Latest Pleistocene and Holocene glacier fluctuations in western Canada

Abstract

We summarize evidence of the latest Pleistocene and Holocene glacier fluctuations in the Canadian Cordillera. Our review focuses primarily on studies completed after 1988, when the first comprehensive review of such evidence was published. The Cordilleran ice sheet reached its maximum extent about 16 ka and then rapidly decayed. Some lobes of the ice sheet, valley glaciers, and cirque glaciers advanced one or more times between 15 and 11 ka. By 11 ka, or soon thereafter, glacier cover in the Cordillera was no more extensive than at the end of the 20th century. Glaciers were least extensive between 11 and 7 ka. A general expansion of glaciers began as early as 8.4 ka when glaciers overrode forests in the southern Coast Mountains; it culminated with the climactic advances of the Little Ice Age. Holocene glacier expansion was not continuous, but rather was punctuated by advances and retreats on a variety of timescales. Radiocarbon ages of wood collected from glacier forefields reveal six major periods of glacier advance: 8.59–8.18, 7.36–6.45, 4.40–3.97, 3.54–2.77, 1.71–1.30 ka, and the past millennium. Tree-ring and lichenometric dating shows that glaciers began their Little Ice Age advances as early as the 11th century and reached their maximum Holocene positions during the early 18th or mid-19th century. Our data confirm a previously suggested pattern of episodic but successively greater Holocene glacier expansion from the early Holocene to the climactic advances of the Little Ice Age, presumably driven by decreasing summer insolation throughout the Holocene. Proxy climate records indicate that glaciers advanced during the Little Ice Age in response to cold conditions that coincided with times of sunspot minima. Priority research required to further advance our understanding of late Pleistocene and Holocene glaciation in western Canada includes constraining the age of late Pleistocene moraines in northern British Columbia and Yukon Territory, expanding the use of cosmogenic surface exposure dating techniques, using multi-proxy paleoclimate approaches, and directing more of the research effort to the northern Canadian Cordillera.

Introduction

Mountain glaciers are sensitive indicators of environmental change. They respond to changes in climate by adjusting their width, length, and thickness. Because most valley glaciers are laterally constrained and ice deforms under its own weight, fluctuations in length are the most common response to long-term changes in climate. These adjustments, however, do not occur immediately; they depend on the response time of the glacier, which for mountain glaciers can range from several years to decades (Nye, 1960, Jóhannesson et al., 1989, Harrison et al., 2003). Nevertheless, the geologic evidence of glacier fluctuations can be used to document Holocene regional climate variability at time scales of decades to centuries.

In western Canada, as elsewhere in the Northern Hemisphere, glaciers expanded from minimum extents in the early Holocene to maximum extents some 150–300 years ago (Porter and Denton, 1967, Denton and Karlén, 1973, Osborn and Luckman, 1988). This long-term, progressive expansion has truncated the surface record of past glacier activity, because recent advances commonly overrode or destroyed lateral and terminal moraines that demarcate former glacier limits. However, fragmentary evidence of these events is preserved in some moraines and glacier forefields where subfossil wood in growth position, detrital wood, paleosols, or tephras are exposed. Glacier retreat in the 20th century and the first decade of the 21st century has provided many such exposures, which can be exploited through stratigraphic, sedimentologic, and geochronologic studies to improve understanding of Holocene glacier history (Osborn and Luckman, 1988, Smith and Desloges, 2000, Koch et al., 2007a, Koch et al., 2007b; Osborn et al., 2007).

Sheared stumps in growth position in glacier forefields provide direct evidence of glacier fluctuations (Menounos et al., 2004). In contrast, detrital wood collected in glacier forefields may have been delivered onto the glacier by mass wasting, or it may have been reworked from older deposits. Ryder and Thomson (1986) provide an excellent discussion of the origin and interpretation of wood collected from glacier forefields.

