Glacier fluctuation and tree-ring records for the last millennium in the Canadian Rockies

Quaternary Science Reviews, Vol. 12, pp. 441-450, 1993. Copyright9 1994ElsevierScienceLtd. Printedin Great Britain.All rightsreserved. 0277-3791/93 $26.00

Pergamon

GLACIER

FLUCTUATION

AND TREE-RING RECORDS FOR THE LAST MILLENNIUM IN THE CANADIAN ROCKIES

B. H.

Luckman

Department of Geography, University of Western Ontario, London, Ontario N6A 5C2, Canada Tree-ring series and records of alpine glacier fluctuations provide complementary evidence to reconstruct decade-to-century climate fluctuations over the last millennium in the Canadian Rockies. Tree-line ring-width chronologies in this area are temperature sensitive and long (ca. 900 years) series from sites adjacent to Bennington and Athabasca Glaciers show synchronous declines in ring-width during the periods ca. 1170-1180, 1280-1290, 1330-1350, 1430-1450, 1530-1540, 1690-1705 and 1810-1825 A.D. The latter two events are replicated in many shorter tree-ring chronologies and immediately precede the two major periods of moraine development (ca. 1700-1725 and 1825-1875 A.D.) during the Little Ice Age. Dendrochronological dating of glacially-overridden trees 0.5-1.0 km upvalley of 18th century maximum positions indicate that Robson and Peyto Glaciers were advancing between 1150 and 1350 A.D. during the earliest phase of the Little Ice Age in the Canadian Rockies. Available tree-ring data suggest that the level and pattern of climate variability during the 18th and 19th centuries extends back over the last millennium. It is also hypothesised that glaciers attained their maximum Holocene extent during the Little Ice Age because of the interaction between these decade-to-century scale fluctuations and the progressive long-term decline of incoming summer solar radiation in the northern hemisphere over the last 10,000 years.

INTRODUCTION

Chronology of Events

In many areas of the Canadian Rockies the most extensive Alpine areas contain several physical and biological Holocene glacier advance was the Little Ice Age (Luckman systems that are sensitive to climate changes and thereby and Osborn, 1979). Heusser (1956) provided the first provide natural archives which may be used to reconstruct comprehensive survey of Little Ice Age glacier fluctuations former climate fluctuations. This paper reviews evidence based primarily on dendrochronological dating of moraines from two such systems in the Canadian Rockies - - records in 11 glacier forefields. Subsequently, Luckman and Osborn of alpine glacier fluctuations and tree-ring series - - that (1979) presented moraine data from 24 glaciers using provide complementary evidence to reconstruct decade-to- lichenometry, dendrochronology and historical data and century scale climate fluctuations over the last millennium9 assigned the term Cavell Advance to the Little Ice Age In some cases the tree-ring sites and glaciers are juxtaposed glacial event of the middle Canadian Rockies. This database and, although they represent different integrations of the has been updated from the time to time (most recently to 33 climate signal and have different temporal resolution, glaciers by Luckman, 1986) and data are presently available examination of these records in concert provides for about 40 glaciers. Luckman (1986) identified three main independent corroboration of. the magnitude, timing and periods of Little Ice Age moraine formation in the Canadian direction of past climate fluctuations. The records will be Rockies based on the age of the oldest moraines. reviewed briefly, beginning with the glacial record, and then - Approximately one-third of the glaciers studied have outer compared 9 Most of the sample sites discussed are in or moraines dating from the first quarter of the 18th century. All adjacent to Banff and Jasper National Parks, Alberta, and are glaciers show outer or readvance moraines dating from the mid 19th century that are close to or (presumably) overrode shown in Fig. 1. the 18th century moraines. These two time intervals are the main periods of outer moraine formation during the Cavell G L A C I E R FLUCTUATIONS DURING T H E LAST Advance in the Canadian Rockies. About 30% of glaciers MILLENNIUM have small moraine fragments on the outermost moraine or outer moraines that have minimum lichenometric or treeThe geological/geomorphic record of glacier fluctuations ring dates that predate 1700. The absence of a clear temporal has been the traditional primary data source for information clustering of these dates may reflect several discrete glacial about climate fluctuations during the last millenium in the events or poor dating control for these moraines but does Canadian Rockies. Human documentary or instrumental indicate some Little Ice Age advances took place prior to climate records barely exceed 100 years (see Luckman, 1700 A.D. Recently Smith and McCarthy (1991) have 9 / 1990) and presently available palynological records have inferred periods of glacter(advance in the mid 14th century, insufficient resolution for a detailed study in this timeframe ca. 1426--1460, 1524-1547, 1670-1710 and ca. 1820-1900 (Luckman, in press)9 Although studies of lacustrine A.D. for glaciers in Peter Lougheed Provincial Park, Alberta sedimentation rates offer promise for the recovery of useful (the area adjacent to the Highwood and Nakiska sites on Fig. paleoenvironmental records (e.g. Leonard, 1986a,b) little 1). These ages are based on calibrated radiocarbon dates chronological work has been done with these deposits. from wood and interpretation of ring-width records from 441

