North America

In Alaska, studies of last glaciation advances have been made in the Brooks Range, the Alaska Range, the Alaska Peninsula, and Kodiak Island (Porter et al., 1983; Hamilton and Thorson, 1983; Mann and Peteet,

1994). Radiocarbon dating of organic material associated with glacial and glaciofluvial deposits supports a time-distance diagram presented by Porter et al. (1983, Fig. 4.7) showing two glacial maxima in the Brooks Range during MIS 2, at ca. 24,000-22,000 14C B.P. and 19,500-17,000 14C B.P. Glaciers in other parts of Alaska appear to have reached their maxima at about the same time as those in the Brooks Range. Deglaciation in coastal southern Alaska began by ca. 16,000-15,000 14C B.P. and continued for at least 1500 14C years, as is shown by the radiocarbon ages of marine fossils in the Bootlegger Cove formation in Cook Inlet (Hamilton and Thorson, 1983).

Farther south, glaciers in the coastal mountains of southwestern British Columbia and the northern Cascade Range advanced to their late Wisconsin maximum limits (Coquitlam advance) at ca. 24,800-18,000 14C B.P. (Porter et al., 1983). The mountain glaciers then receded for about a millennium during the Olympia interstade before the Vashon advance led to maximum expansion of western outlet glaciers of the Cordilleran ice sheet. Although the Puget ice lobe terminated its maximum late Wisconsin advance south of Seattle shortly after ca. 15,000 14C B.P. (Mullineaux et al., 1965; Porter and Swanson, 1998), eastern outlets re-advanced less distance (~50%) than they had at ca. 22,000 14C B.P. (Richmond, 1986). This difference suggests a remarkable influence of either glaciological control (internal flow dynamics, calving) or west-east differences in precipitation supply across the Cordilleran ice sheet during the late Wisconsin.

Moraines in many ranges of the Rocky Mountains mark limits of late Wisconsin (Pinedale stage) glacier advances, but chronologies exist only for those in the Yellowstone Plateau area, the Wind River Range (Wyoming), and the Colorado Front Range. Pierce (1979) mapped three major moraine stages in the Yellowstone River Valley—the Eightmile, Chico, and Deckard Flats moraines (oldest to youngest)—but had little dating control. However, U-series ages from travertine deposits associated with these moraines (Sturchio et al., 1994) now suggest that the Chico stage, with an age in the range of ca. 30,000-22,500 cal. B.P. (ca. 25,000-19,000 14C B.P.) represents the late Wisconsin maximum, whereas the Deckard Flats moraine (ca. 16,500-13,300 14C B.P.) may have formed coevally with the Vashon advance of the Pacific Northwest. The most extensive advance (Eightmile) has an age in the range of 47,000-34,000 cal. B.P. (ca. 40,000-28,000 14C B.P.) and indicates substantial glaciation of the Yellowstone Plateau well before the CLIMAP LGM.

The classic Pinedale sequence of moraines for the Rocky Mountains was first established in the Fremont Valley of the Wind River Range, initially based on

FIGURE 1 Locations of major sites mentioned in the text.

relative weathering criteria (Blackwelder, 1915; Richmond, 1986). Because of the absence of any radiocarbon chronology, surface boulders on the moraines have been dated by cosmogenic exposure methods using 10Be (Gosse et al., 1995). These results generally suggest that glaciers reached their Pinedale maxima between 22,000 and 16,500 cal. B.P. (ca.18,400 and 13,800 14C B.P.). Results from the northern side of the Wind River Range generally confirm the results from Fre-

mont Valley (Phillips et al., 1997). In the Colorado Front Range, three moraines (Winter Park) document the Pinedale glacier maximum, and although it is believed they formed after ca. 30,000 14C B.P. (Nelson et al., 1979), their precise ages are not known.

Late Wisconsin moraines in the Sierra Nevada of California are commonly described as those of the Tenaya (older) and Tioga (younger) glaciations (Porter et al., 1983; Fullerton, 1986). Bursik and Gillespie (1993)

FIGURE 2 Summary of radiocarbon ages bracketing the Marine Isotope Stage 2 (MIS 2)glacial maxima in the American Cordillera. The shaded bar indicates the CLIMAP (Climate: Long-Range Investigation, Mapping, and Prediction) (1981) last glacial maximum (LGM) at 18,000 14C B.P. The arrows indicate minimum ages for glaciation. Based on data from Porter et al. (1983), Hamilton and Thorson (1983), and Mann and Peteet (1994) for Alaska; from Porter et al. (1983) and Richmond (1986) for the southern Canadian Rockies, the Puget lowland, and the Cordilleran ice sheet; from Pierce (1979), Sturchio et al. (1994), Zreda and Phillips (1995), Gosse et al. (1995), Phillips et al. (1997), and Nelson et al. (1979) for the Rockies; from Phillips et al. (1990, 1996), Bur-sik and Gillespie (1993), and Benson et al. (1996) for the Sierra Nevada; from White (1986) for the Ajusco Volcano and the La Malinche and Nevado La Toluca Volcanoes (as interpreted from Heine 1988); from Thouret et al. (1996) and Helmens et al. (1997) for Colombia; from Clapperton (1998) and Clapperton et al. (1995) for Ecuador; from Rodbell (1993), Mercer and Palacios (1977), and Goodman (1999) for Peru; from Seltzer (1994) and Clayton and Clapperton (1997) for Bolivia; from Denton et al. (1999) for the Chilean Lakes Region; and from Clapperton et al. (1995) for the Strait of Magellan.

