Use of lacustrine sedimentary sequences as indicators of Holocene glacial history, Banff National Park, Alberta, Canada

QUATERNARY

RESEARCH

26,218-231

(1986)

Use of Lacustrine Sedimentary Glacial History, Banff

Sequences as Indicators of Holocene National Park, Alberta, Canada

ERIC M. LEONARD Department

of Geology.

Colorado

College,

Colorado

Springs,

Colorado

80903

Received July 27. 1985 Bottom sediments from three lakes in the Canadian Rocky Mountains were examined with the aim of evaluating the usefulness of downvalley sediment studies in reconstruction of Holocene glacial histories. Analyses of organic carbon and carbonate contents of core sediments provide information on changing sedimentation rate and changing relative importance of glacial and nonglacial sediment sources. Sedimentary histories of the three lakes are similar, suggesting that they record regional glacial/climatic forcing, rather than localized events, and thus that they may be useful in reconstructing Holocene glacial history. Lacustrine sediments indicate a period of high sedimentation rates and relatively large glacial sediment contribution prior to 7500-7000 yr B.P., with much reduced rates and decreased glacial sediment contribution between about 6000 and 4000 yr BP, possibly interrupted by a brief period of increased glacial sediment output shortly after 5000 yr B.P. Sometime after 4000 yr B.P., sedimentation rates and glacial sediment output began to rise again, reaching approximately present levels by 2750-2650 yr B.P., and have not since returned to low mid-Holocene levels. In detail over the last 3000 yr there is some indication of a slight decrease in sedimentation rate for more than 1000 yr after about 2200 yr B.P. Sedimentation rates and glacial sediment input into all three lakes rose between about 900 and 750 yr B.P. and have remained very high since. If the lake sediments are interpreted as a proxy record of upvalley glacial activity, they allow the development of a glacial chronology which is at once generally consistent with, and more complete and easily datable than, the surticial glacial record. 1~1986 University

of Washington.

INTRODUCTION

The discontinuous nature of the surficial glacial record presents problems which have long hindered attempts to reconstruct glacial histories. The moraine record is limited almost exclusively to glacial advances, generally to advances after which no subsequent more extensive advance occurred (Porter, 1974; Gibbons el al., 1984). Information on glacial history during intervals between maximum ice stands, intervals constituting by far the majority of Quaternary time, is very difficult to obtain from surticial deposits. Development of Pleistocene marine oxygen isotope chronologies has to some degree alleviated this problem, but the isotope record has proven less useful in documenting the much smaller glacial fluctuations of the Holocene, and understanding of Holocene glacial history

still rests overwhelmingly on the fragmentary moraine record. Karlen (1976, 1981) suggested that continuous Holocene glacial chronologies might be established using downvalley lacustrine sedimentary sequences as indicators of upvalley glacial activity. Glaciogenie lake sediments deposited beyond the outermost Holocene moraines should contain complete sedimentary sequences reflecting, in some fashion, upvalley glacial activity through the Holocene. This record is more difficult to interpret in terms of upvalley glacial activity than is the moraine record, and Karlen’s approach utilizes untested assumptions regarding the relationship between glacial activity and downvalley sedimentation. The approach does, however, offer a potential for circumventing problems inherent in morainebased chronologies. 218

0033-5894/86 $3.00 Copyright All rights

0 1986 by the University of Washington. of reproduction in any form reserved.

NEOGLACIAL

This study examined sedimentary sequences from three lakes in northern Banff National Park, Alberta. The work was aimed at evaluating KarlCn’s method and, if possible, at using the sedimentary record to construct a more continuous and detailed Holocene glacial record for the region than had been possible using the morainal deposits alone. STUDY AREA

Hector, Crowfoot, and Bow Lakes are located in the upper Bow River drainage of Banff Park, immediately east of the Continental Divide (Fig. I). Glaciation in the drainage is now confined to two small icefield complexes along the divide and numerous cirque glaciers. The Bow River, which heads 4 km north of Bow Lake, flows through all three lakes.

CROWFCOT

LAKE

RECORDS

219

At each lake, stream flow and sediment input are strongly augmented by drainage from glaciers to the west (Fig. 1). Bow Lake, northernmost of the three, is a fairly large (2.7 km”) and deep (ca. 50 m maximum) lake. The main source of water and sediment for the lake is a meltwater stream draining 8.5 km2 of the Wapta IcefieldBow Glacier complex to the west (Kennedy, 1975). The nonglacial Bow River is a much less important water and sediment source. Crowfoot Lake is a small (0.23 km?), shallow (ca. 8 m maximum) lake located a few hundred meters down the Bow Rivet from the outlet of Bow Lake. The lake receives water and sediment from two distinct sources and at four points along the lakeshore. The Bow River enters from the northeast as two separate channels. Crow-

LAKE

I km

FIG. I. Map of the study area in the upper Bow River drainage. Lakes investigated in this study are shown in black. Major glaciers are shown in the stippled pattern and named glaciers are identified by letters (B = Bow Glacier, W = Wapta Icefield, C = Crowfoot Glacier, V = Vulture Glacier, Wk = Waputik Icefield, Bf = Balfour Glacier). Altitudes are in meters above sea level. The dot on the insert map indicates the study area.

