Late 18th century drought-induced sand dune activity, Great Sand ...

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Late 18th century drought-induced sand dune activity, Great Sand Hills, Saskatchewan S.A. Wolfe, D.J. Huntley, P.P. David, J. Ollerhead, D.J. Sauchyn, and G.M. MacDonald

Abstract: Geomorphic evidence and optical ages from seven locations indicate that widespread dune activity occurred within the last 200 years in the Great Sand Hills region of southwestern Saskatchewan. Optical ages (n = 36) define an interval of dune activity bracketed by the earliest age of back ridges in the Seward sand hills (185 ± 8 years) and the average age of stabilized dune heads (about 105 years). During this interval, parabolic dunes were active in all areas studied. These ages indicate that the most recent interval of activity was initiated about AD 1800, and continued at a level higher than present for approximately 80 years. The most likely cause of dune activation was lower-than-average precipitation (relative to 1960–1991 values) through the 1700s, culminating in drought in the late 1700s, as evidenced in dendroclimatic records from the Cypress Hills and from the Rocky Mountain foothills. Dunes affected by such climatically induced regional activity require many decades to restabilize. Historical observations show that dunes in this area have been restabilizing throughout the 20th century. For the southern Canadian Prairies, a region with serious concerns about the implications of global warming, this study highlights the sensitivity of sand dunes to drought and cumulative moisture stress. Résumé : Des preuves géomorphologiques et des âges optiques de sept emplacements indiquent qu’il y eut une activité généralisée des dunes au cours des 200 dernières années dans la région des Great Sand Hills du sud de la Saskatchewan. Des âges optiques (n = 36) définissent un intervalle d’activité des dunes encadré par l’âge de la plus précoce des chaînes arrière dans les collines de sable Seward (185 ± 8 ans) et l’âge moyen des crêtes de dunes stabilisées (environ 105 ans). Au cours de cet intervalle, des dunes paraboliques étaient actives dans toutes les régions étudiées. Ces âges indiquent que l’intervalle d’activité le plus récent a débuté vers AD 1800 et a continué à un niveau supérieur au niveau actuel pour environ 80 ans. La cause la plus probable d’activation des dunes était des précipitations inférieures à la moyenne (par rapport à 1960–1991) tout au cours des années 1700, culminant dans la sécheresse de la fin des années 1700, tel que démontré dans les données dendroclimatologiques provenant des collines Cyprès et des contreforts des Rocheuses. Des dunes affectées par une telle activité régionale induite par le climat ont besoin de plusieurs décennies pour se stabiliser de nouveau. Des observations historiques montrent que les dunes de cette région se sont stabilisées tout au cours du 20e siècle. Pour les Prairies canadiennes méridionales, une région pour laquelle le réchauffement planétaire pose de grandes inquiétudes, cette étude souligne la sensibilité des dunes de sable à la sécheresse et au stress hygrométrique cumulatif. [Traduit par la Rédaction]

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Introduction Received May 1, 2000. Accepted July 7, 2000. Published on the NRC Research Press Web site on December 18, 2000. Paper handled by Associate Editor R. Gilbert. S.A. Wolfe.1 Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8, Canada. D.J. Huntley. Department of Physics, Simon Fraser University, Burnaby, BC V5A 1S6, Canada. P.P. David. Départmente de géologie, Université de Montréal, Montréal, QC H3C 3J7, Canada. J. Ollerhead. Department of Geography, Mount Allison University, 144 Main St., Sackville, NB E4L 1A7, Canada. D.J. Sauchyn. Department of Geography, University of Regina, Regina, SK S4S 0A2, Canada. G.M. MacDonald. Department of Geography and Organismic Biology, Ecology, and Evaluation, University of California, Los Angeles, CA 90095–1524, U.S.A. 1

Corresponding author (e-mail: [email protected]).

Can. J. Earth Sci. 38: 105–117 (2001)

Stabilized and partly stabilized sand dunes occur across the Great Plains of the United States and Canada. With the recognition that many were active in the Holocene (Forman et al. 1992; Madole 1994; Holliday 1995; Muhs and Holliday 1995; Muhs et al. 1997a, 1997b; Muhs and Wolfe 1999), these dune areas have received considerable attention. Chronologies from these studies have been constructed through 14C dating of buried soils, which records ages of dune stability, and through thermoluminescence and optical dating of dune sands, which record ages of dune activity. These reconstructions reveal a close association between episodes of past eolian activity and drought intervals recorded in dendrochronological and paleolimnological records (e.g., Stockton and Meko 1983; Fritz et al. 1994; Laird et al. 1996). In addition, modelling studies show that even a modest increase in aridity can reactivate stabilized sand dunes within much of this region (Muhs and Maat 1993; Wolfe

DOI: 10.1139/cjes-38-1-105

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1997). Therefore, dune fields on the Great Plains are extremely sensitive to climatic variability, and the potential for reactivation of stabilized dunes is high, even without climatically induced warming (Muhs and Holliday 1995; Lemmen et al. 1998). On the Canadian Prairies, scattered and mostly stabilized sand dunes are widespread throughout the dry subhumid area called the Palliser Triangle (Fig. 1). Radiocarbon dating is difficult because of the limited organic matter in these eolian deposits. However, with recent developments in optical dating, it is possible to estimate the time since sediment was last exposed to sunlight and thereby develop a chronology of past eolian activity (e.g., Huntley and Lian 1999). Preliminary optical dating studies based upon eight samples from seven locations in the Palliser Triangle were first undertaken by Wolfe et al. (1995), who suggested that the most recent period of widespread dune activity occurred within the last 200 years. This present paper further documents the temporal and spatial extent of this last major period of regional dune activity, based on optical ages of sands from natural blowout exposures and from shallow pits within mostly stabilized dunes. The timing of this activity is compared with moisture trends reconstructed from dendroclimatic records, to determine a probable climatic cause. The southern Canadian Prairies is a region where serious concerns exist regarding the implications of global warming. Sand dunes hold strong potential for defining the impacts of past drought events in this region, as they represent a climatically sensitive component of the prairie landscape (Muhs and Wolfe 1999) and are good geo-indicators of surface moisture resources (Vance and Wolfe 1996). This study highlights the sensitivity of sand dunes in this important rangeland agricultural area to drought and cumulative moisture stress.

