilSl.lrcd ill .sit, u s i n g it portable Nal g i l n l m i l s p c c l r o l l l c t c r o r bv a g a n l n l i t scintillation counter. Samplc preparation in the laboralorx took placc t, ndcr sut~ducd red light. Each slm3ple was sieved to separate thc t)(I-2()(I /xm fraction and carbonate ~as removed with H('l. The remaining mincral components wcrc separated in stages by heavy liquid separation (Mcjdah[. 1985) with liquids of densities 2.7glcm 3 (separation of heavv minerals). 2.62 glcm -~ (separation of quartz and plagioclase) and 2.58 g/cm 3 (separation of K-feldspar). Afterwards the
quartz fraction wits etched with constant stirring in 47% HF for 45 rain. The K-feldspar fraction was etched in 1(1% HF for 45 rain. A SOL2 sunlamp was used for laboratory bleaching (three hours). To avoid too strong heating of the sample discs, they were placed on a copper plate which was cooled with water. Standard volumes (2 mm 3) of quartz and K-feldspar grains were sprinkled in a single grain layer on steel discs of 0.9 cm diameter and fixed with silicon spray. A calibrated ';"Sr beta source (dose rate at the time of irradiation ca. 14.3-15 Gy/min) was used for laboratory irradiation. The TL measurements were recorded with an EMI 9635 QA photomt, ltiplier with a Corning 5-58 blue filter and a Chance-Pilkington HA-3 heat filter in front of it. The TL of quartz was measured without pre annealing, whereas for K-feldspar one minute preannealing at 240°C was required to eliminate thcrmalh' tmslablc TL signals. The estimation of the equivalent dose (ED) was achieved by using the regeneration method (Aitken, 1985: Z611cr el al.. 1988). For this pt, rpose several of the discs wcre bleached for three hours with the SOL2 sunlamp. Aftcr at least one day of storage in darkncss the blcachcd samplc discs wcrc irradiated with diffcrcnt bcta doses to determine the regenerated TL gro\~lh curvc. Unbleached sample discs wcrc irradiated ildditivch with bcta doscs to obtain thc additive TL grmvlh curve. The growth curves of the additive and regcneratcd TL signals v,erc compared (Wagner and Z611cr, 19S7: Z~illcr and Wagncr, 1990) to register any TL sensitivit\ change after bleaching. After beta irradiation the K-feldspar samples were stored for at [ e a s t f o u r w e e k s at roo111 temperature or for o n e week at 75"(" to allm~ loss through anomalous fading. To estimate the anomalous fading of K-feldspar, the T L of somc of the beta irradiated subsamptes wits measured immcdiatch' aftcr irradiation and compared with the TL intensity of the stored subsamplcs of the same beta dose.
477
478
M.M. Barav and L. Zollcr
Concentrations of U and Th were determined by alpha scintillation counting using pulverized subsamples (Z611er and Pernicka, 1989). The K contents of pulverized subsamptes of the bulk sample and those of the K-feldspar fractions were determined by atomic absorption spectrometry. From the concentrations of U, Th and K, the moisture values, the beta attenuation (Mejdahl, 1979) and the contribution of cosmic rays, the effective dose rate was calculated using conversion factors given by Nambi and Aitken (1986).
NTL~ ~
f t3
E
o.2-
/g,'.
o,~
/ g,'.' /,, ,',a..
1 oo
DISCUSSION Systematic comparisons of quartz and K-feldspar TL ages of Late Glacial to Holocene dune sands from Brandenburg showed that in about two-thirds of the investigated cases the ages are consistent within the l~r margin of error. However, for the other one-third the quartz TL age is significantly younger than the K-feldspar age. Because of the better bleachability of latent TL in feldspars compared with that of quartz (main maximum of blue TL emission at ca. 350°C glow curve temperature), insufficient bleaching during transport and deposition can hardly explain the age underestimates from quartz. On the contrary, the feldspar TL ages (even if they were overestimated) should be younger than the quartz TL ages. We investigated the opposite possibility: could the TL of quartz be underbleached through our laboratory technique? According to the manufacturer, the emission spectrum of the sunlamp lies very close to that of sunlight (Fig. 1). For the sample to lamp distance used the intensity of the SOL2 lamp is about 6.5 times stronger than natural sunlight of the northern middle latitudes in July. A comparison of the TL residuals after bleaching with the natural TL signal (Figs 2 and 3) shows that for quartz the percentage of the remaining TL is many times higher than that for K-feldspar. From this point of view underbleaching of feldspar would have had a
T F 300
SOL2
] 400
- Sunlamp:
daylight
i
i 500
i
i 600
Waveelng[th nm] -
r
i 700
-
D65:
FIG. 1. Emission spectra of SOL2 sunlamp and daylight. H6nle. Martinsried, Germanv: Standard D65.
