Infrared stimulated luminescence dating of an Eemian (MIS 5e) site inDenmark using K-feldspar

JAN-PIETER BUYLAERT, SEBASTIEN HUOT, ANDREW S. MURRAY AND PETER VAN DEN HAUTE

BOREAS Buylaert, J.-P., Huot, S., Murray, A.S. & Van den haute, P. 2011 (January): Infrared stimulated luminescencedating of an Eemian (MIS 5e) site in Denmark using K-feldspar. Boreas, Vol. 40, pp. 46–56. 10.1111/j.1502-3885.2010.00156.x. ISSN 0300-9483.

Infrared stimulated luminescence (IRSL) dating of K-feldspars may be an alternative to quartz optically stimu-lated luminescence (OSL) dating when the quartz OSL signal is too close to saturation or when the quartz lumi-nescence characteristics are unsuitable. In this paper, Eemian (MIS 5e) coastal marine sands exposed in a cliffsection on the coast of southern Jutland (Denmark) are used to test the accuracy and precision of IRSL datingusing K-feldspars. This material has been used previously to test quartz OSL dating (Murray & Funder 2003): asmall systematic underestimation ofo10% compared to the expected age of�130 ka was reported. In our study, asingle-aliquot regenerative-dose (SAR) IRSL protocol is used to determine values of equivalent dose (De) and thecorresponding fading rates (g values). A significant age underestimation (of up to �35%) is observed; this is at-tributed to anomalous fading. Using a single site-average fading rate of 3.66�0.09%/decade to correct the IRSLages for all samples provides good agreement between the average fading-corrected K-feldspar age (119�6 ka) andthe independent age control (132–125ka). This is despite the reservations of Huntley & Lamothe (2001) that theirfading correction method is not expected to work on samples older than�20–50 ka. This fading-corrected feldsparresult is not significantly different from the overall revised quartz age (114�7 ka) also presented here. We concludethat fading-corrected IRSL ages measured using K-feldspar may be both precise and accurate over a greater agerange than might be otherwise expected.

Jan-Pieter Buylaert (e-mail: jabu@risoe.dtu.dk), Nordic Laboratory for Luminescence Dating, Department of EarthSciences, Aarhus University, Risø DTU, DK-4000 Roskilde, Denmark, and Laboratory ofMineralogy and Petrology,Geological Institute, Ghent University, B-9000 Ghent, Belgium, and Radiation Research Department, Risø NationalLaboratory for Sustainable Energy, Technical University of Denmark, P.O. 49, DK-4000 Roskilde, Denmark; Se-bastien Huot and Andrew S. Murray, Nordic Laboratory for Luminescence Dating, Department of Earth Sciences,Aarhus University, Risø DTU, DK-4000 Roskilde, Denmark; Peter Van den haute, Laboratory of Mineralogy andPetrology, Geological Institute, Ghent University, B-9000 Ghent, Belgium; received 28th December 2009, accepted25th February 2010.

The optically stimulated luminescence (OSL) signalfrom quartz has been found to provide reliable deposi-tional ages from a few years to several tens of thousandyears for a wide variety of sediments (e.g. Murray &Olley 2002; Duller 2004). It is known to be readily resetby daylight, to have a laboratory-measured thermalstability of 4108 years, and no athermal instability hasbeen reported. Nevertheless, some studies suggest thatolder quartz OSL dates have a tendency to under-estimate the true depositional age (Murray & Funder2003; Stokes et al. 2003; Schokker et al. 2005; Watanukiet al. 2005; Buylaert et al. 2007; Murray et al. 2007;Wallinga et al. 2007; Houmark-Nielsen 2008; Larsenet al. 2009a, b). It is also widely accepted that quartzcannot be used to derive ages much older than�200 ka,because of saturation of the dose–response curve (e.g.Wintle & Murray 2006). The infrared stimulated lumi-nescence (IRSL) signal from feldspar continues to growwith dose to much higher values than quartz and thushas the potential to date older material. Furthermore,feldspar IRSL can be used when the OSL character-istics of quartz are unsuitable (e.g. low sensitivity, orabsence of a strong fast component). However, theIRSL signal suffers from two main disadvantages: (i)when exposed to daylight, the latent IRSL signal tendsto decrease more slowly than the OSL from quartz (e.g.

Klasen et al. 2006); and (ii) the signal shows athermalfading (Spooner 1994; Aitken 1998).