Study area Dune activity The Great Sand Hills is the largest contiguous dune occurrence on the Canadian Prairies (David 1977). It is surrounded by several smaller dune fields (Fig. 1), which together constitute the Great Sand Hills region and occupy a total area of more than 2000 km2. Dune sands are derived from glaciofluvial, glaciolacustrine, and deltaic sediments (David 1977) deposited during the retreat of the Laurentide Ice Sheet approximately 13 000 BP. Consequently, the sediment supply for dune fields in this region is finite, and dune activity is controlled by the availability of sand rather than by replenishment of source sediments (cf. Muhs and Wolfe 1999). Variations in climate influence the availability of sand by affecting moisture conditions and vegetation cover. The Great Sand Hills region is characterized by a dry subhumid climate. Precipitation (~300 mm·a–1) amounts to only about 60% of potential evapotranspiration (~500 mm·a–1) in an average year, and 40% or less in drought years such as 1987 and 1988 (Environment Canada 1995). Most dunes in the region are presently stabilized by vegetation; less than 1% of the area of dune sands is presently active. Vance and Wolfe (1996) related dune activity in subhumid and semiarid environments to moisture availability. They suggested that local reactivation of stabilized dunes may require drought

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conditions for months or years, whereas regional dune reactivation requires durations of several years to decades. Despite the increased aridity that occurred over much of the Canadian Prairies in the late 1980s, the duration of drought conditions was insufficient to promote widespread dune reactivation (Wolfe 1997). During the late Holocene, sand-dune activity on the southern Canadian Prairies varied with changing aridity and accompanying vegetation cover. David (1971) and Wolfe et al. (2000) have reported at least five episodes of late Holocene dune activity between approximately 3700 and 400 BP in the Brandon sand hills of southwestern Manitoba, resulting from dryness in an otherwise humid to subhumid climate. In the Great Sand Hills region, Wolfe et al. (1995) showed that sand dunes were active during the last millennium and particularly the last 200 years; possible periods of dune stability occurred around 2600 BP and between approximately AD 1400 and 1700. Dune morphology All sand dunes in the Great Sand Hills region are of the parabolic type (David 1998). Figure 2 shows the morphology of a typical parabolic dune in this area. The slip face is convex downwind in plan view, and the wings, where developed, point upwind. Generally, the wing ridges rise evenly toward the head, whereas the low area between the wings is deflated. This deflation depression is commonly eroded to a more resistant substrate. A back ridge, where present, connects the wings around the deflation depression to form the upwind extension of the dune. The back ridge is typically a low, arcuate sand accumulation, generally concave downwind and with an uneven crest line (Figs. 2, 4e). Because it is produced by multidirectional winds removing sand from the deflation depression, it is absent from parabolic dunes that develop under unidirectional winds (David 1988). Accumulation of this sand begins with the initiation of dune activity and continues until the upwind portion of the deflation depression is stabilized by vegetation. Back ridges delineate the farthest upwind position of individual dunes and, where dune activity has been triggered by a climatic shift to more arid conditions, can be significant chronological and morphological markers (David et al. 1999). Many dunes in areas of high local water tables in the Great Sand Hills region contain parallel, even-crested ridges between their wings (Fig. 2). Termed “dune-track ridges” (David 1998), these are either arcuate, sometimes slightly sinuous or irregular ridges that connect the wings. Welldeveloped examples are found in the Seward sand hills (Fig. 4e), where the water table fluctuates close to the ground surface. Dune-track ridges develop when a period of dune migration is interrupted by more humid intervals that promote vegetation growth around the base of the dune, while the rest of the dune remains active (David 1998). With a return to more arid conditions, the dune migrates downwind and a low, vegetated ridge (lower than the corresponding back ridge) may remain at the former back base line of the dune. The even crest line of the ridge reflects the limit to which vegetation invaded the back slope of the dune. Dunetrack ridges record only short-term climatic fluctuations, since any significant climate change would stabilize the entire dune, as is now widely observed throughout the region. © 2001 NRC Canada

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Fig. 1. Location of the Great Sand Hills region (inset) and sample sites in southwestern Saskatchewan.

In light of this previous work, the ages of dune-track ridges within a particular dune should postdate the basal age of the back ridge and should become successively younger downwind (David et al. 1999). Sand deposited on the back slope, head, or slip face of a dune should, in turn, postdate the age of the dune-track ridges, as these are typically the final portions of a dune to stabilize. Dendroclimatic records The Great Sand Hills region lies within the mixed-grass prairie and therefore supports no tree species useful for long-term climatic reconstruction. However, two dendroclimatic data sets from outside the region provide reconstructed total annual precipitation records for the last 300– 500 years. The first is an annual precipitation reconstruction for the period AD 1505–1992, developed by Case and MacDonald (1995) from Pinus flexilis James along the Rocky Mountains foothills of southwestern Alberta (Alberta foothills). This record correlates well with the Swift Current instrumental precipitation record. The second data set is an annual precipitation reconstruction for the period AD 1682– 1994, obtained by Sauchyn and Beaudoin (1998) from Picea glauca albertiana in the Cypress Hills, about 75 km south of the Great Sand Hills.

The reconstructed precipitation record of Case and MacDonald (1995) is based on the correlations of three ringwidth chronologies to total annual (August–July) precipitation, averaged from the Fort Macleod and Calgary climate stations (Fig. 1). The climate of the Alberta foothills is transitional between prairie and cordilleran, and the historical annual precipitation typically ranges from 510 to 610 mm. Using a calibration period of 1905–1987 (n = 82), a regression model was constructed that produced an adjusted R2 of 0.45 (Case and MacDonald 1995). Sauchyn and Beaudoin (1998) used mean monthly data for the period 1941–1994 from Maple Creek, 25 km north of the Cypress Hills. They regressed August–July total precipitation against standardized ring widths and obtained an adjusted R2 of 0.37 (n = 45). These reconstructions are discussed in this paper, because they provide the link between the optical ages of dune sands and past periods of drought; methodologies for each reconstruction are provided in the original references.

