Journal of Paleolimnology19: 429–441, 1998. 429c 1998Kluwer Academic Publishers. Printed in Belgium.

Holocene paleovegetation and paleohydrology of a prairie pothole insouthern Saskatchewan, Canada�

Catherine H. YansaDepartment of Geography, University of Wisconsin-Madison, Madison, WI 53706-1491, USA(e-mail: cyansa@geography.wisc.edu)

Received 14 April 1997; accepted 10 July 1997

Key words:Great Plains, Hypsithermal, paleoecology, paleohydrology, plant macrofossils, prairie pothole wetlands

Abstract

The Andrews site represents one of countless prairie potholes found in areas of hummocky moraine on the northernGreat Plains. Sediments from a depth of 5.8 to 3.1 m at this ‘kettle-fill’ site in the Missouri Coteau upland ofsouthern Saskatchewan, Canada, provides a record of vegetation, climate, and hydrologic changes within a small,ca 30 m diameter, closed-drainage basin from ca 10.2 to 5.8 ka. Plant macrofossil analyses of 67 samples, 614Cages, and stratigraphy were used to identify 5 zones, representing the paleohydrological changes that followeddeglaciation in southern Saskatchewan.

Results of this study indicate that with the melting of residual stagnant ice a pond (>2 m deep) with abundantaquatic, emergent, and shoreline plants developed in the basin at ca 10.2 ka and persisted until at least ca 8.8 ka.During this time there was a shift in upland vegetation from a white spruce forest (Zone II) to a deciduous parklandat ca 10 ka (Zone III). As climate warmed, brackish and alkaline conditions developed coincident with shallowing ofthe pond at the end of Zone III. The perennial water phase ended at ca 8.8 ka and was followed by a low-water standlasting ca 1100 years. Prairie fires and slopewash from unstable slopes were dominant sedimentological processesuntil ca 7.7 ka (Zone IV). Water levels began to rise and between ca 7.7 and 5.8 ka a semi-permanent pondwas established in a grassland setting (Zone V). After ca 5.8 ka this prairie pothole wetland became ephemeral,to the point that plant macrofossils could not be preserved, a situation continuing today. Interactions betweenclimate change, variability in local groundwater supply, and sedimentological processes likely account for thepaleohydrologic events reconstructed at the Andrews site.

Introduction

Countless lakes and prairie potholes occur through-out the northern Great Plains region of western NorthAmerica. Water budgets and chemistries at these sitesare variable due largely to the interplay between warmsummer temperatures, low and variable precipitation,strong desiccating winds, local groundwater flow, andtopography. Evapotranspiration rates throughout thisprairie region are commonly high with an annual mois-ture deficit of greater than 30 cm yr�1 (Winter, 1989).In addition, extended intervals of drought occur period-ically, significantly increasing the evapotranspirationrates and altering the water chemistries and hydro-

� Palliser Triangle Global Change Contribution No. 37.

logical regimes of these basins throughout the region.Nevertheless, these closed-drainage basins are ecolog-ically significant, for they act as water traps and arecrucial for maintenance of the biological productivityin this semi-arid grassland region (Hubbard & Linder,1986; Woo & Rowsell, 1993).

Because plants are sensitive to changes in tem-perature and moisture regimes and water chemistry,they have commonly been used as proxy indicators ofpast climatic and hydrologic conditions (Birks, 1973;Birks & Birks, 1980; Vance & Mathewes, 1994).Interpretation of Holocene paleovegetation, paleocli-mate, and paleohydrology is usually based on pollen,diatom, ostracode, and geochemical analyses of sed-iments from lake cores (e.g., Last, 1984; MacDon-

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ald, 1989; Schweger & Hickman, 1989; Smith, 1993).However, because of difficulties in coring compactedlake sediments on the northern Great Plains the old-est postglacial sediments are generally not retrieved(Barnosky et al., 1987; Beaudoin, 1993; Vance et al.,1993). Nevertheless, the lake data do provide a goodpaleoenvironmental record, from pollen analysis andgeochemical studies, for the last 5 ka or 6 ka (e.g.,Mott, 1973; Last & Schweyen, 1985; Last & Slezak,1988), although there are only a few sites providing arecord from the last 9 ka or 12 ka (i.e., Barnosky, 1989;Fritz et al., 1991; Sauchyn & Sauchyn, 1991; Vance &Last, 1994; Grimm, 1995; Laird et al., 1996).