The undesirable effect of athermal fading is that itcauses the IRSL (or TL) age to underestimate the truedepositional age of the sample. For many years, multi-ple aliquot methods were used to date sediments usingfeldspars (Aitken 1998), but the precision of this ap-proach is limited because of inter-aliquot reproduci-bility and, in the case of additive-dose procedures,extrapolation; this is especially true for sand-sizedgrains. Until Huntley & Lamothe (2001) there wasno widely accepted model with which to correct theapparent ages for fading. This and other correctionmethods have since been proposed, and they seem toyield accurate ages for samples younger than�20–50 ka. The development of single-aliquot proto-cols, initially for quartz (Murray & Wintle 2000) andlater for feldspars (Wallinga et al. 2000b; Auclair et al.2003; Huot & Lamothe 2003; Lamothe 2004), has nowmade it possible to measure values of equivalent dose(De) and fading rate (g) more easily than before, andwith much better precision. Single-aliquot IRSL dating,using the blue(-violet) emission band and incorporatingcorrection for anomalous fading, has now been em-ployed in several dating studies (Auclair et al. 2007;Buylaert et al. 2007; Wallinga et al. 2007; Cunha et al.

DOI 10.1111/j.1502-3885.2010.00156.x r 2010 The Authors, Journal compilation r 2010 The Boreas Collegium

2008). However, only a few of these studies have well-known age control, and so it is still difficult to be surethat this approach to dating is accurate.

Before the IRSL signal can be used routinely to datebeyond the quartz OSL age range, it is importantto show that IRSL-derived ages are reliable. As a con-tribution to this testing, we use Eemian (Marine Iso-tope Stage 5e, MIS 5e) coastal marine sands exposedin a cliff section on the coast of southern Jutland(Denmark) to test the accuracy and precision of agesderived using the IRSL from K-feldspars. These sandsprovide a well-known independent age control(132–125 ka; Funder et al. 2002; Murray & Funder2003; Lambeck et al. 2006). The Huntley & Lamothe(2001) correction model is used to correct for anom-alous fading. Huntley & Lamothe warn against the useof their model on samples for which the natural signallies beyond the ‘linear’ part of the dose–response curve,although no clear limit in terms of dose is given (theystate that the samples should not be older than�20–50 ka); the dose/age limits to the applicability ofthis correction method have not been tested.

Here we first describe the IRSL characteristics ofthe material, and then use single-aliquot methods todetermine values of both De and g. We test whetherthere is any apparent benefit in making corrections forfading on an individual aliquot basis, or whether simi-

lar precision can be obtained by using sample- or site-averaged estimates of fading rate. Finally, our IRSLages are corrected for anomalous fading, and are thencompared with the known site age, and with previouslypublished quartz ages.

Site description and age control

The Brunbjerg section investigated in this study isthe northernmost section accessible at the cliffs atGammelmark, in southern Jutland (Denmark). Thesection is about 25m high and reveals a succession ofLate Saalian and Eemian events (Fig. 1). Palaeoecolo-gical evidence provides a well-defined age control at thissite, and full details are discussed in Murray & Funder(2003) and Funder et al. (2002); here, we brieflydescribe the stratigraphy and expected age range putforward by these authors. Above the Late Saalian tillare freshwater sands (between log numbers 25 and 18;Fig. 1) that were deposited shortly before the start ofthe Eemian (132 ka). The overlying lacustrine Anodontaclay and sand represent the start of the interglacial.Subsequently, sea-level rise inundated the area, and theMytilus horizon was deposited during a transgressivephase. This is overlain by a coarsening-upward succes-sion of about 10m of marine sediments (offshore

139±9127±12119±8138±7128±7132±5129±6127±6

Ele

vatio

n (m

a.s

.l.)

5

10

15

20

Cyprina clay

Till

Tapes sand

Transition(clay to sand)

Till?

Sand

Sand

Anodonta clay and sandMH Mytilus horizon

10

MH

15

18

17

25

3

2

4

1

Log number

88±577±5

122±5

125±5123±4121±4

98±4117±4

96±591±3

113±6105±5126±5128±4

108±5103±7

93±12

136±5122±5147±4154±8142±5153±7135±5139±8

71±496±5

119±4

112±499±4

122±3129±4143±5

131±4

120±4114±5115±4118±3

116±4114±5

172±7

Quartz(ka)

K-feldspar(ka)

N.A.

BA

Fig. 1. A.Map of Denmark showing the location of the Gammelmark site (closed triangle). B. The Gammelmark section showing the principalsedimentary units and sampling locations (black circles) (modified fromMurray & Funder 2003). The revised quartz ages and fading-correctedfeldspar ages are also shown. The question mark next to the bottom till indicates that the till was not exposed at this section but at an adjacent,stratigraphically connected, exposure.

BOREAS IRSL dating of an Eemian site using K-feldspar 47

Cyprina clay and shoreface Tapes sand) deposited in ashort time-span presumed to lie between 132 and 125 ka(log numbers 17 to 4). The Tapes sand is overlain by aunfossiliferous sand (log numbers 2 and 3) for whichthe age is uncertain but probably greater than 116 ka.The top of the succession is capped by a till in whichone luminescence sample was taken by Murray &Funder (2003; log number 1). Because of its nature andits uncertain age this sample was not used to evaluatethe potential accuracy of our IRSL chronometer.Murray & Funder (2003) concluded that the entiresediment succession, except for the upper three sam-ples, was deposited between �133 and 125 ka.