220

ERIC

M.

foot Glacier (1.6 km2) and a small neighboring ice body feed two meltwater streams which enter the lake from the southwest. Discharge of the latter streams appears to be less than that of the Bow River, but their turbidity is greater. The relative importance of the sediment sources is not known, but the presence of small deltas at the meltwater stream inlets and their absence at the Bow River inlets suggests that the former may contribute more sediment to the lake. To minimize the influence of Bow River sedimentation and maximize that of Crowfoot Glacier meltwater, cores were taken much closer to the Crowfoot meltwater stream inlet than to the Bow River inlets. Nonetheless, to some extent the core record probably reflects both sources. Hector Lake is the largest (5.5 km*) and deepest (87 m maximum) of the lakes. Water and sediment enter the 5.5-km-long lake from the Balfour Stream at its northwestern (uplake) end and from the Bow River along the eastern shore about 0.5 km uplake from the outlet (Fig. 1). The Balfour Stream drains an area of 40 km*, 41% of which is covered by ice of Waputik Icefield, Balfour and Vulture Glaciers, and several small unnamed ice bodies. The Bow River drains a much larger area but carries virtually no glacial meltwater which has not passed through one or more lakes upvalley. The Bow River has a greater discharge than the Balfour Stream, but transports much less sediment (Smith, 1978), and much of that sediment probably exits the lake rapidly. Based on bottom sediment characteristics, Smith (1978) concluded that except in periods of exceptionally high flow, Bow River sediments are limited to the most distal kilometer of the lake. Sediments throughout the remainder of the lake are almost exclusively of Balfour Stream origin. Kennedy (1975) and Smith (1978) reported the presence of varve-like rhythmic couplets in bottom sediments of Bow and Hector Lakes. Examination of 13’Cs pro-

LEONARD

files from (Leonard, plets from icity (true

laminated cores from each lake 1981, 1986) confirmed that couboth lakes are of annual periodvarves). METHODOLOGY

Karl& (1976, 1981) premised his work on Holocene sedimentary sequences on the assumption of a close relationship between upvalley ice extent and downvalley sediment transport rates. Utilizing this assumption and an assumed inverse relationship between elastic sedimentation rate and organic content of sediments, he interpreted changing organic content in cores from downvalley lakes as an indicator of changes in upvalley glacier extent. Although the assumption of a close and immediate relationship between ice extent and downvalley sedimentation rate may be reasonable, there are few data available against which to test it. Karl& (1981) recognized potential problems, recalling Church and Ryder’s (1972) suggestion that sedimentation rates may peak during “paraglacial” periods following glacial maxima when glacial deposits are exposed to fluvial reworking, rather than during maximum stands. An earlier portion of this study examined relationships between glacial activity and downvalley sedimentation by comparing varve sequences from Hector Lake with local and regional glacial history. The work is described in detail elsewhere (Leonard, 1986), and only the conclusions are summarized here. The comparisons indicate that changes in ice extent of a century or more duration are rather closely mirrored by changing downvalley sedimentation rate. Periods of increased ice extent are also periods of rapid sedimentation, while decreased ice extent is reflected in slower sedimentation. The relationship is complicated by persistence of high “paraglacial” sedimentation rates for up to a century after maximum ice stands, a complication which limits the resolution in glacial chronologies reconstructed from la-

NEOGLACIAL

custrine sequences to about a century (Leonard, 1985). The assumption of an inverse relationship between organic content and elastic sedimentation rate was also tested, by comparing varve thickness (a measure of sedimentation rate) to organic carbon content down one Bow Lake core. A moderately strong inverse relationship was found (Leonard, 1981). Another method of evaluating changing glacial sediment output was suggested by distribution of bedrock lithologies in the Bow drainage (Fig. 2). Major glaciers rest almost entirely on Cambrian carbonate rocks, while the area immediately surrounding the lakes, and underlying the main Bow River valley and much of the rest of the nonglacierized portion of the

LAKE

221

RECORDS

drainage, is underlain by predominantly siliceous sedimentary rocks of upper Proterozoic to Cambrian age. As a result, glaciogenic sediments should be highly calcareous, while lacustrine sediments of nonglacial origin should be relatively carbonate deficient. Changes in carbonate content of lake sediments should thus provide an indication of changing relative importance of glacial and nonglacial sediment sources and, indirectly, of glacial sediment output. Comparison of Bow Lake delta sediments from heavily glacierized and nonglacierized source areas (Table 1) indicates the differences in mineralogy with source area, and supports the suggestion that changing sediment mineralogy reflects changing relative importance of source areas.

5P40’

-

Normal Fault

I 1 lfP30

Thrust Fault. teeth on u,wer block I

116’20’

FIG. 2. Generalized bedrock geology of the upper Bow River drainage (after Price and Mountjoy, 1978a, 1978b). Stippled areas are underlain by Proterozoic and lower Cambrian arenaceous and argillaceous sedimentary rocks. The remainder of the drainage is underlain by middle and upper Cambrian carbonate rocks. Noncalcareous rocks generally underlie the valley of the Bow River, including the areas immediately surrounding Bow (BL), Crowfoot (CL), and Hector (HL) Lakes. Calcareous rocks underlie the glaciers (dashed outline) along and near the Continental Divide.

222

ERIC M. LEONARD

TABLE 1. COMPARISONOFCARBONATECONTENT OFBOWLAKEDELTASEDIMENTSDERIVEDFROM THEBOWGLACIERMELTWATERSTREAMWITH THOSEDERIVEDFROMTHEBOWRIVERDRAINAGE ABOVEBOWLAKE~ Pebble countb Bow Glacier outwash stream Bow River <63 p (silt/clay) fraction Bow Glacier outwash stream Bow River

44.0 (n 4.0 (n

? = ? =

4.2% carbonate 12 samples) 1.6% carbonate 4 samples)

75.7 (n 22.5 (n

i = 2 =

4.2% carbonate 2 samples) 17.3% carbonate 2 samples)

0 Bedrock in the heavily glacierized former drainage is more than 95% carbonates, while noncalcareous rocks underlie more than 75% of the ice-free latter drainage (Fig. 2). b Pebble counts were made on the basis of dilute HCl reaction of 50 freshly split pebbles (l-5 cm longest axis) at each sample site. The procedure may have resulted in an undercount of dolomite pebbles, and a resultant underestimation of pebble carbonate percentage. Carbonate content of the <63 p fraction was made gasometrically with a Chittick device.