2oo
3oo
Temperatur[e°C]
4oo
5oc
FIG. 2. Thermoluminescence (TL) glow curves of quartz grains. NTL=Natural TL, a and b after bleaching with SOL2 for three and six hours, respectively; c,d,e, and f after bleaching with daylight for 4, 8, 16 and 56 hr, respectively.
$ xlO o,5-
~ 0 _
0,4-
:~
0,2-
"7"
0,1-
0,2,-
~oo
[
I
200
300
400
500
Temperature ['C] FIG. 3. Thermoluminescence (TL) glow curves of K-feldspar grams. a=Natural TL (NTL) after one minute preannealing at 240°C and b=residual TL after three hours of bleaching ~ith SOL2 and one minute preannealing.
negligible influence on the ED estimation and hence on TL age, which contrasts sharply with quartz. An extension of bleaching duration under the HOnle SOL2 sunlamp led to a further decrease of the TL residual of quartz between about 280 and 400°C of glow curve temperature, but not of the hardly or non-bleachable TL above about 400°C. Bleaching of TL is a very complex process (McKeever, 1991) during which the spectra of bleaching light and phototransfer processes play an important part. Phototransfer is the optical stimulation of electrons from deeper traps and their retrapping into more shallow traps. Subsequent TL measurement would produce a correspondingly higher TL signal from the lower traps. To carry out bleaching experiments with daylight, a box was constructed of aluminium, painted white inside. The cover of the box consists of a 5 mm thick panc of Suprasil-ll quartz glass. This glass transmits virtually the entire solar spectrum as received at the earth's surface. Therefore, it is suitable for protecting the TL samples from pollution during bleaching outside the laboratory. For those samples for which quartz TL ages were significantly younger than their feldspar
T L Dating o f Dune Sands
47g
;PROFILE 'POST-DUNE' FROM BRANDENBURG
TL ages, we bleached quartz subsamples under the Suprasil-ll pane with different bleaching durations under conditions of bright and cloudless sky in midsummer. The results of these investigations are shown in Fig. 2. After four hours of daylight bleaching the TL signal decreased more than after six hours under the SOL2 sunlamp. The maximum of the natural TL (NTL) between about 300 and 400°C of glow curve temperature could be decreased even more by bleaching with daylight than with the SOL2 sunlamp. After 16 hr of daylight bleaching a TL residual was achieved which could not be bleached further. The results of these measurements seem to point to the underbleaching of quartz by SOL2 sunlamp bleaching. This leads to an underestimation of ED and thus of the TL age of quartz. Comparison of the results of these measurements, standardized for light source intensity (for example 16 hr bleaching with daylight or three hours SOL2 sunlamp), contradicts the hypothesis of simple underbleaching because of too-short bleaching durations. It is much more likely that a stronger phototransfer occurred when using the SOL2 sunlamp instead of daylight. This causes measurement of a higher TL residual between 300 and 400°C temperature of glow curve. This phototransfer is easily demonstrated when heated NTL discs are bleached with the SOL2 sunlamp and a TL signal is measured. The UV maximum around 370 nm in the spectrum of the SOL2 sunlamp (Fig. 1) is probably responsible for the stronger phototransfer in particular quartz grains. Quartz samples with linear additive TL growth could have the ED corrected using the TL residual after bleaching with daylight.
As an illustration of the results we discuss the dating of a sample with correction of the quartz TL age. The rest of the profiles and their dating and analytical results are to be published elsewhere. This profile (Fig. 4) consists of an upper unit of sand containing charcoal layers and a lower unit of sand with a palaeosoil and a peat laver. The charcoal layers give laC ages of 4980--4610 calendar BP (upper layer, Bin. 4317) and 8980-8380 calendar BP (lower layer, Bin. 4318). Therefore these layers could be placed into the Late Preboreal to Lower Boreal (lower layer) and into the Upper Atlanticum to Lower Subboreal (upper layer). Below these sands is a palynologically dated Preboreal soil on top of sand of Younger Dryas age. A peat laver of Aller6d age, subdivided bv a sand ribbon and including the Lake Maria Laach pumice tephra, overlies older sand. The lower peat laver gave a ~4C age of 10, 130+200 BP (Bln. 4304: all data after Schlaak, 1992), which is approximately 900 years younger than the mean HC age for the eruption of Lake Maria laach pumice tephra (Schmincke and v.d, Bogaard, 1990). These ages are uncalibrated. The quartz and feldspar TL ages of the sample above the Preboreal soil (sample 3) are consistent with each other and with the calibrated HC ages. The quartz TL age of the sample below the Preborcal soil (sample 2) is considerably younger than the feldspar TL age. The quartz and feldspar TL ages of the sand below the AllerBd peats differ even more. We obtained a feldspar TL age of 23.4_+2.5 ka and a quartz TL age of 13.9_+2.6 ka. The corrected quartz TL age is 22.(I_+4.11 ka, which is consistent with the feldspar
T L - A g e (ka) Q: Q u a r t z - T L - A g e F: F e l d s p a r - T L - A g e
d, ~th below surface (m)
o
•
."
