North America

The North American glacial record is known in much greater detail than that of South America, but calendar-dated sequences through the last millennium are still rare. In addition, because of the differing latitudinal distribution of landmasses in the two hemispheres, many of the North American sites are poleward of their Southern Hemisphere counterparts. In this chapter, we will concentrate primarily on material that postdates the accounts given in Grove (1988), Calkin (1988), Davis (1988), and Osborn and Luckman (1988).

The literature on LIA glacier fluctuations in Alaska is voluminous and comprehensively reviewed in Calkin (1988), Calkin and Wiles (1992), and Calkin et al. (in press). The most interesting recent work has come from coastal Alaska, for which dendrochronological studies have provided calendar dates for forest beds overridden by glacier advances during the last millennium (Wiles et al., 1995; Wiles and Calkin, 1994; Wiles et al., 1999a; Wiles et al., 1999b). In a study of 16 land-based glaciers on the Kenai Peninsula (59°-60°N), Wiles and Calkin (1994) demonstrated slight differences in the timing of periods of glacier advance and recession on the eastern and western sides of the peninsula. Glaciers on the western (cooler and drier) side of the peninsula were advancing and/or building moraines in ca. A.D. 1420-60, 1640-70, 1750, and between 1880 and 1910. Glaciers on the maritime eastern flank were advancing in ca. A.D. 1440-60, 1650-1710, and 1830-60. Recession from LIA maximum positions occurred between the early 1700s and late 1800s on the west and between the early 1800s and the early 1900s on the east. Examination of meteorological and proxy climate records suggests that the glaciers on the western flank expanded during times of lower arctic temperatures, whereas the advance of the eastern maritime glaciers corresponded with warmer temperatures and increased winter precipitation (Wiles and Calkin, 1994).

The record from coastal glaciers in Prince William Sound (ca. 60°-61°N; Wiles et al., 1999a) shows a consistent pattern of decade-century glacier fluctuations. Widespread glacier advances occurred in the twelfth through thirteenth and late seventeenth to early eighteenth centuries. Two major moraine-building episodes (28 moraines, 15 glaciers) have been identified in the early eighteenth century and the late nineteenth century, with the early eighteenth-century event as the most extensive advance for five glaciers. At three glaciers, the greatest advance was in 1874-95, but moraines of this advance are just within the seventeenth-century moraines at several other glaciers. Glaciers were close to their maxima when the earliest scientific visits to the area took place in the late 1800s.

For the Juneau ice field area (58°-59°N), Calkin and Wiles (1992) reported earlier work showing periods of glacier advance based on sheared stumps at ca. A.D.

1265, 1401, and 1752 at Davidson Glacier and ca. 1600 at Bucher Glacier. Nine other glaciers have mid- to late-eighteenth-century maximum positions and re-advanced to form end moraines in the early to late nineteenth century.

Tidewater-calving glaciers in Alaska have shown spectacular fluctuations with a range of maximum ages that may or may not be synchronous with adjacent land-based termini. Major glacier systems dated include: Glacier Bay, ca. A.D. 1700-50 (in Calkin, 1988); Brady Ford, 1886 (Calkin and Wiles, 1992); Lituya Bay, ca. 1600 (Calkin and Wiles, 1992); La Perouse Glacier, 1890s (Calkin and Wiles, 1992); Icy Bay, 1880 (Calkin et al., in press); Taku Glacier, 1750 (maximum, but has advanced ca. 7.3 km between 1890 and 1990; Motyka and Beget, 1995); and the Yakutat Bay complex, 1250-1500 (Calkin and Wiles, 1992).

Figure 5 (Calkin et al., in press) summarizes glacier history from the forefields of land-based, nonsurging glaciers in coastal southern Alaska (Prince William Sound and adjacent areas). The subsurface record indicates that glaciers were advancing over forested areas during the late 1200s to the early 1300s and from the early seventeenth century to the early eighteenth century. Limited evidence for advances in the mid-1400s at Beare, Grewingk, and Tustemena Glaciers is omitted from Fig. 5. The surface moraine record indicates that well over half the glaciers reached their LIA maxima between 1800 and 1875, although about one quarter of the glaciers reported had their maxima during the eighteenth century. Many glacier forefields contain re-advance moraines dating from the mid- to late nineteenth century.

Using temperature-sensitive tree-ring chronologies, Wiles (1997) identified 1650-1750 as the main cooler interval in the last 300 years in the Gulf of Alaska and inferred that March-May temperatures are the major control of glacier fluctuations in this area. Figure 6

FIGURE 5 Dates of periods of glacier advance and moraine formation in the Gulf of Alaska. (Redrawn from Calkin et al., in press.) The black horizontal bars are calendar-dated records from overridden (subsurface) trees in glacier forefields. At each site, the youngest outermost date provides a minimum-age estimate for glacier overriding at the site. The range of outermost dates provides an indication of the minimum duration of the advance. The histogram shows dates of moraine formation for land-based glaciers in Prince William Sound and the southern Kenai Mountains. Shaded bars are the oldest moraine at each glacier site; open bars represent subsequent (re-advance) moraines.

FIGURE 5 Dates of periods of glacier advance and moraine formation in the Gulf of Alaska. (Redrawn from Calkin et al., in press.) The black horizontal bars are calendar-dated records from overridden (subsurface) trees in glacier forefields. At each site, the youngest outermost date provides a minimum-age estimate for glacier overriding at the site. The range of outermost dates provides an indication of the minimum duration of the advance. The histogram shows dates of moraine formation for land-based glaciers in Prince William Sound and the southern Kenai Mountains. Shaded bars are the oldest moraine at each glacier site; open bars represent subsequent (re-advance) moraines.

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