In this work, we have carried out a minor revision ofthe previously published quartz ages (Murray & Funder2003) by taking into account an improved estimation ofthe cosmic ray contribution and by reducing the influenceof the slow- and medium-component contributions to thefast-component OSL signal. The latter was achieved byusing a background signal derived from the OSL decaycurve immediately following the fast component (Ballariniet al. 2007). This is based on the observations of Jain et al.(2003) and Singarayer & Bailey (2003) that the mediumcomponent is approximately an order of magnitude lesssensitive to blue light stimulation than the fast com-ponent. Thus, if a background interval is calculated after4–5 half-lives of the fast component (i.e. 94–97% decay),the medium component will have decayed only 25–30%and the slow component will not have decayed sig-nificantly. The revised quartz OSL ages are summarized inTable 1 and Fig. 1. The overall mean age, for the entiresequence excluding the upper three samples (log numbers1, 2 and 3) is 120�7ka (n=20; including systematic un-certainties) (Table 1); this is indistinguishable from the119�7ka (n=20) reported by Murray & Funder (2003).They concluded that quartz OSL may systematically un-derestimate the expected age of�130ka by�10%.

Sample preparation and instrumentation

Archived and previously untreated fractions of thesamples used by Murray & Funder (2003) were pro-cessed to extract sand-sized K-feldspar grains. Thesediments were wet-sieved (150–250 mm) and treatedwith HCl and H2O2 to remove carbonates and organicmaterial. A smaller grain size was chosen for fivesamples because they contained insufficient material inthe standard grain-size range (Table 1). A K-feldspar-enriched fraction was then separated using a heavy li-quid solution (ro2.58 g cm�3) and etched for 40minwith diluted HF (10%) to clean the grains and to re-move the outer alpha-irradiated layer. After etching,any contaminating fluorides were dissolved using HCl.Dry grains were mounted on stainless steel discs usingsilicone oil and a 2-mm spray mask (1mm for the dosedistributions; see further in section ‘Evidence of in-

complete bleaching’). All luminescence measurementswere made with Risø TL/OSL DA-15 readers equippedwith IR diodes emitting at 875 nm and 90Sr/90Y betasources calibrated at 0.045 (for fading measurements)and 0.201 (for De measurements) Gy s�1 using quartz(Bøtter-Jensen et al. 2003). Luminescence was detectedin the blue-violet region through a Schott BG39and Corning 7–59 filter combination (transmission320–460 nm). It is worth noting that a more restrictivefilter combination that limits the detection to onlythe blue emission does not significantly influence thefading rate for at least one of these samples (Thomsenet al. 2008).

Luminescence characteristics

Equivalent doses were measured using the single-aliquot regenerative-dose (SAR) procedure (Murray &Wintle 2000), employing the same thermal pretreatment(2501C for 60 s) for the dose and test-dose measurements(Huot & Lamothe 2003; Lamothe 2004; Blair et al. 2005).Aliquots were stimulated for 100 s at 501C, and the sig-nals used for calculations were derived from the initial 1 sof stimulation time less a background based on the last10 s. A representative SAR dose–response curve for analiquot of the sample with log number 2 is shown in Fig.2A. From the shape of the curve it can be seen that thenatural signal (represented by the horizontal line) is wellbelow saturation. Several tests are incorporated in ourmeasurement sequence to test the applicability of ourSAR protocol to this material. One point on the dose–response curve is measured twice to test whether theprotocol is able to correct adequately for any sensitivitychanges occurring during the repeated cycles of irradia-tion, preheating and stimulation (open circle in Fig. 2A).The response to a zero-Gy dose is alsomeasured to checkwhether the curve passes through the origin (open squarein Fig. 2A).

A preheat plateau test was carried out with preheattemperatures varying between 200 and 2801C; fromFig. 2B and C the De value and recycling ratio (ratio ofthe sensitivity-corrected IRSL signals from the two repeatpoints) can be seen to be independent of the preheattemperature. Recuperation (corrected IRSL at zero dose,expressed as a percentage of the natural signal) tends toshow a slight systematic increase with higher preheattemperatures but is significantly less than 10% of thesensitivity-corrected natural IRSL signal at any tempera-ture. A preheat temperature in the middle of the plateau(2501C for 60 s) was selected for all further measure-ments. On average, the recycling ratios were very close tounity (0.978�0.003, n=110), and average recuperationwas 5.04�0.07% (n=110). Finally, dose-recovery tests(Wallinga et al. 2000a; Murray & Wintle 2003) wereundertaken using 13 of the 22 samples. Fifty-nine aliquotswere bleached for 1h in a Honle solar simulator to

48 Jan-Pieter Buylaert et al. BOREAS

Table

1.Summary

oftotaldose

rates(basedonMurray&

Funder

2003),D

evalues,fadingrates,uncorrectedandcorrectedK-feldsparages

andquartzages.

Logno.

Sampleno.