Methods Sampling Samples were collected for optical dating in several localities, both from natural exposures (blowouts) and from shal© 2001 NRC Canada

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Fig. 2. Schematic diagram of a stabilized parabolic dune with a back ridge and dune-track ridges (adapted from Wolfe and David 1997). Terminology for sand dunes in the Great Sand Hills region follows that of Wolfe and David (1997), David (1998), and David et al. (1999).

low pits dug on the surface of stabilized dunes (Figs. 1, 3, 4), to obtain a regional perspective on the last major period of dune activity. Because of the palimpsest nature of eolian deposits (cf. David 1998), samples were only collected from dunes with recognized morphology and internal structure, so that the geological relevance of the optical ages was known. The stratigraphy and sedimentology of each site was described to establish the origin and geomorphic significance of the deposits and to avoid bioturbated sites. Modern samples were collected from two active sand surfaces to test the ability of the optical dating method to give “zero” ages. Blowout exposures were sampled in seven older dune deposits at depths of 3.0–8.0 m below the modern surface (Fig. 3). Of the eleven samples collected from these sites, eight were dune-sand deposits, two were sheet-sand deposits (94-81, 94-82), and one was a basal (eolian–shallow lacustrine) sand (SW6-01). A bison bone was collected from the upper surface of the basal sand deposit for radiocarbon dating. Near-surface samples were collected from 23 pits dug 0.5–1.5 m into eight stabilized dunes (Figs. 1, 4). Samples were collected from the dune heads, back slopes, back ridges, and dune-track ridges of stabilized dunes, and from active surfaces of modern blowouts (Fig. 4) to estimate the last period of eolian activity. Sample sites were located within stabilized dune fields; dunes selected for sampling

were considered morphologically representative of surrounding dunes. Optical dating Optical dating determines the time elapsed since mineral grains were last exposed to sunlight. General descriptions can be found in Radiation Measurements (Vol. 27(5/6), 1997), Aitken (1998), and Huntley and Lian (1999). This study used sand-sized K-feldspar grains with 1.4 eV (infrared) excitation and measurement of the 3.1 eV (violet) emission (Ollerhead et al. 1994; Huntley and Clague 1996), except that the 95-series of samples were measured using a similar apparatus with a higher light-collection efficiency (Baril 1997). A typical example of the additive-dose data is shown in David et al. (1999, see Fig. 4). All aliquots, except some used for control, were heated prior to measurement for 16 h at 120°C; a 5 day, 140°C preheat on one sample (SW601) produced no difference in the equivalent dose (Table 1). Three of the grain separates were analyzed for potassium, and the fractions of the grains that were actually K-feldspar were determined using a scanning electron microscope. From these, values of 13.8 ± 1.2% K for SW6-01, 13.2 ± 1.4% K for 94-32, and 13.0 ± 0.8% K for 94-37 were determined (Huntley and Baril 1997). Since the maximum content possible is 14% K, a value of 13 ± 0.5% K was used to © 2001 NRC Canada

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Fig. 3. Stratigraphic sections and optical ages from sites in the Great Sand Hills region.

calculate the dose rate from the potassium within the grains. The calculated total dose rates are given in Table 1. Uncontrolled systematic errors may cause the optical ages to be too high by 20 years and (or) too low by 5–15%. Because these errors operate in opposite directions, they will partially cancel each other. The effects may completely cancel for samples about 200 years old. For much older samples, the true ages may be as much as 15% larger than calculated. See Appendix A for a more thorough discussion on the accuracy of the optical ages. Dendroclimatic records The total annual (August–July) precipitation reconstructions of tree-ring chronologies from the Alberta foothills (Case and MacDonald 1995) and the Cypress Hills (Sauchyn

and Beaudoin 1998) were used to examine past moisture variability in the region. The annual precipitation records, as well as calculated 3-year and 5-year running means, were compared to the precipitation record at Swift Current using Pearson’s pairwise correlations. Swift Current was used since it is the nearest climate station to the Great Sand Hills region with long-term continuous data. “Dry periods” were determined from the 5-year running mean records and were defined by intervals with a precipitation total of more than one standard deviation below the 1961–1990 mean. This comparison of a subset of the data to the complete set reduces the statistical bias caused by using standard deviation. The first standard deviation of this 30-year period was used, as it permits comparison of past dry periods to the recent historical record (cf. Stockton and Meko 1983). As this ap© 2001 NRC Canada

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Fig. 4. Aerial photographs of study sites in the Great Sand Hills region with sample locations and numbers identified: (a) Burstall sand hills (NAPL A21003-41, 43, 54, 56, taken in 1967); (b) northwest Great Sand Hills (photo by S.A. Wolfe, taken in Sept. 1994); (c) Westerham sand hills (NAPL A17661-121, taken in 1961); (d) Bigstick sand hills (NAPL A7611-56, taken in 1946); (e) Seward sand hills (NAPL A21004-63, taken in 1969). Notation after sample numbers: M, modern; DH, dune head; BS, backslope; DT, dunetrack ridge; BR, back ridge. Open arrows indicate direction(s) of dune-building winds. Black areas in Fig. 4e are ponds behind dunes and between ridges, indicating high water levels at the time of the photograph.

proach uses a 5-year running mean rather than annual data, it provides a conservative definition of dry periods by reducing the influence of single-year droughts. Long-term trends in moisture balance were also determined from the instrumental and reconstructed records by calculating percent cumulative departures from the mean annual precipitation. This helps reveal decade- to century-scale trends that may not be as evident in annual or running-mean records. The approach of summing the successive annual departures results in each record beginning and ending at the respective mean precipitation value.

Results Ages Stratigraphic sections – Burstall The Burstall section occupies the south wall of a 10 m deep by 50 m long blowout through the head and back slope of a stabilized parabolic dune (Fig. 4a). An optical age of 4190 ± 200 years was obtained from K-feldspar grains in a basal unit of grey unoxidized sand at a depth of 6.5 m (Fig. 3). The uppermost portion of the basal sand contains many lithic artifacts and bone fragments of Bison bison, plus © 2001 NRC Canada

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Table 1. Water content, potassium (K), uranium (U), thorium (Th), dose rate (in greys per thousand years), and equivalent doses (in greys) for the samples.