FIGURE 2 Summary of radiocarbon ages bracketing the Marine Isotope Stage 2 (MIS 2)glacial maxima in the American Cordillera. The shaded bar indicates the CLIMAP (Climate: Long-Range Investigation, Mapping, and Prediction) (1981) last glacial maximum (LGM) at 18,000 14C B.P. The arrows indicate minimum ages for glaciation. Based on data from Porter et al. (1983), Hamilton and Thorson (1983), and Mann and Peteet (1994) for Alaska; from Porter et al. (1983) and Richmond (1986) for the southern Canadian Rockies, the Puget lowland, and the Cordilleran ice sheet; from Pierce (1979), Sturchio et al. (1994), Zreda and Phillips (1995), Gosse et al. (1995), Phillips et al. (1997), and Nelson et al. (1979) for the Rockies; from Phillips et al. (1990, 1996), Bur-sik and Gillespie (1993), and Benson et al. (1996) for the Sierra Nevada; from White (1986) for the Ajusco Volcano and the La Malinche and Nevado La Toluca Volcanoes (as interpreted from Heine 1988); from Thouret et al. (1996) and Helmens et al. (1997) for Colombia; from Clapperton (1998) and Clapperton et al. (1995) for Ecuador; from Rodbell (1993), Mercer and Palacios (1977), and Goodman (1999) for Peru; from Seltzer (1994) and Clayton and Clapperton (1997) for Bolivia; from Denton et al. (1999) for the Chilean Lakes Region; and from Clapperton et al. (1995) for the Strait of Magellan.

assigned an age of ca. 26,400 14C B.P. to the Tenaya moraine stage, implying that the maximum glacier extent in the Sierra Nevada had occurred well before the CLIMAP LGM. The Tioga glaciation is generally assumed to have peaked at ca. 22,000-21,000 14C B.P.

(Fullerton, 1986; Gillespie and Molnar, 1995). Phillips et al. (1996) revised this chronology using 36Cl analysis of boulder surfaces to determine four ages of MIS 2 glaciation: Tioga 1, 33,000 36Cl B.P. (29,000 14C B.P.); Tioga 2, 26,000 36Cl B.P. (22,000 14C B.P.); Tioga 3, 20,000 36Cl B.P.

(17,000 14C B.P.); and Tioga 4, 16,000 36Cl B.P. (13,000 14C B.P.).

A good proxy record for glacier advances in the Sierra Nevada comes from analyses of lake sediments taken from Owens Lake, which lies in an intermontane valley immediately north of the range (Benson et al., 1996). Magnetic susceptibility variations in the sediments document intervals when glaciers expanded in the adjacent mountains and delivered rock flour in meltwater to Owens Lake. Twenty-six accelerator mass spectrometry (AMS) radiocarbon dates constrain the sediment accumulation rate, and, from this, it is evident that brief glacier advances occurred at ca. 27,000 and 25,00014C B.P. (Tenaya stages?) prior to a prolonged interval of expanded ice cover at ca. 23,000-15,000 14C B.P. (Tioga glaciation).

11.2.1. Conclusion for North American Ranges

The best chronological data for North American ranges (Fig. 2) suggest that maximum glacier expansion during MIS 2 culminated in most places between ca. 22,000 and 19,000 14C B.P. After a cool interstade, when glaciers receded an unknown distance, many re-advanced by 50-80% of their earlier extents at ca. 15,000 14C B.P. Exceptions to this pattern include the continuous deglaciation in coastal Alaska from ca. 16,000 14C B.P., the Vashon advance of the Cordilleran ice sheet in which western outlets reached their greatest Wisconsin limits, and the suggestion that glaciers in the Sierra Nevada may have reached their maximum extents somewhat earlier in MIS 2 at ca. 26,400 14C B.P.

Some chronology for late Quaternary glacier advances has been obtained for the Mexican volcanoes. Although 10 of the volcanic massifs supported glaciers during MIS 2, the best dated sequences are on Nevado de Toluca, Ajusco, Iztaccíhuatl, La Malinche, and Pico de Orizaba. A sequence of five glacial stages identified on Iztaccíhuatl was proposed as the standard for México and was correlated with the pre-Bull Lake, Bull Lake, and Pinedale stages in the Rocky Mountains of North America (White and Valastro, 1984). White (1986) presented a correlation chart for Mexican glaciations in a synthesis that also included the work of Heine (1984). Although few radiocarbon dates constrain the ages of the proposed glacier advances, the most extensive occurred between ca. 27,200 and 25,100 14C B.P. and was followed by a less extensive glacial advance; a third advance was assigned to the interval of ca.

15,100-11,400 14C B.P. Heine (1988) disagreed with White's scheme, and, on the basis of mapped moraine limits, marker pumice beds, and radiocarbon dating at five of the volcanoes, he concluded that the only MIS 2 glacier advance in México occurred at ca. 12,000 14C B.P. He assigned glacial maximum moraines to the interval of ca. 36,000-32,000 14C B.P. The uncertainty raised by this mismatch of opinions, and the unusual sequence proposed by Heine, will not be resolved until further detailed work is undertaken at these glaciated volcanoes. However, recent unpublished work on Iztaccíhu-atl using 36Cl dating suggests that at least two MIS 2 glacial maxima occurred between 20,000 and 14,000 36Cl B.P. (Phillips, personal communication; Vázquez-Selem and Phillips, 1998), which appears to support the prior work of White.

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