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The Holocene

DOI: 10.1177/0959683609353433 2010; 20; 447 originally published online Feb 22, 2010; The Holocene

John J. Clague, Johannes Koch and Marten Geertsema millennia

Expansion of outlet glaciers of the Juneau Icefield in northwest British Columbia during the past two

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Research paper

IntroductionThe term ‘Little Ice Age’ was first used 70 years ago by Matthes (1939) to identify a recent period when glaciers in the Sierra Nevada in California were more extensive than today. Sixty-five years later, Grove (2004) summarized the large body of literature documenting the greater extent of glaciers during the ‘Little Ice Age’, which is known to span much of the past millennium. Yet, although the term ‘Little Ice Age’ is widely used, its value has recently been questioned on a variety of issues, among them the time of onset and termination of the period (Matthews and Briffa, 2005; Clague et al., 2009).

Grove (2001a, b) concluded that the ‘Little Ice Age’ in the North Atlantic region began before the fourteenth century; else-where glaciers were advancing between the twelfth and fourteenth centuries. However, research performed over the past two decades points to a more complex history, with significant changes in gla-ciers and climate on decadal and centennial timescales throughout the past two millennia (Bradley and Jones, 1993; Holzhauser, 1997; Luckman, 2000; Crowley and Lowery, 2000; Nesje and Dahl, 2003; Koch et al., 2007; Clague et al., 2009). The record of Grosser Aletsch Glacier, for example, shows peaks at about ad 1350, 1650 and 1850, with lesser, but significant advances about ad 450 and ad 800 (Holzhauser, 1997). The Grosser Aletsch record reveals a step change towards more extensive glacier extent at the end of the twelfth century, marking the onset of the European ‘Little Ice Age’.

Confusion over the meaning of ‘Little Ice Age’ parallels that stemming from the use of the term ‘Medieval Warm Epoch’. The latter term was first used by Lamb (1965) for a period of warmth centred on ad 1100–1200. Other researchers, however, subsequently proposed different times for the Medieval Warm Period: ninth through fourteenth centuries (Hughes and Diaz, 1994), ad 900–1250 (Grove and Switsur, 1994), ad 1000–1300

(Crowley and Lowery, 2000), ad 800–1200 (Broecker, 2001), ad 1100–1200 (Bradley et al., 2003b), ad 960–1050 (Cook et al., 2004a), and ad 1000–1450 (Herweijer et al., 2007). The most compelling evidence for the Medieval Warm Period comes from western Europe, and especially Iceland, initially settled by Vikings in ad 874 and Greenland, first settled in ad 982 (Fitzhugh and Ward, 2000). Agriculture reached farther north and to higher elevations in England, Scandinavia, Germany and the European Alps at this time (Lamb, 1995). Researchers who have questioned the utility of the term ‘Medieval Warm Period’ suggest that it is more a Northern European event than a global one, that precipitation was just as important as temperature in distinguishing it from the ‘Little Ice Age’, and that it was inter-rupted by short-lived intervals of cool or wet conditions (Leavitt, 1994; Luckman, 1994; Stine, 1994; Woodhouse and Overpeck, 1998; Hallett et al., 2003; Cook et al., 2004b, 2007; Luckman and Wilson, 2005; Loso et al., 2006; Yalcin et al., 2006; Graham and Hughes, 2007; Graham et al., 2007; Meko et al., 2007; Koch and Clague, 2010).

The usefulness of the term ‘Little Ice Age’ is further called into question by the complex behaviour of glaciers both during the

Expansion of outlet glaciers of the Juneau Icefield in northwest British Columbia during the past two millennia

John J. Clague,1 Johannes Koch2 and Marten Geertsema3

AbstractRadiocarbon and dendrochronological dating of glacially overridden stumps and detrital wood indicates that two outlet glaciers of the Juneau Icefield advanced shortly before the ‘Little Ice Age’. Tulsequah Glacier advanced to within 2.4 km of its all-time Holocene limit between ad 865 and ad 940. Llewellyn Glacier, one of the largest glaciers in British Columbia, advanced sometime between ad 300 and ad 500, and reached to within 400 m of its Holocene limit between ad 1035 and ad 1210, well before the climactic, ‘classical’ ‘Little Ice Age’ advances of the past several centuries. Our data show that some glaciers in western North America were extensive and expanding at times when alpine glaciers have, in the past, been assumed to be restricted. The evidence raises questions about how to define the time of the beginning of the ‘Little Ice Age’ and, perhaps more importantly, about the utility of the term.

KeywordsBritish Columbia, climate change, Coast Mountains, ‘Little Ice Age’, Llewellyn Glacier, Tulsequah Glacier

The Holocene20(3) 447–461© The Author(s) 2010Reprints and permission: sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0959683609353433http://hol.sagepub.com

1Simon Fraser University, Canada2The College of Wooster, Canada3British Columbia Forest Service, Canada

Received 5 June 2009; revised manuscript accepted 28 September 2009

Corresponding author:John J. Clague, Centre for Natural Hazards Research and Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, CanadaEmail: [email protected]



448 The Holocene 20(3)

‘Little Ice Age’ (Matthews and Briffa, 2005) and immediately before it. Glaciers advanced and retreated on timescales of decades to centuries during the past eight centuries (e.g. Bradley and Jones, 1995; Bradley et al., 2003a; Jones and Mann, 2004; Mayewski et al., 2004; Menounos et al., 2009; see also references in these papers). Furthermore, defining the beginning of the ‘Little Ice Age’ at about ad 1200 is itself problematic. Glaciers through-out western North America advanced between 1700 and 1400 years ago (ad 300–600; Reyes et al., 2006). This advance is close in time to the purported first advance of the ‘Little Ice Age’. Because many glaciers in western North America were as exten-sive 1400 years ago as they were 600 years later, why define the beginning of the ‘Little Ice Age’ at ad 1200?

