British Columbia and northwestern. Washington State. Olav B. Lian, Jinsheng Hu, D.J. Huntley, and Stephen R. Hicock. Abstract: The suitability of optical dating ...
Optical dating studies of Quaternary organic-rich sediments from southwestern British Columbia and northwestern Washington State Olav B. Lian, Jinsheng Hu, D.J. Huntley, and Stephen R. Hicock
Abstract: The suitability of optical dating using 1.4 eV (infrared) excitation for determining the time of deposition, or compaction, of organic-rich sediments and peat is assessed with measurements on seven samples from six different lithostratigraphic units. One is of zero age, two have associated I4C ages, three are known to have been deposited during an interglaciation, and one is 1 Ma old. The samples yield satisfactory optical ages ranging from 0 to over 100 ka. We conclude that the Muir Point Formation (southern Vancouver Island) and the Whidbey Formation (northwestern Washington State) were both deposited during 6180 stage 5, as previously deduced from other evidence. The age obtained from the - 1 Ma sample was significantly too low. The optical dzting method is simpler and more precise than thermoluminescence dating, and is recommended for future work.
-
RbumC : L'applicabilitt de la mCthode de datation optique utilisant une source d'excitation de 1,4 eV (infrarouge) pour dkterminer le temps de dCp6t, ou de compaction, des stdiments riches en matikre organique et de la tourbe a kt6 vCrifiCe au moyen de mesures effectukes sur sept Cchantillons prklevks dans six unites lithostratigraphiques difftrentes. Parmi ces unitks, une est d'bge zCro, deux sont associCes a des dges I4C, trois sont reconnues comrne ayant CtC dCposks durant un intreglaciaire et une est bgCe de 1 Ma. Les dges obtenus pour ces Cchantillons par datation optique varient de 0 a plus de 100 ka. Nous concluons que la Formation de Muir Point (partie sud de llile de Vancouver) et que la Formation de Whidbey (partie nord-ouest de 1 ' ~ t a tde Washington) furent dtposkes l'une et l'autre durant le stade 5 de 6180, en accord avec les conclusions tirkes de diffkrentes mkthodes. L'tchantillon de 1'unitC datCe de 1 Ma a fourni par datation optique un dge nettement trop jeune. La mCthode de datation optique est plus simple a utiliser et elle procure des rCsultats plus prCcis que ceux fournis par la mCthode de thermoluminescence, son utilisation est recommandte pour les travaux futurs. [Traduit par la rCdaction]
-
-
Introduction The potential for thermoluminescence (TL) dating of organicrich sediments was discovered more than a decade ago. The first indication was that mineral grains extracted from a sequence of buried Holocene soils (A horizon) from a terrace above the Peace River, British Columbia, gave correct TL ages, whereas the aeolian sediment from which they were formed did not; bioturbation was thought to be the cause of sufficient sunlight exposure prior to burial (Huntley et al. 1983). Subsequently, Divigalpitiya (1982) studied several peats and other organic-rich sediments, with ages ranging from 0 to 800 ka, and obtained reasonable TL ages (Huntley et al. Received October 8, 1994. Accepted March 22, 1995.
O.B. Lian,' J. Hu, and D.J. Huntley. Department of Physics, Simon Fraser University, Burnaby, BC V5A 1S6, Canada. S.R. Hicock. Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada. Corresponding author (e-mail: OLIAN@SFU. CA).
1983). Fine peaty sediments were chosen for this study because they form by the slow accumulation of organic and mineral material in standing or slowly moving water. Mineral sediment introduced to such an environment has usually arrived by aeolian or "low-energy" fluvial processes, processes which are likely to have allowed sufficient sunlight exposure for dating. Since the work of Divigalpitiya (1982), little has been reported on this type of sediment (two exceptions are Wintle et al. 1984 and Berger and Mahaney 1990). In this paper we present an optical dating study, using 1.4 eV (880 nm) excitation, of mineral sediments extracted from four organic-rich lithostratigraphicunits whose ages are well established by other methods, namely a modern peat (Burns Bog), the Sisters Creek Formation, the Cowichan Head Formation, and Salmon Springs Drift, and from two lithostratigraphicunits whose ages are relatively uncertain, namely the Whidbey Formation and the Muir Point Formation. Correct optical ages from the units of known age should lend credibility to the optical ages obtained for the latter units. For some of the samples we provide results from TL experiments for comparison.
