Quaternary Science Reviews, Vol. 7, pp. 339-345, 1988. 0277-3791/88 $0.00 + .50 Printed in Great Britain, All rights reserved. Copyright ~) 1988Pergamon Press plc ZEROING OF THE TL SIGNAL IN SEDIMENT UNDERGOING FLUVIOGLACIAL TRANSPORT. AN EXAMPLE FROM AUSTERDALEN, WESTERN NORWAY A.M.D. Gemmell Department of Geography, University of Aberdeen, Aberdeen AB9 2UF, Scotland Study of levels of natural TL in sediment undergoing fluvial transport in Austerdalen, Norway, shows that bleaching does occur, but does not follow a regular pattern. This is attributed to the incorporation of older, unbleached material into the sediment load as a result of riverbank erosion. Levels of residual TL are high enough to raise doubts about the accuracy of fine-grain TL dating of fluvioglacial sediments which have experienced only a short distance of transport. INTRODUCTION The ability to accurately date sediments using thermoluminescence techniques (TL-dating) has gradually been increasing as techniques have been developed and refined. The advent of such a technique, capable of dating and correlating such materials as unfossiliferous river terrace sediments, loesses and lake deposits, is seen as an invaluable aid by the Quaternary stratigrapher. It would be prudent, however, to adopt this new form of dating cautiously, for, as has been indicated in numerous reviews (e.g. Wintle and Hunt- ley, 1982; Singhvi and Wagner, 1986; Dreimanis et al., 1978; Singhvi and Mejdahl, 1985; Huntley et al., 1983; Huntley, 1985), the ease with which sediments can be dated by TL varies according to a range of environ- mental factors, the most significant of which appears to be the predepositional exposure of the material to light. Without this exposure, the TL 'clock' in the mineral grains making up the sediment will not be zeroed, and erroneously old dates will be obtained unless suitable precautions are taken in the analysis. Fluvial and fluvioglacial sediments are one group of materials of immense significance in the Quaternary record, and yet they have been relatively neglected by TL workers. Those who have investigated such materials have had rather mixed fortunes. Bryant et al. (1983) obtained TL dates of 106 + 11 ka and 107 + 15 ka for fine-grained water-laid sediments, but radio- carbon dating of plant microfossils from the same 36600_ 18ooBP. Contamination horizon gave an age of +2400 of the organic sample was suspected, but repetition of the analyses confirmed the first result. It would thus appear that some factor had affected the TL levels, the most likely being incomplete zeroing at the time of deposition. Such problems are not universal in dating alluvial deposits. Nanson and Young (1987) report good correspondence between TL dates and 14C dates from a sequence of braided river deposits in Australia, though they note that a clay-rich layer within the alluvial sequence did not give good results, the TL dates being some 60% older than the 14C age. This they attributed to residual TL remaining in the sediment at the time of deposition because suspended sediment reduced the penetration of light into the water. Similar conclusions relating discrepant TL dates from Quaternary lacustrine sediments to the filtering effect of suspended sediment upon light penetration into water have been reached in North America by Berger (1984, 1985a, b), Berger et al. (1987) and Huntley et al. (1983). The overall conclusion of this work is that only sediment particles which settle out of suspension slowly are likely to receive sufficient light exposure to be properly zeroed prior to deposition. Even these sediments can only be accurately dated provided such problems as anomalous fading, laboratory overbleach- ing and glow-curve peak shifts are taken into account. A particular factor is that the presence of suspended sediment shifts the wavelengths of light penetrating the water towards the red (Jerlov, 1976; Berger, 1986; Berger and Luternauer, 1987). Kronborg (1983) demonstrated that effective bleaching could occur through a column of lake water at least 7 m deep, but suggested that the inability to obtain accurate dates from fluvioglacial sediments was probably because the TL signal in such materials had not been completely released during transport and deposition. Laboratory experiments by Gemmell (1985) demonstrated that speed of flow and suspension load will also influence rates of bleaching of such materials. In order to test these ideas a number of studies of modern, zero-age river sediments have been conducted (Huntley, 1985; Huntley et al., 1983; Berger and Huntley, 1982). Such sediments should show a very low ED (equivalent dose) if they have been effectively zeroed. Many of the sampled silts gave ED values <5 Gy, but only by using laboratory bleach with an appropriate filter in front of the sunlamp to simulate the attenuation of sunlight by silty water. Even then, one sample gave an ED of about 25 Gy, an aberration which Huntley (1985) attributed to the fact that this sample had been collected in winter at a time of low solar insolation. The modern silt samples noted above were all 339