Alternatively, small kettle depressions (ca 30–80 m diameter) called ‘prairie potholes’, ‘sloughs’, or‘prairie marshes’, commonly provide useful paleoenvi-ronmental data, often based on preserved plant macro-fossils, from terminal Pleistocene to mid-Holocenetime, and complement the pollen-derived lake core data(Yansa, 1995). These small sedimentary basins occuras part of the hummocky and ice-thrust moraine topog-raphy that formed during Late Wisconsinan ice stag-nation (Clayton, 1967; Clayton & Moran, 1974), andsubsequently were partly filled in by 5 to 12 m of post-glacial sediments (Klassen, 1994; Yansa, 1995). Thesediments that accumulated in these kettles often haveexquisitely preserved plant and animal remains, whichare representative of past mesic and wetland environ-mental conditions (Beaudoin, 1992, 1996; Klassen,1994; Yansa, 1996). Discovery of these sites is more byaccident than design with several sub-fossil sequencesfound during excavation of kettle depressions for live-stock watering holes.

Since the late 1950s, there have been severalreports of these ‘kettle-fill’ sites from Saskatchewanand Alberta, Canada (Dew, 1959; Kupsch, 1960;Ritchie & DeVries, 1964; Ritchie, 1966; Beaudoin,1992, 1996; Klassen, 1994; Yansa & Burt, 1995), andNorth Dakota and South Dakota, United States (Moir,1957; Thompson, 1962; McAndrews et al., 1967;Cvancara et al., 1971; Malo, 1988). Although mostof these reports are preliminary, and the interpretationsvary considerably, the fossil remains indicate develop-ment of a deciduous parkland and aquatic environmentat about 10 ka (Beaudoin, 1992, 1996; Klassen, 1994;Yansa, 1995).

Reconstruction of past hydrologicconditions basedon plant macrofossils found in kettle-fill sites are con-sidered to be more reliable than pollen data becausethey arein situ, representative of the species of plantsthat grew in or very near to the depressions at the time

of deposition, and can usually be reliably identifiedto the species level (Birks, 1973, 1980; Yansa, 1995).In addition, plant macrofossils, such as wood, leaves,seeds, and fruits, are commonly used to interpret pastclimate (Birks & Birks, 1980; Mannion, 1986), as wellas changes in the water depth and chemistry (Vance &Mathewes, 1994).

In this paper, the hydrological conditions associatedwith an early postglacial kettle-fill site from southernSaskatchewan, Canada, are presented. Interpretationand reconstruction of this site is based on plant macro-fossils, rather than pollen data. This study provides oneof the most complete records of earliest postglacialvegetation currently available in the northern GreatPlains region, including remains of anin situ Piceaglaucalitter-root layer and underlying diamicton. Fur-thermore, the14C chronology of the Andrews site isthe most reliable and comprehensive of any kettle-fillsequence in this region, thereby allowing a more accu-rate reconstruction of local paleohydrological eventsfrom late-glacial to mid-Holocene times.

Study site

The Andrews site (50�200 N, 105�520 W; 720 m asl)is a rimmed kettle in moderately rolling hummockymoraine of the Missouri Coteau upland of south-ern Saskatchewan, Canada (Figure 1). The MissouriCoteau is a remnant upland consisting of Paleocene(Ravenscrag Formation) sediments that resisted exten-sive erosion during the middle and late Tertiary andoverlain by a thick cover of Quaternary sediments(Christiansen, 1961; Klassen, 1989). The Coteauoccurs as a northwest-trending band of predominant-ly hummocky moraine, averaging 50 km in width andextending nearly 1300 km from south-central SouthDakota to west-central Saskatchewan (Acton et al.,1960; Clayton, 1967). In Saskatchewan, ice stagna-tion, which was largely responsible for the formationof hummocky moraine, began after retreat of the Lau-rentide Ice Sheet approximately 11.7 to 11.3 ka (Clay-ton & Moran, 1982). The clay-rich supraglacial tills inthis area have poor permeability, which contributes todepression-focused recharge of the underlying aquiferby surface waters (Lissey, 1971; Woo & Rowsell,1993).

The relatively fresh-water chemistry of manyprairie potholes of the Missouri Coteau today isattributed to the regional upland setting and depression-focused recharge to the underlying aquifer (LaBaugh

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Figure 1. Map showing the location of the Andrews site on the Missouri Coteau.

et al., 1987; Winter, 1989). Total dissolved solid val-ues ranging from 880 to 1360 mg l�1 were obtainedby Christiansen (1961) from three test holes located afew kilometers from the Andrews site, indicating thatsurface waters in the study area are fresh to slight-ly brackish (Freeze & Cherry, 1979). Although thesebasins may be episodically fed by groundwater, pre-cipitation is still the major contributor, with water lossoccurring primarily through evaporation (Sloan, 1972;Kantrud et al., 1989). Thus, the areal extent of pondwaters within these basins expands and contracts fromseason to season and varies significantly with annu-al changes in precipitation and temperature. Todaymost prairie potholes in southern Saskatchewan areephemeral, being briefly filled with water from springto mid-summer.

