- Email: [email protected]
Comparison of quartz OSL protocols using Lateglacial and Holocene dune sands from Brandenburg, Germany
Quaternary Science Reviews 20 (2001) 731}736
Comparison of quartz OSL protocols using Lateglacial and Holocene dune sands from Brandenburg, Germany夽 A. Hilgers *, A.S. Murray, N. Schlaak, U. Radtke Department of Geography, University of Cologne, Albertus-Magnus-Platz, D-50923 Cologne, Germany Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University, Ris~ National Laboratory, DK-4000 Roskilde, Denmark Hochstrasse 13, D-16244 Altenhof, Germany
Abstract We present the results obtained using various di!erent protocols in the quartz optically stimulated luminescence dating of a Late Glacial and Holocene aeolian dune sequence near Eberswalde, north of Berlin, Germany. The Aller+d &Finow' palaeosol and the Laacher See tephra layer serve as chronostratigraphic markers, and further independent age control is provided by eight C ages. The application of the single-aliquot regenerative-dose protocol (using blue light stimulation) and the multiple-aliquot regenerativedose and additive-dose protocols (using broad-band blue and green light stimulation) are discussed. A comparison of radionuclide analyses using high-resolution gamma-spectrometry and neutron activation analysis is also presented. It is concluded that the two dosimetry methods give indistinguishable results, although the gamma spectrometry data are more precise. In contrast, the single-aliquot regenerative-dose protocol is between 2 and 10 times more precise than the multiple-aliquot protocols, and systematic di!erences in performance are also observed. The uncertainties in the dosimetry calculations are the largest contributor to the overall age uncertainties. The accuracy of the resulting ages is considered very satisfactory when tested against the independent age controls, over the entire age range from the present to 15 ka. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction This investigation compares three widely used approaches to the optically stimulated luminescence (OSL) dating of quartz extracted from geological sediments. Equivalent doses (D ) are estimated using the single aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000), the multiple-aliquot additive-dose (MAA) and the multiple-aliquot regenerative-dose (MAR) protocols. We also used both neutron activation analysis (NAA, undertaken at the Becquerel Laboratory, Sydney, Australia) and high-resolution gamma-spectrometry to estimate dose rates. Eleven samples were taken for luminescence dating from the dune section &PostduK ne' within the Eberswalde ice-marginal valley, north of Berlin in Brandenburg, northeastern Germany. The &PostduK ne' section was chosen for this study because independent age control (see Fig. 1) is provided by eight
夽
Paper published in December 2000. * Corresponding author. Fax: #0049-221-4705124. E-mail address: [email protected] (A. Hilgers).
C ages, the Aller+d &Finow' palaeosol and a layer of Laacher See tephra (12.88 ka varve years, Brauer et al., 1997). The &Finow' palaeosol is discussed by Schlaak (1993), and he details the reasons for ascribing an age range of about 12.9}14 ka (assumed timespan for the Aller+d interstadial from ice core-data from Greenland (Taylor et al., 1993)).
2. Sample preparation and analytical facilities All samples were prepared in subdued red light ('600 nm). After dry-sieving (100}200 lm), the samples were treated with hydrochloric acid, sodium oxalate and hydrogen peroxide in order to remove carbonates, clay and organic material. Solutions of sodium polytungstate (2.62 and 2.7 g cm\) were used to concentrate the quartz fraction. To avoid contamination with any feldspars and to remove the alpha-irradiated layer, the samples were etched in 40% HF for 40 min, followed by a further treatment with hydrochloric acid to remove any acidsoluble #uorides. Finally, the etched quartz was sieved again to provide 100}150 lm grains for dating.
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 5 0 - 0
732
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
Fig. 1. Location of the dune section &PostduK ne', description of the pro"le and sampling positions for C and luminescence dating (F1}F11). The horizontal distance from section A to section B is about 4 m. (modi"ed from Schlaak, 1999, C ages calibrated using CALIB-4.1)
All luminescence measurements were carried out on an automated Ris+ TL/OSL reader (TL-DA-12). For SAR measurements, the reader was equipped with a Sr/Y beta source delivering about 0.025 Gy s\. The aliquots used for the multiple-aliquot measurements were irradiated with a Co gamma source in groups of "ve subsamples with doses in the range 0.4}70 Gy. Thirteen aliquots were used to measure the natural signals in the multiple-aliquot protocols, and all aliquots used in these protocols were normalised using the luminescence signal induced by a brief 0.1 s stimulation with blue and green light. Subsamples used in the MAR protocol were bleached by exposing them to sunlight for several hours prior to irradiation. All multiple-aliquot measurements used a thermal pretreatment of 2203C for 300 s. Samples measured in the SAR protocol were heated to 2603C for 10 s, except for samples F1 and F2, for which a lower preheat of 2403C for 10 s was chosen to avoid thermal transfer e!ects (Murray and Clemmensen, 2001; Hilgers et al., in preparation). The quartz OSL signal was detected through an HA-3 "lter and either two U-340 "lters (MAA, MAR) or three U-340 "lters (SAR). Optical stimulation (100 s) was by blue light-emitting diodes (470$30 nm) for the SAR measurements (B+tter-Jensen et al., 1999), or by broadband blue and green light (420}550 nm) "ltered from a halogen lamp (B+tter-Jensen and Duller, 1992). All OSL signals were measured at 1253C to prevent retrapping in the 1103C TL trap (Wintle and Murray, 1997). All
calculations used the initial signal ("rst 0.4 s of stimulation, Murray and Wintle, 1998) with the average luminescence signal measured in the last 10 s of stimulation subtracted as background signal.