Elevation

(ma.s.l.)

K-feldspar

Quartz(4)

Dose

rate

Demeasured

Fadingrate,g2days

Age

uncorrected

Fading-correctedage(3)

Age

(Gyka�1)�

se(G

y)�

sen

(%/decade)�se

n(ka)�

se(ka)�

se(ka)tot.unc.

(ka)�

se(ka)tot.unc.

1�

981001

20.35

2.40�0.04

282�11

8–

–118�5

172�7

10

93�12

13

2�

981004

18.95

2.32�0.04

185�5

12

3.5�0.4

380�2

116�4

5108�5

73�

981005

18.45

2.32�0.04

181�7

12

4.0�0.5

378�3

114�5

6103�7

84

981006

17.74

2.15�0.04

176�4

21

––

82�2

119�4

6122�5

85

981007

17.28

2.18�0.03

179�5

14

3.5�0.4

382�2

120�4

6113�6

76

981008

16.83

2.35�0.04

184�6

12

––

78�3

114�5

6105�5

77

981009

16.37

2.21�0.04

175�5

12

––

79�3

115�4

6126�5

78

981010

15.91

2.36�0.04

191�3

18

––

81�2

118�3

5128�4

79

981011

15.45

2.35�0.04

181�4

15

3.02�0.09

377�2

112�4

5125�5

710

981012

15.00

2.42�0.04

164�6

12

4.3�0.2

368�3

99�4

5123�4

711(1)

981013

14.54

2.16�0.04

180�3

12

3.6�0.7

383�2

122�3

6121�4

612(1)

981014

14.08

2.25�0.04

199�4

12

––

88�2

129�4

698�4

613(2)

981015

13.62

2.23�0.05

218�5

12

2.9�0.2

398�3

143�5

7117�4

614

981002

13.17

––

––

––

––

96�5

615(2)

981016

12.71

2.51�0.06

225�3

14

3.6�0.5

389�2

131�4

691�3

516�

981017

11.70

3.24�0.06

159�7

11

5.6�1.2

349�2

71�4

488�5

617�

981018

11.15

3.09�0.06

203�9

15

3.69�0.08

366�3

96�5

677�5

618

981019

9.74

2.04�0.03

190�6

12

––

93�3

136�5

7139�9

10

19

981020

9.29

2.04�0.03

170�6

17

3.9�0.4

684�3

122�5

7127�12

13

20

981021

8.83

2.03�0.03

204�5

15

3.6�0.4

3101�3

147�4

7119�8

921

981022

8.38

1.84�0.03

194�9

17

3.1�0.4

6105�5

154�8

10

138�7

10

22

981023

7.92

1.98�0.03

192�5

12

3.4�0.2

397�3

142�5

7128�7

923

981024

7.46

1.96�0.03

204�8

14

3.2�0.3

6104�4

153�7

9132�5

824

981025

7.01

2.20�0.04

204�6

12

3.68�0.06

393�3

135�5

7129�6

825

981026

6.56

2.08�0.04

198�8

12

3.3�0.2

395�4

139�8

8127�6

8

Averagelog4–25

128�3(s

=15)

120�3(s

=13)

Averagelog4–15

119�3(s

=11)

114�4(s

=13)

Averagelog18–25

141�4(s

=11)

130�2(s

=7)

Thegrain

size

is150–250mm

forallsamples,exceptfor(1)and

(2) ,forwhichitis90–150and63–90mm

,respectively.

(3)Usedfadingrate

(g)=

3.66�0.09%/decade(tc=

2days).

(4)Updatedvalues

basedonMurray&

Funder

(2003).

(�)Samplesnotusedfortheaverages

thatare

reported

atthebottom

ofthetable.

Note:se=

standard

error;s=

thestandard

deviation;tot.unc.=

thetotaluncertainty

(see

textfordetails).

BOREAS IRSL dating of an Eemian site using K-feldspar 49

remove the natural IRSL signal and given a dose of180Gy, close to the natural dose in these samples (Table1). This dose was then measured as if it were a naturaldose; the results (expressed as the ratio of measured togiven dose) are summarized in Fig. 2D. The mean ratiois 0.968�0.008 (n=59; s=7%), that is, within 3% ofunity. We conclude that our SAR protocol is able toaccurately recover a known dose given in the laboratorybefore the sample has been heated. The De values mea-

sured using this protocol are summarized in Table 1 andplotted against depth in Fig. 3A. There is no clear trend inthe values of De, although it should be noted that the up-per till sample (log number 1) has a much higher De thanthe samples below; this is presumably because of the pre-sence of poorly bleached grains. Incomplete bleaching isnot surprising for till sediments (Fuchs & Owen 2008). Itis interesting to note that this difference was not observedin the quartz equivalent doses (Murray & Funder 2003).