Sample No. SW1-02b SW2-02b SW3-02b SW4-01b SW4-02b SW5-01b SW6-01b

SW6-02 94-23 94-30e 94-31e 94-32e 94-33e 94-34e 94-35e 94-36e 94-37e 94-38e 94-39e 94-40e 94-41e 94-50 94-51 94-70 94-71 94-81 94-82 94-83 95-01 95-02 95-03 95-04 95-07 95-08 95-10e 95-11e

b

Water content ∆a 0.037 0.062 0.044 0.048 0.063 0.068 0.07

0.051 0.054 0.054 0.060 0.067 0.057 0.063 0.065 0.048 0.070 0.058 0.062 0.055 0.049 0.045 0.040 0.003 0.030 0.047 0.063 0.042 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.035

K (±5%) Whole sample

Surroundings

U (µg·g–1)

Th (±0.1 µg·g–1)

Dose rate (±0.13 Gy·ka–1)

Equivalent dose (Gy)

1.15 1.43 1.28 1.39 1.38 1.49 1.36

1.12 1.41 1.29 1.40 1.38 1.45 1.37

0.77±0.04 1.30±0.04 0.97±0.04 0.96±0.04 0.97±0.04 1.08±0.04 1.28±0.04

— — 2.9 — 3.1 — 4.1

2.49 2.93 2.73 2.83 2.78 2.64 2.60

— — — — 3.5 — — — — 3.9 — — — — — — — — 3.2 — — — — — 3.0 — 2.9 — 3.1

2.00d 2.85 2.70 3.15 3.08 3.13 3.13 3.02 3.12 3.11 3.11 2.99 3.10 2.99 3.08 f 2.79 2.66 2.79 f 2.95 2.83 2.86 2.82 2.88 2.87 2.88 2.85 2.74 2.84 3.07 3.04

0.17±0.03 0.32±0.02 0.25±0.01 1.82±0.15 2.57±0.11 0.57±0.02 10.7±0.8 10.9±0.3c 10.8±0.9d 0.48±0.02 0.23±0.01 0.387±0.015 0.288±0.015 0.363±0.019 0.295±0.015 0.460±0.019 0.390±0.010 0.542±0.019 0.523±0.015 0.477±0.026 0.426±0.025 0.351±0.019 0.052±0.014 0.36±0.02 0.28±0.02 –0.01±0.02 0.27±0.02 0.87±0.05 0.72±0.04 0.46±0.01 0.33±0.01 0.41±0.02 0.37±0.01 0.33±0.01 0.32±0.01 0.43±0.01 0.553±0.016 0.562±0.016

1.35 1.18 1.49 1.49 1.50 1.48 1.48 1.55 1.47 1.48 1.42 1.45 1.39 1.39 1.23 1.15 1.28 1.40 1.39 1.37 1.33 1.59 1.55 1.55 1.59 1.18 1.31 1.48 1.49

1.34 — 1.54 1.50 1.43 1.50 1.47 1.56 1.43 1.48 1.56 1.49 1.54 — 1.23 1.14 1.23 1.25 1.30 1.38 1.37 1.57 1.59 1.55 1.59 1.20 1.27 1.51 1.49

1.12±0.04 0.86±0.05 1.34±0.06 1.22±0.06 1.36±0.06 1.33±0.06 1.12±0.06 1.21±0.06 1.28±0.06 1.35±0.06 1.11±0.06 1.34±0.06 1.17±0.06 1.06±0.09 1.03±0.05 0.90±0.05 0.93±0.06 1.02±0.06 1.05±0.07 1.15±0.07 1.02±0.07 1.07±0.08 1.12±0.09 1.14±0.09 1.00±0.08 1.01±0.09 1.00±0.08 1.16±0.09 1.13±0.09

Note: The first potassium column is for potassium in the actual sample; the second is for a mixture of material collected from within 30 cm above and below the sample. Th contents, where not shown, were calculated using a Th/U ratio determined from the other samples. The grain size selected was 180–250 µm for all samples, except SW5-01, SW6-01, 95-01, 95-02, 95-03, and 95-04, for which it was 90–125 µm. a ∆ = (mass of water) / (dry mass); uncertainty is ± 0.02 for all samples, except SW6-01, for which it is ± 0.03. b Preliminary values reported by Wolfe et al. (1995) and revised herein. c 5 day preheat at 140°C. d Quartz grains: 2 day preheat at 150°C, 2.41 eV (green) excitation. e Data for dose rate and equivalent dose rate published previously by David et al. (1999). f Dose rate used was that which would be applicable if the sample had been buried 0.5 m.

oxidized and iron-cemented root casts, indicating a period of surface stability with vegetation cover. The bones yielded a calibrated radiocarbon age (cf. Stuiver and Reimer 1993) of 2750+−102 257 calendar years before present (S-3553), whereas projectile points found within the blowout were of Pelican Lake type (dated at ca. 2000 BP). Overlying the basal sand is a 1.5–2.0 m thick unit of horizontally stratified sand (Fig. 3). This unit, interpreted as an

eolian sheet-sand deposit, yielded optical ages of 307 ± 21 years at a depth of 5.5 m and 252 ± 17 years at a depth of 3.7 m (Table 2B). The top of this unit contains oxidized root casts similar to the underlying basal deposit, indicating that the surface of the sheet sand was vegetated less than 250 years ago. The unit overlying the sheet sand contains inclined laminations indicative of toeset, foreset, and topset bedding of © 2001 NRC Canada

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Table 2. Locations, depths, and optical ages of the samples. Sample depth (m)

Age (years, before AD 1995)

(A) Zero-age samples (dune sands) Burstall sand hills 94-70 Seward sand hills 94-41a

0–0.02 0–0.02

–4±7b 17±5b

(B) Stratigraphic sections (dune sands) Tunstall sand hills SW1-02c Seward sand hills SW2-02c Bigstick sand hills SW3-02c Central Great Sand Hills SW4-01c Central Great Sand Hills SW4-02c Northwest Great Sand Hills SW5-01c Burstall sand hills SW6-02c Burstall sand hills 94-81 Burstall sand hills 94-82 Burstall sand hills 94-83

6.0 8.0 4.0 3.0 6.0 4.0 3.0 5.5 3.7 3.2

Site

Sample No.