DATING OF CORES

Although varves are present in proximal sediments in Bow and Hector Lakes, in more distal areas where long time span cores could be recovered, sedimentation rates are too low for varve deposition. The presence of Holocene tephra layers allowed development of a general temporal framework for the cores. The Mazama (layer 0) ash from Crater Lake, Oregon, erupted 6700-7000 yr B.P.,’ and/or the Bridge River ash, erupted about 2350 + 50 yr B.P. from the Meager Mountain Complex, British Columbia (Mathewes and Westgate, 1980), were recovered in each core analyzed. Tephras were identified petrographically on the basis of pheoncryst assemblage, refractive index of glass, and character of plagioclase (Nasmith et al.,

Dating of sediment between ash layers was problematic. Small core diameter and low organic and high carbonate contents of the sediment precluded radiocarbon dating. One approach to interpolation is to assume a linear age/depth relationship between dated layers, on the assumption that total sedimentation rate remained approximately constant. Data available in this study indicate that this assumption is not justified (Table 2). Organic sedimentation rate in the lakes studied is somewhat variable, but it is more constant than total sedimentation rate (Table 2), suggesting the method of interpolation used here. Dates were calculated assuming constant organic carbon influx. A l-cm core segment with 0.5% organic carbon was assumed to have taken half as long to have been deposited as a l-cm segment with 1.0% organic carbon. As systemic bulk density measurements were not made, it was not possible to determine absolute organic influx rates. Analysis of one core indicated that core sections with low organic content have essentially constant bulk density, varying by less than 6% in 12 samples (Leonard, 1981). Sediments with high organic content are spongy and have variable bulk densities, making changing percentage of organic carbon a poor indicator of absolute organic influx. The constant organic influx model should provide reasonable results for core sections with less than l-2% organic carbon, but less reliable results for more organic-rich sediments. As sediment deposited during the last 3000 yr in all three lakes was deficient in organic matter, determination of dates for the upper portion of the record should be good. The variable organic content and bulk density of sediments deposited between the Mazama ashfall and 3000 yr BP make dating within this interval more problematic.

1967). 1 Sarna-Wojcicki et al. (1983) have reviewed dating of the Mazama eruption and conclude that the eruption occurred “between about 7000 and 6700” yr ago. In this paper a mean value of 6850 yr B.P. is assumed.

FIELD AND LABORATORY TECHNIQUES

Sediment cores were taken in distal areas of each lake. In Crowfoot Lake cores up to

NEOGLACIAL TABLE

LAKE

223

RECORDS

2. SEDIMENTATION RATES BETWEEN HORIZONS OF KNOWN AGES’ Total sedimentation rate (mg cm-3 yr-i)

Time interval (yr B.P.) Crowfoot Lake Core CF-1 O-2350 2350-6850 Rate change Boundary Lake Core S-7a O-2350 2350-3400 Rate change

Organic sedimentation rate (mg cmm3 yr-i)

Clastic sedimentation rate (mg crne3 yr-i)

15.7 4.6 x3.4

0.290 0.438 x 1.5

15.4 4.2 x3.7

26.2 11.8 x 2.2

0.189 0.131 x 1.4

26.0 11.7 x 2.2

u Ages are from core tops or identified tephra layers (Bridge River = 2350 yr B.P., St. Helens Y = 3400 yr B.P.. Mazama = 6850 yr B.P.). Organic sedimentation rate was calculated assuming that total organic content is 1.6~ organic carbon content. Boundary Lake is a small glacial lake in northernmost Banff National Park. approximately 75 km northwest of Bow Lake.

2 m long were taken with a modified Livingstone piston corer. In Hector and Bow Lakes deep water precluded use of the Livingstone, and cores were taken with a 90-cm-long gravity corer. Cores were selected for analysis on the basis of temporal span (determined by interbedded tephra) and lack of distortion. Analyses were made at OS- to 2.0-cm intervals down each core. Organic carbon determinations were made using potassium dichromate oxidation-digestion methods (Allison, 1965). High dolomite content of the sediment made gasometric determination of carbonate content a slow process. A more rapid, possibly somewhat less accurate, method was developed using x-ray diffraction. Based on diffractogram peak heights, a “carbonate index” was defined as: (H, + H,Y(H,

+ I-I, + H,,,)

where H, is the height of the 29.44” 28 calcite peak, H, the height of the 30.98” 28 dolomite peak, and Hq+m the height of the 26.6”-26.8” 20 quartz plus mica and clay peak. These are the dominant mineral species in the cores. Carbonate index bears an approximately linear relationship to carbonate percentage (determined gasometrically) with a detection limit of about 15% carbonate (Fig. 3). X-radiograph back-

ground variations make it difficult to determine indices less than about 0.05. Scatter in Figure 3 suggests that small sample-tosample differences in carbonate index may not reflect real differences in carbonate content, but differences of 0.1-0.2 or more should indicate significant differences in carbonate content. Downcore index changes greater than about 0.1, if sustained for several samples, should be reflections of changing carbonate content although indices may not provide exact mineral proportions. 0.6. t: 0 z f

:,

*

0.5. . i ,,‘.