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FIG. 1. Location of tree-rlng sites in the Canadian Rockies. The sites at Small River, Robson, Bennington, Cavell, Athabasca, Peyto Lake and Bugaboo are close to glaciers for which moraine chronologies have been developed (see Osbom and Luckman, 1988; Luckman et al., 1992). All other sites except Lake Louise (Luckman et al., 1985) are at treeline. The location of valley-floor sites for which Psuedotsuga menziesii chronologies have been developed are omitted.

GlacierHuctuationand Tree-RingRecords trees associated with the moraines. Most Little Ice Age moraines in Waterton and Glacier (U.S.) National Parks (adjacent to the American border) date from the 19th century (Osbom, 1985; Carrara, 1987). In almost all glacier forefields of the Canadian Rockies there is a sequence of several well developed moraines dating from the mid-to-late 19th century and early 20th centuries a short distance upvalley of the outer 19th century moraine. During the present century there has been considerable glacier recession (e.g. Gardner 1972; Carrara 1987) but rates of recession decreased markedly in the 1960s and 1970s (e.g. Luckman, 1988) and, at some sites, glaciers advanced and built moraines in the 1970s and early 1980s (Luckman et al., 1987). Evidence for glacier fluctuations in the early part of the Little Ice Age has recently been reviewed by Luckman (bz press) and is briefly summarised as follows. Heusser (1956) originally dated the beginning of the Little Ice Age as 450 _+ 150 BP based on an early Libby ~4C date from an overridden forest site at Robson Glacier. Later radiocarbon dates from the same site by Luckman (1986) suggested a kill date for these trees between ca. 1150-1250 A.D. Overriden trees of similar radiocarbon ages were also discovered at Kiwa Glacier in the Premier Range (Luckman, 1986) and Peyto Glacier (Luckrnan et al., 1993). The recent development of long tree-ring chronologies from this area of the Rockies (see below) has enabled the calendar cross-dating of glaciallyoverriden trees at Robson and Peyto Glaciers. At Robson Glacier, 37 trees have been crossdated with a range of outer ring dates between 1142 and 1350 A.D. (Luckman, unpublished data). Thirty four of these dates are between 1214-1350 A.D. At Peyto Glacier, 3 sheared, detrital logs lying on the surface of the forefield have outer ring dates between 1246 and 1324 A.D. (Reynolds 1992; Luckman et al., 1992). Rooted stumps were recovered from this locality in 1992. The trees killed by glaciers at both sites are between 0.5 and 1 km upvalley of the outermost 18th century moraines dated by Heusser (1956). These new data provide calendar dates for a glacier advance in this area during the 12th- 14th centuries. Leonard (1986a) had earlier postulated a glacier advance beginning ca 750-900 BP based on proglacial sedimentation in Bow, Crowfoot and Hector Lakes. Bow Lake is less than 10 km south of Peyto Glacier.

Temporal Resohttion and the Nature of Glacier Fhtctuation Records The major techniques used to date Little Ice Age moraines and glacier fluctuations are radiocarbon dating, dendrochronology, lichenometry and documentary sources. The relative merits, precision and pitfalls of these techniques have been reviewed several times (e.g. Porter, 1981a, 1992; Luckman, 1986; McCarthy and Luckman, 1993; Yamaguchi, 1989) and will not be presented here. Generally, minimum surface ages can be determined with error terms of _+10 years for dendrochronology and lichenometry over the last 300 years at appropriate sites (moraines below treeline or siliceous substrates). In exceptional cases, well controlled lichen growth curves (e.g. Porter, 1981b) or the dating of tilted, scarred or overridden trees (e.g. Luckman, 1988) can