i
."
|
14
• i . "e* = . " • • =
,
|
|
' • " ", • •,
.
s,=nd with wood remnants transition Upper Atlan~cum / Subboreal
C-age: 4 2 5 0 + 8 0 yr BP 4 9 8 0 - 4 6 t 0 yr cal BP
i |
14 C - a g e : 7 7 8 0 _+ 2 0 0 yr BP
.
. "
. .
.
.
. • .|
'
i"" •
•
" ". I * . " '
• . "
" . "'=
i
.
i" •
. *i
" • "',
sand with wood remnants transition Preboreal / Boreal
8 9 8 0 - 8 3 8 0 yr cal BP
Q:
8.4 _+ 1.2
F:
9.6 _+ 1.6
~1:] i l i ~
Q: 8.7 _+1.7 F : 11.8_+1.3
• ."
"=•1
i l i tll
."
' ." ."
Preboreal soil (black) J
Upper Dryas sand
I
14
I I'1111 I'[I I'1 II I I'il l'J
C-age: 1 0 1 3 0 _+ 2 0 0 yr B P
11 t l I l l l l I I I I I l l l i I ~ "
ca. 10 8 0 0 y r cal B P
Q: 13.9_+2.6 corrected Q: 22.0 _+ 4.0 F: 23.4_+ 2.5
" ". I I .
J H I I I T I l I I I I I | I I ] I I
3
"..
"
.
'
•
/Peat,
L ~,(e
.'
.
Allerod
to Upper Dryas
M..;. =..~.-. =.l~
~ u , =
--;~.=.~.. Wur,,l~
L~,,.~
Lower Dryas sand
FIG. 4. Schematic profile fl~r posl-dun¢. Brandenburg. ~,howing TL age~, for quartz and Kqeltlspars from ,,andy unils and radiocarbon ages on charcoal fragments and peat.
480
M.M. Bara~. and L. Zollcr
T L age. This proves our assumption that inadequate bleaching is the reason for the rejuvenation of some quartz T L ages when c o m p a r e d with feldspar TL ages for the same sample. The T L age underestimates of K-feldspars extracted from aeolian sands, as reported from Late Glacial sands in the Netherlands, Belgium and D e n m a r k (Griin et al., 1989; Dijkmans et al., 1992) was not seen here. Wintle and Duller (1991) and Balescu and L a m o t h e (1992) suggested that self-absorption of luminescence in K-feldspars leads to age underestimation, whereas the blue TL emission which has been used for our TL m e a s u r e m e n t s is not subject to this effect. In addition, through our preannealing technique we made sure that just T L signals with sufficient thermal stability were used for dating.
CONCLUSIONS
For T L dating of Late Glacial to Holocene dune sands it is r e c o m m e n d e d first to test which mineral separates exhibit the best T L reproducibilty. In normal cases these would be expected to be K-feldspars because of their higher TL intensity and smaller errors in dose rate calculation. Thermoluminescence m e a s u r e m e n t s of Kfeldspar are promising when using adequate laboratory techniques (preannealing to eliminate thermally unstable T L and TL m e a s u r e m e n t s using a blue filter) and yield stratigraphically consistent and chronologically correct ages for Late Glacial to Holocene dune sands. For TL dating of thc quartz fraction we suggest bleaching with daylight. The bleaching duration is dependent on the daylight intensity (in our case 16 hours of bright, cloudless sky in mid-summer). This could be estimated experimentally by continuing the bleaching until n() more significant decrease of TL residuals takes place. Thc problems mentioned could perhaps be overcome bx using the selective bleach technique as suggested b\ Prescott and Purvinskis (1991) and Hutton et al. (1t,~93).
REFERENCES
Aitken, M.J. (1985). Tlu'rnudumine~cume Dating. Academic Press, London, 359 pp.