The Andrews site has a mean annual precipitationof ca 350 mm, a cold month mean temperature of�14 �C, a warm month mean temperature of 20�C,and an average frost-free period of 105 days (Elliset al., 1965; Clayton et al., 1977). The Andrews site issituated near the northern boundary of the mixed-grassprairie zone (Risser et al., 1981).

Based on the present-day physiological adaptationsand tolerances it is possible to use plants to infer whatthe local climatic and hydrologic conditions were likein the past. According to Kantrud et al.’s (1989) prairiewetland vegetation classification system, depressionsin the study area are presently classified as ‘wet mead-ows’. The wet meadow at the Andrews site todaycontains an abundance ofHordeum jubatumL. andGrindelia squarrosa(Pursh) Dunal, which both indi-

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cate slightly brackish soil conditions, correlating withthe water chemistry described for this area (Dodd& Coupland, 1966; Stewart & Kantrud, 1972). Thespecies of plant fossils identified at the Andrews site arealso indicative of fresh to brackish water, but includespecies found today in and around more permanentwater. Wetland plants commonly produce large seedbanks in response to changes in water levels (van derValk & Davis, 1979). Consequently, the macrofossilsfrom the Andrews site can be taken to represent a ‘fossilseed bank’.

There are 42 taxa of fossil plants representing 20families of vascular plants and 2 families of non-vascular plants found at this site. These plants are rep-resentative of upland, wet meadow, and emergent andsubmerged aquatic habitats. Based on14C ages, thedeposits range from 10.2 to 5.8 ka.

Materials and methods

Autochthonous plant and invertebrate macrofossilswere exposed during the excavation of a livestockwatering hole. The watering hole is about 30 m indiameter and over 5 m deep and did not become filledwith water until a few weeks after sample collection.Samples were collected at 2 to 5 cm intervals from5.8 m of exposed section and stored in labeled plasticbags at 4�C until processed to prevent microbial andfungal decomposition. Of these, 67 samples from thepostglacial sediments (3.1 to 5.1 m) and underlyingdiamicton (5.1 to 5.8 m) contained plant macrofossils(Figure 2). Samples collected from the upper 3.05 mof the section lacked identifiable plant remains.

Fifty cm3 of sediment from each sample wasprocessed. To each sample was added approximately900 ml of distilled water and 20 ml of Quaternary OTM

detergent. The mixture was gently heated to 40�C andagitated for up to 2 hours on an oscillating hot plateto deflocculate clay minerals, then washed with waterthrough 1 mm, 250 mm, and 125 mm mesh screens.Following the methods of A. Beaudoin (pers. com-mun., 1994), the fossils were stored in labeled glassvials filled with a 1:1:1 ratio of 95% ethanol, distilledwater, and glycerine. Three drops of a dilute phenolsolution were added to the vials to prevent fungaland microbial decomposition. Samples are present-ly stored in the University of Saskatchewan Paleob-otanical Collection (USPC), Department of Geologi-cal Sciences, University of Saskatchewan, Saskatoon,Saskatchewan, S7N 5E2, Canada.

Figure 2. Stratigraphic column of the Andrews site illustrating themacrofossil zones,14C ages, lithology, and fossils.

Plant macrofossils were identified by using thekeys, line drawings, and photographs of Martin &Barkley (1961), Berggren (1969, 1981), Montgomery(1977), and Levesque et al. (1988). Final speciesidentification of fossil remains involved direct com-parisons with extant reproductive and vegetative plantremains in a collection prepared for this study, which ispresently housed at the Fraser Herbarium, Universityof Saskatchewan. Nomenclature and interpretation ofecological requirements for the species of fossil plantsdiscussed in this paper follow that of Looman andBest (1987), unless otherwise noted. The fossil speciesabundance from the Andrews site samples were plotted

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on a plant macrofossil diagram usingTilia.graphTM

(Grimm, 1992).For samples selected for14C dating, the datable

organics were rinsed with distilled water, air dried, andstored in clean glass vials or plastic bags. Six14C ageswere obtained from uncharredPicea glauca(Moench)Voss (white spruce) wood using conventional datingmethods and from non-aquatic seeds, charcoal flakes,and plant fragments using accelerator mass spectrom-etry (AMS) (Table 1). There was no evidence of fungalcontamination in the wood samples selected for dating,and the seeds and plant remains used for AMS14C agedetermination showed no signs of post-depositionalabrasion. Both calibrated and corrected (for�13C frac-tionation) ages were obtained for the samples datedusing AMS 14C techniques (Table 1). Because mostother paleoenvironmental studies lack calibrated14Cages, the14C ages used to provide a chronologic frame-work for the section (Figure 2) are those corrected for�13C fractionation.