443

provide more precise estimates of the date of moraine formation. Interpretation of the climatic significance of moraine dates has also been the topic of some discussion (e.g. Porter, 1981a; Oerlemans, 1989). Well-dated moraine sequences and/or stratigraphy provide a good general outline of the climatic history of the latter part of the Little Ice Age. The main limitation of these records for studies of decade-to-century climatic fluctuations in the last millennium is that they are incomplete, of variable quality over time, and only record selected events. Moraine sequences provide information regarding the maximum extent of glaciers and subsequent recession only for those areas which were not overriden by a later advance. Therefore, although the period of recession following the 19th century maximum positions can usually be reconstructed in some detail from available materials (documentary sources, historical photographs/drawings, dated morphological evidence etc.), information about the fluctuations of glaciers between advances or prior to ca. 1700 is rarely found. One of the most difficult unresolved questions in reconstructing glacier fluctuations of the last millennium is whether (or how far) glacier fronts receded between documented glacier advances. Is the rapid frontal retreat of glaciers in the present century typical of other warm periods during the last millennium or an unusual and significant event in the glacial history of the last 2000-3000 years? Therefore, although glacier fluctuation data provide a framework for the reconstruction of decade-to-century scale climate fluctuations for part of the last millennium, these records must be supplemented by other, continuous, proxy climate records that allow development of a more complete picture of the range of conditions during this period. TREE-RING RECORDS

bztrodttction Tree-ring series have highly desirable properties as proxy climate data because they provide precisely dated calendar records and vary in width, density and chemical composition in response to a variety of environmental factors (see Fritts, 1976; Gray, 1981; Schweingruber, 1988a). Until recently, tree-ring studies in the Canadian Rockies had received little attention except as a dating tool (Luckman and Innes, 1991) because the trees are relatively short-lived compared with other mountain areas in western North America (Hughes et al., 1982) and were considered complacent. Development of Records Initial dendrochronological studies by Schulman (1947) focused on moisture-sensitive sites using Psuedotsuga menziesii at the montane forest/parkland boundary in valley floor sites. Psuedotsuga menziesii chronologies from the Canadian Rockies are the northernmost sites in the first major dendrochronological network collected by the Laboratory of Tree-Ring Research in Tucson during the 1960s. These data have subsequently been used by Fritts and co-workers for climatic reconstruction of several parameters over the period 1600-1965 in the western United States (e.g. Fritts et al., 1979; Fritts, 1971, 1991). Parker and Henoch

444

B.H.Luckman

(197 I) published the first dendrochronological investigation at a treeline site in their pioneer densitometric paper on Picea engehnannii at Peyto Lake. In 1983 four Picea engehnannii treeline sites (Vermilion, Peyto Lake, Sunwapta and Bell Mountain, Fig. 1) were sampled by F. H. Schweingruber as part of his network of densitometric chronologies in the North American Cordillera (Schweingruber, 1988b). Preliminary annual temperature reconstructions based on this data set have recently been published by Schweingruber et al. (1991). The first tree-ring studies from the University of Western Ontario employed dendrogeomorphic techniques to date moraines (e.g. Luckman, 1977, 1988). The initial dendrochronological work addressed the potential of densitometric and isotopic studies as proxy climate sources (Luckman et ai., 1985). Over the last 10 years, studies have focused on the development of ring-width chronologies for treeline sites because of the well-documented temperature sensitivity of treeline species (e.g. Jacoby and Cook, 1981; Schweingruber, 1988a) and the potential to develop long chronologies using several species (Luckman et al., 1984; Luckman and Colenutt, 1992a). During these investigations the oldest living individuals of three of the four treeline species have been found at sites in the Canadian Rockies - Pbzus albicaulis Engelm. >882 years at Bennington Glacier, Picea engebnannii Parry >730 years at Peyto Glacier and Larix lyallii Parl. >728 years at Storm Mountain. Chronologies from living trees can be extended by the inclusion of crossdated ring-series from dead snags or stumps at and above present treeline or from stumps and logs buried by (or included within) Little Ice Age tills. The present UWO sample site network includes over 20 sites at which ring-width chronologies have been developed (17 Picea, 6 Larix, 1 Pinus, 1 Abies lasiocarpa, Fig. 1). Comparative studies of ring-width characteristics indicate differences in sensitivity (Fritts, 1976), ring-width characteristics and response between Larix and Picea (Colenutt and Luckman, 1991; Luckman and Colenutt, 1992a). The chronologies at all sites exceed 300 years in length. Some preliminary data for 12 chronologies over the 1700-1980 period are presented in Luckman (1992). Four chronologies extend back beyond 1200 A.D. and, when viewed in combination, provided the first detailed proxy climate information for the period prior to ca. 1600 A.D. The characteristics of these sites will be briefly reviewed prior to the evaluation of the record.