Balescu, S. and Lamothe, M. (1992). The blue emission of K-feldspar coarse grains and its potential for overcoming TL age underestimation. Quaternary Science Reviews, 11, 45-51. Dijkmans, J.W.A., Mourik, J.M. and Wintle, A.G. (1992). Thermoluminescence dating of aeolian sands from polycyclic soil profiles in the southern Netherlands. Quaternary Science Reviews, 11, 85-92. Grtin, R., Packman, S.C. and Pve, K. (1989). Problems involved in Tk-dating of Danish Cover sands using Kfeldspar. In: Aitken, M.J. (ed.), Long and Short Range Limit,~ in Luminescence Dating. Occasional Publication No. 9. Research Laboratory for Archaeology and the
History of Art, Oxford. Hutton, J.T., Keller, J.M., Mojarrabi, B., Prescott, J.R., Purvinskis. R.A., Robertson, G.B. and Scholeficld, R.B. (1993). Comparison of thermoluminescence dating methods for some Australian sites. -lst AustralianNew Zealand Meeting on Quaternary Dating, 8th-lllth Febr. 1993, ANU Canberra, Abstracts:41. McKeever, S.W.S. (1991). Mechanisms of thermoluminescence production: some problems and a few answers. Nuclear Tracks and Radiation Measurements. 18, 5-12. Mejdahl. V. (1979). Thermoluminescence dating: Beta-dose attenuation in quartz grains. Archaeometrv. 21, 61-72. Mejdahl, V. (1985). Thermoluminescence dating of partially bleached sediments. Nuclear Tracks and Radiation Measurements, 10, 711-715. Nambi, K.S.V. and Aitken, M.J. (1986). Annual dose conversion factors for TL and ESR dating. Archaeometrv. 28, 2(12-2115. Prescott, J.R. and Purvinskis. R.A. (1991). Zero thcrmoluminescence for zero age. Ancient TL. 9, 19-2(I. Schlaak, N. (19921. Studie zur Landschaftsgc.lesc im Raum Nordbarnim und Eberswalder Urstromtal. Berliner Geogr. Arb., 76, 1-16(I. Schmincke, H.-U. and v.d. Bogm,rd, P. (19901. Die Entwicklungsgeschichte des Mittclrhcinraumcs und die Eruptionsgeschichte des Osteifel-Vulkanfcldes. In: Schirmcr, W. (cd.). Rheingeschichte =wischen Mosel trod Maas, pp. 16(-,-190. DEUQUA-Fiihrcr. Wagner, G.A. and Z61ler, k. (19,~71. Thcrmolumincszcnz - Uhr ffir Artcfaktc und Scdimente. Phvsik itt anserer Zeit, 18, 1-9. Wintle, A.G. and Duller, G.A.T. (19911. The effect of optical absorption on luminescence dating. Ancient TL, 9, 37-39. Zoller. k. and Pcrnicka, E. (1989). A note on overcounting in alpha-cot.ntcrs and its elimination. Ancient TL, 7, 11-14. Z611cr, k. and Wagner, G.A. (19901. Thcrmolumincsccnce dating of loess - - recent developments. Quaternat 3' International, 7/8, 119-128. Z611cr, k., Strcmmc, H.E. and Wagner, G.A. (19881. Thcrmohmfincszenz-Daticrung an k6[3-Pal~.iobodcn-Scqucnzcn yon Nicdcr-, Mittcl- und Obcrrhcin. ('heroical Geology, 73, 39-62.
( ' o p \ righl ~
4,~11 ] t)v~ I~ltJ4 t:Nc ~.icr Science [_ld
Ptmtcd in (ircal la,rilaiu +\ll tlght,, tc',crxcd 0 2 7 - 37t)1 t14 $2[~ DD
0277-3791 (94)E0{142-9
METHODOLOGICAL ASPECTS OF THERMOLUMINESCENCE DATING OF LATE GLACIAL AND HOLOCENE DUNE SANDS FROM BRANDENBURG, GERMANY M. Musa B a r a v and L u d w i g Z611er
f"orscl;tmg.s.stellc Archiiometrie der Heidelberger Akademie der Wi.s.sensclu(ften am Ma.v-Phmck-lnstitut fiTr Kernl?hysik, Pos(fach lO 39 80, 69029 Heidelberg, Germany
Scvcral prolilcs of Late Glacial-Holoccnc dune sands in the glacial outwash plains of Brandenburg were sampled to systematically investigate and compare thermoluminesccnce ages from the 9(1-2()() #m quartz and K-feldspar fractions. An ultraviolet pcak in the emission spectrum of thc H6nlc SOL2 sunlamp, which is not normally abundant in natt, ral sunlight after passing through the atmosphere, is thought to be responsible for a stronger phololransfcr of charges in quartz compared with daylight bleaching. To eliminatc these errors as far as possible, a two day blcaching under natural sunlight is suggested.