3. Equivalent dose estimation Representative SAR, MAA and MAR growth curves showing the determination of the equivalent dose for sample F5 are given in Fig. 2, and a summary of the D estimates presented in Table 1. All data points used in the MAA and MAR analyses have been normalised as discussed above. Despite this, the scatter is considerable, and the advantages of the SAR protocol, at least in terms of precision, are evident. Each SAR estimate of D in Table 1 is presented as a mean and standard error of at least 18 aliquots (cf. typically 48 used to provide only one average multiple aliquot estimate). The reproducibility of the SAR estimates of D , with relative standard deviations of 6}12%, argues for a homogeneous distribution of dose in the samples, and so supports the expected complete bleaching of the dune sediments during the aeolian transport. The signi"cant larger errors in the equivalent doses obtained for the MA measurements (&10}50%, see Table 1) are to be expected from the known large scatter in speci"c luminescence and growth curve shape (e.g. Duller et al., 2000). These sources of variability are not present in single-aliquot determinations.
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
733
Fig. 2. Comparison of growth curve obtained for (a) multiple-aliquot regenerative-dose protocol (13 natural (open circles) and 35 laboratory irradiated ("lled circles) aliquots of sample F5), (b) multiple-aliquot additive-dose protocol (13 natural (open circles) and 34 laboratory irradiated ("lled circles) aliquots of sample F5), (c) single-aliquot regenerative-dose protocol (one aliquot of sample F5, natural OSL-signal shown as horizontal dashed line), and (d) growth curve derived from (c) after correction for sensitivity changes using the OSL response to a test dose (Murray and Wintle, 2000). The insets in (c) and (d) present the lower regenerated doses for clarity. Open squares show OSL-signals detected after administering a zero dose (in the 1st, 13th, and 20th cycle) to monitor recuperation e!ects. Open triangles and diamond give the OSL-response on the lowest regenerated dose repeated in the 7th, 14th, and 21st cycles showing good reproducibility after correction and so indicating a successful application of the correction procedure.
Fig. 3. Comparison of estimates of equivalent doses (D ) from mul tiple-aliquot protocols (regenerative dose shown as "lled circles, additive dose shown as open circles) with those derived from the singlealiquot regenerative-dose protocol. The solid lines have a slope of one. The two lowest values are shown inset for clarity.
Fig. 3 presents the various multiple-aliquot estimates of D plotted against those derived from the SAR proto col. The larger uncertainties on the multiple-aliquot data make it impossible to state categorically that the dose estimates of the multiple-aliquot protocols di!er from the
SAR protocol in any individual case. Nevertheless systematic trends can be examined. At low doses (samples F1}F5) the ratio of MAR to SAR values is 1.04$0.02. However, at higher doses (samples F6}F11) this ratio increases to 1.32$0.04. Such a distinct systematic discrepancy between a protocol which makes no correction for sensitivity changes (MAR) and one which explicitly measures these changes and corrects for them (SAR) is not surprising. The sensitivity change observed between the SAR natural and the "rst regeneration cycle was by a factor of 1.08$0.01 (average of samples F1}11). This increase in sensitivity must have been present in the MAR data set as well, but was not detected in the MA measurement routine and was not corrected for. No such systematic tendency is detected in the MAA data (MAA/SAR"1.01$0.08), but it should be noted that the dose range of comparison is less.
4. Dose rate estimation Both neutron activation analysis (NAA) and highresolution gamma-ray spectrometry were used to
734
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
Table 1 Summary of average dose rates, equivalent doses, and resulting luminescence ages Sample
average dose rate, Gy ka\
SAR D in Gy
MAR D in Gy
MAA D in Gy
SAR, age in Ka
MAR, age in Ka
MAA, age in ka
F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11
1.40$0.08 1.27$0.08 1.23$0.08 1.20$0.08 1.20$0.08 1.18$0.08 1.33$0.08 1.36$0.14 1.37$0.14 1.56$0.11 1.20$0.08
0.193$0.006 0.347$0.004 3.74$0.08 5.36$0.12 9.4$0.3 11.1$0.2 13.7$0.4 14.6$0.4 15.1$0.2 18.5$0.4 18.0$0.5
0.19$0.11 0.34$0.10 4.1$0.5 5.9$0.7 9.7$1.3 13$2 18$2 21$4 21$3 23$4 25$5
0.23$0.05 0.34$0.03 3.1$0.2 6.9$1.2 8.0$0.7 10$2 NA NA NA NA NA
0.139$0.012 0.274$0.020 3.0$0.3 4.5$0.4 7.9$0.8 9.4$0.8 10.3$0.9 10.9$1.4 11.2$1.3 11.9$1.1 15.1$1.4
0.14$0.09 0.27$0.10 3.4$0.6 5.0$0.9 8.2$1.6 11.1$2.4 13.6$2.3 16$5 16$4 15$4 21$6
0.17$0.05 0.27$0.04 2.5$0.3 5.8$1.4 6.7$1.0 8.6$2.3
SAR, MAR and MAA protocols described in the text. NA * not available.