This discussion and the questions that arise from it provide context for our research on glacier activity immediately before, and during, the earliest part of the past millennium. We have found evidence in western Canada for major advances of glaciers at this time, before the start of the ‘Little Ice Age’ as it is currently defined. The purpose of this paper is to present evidence that Llewellyn Glacier, an outlet glacier of the Juneau Icefield and one of the largest glaciers in British Columbia, expanded to near its all-time Holocene limit during the first two centuries of the last millennium. Further, nearby Tulsequah Glacier advanced into for-est 3 km beyond its present terminus more than a century earlier, near the end of the first millennium ad.

Study sitesTulsequah and Llewellyn glaciers are outlet glaciers of the Juneau Icefield – the fifth largest icefield in North America, with an area of 3900 km2, a maximum length of 140 km from north to south, and a maximum width of 75 km from east to west (Figure 1). The Juneau Icefield supports over 40 large valley glaciers and 100 smaller ones. Of the 20 largest glaciers, only one, Taku Glacier, was advancing at the beginning of this cen-tury (Motyka and Echelmeyer, 2003), and it too seems to have begun to recede.

Tulsequah Glacier flows 30 km from peaks up to 2100 m a.s.l. (above sea level) near the Alaska–British Columbia border to its terminus at about 170 m a.s.l. in lower Tulsequah River valley (Figures 1 and 2). The snout of the glacier is 1.5 km wide and terminates in a growing proglacial lake at the head of the Tulsequah River braidplain. A valley train extends 22 km from the proglacial lake south to Taku River. Tulsequah Glacier has retreated and thinned considerably during the past century (Kerr, 1948), and two ice-dammed lakes that release frequent jökul-hlaups have formed at its margin (Geertsema and Clague, 2005). In the nineteenth century, its terminus was about 5 km farther downvalley than today.

Our Tulsequah study site is a till-covered, 90 m high rock ridge within the braidplain and at the northeast side of the valley and 2.6–3.8 km downvalley from the 2001 glacier snout. The ridge is 1.6–2.8 km inside the Holocene glacial limit, which is defined by a prominent trimline and lateral moraines. We recovered wood samples for dating from large stumps rooted in a soil on rock. The stumps were partly covered in till. The ridge supports a forest of subalpine fir (Abies lasiocarpa (Hooker) Nuttall).

Llewellyn Glacier flows about 30 km east-northeast from peaks up to 2300 m a.s.l. to a terminus at 730 m a.s.l. near the

south end of Atlin Lake (Figures 1 and 3). The glacier has three terminal lobes – two smaller ones on the north and south, and a third, larger one on the east (Figures 3, 4 and 5). The east, or main, lobe calves into water along much of its nearly 10 km long termi-nal perimeter. The south lobe, which has a 7 km perimeter, termi-nates on a rock slope that drops down to the east. The north lobe, which we did not study, has a length of about 3.8 km and a narrow (c. 200 m) snout.

Conspicuous trimlines and lateral and end moraines 0.5–3 km outside the glacier terminus delineate the maximum Holocene extent of Llewellyn Glacier (Figure 3). Three valley trains extend north, northeast and east from the glacier margin at its maximum extent. At the Holocene maximum, meltwater and sediment from the north and main lobes discharged into Atlin Lake; meltwater and sediment from the south lobe and from Sloko Glacier, another outlet glacier of the Juneau Icefield, entered Sloko Lake (Figure 3).

Our Llewellyn study site is the glacier forefield east of the main and south lobes of Llewellyn Glacier, an area of about 20 km2 (Figures 3 and 4). About one-quarter of this area is covered by proglacial lakes, which have expanded as the glacier retreated during the twentieth century. The other three-quarters of the fore-field include (1) a flat valley floor underlain by thick drift, (2) a till-covered slope to the east, and (3) bare bedrock hills to the west, adjacent to the present glacier terminus. We documented stream-cut sediment exposures on the valley floor and gully sec-tions on the till-covered slopes to the east. We also collected samples of detrital wood and outer rings of stumps in growth posi-tion for radiocarbon dating.

Treeline in the vicinity of Llewellyn Glacier lies at 1000–1400 m a.s.l. Vegetation on the till-covered slope east of Llewellyn Glacier is a parkland mosaic of subalpine fir, white spruce (Picea glauca (Moench) Voss), alder, shrubs and herbs. All our dendro-chronological and radiocarbon samples are subalpine fir.

MethodsFeatures in the Llewellyn Glacier forefield were mapped on 1:31 680-scale black-and-white aerial photographs taken in 1972 (BC5616: 149–153, 178–282; BC5617: 5–10, 129–135) and trans-ferred to 1:20000-scale BC TRIM (Digital Terrain Resource Information Management) topographic maps (104N001, 104N011 Survey topographic maps).

We conducted field studies at Tulsequah Glacier in October 2001 and at Llewellyn Glacier in October 2001 and July 2004. We documented the stratigraphy of sediments at natural exposures and collected samples for dendrochronological analysis and radiocar-bon dating. Twenty-one samples of wood were radiocarbon dated by conventional (radiometric) and accelerator mass spectrometry (AMS) methods at the Arizona Accelerator Mass Spectrometry Laboratory, Beta Analytic Ltd, the Geological Survey of Canada Radiocarbon Laboratory and the Keck Carbon Cycle AMS Facility. Radiocarbon ages were calibrated using the IntCal04 curve of Reimer et al. (2004) in OxCal 4.1 (Bronk Ramsay, 2009). Calibrated ages, rounded off to the nearest decade or mid-decade year and shown in brackets in the paper, are 2s ranges. Wiggle matching was done by performing a Chi-squared fit to the 14C data with the OxCal 3.1 program of Bronk Ramsay (2005), based on the method described in Bronk Ramsay et al. (2001).



