Zeroing tests and application of ...

Zeroing tests and application of thermoluminescence dating to Fraser River delta sediments' GLENN W. BERGER Department of Geology, Western Washington University, Bellingham, WA 98225, U.S.A. AND

JOHN L. LUTERNAUER

A N D JOHN

J. CLAGUE

Geological Survey of Canada, 100 West Pender Street, Vancouver, B . C . , Canada V6B IR8

Received March 27, 1990

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Revision accepted August 9, 1990 Thermoluminescence (TL) systematics of known-age sediments from the Fraser River delta in southwestern British Columbia have been determined to test the hypothesis that "zeroing" of TL mineral clocks occurs in deltaic and fluvial environments. Analyzed sediments include suspended silt carried by the Fraser River, surface mud from three different environments on the modem Fraser River delta, and independently dated floodplain and prodelta sediments from two cores. Derived TL dating procedures were also applied to unknown-age proglacial muds from one of these cores. The suspended silt and surface muds yielded minimum, relict TL ages of about 300 years, with typical "zero-point" equivalent-dose values of 0.5-1 Gy. Zero-point-corrected TL ages of two samples of Holocene floodplain sediments from one core and two samples of latest Pleistocene - early Holocene prodelta sediments from a second core agree with independent age estimates. In contrast, the older proglacial deposits could not be dated by the TL methods used here because plateaus in equivalent-dose values were not observed. La systématique de thermoluninescence (TL) pour des sédiments d'âge connu du delta de la rivière Fraser, en ColombieBritannique, a été déteminée dans le but de vérifier l'hypothèse de la « remise à zéro>, des chronomètres fournis par les minéraux thermoluminescents présents dans les milieux deltaïques et fluviatiles. Les sédiments étudiés comprennent le limon en suspension transporté par la rivière Fraser, les boues de surface de trois milieux différents sur le delta de la rivière Fraser, et les matériaux de deux carottes d'une plaine d'inondation et d'un prodelta datés par des méthodes indépendantes. Quelques méthodes de datation dérivées de la TL ont été également appliquées aux boues proglaciaires d'âge inconnu d'une de ces carottes. Le limon en suspension et les boues de surface ont fourni par TL des âges relictuels d'environ 300 ans, avec des valeurs typiques de dosage équivalent de « remise à zéro » de 0,5 à 1 Gy. Les âges obtenus par TL, corrigés par une remise à zéro, pour deux échantillons d'une carotte de sédiments de la plaine d'inondation holocène et pour deux échantillons d'une seconde carotte de sédiments prodeltaïques de la fin du Pléistocène - Holocène précoce, s'accordent avec les âges déterminés par les autres méthodes indépendantes. D'autre part, il fut impossible de dater par les méthodes de TL utilisées ici les dépôts proglaciaires, car les plateaux en valeurs de dosage équivalent n'ont pas été observés. [Traduit par la revue] Can, J. Earth Sci. 27, 1737-1745 (1990)

Introduction There is a pressing need for a reliable dating method that can be applied to Quatemary deposits older than the 40 ka limit of conventional radiocarbon dating. In response, thermoluminescence (TL) methods have been developed to directly date Quaternary sediments (see reviews by Aitken 1985; Berger 1988; Forman 1989). To use TL for this purpose, however, it is necessary to identify and characterize depositional environments that permit effective "zeroing" of TL minera1 clocks. Two general approaches have been used to identify such environments: (i) examination of the TL systematics of sediments that have been recently deposited, or are in the process of being deposited, and (ii) application of TL dating methods to independently dated sediments. Earlier TL work on subaqueous sediments (Berger 1988) has shown that light-sensitive TL is often ineffectively zeroed and, therefore, TL age estimates from such sediments commonly exceed m e deposition ages. Huntley (1985) was the fîrst to systernatically examine the TL from subaqueous suspensions and modem beach deposits. He observed a variable, relict, light-sensitive TL component that might lead to overestimates of TL age of at least 2-10 ka. The work of these authors further showed that only the partial-bleach technique of Wintle and Huntley (1980) is appropriate for dating waterlaid sediments and that considerations of ambient light spectra during deposi'Geological Survey of Canada Contribution 22990.

