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FIGURE 7.1 5 Summary diagram showing the principal episodes of glaciation in the northern and southern hemispheres over the past ~3 M years based on glacial geologic studies in the various regions indicated. Ice advances are indicated schematically by an upward pointing triangle, the relative dimensions of which signify the magnitude of each ice advance. Note that timescale (shown at top) is nonlinear; marine oxygen isotope stages are indicated at bottom with even-numbered stages (times of major ice accumulation on the continents) shaded (however, note that not all such stages are equal in magnitude — see section 6.3.2). Arrows indicate dating uncertainties; in spite of these, numerous episodes of globally significant ice advances are clearly identifiable. I = U.S. Cordilleran ice sheet; 2 = U.S. mountain glaciers; 3 = U.S. Laurentide ice sheet; 4 = Canadian Cordilleran ice sheet; 5 = Canadian Laurentide ice sheet (a = SW margin, b = NW margin); 6 = N.E. Russia; 7 = Poland/western (former) Soviet Union; 8 = N.W. Europe; 9 = European Alps; 10 = Southern Andes; 11 = New Zealand; 12 = Tasmania; 13 = Southern Ocean and sub-Antarctica; 14 = Antarctica (Ross Embayment); IS = New Guinea; 16 = E. Africa (simplified from charts accompanying Bowen et al„ 1986; Clapperton, 1990).

FIGURE 7.16 Summary of glacier expansion phases in different areas of the world during the Holocene. This compilation shows the complexity of the records and the difficulty of discerning worldwide synchronous episodes on this timescale (possible times of widespread advances are indicated by black bars at bottom of fig-ure).This difficulty may be due to climatic fluctuations that are regional, not hemispheric or global in extent, or to poor dating, or to problems inherent in a discontinuous and incomplete data set. A general absence of glacier advances in the early to mid-Holocene is apparent, as is the onset of Neoglaciation after ~5000 yr B.P. (Grove, 1988).

1 Alaska _

2. Baffin

3. N. Scandinavia

5. Colorado FR Himalaya

7. New Guinea

6. Patagonia 9 New Zealand

FIGURE 7.16 Summary of glacier expansion phases in different areas of the world during the Holocene. This compilation shows the complexity of the records and the difficulty of discerning worldwide synchronous episodes on this timescale (possible times of widespread advances are indicated by black bars at bottom of fig-ure).This difficulty may be due to climatic fluctuations that are regional, not hemispheric or global in extent, or to poor dating, or to problems inherent in a discontinuous and incomplete data set. A general absence of glacier advances in the early to mid-Holocene is apparent, as is the onset of Neoglaciation after ~5000 yr B.P. (Grove, 1988).

earlier glacial events was often destroyed or buried. This has made worldwide correlations very difficult, compounded by the dating difficulties discussed earlier. Hence, whether there were globally synchronous glacial episodes is somewhat unclear. Certainly there were many periods of alpine glacier expansion throughout the Holocene (see Fig. 7.16) and these varied in magnitude from one area to another. However, on the basis of current evidence, there is little support for the notion that Holocene glacier fluctuations were synchronous throughout the world (Grove, 1979; Rothlisberger, 1986; Wigley and Kelly, 1990) and even less for a 2500-yr periodicity in mountain glacier fluctuations (as proposed by Denton and Karlen, 1973a). Until more detailed studies are carried out (along the lines of those by Patzelt [1974] and Rothlisberger [1976] in the European Alps) perhaps supplemented by the continuous time series offered by palynological and lake sediment studies (Karlen, 1981; Burga, 1988; Leeman and Niessen, 1994b), it seems probable that the record of glacier fluctuations in many areas will remain incomplete.

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