Long Chronologies The longest chronology developed so far is at the Peyto Glacier and extends from 760-1990 A.D. (Reynolds, 1992). It is derived from living Picea (1260-1990 A.D., only one iree prior to 1550) growing downvalley of the Little Ice Age terminus of the glacier and from snag material (mainly Picea) sampled within the glacier forefield. Two main snag populations have been identified; three trees killed by a glacier advance between 1246 and 1324 A.D. and 14 trees with outer ring dates between 1716 and 1836 A.D. (13 between 1765 and 1836) that were killed during the advance to the Little Ice Age maximum position (Reynolds, 1992; Luckman et al., 1992). Presently this chronology does not

have much sample depth (3 or 4 trees) prior to 1550 but samples taken in 1992 from a population ofPinus albicaulis snags on an adjacent talus slope will cover the period ca. 1000-1600 A.D. A second chronology was developed from snags (mainly Picea) overridden by Robson Glacier and covering the period 865-1350 A.D. This has greater sample depth (>5 trees between 1075 and 1275) but, because the trees were progressively overridden by the glacier, there is not a strong common signal for most of the later part of the record. The two best replicated long chronologies come from sites at Bennington and Athabasca Glaciers (Fig. 2). Both sites are on south-facing slopes overlooking Little Ice Age moraines. The Bennington site chrono!ogy utilises Pinus albicaulis growing on a blocky quartzite talus slope overlooking the lateral moraine of the glacier. The Athabasca chronology is developed from a stand of Picea engelmannii (up to 680 years, Luckman et al., 1984) at the foot of the slope adjacent to the terminal moraine. These data are supplemented with ring-width series from snags lying on the surface higher up the slope. Crossdating of these snags suggests that the treeline was higher than present at this site ca. 1400-1700 A.D. (Many trees died in the last half of the 17th century) and there is limited evidence for higher treeline prior to 1200 A.D. (Luckman, in press). Six Lar/x chronologies have been developed from treeline sites in Banff National Park and adjacent areas, close to the present northern limit for this species (Colenutt and Luckman, 1991). The chronologies are, from north to south, Larch Valley 1347-1986; Sunshine 1438-1987; Floe Lake 1523-1987; Marmot Basin 1588-1987; Nakiska 1568-1987; and Tyrwhitt 1406-1987 (Colenutt, 1992, see Fig. 1). These chronologies have a strong common signal (Colenutt, 1992; Luckman, 1992; Fig. 3) but sample depth at individual sites is limited prior to 1600.

blterpretation of the Tree-Rh~g Record Dendrochronological studies in this area to date have concentrated on the development of a network of sites and the building of long ring-width chronologies. Calibration of these records with instrumental climate data is difficult because climate stations are few (Luckman, 1990) and in valley floor locations far below or away from the tree-ring sites. Exploratory correlation studies matching individual chronologies with appropriate single station climate records have yielded promising but mixed results (Luckman et al., 1985; Hamilton, 1987; Colenutt and Luckman, 1991; Colenutt, 1992). These studies show that the strongest signal in both spruce and larch chronologies is summer temperature which is consistent with studies of similar treeline species elsewhere (e.g. Jacoby and Cook, 1981; Schweingruber, 1988a). Development of a tree-ring chronology emphasises the common signal between trees at a site with the underlying assumption that the primary control of this signal is 'climate (see Graybill, 1982). However, a tree-ring chronology or climate reconstruction from a single site contains both local and regional information, not all of which may be climate related (e.g. insect infestations). Identification of a regional climatic component of the signal is usually justified by

445

Glacier Fluctuation and Tree-Ring Records

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FIG. 2. Comparison of indexed ring-width chronologies for sites near Bennington and Athabasca Glaciers. These chronologies utilise Pinus albicaulis at Bermington and Picea engelmannff at Athabasca. Although most of the Bennington samples are from living trees many records are truncated in the 19th century because of measurement problems with extremely narrow and possible missing rings in old trees. The chronologies were developed using ARSTAN (Holmes, 1992) using only negative exponential or straight line indexing without secondary modelling to preserve the low-frequency record. The numbers below the graph are correlation coefficients for each 50 year period. The upper values (A) utilise the indexed ring-width chronology data shown. The lower values (B) refer to correlations between filtered 'COFECHA' chronologies. COFECHA chronologies (Holmes, 1992) are standardised using a cubic smoothing spline with a 50% cutoff at 32 years and the persistence in these smoothed series is removed by autoregressive modelling. These series provide a useful summary of the high frequency signal in these records. Note that the period 1490-1540 is shown on both diagrams.