Transcript
Quaternary Science Reviews, Vol. 7, pp. 339-345, 1988. 0277-3791/88 $0.00 + .50 Printed in Great Britain, All rights reserved. Copyright ~) 1988 Pergamon Press plc
ZEROING OF THE TL SIGNAL IN SEDIMENT UNDERGOING FLUVIOGLACIAL TRANSPORT. AN EXAMPLE FROM AUSTERDALEN, WESTERN NORWAY
A.M.D. Gemmel l Department of Geography, University of Aberdeen, Aberdeen AB9 2UF, Scotland
Study of levels of natural TL in sediment undergoing fluvial transport in Austerdalen, Norway, shows that bleaching does occur, but does not follow a regular pattern. This is attributed to the incorporation of older, unbleached material into the sediment load as a result of riverbank erosion. Levels of residual TL are high enough to raise doubts about the accuracy of fine-grain TL dating of fluvioglacial sediments which have experienced only a short distance of transport.
INTRODUCTION
The ability to accurately date sediments using thermoluminescence techniques (TL-dating) has gradually been increasing as techniques have been developed and refined. The advent of such a technique, capable of dating and correlating such materials as unfossiliferous river terrace sediments, loesses and lake deposits, is seen as an invaluable aid by the Quaternary stratigrapher. It would be prudent, however, to adopt this new form of dating cautiously, for, as has been indicated in numerous reviews (e.g. Wintle and Hunt- ley, 1982; Singhvi and Wagner, 1986; Dreimanis et al., 1978; Singhvi and Mejdahl, 1985; Huntley et al., 1983; Huntley, 1985), the ease with which sediments can be dated by TL varies according to a range of environ- mental factors, the most significant of which appears to be the predepositional exposure of the material to light. Without this exposure, the TL 'clock' in the mineral grains making up the sediment will not be zeroed, and erroneously old dates will be obtained unless suitable precautions are taken in the analysis.
Fluvial and fluvioglacial sediments are one group of materials of immense significance in the Quaternary record, and yet they have been relatively neglected by TL workers. Those who have investigated such materials have had rather mixed fortunes. Bryant et al. (1983) obtained TL dates of 106 + 11 ka and 107 + 15 ka for fine-grained water-laid sediments, but radio- carbon dating of plant microfossils from the same
36600_ 18ooBP. Contamination horizon gave an age of +2400 of the organic sample was suspected, but repetition of the analyses confirmed the first result. It would thus appear that some factor had affected the TL levels, the most likely being incomplete zeroing at the time of deposition. Such problems are not universal in dating alluvial deposits. Nanson and Young (1987) report good correspondence between TL dates and 14C dates from a sequence of braided river deposits in Australia, though they note that a clay-rich layer within the alluvial sequence did not give good results, the TL dates being some 60% older than the 14C age. This they
attributed to residual TL remaining in the sediment at the time of deposition because suspended sediment reduced the penetration of light into the water.
Similar conclusions relating discrepant TL dates from Quaternary lacustrine sediments to the filtering effect of suspended sediment upon light penetration into water have been reached in North America by Berger (1984, 1985a, b), Berger et al. (1987) and Huntley et al. (1983). The overall conclusion of this work is that only sediment particles which settle out of suspension slowly are likely to receive sufficient light exposure to be properly zeroed prior to deposition. Even these sediments can only be accurately dated provided such problems as anomalous fading, laboratory overbleach- ing and glow-curve peak shifts are taken into account. A particular factor is that the presence of suspended sediment shifts the wavelengths of light penetrating the water towards the red (Jerlov, 1976; Berger, 1986; Berger and Luternauer, 1987). Kronborg (1983) demonstrated that effective bleaching could occur through a column of lake water at least 7 m deep, but suggested that the inability to obtain accurate dates from fluvioglacial sediments was probably because the TL signal in such materials had not been completely released during transport and deposition. Laboratory experiments by Gemmell (1985) demonstrated that speed of flow and suspension load will also influence rates of bleaching of such materials.