Table 2 Summary of radionuclide analyses, and ratios of derived beta and gamma dose rates Sample
U (Bq kg\) NAA
Gamma
Th (Bq kg\) NAA
Gamma
K (Bq kg\) NAA
Gamma
Beta dose ratio, NAA/Gamma
Gamma dose ratio, NAA/Gamma
F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11
6.07$0.67 4.52$0.50 4.91$0.54 3.75$0.56 4.91$0.54 6.07$0.67 4.01$0.60 19.4$1.0 19.6$1.0 9.30$2.33 4.52$1.36
6.5$3.2 10.6$3.0 10.4$2.2 2.7$3.2 6.8$1.8 5.7$1.5 6.0$3.2 23.5$5 13.7$3.0 15.2$3.7 7.4$2.4
9.12$0.46 8.14$0.41 6.27$0.31 6.31$0.32 4.88$0.24 4.92$0.25 7.41$0.37 11.4$0.6 12.5$0.6 15.9$0.8 7.12$0.36
9.28$0.26 8.06$0.24 6.01$0.32 6.02$0.24 5.08$0.14 4.91$0.22 6.68$0.24 11.5$0.4 11.5$0.5 14.3$0.3 6.60$0.35
291$15 260$13 270$14 254$13 274$14 268$13 316$16 326$16 338$17 307$15 276$14
302$7 274$7 265$11 288$7 281$5 279$9 312$8 334$9 327$14 331$8 270$12
0.95$0.13 0.91$0.12 0.99$0.15 0.90$0.12 0.97$0.12 0.98$0.13 1.00$0.14 1.01$0.13 1.10$0.15 0.91$0.12 1.00$0.15
0.92$0.05 0.90$0.05 0.98$0.07 0.89$0.05 0.96$0.06 1.00$0.06 0.98$0.06 1.09$0.06 1.13$0.07 0.93$0.07 0.98$0.08
measure the radionuclide concentrations of the sediments. Table 2 summarises these analyses. The results for thorium and potassium contents show quite good agreement for both techniques with averages of the ratios (NAA/gamma; n"11) of 1.04$0.02 for Th and 0.97$0.01 for K. Comparing the uncertainties of the analyses the gamma-ray spectrometry provides slightly more precise results than NAA for Th (average of the relative standard deviation 3.6% for gamma spectrometry and 5% for NAA) and K (2.9% for gamma and 5% for NAA). For both types of analyses the uranium contents yielded the results with the largest uncertainties (38% for gamma and 13% for NAA). Furthermore, the U determinations show a signi"cant discrepancy of both methods with an average ratio (U content NAA/gamma; n"11) of 0.83$0.10. However, it should be noted that the gamma spectrometry dose rates do not depend on the ; analyses; almost all of the U-series dose comes from
Ra, which is measured directly by gamma spectrometry with a mean relative standard deviation of &4%. The unweighted averages of the ratio of the 11 NAA based dose rates to those based on gamma spectrometry are 0.97$0.02 (beta) and 0.98$0.02 (gamma) giving an overall ratio of 0.98$0.02 (beta#gamma; n"11), and so we cannot safely conclude that there is any systematic di!erence between the two types of analyses. The relative standard deviation of the average of the total (NAA/gamma) dose rate ratio is 6%, comparable with the average of the estimated individual uncertainties on each ratio of 11%, which suggests that the individual estimates of analytical uncertainty are also reliable (both "gures exclude water content uncertainties). Table 1 summarises the weighted average dose rates for each sample (including the cosmic ray dose and water content e!ects, and the contribution from their uncertainties). The water content used for dose rate estimation is assumed from
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
735
measured values: the dose rates change by about 1.6%, if the assumed water content increases or decreases by 30% for the drier samples F1}F3 (water content 6$2%); for the wetter samples F8 and F9 (water content 30$10%) the relative change is about 7%. Table 1 also summarises the ages resulting from the estimates of D derived from the di!erent analytical pro tocols, in combination with the average dose rates. In all cases, the limiting factor on the MAR and MAA ages is the uncertainty on the estimate of D . In contrast, the limiting factor in all the SAR ages is the uncertainty on the dose rate. We must now turn to the independent age controls to assess the overall reliability of these uncertainties.
5. Discussion and conclusion In Fig. 4 the luminescence ages based on the SAR protocol are compared with the C ages, and the &Finow' palaeosol and Laacher See tephra chronometric markers. It should be emphasised here that the C samples and the luminescence samples were taken at di!erent times and from di!erent locations in the dune system. This comparison thus includes any additional uncertainty in the "eld extrapolation of stratigraphic relationships. Nevertheless, with the exception of the age for sample F6, the stratigraphy/age relationship between the luminescence ages and the independent chronological controls is excellent. (Note that there is no stratigraphic uncertainty in the case of the &Finow' palaeosol or the Laacher See tephra, which were unambiguously identi"ed at the time the luminescence samples were taken.) The youngest luminescence dates (F1, 139$12 years and F2, 274$20 years) are entirely consistent with the immediately underlying C age of 590$80 years (Fig. 1). It is di$cult to place a limit on how young these samples could be, but the immediately overlying vegetation has stabilised with the accompaniment of weak podzol development; there are no recent accounts of sand mobilisation in this area and the dune is now located in a forest. It seems very likely that the last mobilisation is at least many decades old. We consider that these young ages provide strong evidence that the resetting process in these sediments was complete, and that the last mobilisation of these sediments probably took place towards the end of the Little Ice Age. Similar late Little Ice Age results have been reported using the SAR protocol from the top of an aeolian sand sequence in western Denmark by Murray and Clemmensen (2001). They were also able to obtain a sample known to be younger than 35 years, for which they obtained an age of 36$5 years. Samples F10 and F11 (11.9$1.1 and 15.1$1.4 ka) encompass the &Finow' palaeosol, which Schlaak (1993) argues must have formed in the Aller+d interstadial (12.9}14 ka, Taylor et al., 1993). This is supported
Fig. 4. Comparison of luminescence ages and C ages with sampling depth. The assumed Aller+d-age of the &Finow' palaeosol and the Laacher See tephra age are also shown.