Regenerative Dose (Gy)0 100 200 300

Cor

rect

ed IR

SL

(Lx/T

x)

0

2

4

6

8

10A

De = 166 Gy

log no.2

Average De (n=18) = 198±5 Gy

log no.25

Preheat temperature (°C)200 225 250 275 300

De

(Gy)

0

100

200

300B

log no.25

Preheat temperature (°C)200 225 250 275 300

Rec

yclin

g R

atio

0.50

0.75

1.00

1.25

Rec

uper

atio

n (%

Nat

ural

)

0

5

10

15

20C

0.50 0.75 1.00 1.25 1.500

5

10

15D

Fre

quen

cy

Measured to given dose ratio

Average=0.968±0.008RSD=7%n=59

Fig. 2. A. Single-aliquot regenerative-dose dose–response curve for a single aliquot of the sample with log number 2 showing a recycled point(open circle) and a zero-dose point (open square). Preheat 2501C for 60 s for both the regeneration and test dose. B. Preheat plateau test (60-spreheat) for the sample with log number 25; three aliquots were measured at each temperature and error bars represent 1 standard error. C.Recycling and recuperation for the same aliquots. D. Histogram summarizing the dose-recovery ratios for 13 samples (minimum of threealiquots per sample); the dashed line is drawn at unity. RSD is the relative standard deviation.

De (Gy)

Ele

vatio

n (m

a.s

.l.)

5

10

15

20

Dose rate(Gy ka–1)

Age (ka)

0 100 200 300 0 1 2 3 0 50 100 150 200

A B CTill

Fig. 3. A. Equivalent dose against depth.B. Total dose rates against depth. The dotted lineinterpolates between log numbers 15 and 18.C. Fading-corrected K-feldspar ages (circles) andquartz ages (squares) against depth. The opencircles and squares for samples with log numbers16 and 17 are obtained using the interpolateddose rates from (B). The solid line represents aregression line through the K-feldspar ages fromlog number 4 to log number 15.

50 Jan-Pieter Buylaert et al. BOREAS

Dosimetry

The external dose rates to K-feldspar grains are thesame as those to quartz grains (with minor correctionsfor grain-size differences), and are taken from Murray& Funder (2003); the details of the measurementsand a discussion of the possible origin of the outliers(log numbers 16 and 17) are given there. We havealso revised the cosmic ray dose rates slightly follow-ing Prescott & Hutton (1994). The internal dose rateto K-feldspar grains makes up a significant portion(�30%) of the total dose rate, and is derived bothfrom the structural potassium in the mineral and fromminor amounts of internal Rb, U and Th. The con-tribution from internal beta activity from 40K is basedon an assumed effective potassium content of12.5�0.5% (following Huntley & Baril 1997), and thebeta dose rate contribution from 87Rb is calculatedassuming a Rb content of 400�100 ppm (Huntley& Hancock 2001). A small internal alpha contributionof 0.10�0.05Gy ka�1 from internal U and Th is alsoincluded in our dose rates, derived from U and Thconcentration measurements made by Mejdahl (1987)using neutron activation analysis (NAA) on multi-grain aliquots of K-feldspar grains from variousScandinavian sites (Mejdahl 1987: figs 2, 3). The con-centrations were converted into dose rates using theconversion factors of Adamiec & Aitken (1998). The Uand Th concentrations from Mejdahl (1987) are some-what lower than those reported by Zhao & Li (2005)for Chinese samples. However, it is likely that theamounts of U and Th are variable between K-feldsparsof different provenance. Because our study site is locatedin Denmark and the samples used by Mejdahl (1987)are all from the Scandinavian region we have adoptedthese concentrations. The total dose rates are reportedin Table 1 and plotted against depth in Fig. 3B. Thetotal dose rate to the etched K-feldspar grains is, de-pending on the grain size, between �0.4 and 0.8Gyka�1

higher (�20 and �80%) than the total dose rate to theetched 90–150mm quartz grains used by Murray & Fun-der (2003).

Anomalous fading and corrected ages

Anomalous fading is the term used to describe theunexpected loss of luminescence with time displayedby some minerals for which kinetic studies show thatthe charge should in fact remain trapped, for a con-siderably longer time, at the relevant defect. The mostwidely accepted model for anomalous fading is quan-tum mechanical tunnelling of electrons to nearby holes;as a consequence of this mechanism the luminescencelost can be approximated by logarithmic decay (see e.g.Aitken 1985: appendix F; Visocekas 1985, 2002). Thereis increasing evidence that anomalous fading is a ubi-

quitous phenomenon in feldspars (e.g. Spooner 1994;Balescu et al. 2001, 2003; Huntley & Lamothe 2001;Wallinga et al. 2007; Thomsen et al. 2008) and it hasbeen recommended that testing for its presence shouldbe routine practice. Because experimentally observedanomalous fading can be approximated by logarithmicdecay, it is convenient to express the fading rate as thepercentage of signal lost for a storage period of onedecade of time (the g value; Aitken 1985: appendix F;the storage periods are expressed as decades relative tothe laboratory irradiation period).