0.4, ,’

k g

0.3.

E 5

0.2.

-

, /’ /

./

y=.oo973x r=0.9

- ,174 16

/ ’ I /’

0.1. . ,,’

-. 7 -I/._-..

0.0

0

10

20

30

40

PERCENT

_~

50

60

70

80

_~~

1

90

100

CARBONATE

FIG. 3. Relationship between x-radiographically determined “carbonate index” (see text) and gasometrically determined carbonate content for 12 samples from Bow Lake core B-111-2. The reason why one sample with moderately high carbonate content (44.0%) has an anomalously low carbonate index (0.07) is not known.

224

ERIC M. LEONARD

SEDIMENTARY

RECORDS

Hector Lake

One Hector Lake core (H-13~) penetrated a Holocene tephra layer. The 39-cmlong gravity core was taken in about 30 m of water in the northeastern portion of the lake, in an area dominated by Balfour Stream sedimentation. Bridge River ash was present at a depth of 31 cm. Organic carbon was measured at OS-cm intervals down the core, and carbonate index at l-cm intervals (Fig. 4). The two curves show strong general parallelism. Low organic content and high carbonate index are prevalent in the uppermost 11 cm, preceded by an interval (11-27 cm depth) of higher organic carbon content and less carbonate. The interval for several centimeters both above and below the Bridge River ash layer shows a pronounced decline in organic content and some increase in carbonate. In the basal samples (below 36 cm) there is a return to high organic content but no change in carbonates. The profiles in Figure 4 show periods of relatively high elastic sedimentation rate (indicated by low organic content) and

large glacial sediment contribution (indicated by high carbonate index) at the core top and around the time of the Bridge River ashfall. A period of lower elastic sedimentation rates and reduced glacial sediment output occurred in between. Sedimentation rate and glacial sediment contribution appear to have been at their lowest during the interval when sediments between 12 and 18 cm depth were deposited. Applying the constant organic influx model, using the Bridge River ash layer and the core top as fixed dates, elastic sedimentation rates appear to have been low prior to 2750 yr B.P., relatively high between 2750 and 2200 yr B.P., lower during the time interval between 2100 and 800 yr B.P. (especially 1300 to 900 yr B.P.), and high since 800 yr B.P. Varve measurements on sediment from more proximal areas of the lake (Leonard, 1986), yield additional data on the most recent interval of rapid sedimentation. During that interval, highest sedimentation rates prevailed between about 800 and 700 yr B .P., and during the last 400 yr, with somewhat lower rates between 700 and 400 yr B.P. Bow Lake

ORGANIC CARBON (%I

CARBONATE INDEX

0

10

ii 2

I t :

20

30

40

4. Organic carbon and carbonate index profiles of Hector Lake core H-13~1. FIG.

Several gravity cores from the eastern portion of Bow Lake reached the Mazama ash layer. In each, the tephra stopped corer penetration. Due to the corer configuration, the bottom several centimeters of sediment penetrated were badly distorted. Thus, although Mazama ash was retained in the base of each core, a section of 3-5 cm immediately above the ash could not be analyzed. Core B-4a, which was studied in detail, penetrated the Bridge River ash at 33-34 cm depth and was stopped by the Mazama ash at approximately 50 cm. The core was analyzed for organic carbon at 0.5-cm intervals, and for carbonate index every cm (Fig. 5). The basal section of the core is very organic rich (up 8% organic carbon). Organic content drops off abruptly upward and about 5 cm below the Bridge River Ash

NEOGLACIAL ORGANIC

CARBON

(96)

MAZAMA “”

CARBONATE

INDEX

ASH

LAKE RECORDS

225

lowed by a gradual, if variable, increase to the top of the core. In the top 13 cm of the core, organic content is broadly lower than in the section below (this change shows more clearly in the B-4a curve in Fig. 7). These relatively small variations in organic content in the upper section of the core are not paralleled by changing carbonate index. Reasons for this discrepancy are unclear. Application of the constant organic influx model suggests that the early period of very low sedimentation rate terminated about 2650 yr B.P. Much higher sedimentation rates have prevailed since that time. Some reduction in sedimentation rate may have occurred after about 2200 yr B. P. Since about 900 yr B.P., rates have been generally higher, reaching highest levels during the last 300 yr.2 Crowfoot Lake

”

FIG. 5. Organic carbon and carbonate index profiles of Bow Lake core B-4a. Note the change in scale along the abscissa.

reaches relatively low levels (~1%) which continue to the core top. The upward decrease in organic matter is accompanied by an equally pronounced increase in carbonates. These changes indicate that much lower sedimentation rates prevailed during the interval between the Mazama and Bridge River ash falls than has been the case since deposition of the Bridge River tephra, and that the transition was marked by a sharp increase in sediment derived from the icefield complex along the Continental Divide. Within the high sedimentation rate upper section of the core, trends are less clear than in the Hector Lake core. The organic carbon profile indicates relatively high sedimentation rates between 31 and 35 cm depth, around the time of the Bridge River ashfall. The high elastic influx may, however, reflect tephra influx rather than increased glacial sediment output. Above this level there is a slight decrease in sedimentation rate (increased organics) fol-

Three piston cores from the southern portion of Crowfoot Lake reached the Mazama ash. Core CF- 1, which showed the least distortion was analyzed, along with the basal section of core CF-3, which extended slightly further back in time. At a depth of 106 cm, core CF-1 reached, but did not penetrate, the Mazama ash layer. Disseminated Bridge River ash was present at 48-51 cm. Core CF-3 extended 11 cm below the 123- to 126-cm-deep Mazama Ash. Organic carbon content and carbonate index were measured at l- and 2-cm intervals, respectively, down core CF-1, and 2 A second Bow Lake core (B-6v) taken from the southeast portion of the lake was also examined in some detail. Bridge River Ash was recovered at the very base of that core (ca. 35 cm depth), but several centimeters immediately above the ash were lost due to the core catcher configuration. The organic carbon profile down this core was in general similar to that of the upper portion of core B-4a, although the most recent upward decrease in organic content, evident in the top 13 cm of B-4a, may have occurred somewhat earlier. Because the core was not complete down to a dated ash layer, however, dating of this event by interpolation using the constant organic influx model was not possible.