(C) Stratigraphic sections (basal sands) Burstall sand hills SW6-01c 6.5

68±12 109±8 91±5 640±60 930±50 216±11 168±9 307±21 252±17 163±7 4120±350 4190±200d,e 5400±600 f

(D) Seward sand hills (dune sands) Dune 1 BR1g 94-30a Dune 1 DT2 94-31a Dune 1 DT3 94-32a Dune 1 DT4 94-33a Dune 1 DT5 94-34a Dune 2 BR1 94-35a Dune 2 BR1 95-10a Dune 2 BR1 95-11a Dune 2 DT2 94-36a Dune 2 DT3 94-37a Dune 2 DT4 94-38a Dune 2 DT5 94-39a Dune 2 BS6 94-40a

0.5 0.5 0.5 0.5 0.5 0.5 1.0 1.5 0.5 0.5 0.5 0.5 0.5

123±6 94±6 116±7 94±6 152±8 125±5 180±8 185±8 174±8 168±7 160±10 137±9 117±7

(E) Other stabilized dunes Westerham BR1 Westerham DH1 Burstall DH1 Bigstick DH1 Bigstick DH1 Bigstick BR1 NW GSH DH1 NW GSH BR1 NW GSH BR1 NW GSH DH1

0.5 0.5 0.5 0.5 0.6 0.7 0.75 0.9 0.75 1.0

129±9 105±8 92±7 85±5 117±6 151±5 115±5 143±9 129±6 116±5

a

94-50 94-51 94-71 94-23 95-07 95-08 95-01 95-02 95-03 95-04

Data published previously by David et al. (1999). Dose rate used was that which would be applicable if the sample had been buried 0.5 m. c Preliminary values reported by Wolfe et al. (1995) and revised herein. d 5 day preheat at 140°C. e If corrected for anomalous fading using the model described by Aitken (1985, see Appendix E), and the fading data given in Table A1 of the Appendix for this sample, the age becomes 4760 ± 330 years. f Quartz grains. g DH, dune head; BS, backslope; BR, back ridge; DT, dune-track ridge. b

dune sand (Fig. 3). Foresets represent slip-face deposits, whereas overlying topsets are meso-bed forms that accumulated across the top of the dune. This unit is capped by a unit of subhorizontally bedded sand, about 1 m thick. The lowermost toeset deposit yielded an age of 163 ± 7 years at 3.2 m depth, and the base of the overlying foreset yielded a similar age of 168 ± 9 years at 3.0 m depth. An age of 92 ± 7 years, obtained from a depth of 0.5 m on the head of a stabilized parabolic dune about 700 m north of this section (Fig. 4a), provides an estimate of the timing of stabilization. In summary, basal sand was deposited at least 4000 years ago in the Burstall sand hills and the area was subsequently occupied by people around 2750 calendar years before present, by which time the local surface was vegetated. Eolian sheet sand was deposited between 250 and 300 years ago, with vegetation again covering the surface after this interval. Sand-dune activity in the past 165–170 years resulted in eolian deposition over the formerly vegetated surface. Stabilization occurred as recently as 90 years ago. Other stratigraphic sections Six other exposures were investigated within parabolic dunes at Tunstall, Bigstick, Seward, and Great sand hills (Fig. 1). At all sites, active blowouts had exposed sediments underlying the base of the dunes. These lower sediments, similar in texture to the overlying dune sands, were grey in colour and contained root casts stained and cemented with iron oxide. Buried soil was only observed at Bigstick (Fig. 3). The absence of buried soil may be due either to weak soil development on stabilized dunes or to the erosion of stabilized surfaces during dune reactivation. All dated samples were collected from inactive slip-face deposits, contained more than 99.5% sand, were well sorted, and had mean grain sizes of fine sand (125–250 µm). These sands were damp, with moisture contents ranging from 3.7 to 6.8% by weight and averaging 5.4% (Table 1, first six samples). The ages of dune activity at these sites ranged from 930 ± 50 to 68 ± 12 years (Fig. 3, Table 2). The ages indicate that many presently stabilized sand dunes in the Great Sand Hills region were active in the last millennium and as recently as the last century. Stabilized sand dunes – Seward sand hills The most detailed chronology was obtained from the Seward sand hills (Fig. 1), an area of well-developed parabolic dunes with well-defined back ridges and dune-track ridges (Fig. 4e). A detailed description of the study site and the ages obtained is provided by David et al. (1999). All of the ages from these shallow-pit samples occur within the last 200 years (Table 2D). The oldest age (174 ± 8 years) from the dune-track ridges is from the one closest to the back ridge of Dune 2. As expected, those of the remaining dunetrack ridges are successively younger downwind, toward the back slope of Dune 2 (117 ± 7 years). An age of 109 ± 8 years from a dune section 500 m north of Dune 2 is also consistent with the expectation that the ridges are older than the stabilized dunes. The ages obtained from Dune 1 and the back ridge of Dune 2 do not follow expectations. Those from Dune 1 may date minor depositional events, possibly correlating with droughts (ca. 95 and 120 years ago) that occurred after initial ridge formation. Those from Dune 2 (the © 2001 NRC Canada

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smaller of the two dunes) date the formation of the dunetrack ridges (David et al. 1999). Ages of basal deposits from the back ridge of Dune 2 provide the most reliable estimate of the onset of eolian activity in this area. Samples collected at depths of 1.0 and 1.5 m yielded ages of 180 ± 8 and 185 ± 8 years, respectively. Ages obtained from other stratigraphic sections (e.g., 216 ± 11 years) cannot be ascribed to the onset of renewed eolian activity because they are in a basal position and only indicate that dune activity also occurred in the area prior to 200 years ago. Thus, ages obtained from the Seward sand hills provide the earliest evidence of large-scale renewed dune activity, at about 185 years ago, with most of the dune morphology having formed between 175 and 110 years ago. Other stabilized dunes Shallow-pit samples from stabilized dunes in the Burstall, Westerham, Bigstick, and northwestern Great sand hills (Fig. 4) yielded ages ranging from 151 to 85 years (Table 2E). Ages from the back ridges at these location were obtained from shallow pits to determine the most recent time of dune activity, rather than the onset of activity, which is obtained from basal deposits. In all cases, sand at the dune head was deposited about 30 years after sand in the back ridge (Table 2E), reinforcing the expectation that dune heads are typically the last parts of the dunes to stabilize. Historical and reconstructed precipitation records Figure 5 depicts the total annual precipitation, expressed as departures from the mean, from the instrumental record at Swift Current and reconstructed precipitation using tree-ring chronologies from the Alberta foothills (Case and MacDonald 1995) and the Cypress Hills (Sauchyn and Beaudoin 1998). Comparison of the 5-year running means for the last century reveals that the three data sets show similar trends, including periods of low precipitation centred on the early 1920s, mid-1930s, late 1950s, and early 1960s. All these periods are recognized as significant drought events (Williams and Rowe 1986; Maybank et al. 1995). Both reconstructed records are significantly correlated with the historical record using both 5-year and 3-year running means (Pearson’s pairwise test, p ≤ 0.01). Although subregional differences in precipitation trends exist, the dendroclimatic data indicate that, prior to the 20th century, the prairies experienced lower average annual precipitation and more frequent dry periods (Figs. 5b, 5c). Precipitation in the Cypress Hills was less in the 1700s than in the 1800s or 1900s and extended dry periods occurred through the latter part of the 1700s. The Alberta foothills record also shows less precipitation and more dry periods in the 1700s, particularly near the end of that century. Both sets of observations indicate that the latter part of the 1700s was the driest period in the last 300 years.