comparison with other local or regional proxy climate series. correlation of the two chronologies shown or by utilising Where several chronologies are available for an area, 'Cofecha' chronologies to evaluate the high frequency signal temporal synchroneity is a powerful argument for a common (see Fig. 2). Over the period 1150 to 1950 the correlation forcing function. As climatic factors are likely to be the only coefficients between these two series are 0.46 (index controls of ring-width variation with considerable spatial chronology) and 0.63 (Cofecha). For several long periods extent, then similar temporal patterns of variation in these "e.g. early 1500s and 1630-1920, the records are almost chronologies indicate a common climatic forcing. In areas identical with correlations of 0.63 (index) and 0.71 where climate stations are sparse, identification of tree-ring (Cofecha) over the period between 1650 and 1900. The climate relationships using regional databases for both tree- largest discrepancy is in the early 1950s when tree-growth at ring and climate data offer a promising approac h to the Bennington site is strongly suppressed. Preliminary calibration (e.g. Schweingruber et al., 1991; Jacoby and records from other Picea chronologies in this area do not D'Arfigo, 1989). This work has not yet been completed for contain this signal, suggesting it is a local effect. There are the Canadian Rockies and therefore the discussion below also some differences between the chronologies in the period focuses on the interpretation of the ring-width chronologies ca. 1150-1630. Both chronologies show a significant ring-width decline presently available. between 1800 and 1820 that is replicated in other Athabasca and Bennington Chronologies chronologies in the area (Luckman and Colenutt, 1992a,b) The chronologies from the Athabasca and Bennington and is a well-marked feature of Picea chronologies along the sites involve two different species growing at sites 100 km northern treeline in North America (Jacoby et al., 1988; apart on either side of the Continental Divide. Jacoby and D'Arrigo, 1989). Jacoby et al. (1988) present a Superimposition of the two indexed chronologies (Fig. 2) variety of evidence indicating a major northern hemisphere empbasises the strong common signal between these sites cooling event at this time. However, this abrupt early 19th both in terms of individual 'marker' years and longer-term century decline is not a unique event in the ring-width growth patterns. These chronologies may be compared by records at the Athabasca and Bennington sites. A similar

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FIG. 3. Comparisonof long tree-ring chronologies and glacier fluctuations over the last millennium in the Canadian Rockies.The five indexed ring-width chronologiesare plotted at the same vertical scale and smoothedwith a 25-yearmean. Periodswhen tree-ring indices exceed the mean for each record are shaded. The LARIXchronologyis the mean of 6 Lar/xchronologies(for other details of chronologies see text). Dates of Little Ice Age morainesare based on Luckman(1986) plus more recent data for Peyto and Robson Glaciers.The light shaded colunmsrefer to data for the oldestmoraineat each site; the open histogramsinclude all dated moraines.

decline occurs in both chronologies in the period 1690-1705. Both these events immediately precede the two major moraine building episodes (1700-1725, 1825-1875; Fig. 3) at the maximum downvalley extent of glaciers during the Little Ice Age in the Canadian Rockies. Similar declines occur in ring-width chronologies from treeline sites that are not adjacent to glaciers (Luckman and Colenutt, 1992a; Fig. 3) indicating that this is not a local, glacier induced, signal. This temporal concurrence confirms the inference that these two periods of declining ring-width are associated with colder summer temperatures and positive glacier mass balances. Similar synchronous declines in ring-width occur elsewhere in the Athabasca/Bennington record, notably ca. 1100-1110, 1170-1180, 1280-1190, 1430-1450, 1530-1540 and, though slightly offset, ca. 1330-1360. As the first three of these are also associated with documented evidence of glacier advance (see Fig. 3) it would seem reasonable to infer that the events during the 14th to 16th centuries may similarly be associated with glacier events for which evidence is poorly preserved. C O M P A R I S O N OF T R E E - R I N G AND G L A C I E R RECORDS The regional nature of the signal in the ring-width records of the Athabasca/Bennington sites is confirmed by the broader summary compilation of ring-width chronologies and glacier fluctuation data shown in Fig. 3. The moraine

data shown indicate the temporal distribution of dated moraines built between 1500 and 1925 for 33 glacier forefields as compiled by Luckman (1986). Moraine ages are grouped into 25 year intervals and further differentiated into oldest and 'other' moraines at each site. The dated periods of glacier advance at Robson and Peyto Glaciers are also shown. The ring-width chronologies shown are preliminary ARSTAN chronologies with very conservative standardisation that have been smoothed with a 25 year running mean. Two sources have been used for the Peyto site: the post-1700 chronology utilises data collected by Schweingruber from a treeline site about 400 m above the glacier forefield and the earlier chronology is from Reynolds (1992). The Athabasca, Bennington and Robson chronologies are as described above but the LARIX chronology is a composite of the 6 Larix chronologies developed by Colenutt (1992) to provide greater replication of the early record. The three long chronologies based mainly on living trees (Athabasca, Bennington and LARIX) are from different treeline species growing at a variety of sites extending about 400 km along and on both sides of the Continental Divide. There is a strong synchroneity in the pattern of ring-width variation between sites but there are differences in the nature and amplitude of response between species. Luckman and Colenutt (1992a) cite mean sensitivities for tree-ring series ofPicea, Pinus and Larix at treeline in this area as 0.18, 0.19