Results and interpretation

Early postglacial hydrological setting

Five zones (Figures 2, 3; Table 2), indicative ofthe hydrologic changes that occurred at the Andrewssite following deglaciation, were reconstructed fromchanges in plant macrofossil assemblages, often asso-ciated with changes in sediment stratigraphy, and14Cages. The exposed stratigraphic sequence begins withZone I, which is a diamicton containing rare and frag-mentary fossil plant remains that are likely allochtho-nous. The earliest unequivocal evidence of postglacialvegetation occurs in the Zone II litter layer.Here,in situPicea glaucaroots penetrating a 10-cm-thick litter lay-er into the underlying Zone I diamicton indicate that awhite spruce forest was established by ca 10.2 ka (Fig-ure 2; Table 1). This forest (Zone II) is thought to havebeen growing on a blanket of till overlaying stagnantice on the Missouri Coteau (Yansa & Basinger, submit-ted); a pattern seen in similar kettle deposits in NorthDakota (Clayton, 1967) and the Great Lakes region(Warner et al., 1991).

Parts of this white spruce forest collapsed into thepond formed by the melting of the underlying ice atca 10.2 ka. The continued presence ofPicea glaucaand other woodland plants in the lacustrine sedimentsof Zone II indicates the persistence of a spruce for-est for some time after kettle formation. The spruce

was eventually replaced byPopulusspp. andBetulasp. that surrounded the pond which was rich in aquat-ic and emergent vegetation between ca 10 and 8.8 ka(Zone III). The plant macrofossils and stratigraphy ofthe uppermost lacustrine sediments of this zone indi-cate that the pond began to shallow and became morebrackish and alkaline by about 8.8 ka as the climatewarmed (Table 1).

Zone IV is characterized by charcoal-rich sandyclay indicating the prevalence of prairie fires and slope-wash from unstable slopes until ca 7.7 ka (Table 1).Following deposition of these sediments, water levelsbegan to rise, forming a semi-permanent pond in agrassland setting (Zone V) that persisted from ca 7.7to 5.8 ka (Table 1). After approximately 5.8 ka, thisprairie pothole became ephemeral and plant macro-fossils were no longer preserved. The wetland phaseof the Andrews site is best represented by the plantmacrofossils in Zones II, III, IV, and V and were usedin reconstructing the paleohydrology of the site.

Plant macrofossil zones

Zone II: pond development (500–445 cm; ca 10.2 ka)Around 10.2 ka the stagnant ice melted to the point thatthe till and forest overlying these ice pockets collapsedcreating a series of ponds and lakes on the MissouriCoteau.In situ Picea glaucatrunks buried in lami-nated lacustrine silty clays of the uppermost part ofZone II (Figures 2, 3), indicates that the spruce forestvegetation probably occupied kettle slopes and knollsduring this time.

During the initial stages of pond development, avegetation comparable to a sedge fen appears to havebeen associated with a fairly deep-water pond. Twothree cm-thick layers, composed primarily ofDrepan-ocladus polycarpus(Bland. ex Voit) Roth, a mosswhich preferentially grows in carbonate-rich aquaticenvironments (Grout, 1965), occurs near the base ofthe laminated sediments of Zone II (Figure 3) and sig-nals the start of the wetland phase. The moss may havebeen associated with floating and submerged aquaticplants, emergent macrophytes that fringed the pond,and wet meadow shoreline plants, as evident in sim-ilar modern environments today (Stewart & Kantrud,1972).

Floating and submerged aquatic taxa (Figure 3;Table 2) such asHippuris vulgarisL., Lemna trisul-ca L., Myriophyllum verticillatumL. var. pectinatumWallr., andPotamogeton filiformisPers. suggest quiet,pH neutral to slightly alkaline, and fresh to slightly

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Table 1. Radiocarbon ages of significant plant macrofossil-bearing levels of the Andrewssite. Reference is made to stratigraphic depth, sample number, method of dating (correctedfor � 13C fractionation or calibrated using CALIB 3.0 (Stuiver & Reimer (1993)), and thematerials dated

Depth Sample 14C Age 14C Age Materials dated

(cm) # (corr.) ka (calibrated)

310 TO-5018 5.77�0.08 4670, charcoal flakes and plant

AMS 4645, 4610 rootlets

395 TO-4780 7.67�0.08 6460 charcoal flakes and plant

AMS rootlets

410 TO-5019 8.79�0.14 7905, Chenopodium salinumseeds

AMS 7755, 7745 andScirpus americanusfruits

455 AECV— 10.2�0.14 — Piceawood (trunk)

2048C Conventional

490 GSC-5822 10.2�0.08 — Piceawood (trunk)

Conventional

500 AECV- 10.23�0.14 — Piceawood (root)

2047C Conventional

Table 2. Species of macrofossils not shown in Fig-ure 3, but described in text, and the zones in whichthey are found