In order to test these ideas a number of studies of modern, zero-age river sediments have been conducted (Huntley, 1985; Huntley et al., 1983; Berger and Huntley, 1982). Such sediments should show a very low ED (equivalent dose) if they have been effectively zeroed. Many of the sampled silts gave ED values <5 Gy, but only by using laboratory bleach with an appropriate filter in front of the sunlamp to simulate the attenuation of sunlight by silty water. Even then, one sample gave an ED of about 25 Gy, an aberration which Huntley (1985) attributed to the fact that this sample had been collected in winter at a time of low solar insolation.
The modern silt samples noted above were all
339
340 A.M.D. Gemmel l
collected from major rivers. Once entrained it is possible for silt-sized grains to be transported in suspension for considerable distances, but the above studies presented no information about the distance that the silt had travelled in the river prior to sampling. The experiment reported in this paper was devised in order to test for progressive loss of TL signal in such sediment during transport by a silt-laden stream, with the objective of assessing the rate at which bleaching might occur.
FIELD WORK AND SAMPLE COLLECTION
The selected field area was Austerdalen, a glacial valley in western Norway (Fig. 1). At the head of the valley, the glacier Austerdalsbreen descends from the Jostedalsbreen ice cap. Meltwater derived from this glacier is the principal source of supply for the river draining the valley. At present, the glacier terminates in a small lake from which the river issues (Fig. 2), flowing initially over a rock bar out onto the wider valley train infilling the floor of the trough. In this latter section, the river alternates between small-scale braided behaviour and single-channel flow, the latter being associated with those reaches where it cuts through moraine ridges (King, 1959).
The area was visited for the purpose of collecting samples in September 1985. Sampling points were established at seven locations along the course of the
o~/ ~ x. j ' i ~
FIELD AREA ~ f
(.o~: /
JOSTEDALSBREEN
AUST'E;RDALEN
mlli'iYt GLACIER 0 50kin ! i
FIG. 1. Location map of the field area.
river (Fig. 2). At each of these locations, a sample was taken by dipping a 300 ml bottle into the river by hand. As soon as the bottle was full, it was sealed, wrapped in black polythene, and placed in a light-tight container. It was kept in that container until it arrived in the laboratory for analysis. This procedure was repeated on five separate dates. At present, analysis has been completed for three of those days, and the results are presented below.
Sampling always followed the same sequence, start- ing at the glacier snout, and working downstream. The objective of this was to try and minimise the effect of fluctuations in weather and light on the sediment samples to maximise comparability of results for any given day.
LABORATORY ANALYSIS
In the laboratory, the bottles containing the water plus sediment were opened and the contents dried at 60°C for at least 7 days. The 2-8 micron fraction of the material was isolated by suspension in acetone, and deposited upon A1 discs. No pretreatments were applied. Because the sediment load in most samples was very low, only two or three (in one case only one) discs could be made from most of the samples. No attempt was made to determine the mineralogy of the sediment on the discs.
The samples were glowed out at a heating rate of 5°C sec-~ in an atmosphere of oxygen-free nitrogen. Corn- ing 7-59 and 4-69 filter combinations were used for the analysis of samples from two of the days (21/9 and 24/9) while the samples from 26/9 were examined using Schott UG11 and Chance-Pilkington HA-3 filters.
RESULTS
Trends in Natural TL Because of the paucity of material, it was not
possible to perform full TL analysis of the samples, so it was decided that the first step was to determine trends in natural TL. The anticipated result if models of bleaching of river sediments are reliable would have been for levels of TL to decline with distance down- stream, most probably along some form of decay curve (Huntley et al., 1983). This result would, however, be dependent upon the sediment not already having been zeroed within the glacier system prior to release into the meltwater flow.