Clague et al. 449

We systematically searched the Llewellyn Glacier forefield for detrital and in situ fossil wood for dendrochronological analysis.A few trees on the north lateral moraine of the south lobe of Llewellyn Glacier were cored to provide a minimum age for moraine stabilization. We made corrections for the time it takes trees to grow to coring height (age-height corrections; McCarthy et al., 1991; Winchester and Harrison, 2000; Koch, 2009) based on the width of pith rings and seedling ages in the forefield. We also corrected for ecesis, the time from surface stabilization to seedling germination (Sigafoos and Hendricks, 1969; McCarthy and

Luckman, 1993; Koch, 2009). This time was estimated by sampling living trees at two locations that were ice covered in 1974 and 1987, respectively.

Tree cores and disks from fossil wood were air-dried, mounted and sanded with progressively finer grades of sandpaper to enhance the definition and contrast of annual tree-ring boundaries. Rings were measured on a Velmex stage with a precision of ± 0.001 mm using the software MeasureJ2X. Two to four radii were measured to replicate data and reduce the possibility of miss-ing rings or ‘false’ rings (Stokes and Smiley, 1996). ‘Floating’

Figure 1. Location of Tulsequah and Llewellyn glaciers and Juneau Icefield




chronologies were developed for sites where more than one tree was sampled. The floating chronologies were visually compared to cor-relate marker rings and were checked and verified using the International Tree-Ring Data Bank (ITRDB) software program COFECHA (Holmes, 1983). The verified series were crossdated (50-year dated segments lagged by 25 years, with a critical level of correlation (99%) set at 0.32) to create master ring-width chronolo-gies (Holmes, 1983). Series too old to cross-date with the living ring series derived from living trees growing on the north lateral moraine of the south lobe of Llewellyn Glacier were radiocarbon dated.

Results

Tulsequah Glacier

In situ stumps and stems of subfossil trees are present on the west flank of the bedrock ridge about 3 km downvalley of the present terminus of Tulsequah Glacier. The stumps are rooted on bedrock and weathered till, and are partly covered by non-weathered till. The stumps and stems are up to 30 cm in diameter and 1.2 m in circumference.

We obtained radiocarbon ages on a disk of a tree stem contain-ing 218 annual rings. A sample comprising the pith and a few adjacent rings, and another consisting of the outermost 15 rings, yielded radiocarbon ages of, respectively, 1272 ± 52 14C yr BP

(ad 660–870) and 1228 ± 39 14C yr BP (ad 685–885) (Figure 6; Table 1). Wiggle-matching of the two ages indicate that the tree died between ad 865 and ad 940 (most likely death ages are ad 885 and ad 935).

Llewellyn Glacier

Llewellyn Glacier presently terminates 1 km from the rising till-covered rock slope to the east. When the glacier advanced beyond its present limit, it reached this rising slope, overrode forest, and impounded a lake to the south. The lake was impounded on the north by the main lobe of Llewellyn Glacier and on its south by ice of the south lobe. It overflowed along the edge of the south lobe and the snout of Sloko Glacier into the valley that drains northeast to Sloko Lake.

We found evidence for three lakes impounded by Llewellyn Glacier when it was more extensive than today. We also have independent evidence for at least three incursions of the main and south lobes of Llewellyn Glacier into forests well beyond the present glacier margin during the past 2000 years. We briefly present the evidence for these events and their ages below.

Sample height and ecesis correctionsAverage annual height growth rates of seedlings in the glacier forefield range from 1.73 to 2.44 cm/yr. For samples of subfossil

Figure 2. Tulsequah Glacier and its forefield, showing location of the bedrock ridge on which glacially overridden, in situ stumps were found. Infrared Landsat 5 image (29 July 2006)



Clague et al. 451

wood with wide pith rings, we corrected dendrochronological ages by adding the higher value to ring counts; we used the lower value to correct samples with narrow pith rings. Two saplings at a site that was covered by ice in 1987 yielded 5 and 4 rings, thus ecesis there is no more than 12 years. At two sites that were covered by ice in 1974, seedlings yielded 12, 10 and 9 rings, giving a maxi-mum ecesis value of 18 years.

The oldest cored living trees on the outermost north lateral moraine of the south lobe of Llewellyn Glacier contain 174 rings. Assuming 15 years for ecesis and 5 years for growth to coring height, this moraine stabilized no later than the second decade of the nineteenth century.

Oldest documented lake phaseSeveral bluffs along the north-flowing stream on the valley floor about 1 km south of the 2004 glacier terminus (sites 1, 2, and 3 in Figures 4 and 7) provide exposures of lacustrine sediments overlain by till. At site 1, about 9 m of sand and minor silt, with abundant lenses of gravel and sandy diamicton containing stones up to 1.5 m across, form the lower part of the bluff (Figures 7 and 8). The sand is planar-bedded, laminated and rippled. Some strata dip about 10° to the south. Bark, twigs, branches, and other plant detritus are common within the sediments. A branch in gravely sand 8 m above the base of the bluff returned a radiocarbon age

of 670 ± 60 14C yr BP (ad 1250–1410; Figure 6; Table 1), and a twig 0.9 m lower, in the same unit, gave an age of 935 ± 38 14C yr BP (ad 1020–1185).