Macrofossil species Zone(s)

Potamogeton filiformis II, III, IV

Potamogeton pectinatus II, III, V

Carexcf. C. atherodes II, III

Carexcf. C. rostrata I, II, III

Scirpus americanus II, III

Scirpuscf. S. validus II, III

Lemna trisulca II, III

Wolffia arrhiza I, II, III

Populus balsamifera III

Populus tremuloides III, IV, V

Chenopodiumcf. C. berlandieri I, II

Chenopodium salinum II, III, IV, V

Rorippacf. R. truncata II, IV, V

Epilobiumcf. E. ciliatum II

Astercf. A. novae-angliae II, III

brackish waters (Harms, in prep.). A similar waterchemistry is indicated by the emergent plants of theZone II lacustrine sediments at the Andrews site (Fig-ure 3; Table 2), which includeTypha latifoliaL., Epi-lobium cf. E. ciliatum Raf., Carex cf. C. atherodesSpreng.,Carexcf. C. rostrataStokes,Scirpus ameri-canusPers., andScirpuscf. S. validusVahl. The wetmeadow vegetation (Figure 3; Table 2) is represent-ed by species such asMentha arvensisvar. villosa(Benth.) S. R. Stewart,Chenopodiumcf.C. berlandieri

Moquin, andAstercf. A. novae-angliaeL. Today thesetaxa commonly grow in soils influenced by slightlyalkaline water.

Plant macrofossils and general stratigraphy indicatethe development of a deep pond in the uppermost partof Zone II that persisted throughout much of Zone III.More or less permanent standing water is required forthe establishment of submerged and floating aquaticplants (Stewart & Kantrud, 1972; Shay & Shay, 1986),such as thePotamogeton,Lemna, Hippuris, andMyrio-phyllum taxa identified in these two zones as fossils.In addition, preservation of lamina in the dark olivegray (5Y 3/2, Munsell) lacustrine silty clay sedimentof uppermost part of Zone II and Zone III suggests adeep and permanent pond. The laminated sedimentsand subfossils of aquatic plants, insects, ostracodes,and mollusks from this and other nearby sites indicatethat this pond was perennial with a depth of at least2 m (McAndrews et al., 1967), and perhaps in excessof 3 m (Bickley & Clayton, 1972; Last, 1989).

Zone III: permanent pond (445–400 cm;ca 10–8.8 ka)Zone III includes the uppermost 45 cm of lacustrinesediments that overlie the spruce trunks and rangesfrom ca 10.2 ka, or shortly afterwards if the woodpredates the sediments, to 8.8 ka in age (Figure 2;Table 1). The near absence ofPicea glaucamacro-fossils and first appearance of remains fromBetulacf. B. occidentalisHook., Populus balsamiferaL.,and P. tremuloidesMichx. (Figure 3; Table 2) indi-

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cate a dramatic shift in the local forest composition,which may be due to climatic warming (Ritchie, 1986;Barnosky et al., 1987). The significant decrease in theabundance ofPicearemains from this zone may indi-cate that the mean annual summer temperature hadrisen above the 17�C isotherm, which today governsthe southern limit ofPicea glauca(Ritchie & Harrison,1993). In comparison,Populus tremuloidesis the mostdrought-tolerant of these species and is found withinthe study area today. Significantly, the shift in vegeta-tion from between Zones II and III at the Andrews site(Figure 3) coincides with the transition from the coolerterminal Pleistocene to warmer Holocene.

The aquatic, emergent, and wet-meadow plantsindicate a gradual trend towards more alkaline andbrackish water conditions (Figure 3; Table 2), whichseem to be associated with the presumed increase intemperature during deposition of sediments assignedto Zone III. Reduction in the number ofHippuris vul-garis, Myriophyllum verticillatumvar.pectinatum, andWolffia arrhiza(L.) Wimm. macrofossils in this zonesuggests that the water had a greater pH and salinitythan these aquatic plant species tolerate (Kantrud et al.,1989; Harms, in prep.). The increased abundance ofScirpus americanusandChenopodium salinumStand-ley seeds and appearance ofZannichellia palustrisL.andRumex maritimusL. var. fueginus(Phil.) Dusenmacrofossils is a good indication that the water wasbecoming more alkaline and brackish. The absence ofRuppiaL., Scirpus paludosusNels., andSalicorniarubra A. Nels. in the Andrews site fossil record mayindicate that the water within the basin was brackish,but not saline. These plant species are found growingin and around saline water today (Dodd & Coupland,1966; Hammer & Heseltine, 1988), and have beenused as indicators of peak Holocene aridity in oth-er paleoenvironmental records on the northern GreatPlains (Watts & Bright, 1968; Vance et al., 1993).