GlacierFluctuationand Tree-RingRecords and 0.34 respectively. These differences are reflected in the chronologies shown in Fig. 3. The two Picea chronologies (Peyto, Athabasca) show a limited range of response whereas Larix is more highly variable (partially due to the occurrence of missing rings - - see Colenutt and Luckman, 1991). The Phms chronology has a slightly greater amplitude of variation than the Picea chronologies. The LARIX and Peyto records shown in Fig. 3 parallel the fluctuations in the Athabasca and Bennington chronologies discussed earlier. All chronologies show significantly narrower ring-widths ca. 1450-1500, ca. 1600, ca. 1680-1720 and for most of the 19th century with intervening periods of higher growth, particularly in the 16th and late 18th centuries. Prior to 1400 sample depth is limited. Nevertheless, the Peyto, Robson, Athabasca and Bennington chronologies all show a decrease in ring-width in early decades of the l l00s and the Bennington chronology (which has the best replication in the period 1200-1300) shows reduced ring-widths at the time of the glacier advances at Peyto and Robson in the late 1200s and early 1300s. The LARIX, Bennington and Athabasca chronologies all show generally better growth conditions during the late 1300s and early 1400s when many snags become established at and above present treeline at the Athabasca site. Most of these snags died in the late 1600s (Luckman, in press).

447

a coolingepisodein the 1960sand 1970s(D'Arrigoand Jacoby, 1992. pp. 303-304). Although there are minor differences in detail between the Canadian Rockies chronologies, this quotation could be used to describe the ring-width records shown in Fig. 3. It therefore seems reasonable to interpret these records as a temperature signal. This inference is supported by the relatively good agreement with known climate fluctuations (based on the glacier records) over the last 300 years. These ring-width series provide the first useful qualitative proxy data for the climatic history in the Canadian Rockies between ca. 1100 and 1700 A.D. Moreover, given the strong matching of the last 400 years with Jacoby's arctic chronologies over distances of several thousand kilometres, the chronologies from the Canadian Rockies may well be representative of climatic conditions over a wide area of Western Canada during the last millennium.

WHY WAS T H E L I T T L E ICE AGE T H E MOST EXTENSIVE H O L O C E N E GLACIER ADVANCE IN T H E SOUTHERN CANADIAN ROCKIES?

The data presented in Fig. 3 show a strong relationship between known glacier fluctuations and changes in tree-ring width for a variety of treeline species. The synchronous blterpretation of the Rh~g-Width Record The chronologies presented in Figs 2 and 3 are pattern of ring-width variation between various chronologies preliminary and unsophisticated but the strong common shows a common forcing function that must be climate pattern and signal will clearly be retained in future climate driven and the discussion above suggests that this is probably reconstructions developed from these data. Initial single temperature. However, in the examination of these records station calibrations suggest that the major control in ring- there is no clear indication of extended periods that could be width variation at these sites is summer temperatures defined - - from the tree-ring record - - as a 'Little Ice Age' (Luckman et al., 1985; Colenutt and Luckman, 1991; or a 'Little Climatic Optimum'; i.e. to rephrase Bradley and Colenutt, 1992). Strong additional support for this Jones "there is no evidence for a . . . 300--400 year l o n g . . . interpretation comes from comparison with results from cold interval to which we can ascribe the term 'Little Ice Picea at arctic treeline sites investigated by Jacoby and co- Age' (1992a, p. 3). This observation prompts some workers. Initial results from the Rockies (Luckman, 1990, speculation about why the Little Ice Age was the most Fig. 7) indicated very strong correlations between ring-width extensive Holocene glacier advance in the Canadian records from Picea engehnannii at treeline near Lake Louise Rockies. In recent years there have been many discussions of the and the TFHH Picea glauca chronology (Jacoby and Cook, 1981) near Dawson City, Yukon, almost 1800 km north of possible forcing mechanisms of decade-to-century scale Lake Louise. The TTHH chronology contains a strong climatic variations focusing on volcanic eruptions, regional temperature signal and was used to infer variations in the receipt of solar radiation and the oceanic temperature variation over the last 400 years in the Yukon thermohaline circulation (e.g. Grove, 1987; Wigley, 1991; and adjacent areas (Jacoby and Cook, 1981). More recently, Overpeck, 1991; Overpeck and Rind, 1992; Bradley and Jacoby and D'Arrigo (1989) (D'Arrigo and Jacoby, 1992) Jones, 1992a). These possible causes interact at varying have published temperature reconstructions for northern spatial and temporal scales to provide the complexity that we North America using ring-width data from a network of sites recognise in the climate history of the last millennium. along the latitudinal treeline. The more recent reconstruction However, in addition to these decade-to-century scale (D'Arrigo and Jacoby, 1992) uses 6 Picea glauca fluctuations, changes are occurring at longer timescales that chronologies from the northern treeline between Alaska and may be critically important in understanding the climatic Hudson Bay, plus a Thuja occidentalis chronology from history of this time-period. Examination of the solar Gasp6, that extend back to 1600 A.D. The major features of radiation curve for the northern hemisphere summer during the Holocene indicates an 8% decrease in summer solar their reconstruction are radiation over this period (Kutzbach, 1987). Reconstructed below averagetemperaturesin the early 1600s. a cooling in the early treeline fluctuations in the Watchtower Basin, Jasper 1700s,a relativewarmingin the later 1700s,an abrupttransitionto cold temperaturesin the earlyto mid 1800s... and the generalwarmingtrend National Park (Luckman and Kearney, 1986; see Fig, 4), overthe past century.., superimposedon this recentwanning trend is indicate a strong parallel trend between treeline elevation