INTRODUCTION
Thc Brandenburg profile "post-dune" was selected becausc both independent age control is available and the gcomorphological rcsponsc to significant palaeoecological change and to human activities is preserved in the sedimentary record. The purpose of the sampling was a systematic investigation of the thermoluminescence (TL) agcs of quartz and K-feldspar to find out whether their TL ages arc consistent with each other and with independent age controls. Another aim of the study was to investigate whether TL age underestimates, such as those reported from K-feldspars extractcd from l)t, tch and Danish cover sands, can be circt, mvcntcd by the use of appropriate laboratory techniques. Thc TL blcaching of quartz under natural stmlight and artificial laboratory light (Dr H6nlc SOL2 sunlamp) werc compared to evaluate their influence on the TL itgcs.
SAMPLING AND E X P E R I M E N T A l . CONDITIONS
Samples werc collected using lightproof tins pushed into the freshh cleaned profilc. AI lhe same time samples were collcctcd in moiSlUl-C-tigh! bags for the determination of the moisture content and radioactivity measurements of the samples in the laboratory. The gamma dose rate was also nlCilSl.lrcd ill .sit, u s i n g it portable Nal g i l n l m i l s p c c l r o l l l c t c r o r bv a g a n l n l i t scintillation counter. Samplc preparation in the laboralorx took placc t, ndcr sut~ducd red light. Each slm3ple was sieved to separate thc t)(I-2()(I /xm fraction and carbonate ~as removed with H('l. The remaining mincral components wcrc separated in stages by heavy liquid separation (Mcjdah[. 1985) with liquids of densities 2.7glcm 3 (separation of heavv minerals). 2.62 glcm -~ (separation of quartz and plagioclase) and 2.58 g/cm 3 (separation of K-feldspar). Afterwards the
quartz fraction wits etched with constant stirring in 47% HF for 45 rain. The K-feldspar fraction was etched in 1(1% HF for 45 rain. A SOL2 sunlamp was used for laboratory bleaching (three hours). To avoid too strong heating of the sample discs, they were placed on a copper plate which was cooled with water. Standard volumes (2 mm 3) of quartz and K-feldspar grains were sprinkled in a single grain layer on steel discs of 0.9 cm diameter and fixed with silicon spray. A calibrated ';"Sr beta source (dose rate at the time of irradiation ca. 14.3-15 Gy/min) was used for laboratory irradiation. The TL measurements were recorded with an EMI 9635 QA photomt, ltiplier with a Corning 5-58 blue filter and a Chance-Pilkington HA-3 heat filter in front of it. The TL of quartz was measured without pre annealing, whereas for K-feldspar one minute preannealing at 240°C was required to eliminate thcrmalh' tmslablc TL signals. The estimation of the equivalent dose (ED) was achieved by using the regeneration method (Aitken, 1985: Z611cr el al.. 1988). For this pt, rpose several of the discs wcre bleached for three hours with the SOL2 sunlamp. Aftcr at least one day of storage in darkncss the blcachcd samplc discs wcrc irradiated with diffcrcnt bcta doses to determine the regenerated TL gro\~lh curvc. Unbleached sample discs wcrc irradiated ildditivch with bcta doscs to obtain thc additive TL grmvlh curve. The growth curves of the additive and regcneratcd TL signals v,erc compared (Wagner and Z611cr, 19S7: Z~illcr and Wagncr, 1990) to register any TL sensitivit\ change after bleaching. After beta irradiation the K-feldspar samples were stored for at [ e a s t f o u r w e e k s at roo111 temperature or for o n e week at 75"(" to allm~ loss through anomalous fading. To estimate the anomalous fading of K-feldspar, the T L of somc of the beta irradiated subsamptes wits measured immcdiatch' aftcr irradiation and compared with the TL intensity of the stored subsamplcs of the same beta dose.
477
478
M.M. Barav and L. Zollcr
Concentrations of U and Th were determined by alpha scintillation counting using pulverized subsamples (Z611er and Pernicka, 1989). The K contents of pulverized subsamptes of the bulk sample and those of the K-feldspar fractions were determined by atomic absorption spectrometry. From the concentrations of U, Th and K, the moisture values, the beta attenuation (Mejdahl, 1979) and the contribution of cosmic rays, the effective dose rate was calculated using conversion factors given by Nambi and Aitken (1986).