Can. J. Earth Sci. 32: 1194- 1207 (1995). Printed in Canada 1 Imprim6 au Canada
I
1195
Lian et al
Luminescence dating of sediments works on the principle that minerals contain impurities and structural defects, some of which may act as traps for electrons. Interaction of cr, P and y (from the decay of U, Th, and 40K in the sediment) and cosmic-ray radiation with matter results in free electrons that migrate through the crystal lattice; some of these electrons become trapped at defects, and, over time, the number of these trapped electrons increases. If the sediment is uncovered by natural erosion, for example, and exposed to sunlight, light-sensitive electron traps will be emptied. With subsequent burial, these traps will fill again. Luminescence, whether stimulated by heat (as used for TL dating) or by light (for optical dating), is used to obtain a measure of the number of trapped electrons and, in combination with a measurement of the ambient dose rate, the time when the mineral was last exposed to sunlight (usually the time of deposition) may be estimated. Aitken (1985) gives a detailed discussion of the principles of TL dating, Berger (1988) gives a good review relevant to TL dating of sediments, and Wintle (1993) and Aitken (1994) review recent developments in optical dating of sediments. Optical dating, first introduced by Huntley et al. (1985), has a number of advantages over TL dating regarding the determination of past radiation doses, both conceptually and experimentally. Minerals have electron traps with a great range of sensitivities to light. Some traps are emptied by as little as a few seconds of sunlight exposure, whereas for others sunlight has little or no effect (see Godfrey-Smith et al. 1988). All are sampled in TL dating, whereas in optical dating the traps that are most readily emptied by sunlight are sampled preferentially, and traps that are not emptied by sunlight are not sampled at all. In general, those traps which are most readily emptied by sunlight and dominate the luminescence under optical excitation make no significant contribution to the TL. Optical dating using light from an argon laser (2.41 eV, 514 nm) has been shown to be capable of giving ages consistent with other evidence (Huntley et al. 1985; Smith et al. 1990; Stokes and Gaylord 1993). The discovery by Hutt et al. (1988) that infrared light of about 1.4 eV could be used for the excitation of K-feldspars was quite unexpected, because such photons do not have sufficient energy to excite electrons directly from the relevant traps to the conduction band. Nevertheless, dating using 1.4 eV excitation is possible, and is important because the appropriate apparatus can be made simply and cheaply. A rigorous demonstration of procedures needed to obtain correct ages using this method has not yet been provided, although progress in this direction has been reported by Hutt et al. (1988), Spooner et al. (1990), Duller (1992), Aitken and Xie (1992), Edwards (1993), and Ollerhead et al. (1994). Huntley et al. (1993) showed that satisfactory ages could be obtained from inclusions, which were thought to be K-feldspars, within quartz grains, over a time span of 400 ka. The upper limit of this method is, at present, not known. With any of the above methods, recognition of the potential for errors in methodology due to unforeseen effects is important. For this reason, the reader should always exercise caution in accepting luminescence ages. Tests of the various methods that different laboratories use on samples of known age are remarkably scarce. We help to fill this gap with this study.
Fig. 1. Map showing the locations mentioned in the text.
VANCOUVER
PA ClFlC OCEAN
Lithostratigraphic units and sample locations Burns Bog peat Burns Bog (Fig. 1) comprises a modern peat that is presently forming on the Fraser delta (Hebda 1977; Clague et al. 1983). The sample used for this study @BP4) was a subsample of that collected from a depth of 25 cm by Divigalpitya (1982) (his sample BBP-3). On the basis of the estimated sedimentation rate (Hebda 1977), the sample age is calculated to be younger than about 500 a. It was selected to test whether or not our methods yielded an age of zero for a very young sample. Sisters Creek Formation The Sisters Creek Formation (Hicock and Lian 1995) includes sediments deposited during the Port Moody interstade, a nonglacial interval that occurred in the Fraser Lowland and adjacent mountain valleys of southwestern British Columbia during the Fraser Glaciation (6180 stage 2) (Hicock et al. 1982; Hicock and Armstrong 1985; Hicock and Lian 1995). The duration of the Port Moody interstade in the western Fraser Lowland presently is defined by several radiocarbon ages between 18 700 f 190 BP (GSC-2344) and 17 600 f 130 BP (Beta-38907). Because all of the radiocarbon ages are closely spaced, the Port Moody interstade may have lasted no more than 2-3 ka (Hicock and Lian 1995). Our sample, HCP1, was collected at Hollyburn Creek (Fig. 1) from a 25 cm thick compressed woody peat bed (Figs. 2 and 3) that overlies fluvial sand and channel boulder
Can. J. Earth Sci. Vol. 32. 1995
1196
Fig. 2. Stratigraphic sections and sample locations. Lynn Valley section: the section shown is from the east bank of Lynn Creek; the radiocarbon ages are from a section exposed at the west bank (see Armstrong 1990, p.87). Sumner section: the pluses (+) and minuses (-) indicate normal and reversed magnetization, respectively (from Easterbrook et al. 1981, Fig. 2, p. 88); the fission-track age is from Westgate et al. (1987). Muir Point section: the type section A is based on Fig. 2 of Alley and Hicock (1986); section B was measured during this study and is exposed about 200 m east of type section A. Note the various vertical scales. Hollyburn Creek
Useless Bay
boulder gravel (Holocene?)
Lynn Valley
Sumner
I
boulder gravel (Holocene)
/3(bF$267:;Fp
,480
+ 90 ka (SSP-1; TL age)
s a n d
....
,,,,, ,,,,,volcanic ash (Lake Tapps tephra) ,
>>:.>j>.I.. C ..
C:zci.;< pi;
