Info

*Berendon Glacier

FIGURE 10 Comparison of moraine chronologies for western Canada and southern Alaska. Data sources are indicated on the diagram. Asterisks indicate additions to the original published sources.

*Berendon Glacier

FIGURE 10 Comparison of moraine chronologies for western Canada and southern Alaska. Data sources are indicated on the diagram. Asterisks indicate additions to the original published sources.

nor exceptions, the LIA maxima in the Coast Ranges and on Vancouver Island occurred during the mid- to late nineteenth century. However, available dating from glaciers in the Cascades and Olympic Mountains in the adjacent areas of the United States show sixteenth- to eighteenth-century moraines outside the conspicuous nineteenth-century moraines. At Mount Rainier (47°N), the Nisqually Glacier has been monitored since 1857 (Porter, 1981b), and Burbank (1981) reports minimum tree-ring and lichenometric dates of 1519-28, 1552-76, 1613-24, 1640-66, 1691-95, 1720, 1750, and 1768-77 for pre-nineteenth-century moraines on Mount Rainier. For the Dome Peak area of the Cascades (48°N), Miller (1969) describes a possible thirteenth-century moraine (supporting living trees >680 years old) at Chickamin Glacier and late sixteenth- to early seventeenth-century ages were determined for LIA maximum moraines at the Cascade, La Conte, and Dana Glaciers. At Mount Baker (ca. 49°N), Fuller (1980; Fuller et al., 1983) used dendrochronology to identify sixteenth- (two valleys) and seventeenth-century (three valleys) moraines down-valley of nineteenth-century moraines. Heikkinen (1984) similarly reports moraines from the early 1500s and 1740 outside the nineteenth-century moraines of the Coleman Glacier. For Blue Glacier (ca. 48°N) in the Olympics, Heusser (1957) reports an older moraine fragment dating at ca. 1650, but nineteenth-century moraines (dated by a tilted tree ca. 1850 at Blue Glacier) usually obscure these earlier features at Blue and Hoh Glaciers.

8.3.4. North American Summary

Evidence from throughout the western cordillera suggests that the earliest LIA advances occurred during the twelfth through fourteenth centuries, but the LIA maxima generally occurred during the seventeenth through nineteenth centuries. There are both inter- and intraregional variations in the dating of the LIA maximum position. Mid- through late-nineteenth-century advances occurred throughout the region and appear to have been the most extensive in the more heavily glaciated regions of the British Columbia Coast Mountains and in parts of southern Alaska. However, large numbers of glacier forefields remain uninvestigated. The general absence of multiple till units and/ or intervening organic deposits dating from the last millennium suggests that glaciers may have remained at or close to their maximum positions throughout this interval until the mid- through late nineteenth century, although there may have been several intervals of glacier advance during this period. There is no evidence for periods of glacier recession during the last millennium comparable to those of the twentieth century.

Examination of the coastal Alaskan, Rockies, and British Columbian chronologies suggests a strong similarity between the glacier histories of these areas that implies a common climate control. However, contemporary mass balance data suggest more complex patterns and relationships that change over time. Hodge et al. (1998) recently discussed the relationships between Pacific climate indices and mass balance data for Gulkana Glacier (interior Alaska, 63°N), Wolverine Glacier (Kenai Peninsula in Alaska, ca. 60°N), and South Cascade Glacier (Washington, 48°N). The mass balance data for the Peyto Glacier (Canadian Rockies, 51°N) were briefly discussed by Luckman (1998b, 2000b), and all four glaciers have over 30 years of mass balance data. There appears to be no simple relationship between the mass balance records of the two Alaskan glaciers (Gulkana and Wolverine Glaciers), which are only 300 km apart (Hodge et al., 1998). However, the winter balance records for Peyto, Wolverine, and South Cascade Glaciers all show clear "step changes" coincident with the interdecadal 1976-77 regime shift over the North Pacific (Ebbesmeyer et al., 1991). Although, Peyto and South Cascade Glaciers both show sharp decreases in winter balance, Wolverine Glacier shows a marked increase. This negative correlation between the mass balances of Alaskan glaciers and those in southern Canada and the Pacific Northwest was recognized by Walters and Meier (1989) and was attributed to the steering of storm tracks by the Alaskan Low. This teleconnection pattern breaks down after 1989, when all four glaciers show strongly negative balances (Hodge et al., 1998; Luckman, 1998b, 2000b).

0 0

Post a comment