226

ERIC M. LEONARD ORGANlC

CARBON

CARBONATE

[B)

1NDEX

may have been simply the result of an influx of reworked ash, although the presence of a minor peak in carbonate index at f that level suggests a brief increase in glacial sediment output. Deposition of relatively organic-deficient sediments has persisted to the present, with a final further drop in organic content occurring between 27 and 18 cm depth in the core, accompanied by a major increase in carbonates. Coarse sand and gravel are present in the organic-deficient basal 4 cm of core CF-3, but absent higher in the cores. The coarse material prevented deeper coring, but probing indicated that the coarse layer had a thickness of about 60 cm with approximately 30 cm of fine-grained sediment present below. Dating of sedimentary events is rela140 tively simple in the organic-deficient upper I FIG. 6. Organic carbon and carbonate index profiles of Crowfoot Lake core CF-1 (solid lines) and the portion of the core, but the organic-rich, spongy nature of the mid-Holocene sedibasal 20 cm of core CF-3 (dashed lines). ments lower in the core makes interpolation of dates problematic. Assuming a conthe basal 20 cm of CF-3 (Fig. 6). Low or- stant organic influx model for the upper ganic and high carbonate contents pre- portion of the core, deposition of low orvailed prior to the Mazama ashfall, fol- ganic content sediments (at 55 cm depth) lowed by an interval of deposition of much began about 2750 yr B.P. This date is a more organic-rich, carbonate-deficient sed- useful reference point for dating of earlier, iments ending sometime before the deposimid-Holocene, events. tion of the Bridge River ash. This interval Timing of the deposition of coarse, verywas interrupted by a brief period of deposiorganic-deficient, carbonate-rich sediments tion of more organic-deficient, carbonateat the base of core CF-3 is uncertain. As rich sediments evident at 81-83 cm depth these sediments extend to within about 6 in core CF- 1. A long period of renewed de- cm of the Mazama Ash, itself at a depth of position of organic-deficient sediments 123-126 cm, it is probable that their depobegan shortly before deposition of the sition did not end until ca. 7500-7000 yr Bridge River tephra, accompanied by a B.P. The ensuing drop in sedimentation slight increase in carbonate index.3 A fur- rate continued until shortly after Mazama ther, minor, drop in organic content around time (ca. 6000 yr B.P.), ushering in the midthe time of the Bridge River ash deposition Holocene period of very low sedimentation rates. The interval of slow sedimentation 3 Although this appears to be only a minor increase appears to have lasted until sometime beincarbonate content, it may in fact be more significant than it appears, due to the lack of sensitivity of car- tween 4000 and 3000 yr B.P. The brief period of more rapid sedimentation during bonate index measurements to low carbonate content samples (see above). Carbonate indices lower in the this mid-Holocene interval occurred at core are essentially zero and Chittick analyses of two 4950-4800 yr B.P., assuming a linear age/ of those samples indicates carbonate contents of less depth relationship between 2750 yr B.P. than 10%. The upcore increase to carbonate indices of and the Mazama ash layer, or 5000-4900 yr 0.07 to 0.12 suggests at least 25% carbonate in these BP, assuming constant organic influx (but samples. 8

0

6

4

2

0

0

020.406081.0

NEOGLACIAL

not correcting for possible changes in bulk density). These estimates are at best first approximations, but suggest that the peak in sedimentation occurred around or shortly after 5000 yr B.P. Following the return to much higher sedimentation rates by 2750 yr B.P., rates remained high and nearly constant until 7.50-500 yr B.P., when they increased once more. HOLOCENE SEDIMENTARY HISTORY OF THE UPPER BOW RIVER DRAINAGE

Comparison of sedimentary records from the three lakes allows identification of patterns present throughout the upper Bow drainage and isolation of events limited to a single lake, facilitating development of a basin-wide sedimentary chronology. Laketo-lake correspondence is good, with almost all major changes in one lake record also appearing in records from the others. The broadest pattern is of rapid sedimentation in the early portion of the record (a period covered by only the Crowfoot Lake cores), lasting until approximately the time of Mazama Ash deposition, followed by a period of much lower sedimentation rates (spanned by both Crowfoot and Bow Lake cores), ending some time before deposition of the Bridge River Ash. Sedimentation rates then rose to approximately their present level (a rise seen in all three lake records), shortly before the Bridge River ashfall. Since Bridge River time, sedimentation rates have remained relatively high, with a tinal increase occurring near the top of each core. Carbonate content data display a similar pattern, indicating that periods of high sedimentation rate (low organic content of sediment) are also periods of high glacial sediment output (high carbonate percentage), supporting the idea that major changes in sedimentation rate in the lakes result from changes in glacial sediment output. Dating of these changes in sedimentation is not precise. At Crowfoot Lake sedimentation rates were high until ca. 7500-7000 yr BP and then dropped to very low levels