The sediments and plant macrofossils from Zone IIIindicate that the pond became more alkaline and brack-ish as it slowly shallowed by 8.8 ka (Figure 3). Achenesfrom emergent plants such asCarexspp. andScirpusspp. are more abundant in Zone III than in any otherzone at the Andrews site (Figure 3; Table 2) and mayindicate the expansion of the emergent fringe towardsthe center of the pond during periodic draw-downs ofwater within the basin. The increase in the macrofossilabundance ofTypha latifolia, Chenopodium salinum,andRumex maritimusvar.fueginusmay be due to fluc-tuating shoreline levels within the basin, as interpretedfrom other paleoenvironmental studies (Watts & Win-

ter, 1966; Vance, 1991). The abundance ofPotamoge-ton pectinatusL. macrofossils in Zone III may indicateclear, alkaline water high in Cl or SO4, as observed incomparable modern environments (H. J. B. Birks, pers.commun., 1997). This aquatic plant species is foundtoday where water depth is 1 to 4 m in the basin center(Hammer & Heseltine, 1988).

The appearance of marl and the greater abundanceof shoots and oogonia ofCharasp. in Zone III (Fig-ure 3) indicate increased algal production and precip-itation of carbonates under a highly alkaline, brack-ish, warm water regime (Kupsch, 1956; Hutchinson,1975). Given such environmental conditions, thesecharophytes can develop large populations over a shorttime interval (Garcıa, 1994). The transition from lami-nated to non-laminated marl sediments at the top 5 cmof Zone III suggests a significant lowering of waterlevels after 8.8 ka as laminae are no longer preservedand macrofossils are abraded. It is likely that marlformation stopped when water levels dropped below1 m, as reported for comparable modern environments(Hutchinson, 1975; Hammer & Heseltine, 1988), andsimilar paleolimnologic deposits (Vance et al., 1993).The presence of authigenic pyrite-coated roots withinthe top 5 cm of Zone III suggests reducing conditionsand decomposition of significant amounts of organicmatter under a low-water regime, as reported for oth-er paleoenvironmental sites of similar age (Schweger& Hickman, 1989; Last & Sauchyn, 1993). However,excellent preservation of plant macrofossils in the lam-inated lacustrine sediments of Zone III, including theseeds used to obtain the14C age of ca 8.8 ka (Table 1),suggests that during the remainder of the Holocenethe water table was high enough to prevent completedesiccation of the basin sediments.

Zone IV: low-water stand (400–390 cm;ca 8.8–7.7 ka)Zone IV is represented by a 10-cm unit of black (5Y2.5/1, Munsell) sandy clay (Figure 2). The occurrenceof sand may be attributed to slope instability withslopewash and/or eolian deposition. Abundant char-coal in this zone suggests the prevalence of fires fromca 8.8 to 7.7 ka (Table 1) associated with arid con-ditions. Such fires would have contributed to slopeinstability and sand deposition by removing vegetationcover.

The only fossil remains recovered from Zone IVwere pieces of charcoal, rootlets, and a few charredseeds. Charcoal and plant fragments contained in thesandy clay at the top of this zone provide an age esti-

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Figure 3. Plant macrofossil diagram showing 22 of the 42 taxa identified.

mate of ca 7.7 ka (Table 1). Because these dated mate-rials may have been redeposited from older sediments,this age of ca 7.7 ka may be considered as a limitingdate. The difference in age between the fossils dated inthe uppermost part of Zone III and those from Zone IVindicates that a low-water stand lasted for at least 1100years at the Andrews site. Although fossil evidence islacking, grassland vegetation was probably establishedin the area with the onset of drought conditions.

Zone V: semi-permanent pond (390–310 cm;ca 7.7–5.8 ka)More abundant and better preserved fossils in strati-graphic levels14C dated between ca 7.7 and 5.8 ka(Figure 2; Table 1) indicates that the period of ariditythat began ca 8.8 ka at the Andrews site was beginning

to wane and water levels were rising. Between ca 7.7and 5.8 ka (Table 1), the Andrews site basin appears tohave been a semi-permanent prairie pond (Figure 3).Water levels in Zone V are believed to have been low-er than those in Zone III, for the dark olive gray (5Y3/2, Munsell) sediments of Zone V are non-laminated.Although distinct marl layers are not present, abun-dant carbonate nodules andCharasp. macrofossils inthis zone indicate that the water was rich in carbonatessimilar to that during Zone III.