tion must govem the choice of optical bleaching procedures employed with this technique. Because no previous TL dating study focused on fluvialdeltaic depositional environments, we began a study of the TL systematicsof suspended grains and known-age sediments from the Fraser River delta (Berger and Lutemauer 1987). Our objectives were (i) to document the underwater ambient light spectra within the sediment plume of the Fraser River, (ii) to measure the TL responses of suspended grains and recently deposited sediments, and (iii) to apply consequent TL dating procedures to known-age and unknown-age muds in two cores recovered from the Fraser River delta. The results of Our measurements of underwater light spectra are reported separately (Berger 1990) as part of a broader study of the effectiveness of TL zeroing in modem or "zero-age" eolian and waterbome sediments. We report here TL results for suspended grains, contemporary fluvial-deltaic sediments, and Quaternary sediments from cores. Specific questions we examined are as follows: Are there freshly deposited feldspar grains on the Fraser delta and, by implication, on other marine deltas that yield equivalent radiation doses (De)equal to zero? If so, can older fine-grained sediments from this delta be accurately dated by TL?

Principles of thermolumiiiescence dating The TL in fine-grained (4-11 pm) polymineralic sediments is generally dominated by that from detrital feldspars (Berger

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CAN. J. EARTH SCI. VOL. 27. 1990

O

INTERTIDAL 8 UPPER FORESLOPE SURFACE

I, SLOPE-BASE FAN SURFACE FLOODPLAIN DEPOSIT IN CORE

O

PRODELTA DEPOSlT IN CORE

FIG. 1. Sample locations, Fraser River delta, British Columbia (see Table 2 for sarnple and site information). Undenvater light spectra and TL results for suspensions at sites lb and 2 are described elsewhere (Berger 1990). 1984). This TL consists of light-sensitive and light-insensitive components. After burial, emptied light-sensitive charge traps (e.g., crystal defects) are replenished at a constant rate by low-level-background ionizing radiation in the sediment. Laboratory heating releases al1 trapped charges (e.g., electrons), yielding a TL signal proportional to the sum of the postdepositional radiation dose and a relict dose. In the most general case the TL representing this relict dose can have contributions from both the light-insensitive charge traps and any light-sensitive traps that were not emptied at burial. The partial-bleach technique (Wintle and Huntley 1980) permits the measurement of the postdepositional, equivalent radiation dose (De), which appears directly in the practical age equation t = D,/(D,

I

SUSPENDED SEDIMENT

+ D@ + Dy + Der)

The alpha, beta, and gamma dose-rate components in the denominator can be calculated from the dose-rate equations and revised conversion coefficients given by Berger (1988), and the cosmic-ray component (D,,) can be estimated from Prescott and

Hutton (1988). De values are measured as the extrapolated intercepts of the two TL dose-response curves in the partialbleach technique, as illustrated below.

Methods Samples Six types of known-age sediment were collected and analyzed: suspended sediment from 60 L of water collected in the main Fraser River channel (FRSH87-1; see Berger 1990 for details); surface intertidal (FRMDF4B, FRMDF-6B) and upper delta foreslope (FDF-1, FDF-3) muds; surface mud from a submarine fan at the base of the delta slope (VEC-02, VEC-03); and subsurface floodplain and prodelta muds in two cores (GILRD, FD87A1) (Figs. 1,2). In addition, four samples of a proglacial sediment sequence (silt and silty clay) of unknown age were collected below the upper diarnicton in core FD87A1. The tidal flat, foreslope, and submarine-fan samples were collected 1-6 cm below the sediment surface. These materials probably have been deposited within the last 100 years, based on estimates of sedimentation rates in the delta

BERGER ET AL.

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SITE 0 2 3 (ADAPTED FROM WILLIAMS AND ROBERTS 1 9 8 9 )

GlLRD

-

FD87A 1

AGE ( k a ) CALENDAR

TL

sv: PEAT

X X X

TEPHRA

FIG. 2. Stratigraphy of core D23 (10-13 m from GILRD) and core FD87A1, showing TL samples, TL ages, and relevant independent calendar-age estimates. Calendar ages were obtained from radiocarbon ages using the calibration curves of de Jong et al. (1986), Linick et al. (1986), and Stuiver et al. (1986). The age estimate of the tephra in core D23 is based on an assumed radiocarbon age of 6700 BP (see text). Unconverted radiocarbon ages from the two cores: core D23,5500 I 70 (GSC-4238); core FD87A1 (in descending order), 7470 2 60 (TO-780), 8310 I 70 (TO-781), 9150 I 70 (TO-782), 9410 2 70 (TO-783), 9950 I 80 (TO-784), and 11 920 2 90 (TO-1094). Cores D23 and GILRD have similar stratigraphy, but depths of correlative contacts differ by 2 0 . 5 m. Core GILRD was obtained for TL dating because the sediment in core D23 was exposed to light during subsampling for the study of Williams and Roberts (1989).