LAKE

RECORDS

227

by about 6000 yr B.P. Subsequent return to higher rates, evident in both Crowfoot and Bow Lake cores, probably began between 4000 and 3000 yr B.P. Assuming constant organic influx for the upper portion of the cores, the return to high rates was completed by 2750-2650 yr B.P. in each lake. During the mid-Holocene interval of low sedimentation rates, there is evidence of a brief period of increased sedimentation rate and glacial sediment output in both Crowfoot and Bow Lake cores. Neither the dating of this event, nor its lake-to-lake synchronism is certain, but at Crowfoot Lake the event probably occurred shortly after 5000 yr B.P. Sedimentation rates during the last 3000 yr have remained much higher than those of the mid-Holocene. Changes in sediment organic carbon content over the last three millennia are plotted in Figure 7, with ages calculated assuming constant organic influx. Relatively high sedimentation rates

FIG. 7. Comparison of organic carbon profiles from Hector, Crowfoot, and Bow Lake cores over the time interval since approximately 3000 yr B.P. Stippling indicates intervals of deposition of sediments with lower than the mean post-2750 yr B.P. organic content in each lake. Time scale is determined on the basis of the constant organic influx model (see text) taking the core top and the base of the Bridge River ash as fixed dates of 0 and 2350 yr B.P.. respectively.

228

ERIC M. LEONARD

prevailed in all three lakes around the time of the deposition of the Bridge River ash. At Hector Lake sedimentation rate and glacial sediment output decreased somewhat after about 2200 yr B .P., reaching late Holocene minima between about 1300 and 900 yr B.P. It is not clear whether a similar reduction in glacial sediment ouput occurred in the other lakes at that time. A final rise in sedimentation rates occurred in all three lakes, but timing of this rise appears slightly variable between the lakes, ranging from about 900 yr B.P. in Bow Lake to 750 yr B.P. in Crowfoot Lake. It is unclear whether the onset was really slightly diachronous, or whether the apparent differences in timing result from the crudeness of the assumptions involved in dating the cores. INFERRED GLACIAL CHRONOLOGY FOR THE UPPER BOW RiVER DRAINAGE

As noted above, there is reason to believe that changes in glacial sediment output a century or more duration are closely related to changing upvalley ice extent. Because they reflect glacial sediment output, the organic carbon and carbonate curves discussed in the previous section may thus be viewed, at least in general outline, as proxy records of ice extent. In broad outline the core records suggest a period of relatively increased ice extent shortly before 7500-7000 yr B.P. The presence of coarse detritus in this section of the Crowfoot Lake core suggests that during this interval the type-Crowfoot moraine (Luckman and Osborn, 1979) immediately adjacent to the lake was deposited. The moraine is now densely vegetated and stabilized and no coarse material is being supplied from it to the lake. If unstable and unvegetated, as at its time of deposition, it would provide the most reasonable source of course sediment for the distal portion of the lake. Another possibility is that deposition of coarse material resulted from stream incision into the moraine some time

after moraine deposition and stabilization. That deposition of the coarse material represents a discrete event, rather than the end of the paraglacial effect following late Wisconsin deglaciation, is suggested by the presence of fine-grained lake sediments below the coarse layer. Major changes in sedimentation immediately before and following deposition of the Mazama ash suggest the onset of a midHolocene interval of greatly reduced ice extent. This period of reduced ice extent, lasted until 4000-3000 yr B.P., possibly interrupted by a brief readvance around 5000 yr B.P. Dramatic increases in sedimentation rate and carbonate content of sediments by about 3000 yr B.P. suggest greatly increased ice extent, and the persistence of those characteristics in the sediments suggests that since that time ice has been continuously much more extensive than it was in the mid-Holocene. In detail, advances culminating around the time of the deposition of Bridge River ash (2350 yr B.P.) may have been followed by a period of slightly reduced ice extent, with minimum extent occurring 1300-900 yr B.P. Between 900 and 750 yr B.P., changes in sedimentation in all three lakes indicate the onset of the latest period of glacial advances, with sedimentation rates and presumed ice advance culminating during the 13th and 16th through 19th centuries A.D. COMPARISON OF THE LACUSTRINE RECORD WITH OTHER PALEOCLIMATIC RECORDS

A check on the usefulness of lacustrine sediments as indicators of upvalley glacial activity is provided by comparison of lacustrine records with moraine sequences and other paleoclimatic records. To be useful in circumventing the limitations of the moraine record, lake sediments should yield an inferred glacial history both consistent with other glacial and nonglacial paleoclimatic records and more detailed and datable than the moraine record.