by the two adjacent C ages from this layer, of 12.0$0.8 and 13.4$0.4 ka (see Fig. 1). Sample F9 (11.2$1.3 ka) was taken from the dune sands immediately above the layer of Laacher See tephra (12.88 ka; Brauer et al., 1997) and peat with Younger Dryas (Schlaak, 1993; 11.65}12.85 ka, Taylor et al., 1993) pollen assemblages. The good agreement between our OSL ages and the independent controls in these strata also supports the assumption that these aeolian sands were well bleached prior to deposition. The OSL ages also date this onset of aeolian remobilisation to the Younger Dryas/Holocene transition, a period known to include high aeolian activity in the northern European sandbelt (e.g. Koster, 1988; Radtke, 1998). The OSL ages show that this Late Glacial aeolian sedimentation continued into the Holocene in the Eberswalde valley, probably assisted by human impact on the landscape (e.g. forest clearing); similar conclusions have been drawn at other sites within the sandbelt in northern Germany (e.g. Radtke, 1998). The alternation of aeolian sand and organic rich layers documents the alternation of phases of remobilisation and stabilisation of dune sand deposits. That this process continued until recent times is shown by samples F1 and F2. It would be unwise to assume that the apparent break in sedimentation between F3 (3.0$0.3 ka) and F2 (274$20 a) is real; the two sections are not connected (horizontal separation of about 4 m), and some stripping may have occurred, especially at Section B (Fig. 1). Except for one outlier (F6), we conclude that our data show that OSL ages obtained using the SAR protocol are both precise and accurate. Stratigraphic uncertainty may have contributed to this single inconsistency. The limiting factor in the uncertainties on our OSL ages arises
736
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
from the dose rate determinations, and these in turn are limited, for most samples investigated in this study, by uncertainties on radionuclide analyses. For the drier samples (water contents of &6% or less; all samples except F8}F10) the overall uncertainty is reduced by only &4% when the water content uncertainty is assumed to be zero. For the three wetter samples (water contents of 10}30%) the contribution to overall uncertainity from the assumption about water contents become more signi"cant (reduction of the overall error by &33%), but not dominating. If OSL ages on Lateglacial and Holocene aeolian material are to become signi"cantly more precise, then attention must be paid to these sources of uncertainty. Only then can the accuracy be further tested.
Acknowledgements This work was partly funded by the Deutsche Forschungsgemeinschaft (DFG, grant Ra 383/7-1). Our thanks to Prof. Dr. G. Schmitt and B. Bannach for providing access to the Co gamma source at the Department of Nuclear Medicine at the University of DuK sseldorf. The visit of A.H. to Ris+ National Laboratory was funded by the `KaK the-Hacka Foundation (University of Cologne).
References B+tter-Jensen, L., Duller, G.A.T., 1992. A new system for measuring OSL from quartz samples. Nuclear Tracks Radiation Measurements 20, 549}553. B+tter-Jensen, L., Mejdahl, V., Murray, A.S., 1999. New light on OSL. Quaternary Science Reviews (Quaternary Geochronology) 18, 303}309.
Brauer, A., Endres, C., Negendank, J.F.W., 1997. Lake Meerfelder Maar annually laminated record - varve chronology and Late Glacial and early Holocene environmental changes. WuK rzburger Geographische Manuskripte 41, 39}40. Duller, G.A.T., B+tter-Jensen, L., Murray, A.S., Truscott, A.J., 2000. Optical dating of single sand-sized grains of quartz: sources of variability. Radiation Measurements, 32, 453}457. Hilgers, A., Murray, A.S., Schlaak, N., Radtke, U., Luminescence dating of Late Glacial and Holocene dune sands from Brandenburg, north-eastern Germany, using a single-aliquot regenerative-dose protocol for quartz. Earth and Planetary Science Letters, submitted for publication. Koster, E.A., 1988. Ancient and modern cold-climate aeolian sand deposition: a review. Journal of Quaternary Science 3, 69}83. Murray, A.S., Clemmensen, L., 2001. Luminescence dating of Holocene aeolian sand movement, Thy, Denmark. Quaternary Science Reviews (Quaternary Geochronology) 20, 751}754. Murray, A.S., Wintle, A.G., 1998. Factors controlling the shape of the OSL decay curve in Quartz. Radiation Measurements 29, 65}79. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57}73. Radtke, U., (Ed.), 1998. Lumineszenzdatierung aK olischer Sedimente * BeitraK ge zur Genese und Altersstellung jungquartaK rer DuK nen und LoK sse in Deutschland. KoK lner Geographische Arbeiten 70, 124 p. Schlaak, N., 1999. PostduK ne (Eberswalder Urstromtal, Exkursionspunkt 3). In: Schmidt, R., Bork, H.-R., Fischer-Zujkov, U. (Eds.): PalaK oboK den und Kolluvien auf glazialen Sedimenten Nordostdeutschlands. Zalf-Bericht 37, 62}69. Schlaak, N., 1993. Studie zur Landschaftsgenese im Raum Nordbarnim und Eberswalder Urstromtal. Berliner Geographische Arbeiten 76, 145 p. Taylor, K.C., Lamorey, G.W., Doyle, G.A., Alley, R.B., Grootes, P.M., Mayewski, P.A., White, J.W.C., Barlow, L.K., 1993. The &#ickering switch' of late Pleistocene climate change. Nature 361, 432}436. Wintle, A.G., Murray, A.S., 1997. The relationship between quartz thermoluminescence, photo-transferred thermoluminescence, and optically stimulated luminescence. Radiation Measurements 27, 611}624.