region: 0.4 mmla on Boundary Bay tidal flat (Kellerhals and Murray 1969), 1.8 c d a in estuarine marshes (Hutchinson 1988), -3 c d a in the Strait of Georgia off the front of the Fraser delta (Pharo and Barnes 1976), and -30 crnla at depths of 90-180 mon the active delta slope (Pharo and Barnes 1976). Laboratory procedures The laboratory and statistical procedures of Berger (1985a, 1987) and Berger et al. (1987~)were followed. An anomalous fading TL component (from 5 to 20%, depending upon the sample) was removed by heating subsamples at 75°C for 8 days (Berger 1987). A 60Co gamma irradiation facility at Simon Fraser University (Burnaby, British Columbia) was used for

the four deepest samples from core FD87A1, and a 125 mCi 90Sr beta irradiation source at Western Washington University was used for al1 other sarnples. The two samples from core GILRD contained significant organic matter; consequently, the effective dose-rate components were corrected, following Divigalpitiya (1982), using the equation (~i)com= (~i)uncom/[l+ A d 1 + Ao)W + AoZl are the H values given by In this equation, i = a, p, or -y; Berger (1988); and HP are the H values for organics given by Divigalpitiya (1982). The H values are the ratios of stopping powers (for a and p particles), or of absorption coefficients

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CAN. J. EARTH SCI. VOL. 27, 1990

Apparent TL Age (ka) O

1

2

FRSH87- 1 FRMDF-4B FRMDF-6B

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FDF- 1 FDF-3 VEC- 02 VEC-03 1 50

250

350

450

Temperature (OC) FIG. 3. Examples of glow curves for core sample FD87A1-510. N is an unirradiated subsample; N + B73Gy represents N with an added laboratory beta dose of 73 Gy (1 Gy = 100 rad); SLlh and SL24h represent subsamples equivalent to N after laboratory illumination ("opticai bleaching") of 1 and 24 h, respectively. TL signals were recorded through a UV-passglass filter (Schott UG-Il) and a heatabsorbing optical filter (KG3), with a sample heating rate of S°C/s. The UG-11 filter was chosen to minimize any potential quartz TL signal, but its efficacy is uncertain (Berger 1988). Positions of the peaks of the N glow curves, their dose responses, and optical-bleaching behaviour indicate that the dominant TL signal is from feldspars (Berger 1984). (y rays), of water to dry sediment. The water delta value is A, = mass water 1 mass dehydrated sample; the organic delta value is A, = mass organics 1 mass ashed sample. The latter was

assigned a value equal to one-half the loss-on-ignition value (Hakanson and Jansson 1983, p. 76). A 570 nm high-pass optical filter (Schott OG-570) and bleach times of 1 and 24 h were used with a 275 W Hg-vapour lamp for optical bleaching in the partial-bleach experiments to determine D, values. For these experiments, 1 h of illumination corresponds to an integrated optical energy fluence of about 650 d l cm2. The use of two such different bleach times can, in principle, indicate whether a sarnple has been zeroed in nature (Berger 1985a, 1990). Bleach times of 1 and 24 h were chosen to reduce, in old samples, the peak TL by about 30 and 50%, respectively, as shown in Fig. 3 for the unirradiated (N) subsample of core sarnple FD87A1-510. Generally, after 8 days of heating to remove an anomalous fading TL component, al1 samples showed glow-cuwe shapes simil& to those-in Fig. 3.