NTL~ ~
f t3
E
o.2-
/g,'.
o,~
/ g,'.' /,, ,',a..
1 oo
DISCUSSION Systematic comparisons of quartz and K-feldspar TL ages of Late Glacial to Holocene dune sands from Brandenburg showed that in about two-thirds of the investigated cases the ages are consistent within the l~r margin of error. However, for the other one-third the quartz TL age is significantly younger than the K-feldspar age. Because of the better bleachability of latent TL in feldspars compared with that of quartz (main maximum of blue TL emission at ca. 350°C glow curve temperature), insufficient bleaching during transport and deposition can hardly explain the age underestimates from quartz. On the contrary, the feldspar TL ages (even if they were overestimated) should be younger than the quartz TL ages. We investigated the opposite possibility: could the TL of quartz be underbleached through our laboratory technique? According to the manufacturer, the emission spectrum of the sunlamp lies very close to that of sunlight (Fig. 1). For the sample to lamp distance used the intensity of the SOL2 lamp is about 6.5 times stronger than natural sunlight of the northern middle latitudes in July. A comparison of the TL residuals after bleaching with the natural TL signal (Figs 2 and 3) shows that for quartz the percentage of the remaining TL is many times higher than that for K-feldspar. From this point of view underbleaching of feldspar would have had a
T F 300
SOL2
] 400
- Sunlamp:
daylight
i
i 500
i
i 600
Waveelng[th nm] -
r
i 700
-
D65:
FIG. 1. Emission spectra of SOL2 sunlamp and daylight. H6nle. Martinsried, Germanv: Standard D65.
2oo
3oo
Temperatur[e°C]
4oo
5oc
FIG. 2. Thermoluminescence (TL) glow curves of quartz grains. NTL=Natural TL, a and b after bleaching with SOL2 for three and six hours, respectively; c,d,e, and f after bleaching with daylight for 4, 8, 16 and 56 hr, respectively.
$ xlO o,5-
~ 0 _
0,4-
:~
0,2-
"7"
0,1-
0,2,-
~oo
[
I
200
300
400
500
Temperature ['C] FIG. 3. Thermoluminescence (TL) glow curves of K-feldspar grams. a=Natural TL (NTL) after one minute preannealing at 240°C and b=residual TL after three hours of bleaching ~ith SOL2 and one minute preannealing.
negligible influence on the ED estimation and hence on TL age, which contrasts sharply with quartz. An extension of bleaching duration under the HOnle SOL2 sunlamp led to a further decrease of the TL residual of quartz between about 280 and 400°C of glow curve temperature, but not of the hardly or non-bleachable TL above about 400°C. Bleaching of TL is a very complex process (McKeever, 1991) during which the spectra of bleaching light and phototransfer processes play an important part. Phototransfer is the optical stimulation of electrons from deeper traps and their retrapping into more shallow traps. Subsequent TL measurement would produce a correspondingly higher TL signal from the lower traps. To carry out bleaching experiments with daylight, a box was constructed of aluminium, painted white inside. The cover of the box consists of a 5 mm thick panc of Suprasil-ll quartz glass. This glass transmits virtually the entire solar spectrum as received at the earth's surface. Therefore, it is suitable for protecting the TL samples from pollution during bleaching outside the laboratory. For those samples for which quartz TL ages were significantly younger than their feldspar
T L Dating o f Dune Sands
47g
;PROFILE 'POST-DUNE' FROM BRANDENBURG
TL ages, we bleached quartz subsamples under the Suprasil-ll pane with different bleaching durations under conditions of bright and cloudless sky in midsummer. The results of these investigations are shown in Fig. 2. After four hours of daylight bleaching the TL signal decreased more than after six hours under the SOL2 sunlamp. The maximum of the natural TL (NTL) between about 300 and 400°C of glow curve temperature could be decreased even more by bleaching with daylight than with the SOL2 sunlamp. After 16 hr of daylight bleaching a TL residual was achieved which could not be bleached further. The results of these measurements seem to point to the underbleaching of quartz by SOL2 sunlamp bleaching. This leads to an underestimation of ED and thus of the TL age of quartz. Comparison of the results of these measurements, standardized for light source intensity (for example 16 hr bleaching with daylight or three hours SOL2 sunlamp), contradicts the hypothesis of simple underbleaching because of too-short bleaching durations. It is much more likely that a stronger phototransfer occurred when using the SOL2 sunlamp instead of daylight. This causes measurement of a higher TL residual between 300 and 400°C temperature of glow curve. This phototransfer is easily demonstrated when heated NTL discs are bleached with the SOL2 sunlamp and a TL signal is measured. The UV maximum around 370 nm in the spectrum of the SOL2 sunlamp (Fig. 1) is probably responsible for the stronger phototransfer in particular quartz grains. Quartz samples with linear additive TL growth could have the ED corrected using the TL residual after bleaching with daylight.