In the case of Austerdalsbreen, there are no sig- nificant supraglacial stream systems, and the vast majority of meltwater flow comes from englacial and subglacial sources. This basal melt will incorporate sediment which has been dragged or crushed under- neath the ice. The debate as to the effect of this grinding process on the level of TL in the sediment is complex. A number of workers (e.g. Lewis, 1968; Morozov and Shelkoplyas, 1980; Butrym, 1986) suggest that such grinding will act to lower levels of TL, and may even be a zeroing mechanism. Conversely, Berger
FIG. 2. Map of Austerdalen, showing the sampling points for suspended sediment. The moraine ridge marker A is believed to date from 1750, that marked E from 1865 and that marked H from 1928 (King, 1959).
10
n. 5 I -
¢_
--X.,- 21/9/85 --0- 24/9/85 --A-- 26/9/85 ....... MEAN TL AFTER IIDAYS SUNLIGHT
D I S T A N C E D O W N S T R E A M FROM G L A C I E R ( K M )
FIG. 3. Fluctuations in the level of natural TL in suspended sediment samples, Austerdalen. The values shown are for TL at 375°C.
342 A.M.D. Gemmell
(!984) found that the TL characteristics of a lodgement till did not support the idea of mechanical abrasion resetting the TL 'clock', while Busuoli (1978) thought a rise in TL signals might result from such 'mutual friction'. As can be seen from Fig. 3, the levels of TL, after second glow normalisation, demonstrate clearly that the sediment issuing from the glacier snout has not been completely zeroed, and that further bleaching takes place during transport by the stream. No measurements were taken of the rate of stream flow, but it is likely that a particle being carried in suspension by the stream would be able to cover the 4 km length of the survey reach in a matter of hours rather than days.
The trends of zeroing on the 21st and the 24th are very similar, showing an initial rapid decline and then a much gentler rate of decay with occasional oscillations. The trend revealed on the 26th, measured using a U G l l filter and so reflecting feldspar concentrations rather more closely than the other curves, is again one of overall decline, starting and finishing at levels similar to those of the other two days. The main difference comes in the major rise between the first two sample points, and the subsequent relatively elevated level of the curve for the next couple of kilometres of travel.
The fact that TL levels increase so rapidly between the first two sample points when measured with a U G l l , and yet fall rapidly over that same distance when measured with a Corning 7-59 (essentially a blue filter) is a puzzle. A possible explanation is that the first sampling point is on a small meltwater stream issuing from the snout of the glacier, and is only a few metres from the ice edge. The second sampling point is by the outflow from the proglacial lake. The lake will act as a sump, or even a 'mixing bowl', for suspended sediment from all the meltwater streams issuing from the glacier, and therefore the mineralogy of that suspended load may not be the same as that of the small stream on which the first sampling point is situated. The second factor is the possibility of differential settling of sediment within the lake. Gemmell (1985) observed that even in flowing water, there was a tendency for quartz to settle and the suspended sediment to become increasingly feldspar-rich, with a corresponding decline in quartz signal and increase in that from feldspar.
Bleaching Experiments In an attempt to determine whether or not the
flattening out of the TL curves meant that the sediment had been effectively bleached over the 4 km transport distance studied, some discs from samples taken on 24/9 were exposed to light. The discs were placed on a north-facing window ledge (inside the window, which was of ordinary window-glass), one disc being removed for analysis after five days and the others after eleven days exposure. Each was glowed out, normalised using a second-glow procedure, and the results compared with the value of natural TL for that sample. The results are given in Table 1.
The bleached level of TL achieved after 11 days exposure to light is almost identical in both cases, and
TABLE 1. Bleach test on suspended sediment samples
Length of bleach 5 days 11 days
Glacier snout samples 0.579 0.408 Lake outfl.ow sample Not tested 0.822
Values quoted are for TL(bleach)/TL(natural) (375°C).
has therefore been marked on Fig. 3. It can be seen from this that the sediment at the glacier outflow is certainly not zeroed, and while significant progress towards full zeroing occurs during the passage of sediment through the lake, further bleaching is still possible. Even if the eleven day artificial bleach did zero the sediment - - which cannot be confirmed with present evidence - - the material in the stream was not reduced to that level even after transit through the full length of the test section.