Results and discussion The results for al1 samples are presented in Tables 1 and 2 and are discussed below. "Zero-age" samples Table 2 and Fig. 4 show that the suspended-sediment and surface samples yielded near-zero TL ages only with the shorter bleach time. With the exception of the VEC samples collected from a submarine fan at the base of the Fraser delta slope, there is a "zero-point" error of about 1.0 + 0.5 Gy for the 24 h bleach-time data. If similar older sediments are to be dated accurately using the longer bleach time, it is logical to subtract this zero-point correction from measured De values. The more variable De values for the two submarine-fan samples (VEC-02

FIG. 4. Equivalent-dose values and TL apparent ages for the "zero-age" samples, showing results for 1 h (O) and 24 h (O) bleach times. An average dose rate of 1.7 Gy/ka (Table 1) was used to scale these De values to TL apparent ages. The dose-response curves for ail samples except those from core FD87Al were fitted by weighted quadratics.

and VEC-03) suggest a less effective zeroing of TL in this depositional environment. Nevertheless, such variable zeropoint De values would probably contribute less than 10%to the measured De values in submarine-fan muds older than about 20 ka. Possibly, VEC-02 and VEC-03 include older sediment emplaced by turbidity currents or dredging activity (McKenna and Luternauer 1987; Kostaschuk et al. 1989; see Berger 1990 for additional TL data from this site). An exarnple of the partial-bleach data curves and corresponding "plateau" plot of De values is shown in Fig. 5 for sample FRMDF-4B. Errors in De values for this and al1 other samples are larger than expected, perhaps because the subsamples were not completely uniform (reflected in the data-point scatter in the TL growth cuwes). These errors make it difficult to resolve the lower age limit of the TL method for these samples, but the minimum equivalent dose appears to be about 0.4 Gy (using the 1 h bleach time). This is similar to the value reported by Berger (1990) and is equivalent to a relict TL age of Ca. 300 a for these samples. Known-age samplesfrom cores The above zero-point De value was subtracted h m the plateau De values obtained for the two known-age floodplain samples from core GILRD (e. g., Fig. 6). The resulting TL ages of 5.1 + 1.4 and 8 .O + 1.1 ka agree with a calendar-converted radiocarbon age of 5.8 ka (Fig.2) on organic detritus associated with the upper sarnple (Williams and Roberts 1989) and with a TL age of 7.4 + 0.5 ka for Mazama tephra (Berger 1985 b). The generally cited ages of 6600-6800 BP for Mazama tephra (Powers and Wilcox 1964; Bacon 1983) are in uncorrected radiocarbon years. These convert to calendar ages of 74507650 a (Linick et al. 1986) and thus are statistically equivalent to the TL age estimate for the tephra. Results for one of the two known-age samples from core FD87A1 (510 and 573) are shown in Fig. 7. There are no

1741

BERGER ET AL.

TABLE 1. Dosimetry datao

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FRMDF-4B FRMDF-6B FDF- 1 FDF-3 WC-O2 WC-O3 GILDR2-2 GILRD1-2 FD87A1-510 FD87A1-573 FD87A1-644 FD87A1-852 FD87AI-966 FD87A1-1201

ct

wateP

K20 (wt.%)

(ks. cm2)-''

(ks . cm2)-' '

cm

b value (PGY.m21d

Cosmic ray (GyIkaY

Dose rate (~ylkay

0.4510.10 0.4810.10 0.40IO.10 0.5010.10 0.85IO. 10 0.7010.08 1.9350.20 2.40I0.25 1.0210.05 0.85I0.05 0.5520.10 0.3210.08 0.5010.10 0.28I0.05 0.3310.08 0.4220.08 0.3210.08 0.4010.10 0.33IO.10 0.40I0.10 0.23IO. 10

1.3910.03 1.63I0.05 1.5510.05 1.6010.05 1.75?0.05 1.7210.05 1.6120.05 1.4810.05 1.9810.05 2.02k0.06 1.9010.05 1.95k0.03 2.2810.05 1.79I0.15 1.6720.05 1.4710.05 I.62I0.05 2.0510.05 1.60I0.40 1.9520.05 1.7010.30

0.44010.010 0.495I0.010 0.42210.009 0.44510.009 0.52410.012 0.51310.012 0.40910.009 0.43410.009 0.491 IO.010 0.53010.012 0.44010.009 0.448t0.015 0.408I0.009 0.419I0.009 0.28610.006 0.29310.008 0.276I0.007 0.27410.007 0.350I0.030 0.35020.008 0.33610.010