Discussion Long-term precipitation trends Reconstructed long-term precipitation records for the Cypress Hills and Alberta foothills show that the climate of the southern Canadian Prairies was drier in the 1700s than during the 1800s and the historical (instrumental) climate record (Figs. 5b, 5c). Percent cumulative departures from the

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mean annual precipitation indicate that the Alberta foothills record and the instrumental Swift Current record contain similar wetting and drying trends throughout the 1900s, with a drying trend evident for the latter part of that century (Fig. 6a). The precipitation trend in the Cypress Hills is also similar back to about 1920, after which it departs due to lower precipitation at the end of the 1800s (Figs. 5b, 6a). As a result, the Swift Current cumulative departure record is strongly correlated with the Alberta foothills record (Pearson’s pairwise correlation, r = 0.74, n = 105, p = 0.01), but not the Cypress Hills record. Long-term cumulative departures from the mean reconstructed precipitation for the Alberta foothills record of Case and MacDonald (1995) show a trend of increasing moisture from AD 1500 to 1600, followed by below-normal precipitation through to AD 1800 (Fig. 6a). The greatest deficit in cumulative departure from the mean occurs just before AD 1800. Although there are differences between the Cypress Hills record and the Alberta foothills record, the two records both document a cumulative drying trend through the 1700s, with the greatest precipitation departure occurring just after AD 1800. Sand-dune activity Optical ages from eolian deposits in the Great Sand Hills region (Fig. 6b) provide evidence for an interval of dune activity defined by the earliest age of back ridges in the Seward sand hills (185 ± 8 years) and the age of stabilized dune heads (average = 105 years; youngest = 85 ± 5 years). During this interval, parabolic dunes were active in all areas studied. This consistency of ages indicates widespread dune activity, despite local differences in substrate, vegetation cover, and water table. Although evaluating the total area of the Great Sand Hills region that was active at this time is difficult, Wolfe (unpublished data, 1999) has estimated that 10–20% of the region was composed of active sand dunes, compared with less than 1% today. Comparison of the reconstructed precipitation records with the distribution of optical ages (Fig. 6) indicates that the onset of widespread dune activity was preceded by a relatively dry climate throughout the 1700s and by a severe dry period in the 1790s. This suggests that drought in the late 1700s was sufficient to initiate widespread sand-dune activity in the Great Sand Hills region. A lag is apparent between peak dryness (ca. AD 1800) and the onset of dune activity (ca. AD 1810), although this likely varied with local vegetation and water table conditions. Uncertainty in the optical ages, due primarily to anomalous fading (Appendix A) prohibits any additional interpretation of lag response times. It appears, however, that many sand dunes were stabilized by about AD 1890 (105 years ago). A net trend in dune stabilization has also been observed from aerial photographs since 1944 (Wolfe et al. 1995), suggesting that stabilization of dunes has continued throughout the 20th century. The absence of optical ages from AD 1500 to 1600 coincides with higher than average annual precipitation and comparatively few dry periods in the Alberta foothills record (Fig. 5c). The 1600s were drier, with numerous dry periods, comparable with those of the l790s, at the start of the century. However, only a few optical ages fall within this period. These include the sheet-sand deposit in the Burstall © 2001 NRC Canada

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Fig. 5. Historical and reconstructed annual precipitation trends (August–July), depicted as departures from the mean: (a) 104 year (AD 1886–1990), 5-year running mean precipitation from the instrumental record at Swift Current, Saskatchewan; (b) 312 year (AD 1682– 1994) dendroclimatic reconstruction of 5-year running mean precipitation from the Cypress Hills uplands (after Sauchyn and Beaudoin 1998); (c) 487 year (AD 1505–1992) dendroclimatic reconstruction of 5-year running mean precipitation from the Alberta foothills (after Case and MacDonald 1995). Also shown are mean precipitation values for each century and the entire record; horizontal dashed line signifies the threshold for dry periods, defined as one standard deviation below the 1961–1990 mean; bold line segments signify dry periods.

sand hills (307 ± 21 to 252 ± 17 years) and a stratigraphic section in the northwestern Great Sand Hills (216 ± 11 years; Fig. 3). This suggests that, although the 1600s and 1700s may have been relatively dry, sand dunes remained stable due to accumulated moisture from the previous period. Alternatively, eolian sands may simply have been reworked by subsequent activity, leaving little older material in situ.

Recent trends It is unlikely that the drought of the 1790s would have been sufficient to initiate widespread sand-dune activity independent of antecedent climatic conditions. This drought was preceded by several dry periods and lower than average precipitation throughout the 1700s, resulting in cumulative drying in the region and increased susceptibility to dune activity. By comparison, the historic droughts of the 1930s and © 2001 NRC Canada

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Fig. 6. Summary of moisture trends and optical ages: (a) Long-term moisture trends for the southern Canadian Prairies derived from tree-rings and instrumental records (see Fig. 5) and depicted as percent cumulative departures from the mean; (b) Optical age determinations from stabilized dunes and stratigraphic sections in the Great Sand Hills region, the shaded area indicating the period of widespread dune activity; uncontrolled systematic errors may cause the optical ages to be too high by 20 years and (or) too low by 5–15% (see Appendix A).

1980s were insufficient to initiate the degree of dune activity that occurred in the 1800s. At most, these recent droughts caused minor increases in the area of bare sand on dunes that were already partly active. The 1900s appear to have been sufficiently moist to preserve stabilized dunes and continue the process of stabilization in previously active dune areas (Wolfe et al. 1995). Reconstruction of past precipitation, as reconstructed from Case and MacDonald (1995), suggests that the average annual precipitation occurring in the last century (431 mm) is roughly equivalent to that for the last 500 years (435 mm; Fig. 5). Within the instrumental climate record, however,

there has been a slight decrease in annual precipitation on the Canadian Prairies between 1948 and 1992, while temperatures have risen by a statistically significant 0.9°C (Environment Canada 1995). Annual precipitation at Swift Current has decreased by about 45 mm over the last century, with precipitation during the May–July growing season declining by 33 mm (Judiesch and Cutforth 1996). This suggests that the present trend may be toward decreased moisture availability on the southern Canadian Prairies. Given such conditions, the present level of dune activity may have reached a minimum, and future trends could be either a maintenance of present levels or increased activity. © 2001 NRC Canada

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However, there would have to be a longer term shift toward increased aridity than has so far occurred to produce a return to widespread sand-dune activity in the region.