Also worthy of comment is the fact that the level of TL occasionally is seen to rise downflow. This is probably due to the re-entrainment of 'older' sediment from the bed and banks of the stream as it flows over its floodplain, a phenomenon anticipated by Gemmell (1985). Jungner (1983) noted the problems caused by mixtures of unbleached and bleached grains in sedi- ments, and the present study indicates that precautions should be taken in respect of this when trying to date fluvioglacial sediments.
DISCUSSION
The results obtained from the experiments outlined above indicate that it should not be assumed that fluvioglacial sediments have been adequately zeroed prior to deposition. An attempt has been made to calculate EDs for the sediment samples, and to investigate whether they follow the same fluctuations as TL(natural). Because so few discs could be prepared from each water sample, the normal methods of calculating TL could not be employed. Instead each disc was treated as a sample in itself. Once TL(natural) had been determined, then the disc was given a laboratory dose using an SR-90 beta source, stored overnight and then glowed out. This was repeated for doses of decreasing size to build a multiple-glow regeneration curve (Fig. 4). Some changes in sensi- tivity, possibly due to predose effects, are revealed by glow curves (Fig. 5). Most growth curves fitted very closely to a straight line, and no attempt was made to introduce a correction for sensitivity change.
As the start point for the regeneration curve is heat- zeroed rather than light-zeroed, it was suspected that the dose required to regenerate TL(natural) would be too high to be realistic. To compensate for this, each disc was given further doses of irradiation, but then exposed to light for a fixed time prior to glowing out (Fig. 4). The results were used to construct a second growth curve. The two curves were used as an R-Beta type of analysis, with the intersection between the lines marking the level to which light could reduce the
Zeroing of the TL Signal 343
X J I -
f A
. A J '
,~ ED DOSE
FIG. 4. The use of the multiple glow technique to determine a value for the equivalent dose of suspended sediment from a single disc. Line A is the natural + dose growth curve, line B the natural + dose + light growth curve. A full explanation is given in the
text.
_1 I.-
o o 5"o
I
I
l I
1 l I i I
/ 4
I , I , I I ' I I
i t ~\ x \
i x x \ x
S T R E A M S A M P L E 2
. . . . . . . . CURVE A , BACKGROUND TL
8 , 2ndGLoW + 5 0 0 s e c s . B C , 2ndGLoW + t O 0 0 s e c s , I B
FIG. 5. Glow curves for a single disc. The sample being analysed was collected at the outflow from the proglacial lake (sampling point 2) on 24/9/85.
sample. The E D was considered to be the dose needed to regenerate TL(natural) from the zero point, unless the intersection between the two growth curves was to the left of the Y-axis. In the latter situation, which did not occur very often, the E D was taken to be that shown by the initial regeneration curve.
This technique was used on samples from 24/9 and
26/9, the results being presented in Table 2. The bleach time used in the latter case was 18 hours under a mercury U V lamp (with emissions peaking at 365 nm). Because of worries about overbleaching, the samples from 24/9 were given a much shorter exposure, being placed in natural daylight on a north-facing window ledge for I hour only. Subsamples taken from the same
344 A.M.D. Gemmell
TABLE 2. Equivalent dose values for suspended sediment samples
24/9/85 26/9/85 Distance from Plateau ED Plateau ED glacier (km) (°C) (Gy) (°C) (Gy)
Note: the values for the ED are mean values for the subsamples obtained from the sample at that location.
bottle gave results generally consistent to within 15%, so there is some measure of confidence in the tech- nique.
The downstream fluctuations in ED values on the two dates show substantial similarities (Fig. 6). The most significant discrepancy is seen at the snout of the glacier. The reason for this is unknown. At that sample point, the stream is fed mainly by basal melt, but on occasion a supraglacial melt-out till (Lawson, 1979) may flow into it. The sediment load of the stream would then be 'spiked' by material which had received a greater degree of exposure to solar radiation than the normal basal sediment (Gemmell, 1988). If such a flow fed the stream between the 24th and 26th, then the difference in ED levels on the two dates could be explained.
