For the Colombian Andes, a radiocarbon-dated framework of pre-Late-glacial maximum was presented by Helmens et al (1997). During each of three pre-LGM advances, glaciers extended beyond the LGM. In other parts of the Andes, in New Zealand, and in some Eurasian mountain ranges, glaciers advanced farther earlier in the last glacial cycle than at the LGM (Gillespie and Molnar, 1995; Bon-darev et al, 1997; Clapperton et al, 1997; Fitzsimons, 1997; Clapperton, 1998). The interval of greatest glacier extent is not dated, but it is commonly assumed to correspond to the cooling recorded in isotope stage 4, with an approximate duration of 10,000 years. As a comparison, isotope stage 2 lasted for about 16,000 years, during which the northern hemisphere ice sheets grew to their greatest extent. Assuming that sea-surface temperatures were not as low during isotope stage 4 as during isotope stage 2, but that precipitation was generally higher, it is conceivable that local glaciers grew to their maximum extent during isotope stage 4 (Clapperton, 1997).
The timing and extent of local glaciers during the LGM are uncertain and variable, and the concept of a single LGM is misleading
(Clapperton, 1997). For the Colombian Andes, Helmens et al. (1997) reported advances at 23,900-19,500yrbp, and between 18,000 and 15,500yrbp. Glaciers in the more arid Andes and Bolivia, on the other hand, reached their greatest extent after ca. 13,300 yr bp (Clapperton et al, 1997). In southern Chile, Lowell et al (1995) dated glacier advances at 23,000, 21,000 and 17,000yrbp. In New Zealand, the LGM glaciers reached their maximum at 23,30018,000 yr bp (Fitzsimons, 1997). In the European Alps, glacier expansion to the Wiirm (LGM) maximum occurred after 24,000yrbp (van Husen, 1997).
In some regions, local glaciers underwent glacier readvance after the LGM and prior to the 13,500yrbp warming. In the European Alps, a glacier advance (Gschnitz) culminated at ca. 16,000-14,000yrbp (van Husen, 1997). In Scotland, the Wester Ross Advance occurred between 17,000 and 12,800yrbp (Benn, 1996; Ballantyne, 1997) and may have been contemporaneous with Heinrich layer 1 in the North Atlantic at 15,000-14,000 yr bp (Bond and Lotti, 1995). In the Southern Alps of New Zealand, glaciers advanced at ca. 15,00014,000 yr bp (Fitzsimons, 1997) and this advance was probably simultaneous with a glacier advance in southern Chile (Clapperton, 1995), with a maximum extent at 14,900-13,900 yr bp (Lowell et al, 1995). Glaciers in SE Peru experienced their maximum isotope stage 2 positions at 14,000-13,900yrbp (Mercer and Palacios, 1977; Clapperton, 1993). In the interval 18,000-13,500 yr bp, many glaciers seem to have advanced between 14,500-13,500 yr bp, and this readvance was terminated by abrupt global warming at 13,500 yr bp.
The interval between 13,500 and 10,000 yr bp was characterized by rapidly receding glaciers as a response to global warming, and most of the main valleys in Scotland, the Alps, southern Chile and New Zealand were deglaciated by ca. 13,000yrbp (Sutherland, 1984; Porter, 1981b; Lowell et al, 1995). In Columbia and Bolivia, however, Helmens et al (1997), Clapperton (1995) and Clapperton et al (1997) reported the expansion of local glaciers between 13,500 and 12,500 yr bp. A cooling at around 12,500yrbp is reflected in North
Atlantic pollen diagrams (Levesque et al, 1993; Lowe et al, 1995), in glacier expansion in the Alps (the Daun Stadial) and Scandinavia (the Older Dryas stadial), and in Greenland ice cores (Alley et al, 1993). Between 11,000 and 10,000yrbp, the North Atlantic region in particular was plunged into a severe cooling (the Younger Dryas). Throughout Europe local glaciers readvanced, and outlet glaciers along the western margin of the Scandinavian ice sheet expanded by tens of kilometres (Mangerud, 1991). In recent literature, the question of whether the Younger Dryas event was global or not has been a continuous issue. Compared with the evidence presented from Europe, however, the amplitude of glacier expansion outside the amphi-Atlantic region was small. However, in the Colorado Front Range, Menounos and Reasoner (1997) found evidence in lake sediments for a Younger Dryas glacier episode.
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