0.17210.029 0.24510.034 O.14110.023 0.17310.026 O.15410.033 O.15410.028 O.10220.021 O.14210.023 O.19410.028 0.23120.035 0.14510.025 0.20010.028 0.17720.026 0.18110.027 0.10110.017 0.131 20.022 0.10120.017 0.11510.018 O.13820.025 0.08920.019 O.162I0.025

0.76920.043 0.77050.063 0.84610.081 0.82810.071 0.76110.071 0.7420.10 0.86210.095

0.15I0.02 0.15k0.02 O.1520.02 0.1520.02 0.14I0.02 0.14k0.02 O.1410.02

1.6020.15 1.7710.15 1.7420.46 1.68I0.36 1.51IO.11 1.61IO.17 1.00210.076

0.61310.080

0.1310.02

1.68210.059

0.6740.12

0.0220.01

1.9510.11

0.840I0.061

0.0210.01

2.1320.15

1.0I0.2

0.0210.01

1.7410.13

1.10k0.15 1.0I0.2

0.02I0.01 0.0210.01

1.7610.13 1.89IO.18

1.0I0.2

0.0210.01

2.03IO.17

Sarnple

"Al1 errors are 1 1 SD. For most of the core samples, a second row of data is shown, corresponding to sediment 10-20 cm above or below

the TL sample. Sample FRSH87-1 was too small to make both radioactivity and TL analyses. The numeric suffix for core FD87A1 samples equals the depth in feet.

bMass water / mass dry sample (A value of Wintle and Huntley 1980). A values of the organic fraction of the GILRD samples were also measured (see text). These values are as follows: 0.20 I 0.02 and 0.35 I 0.03 (GILRD2-2 and adjacent sediment, respectively);0.070 I 0.005 and 0.010 0.005 (GILRDI-2 and adjacent sediment respectively). 'C, is the total a particle count rate, and CThis that part of the count rate due to the 232Thchah (Huntley and Wintle 1981). dRelativealpha efficiency factor (e.g., Berger 1988). This value has been only estimated for samples FD87A1-644, -966, and -1201. 'Cosmic-ray dose-rate component (estimated from data of Prescott and Hutton 1988 for geomagnetic latitude 50"). An effective burial depth equal to half the present depth was assumed for al1 FD87A1 samples. *Only a fraction of the total gamma dose rate was used for surface samples (after Aitken 1985, p. 72). Specifically, the assumed values are 0.55 I 0.05 for FRMDF and FDF samples and 0.60 10.05 for VEC samples.

*

21O

TEMPERATURE (OC) 270

r 2

330 1

1

BLEACH r l h

FRMDF-4B 270°C

4-1 1 p m

4

I

- 1O

10

30

Applied

4

/

/

O 50

70

Dose (Gy)

FIG. 5 . Partial-bleach and equivalent-dose (inset) plots for sample FRMDF4B from the Sturgeon Bank tidal flat, showing near-zerointercepts, as would be expectedfor sediment whose light-sensitive TL has been almost completely removed during deposition. The error bars in the inset are I1 SD.

CAN. J. EARTH SCI. VOL. 27, 1990

TABLE 2. Equivalent-dose values and TL age estimatesa

Sample FRSH87-1 FRMDF-4B FRMDF-6B

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Temperature interval C0C)c

De

FDF-1 FDF-3

GILRD2-2

0.37?0.31 1.02I0.22 0.4810.45 0.6120.28 0.5410.27 0.61IO. 15 0.5720.67 1.9510.30 0.4450.35 1.2810.42

230-280 230-280 250-290 250-300

5.311.4

280-320 na na 270-340 270-310 280-320 270-320 270-320

-

GILRD1-2 13.322.0 18.214.1 21.923.1 26.014.4 28.412.2

FD87A1-510 FD87A1-573

Age (ka)d

Remarkse Suspended silt, 3.8 m water depth, Fraser River channel Sandy silt, 3-5 cm below surface of Sturgeon Bank tidal flat

-

0.2810.26 0.41+-O.19

Clayey silt, 0.5-3.0 cm below surface of Sturgeon Bank tidal flat Silt, 2-6 cm below sea floor, 16.8 m water depth, Fraser delta slope Clayey silt, 1-5 cm below sea floor, 16.2 m water depth, Fraser delta slope Silty clay, 2-6 cm below sea floor, 242m water depth, fan at base of Fraser delta slope Same as VEC-02, but 239 m water depth 5.111.4