Conclusions Severe drought in the late 1700s, preceded by at least a century of below-average precipitation, resulted in widespread sand-dune activity over much of the Great Sand Hills region. Synchronous dune activity began in the early 1800s and resulted in the formation or reactivation and migration of parabolic dunes. It is suggested that cumulative drying was responsible for both the comparatively rapid onset of dune activity and its regional extent. Had a drought of similar magnitude occurred during a comparatively moist period, it would not likely have triggered the same amount of dune activity. Following formation or reactivation, dunes remained fully active for about 80 years. Stabilization began in the late 1800s and has continued throughout the 20th century, despite periodic drought intervals. Although regional reactivation of sand dunes may require several decades, the present trend toward a warmer, drier climate suggests that the region could experience a shift in the future from continued stabilization to increased levels of dune activity.

Acknowledgments Don Lemmen, Sonya Utting, and Susan Ball assisted with sample collection for optical age determinations. Bison bones collected from the Burstall site were identified by Brian Kooyman of the University of Calgary, while lithic artifacts were identified by Stanley Mathison of Mountain View, Alberta. Samples for optical age determinations were carefully prepared, treated, and measured by George Morariu. Financial support by the Natural Sciences and Engineering Research Council of Canada and the National Science Foundation (U.S.A.) are gratefully acknowledged. This paper benefitted from critical reviews by Don Lemmen, Cheryl McKenna Neuman, Willem Vreeken, Lynda Dredge, and Erica Kotler, and from editing by Bob Davie.

References Aitken, M.J. 1985. Thermoluminescence dating. Academic Press, London. Aitken, M.J. 1998. An introduction to optical dating. Oxford University Press, Oxford, U.K. Baril, M.R. 1997. Optical dating of tsunami deposits. M.Sc. thesis, Simon Fraser University, Burnaby, B.C. Case, R.A., and MacDonald, G.M. 1995. A dendroclimatic reconstruction of annual precipitation on the western Canadian Prairies since A.D. 1505 from Pinus flexilis James. Quaternary Research, 44: 267–275. David, P.P. 1971. The Brookdale road section and its significance in the chronological studies of dune activities in the Brandon Sand Hills of Manitoba. In Geoscience studies in Manitoba. Edited by A.C. Tarnock. Geological Association of Canada, Special Paper 9, pp. 293–299. David, P.P. 1977. Sand dune occurrences of Canada: a theme and resource inventory study of eolian landforms of Canada. Indian and Northern Affairs Canada, National Parks Branch, Ottawa. Contract No. 74-230.

Can. J. Earth Sci. Vol. 38, 2001 David, P.P. 1988. The coeval eolian environment of the Champlain Sea Episode. In The late Quaternary development of the Champlain Sea basin. Edited by N.R. Gadd. Geological Association of Canada, Special Paper 35, pp. 291–305. David, P.P. 1998. Eolian processes and landforms. In Geomorphic systems of the Palliser Triangle, southern Canadian Prairies: description and response to changing climate. Geological Survey of Canada, Bulletin 521, pp. 25–39. David, P.P., Wolfe, S.A., Huntley, D.J., and Lemmen, D.S. 1999. Activity cycle of parabolic dunes based on morphology and chronology from Seward sand hills, Saskatchewan. In Holocene climate and environmental change in the Palliser Triangle. Edited by D.S. Lemmen and R.E. Vance. Geological Survey of Canada, Bulletin 534, pp. 223–238. Environment Canada. 1995. The state of Canada’s climate: monitoring variability and change. State of the Environment Report No. 95-1. Forman, S.L., Goetz, A.F.H., and Yuhas, R.H. 1992. Large-scale stabilized dunes on the High Plains of Colorado: understanding the landscape response to Holocene climates with aid of images from space. Geology, 20: 145–148. Fritz, S.C., Engstrom, D.R., and Haskell, B.J. 1994. ‘Little Ice Age’ aridity in the North American Great Plains: a highresolution reconstruction of salinity fluctuations from Devils Lake, North Dakota, U.S.A. The Holocene, 4: 69–73. Holliday, V.T. 1995. Late Quaternary stratigraphy of the Southern High Plains. In Ancient peoples and landscapes. Edited by E. Johnson. Museum of Texas Tech University, Lubbock, Tex., pp. 289–313. Huntley, D.J., and Baril, M.R. 1997. The K-content of the Kfeldspars being measured in optical dating or in thermoluminescence dating. Ancient TL, 15: 11–13. Huntley, D.J., and Clague, J.J. 1996. Optical dating of tsunami-laid sands. Quaternary Research, 46: 127–140. Huntley, D.J., and Lian, O.B. 1999. Using optical dating to determine when a sediment was last exposed to sunlight. In Holocene climate and environmental change in the Palliser Triangle. Edited by D.S. Lemmen and R.E. Vance. Geological Survey of Canada, Bulletin 534. pp. 211–222. Judiesch, D., and Cutforth, H. 1996. Weather trends at Swift Current: what’s been going on for the last century. Semiarid Prairie Agricultural Research Centre, Swift Current, Sask., Research Newsletter No. 20, pp. 1–2. Laird, K.R., Fritz, S.C., Maasch, K.A., and Cumming, B.F. 1996. Greater drought intensity and frequency before AD 1200 in the northern Great Plains, USA. Nature, 384: 552–554. Lemmen, D.S., Vance, R.E., Campbell, I.A., David, P.P., Pennock, D.J., Sauchyn, D.J., and Wolfe, S.A. 1998. Geomorphic systems of the Palliser Triangle, southern Canadian Prairies: description and response to changing climate. Geological Survey of Canada, Bulletin 521. Madole, R.F. 1994. Stratigraphic evidence of desertification in the west-central Great Plains within the past 1000 yr. Geology, 22: 483–486. Maybank, J., Bonsal, B., Jones, K., Lawford, R., and O’Brien, E.G. 1995. Drought as a natural disaster. Atmosphere-Ocean, 33: 195–222. Muhs, D.R., and Holliday, V.T. 1995. Evidence of active dune sand on the Great Plains in the 19th century from accounts of early explorers. Quaternary Research, 43: 232–237. Muhs, D.R., and Maat, P.B. 1993. The potential response of eolian sands to greenhouse warming and precipitation reduction on the Great Plains of the U.S.A. Journal of Arid Environments, 25: 351–361. © 2001 NRC Canada