Measuring the fading rate

The fading rate was measured using a SAR protocolthat employed the same preheating conditions as usedfor De determinations. Fading measurements werecarried out on 17 samples, and a minimum of threealiquots were used per sample. After being used formeasuring De, aliquots were bleached for 1 h in a HonleSOL2 solar simulator; the SAR cycle was then repeatedseveral times using a regeneration dose of 45Gy and atest dose of 11Gy, but with varying storage periodsafter the regeneration doses. Following Auclair et al.(2003), the aliquots were preheated immediately afterirradiation. The delays after administration of the45-Gy regeneration dose ranged from 0.2 h (promptmeasurement) to 24 h (longest delay); in this way theindividual g values from a set of 12 aliquots weredetermined in only three days. For all the fading mea-surements the measurement chamber of the Risø readerwas filled with helium instead of nitrogen gas; thisimproved the reproducibility of the preheat treatment(Huot 2007). The g value, calculated using equation (4)of Huntley & Lamothe (2001), was normalized to ameasurement delay time of 2 days after irradiation(Huntley & Lamothe 2001).

A representative example of a fading measurementon an aliquot from the sample with log number 24 isshown in Fig. 4A. All the individual fading measure-ments (n=60) are shown as a radial plot in Fig. 4B,and the weighted mean of all g values is 3.66�0.09%/decade (tc=2 days). Assuming that all the aliquotshave the same fading rate (same provenance area of theK-feldspar grains), the over-dispersion in these data is10�3%. Over-dispersion values were calculated usingthe central age model (Galbraith & Laslett 1993;Galbraith et al. 1999). The sample-averaged g valuesare given in Table 1; the lowest g value is 2.9�0.2%/decade (log number 13) and the highest 5.6�1.2%/dec-ade (log number 16).

Age correction

The uncorrected feldspar ages severely underestimate(by �30%) both the quartz ages and the independent

BOREAS IRSL dating of an Eemian site using K-feldspar 51

age information (Table 1), with the exception of sam-ple log number 1, which was taken from till. To obtainthe fading-corrected ages for our samples we used thecorrection method developed by Huntley & Lamothe(2001: equation A5). This procedure is expected to beapplicable only to young samples, where the observednatural IRSL signal lies within the relatively linearpart of the growth of luminescence with dose. Weconsider that this condition may be met for our sam-ples (see Fig. 2A). However, Huntley & Lamothe(2001) argue that the correction procedure is notexpected to be applicable to samples exceeding�20–50 ka.

Because of the observed over-dispersion in thefading-rate data (Fig. 4B) and in the uncorrected ages(not shown) the following questions arise. Should in-dividual fading rates or an average fading rate be usedfor age correction? Should correction be carried out onthe individual aliquots or on the average De for eachsample? In other words, are the De and g values fromeach aliquot inversely correlated? We have addressedthis issue using those aliquots for which values of bothDe and g were measured (54 aliquots from 15 samples;excluding the 6 aliquots from the outlier samples (lognumbers 16 and 17; see ‘Discussion’ below)). The De

values, normalized for dose-rate differences of the dif-ferent samples, are plotted against their correspondingg value in Fig. 5A; no clear trend is visible in the data.In order to confirm this, we first corrected the agesfrom the individual aliquots using their individualfading rates, and then averaged the ages per sample(Fig. 5B). Secondly, the ages from individual aliquotswere corrected using the fading rate averaged across allaliquots from a particular sample, and then the ageswere averaged per sample (Fig. 5C). Finally, the aver-age De value for each sample was calculated, and theage based on this mean dose derived using the site-

averaged fading rate (Fig. 5D). From Fig. 5B–D it canbe seen that all three approaches yield average ages anddispersions indistinguishable from one another.Therefore, for calculation of the final fading-correctedages we use the site-averaged fading rate of all the ali-quots (g=3.66�0.09%/decade, n=60) and the aver-age De per sample.

The fading-corrected ages for all 21 accepted samples(excluding log numbers 1, 16 and 17) are shown in aradial plot and a histogram in Fig. 6A and B, respec-tively. The average age (weighted or unweighted) is128�3 ka (n=21) with a standard deviation of 15 ka.

Evidence of incomplete bleaching?