The sand unit is truncated and overlain by 2.7 m of massive to weakly stratified, sandy diamicton containing faceted and striated stones. Clastic dykes extend downward into the stratified sandy sediments from the base of the diamicton. We interpret the clastic dykes to be glaciotectonic fractures that formed and were infilled with diamicton after the site was overridden by the east lobe of Llewellyn Glacier.

The stratigraphy of the river bluff at site 2, 100 m south of site 1, complements and supplements the stratigraphy described above (Figure 7). The glaciolacustrine sediments at site 2 are 3.6 m thick and, as at site 1, are unconformably overlain by massive to weakly stratified diamicton, which we interpret to be till. However, the glaciolacustrine sediments at site 2, unlike those at site 1, are severely deformed – stratification is folded and broken, and load structures and clastic dykes are common. The deformation probably was caused by glacier overriding. Whereas the glaciolacustrine sediments extend down to the base of the bluff at site 1, those at site 2 are underlain by up to 3 m of horizontally bedded sand and gravel comprising mainly rounded pebbles and granules. The contact of the sand-gravel unit with the overlying deformed glaciolacustrine sediments is

Figure 3. The lower part of Llewellyn Glacier and its forefield. The image was produced by draping an infrared Landsat 5 image (29 July 2006) on a digital elevation model created from TRIM data




gradational over a vertical distance of several centimetres. The horizontally stratified sediments at the base of the exposure could be part of the overlying glaciolacustrine sequence, but they resemble fluvial sediments and we interpret them as such. If so, they record unobstructed, northward river flow immedi-ately before Llewellyn Glacier advanced across the valley and impounded the drainage.

A third bluff (site 3) about 50 m northwest of site 1, exposes 6 m of planar-bedded lacustrine sand dipping 24° east (Figure 7). Low-angle thrust faults indicate displacement towards the east due to glacier overriding. Injection and loading structures provide additional evidence for glaciotectonic deformation. The sand is unconformably overlain by weakly stratified, matrix-supported diamicton, which we interpret to be till. Radiocarbon ages of 994 ± 38 14C yr BP (ad 980–1160) and 1210 ± 40 14C yr BP (ad 685–935) were obtained on twigs near the top of the lacustrine sand unit (Figure 6; Table 1).

The stratigraphy of the valley-floor sediments at sites 1, 2 and 3 is similar, but the ages of the lacustrine sediments appear to be different. The four radiocarbon ages at sites 1 and 3 range from 670 ± 60 14C yr BP to 1210 ± 40 14C yr BP (Figure 6; Table 1). Some of the dated plant fossils, however, may have been recycled from older sediments or soils; if so, they could be significantly older than the lacustrine sediments from which they were col-lected. All of the ages, therefore, must be considered maxima for the age(s) of the lake(s) in which the sediments were deposited.

Figure 4. Map of the terminus and forefield of Llewellyn Glacier, showing its Holocene glacial limit and locations of stratigraphic sections, radiocarbon-dated in situ tree stumps, and transect shown in Figure 10. Based on BC TRIM map 104N001 (contour interval = 20 m)

Intermediate-age lake phaseLacustrine and deltaic sediments are overlain by till at the south end of the Llewellyn Glacier forefield (site 4 in Figure 4). These sediments are exposed in gullies below the trimline of the glacier’s south lobe. They coarsen upward from laminated sand to inclined, parallel-bedded gravel. Gravel beds dip up to 24° eastward. The gravel is unconformably overlain by up to 2 m of matrix-sup-ported diamicton containing striated and faceted stones.

This sequence is similar to, but thinner than, that at site 5, which is described below. It records an advance of Llewellyn Glacier toward the site, followed by glacier overriding. The gravely foreset beds at the top of the sequence probably were deposited shortly before the site was overridden.

The sediments at site 4, however, may be older than those at site 5. We obtained three radiocarbon ages on twigs collected from a single layer of sand about 70 cm above the base of the section. The ages are tightly clustered, ranging from 434 ± 37 14C yr BP to 454 ± 37 14C yr BP (Figure 6; Table 1). The weighted mean of the three samples is 443 ± 23 14C yr BP (ad 1420–1470). Each of the three ages and the weighted mean are significantly older than the single radiocarbon age from site 5 (295 ± 38 14C yr BP). All of these ages, however, are derived from detrital wood and thus must be considered maxima for the age of their enclosing sediments. Although we cannot rule out the possibility that the two sequences are correlative, the four samples (one from site 5 and three from site 4) are delicate twigs that were carefully chosen to minimize the possibility of reworking.

‘Classical’ ‘Little Ice Age’ lakeA stream flowing east from the south lobe of Llewellyn Glacier has dissected a delta nested against the rock slope about 700 m east of the glacier terminus (site 5 in Figure 4). A c. 100 m long exposure, up to 20 m high, provides a section through the delta (Figure 9). Most of the exposed sediments are inclined, parallel-bedded gravel and sand, with lenses of sandy matrix-supported diamicton. We interpret these sediments to be delta foreset beds. These beds gradually steepen upward and towards the apex of the delta, where they dip up to 22° south and extend up to about 790 m a.s.l., 55 m above the valley floor to the east. The uppermost sediments are gravel and gravely sand. In contrast, the lowest and most distal sediments with respect to the delta apex are nearly flat-lying sand deposited in a lower delta slope environment. A branch recovered from sand 5 m above the base of the exposure at the downstream end yielded a radiocarbon age of 295 ± 38 14C yr BP (ad 1480–1665; Figure 6; Table 1).