Comparison of quartz OSL protocols using Lateglacial and Holocene dune sands from Brandenburg, Germany夽 A. Hilgers *, A.S. Murray, N. Schlaak, U. Radtke Department of Geography, University of Cologne, Albertus-Magnus-Platz, D-50923 Cologne, Germany Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University, Ris~ National Laboratory, DK-4000 Roskilde, Denmark Hochstrasse 13, D-16244 Altenhof, Germany
Abstract We present the results obtained using various di!erent protocols in the quartz optically stimulated luminescence dating of a Late Glacial and Holocene aeolian dune sequence near Eberswalde, north of Berlin, Germany. The Aller+d &Finow' palaeosol and the Laacher See tephra layer serve as chronostratigraphic markers, and further independent age control is provided by eight C ages. The application of the single-aliquot regenerative-dose protocol (using blue light stimulation) and the multiple-aliquot regenerativedose and additive-dose protocols (using broad-band blue and green light stimulation) are discussed. A comparison of radionuclide analyses using high-resolution gamma-spectrometry and neutron activation analysis is also presented. It is concluded that the two dosimetry methods give indistinguishable results, although the gamma spectrometry data are more precise. In contrast, the single-aliquot regenerative-dose protocol is between 2 and 10 times more precise than the multiple-aliquot protocols, and systematic di!erences in performance are also observed. The uncertainties in the dosimetry calculations are the largest contributor to the overall age uncertainties. The accuracy of the resulting ages is considered very satisfactory when tested against the independent age controls, over the entire age range from the present to 15 ka. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction This investigation compares three widely used approaches to the optically stimulated luminescence (OSL) dating of quartz extracted from geological sediments. Equivalent doses (D ) are estimated using the single aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000), the multiple-aliquot additive-dose (MAA) and the multiple-aliquot regenerative-dose (MAR) protocols. We also used both neutron activation analysis (NAA, undertaken at the Becquerel Laboratory, Sydney, Australia) and high-resolution gamma-spectrometry to estimate dose rates. Eleven samples were taken for luminescence dating from the dune section &PostduK ne' within the Eberswalde ice-marginal valley, north of Berlin in Brandenburg, northeastern Germany. The &PostduK ne' section was chosen for this study because independent age control (see Fig. 1) is provided by eight
夽
Paper published in December 2000. * Corresponding author. Fax: #0049-221-4705124. E-mail address: [email protected] (A. Hilgers).
C ages, the Aller+d &Finow' palaeosol and a layer of Laacher See tephra (12.88 ka varve years, Brauer et al., 1997). The &Finow' palaeosol is discussed by Schlaak (1993), and he details the reasons for ascribing an age range of about 12.9}14 ka (assumed timespan for the Aller+d interstadial from ice core-data from Greenland (Taylor et al., 1993)).
2. Sample preparation and analytical facilities All samples were prepared in subdued red light ('600 nm). After dry-sieving (100}200 lm), the samples were treated with hydrochloric acid, sodium oxalate and hydrogen peroxide in order to remove carbonates, clay and organic material. Solutions of sodium polytungstate (2.62 and 2.7 g cm\) were used to concentrate the quartz fraction. To avoid contamination with any feldspars and to remove the alpha-irradiated layer, the samples were etched in 40% HF for 40 min, followed by a further treatment with hydrochloric acid to remove any acidsoluble #uorides. Finally, the etched quartz was sieved again to provide 100}150 lm grains for dating.
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 5 0 - 0
732
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
Fig. 1. Location of the dune section &PostduK ne', description of the pro"le and sampling positions for C and luminescence dating (F1}F11). The horizontal distance from section A to section B is about 4 m. (modi"ed from Schlaak, 1999, C ages calibrated using CALIB-4.1)
All luminescence measurements were carried out on an automated Ris+ TL/OSL reader (TL-DA-12). For SAR measurements, the reader was equipped with a Sr/Y beta source delivering about 0.025 Gy s\. The aliquots used for the multiple-aliquot measurements were irradiated with a Co gamma source in groups of "ve subsamples with doses in the range 0.4}70 Gy. Thirteen aliquots were used to measure the natural signals in the multiple-aliquot protocols, and all aliquots used in these protocols were normalised using the luminescence signal induced by a brief 0.1 s stimulation with blue and green light. Subsamples used in the MAR protocol were bleached by exposing them to sunlight for several hours prior to irradiation. All multiple-aliquot measurements used a thermal pretreatment of 2203C for 300 s. Samples measured in the SAR protocol were heated to 2603C for 10 s, except for samples F1 and F2, for which a lower preheat of 2403C for 10 s was chosen to avoid thermal transfer e!ects (Murray and Clemmensen, 2001; Hilgers et al., in preparation). The quartz OSL signal was detected through an HA-3 "lter and either two U-340 "lters (MAA, MAR) or three U-340 "lters (SAR). Optical stimulation (100 s) was by blue light-emitting diodes (470$30 nm) for the SAR measurements (B+tter-Jensen et al., 1999), or by broadband blue and green light (420}550 nm) "ltered from a halogen lamp (B+tter-Jensen and Duller, 1992). All OSL signals were measured at 1253C to prevent retrapping in the 1103C TL trap (Wintle and Murray, 1997). All
calculations used the initial signal ("rst 0.4 s of stimulation, Murray and Wintle, 1998) with the average luminescence signal measured in the last 10 s of stimulation subtracted as background signal.