Figure 6B shows the fading-corrected feldspar ages as ahistogram with the revised quartz ages inset. The feld-spar histogram appears to be bimodal, with one modeat �125 ka and another one at �145 ka. The highermode originates mainly from the ages of the bottomeight samples (Figs 1, 3C), all from the meltwater sands;these are presumed to be a product of the Late Saaliandeglaciation, and should have essentially the same ageas the overlying marine transgression (see section ‘Sitedescription and age control’ and Murray & Funder2003), and this is reflected in the quartz data. Never-theless, the average age for the bottom eight feldsparsamples is 141�4 ka, which is significantly older (by�10 ka) than the expected age. Meltwater transport is aless favourable bleaching environment than the pro-cesses that deliver sand to shallow marine deposits.For shallow marine sands there is already some evi-dence in the literature that feldspars are well-bleached(Huntley & Clague 1996; Hong et al. 2003). It is possi-ble that the feldspar signal in the bottom meltwatersands, in contrast to that from the quartz, was not

Delay (h)

0.1

2.70

2.75

2.80

2.85

2.90

2.95

3.00log no.24

g2days = 3.1±0.2 %/decade

A BWeighted mean g2days =3.66±0.09%/decadeOver-dispersion = 10±3%

1 10

L x/T

x

n = 60

Fig. 4. A. Anomalous fading data for an aliquot from the sample with log number 24. The measured Lx/Tx ratios are plotted versus the loga-rithm of time elapsed since the end of the irradiation (45Gy). The dashed lines represent an error envelope (1s). The same experimental con-ditions (filter set, preheat, stimulation temperature and time) were used as for De determinations. B. Radial plot of all g values.

52 Jan-Pieter Buylaert et al. BOREAS

g2days value (%/decade)0 1 2 3 4 5 6

Nor

mal

ised

De

(Gy)

0

100

200

300

400An = 54 B Weighted mean =

128±4 kaOver-dispersion =11±3 %

C Weighted mean =129±4 kaOver-dispersion =11±3 %

D Weighted mean =131±5 kaOver-dispersion =12±3 %

Fig. 5. A. Individual De values (normalized for dose-rate differences) plotted against their corresponding g value. (B–D) Radial plots showingthe corrected K-feldspar ages for 15 samples using only those aliquots for which g values were measured (see Table 1; excluding data from lognumbers 16 and 17). B. The age from each aliquot was corrected using its individual fading rate, and then the ages were grouped by sample. C.Every aliquot was corrected using the sample-averaged fading rate, and then the ages were grouped by sample. D. The sample-averaged De wascalculated and then corrected using the average fading rate for all aliquots (3.66�0.09%/decade, tc=2 days).

Quartz age (ka)

Fre

quen

cy

Average age =120±3 ka(σ=13 ka; n=22)

Corrected IRSL age (ka)0 40 80 120

A

160

Fre

quen

cy

0

3

6

9Expectedage range

Average age = 128±3 ka(σ=15 ka; n=21)

B

Fig. 6. Fading-corrected K-feldspar ages shown (A) as a radial plot and (B) as a histogram, excluding the till sample (log number 1) and the twooutliers (log numbers 16 and 17). The expected age range (132–125ka) is also shown in (B). The inset shows a histogram of the revised quartz ages.

BOREAS IRSL dating of an Eemian site using K-feldspar 53

completely bleached at the time of deposition. (Theaverage quartz age of these lower eight samples is130�2 ka, in excellent agreement with the expected ageof 132–125 ka.)

In order to investigate the bleachability of the differ-ent depositional environments in more detail we mea-sured small-aliquot (1mm) dose distributions for three

samples. Visual inspection after measurement indicatedthat there were �20–50 grains per disc as calculated byDuller (2008). One sample (log number 1) comes from alayer that we expect to show signs of incompletebleaching (the top till layer); another sample comesfrom the beach sand layer, which we expect to be wellbleached (log number 7); and a third sample comesfrom the meltwater sand layer, about which we are un-sure (log number 23). The three histograms are shownin Fig. 7. While there may be some evidence forincomplete bleaching in the till sample from the top ofthe section (Fig. 7A), neither of the two samples fromthe beach and meltwater sands shows any significantevidence for a high-dose tail (Fig. 7B, C). Nevertheless,both distributions have relative standard deviations(RSDs) of �20%, which means that there could be anundetectable residual dose of 20–40Gy in these sam-ples. The relative importance of any such residual willof course be increased when it is included in the doseto be corrected for fading (as it must be, in the absenceof information about the size of the residual on asample-by-sample basis). Nevertheless, we concludethat we are unable to detect any incomplete bleachingin either the beach sands or the meltwater sands. Incontrast, the expected poor bleaching in the overlyingtill sample is observable.

Discussion

From a consideration of the preheat plateau, recyclingratios and dose-recovery ratios we conclude that ourSAR-IRSL protocol is suitable for the determinationof De for our samples. By using this SAR protocoland flushing the luminescence reader with helium wewere able to determine precise (typical relative standarddeviation �18%) values of g for a large number of ali-quots in an efficient manner, and show that there is noapparent benefit in using aliquot-specific estimates offading at this site.

The fading-corrected feldspar ages and revised quartzages are plotted against depth in Fig. 3C. The fading-corrected age of the upper till sample (log number 1) isnearly twice the quartz age and presumably reflects thegreater difficulty of bleaching K-feldspar grains in suchan environment, as is also suggested by the histogram ofDe values in Fig. 7A. The two previously identified out-liers in the dosimetry data (Fig. 3B) at �11.5m (lognumbers 16 and 17) can also be seen in the uncorrectedand corrected feldspar ages (Table 1, Fig. 3C). Inter-polation of the dose rate between log numbers 15 and 18makes both quartz ages and the bottom feldspar agemore consistent with the quartz and feldspar age–depthtrends. Thus these observations support the interpreta-tion of Murray & Funder (2003) that the measured pre-sent-day dose rates of these samples do not accuratelyreflect the dose rates during the entire burial period.