Organic clayey silt, 5.1 m deep in core, eastem Lulu Island

-

Clayey silt, 7.45 m deep in core, eastem Lulu Island

8.051.1

-

Silty clay, 155 m deep in core, southem Fraser delta

10.351.9

-

Silty clay, 175 m deep in core, southem Fraser delta

13.3k1.4

*

"Al1 errors are 1 SD. The 2-1 1 pm grain-size fraction was used for the first sample, and the 4-11 pm fraction for the others. bThe upper and lower weighted mean De values correspond to 1 h and 24 h optical bleach times, respectively. The cited error is the arithmetic mean of errors in D, values over the temperature range selected as the plateau (Berger and Huntley 1989) and may sometimes be an overestimate of the true statistical uncertainty in the weighted mean De value. 'This temperature interval in the glow curves corresponds to the effective plateau in De values. na, no acceptable plateau. dTL age equals De divided by the dose rate in Table 1 . For the last three samples, a "zero-point" value of 1 Gy was subtracted from the De values before age calculation. For GILRD2-2, a "zero-point" value of 0.5 Gy was subtracted. 'Additional core samples (see Table 1): -644, silt with fine sand laminations, 196 m deep in core; -852, silty clay, 260 m deep in core; -966, silty clay, 294 m deep in core; -1201, laminated silty clay, 366 m deep in core.

21 O

-

TEMPERATURE ("C) 270 330

BLEACH

. A l h

-20

20

60

1 O0

Applied

1 40

,8

180

220

Dose (Gy)

FIG. 6. Partial-bleach and equivalent-dose plots for core sample GILRD2-2. An acceptable plateau in the equivalent-dose values was obtained only for the short-bleach-time data.

BERGERETAL

TEMPERATURE (OC)

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220

280

340

Applied

@

Dose (Gy)

FIG. 7. Partial-bleach and equivalent-dose plots for core sample FD87A1-510, showing acceptable plateaus for both the short- and long-bleach-time data (see also Fig. 3). Weighted saturating exponentials (Berger et al. 1987a) were used to fit the TL growth curves for al1 samples from this core.

Applied y Dose (kGy)

250

3 10 TEMPERATURE ('C)

370

FIG.8. Partial-bleach growth curves for core sample FD87A1-966 and equivalent-dose plots (inset) for al1four samples of unknown age from this core. In this example the intersection of the growth curves is not statistically below the zero-TL axis. No acceptable plateaus in De values were observed for these samples. significant differences between the short- and long-bleach-time sistent and encouraging. They show that sediments deposited in D, values for these samples. Because the long-bleach-time data a variety of deltaic and fluvial environments are sufficiently have smaller errors, they were used to calculate the TL ages zeroed for accurate TL dating to be possible. For nonfan. (Table 2). The resulting zero-point-corrected TL ages (10.3 2 deposits older than 12-15 ka, the observed zero-point error 1.9 and 13.3 2 1.4 ka) are consistent with converted radiocarprobably can be neglected. However, even for the two Holocene bon ages on shells and wood from bounding strata (Fig. 2). test samples from core GILRD, subtraction of the observed Furthermore, the 13 ka result is a reasonable minimum age for zero-point De values from the plateau values makes almost no deglaciation, since drift is present just below sample F D ~ ~ A I - difference (Table 2), because the errors in the plateau De values 573. for these samples are similar in magnitude to the zero-point In summary, the results from the zero-age samples are convalues. The large errors in plateau De values observed here

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CAN. J. EARTH SCI. VOL. 27, 1990

probably could be reduced by replicate experiments or by improvements in sample preparation.

temperature feldspar signal or because the sediments were deposited without effective zeroing of TL feldspar clocks.

Unknown-age samples TL ages could not be calculated for the deeper samples in core FD87A1 because no plateaus in De values were observed (Fig. 8). This is partly a consequence of increasing errors at higher glow-curve temperatures and may be related to the presence of a large quartz fraction in these samples (quartzlfeldspar ratios of 2:1 to 3: 1 were measured in a few samples throughout the core). Specifically, the light-sensitive TL of quartz in waterlaid sediments is not zeroed at the time of deposition (Berger 1988), and at the higher useful glow-curve temperatures for the older samples the thermally stable TL signal from feldspars (peak