3. Equivalent dose estimation Representative SAR, MAA and MAR growth curves showing the determination of the equivalent dose for sample F5 are given in Fig. 2, and a summary of the D estimates presented in Table 1. All data points used in the MAA and MAR analyses have been normalised as discussed above. Despite this, the scatter is considerable, and the advantages of the SAR protocol, at least in terms of precision, are evident. Each SAR estimate of D in Table 1 is presented as a mean and standard error of at least 18 aliquots (cf. typically 48 used to provide only one average multiple aliquot estimate). The reproducibility of the SAR estimates of D , with relative standard deviations of 6}12%, argues for a homogeneous distribution of dose in the samples, and so supports the expected complete bleaching of the dune sediments during the aeolian transport. The signi"cant larger errors in the equivalent doses obtained for the MA measurements (&10}50%, see Table 1) are to be expected from the known large scatter in speci"c luminescence and growth curve shape (e.g. Duller et al., 2000). These sources of variability are not present in single-aliquot determinations.
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
733
Fig. 2. Comparison of growth curve obtained for (a) multiple-aliquot regenerative-dose protocol (13 natural (open circles) and 35 laboratory irradiated ("lled circles) aliquots of sample F5), (b) multiple-aliquot additive-dose protocol (13 natural (open circles) and 34 laboratory irradiated ("lled circles) aliquots of sample F5), (c) single-aliquot regenerative-dose protocol (one aliquot of sample F5, natural OSL-signal shown as horizontal dashed line), and (d) growth curve derived from (c) after correction for sensitivity changes using the OSL response to a test dose (Murray and Wintle, 2000). The insets in (c) and (d) present the lower regenerated doses for clarity. Open squares show OSL-signals detected after administering a zero dose (in the 1st, 13th, and 20th cycle) to monitor recuperation e!ects. Open triangles and diamond give the OSL-response on the lowest regenerated dose repeated in the 7th, 14th, and 21st cycles showing good reproducibility after correction and so indicating a successful application of the correction procedure.
Fig. 3. Comparison of estimates of equivalent doses (D ) from mul tiple-aliquot protocols (regenerative dose shown as "lled circles, additive dose shown as open circles) with those derived from the singlealiquot regenerative-dose protocol. The solid lines have a slope of one. The two lowest values are shown inset for clarity.
Fig. 3 presents the various multiple-aliquot estimates of D plotted against those derived from the SAR proto col. The larger uncertainties on the multiple-aliquot data make it impossible to state categorically that the dose estimates of the multiple-aliquot protocols di!er from the
SAR protocol in any individual case. Nevertheless systematic trends can be examined. At low doses (samples F1}F5) the ratio of MAR to SAR values is 1.04$0.02. However, at higher doses (samples F6}F11) this ratio increases to 1.32$0.04. Such a distinct systematic discrepancy between a protocol which makes no correction for sensitivity changes (MAR) and one which explicitly measures these changes and corrects for them (SAR) is not surprising. The sensitivity change observed between the SAR natural and the "rst regeneration cycle was by a factor of 1.08$0.01 (average of samples F1}11). This increase in sensitivity must have been present in the MAR data set as well, but was not detected in the MA measurement routine and was not corrected for. No such systematic tendency is detected in the MAA data (MAA/SAR"1.01$0.08), but it should be noted that the dose range of comparison is less.
4. Dose rate estimation Both neutron activation analysis (NAA) and highresolution gamma-ray spectrometry were used to
734
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
Table 1 Summary of average dose rates, equivalent doses, and resulting luminescence ages Sample
average dose rate, Gy ka\
SAR D in Gy
MAR D in Gy
MAA D in Gy
SAR, age in Ka
MAR, age in Ka
MAA, age in ka
F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11
1.40$0.08 1.27$0.08 1.23$0.08 1.20$0.08 1.20$0.08 1.18$0.08 1.33$0.08 1.36$0.14 1.37$0.14 1.56$0.11 1.20$0.08
0.193$0.006 0.347$0.004 3.74$0.08 5.36$0.12 9.4$0.3 11.1$0.2 13.7$0.4 14.6$0.4 15.1$0.2 18.5$0.4 18.0$0.5
0.19$0.11 0.34$0.10 4.1$0.5 5.9$0.7 9.7$1.3 13$2 18$2 21$4 21$3 23$4 25$5
0.23$0.05 0.34$0.03 3.1$0.2 6.9$1.2 8.0$0.7 10$2 NA NA NA NA NA
0.139$0.012 0.274$0.020 3.0$0.3 4.5$0.4 7.9$0.8 9.4$0.8 10.3$0.9 10.9$1.4 11.2$1.3 11.9$1.1 15.1$1.4
0.14$0.09 0.27$0.10 3.4$0.6 5.0$0.9 8.2$1.6 11.1$2.4 13.6$2.3 16$5 16$4 15$4 21$6
0.17$0.05 0.27$0.04 2.5$0.3 5.8$1.4 6.7$1.0 8.6$2.3
SAR, MAR and MAA protocols described in the text. NA * not available.