Wolfe et al. Muhs, D.R., and Wolfe, S.A. 1999. Sand dunes of the northern Great Plains of Canada and the United States. In Holocene climate and environmental change in the Palliser Triangle. Edited by D.S. Lemmen and R.E. Vance. Geological Survey of Canada, Bulletin 534, pp. 183–197. Muhs, D.R., Stafford, T.W., Jr., Swinehart, J.B., Cowherd, S.C., Mahan, S.A., Bush, C.A., Madole, R.F., and Maat, P.B. 1997a. Late Holocene eolian activity in the mineralogically mature Nebraska Sand Hills. Quaternary Research, 48: 162–176. Muhs, D.R., Stafford, T.W., Jr., Been, J., Mahan, S.A., Burdett, J., Skipp, G., and Muhs Rowland, Z. 1997b. Holocene eolian activity in the Minot dune field, North Dakota. Canadian Journal of Earth Sciences, 34: 1442–1459. Ollerhead, J., Huntley, D.J., and Berger, G.W. 1994. Luminescence dating of sediments from Buctouche Spit, New Brunswick. Canadian Journal of Earth Sciences, 31: 523–531. Sauchyn, D.J., and Beaudoin, A.B. 1998. Recent environmental change in the southern Canadian prairies. The Canadian Geographer, 42: 337–353. Stockton, C.W., and Meko, D.M. 1983. Drought recurrence in the Great Plains as reconstructed from long-term tree-ring records. Journal of Climate and Applied Meteorology, 22: 17–29. Stuiver, M., and Reimer, P.J. 1993. Extended 14C database and revised CALIB 3.0 14C age calibration program. Radiocarbon, 35: 215–230. Vance, R.E., and Wolfe, S.A. 1996. Geological indicators of water resources in semi-arid environments: southwestern interior of Canada. In Geoindicators: assessing rapid environmental changes in earth systems. Edited by A.R. Berger and W.J. Iams. A.A. Balkema, Rotterdam, pp. 251–263. Williams, G.D.V., and Rowe, K. 1986. Agricultural drought. In An applied climatology of drought in the Prairie Provinces. Atmospheric Environment Service, Canadian Climate Centre, Report No. 86-4. Wolfe, S.A. 1997. Impact of increased aridity on sand dune activity in the Canadian Prairies. Journal of Arid Environments, 36: 421–432. Wolfe, S.A., and David, P.P. 1997. Canadian landforms example – 34. Parabolic dunes: examples from the Great Sand Hills, southwestern Saskatchewan. The Canadian Geographer, 41: 207–213. Wolfe, S.A., Huntley, D.J., and Ollerhead, J. 1995. Recent and late Holocene sand dune activity in southwestern Saskatchewan. In Current research 1995-B. Geological Survey of Canada, pp. 131–140. Wolfe. S.A., Muhs, D.R., David, P.P., and McGeehin, J.P. 2000. Chronology and geochemistry of late Holocene eolian deposits in the Brandon Sand Hills, Manitoba, Canada. Quaternary International, 67: 61–74.

Appendix A. Accuracy of the optical ages The two sand samples collected from the active surfaces for zero-age determinations came from the easternmost end of blowout dunes, on the assumption that grains transported to this location would have locally undergone comparatively long transport distances and exposure times to sunlight. Sample locations would be equivalent to the head of the parabolic dune depicted in Fig. 2. The uppermost surface grains were collected from dry, active ripples, and we feel confident that the grains collected were recently transported, rather than being older deposits that were newly exposed by

117 Table A1. Results of anomalous fading tests, showing the ratio of luminescence intensities for different delay times between irradiation and measurement. Sample No.

Delay time ratios

Intensity ratios

94-38 94-38 SW4-01 SW6-01

12 118 119 1001

0.956±0.015 0.925±0.019 0.926±0.019 0.953±0.016

days days days days

/ / / /

2 days 2 days 2 days 22 days

Note: Preheat was 16 h at 120°C for all samples, except SW6-01, for which it was 7 days at 140°C.

surface erosion. The ages were –4 ± 7 years (94-70) and 17 ± 5 years (94-41) (Table 2A). Such zero-age testing is an essential component of a dating program. A non-zero age can indicate that the sample was not adequately exposed to sunlight before burial, or it may suggest that further development of laboratory techniques is needed. We are uncertain which applies here, as few studies have been done in the time scale of a decade or two. However, it has been shown that the state in which a sample is left after a sunlight exposure is dependent on the light spectrum during that exposure (Huntley and Clague 1996). In that case, full sunlight exposure left electrons in traps that were sampled during the measurements for optical age determinations using 1.4 eV excitation with K-feldspars. The same effect would lead to a non-zero age using the technique used here. Dune samples from two other regions have given similar non-zero ages by this technique. We therefore conclude that all the optical ages in this paper may be affected by a zero-age error, possibly as high as 20 years. No correction for this has been made, because the exact value is unknown and is likely to be variable. For ages greater than 1000 years, this “zero-age error” will be inconsequential. Anomalous fading tests were conducted on samples 9438, SW4-01, and SW6-01. The ratios of intensities for different delay times between irradiation and measurements are shown in Table A1. The data indicate the presence of fading from which, based on the tunnelling model described by Aitken (1985), we predict that all our ages could be lower (younger) than the true ages by 5–15%. The exact amount depends on the age and the time elapsed between laboratory irradiation and final measurement; the latter ranged from 4 to 49 days but was typically 2 weeks. The same fading coefficient accounts for the K-feldspar age being lower than an optical age obtained from quartz for SW6-01 (Table 2C). The implication of the anomalous fading measurements, and the difference between the quartz and feldspar ages obtained for sample SW6-01, is that all the feldspar ages are almost certainly affected by anomalous fading. We have not attempted to make corrections for this because an established procedure for doing so has not yet been developed. The results of the fading measurements (Table A1), combined with the current theory of anomalous fading (Aitken 1985) and the difference between the quartz and feldspar ages obtained for sample SW6-01, show that the true ages are 5–15% higher than calculated for these samples.

© 2001 NRC Canada