The inclined gravel-sand unit is truncated near the top of the exposure by 4–5 m of diamicton. The lowest 1–2 m of the diamicton is matrix-supported, weakly stratified and contains some sandy horizons. Many of the stones are striated and fac-eted. The basal diamicton is gradationally overlain by up to 3 m of clast-supported diamicton that also contains striated and faceted stones; it differs from the lower diamicton in being more stoney and nearly massive. The unconformity between the basal diamicton and the underlying delta foresets shows that the latter were eroded before the former was deposited. The presence of striated and faceted clasts within the two diamictons indicate a glacial origin. The most likely scenario is that the south lobe of Llewellyn Glacier overrode the delta sometime after ad 1480.



Clague et al. 453

Glacier advances inferred from in situ and detrital woodThe outer rings of the stem of a subfossil tree rooted on bedrock about 10 m above the valley floor east of the main lobe of Llewellyn Glacier (site 6, Figure 4) yielded a radiocarbon age of 1730 ± 40 14C yr BP (ad 225–410). A floating chronology (r = 0.679) derived from this tree and a second nearby tree, also rooted on bedrock, spans 332 years (Figure 10, Table 2). These data indi-cate that the main lobe of Llewellyn Glacier advanced beyond this site between ad 225 and 410 and that the terminus was west (inboard) of the site for at least the previous 350 years.

We recovered numerous pieces of wood, ranging from small torn fragments to 2 m long boles, some with attached roots, from along two streams draining the south lobe of Llewellyn Glacier (sites 7 and 8, Figure 4). The wood was traced to a woody horizon separating two tills. One of the wood fragments yielded a radio-carbon age of 1560 ± 40 14C yr BP (ad 415–585). A floating chro-nology of six samples from this area spans 324 years and, together with the radiocarbon age, suggests that the south lobe advanced beyond this area between ad 415 and 585 and had a more restricted extent for at least the previous 330 years (Figure 10).

Two small, in situ stumps were sampled on the stoss side of a small hill (site 9, Figure 4) about 1600 m from the 2004 terminus of the south lobe. A floating chronology based on the two trees spans 183 years (Figure 10). One of the stumps yielded a radiocar-bon age of 950 ± 15 14C yr BP (ad 1025–1155). Both stumps were sheared off by Llewellyn Glacier when it advanced into forest with trees at least 190 years old.

A floating chronology constructed from two logs collected from a wood layer plastered on a steep bedrock slope east of the main lobe of Llewellyn Glacier (site 10, Figure 4) spans 214 years

(Figure 10). It was successfully cross-dated with the floating chro-nology built from the two samples at site 9, mentioned above (r = 0.637). The correlation suggests that the east and south lobes advanced into forest about ad 1100. Additional evidence for this advance is provided by three radiocarbon ages on overridden trees on the slope east of Llewellyn Glacier. The ages range from 930 ± 40 to 980 ± 40 14C yr BP (ad 995–1175); the oldest age was obtained on an in situ stump about 400 m from the end moraine marking the maximum Holocene advance.

The rising rock slope east of the main lobe of Llewellyn Glacier is blanketed by till. Stumps of subfossil trees are rooted on exposed rock, on a thin reddish-brown regolith consisting of angular rock fragments, or on a discontinuous layer of colluvium that covers the regolith and rock and underlies till (Figure 11). The colluvium is a silty, matrix-supported diamicton with angular to rounded clasts up to boulder size. The yellowish-brown to reddish-brown colour of the diamicton contrasts markedly with the light olive-gray colour of the overlying till. Its distinctive colour reflects that of the local bedrock. The colluvium, however, contains some exotic, granitic pebbles and cobbles, presumably derived from an older till.

A paleosol is developed on the colluvium. The upper layer of the paleosol (Ah horizon) is a dark brown organic silt with abun-dant wood fragments, charred wood, charcoal and roots. This Ah horizon overlies oxidized colluvium, which has brighter yellow and orange hues than the underlying less weathered colluvium.

We obtained six radiocarbon ages from wood samples collected at or near the top of the colluvium, directly below till (sites 11, 12, and 13; Figure 4). Three of these ages – 780 ± 70 14C yr BP (ad 1045–1385), 820 ± 70 14C yr BP (ad 1040–1285) and 980 ± 40 14C yr BP (ad 990–1155) (Figure 6; Table 1) – were obtained on outer

Figure 5. Panoramas of the forefield of Llewellyn Glacier, where we conducted our study. (a) Main lobe of glacier; (b) south lobe. Panoramas constructed from photographs taken in July 2004




rings of stumps rooted in the paleosol at the top of the colluvium and directly below till. The weighted mean of these three ages is 910 ± 31 14C yr BP (ad 1030–1210). A fourth radiocarbon age, 1000 ± 50 14C yr BP (ad 900–1160), was obtained on a charred root or branch at the contact between the regolith and overlying till. The fifth age, 970 ± 60 14C yr BP (ad 970–1210), came from a twig at the top of the colluvium. One additional age of 930 ± 40 14C yr BP (ad 1020–1205) was obtained from a branch within till. The six ages have a restricted range, from about 780 to 1000 14C yr BP. With one exception, the ages on the in situ stumps (c. 780–980 14C yr BP) are slightly younger than those on detrital material (c. 930–1000 14C yr BP), but this difference is barely significant at the 2s level.

Five of the in situ stumps on the rising bedrock slope east of Llewellyn Glacier cross-dated, providing a floating chronology spanning 257 years (Figure 10). One of the five cross-dated stumps yielded the above-mentioned age of 780 ± 70 14C yr BP.