As an illustration of the results we discuss the dating of a sample with correction of the quartz TL age. The rest of the profiles and their dating and analytical results are to be published elsewhere. This profile (Fig. 4) consists of an upper unit of sand containing charcoal layers and a lower unit of sand with a palaeosoil and a peat laver. The charcoal layers give laC ages of 4980--4610 calendar BP (upper layer, Bin. 4317) and 8980-8380 calendar BP (lower layer, Bin. 4318). Therefore these layers could be placed into the Late Preboreal to Lower Boreal (lower layer) and into the Upper Atlanticum to Lower Subboreal (upper layer). Below these sands is a palynologically dated Preboreal soil on top of sand of Younger Dryas age. A peat laver of Aller6d age, subdivided bv a sand ribbon and including the Lake Maria Laach pumice tephra, overlies older sand. The lower peat laver gave a ~4C age of 10, 130+200 BP (Bln. 4304: all data after Schlaak, 1992), which is approximately 900 years younger than the mean HC age for the eruption of Lake Maria laach pumice tephra (Schmincke and v.d, Bogaard, 1990). These ages are uncalibrated. The quartz and feldspar TL ages of the sample above the Preboreal soil (sample 3) are consistent with each other and with the calibrated HC ages. The quartz TL age of the sample below the Preborcal soil (sample 2) is considerably younger than the feldspar TL age. The quartz and feldspar TL ages of the sand below the AllerBd peats differ even more. We obtained a feldspar TL age of 23.4_+2.5 ka and a quartz TL age of 13.9_+2.6 ka. The corrected quartz TL age is 22.(I_+4.11 ka, which is consistent with the feldspar
T L - A g e (ka) Q: Q u a r t z - T L - A g e F: F e l d s p a r - T L - A g e
d, ~th below surface (m)
o
•
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i
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|
14
• i . "e* = . " • • =
,
|
|
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.
s,=nd with wood remnants transition Upper Atlan~cum / Subboreal
C-age: 4 2 5 0 + 8 0 yr BP 4 9 8 0 - 4 6 t 0 yr cal BP
i |
14 C - a g e : 7 7 8 0 _+ 2 0 0 yr BP
.
. "
. .
.
.
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.
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sand with wood remnants transition Preboreal / Boreal
8 9 8 0 - 8 3 8 0 yr cal BP
Q:
8.4 _+ 1.2
F:
9.6 _+ 1.6
~1:] i l i ~
Q: 8.7 _+1.7 F : 11.8_+1.3
• ."
"=•1
i l i tll
."
' ." ."
Preboreal soil (black) J
Upper Dryas sand
I
14
I I'1111 I'[I I'1 II I I'il l'J
C-age: 1 0 1 3 0 _+ 2 0 0 yr B P
11 t l I l l l l I I I I I l l l i I ~ "
ca. 10 8 0 0 y r cal B P
Q: 13.9_+2.6 corrected Q: 22.0 _+ 4.0 F: 23.4_+ 2.5
" ". I I .
J H I I I T I l I I I I I | I I ] I I
3
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"
.
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/Peat,
L ~,(e
.'
.
Allerod
to Upper Dryas
M..;. =..~.-. =.l~
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--;~.=.~.. Wur,,l~
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Lower Dryas sand
FIG. 4. Schematic profile fl~r posl-dun¢. Brandenburg. ~,howing TL age~, for quartz and Kqeltlspars from ,,andy unils and radiocarbon ages on charcoal fragments and peat.
480
M.M. Bara~. and L. Zollcr
T L age. This proves our assumption that inadequate bleaching is the reason for the rejuvenation of some quartz T L ages when c o m p a r e d with feldspar TL ages for the same sample. The T L age underestimates of K-feldspars extracted from aeolian sands, as reported from Late Glacial sands in the Netherlands, Belgium and D e n m a r k (Griin et al., 1989; Dijkmans et al., 1992) was not seen here. Wintle and Duller (1991) and Balescu and L a m o t h e (1992) suggested that self-absorption of luminescence in K-feldspars leads to age underestimation, whereas the blue TL emission which has been used for our TL m e a s u r e m e n t s is not subject to this effect. In addition, through our preannealing technique we made sure that just T L signals with sufficient thermal stability were used for dating.