448

B.H. Luckman

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and the solar radiation curve for the period ca. 8500- present (similar relationships have recently been described from Sweden, see Kullman, 1992). Over the Holocene, treeline elevation at the Watchtower site has been generally decreasing but shorter period fluctuations of higher and lower treeline elevation (mainly century-to-millennial scale) are superimposed on this trend. During the Hypsithermal these oscillations were from above present levels to present levels whereas during the Neoglacial the range has been from at, or slightly above, present levels to below present. Glacier fronts continue to recede in the present century exposing evidence of earlier glacier fluctuations in the form of buried stumps and paleosols (Luckman et al., 1993; Osbom and Luckman, 1988). At all of the glacier forefields examined so far in the Canadian Rockies there is no site where older in sittt stumps or paleosols occur downvalley of younger material in the same forefield. One interpretation of this relationship is that, in this area, glacier advances have been progressively more extensive during the Neoglacial,

culminating in the Little Ice Age. As incoming summer solar radiation is a major control of glacier ablation (and therefore a significant variable in the deterrriination of glacier mass balance), this pattern of events is entirely consistent with the long-term changes in summer solar radiation over the Holocene: a long-term decrease in summer radiation inputs would, all other things being equal, lead to a decrease in summer ablation and hence an increase in mass balance. Therefore, in seeking a cause for the more extensive glacier advances in the Late Holocene it is not necessary to invoke a significant shift of climate between a period of the 'Little Climatic Optimum' and the 'Little Ice Age'. The preliminary chronologies shown in Fig. 3 suggest that there has been considerable decade-to-century scale climatic fluctuation throughout the last millennium. The periods of glacier advance identified in the figure are defined by decade-tocentury fluctuations which are superimposed on a long-term trend of decreasing incoming solar radiation during the summer (see Nesje and Johannesson, 1992). As more