The tree-ring series built from the five stumps allows a detailed reconstruction of the advance of the main lobe of Llewellyn Glacier in the eleventh through early thirteenth centuries. Three of the five samples retain their outermost ring. The tree nearest the glacier died 37 years before one of the trees farther away, which

Figure 6. Probability distributions of the 21 radiocarbon ages reported in this paper (see also Table 1). Site numbers appear to the right of the distributions

in turn died 49 years before the tree farthest from the glacier. The sites are separated by a distance of about 450 m in the direction the glacier advanced. The glacier advanced 180 m from the proxi-mal site to the intermediate one in 37 years (average rate of advance = 5 m/yr) and 270 m from the intermediate site to the most distant one in 49 years (average rate of advance = 5.5 m/yr). A tree-ring series based on these stumps and samples from the wood layer at site 10 cross-dates with the floating chronology from the small hill at site 9 (r = 0.597), indicating that both the east and south lobes were advancing for most of the eleventh, twelfth and early thirteenth centuries.

We radiocarbon dated two samples of wood recovered from a stream channel in front of the south lobe of Llewellyn Glacier (site 14, Figure 4). Outer rings of an in situ stump yielded a radiocarbon age of 580 ± 40 14C years BP (ad 1295–1420). The stump contains 154 rings and may cross-date with a piece of radiocarbon-dated detrital wood at the same site (650 ± 40 14C yr BP, ad 1280–1400), and with two logs at site 15, 200 m to the east (r = 0.492), but the overlap is only 73 years (Figure 10).

A floating chronology spanning 257 years was established from several samples of detrital wood found in till near the base of the bedrock slope outside the main lobe of Llewellyn Glacier (site 16, Figure 4), inboard of the sites 11, 12 and 13, discussed above (Figure 10). The first 104 years of the chronology crossdate with the samples at sites 14 and 15 (r = 0.546).

The outermost moraine of Llewellyn Glacier and an associated trimline extend around the north flank of a nunatak that separates the south from the main lobe of the glacier. Five in situ trees and five pieces of detrital wood were sampled at five sites (17, 18, 19, 20 and 21; Figure 4) just inside the trimline and moraine. Their location indicates that the trees were killed at the time of the maximum advance. A floating chronology comprising the ten samples spans 309 years (r = 0.578) (Figure 10). The inner 132 rings of this chronology crossdate with the floating chronology derived from sites 14, 15 and 16 (r = 0.512).

InterpretationThe radiocarbon-dated subfossil tree on the rock ridge in the Tulsequah Glacier forefield was overridden by the glacier some-time between ad 865 and ad 940 (Figure 12). The ridge had been ice-free for more than 218 years prior to that time (i.e. back to at least ad 720). The remnants of the overridden forest are about 50 m above the valley floor and 2.4 km inside the Holocene limit of the glacier. The evidence suggests that Tulsequah Glacier was much more extensive 1100 years ago than it is today.

The main and south lobes of Llewellyn Glacier advanced into mature forest between ad 300 and ad 500 (Figure 12). The glacier was no more extensive than today immediately before that time.

The south lobe of Llewellyn Glacier advanced into forest that was at least 190 years old around ad 1100 (Figure 12). At about the same time, the main lobe overrode forest at least 240 years old and, based on the tree-ring cross-dating, continued to advance for at least another 150 years. The most likely time of initial over-riding, based on the probability distribution of the calibrated radiocarbon ages on outer rings of the in situ stumps, is ad 1030–1210. By the end of this advance, the main lobe of Llewellyn Glacier terminated within 400 m of its maximum Holocene margin. The south lobe reached at least 1000 m east of



Clague et al. 455

Tab

le 1

. R

adio

carb

on a

ges

repo

rted

in th

e pa

per

Rad

ioca

rbon

age

C

alen

dric

age

(ad

)b

Mos

t lik

ely

age(

s) (ad

)c

Lab

orat

ory

num

berd

S

ite

no.

Lat

itud

e (N

), L

ongi

tude

(W

) E

leva

tion

(m

) D

ated

mat

eria

l C

omm

ent

(14 C

yr

BP

)a

(F

igur

e 4)

295

± 38

14

80–1

665

1535

, 164

0 A

A54

468

5

59°0

3.7’

133°5

8.4’

76

3 B

ranc

h L

ate

‘Lit

tle

Ice

Age

’ del

ta43

4 ±

37

1415

–162

0 14

45

AA

5447

2e

4

59°0

3.0’

133°5

7.0’

76

0 Tw

ig

‘Lit

tle

Ice

Age

’ lak

e44

0 ±

45

1405

–162

5 14

45

AA

5477

0e

4

59°0

3.0’

133°5

7.0’

76

0 Tw

ig

‘Lit

tle

Ice

Age

’ lak

e45

4 ±

37

1405

–161

5 14

45

AA

5446

9e

4

59°0

3.0’

133°5

7.0’

76

0 Tw

ig

‘Lit

tle

Ice

Age

’ lak

e58

0 ±

40

1295

–142

0 13

30, 1

395

Bet

a-24

4625

14

59°0

3.0’

133°5

6.9’

76

0 T

ree

stum

pf

Bet

wee

n tw

o ti

lls

650

± 40

12

80–1

400

1300

, 137

5 B

eta-

2446

24

14

59°0

3.0’

133°5

6.9’

75

5 P

iece

of

woo

d B

etw

een

two

till

s67

0 ±

60

1250

–141

0 12

90, 1

375

Bet

a-20

0738

1

59°0

4.0’

133°5

7.2’

73

5 B

ranc

h E

arly

‘L

ittl

e Ic

e A

ge’ (

?) la

ke78

0 ±

70

1045

–138

5 12

65

GS

C-6

634

11

59°0

4.9’

133°5

6.2’