CONCLUSIONS
For T L dating of Late Glacial to Holocene dune sands it is r e c o m m e n d e d first to test which mineral separates exhibit the best T L reproducibilty. In normal cases these would be expected to be K-feldspars because of their higher TL intensity and smaller errors in dose rate calculation. Thermoluminescence m e a s u r e m e n t s of Kfeldspar are promising when using adequate laboratory techniques (preannealing to eliminate thermally unstable T L and TL m e a s u r e m e n t s using a blue filter) and yield stratigraphically consistent and chronologically correct ages for Late Glacial to Holocene dune sands. For TL dating of thc quartz fraction we suggest bleaching with daylight. The bleaching duration is dependent on the daylight intensity (in our case 16 hours of bright, cloudless sky in mid-summer). This could be estimated experimentally by continuing the bleaching until n() more significant decrease of TL residuals takes place. Thc problems mentioned could perhaps be overcome bx using the selective bleach technique as suggested b\ Prescott and Purvinskis (1991) and Hutton et al. (1t,~93).
REFERENCES
Aitken, M.J. (1985). Tlu'rnudumine~cume Dating. Academic Press, London, 359 pp.
Balescu, S. and Lamothe, M. (1992). The blue emission of K-feldspar coarse grains and its potential for overcoming TL age underestimation. Quaternary Science Reviews, 11, 45-51. Dijkmans, J.W.A., Mourik, J.M. and Wintle, A.G. (1992). Thermoluminescence dating of aeolian sands from polycyclic soil profiles in the southern Netherlands. Quaternary Science Reviews, 11, 85-92. Grtin, R., Packman, S.C. and Pve, K. (1989). Problems involved in Tk-dating of Danish Cover sands using Kfeldspar. In: Aitken, M.J. (ed.), Long and Short Range Limit,~ in Luminescence Dating. Occasional Publication No. 9. Research Laboratory for Archaeology and the
History of Art, Oxford. Hutton, J.T., Keller, J.M., Mojarrabi, B., Prescott, J.R., Purvinskis. R.A., Robertson, G.B. and Scholeficld, R.B. (1993). Comparison of thermoluminescence dating methods for some Australian sites. -lst AustralianNew Zealand Meeting on Quaternary Dating, 8th-lllth Febr. 1993, ANU Canberra, Abstracts:41. McKeever, S.W.S. (1991). Mechanisms of thermoluminescence production: some problems and a few answers. Nuclear Tracks and Radiation Measurements. 18, 5-12. Mejdahl. V. (1979). Thermoluminescence dating: Beta-dose attenuation in quartz grains. Archaeometrv. 21, 61-72. Mejdahl, V. (1985). Thermoluminescence dating of partially bleached sediments. Nuclear Tracks and Radiation Measurements, 10, 711-715. Nambi, K.S.V. and Aitken, M.J. (1986). Annual dose conversion factors for TL and ESR dating. Archaeometrv. 28, 2(12-2115. Prescott, J.R. and Purvinskis. R.A. (1991). Zero thcrmoluminescence for zero age. Ancient TL. 9, 19-2(I. Schlaak, N. (19921. Studie zur Landschaftsgc.lesc im Raum Nordbarnim und Eberswalder Urstromtal. Berliner Geogr. Arb., 76, 1-16(I. Schmincke, H.-U. and v.d. Bogm,rd, P. (19901. Die Entwicklungsgeschichte des Mittclrhcinraumcs und die Eruptionsgeschichte des Osteifel-Vulkanfcldes. In: Schirmcr, W. (cd.). Rheingeschichte =wischen Mosel trod Maas, pp. 16(-,-190. DEUQUA-Fiihrcr. Wagner, G.A. and Z61ler, k. (19,~71. Thcrmolumincszcnz - Uhr ffir Artcfaktc und Scdimente. Phvsik itt anserer Zeit, 18, 1-9. Wintle, A.G. and Duller, G.A.T. (19911. The effect of optical absorption on luminescence dating. Ancient TL, 9, 37-39. Zoller. k. and Pcrnicka, E. (1989). A note on overcounting in alpha-cot.ntcrs and its elimination. Ancient TL, 7, 11-14. Z611cr, k. and Wagner, G.A. (19901. Thcrmolumincsccnce dating of loess - - recent developments. Quaternat 3' International, 7/8, 119-128. Z611cr, k., Strcmmc, H.E. and Wagner, G.A. (19881. Thcrmohmfincszenz-Daticrung an k6[3-Pal~.iobodcn-Scqucnzcn yon Nicdcr-, Mittcl- und Obcrrhcin. ('heroical Geology, 73, 39-62.