Table 2 Summary of radionuclide analyses, and ratios of derived beta and gamma dose rates Sample
U (Bq kg\) NAA
Gamma
Th (Bq kg\) NAA
Gamma
K (Bq kg\) NAA
Gamma
Beta dose ratio, NAA/Gamma
Gamma dose ratio, NAA/Gamma
F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11
6.07$0.67 4.52$0.50 4.91$0.54 3.75$0.56 4.91$0.54 6.07$0.67 4.01$0.60 19.4$1.0 19.6$1.0 9.30$2.33 4.52$1.36
6.5$3.2 10.6$3.0 10.4$2.2 2.7$3.2 6.8$1.8 5.7$1.5 6.0$3.2 23.5$5 13.7$3.0 15.2$3.7 7.4$2.4
9.12$0.46 8.14$0.41 6.27$0.31 6.31$0.32 4.88$0.24 4.92$0.25 7.41$0.37 11.4$0.6 12.5$0.6 15.9$0.8 7.12$0.36
9.28$0.26 8.06$0.24 6.01$0.32 6.02$0.24 5.08$0.14 4.91$0.22 6.68$0.24 11.5$0.4 11.5$0.5 14.3$0.3 6.60$0.35
291$15 260$13 270$14 254$13 274$14 268$13 316$16 326$16 338$17 307$15 276$14
302$7 274$7 265$11 288$7 281$5 279$9 312$8 334$9 327$14 331$8 270$12
0.95$0.13 0.91$0.12 0.99$0.15 0.90$0.12 0.97$0.12 0.98$0.13 1.00$0.14 1.01$0.13 1.10$0.15 0.91$0.12 1.00$0.15
0.92$0.05 0.90$0.05 0.98$0.07 0.89$0.05 0.96$0.06 1.00$0.06 0.98$0.06 1.09$0.06 1.13$0.07 0.93$0.07 0.98$0.08
measure the radionuclide concentrations of the sediments. Table 2 summarises these analyses. The results for thorium and potassium contents show quite good agreement for both techniques with averages of the ratios (NAA/gamma; n"11) of 1.04$0.02 for Th and 0.97$0.01 for K. Comparing the uncertainties of the analyses the gamma-ray spectrometry provides slightly more precise results than NAA for Th (average of the relative standard deviation 3.6% for gamma spectrometry and 5% for NAA) and K (2.9% for gamma and 5% for NAA). For both types of analyses the uranium contents yielded the results with the largest uncertainties (38% for gamma and 13% for NAA). Furthermore, the U determinations show a signi"cant discrepancy of both methods with an average ratio (U content NAA/gamma; n"11) of 0.83$0.10. However, it should be noted that the gamma spectrometry dose rates do not depend on the ; analyses; almost all of the U-series dose comes from
Ra, which is measured directly by gamma spectrometry with a mean relative standard deviation of &4%. The unweighted averages of the ratio of the 11 NAA based dose rates to those based on gamma spectrometry are 0.97$0.02 (beta) and 0.98$0.02 (gamma) giving an overall ratio of 0.98$0.02 (beta#gamma; n"11), and so we cannot safely conclude that there is any systematic di!erence between the two types of analyses. The relative standard deviation of the average of the total (NAA/gamma) dose rate ratio is 6%, comparable with the average of the estimated individual uncertainties on each ratio of 11%, which suggests that the individual estimates of analytical uncertainty are also reliable (both "gures exclude water content uncertainties). Table 1 summarises the weighted average dose rates for each sample (including the cosmic ray dose and water content e!ects, and the contribution from their uncertainties). The water content used for dose rate estimation is assumed from
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
735
measured values: the dose rates change by about 1.6%, if the assumed water content increases or decreases by 30% for the drier samples F1}F3 (water content 6$2%); for the wetter samples F8 and F9 (water content 30$10%) the relative change is about 7%. Table 1 also summarises the ages resulting from the estimates of D derived from the di!erent analytical pro tocols, in combination with the average dose rates. In all cases, the limiting factor on the MAR and MAA ages is the uncertainty on the estimate of D . In contrast, the limiting factor in all the SAR ages is the uncertainty on the dose rate. We must now turn to the independent age controls to assess the overall reliability of these uncertainties.
5. Discussion and conclusion In Fig. 4 the luminescence ages based on the SAR protocol are compared with the C ages, and the &Finow' palaeosol and Laacher See tephra chronometric markers. It should be emphasised here that the C samples and the luminescence samples were taken at di!erent times and from di!erent locations in the dune system. This comparison thus includes any additional uncertainty in the "eld extrapolation of stratigraphic relationships. Nevertheless, with the exception of the age for sample F6, the stratigraphy/age relationship between the luminescence ages and the independent chronological controls is excellent. (Note that there is no stratigraphic uncertainty in the case of the &Finow' palaeosol or the Laacher See tephra, which were unambiguously identi"ed at the time the luminescence samples were taken.) The youngest luminescence dates (F1, 139$12 years and F2, 274$20 years) are entirely consistent with the immediately underlying C age of 590$80 years (Fig. 1). It is di$cult to place a limit on how young these samples could be, but the immediately overlying vegetation has stabilised with the accompaniment of weak podzol development; there are no recent accounts of sand mobilisation in this area and the dune is now located in a forest. It seems very likely that the last mobilisation is at least many decades old. We consider that these young ages provide strong evidence that the resetting process in these sediments was complete, and that the last mobilisation of these sediments probably took place towards the end of the Little Ice Age. Similar late Little Ice Age results have been reported using the SAR protocol from the top of an aeolian sand sequence in western Denmark by Murray and Clemmensen (2001). They were also able to obtain a sample known to be younger than 35 years, for which they obtained an age of 36$5 years. Samples F10 and F11 (11.9$1.1 and 15.1$1.4 ka) encompass the &Finow' palaeosol, which Schlaak (1993) argues must have formed in the Aller+d interstadial (12.9}14 ka, Taylor et al., 1993). This is supported
Fig. 4. Comparison of luminescence ages and C ages with sampling depth. The assumed Aller+d-age of the &Finow' palaeosol and the Laacher See tephra age are also shown.