0 200 400 600 800 10000

5

10

15

20

A

Fre

quen

cy

De (Gy)

log no.1De=305±18GyRSD=48%n=64

0 50 100 150 200 250 300 3500

5

10

15

20

Blog no.7De=165±3GyRSD=17%n=64

Fre

quen

cy

De (Gy)

0 50 100 150 200 250 300 3500

4

8

12log no.23De=177±4GyRSD=20%n=64

C

Fre

quen

cy

De (Gy)

Fig. 7. Histograms of De values measured with small aliquots (1mmdiameter,�20–50 grains per disc) on (A) the upper till (log number 1),(B) the beach sand (log number 7) and (C) the meltwater sand (lognumber 23). RSD is the relative standard deviation.

54 Jan-Pieter Buylaert et al. BOREAS

We restrict the remaining discussion of the accuracy ofthe fading-corrected feldspar and revised quartz ages tothe samples with log numbers 4 to 17 (in fact 4 to 15 be-cause of the outliers, 16 and 17; Murray & Funder 2003)because for these samples from the Eemian marine layersthere is no uncertainty in the age. No significant trend ofK-feldspar age with depth can be observed (see the linearregression line in Fig. 3C). The average fading-correctedfeldspar age for these samples is 119�3ka (n=11,s=11ka; random uncertainties only), which is notsignificantly different from the average quartz age of114�4ka (n=12, s=13ka) but in slightly betteragreement than the quartz age (132–125ka).

The standard errors on the average values givenabove, and in the figures and Table 1, do not includesystematic components. This additional uncertaintywas discussed by Murray & Funder (2003) for sand-sized quartz, and the same uncertainty is present in theK-feldspar ages presented here. Furthermore, an addi-tional systematic uncertainty associated with the internal40K contribution (�4%; Huntley & Baril 1997) must beincorporated. There must be some systematic uncertaintyresulting from inaccuracy in the fading correction proce-dure, but this is unknown. Summing all known systema-tic uncertainties in quadrature gives an overall systematicuncertainty on the feldspar age of�5ka (�4%). Becausethe random uncertainty on the mean (discussed above) isrelatively small, the total uncertainty associated withthese average fading-corrected feldspar ages is not sig-nificantly larger, in the range of 5–7ka.

A first version of this paper was completed in 2007, andsince then further tests based on this approach have beenundertaken on another Eemian marine site in northernSiberia (Buylaert et al. 2008) with similarly satisfactoryconclusions. Our results are also used as reference data ina recent study of the behaviour of the IRSL signal stimu-lated at high temperature (Buylaert et al. 2009).

Conclusions

In this paper we have tested the potential of using theIRSL signal from K-feldspar grains to date a siteknown to be of early MIS 5e age. The luminescencecharacteristics of our samples are satisfactory, particu-larly as suggested by the mean dose-recovery ratio of0.968�0.008 (n=59). Uncorrected K-feldspar ages se-verely underestimate the known age of the sediments(by up to 35%). We have shown that a site-specificoverall fading rate of 3.66�0.09%/decade (n=60)provides age corrections at least as precise as those ob-tained by correcting each aliquot individually; these es-timates of the anomalous fading rates were obtainedusing time delays of no more than 24 hours. After cor-rection, the overall average K-feldspar age (119�6 ka,total uncertainty) for the Eemian layer is in agreementwith the expected age range of 132–125 ka and is not

significantly different from the revised quartz result(114�7 ka, total uncertainty). From this study, it seemsthat the fading correction does not give rise to un-acceptable errors for apparent doses up to �200Gy,suggesting that the age limit of �20–50 ka recom-mended by Huntley & Lamothe (2001) is, in practice,unnecessarily restrictive. We conclude that our K-feld-spar IRSL ages are at least as accurate as quartz ages,and consistent with the expected age.

Acknowledgements. – One author (JPB) received a PhD grant fromthe Institute for the Promotion of Innovation through Science andTechnology in Flanders (IWT-Vlaanderen), and financial supportfrom the Nordic Centre of Excellence Programme of the Joint Com-mittee of Nordic Natural Science Research Councils. The technicalassistance of Mette Adrian, Nadia Hegazy and Vicki Hansen is verymuch appreciated. Dr. Ya-Wen Chen is thanked for help with thefading measurements. Anni T. Madsen is thanked for the map ofDenmark. We would like to thank Professor Ann Wintle and Dr.Nicolaj Krog Larsen for their helpful reviews.

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56 Jan-Pieter Buylaert et al. BOREAS