GlacierHuctuationand Tree-RingRecords

extensive glacier advances remove most evidence of earlier

449

preliminary tree-ring series also suggests that there may have

events, only the latest H o l o c e n e glacial episodes are been periods of glacier advance in this area between 1300 preserved in any details.* a n d 1700 A.D. that are not presently k n o w n from the At a recent conference on the Climate of the Little Ice Age 9 morphological and stratigraphic records. (Mikami, 1992) there was considerable discussion about the definition of this period (Bradley and Jones, 1992b). Opinion ACKNOWLEDGEMENTS was almost equally divided between dates of the late 13th The assembly of tree-ring databases is labour intensive and I would century and the early 16th century for the starting point of the Little Ice Age. A simple (and perhaps naive) interpretation particularlylike to thank the followingfor measurementof tree-ring data; I.M. Besch, D. Boyes, M.E. Colenun,S. Daniels,G.W. Frazer; Geography of this division is that the local definition of the Little Ice Age 312, 1990-2; J.P. Hamilton, R. Heipel, L.A. Jozsa (Forintek Canada reflects the chronology of glacier events that are preserved in Corporation); P.E. Kelly, D.P. McCarthy, W. Quinton,J.R. Reynolds,B. a given area. Evidence for the older less extensive glacier Schaus, D. Smith, C. Somr, A. Tarrnsov,V. Wyatt and R. Young. Several of the preceding, plus F.F. Dalley, C.J. Rowley and D.C. Luckman, also events is best k n o w n from regions with good historical assisted in the field. M.E. Colenutt, L.A. Jozsa, J.R. Reynolds and F.H. records (e.g. European Alps) or sites of intensive research Schweingruberprovidedtree-ringchronologies.Financialsupporthas been where glaciers advanced into formerly forested areas (e.g. givenby the NaturalSciencesand EngineeringResearchCouncilof Canada and the Universityof Western Ontario. I thank Parks Canadaand the B.C. Alps, New Zealand and North American Cordillera - - see ProvincialParks for support and the granting of research permits; Gordon Grove, 1987). Investigators in these regions include 13th and Shields and Tricia Chalk, Geography, U.W.O., for the cartography; P.T. 14th century advances in the 'Little Ice A g e ' whereas Davis and an unidentifiedreferee for theircommentson the manuscript;and J.T. Overpeck for the invitationto participatein this symposium. workers in other areas only identify the later 17-19th century advances and place the i n c e p t i o n of the Little Ice Age REFERENCES correspondingly later. In the Canadian Rockies the Cavell Advance was originally defined to include these 13th and Bradley, R.S. and Jones, P.D. (1992a). When was the "Little Ice Age". In: 14th century events (Heusser, 1956; Luckman and Osborn, Mikami, T. (ed.), Proceedings of the Conference on the Little Ice Age Climate, pp. 1-4. Tokyo MetropolitanUniversity. 1979; see also Leonard, 1986a). Bradley, R.S. and Jones, P.D. (eds) (1992b). Climate since A.D. 1500. Routledge, New York, 679 pp. Carrara, P.E. (1987). Holoceneand latest Pleistoceneglacial chronology, CONCLUDING REMARKS Glacier NationalPark, Montana. Canadian Journal of Earth Sciences, 24, 387-395. In examining decade-to-century scale climate Colenutt, M.E. (1992). An investigation into the dendrochronological fluctuations and episodes such as the 'Little Ice Age' it is potential of alpine larch. Unpublished M.Sc. thesis, University of Westem Ontario,226 pp. important not to ignore longer scale climate fluctuations that Colenun,M.E. and Luckman,B.tt. (1991). Dendrochronologicalstudiesof may significantly influence the response of natural systems Larlx lyallii at Larch Valley, Alberta. Canadian Journal of Forest to these changes. Climate fluctuations result from an Research, 21, 1222-1233. integrated response of the climate system to forcing factors D'Arrigo, R. and Jacoby, G.C. (1992). Dendroclimatic evidence from northern North America. In: Bradley, R.S. and Jones, P.D. (eds), that operate at all spatial and temporal scales, not simply Climate since A.D. 1500, pp. 296-311. Routledge,New York. those that can be observed in decade-to-century scale Fritts, H.C. (1971). Dendroclimatologyand dendroecology. Quaternary Research, 1,419--449. records. The pattern of climatic variation during the last Fritts, H.C. (1976). Tree Rings and Climate. Academic Press, Londonand millennium can be modelled in terms of decade-to-century New York, 568 pp. forcing, but the extended glacier advances of the Little Ice Fritts, H.C. (1991). Reconstructing Large-Scale Climate Patterns from Tree-Ring Data. Universityof ArizonaPress, Tucson,286 pp. Age may also reflect the influence of changes over longer Fritts, H., Lofgren, G.R. and Gordon, G.A. (1979). Variationsin climate timescales. The evidence presented above emphasises the since 1602 as reconstructedfrom tree rings. Quaternary Research, 12, need to use several proxy data series in reconstructing 18-46. regional climatic history to evaluate the possible controls of Gardner,J.S. (1972). Recentglacieractivityand some associatedlandforms in the Canadian Rockies. In: Slaymaker, H.O. and McPherson, H.J. climate variability at differing temporal scales. For the (eds), Mountain Geomorphology, pp. 65-72. Tantalus Press,

Canadian Rockies, the preliminary tree-ring records presented above suggest that the relatively well-documented pattern of climate fluctuations of the past few centuries probably extends back over the last millennium. Although glacier fluctuation data provide a d i s c o n t i n u o u s proxyclimate record over this period they complement and support p a l e o e n v i r o n m e n t a l inferences derived from dendrochronological studies. However, evaluation of these

*This is also true of Little Ice Age events. Althoughit is probablethat Little Ice Age morainespredating 1700are knownfrom severalsites in the Canadian Rockies (see Luckmanand Osbom, 1979), in most cases these moraines are restricted to locally preserved fragments along (usually lateral) morainecrests that were last reoccupiedor built in the 19thcentury. Although older morainesare preserved at these sites, the most extensive Little Ice Age advance was during the 18th or 19th century (see also Luckman, 1977, 1988).

Vancouver.

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