Based on comparison of fossil and modern plantrepresentatives, the hydrological regime of Zone V atthe Andrews site appears to be intermediate to thatinferred for Zone III and the present water levels.Abundant well-preserved buds ofPopulus tremuloides

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Figure 3. Continued

and fruits ofTypha latifoliain the lower to mid part ofZone V suggests that moisture levels were somewhathigher than today (Figure 3), because these speciespresently grow in and near prairie potholes and lakes,but not at the Andrews site itself.

Macrofossil remains from mudflat plants includ-ing Rorippacf. R. truncata(Jepson) Stuckey,Menthaarvensisvar. villosa, Ranunculus sceleratusL., Lyco-pus americanusMuhl., andPotentilla norvegicaL. arecommon to this zone (Figure 3; Table 2). These speciesare well adapted to disturbances, such as fluctuatingwater levels, and may have expanded from the shore-line towards the center of the basin during low waterphases (Stewart & Kantrud, 1972). Likewise, the onlyemergent plant in Zone V isTypha latifolia(Figure 3),

which is known for its ability to produce abundantseeds in a disturbed habitat and quickly re-vegetate thesite when shallow (ca 50 cm), slightly brackish wateris present (Shay & Shay, 1986; Kantrud et al., 1989).

Increased fungal and microbial degradation of thefossil remains at the top of Zone V and decreased diver-sity and abundance of macrofossils recovered (Fig-ures 2, 3), indicate that conditions in the upper part ofthe section were not conducive to macrofossil preser-vation.

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Discussion and conclusions

Plant macrofossils and stratigraphy of a prairie pot-hole in southern Saskatchewan indicate that hydrolog-ic and climatic conditions were significantly differ-ent in late-glacial to mid-Holocene times than today.The Andrews site is presently a kettle basin with anephemeral water regime situated in the semi-arid north-ern Great Plains. In comparison, the fossil and strati-graphic records of this site suggests a paleohydrologicsequence of: a deep (>2 m) perennial pond (Zones IIand III); a low-water stand (Zone IV); and a semi-permanentpond (Zone V). Six14C ages for 2 m of plantmacrofossil-bearing sediments present a tight radio-carbon chronology in which to reconstruct postglacialpaleoenvironmental events.

The wetland phase at the Andrews site beganca 10.2 ka with vegetation like that of a calcareousdeep-water sedge fen situated in a white spruce forest(lacustrine deposits of Zone II). This zone is transition-al between a white spruce forest of Zone II litter (priorto 10.2 ka) and a deep-water pond and deciduous park-land environment of Zone III (ca 10 to 8.8 ka). Otherkettle-fill sites in southern Saskatchewan and southernAlberta have yielded similar plant macrofossil assem-blages and lacustrine sedimentary sequences indicativeof an early Holocene high-water stand (e.g., Beaudoin,1992, 1996; Klassen, 1994; Beaudoin et al., 1996).Geochemical and sedimentological data from CeylonLake, an empheral salt lake situated 150 km southeastof the Andrews site in southern Saskatchewan, sug-gests that this lake contained relatively fresh and deepwater (up to 20 m) during the early Holocene (Last,1990).

The early Holocene landscape of the Andrewssite (Zone III), and presumably other sites within theregion, is suggested here to have been a ‘park oasis’,with abundant moisture for plant, animal, and humanpopulations. This environment has no exact modernanalogue (Yansa, 1995), but comparable deep-watercalcareous ponds surrounded by deciduous trees maypresently be found in areas of Humid Continental cli-mate like north-central Minnesota (Bickley & Clay-ton, 1972). The mechanism responsible for very highwater levels during the early Holocene at the Andrewssite is unclear, but is probably due to the melting ofresidual stagnant ice (Clayton, 1967). High evapotran-spiration rates associated with climatic warming is theprobable cause for the shallowing and development ofcarbonate-rich and brackish water in the Andrews sitepond by 8.8 ka.

A period of maximum Holocene aridity, knownas the Hypsithermal, is proposed for the period fromca 9 to 4 ka on the northern Great Plains (Andersonet al., 1989). Fluctuating lake levels throughout thisinterval have been reconstructed from paleoenviron-mental studies in Saskatchewan (Sauchyn & Sauchyn,1991; Last, 1990; Yansa, 1995; Vance, 1997), Alber-ta (Vance et al., 1992; MacDonald, 1989; Schweger& Hickman, 1989), Montana (Barnosky, 1989), andNorth Dakota (Fritz et al., 1993; Grimm, 1995). Whilecharcoal has been identified and described in sedimentsfrom other kettle-fill sites, only the Andrews site has acharcoal-rich sandy clay unit indicating a peak in arid-ity (Zone IV). The timing of the Hypsithermal at theAndrews site is bracketed between ca 8.8 and 7.7 ka,and lags somewhat behind reports from the MackenzieDelta, central Alberta, and Montana where dry lakebasins and drought-tolerant taxa have been reportedbetween 10 and 8 ka (Ritchie et al., 1983; Barnosky,1989; Schweger & Hickman, 1989). The delay in theonset of peak Holocene aridity until the mid-Holocenehas also been reported for the American Midwest andthere it has been identified as time-transgressive corre-sponding with the retreat of the Laurentide Ice Sheetacross north-central North America (Wright, 1976;Winkler et al., 1986). The Hypsithermal, therefore,should be viewed as a diachronous inferred climaticperiod (H. J. B. Birks, pers. commun., 1997).