78

5 T

ree

stum

pf

Gla

cier

adv

anci

ng82

0 ±

70

1040

–128

5 12

20

GS

C-6

630

11

59°0

4.9’

133°5

6.2’

80

5 T

ree

stum

pf

Gla

cier

adv

anci

ng93

0 ±

40

1020

–120

5 10

45, 1

085,

113

5 G

SC

-663

2 12

59°0

4.9’

133°

56.2

’ 81

7 B

ranc

h in

till

M

axim

um a

ge f

or g

laci

er a

dvan

ce93

5 ±

38

1020

–118

5 10

45, 1

090,

112

0, 1

150

AA

5447

3 1

59°0

4.0’

133°5

7.2’

73

4 Tw

ig

Ear

ly ‘

Lit

tle

Ice

Age

’ (?)

lake

950

± 15

10

25–1

155

1115

U

CIA

MS

-450

09

9

59°0

3.5’

133°5

7.4’

75

8 T

ree

stum

pf

Gla

cier

adv

anci

ng97

0 ±

60

970

–121

0 10

30, 1

115,

114

5 B

eta-

2007

42

13

59°0

4.8’

133°5

6.2’

78

5 Tw

ig in

col

luvi

um

Max

imum

age

for

gla

cier

adv

ance

980

± 40

9

90–1

155

1025

B

eta-

2007

40

13

59°0

5.0’

133°5

6.2’

79

9 T

ree

stum

pf

Max

imum

age

for

gla

cier

adv

ance

994

± 38

9

80–1

155

1025

A

A54

467

3

59°0

4.1’

133°5

7.2’

73

3 Tw

ig

Ear

ly ‘

Lit

tle

Ice

Age

’ (?)

lake

1000

± 5

0 9

00–1

160

1020

B

eta-

2007

41

13

59°0

5.0’

133°5

6.2’

80

0 B

ranc

h or

roo

t M

axim

um a

ge f

or g

laci

er a

dvan

ce12

10 ±

40

695

–935

79

5, 8

55

Bet

a-20

0739

3

59°0

4.1’

133°5

7.2’

73

3 Tw

ig

Ear

ly ‘

Lit

tle

Ice

Age

’ (?)

lake

1228

± 3

9 6

85–8

85

775

AA

4637

5

58°4

8.1’

133°4

1.4’

19

0 T

ree

stem

g O

uter

rin

g ag

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gla

cial

ly o

verr

idde

n tr

ee12

72 ±

52

660

–870

70

5, 7

65

AA

4637

6

58°4

8.1’

133°4

1.4’

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0 T

ree

stem

g P

ith

age

of g

laci

ally

ove

rrid

den

tree

1560

± 4

0 4

15–5

85

445,

475

, 535

B

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2446

23

7

59°0

3.4’

133°5

8.4’

76

0 T

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stem

h B

etw

een

two

till

s17

30 ±

40

225

–410

26

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80, 3

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a-24

5591

6

59°0

6.6’

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a Lab

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prog

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% c

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the

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age.

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orat

ory;

Bet

a, B

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lyti

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, Geo

logi

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out

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its present margin around ad 1100 and appears to have advanced 500 m farther in the fourteenth and fifteenth centuries. Because the overridden forest was rooted in a well developed soil with an oxidized C horizon, it seems likely that Llewellyn Glacier had not been that extensive for thousands of years before ad 1000. In fact, the only evidence for an earlier, more extensive glaciation is

granitic stones in the colluvium on which the soil developed. These stones probably were reworked from Late Pleistocene glacial sediments.

The main lobe advanced beyond its twelfth and thirteenth cen-tury limits around ad 1500. This advance culminated in the late seventeenth century (Figure 12).

Figure 7. Interpreted stratigraphy at sites 1, 2, 3, 4 and 5 in the Llewellyn Glacier forefield (see Figure 4 for locations and text for discussion)

Figure 8. Section at site 1 in the Llewellyn Glacier forefield; the bluff is about 11.5 m high (see Figure 4 for location and text for discussion)



Clague et al. 457

Radiocarbon ages on detrital plant material recovered from glaciolacustrine sediments on the valley floor and south and west margins of the glacier forefield suggest that lakes were impounded against the main lobe of Llewellyn Glacier one or more times dur-ing the past millennium. A lake during the first half of the past millennium is suggested by ages at sections in the centre of the valley ranging from 670 ± 60 to 994 ± 38 14C yr BP (ad 980–1410). Three, tightly clustered radiocarbon ages suggest a lake was present between ad 1420 and ad 1480. A younger lake is sug-gested by a radiocarbon age of 295 ± 38 14C yr BP (ad 1480–1665) at site 5. Each of these radiocarbon ages, however, is derived from detrital wood and thus must be considered a maximum for the age of the enclosing lake sediments. A glacier-dammed lake was pres-ent in the valley in the past century and left strandlines up to 760

m a.s.l. on the slopes east of Llewellyn Glacier (Figure 13). The remnants of this lake persist at the present margin of the glacier.

