Environmental change in Garibaldi Provincial Park, Southern Coast Mountains, British Columbia

Geoscience Canada, Sept, 2004 by Johannes Koch, Brian Menounos, John J. Clague, Gerald D. Osborn

The PDO and ENSO exert their strongest control in the mountains of western North America during winter (Mantua and Hare, 2002). Glaciers that have a persistent positive net mass balance will adjust their areas, lengths, and thickness to accommodate the increased mass. They typically leave moraines and trimlines, delimiting their former extents long after they have receded. Conspicuous moraines and trimlines in Garibaldi Park delineate the maximum recent cover of ice during the Little Ice Age (Fig. 2, 3).

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The time of the Little Ice Age maximum in Garibaldi Park may differ from glacier to glacier by as much as decades due to differences in glacier response times, but Figures 2 and 3 give a sense of the amount of ice loss in the park since the most extensive advance of the Little Ice Age in the early eighteenth century. At least 630 [km.sup.2] (33 %) of the park was covered by ice in the early eighteenth century. Ice cover today is 390 [km.sup.2], a loss of 38 % in less than 300 years (Koch et al., 2003a). Moraines dating to the late nineteenth century are very close to Little Ice Age maximum ice limits, thus most of the ice loss has happened in the last 150 years. In fact, much of the thinning and retreat date to the last 80 years (Fig. 3) and occurred during two intervals, one between about 1930 and 1960 and another starting in the late 1970s and continuing to the present. The two retreat phases were separated by a stillstand or minor re-advance of many glaciers in the late 1960s and early 1970s (Fig. 3). Glaciers in other parts of the Canadian Cordillera have fluctuated in a similar manner to those in Garibaldi Park since the late nineteenth century (e.g., Osborn and Luckman, 1988; Luckman, 2000).

Ages of glacier advances can be estimated by dating plant remains in lateral and terminal moraines, or by determining the age of trees or lichens growing on the moraines (Fig. 4, 5; Luckman, 2000). Changes in tree growth may reveal the duration of an advance, as both react to the same forcing (Luckman, 2000). Cooler climate leads to glacier advance and, commonly, to narrow rings in temperature-sensitive trees (Luckman, 1996, 1998). Some tree species, for example mountain hemlock (Peterson and Peterson, 2001), may likewise form narrow rings during years when the snowpack is thick and lingers late into the summer.

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The oldest tree on a moraine provides a minimum estimate for the time of moraine stabilization (Fig. 4; Lawrence, 1946). The tree ages need to be adjusted to account for the time between moraine stabilization and seedling germination ("ecesis"; McCarthy and Luckman, 1993; Winchester and Harrison, 2000). Photographic records and field studies indicate that the ecesis interval in Garibaldi Park is generally less than 20 years, but in some situations can be as long as 70 years (Koch, unpublished data).

The age of a glacial advance can also be estimated by dating living trees damaged during the advance, dead trees in growth position overridden by the glacier (in situ stumps), and detrital wood within moraines or lying on the surface in the glacier forefield (Fig. 5; Luckman, 1998, and references therein). The most valuable ages are those derived from in situ trees, especially if the rings can be cross-dated into living chronologies to obtain the year when the tree was damaged or killed. Detrital wood and damaged and dead trees in growth position are present in the forefields of glaciers in Garibaldi Park, allowing us to acquire a more complete Holocene glacier record than his been possible in other parts of the Coast Mountains to date. Radiocarbon dating of these materials has revealed advances around 6000 and 3000 years ago and during the last 500 years (Fig. 6; Koch et al., 2004). Evidence for these events is provided by in situ stumps and detrital wood from several glaciers in Garibaldi Park. Other, more poorly constrained advances occurred about 8000 (Menounos et al., 2004), 4500, and 1500 years ago. The 8000- and 4500-year advances are inferred from detrital wood in the forefields of two glaciers in the park. Evidence for the 1500-year-old event has been found at only one glacier in the park, but has been documented elsewhere in the southern Coast Mountains (Reyes et al., 2004).