It is not known why the water levels rose at theAndrews site after 7.7 ka and persisted until ca 5.8 ka(Zone V). Perhaps increased precipitation associatedwith shifts in atmospheric circulation patterns wereresponsible. Vance et al. (1993) interprets brief moistintervals occurring during a largely arid period between7.3 and 6 ka, based upon the Chappice Lake record ofsoutheastern Alberta.

Termination of the Andrews site fossil record atabout 5.8 ka coincides with other reports of persis-tent drought conditions occurring at this time. A pro-longed period of aridity between ca 6 and 4 ka hasbeen reconstructed for Chappice Lake (Vance, 1991;Vance et al., 1992). At about 6 ka, the water of CeylonLake, southern Saskatchewan, shifted from carbonate-dominated to saline or hypersaline as the lake shal-lowed (Last, 1990). A change in diatom assemblagesbetween ca 6.5 and 5.2 ka at Harris Lake,Cypress Hills,Saskatchewan, has been attributed to greater aridity(Wilson et al., 1997).

Water level perturbations of Holocene lakes andprairie potholes of the northern Great Plains have beenattributed to climate forcing (Harrison & Metcalfe,

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1985; Anderson et al., 1989; Vance et al., 1994).Lake basins that went dry or were at low levels dur-ing the mid-Holocene on the northern Great Plainsbegan to fill with water ca 4 ka to reach current lev-els by about 3 ka (Last & Slezak, 1988; Barnosky,1989; Schweger & Hickman, 1989; Vance & Last,1994). Prairie potholes, like the Andrews site, beinggenerally shallower than lakes were unable to recoversufficiently high and persistent water levels to preservefossils in the late Holocene. It may be that by aboutthis time the Andrews site and other kettles withinthe region became filled with enough sediment so thatthe water-holding capacity of the basin was reducedand hence fossil preservation was limited to belowthe water table (A. Beaudoin, pers. commun., 1996).Furthermore, local hydrological conditions may havealtered the water table at this and other sites on thenorthern Great Plains so that some basins remainedfilled with water while others dried (Last & Slezak,1986; Last & Sauchyn, 1993).

The absence of saline-tolerant taxa in the plantmacrofossil record of the Andrews site suggests thatgroundwater discharge was never dominant at the site.However, periodic discharge into the Andrews sitebasin may have occurred, because the macrofossilassemblages of Zones II, III, and V include plant taxadefined by Kantrud et al. (1989) as characteristic ofmodern recharge wetlands as well as those restrictedto discharge wetlands. Groundwater regimes of prairielakes and potholes of today are poorly understood, andour knowledge of past regimes, given the fragmentarynature of the record, is even poorer (LaBaugh et al.,1987; Vance et al., 1993). Only by sampling a range ofhydrologic regimes, from discharge-dominated salineprairie lakes to recharge-dominated prairie potholeslike the Andrews site, may regional paleohydrolog-ic dynamics and climate change be better understood(Lemmen et al., 1993).

Acknowledgments

This research was conducted at the University ofSaskatchewan as a M.S. project under the supervisionof J. F. Basinger whose help is gratefully acknowl-edged. I thank A. Andrews for permission to collectsample on her property;L. Fritz and S. J. Yansa for fieldassistance; V. H. Harms and P. A. Ryan for permittingpreparation of a comparative collection from speci-mens housed in the Fraser Herbarium; D. Lemmen andA. Beaudoin for arranging for radiocarbon age deter-

minations; and A. E. Aitken, A. K. Burt, B. A. LePage,C. Greenwood, and M. J. Reeves for technical adviceand assistance. This manuscript has benefited consid-erably from critical comments made by H. J. B Birks,A. Beaudoin, V. Holliday, B. A. LePage, and an anony-mous reviewer on an earlier version. Funding was pro-vided by the Palliser Triangle Global Change Project(to J. F. Basinger, A. E. Aitken, and C. H. Yansa), Geo-logical Survey of Canada; and the Natural Sciencesand Engineering Research Council of Canada (operat-ing grant #0GP0001334 to J. F. Basinger).

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