NEOGLACIAL

Comparison of lacustrine and moraine records in the Canadian Rockies is hampered by the incompleteness of the latter. There is widespread moraine evidence of two Holocene(?) glacial advances in the region (Luckman and Osborn, 1970; Osborn 1982). The recent Cave11 Advance commenced 1000-800 yr B.P. (Luckman, 1985), culminating in maximum ice stands during the early 18th and mid-19th centuries A.D. The earlier Crowfoot Advance predated deposition of the Mazama Ash, but no close maximum date is available, and the advance may in fact be of latest Pleistocene age (Luckman and Osborn, 1979). No moraines of intermediate age have been identified, but recent radiocarbon dating of overridden organic material at Boundary Glacier in northernmost Banff Park suggests that an early Neoglacial advance was underway by 4200-3800 yr B.P. (Gardner and Jones, 1985). Virtually no information on the magnitude and timing of glacial recessions is preserved in the surficial record. An earlier portion of this study (Leonard, 1986) concluded that Cave11 Advance glacial fluctuations of the last 800 yr were closely paralleled by changes in sedimentation rate. The 1000-800 yr B.P. date for inception of the Cave11 Advance corresponds closely to the 900-750 B.P. rise in sedimentation rate noted in all three lakes. The earlier Holocene surticial record is so sketchy and poorly dated that a direct comparison is difficult. Advancing ice at Boundary Glacier may be correlative with ice advances beginning between 4000 and 3000 yr B.P. inferred from the lake sediments. As no surficial evidence of maximum ice stands of this advance is preserved, a direct comparison to post-2750 yr B.P. maximum stands inferred from the lake record is not possible. Dating of the earlier Crowfoot Advance moraines is permissive of the inference from lacustrine evidence that the advance occurred shortly before Mazama ash deposition, but is not sufficiently precise to aid in evaluation of that inference.

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Radiometrically dated post-Mazama glacial chronologies elsewhere in the southern Canadian Cordillera yield sequences very similar to that inferred from the Banff lacustrine record. In the Coast Mountains of British Columbia there is evidence of three glacial advances corresponding closely in time with the inferred Banff advances of the last several hundred years and ca. 2800-2200 yr B.P., and with the inferred possible brief advance about 5000 yr B.P. (Fulton, 1971; Ryder and Thompson, 1986). In the Interior Ranges of British Columbia there is evidence of advances during the two later periods, and possibly during the earlier period (Alley, 1976; Duford and Osborn, 1978; Osborn. 19841. Paleobotanical data provide another proxy climatic record to which interpretation of the lacustrine data may be compared. In a review of Holocene paleobotanic studies in the Canadian Rockies, Osborn (1982) identified four general features from a somewhat disparate set of paleoclimatic interpretations: (1) the warmest and driest period of the Holocene occurred during an interval variously estimated at IOOO- to 2500-yr duration preceding about 6000 yr B.P. (2) A shift toward cooler and wetter climate occurred about 6000 yr B.P. (3) A second shift in the same direction occurred around 3000 yr B.P. Relative importance of these two shifts toward cooler and wetter conditions differs from study to study. (4) Following continuously relatively cool conditions since about 3000 yr RI?. the last “several centuries” have been the coolest and wettest of the Holocene. The later portion of Osborn’s generalized paleoclimatic record corresponds closely to the glacial history inferred from the lacustrine record. The major increase in ice extent suggested to have occurred between about 4000 and 2750 yr B.P., the persistence of much greater than mid-Holocene ice extent since that time, and the further expansion of ice since about 900-750 yr B.P. all closely parallel climatic changes suggested by paleobotanical evidence. A discrepancy exists between the two

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230

records for the earlier portion of the Holocene. While paleobotanical records indicate warm/dry conditions for the 1000-2500 yr preceding 6000 yr B.P., with a change toward cooler conditions around 6000 yr BP., the lake record suggests high glacial sediment output, and possibly deposition of the type Crowfoot moraine, shortly before 7000 yr B.P., with reduced ice extent after 6000 yr B.P. There are several possible reasons for the discrepancy. First, although it seems unlikely that the consensus of palynologists is incorrect that the period before 6000 yr B.P. was one of generally warm/dry conditions, it is possible that the inferred glacial advance during that period was a short-lived event which does not show clearly in most pollen records. Kearney and Luckman’s (1981) tree-line record from Jasper Park indicates a brief cool period between 8000 and 7500 yr B.P., splitting the Holocene thermal maximum. Possibly the Crowfoot Advance occurred during this interval. Second, the coarse, organic-deficient basal sediment in the Crowfoot core might represent the final phase of rapid paraglacial sedimentation following earlier Holocene or latest Pleistocene deglaciation. As noted above, however, the presence of fine sediment below the coarse material suggests a discrete event rather than a final deglaciation effect. Third, the sediment might reflect a more localized event, a drainage or sediment distribution pattern change, a mass-movement event, or stream incision into an older and already stabilized type Crowfoot moraine. The coarse layer was encountered in sediment probes throughout the lake, and must be at least lake-wide in significance. The first two possibilities involve events of probable regional significance and should be evident in cores from other lakes spanning the same time interval. Examination of such cores would aid greatly in clarifying the event. CONCLUSIONS

This study addressed two basic questions: first, is it possible to develop a con-

sistent basin-wide lacustrine sedimentation chronology which might be used as a proxy indicator of upvalley glacial history and, second, is the glacial chronology inferred from the lake sediments consistent with what is known of glacial and climatic history from other lines of evidence and useful in augmenting the other types of evidence. The current study suggests that both questions can be answered, at least preliminarily, in the affirmative. The three lakes examined provide essentially consistent chronologies, which are also in general consistent with both surficial glacial and paleobotanical evidence in the region, at least for the period since 6000 yr B.P. At the same time, the lacustrine record is more complete and detailed, preserves records of ice recessions as well as advances, and is generally more accurately datable than the moraine record. As such it appears to offer a method of circumventing the problems associated with the discontinuity of the moraine record. A significant discrepancy exists between the lacustrine and paleobotanical records concerning conditions shortly before the deposition of the Mazama ash. This discrepancy may serve as a reminder that the lacustrine record is only an indirect record of upvalley glacial activity and may sometimes provide “misinformation” on glacier variations, especially if interpretations are based on a single core. On the other hand, it is not certain that the inferred glacial history is incorrect, and it is also possible that the sediments record a glacial/climatic event sufficiently short lived that it does not appear in most of the paleobotanical records. ACKNOWLEDGMENTS This study was part of the author’s doctoral research. Thanks are due to John Andrews for his guidance, to Nel Caine, Bill Bradley, Peter Birkeland, and Norm Smith for their help, to Paul Russell, Bruce Morrison, and Peter Hodge for field assistance, and to Gerry Osborn for very helpful comments on this text. Financial support was provided by the National Science Foundation (EAR-7906316) and the University of Colorado Department of Geological Sciences.