DiscussionOur data call into question aspects of the term ‘Little Ice Age’, especially its duration. Reyes et al. (2006) documented a short-lived period of extended glaciers in British Columbia and Alaska between about 1700 and 1400 years ago, which they termed the ‘First Millennium ad Advance’. Some of the gla-ciers that they discussed were as extensive at ad 600 as they were 600 years later, at the beginning of the ‘Little Ice Age’ as defined by Grove (2004). Llewellyn Glacier also advanced between about 1700 and 1400 years ago, although it did not

Figure 9. Section at site 5 in the Llewellyn Glacier forefield (see Figure 4 for location and text for description of stratigraphy). A branch 5 m above the base of the exposure at the downstream end (left) yielded a radiocarbon age of 295 ± 38 14C yr BP

Figure 10. Floating tree-ring chronologies and related radiocarbon ages from the Llewellyn Glacier forefield. Bars indicate length of individual chronologies (see Table 2)




Figure 11. Schematic transect up the bedrock slope east of the main lobe of Llewellyn Glacier, showing regolith and weathered slope deposits (thickness exaggerated) overlain by till. See Figure 4 for location of transect

Table 2. Characteristics of tree-ring chronologies from Llewellyn Glacier

Site(s), Figure 4 Radiocarbon age (14C yr BP) Calendric age (ad) No. of trees/no. of radii Interseries correlation Length of chronology (yr)

6 1730 ± 40 225-410 2/4 .679 3327 & 8 1560 ± 40 415-585 6/11 .564 3246-8 1730 and 1560 225-585 8/15 .521 163 yr overlap9 950 ± 15 1025-1155 2/4 .713 18310 2/4 .698 2149 & 10 950 ± 15 1025-1155 4/8 .537 141 yr overlap11 780 ± 70 1045-1385 5/8 .647 25710, 11 780 ± 70 1045-1385 7/12 .601 153 yr overlap9-11 950 and 780 1025-1385 9/16 .597 99 yr overlap14a 580 ± 40 1295-1420 1/2 .815 15414b 650 ± 40 1280-1400 1/2 .802 14714a, 14b 580 and 650 ~1280-1420 2/4 .745 120 yr overlap15 2/4 .715 12914a, 14b, 15 580 and 650 ~1280-1420 4/8 .492 73 yr overlap14b, 15 650 ± 40 1280-1400 3/6 .601 114 yr overlap16 4/8 .534 25714-16 580 and 650 ~1280-1420 8/16 .546 oldest 104 rings (16)17-21 10/18 .578 30914-21 580 and 650 ~1280-1420 14/26 .512 oldest 132 rings (17-21) living trees 1830-2004 5/10 .581 174

achieve as great an extent as it did several hundred years later (between ad 1034 and 1208).

The rationale for formally defining an advance in the first mil-lennium ad that is separate from the first advance of the ‘Little Ice Age’ is that the two are separated by 600 years, a period during which glaciers presumably were less extensive than both earlier and later. The evidence presented in this paper, however, demon-strates that both Tulsequah and Llewellyn glaciers were advancing midway through this 600 yr interval, as were many other glaciers in western North America (Wiles et al., 1999a, b, 2008; Luckman, 2000; Calkin et al., 2001; Koch and Clague, 2010).

Our evidence further reveals differences in the behaviour of outlet glaciers of the Juneau Icefield during this period. An advance of Llewellyn Glacier that was relatively larger than the ad 865–940 advance of Tulsequah Glacier occurred at least 100 years later, between ad 1030 and AD 1210. It was one of the largest advances of Llewellyn Glacier during the past millennium.

Pigeonholing events with phrases such as ‘Little Ice Age’ may actually impede progress towards a full understanding of real-world complexity of glacier activity (Clague et al., 2009). We return here to a point raised in the introduction – the question of when the ‘Little Ice Age’ began. Some researchers have argued that the ‘Little Ice Age’ be linked to the most severe climates of the past millen-nium, which commenced around ad 1550 (Lamb, 1963, 1966; Bradley and Jones, 1995). This definition corresponds to what Ogilvie and Jónsson (2001) called the ‘orthodox’ or ‘classical’ view (Lamb, 1977). This view is based, however, on climate, not glaciol-ogy. The term ‘Little Ice Age’ should be restricted to glacier activity, as that is how Matthes (1939) originally defined it. Even so, some climatologists recognize a longer interval for the ‘Little Ice Age’, with a beginning in the early thirteenth century (Jones and Mann, 2004). With the discovery that Llewellyn Glacier likewise was advancing to near its all-time Holocene limit sometime between ad 1030 and 1210, we argue for more explicit recognition of centen-nial- and decadal-scale variability in glacier activity in the late Holocene than can be captured by terms such as the ‘Little Ice Age’.

Figure 12. Time–distance diagram showing activity of Llewellyn Glacier over the past 2000 years and the advance of Tulsequah Glacier late in the first millennium ad. FMA, First Millennium ad advance; LIA max, maximum (classical) advance of ‘Little Ice Age’



Clague et al. 459

Conclusion

Our work at Tulsequah and Llewellyn glaciers contributes to an emerging, more sharply focused view of the complexity of glacier activity in western North America during the Holocene. During the past two millennia, and almost certainly earlier, glaciers advanced and retreated on decadal and centennial timescales, much as they have during the historic period when they have been photographed and monitored. Tulsequah Glacier advanced to near its Holocene limit between ad 865 and ad 940. Llewellyn Glacier advanced sometime between ad 300 and ad 500, and reached to within 400 m of its Holocene limit between ad 1035 and ad 1210, before the classical ‘Little Ice Age’ advances of the past several centuries. Our data show that some glaciers in western North America were extensive and expanding at times when alpine glaciers have, in the past, been assumed to be restricted. Continued use of terms such as ‘Little Ice Age’, which were defined at a time when our under-standing of Holocene glacier activity in western North America was far less advanced than it is today, can be questioned.

AcknowledgementsReviews by Brian Luckman and two anonymous journal referees improved the paper. Vanessa Egginton, Karen Geertsema, Michelle Hanson, Robin McKillop, John Orwin, Jim Pojar and Kenna Wilkie assisted in the field. Richard Franklin drafted some of the figures. Roger Wheate provided some satellite imagery used in Figures 2 and 3.

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