Downstream fluctuations in ED levels suggest that re-entrainment of older material is taking place. To check this, bank material was sampled at three locations, and ED values calculated. When these values are plotted against those from suspended sediment samples (Fig. 6), it can be seen that incorporation of
such material into the river by bank erosion can easily explain the fluctuations in level below the lake outflow.
As a further check on bleaching of stream sediments, two samples of mudbank material were taken from the floodplain adjoining the river, one 1.2 km below the glacier snout, the other 8 km from the snout. These are samples of material dropped when the river flooded, and are likely to have been deposited fairly rapidly. Solar exposure during deposition would have been correspondingly brief, and would have been insig- nificant compared with exposure during transport. Both samples bury floodplain vegetation, and have not themselves been colonised by plants, so their age can be no more than a few tens of years.
The mudbank samples were analysed using a form of the total bleach technique (Singhvi et al . , 1982), using a light exposure of three days natural daylight. The mudbank nearer the glacier gave an ED of 243 + 29 Gy. As the sampling point lies just downflow from a section in which the stream is actively undercut- ting its bank, this high value may reflect the input of 'old' material to the flow. The sample taken 8 km from the glacier had an ED of 44 + 17 Gy, suggesting that bleaching still continues downstream from the studied reach. These EDs are similar to those obtained by Vlasov et al. (1978) from modern river sediments in Russia.
CONCLUSIONS
The study of the TL of fluvioglacial sediment presently undergoing transportation reveals that the simple bleach curves produced during laboratory tests on samples are unlikely to reflect the experience of the material in nature. There, reincorporation of older
2 5 0 '
2 0 0 ,
A 1 5 0 , >- co
t 0 0 '
5 0 '
- - 0 - - 2 4 / 9 / 8 5 (7°59 WITH 4 - 6 9 )
- - A - - 2 6 / 9 / 8 5 (UG 11 WITH HA:5)
- - I I - - RIVER BANK SAMPLES ( R - BETA METHOD)
, '1 ' . . . . . . . . . . ~-.-~'----- ~ o - * ,,,' 1 lj i i ' 1 .L
0 1 2 3 4 5
DISTANCE DOWNSTREAM FROM GLACIER (KM)
FIG. 6. Fluctuations in equivalent dose for suspended sediment with distance downstream from Austerdalsbreen. The river bank samples were collected where the stream had cut sections in moraine ridges.
Zeroing of the TL Signal 345
sediment into the flow produces a much less regular pattern of zeroing with transport distance. The 'zero- age' samples analysed here gave equivalent doses of up to 240 Gy, despite having some quite well-developed plateaux. Such sediments do indeed bleach during transport, but the rate of bleach is going to be affected by the speed of flow of the river (Gemmell, 1985). Speed of flow not only influences turbulence, and hence the turbidity of the water, but also the ability of the river to erode its banks and bed. Faster flow will give more erosive power, and potentially increase the proportion of 'older' sediment carried by the river. The age of that sediment which is being eroded or re- entrained by a river will be an important influence on the TL-age of modern fluvial and fluvioglacial sedi- ments.
The conclusion from this study must be that the incorporation of 'old' material into fluvial sediments during transport is a significant problem for TL-dating of such materials, particularly in the case of relatively young sediments. If the values quoted by Huntley (1985) for Canadian river silt samples where a 'zero- age' ED of only 5 Gy would lead to an error of about 2 ka in TL age are applied to the Austerdalen river sediments, then significant errors would be created in dating samples up to 500 ka in age. As this may be beyond the age at which some sediments can be dated by TL (see discussion in Debenham, 1985a,b,c; Mejdahl, 1985a,b; Wintle, 1985), the present study raises doubts about the suitability for dating of fine- grain fluvioglacial sediments which have only had a short transport distance. Possibly special techniques such as proposed by Jungner (1983) or by Berger et al. (1987) are essential if such sediments are to be dated accurately by TL.
ACKNOWLEDGEMENTS
The assistance of Ric Gard in the field, Maureen Lamb in the laboratory and Kay Leiper with the illustrations is gratefully acknowledged. The work was financially supported by the Carnegie Trust for the Universities of Scotland, and 20th International Geography Conference Fund of the Royal Society and the University of Aberdeen.
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