NEOGLACIAL

REFERENCES Alley, N. F. (1976). Post-Pleistocene glaciations in the interior of British Columbia (abst.). In “Geomorphology of the Canadian Cordillera and its bearing on Mineral Deposits,” pp. 6-7. Cordilleran Section. Geological Association of Canada, Vancouver, B.C., Canada. Allison, L. E. (1965). Organic carbon. In “Methods of Soil Analysis” (C. A. Black. Ed.), pp. 1367-1378. American Association of Agronomy, Madison, Wise. Church. M., and Ryder, J. M. (1972). Paraglacial sedimentation: A consideration of fluvial processes conditioned by glaciation. Geologicnl Society of America

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Duford. J. M., and Osborn, G. D. (1978). Holocene and latest Pleistocene cirque glaciations in the Shuswap Highland. Britisth Columbia. Canudian Jorrrnal of Earth Sciences 1.5, 865-873. Fulton. R. J. (1971). “Radiocarbon Geochronology of Southern British Columbia.” Geological Survey of Canada Paper 71-37. Gardner, J. S., and Jones, N. K. (1985). Evidence for a Neoglacial advance of the Boundary Glacier, Banff National Park, Alberta. Canadian Jownul of Earth Sciences 22, 1753- 17.55. Gibbons, A. B.. Megeath, J. D.. and Pierce, K. L. (1984). Probability of moraine survival in a succession of glacial advances. Geology 12, 327-330. Karl&, W. (1976). Lacustrine sediments and tree-line variations as indicators of climatic fluctuations in Lappland, northern Sweden. Geogucrfiskrr Aunaler 58A. l-34. Karl&, W. (1981). Lacustrine sediment studies: A technique to obtain a continuous record of Holocene glacier variations. Geogrujiska Annaler 63A, 273-281. Kearney. M. S., and Luckman, B. H. (1981). Evidence for late-Wisconsin-early Holocene climatic/ vegetational change in Jasper National Park, Alberta. In “Quarternary Paleoclimate” (W. C. Mahaney, Ed.). pp. 85- 105. Geoabstracts. Norwich, U.K. Kennedy, S. K. (1975). “Sedimentation in a GlacierFed Lake.” Unpublished M.S. thesis, University of Illinois, Chicago Circle. Leonard, E. M. (1981). “Glaciolacustrine Sedimentation and Holocene Glacial History, Northern Banff National Park, Alberta.” Unpublished Ph.D. dissertation. University of Colorado. Leonard. E. M. (1985). Glaciological and climatic controls on lake sedimentation, Canadian Rocky Mountains. Zeirschrift fiir Gletcherkunde und Gla:iu/geo/ogie 21, 35-42.

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Leonard, E. M. (1986). Varve studies at Hector Lake. Alberta, Canada, and the relationship between glacial activity and sedimentation. Quaternarv Research 25, 199-214. Luckman, B. H. (1985). Reconstructing Little Ice Age events in the Canadian Rockies (abst.). In “CANQUA Symposium on the Paleoenvironmental Reconstruction of the Late Wisconsin Deglaciation and the Holocene,” p. 40. Luckman, B. H., and Osborn. G. D. (1979). Holocene glacier fluctuations in the middle Canadian Rocky Mountains. Quaternary Research 11, 52-77. Mathewes. R. W., and Westgate. J. A. (1980). Bridge River tephra: Revised distribution and significance for detecting old carbon errors in radiocarbon dates of limnic sediments in southern British Columbia. Canadian Journal of Earth Sciences 12, 1454- 1461, Nasmith, H., Mathews. W. H.. and Rouse, G. E. (1967). Bridge River Ash and some other recent ash beds in British Columbia. Canudian Jourrrul qt Earth Sciences 4, 163- 170. Osborn. G. (1982). Holocene glacier and climate fluctuations in the southern Canadian Rocky Mountains: A review. Striue 18, 15-25. Osborn, G. (1984). 2000 Year History of the Bugaboo Glacier, Purcell Mountains. British Columbia. American bienniul

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68-72. Price. R. A., and Mountjoy, E. W. (1978a). “Geology. Hector Lake (East Half).” Geological Survey ot Canada Map 1463A. Price, R. A., and Mountjoy, E. W. (1978b). “Geology. Hector Lake (West Half).” Geological Survey of Canada Map 1464A. Ryder, J. M., and Thompson, B. (1986). Neogiaciation in the southern Coast Mountains of British Columbia: Chronology prior to the late Neoglacial maximum. Canndion Journul of Eorfh Sciencc,.v 23, 273-287. Sarna-Wojcicki. A. M.. Champion, D. E.. and Davis. J. 0. (1983). Holocene volcanism in the conterminous United States and the role of silicic volcanic ash layers in correlation of latest Pleistocene and Holocene deposits. In “Late Quaternary Environments of the United States.” Vol. 3. “The Holocene” (H. E. Wright. Ed.), pp. 52-77. Smith. N. D. (1978). Sedimentation processes and patterns in a glacier-fed lake with low sediment input. Canadicrn Journrrl of Earth .Scienc,c.\ 15, 741-756.