Proglacial lake sediments afford an additional, indirect but continuous proxy of upvalley glacier fluctuations (Karlén, 1981, Leonard, 1986, Souch, 1994, Leonard and Reasoner, 1999, Menounos, 2002). Lengthy periods characterized by high sedimentation rates or high clastic content generally coincide with times of greater ice extent (Leonard, 1997). However, changes in sediment delivery from alpine glaciers, proglacial sources, or non-glacierized terrain may obscure the relation between proglacial lake sedimentation and glacier activity (Hallet et al., 1996, Leonard, 1997, Menounos, 2002). Lake sediment records in combination with more direct evidence from glacier forefields offer considerable scope for reconstructing alpine glacier activity during the Holocene. In some cases, the combined use of clastic records with detrital wood strengthens the case for regional glacier activity if both records accord (Menounos et al., 2004, Menounos et al., 2008).

Twenty years have elapsed since the last major review of Holocene glacier fluctuations in western Canada (Osborn and Luckman, 1988). Over that time, many studies and discovery of new sites have contributed to an improved understanding of glacier activity in this region. Accelerated glacier retreat in the 1990s exposed new evidence at sites studied previously, and many new proglacial lake sediment records have become available. In this paper we summarize and interpret the evidence of the latest Pleistocene and Holocene glacier fluctuations in western Canada that has appeared in the past 20 years. Further, we consider some of the forcing factors responsible for decadal to millennial changes in glacier cover. Although the focus of the review is on western Canada, we refer to sites in the United States close to the International Boundary. We limit our review to evidence that is most directly associated with glacier fluctuations: lateral and end moraines, detrital and in situ (growth position) wood in glacier forefields, and proglacial lake sediment records. We present the evidence from the oldest to youngest periods and from maritime to continental localities. Ages are reported in both radiocarbon years (¹⁴C yr BP) and calibrated radiocarbon years before AD1950 (ka) from 16 to 1.0 ka. We calibrated radiocarbon ages with the calibration program CALIB 5.02 (Stuiver et al., 2005) and report the 95% confidence limits of these calibrated ages. For the past millennium we use AD for true calendar ages and cal yr AD for calendar-equivalent radiocarbon years.

The most recent definition of the base of the Holocene is 11.7 ka (Walker et al., 2008). According to this definition Younger Dryas-equivalent and older advances described in this paper are latest Pleistocene in age, and events subsequent to the Younger Dryas interval are Holocene in age.

The Canadian Cordillera is a broad mountainous region that extends from the International Boundary to the Arctic Ocean (Fig. 1). The region includes many rugged mountain ranges with local relief exceeding 600 m. The tallest mountain in the Cordillera is Mount Logan in southwest Yukon (5959 m asl), but the highest peaks in most mountain ranges are 2000–3500 m asl. The Cordillera can be subdivided into Western, Interior, and Eastern systems (Bostock, 1949). The Western System includes the Insular, Cascade, Coast, and St. Elias mountains. The Interior System includes the Purcell, Selkirk, Cariboo, and Monashee mountains in the south and the Hazelton, Skeena, Cassiar, Omineca, and Ogilvie mountains in the north (Fig. 1). These interior mountain ranges border extensive areas of low relief, referred to as the Interior Plateaus in British Columbia and the Yukon Plateaus in Yukon Territory. The Eastern System includes the Rocky Mountains in the south and the Selwyn, Mackenzie, and Richardson mountains in the north (Fig. 1). The Canadian Rocky Mountains are bordered by the Rocky Mountain Trench to the west and the Rocky Mountain Foothills to the east. The Tintina Trench, which is the northern extension of the Rocky Mountain Trench, borders the Selwyn Mountains.

Glaciers today cover about 30,000 km², or 3%, of the landmass of British Columbia and 12,500 km², or 2.6%, of Yukon (Schiefer et al., 2007, Moore et al., in press). The Coast and St. Elias Mountains contain the largest glaciers and are most heavily glacierized mountain ranges in the Canadian Cordillera. Glaciation levels rise eastward, largely due to the diminished influence of maritime air masses (Østrem, 1966). Median elevations of glaciers range from 1540 m asl in the Insular Mountains to 2550 m asl in the southern Rocky Mountains (Schiefer et al., 2008).