by the two adjacent C ages from this layer, of 12.0$0.8 and 13.4$0.4 ka (see Fig. 1). Sample F9 (11.2$1.3 ka) was taken from the dune sands immediately above the layer of Laacher See tephra (12.88 ka; Brauer et al., 1997) and peat with Younger Dryas (Schlaak, 1993; 11.65}12.85 ka, Taylor et al., 1993) pollen assemblages. The good agreement between our OSL ages and the independent controls in these strata also supports the assumption that these aeolian sands were well bleached prior to deposition. The OSL ages also date this onset of aeolian remobilisation to the Younger Dryas/Holocene transition, a period known to include high aeolian activity in the northern European sandbelt (e.g. Koster, 1988; Radtke, 1998). The OSL ages show that this Late Glacial aeolian sedimentation continued into the Holocene in the Eberswalde valley, probably assisted by human impact on the landscape (e.g. forest clearing); similar conclusions have been drawn at other sites within the sandbelt in northern Germany (e.g. Radtke, 1998). The alternation of aeolian sand and organic rich layers documents the alternation of phases of remobilisation and stabilisation of dune sand deposits. That this process continued until recent times is shown by samples F1 and F2. It would be unwise to assume that the apparent break in sedimentation between F3 (3.0$0.3 ka) and F2 (274$20 a) is real; the two sections are not connected (horizontal separation of about 4 m), and some stripping may have occurred, especially at Section B (Fig. 1). Except for one outlier (F6), we conclude that our data show that OSL ages obtained using the SAR protocol are both precise and accurate. Stratigraphic uncertainty may have contributed to this single inconsistency. The limiting factor in the uncertainties on our OSL ages arises
736
A. Hilgers et al. / Quaternary Science Reviews 20 (2001) 731}736
from the dose rate determinations, and these in turn are limited, for most samples investigated in this study, by uncertainties on radionuclide analyses. For the drier samples (water contents of &6% or less; all samples except F8}F10) the overall uncertainty is reduced by only &4% when the water content uncertainty is assumed to be zero. For the three wetter samples (water contents of 10}30%) the contribution to overall uncertainity from the assumption about water contents become more signi"cant (reduction of the overall error by &33%), but not dominating. If OSL ages on Lateglacial and Holocene aeolian material are to become signi"cantly more precise, then attention must be paid to these sources of uncertainty. Only then can the accuracy be further tested.
Acknowledgements This work was partly funded by the Deutsche Forschungsgemeinschaft (DFG, grant Ra 383/7-1). Our thanks to Prof. Dr. G. Schmitt and B. Bannach for providing access to the Co gamma source at the Department of Nuclear Medicine at the University of DuK sseldorf. The visit of A.H. to Ris+ National Laboratory was funded by the `KaK the-Hacka Foundation (University of Cologne).
References B+tter-Jensen, L., Duller, G.A.T., 1992. A new system for measuring OSL from quartz samples. Nuclear Tracks Radiation Measurements 20, 549}553. B+tter-Jensen, L., Mejdahl, V., Murray, A.S., 1999. New light on OSL. Quaternary Science Reviews (Quaternary Geochronology) 18, 303}309.
Brauer, A., Endres, C., Negendank, J.F.W., 1997. Lake Meerfelder Maar annually laminated record - varve chronology and Late Glacial and early Holocene environmental changes. WuK rzburger Geographische Manuskripte 41, 39}40. Duller, G.A.T., B+tter-Jensen, L., Murray, A.S., Truscott, A.J., 2000. Optical dating of single sand-sized grains of quartz: sources of variability. Radiation Measurements, 32, 453}457. Hilgers, A., Murray, A.S., Schlaak, N., Radtke, U., Luminescence dating of Late Glacial and Holocene dune sands from Brandenburg, north-eastern Germany, using a single-aliquot regenerative-dose protocol for quartz. Earth and Planetary Science Letters, submitted for publication. Koster, E.A., 1988. Ancient and modern cold-climate aeolian sand deposition: a review. Journal of Quaternary Science 3, 69}83. Murray, A.S., Clemmensen, L., 2001. Luminescence dating of Holocene aeolian sand movement, Thy, Denmark. Quaternary Science Reviews (Quaternary Geochronology) 20, 751}754. Murray, A.S., Wintle, A.G., 1998. Factors controlling the shape of the OSL decay curve in Quartz. Radiation Measurements 29, 65}79. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57}73. Radtke, U., (Ed.), 1998. Lumineszenzdatierung aK olischer Sedimente * BeitraK ge zur Genese und Altersstellung jungquartaK rer DuK nen und LoK sse in Deutschland. KoK lner Geographische Arbeiten 70, 124 p. Schlaak, N., 1999. PostduK ne (Eberswalder Urstromtal, Exkursionspunkt 3). In: Schmidt, R., Bork, H.-R., Fischer-Zujkov, U. (Eds.): PalaK oboK den und Kolluvien auf glazialen Sedimenten Nordostdeutschlands. Zalf-Bericht 37, 62}69. Schlaak, N., 1993. Studie zur Landschaftsgenese im Raum Nordbarnim und Eberswalder Urstromtal. Berliner Geographische Arbeiten 76, 145 p. Taylor, K.C., Lamorey, G.W., Doyle, G.A., Alley, R.B., Grootes, P.M., Mayewski, P.A., White, J.W.C., Barlow, L.K., 1993. The &#ickering switch' of late Pleistocene climate change. Nature 361, 432}436. Wintle, A.G., Murray, A.S., 1997. The relationship between quartz thermoluminescence, photo-transferred thermoluminescence, and optically stimulated luminescence. Radiation Measurements 27, 611}624.