As the Andes extend for more than 7000 km between latitudes 100N and 550S, they cross most of the zonal climate belts between equatorial and cool temperate regions of an entire hemisphere. Because mountain glaciers and ice caps formed throughout this mountain transect during the last glacial cycle (Clapperton, 1983, 1993a,b), chronologies of morainic deposits obtained in key parts of the Andes provide an opportunity to examine the interhemispheric synchrony of glacier advances and, thus, climate change (see, e.g., Lowell et al.,
1995). Although spectacular moraines are present at all latitudes, only a few sequences have a series of radiocarbon ages to constrain the timing of glaciation (e.g., Helmens, 1988; Helmens et al., 1996, 1997; Thouret et al., 1996; Lowell et al., 1995; Clapperton et al., 1995; Denton et al., 1999).
Radiocarbon ages for MIS 2 glacier advances in the northern Andes were obtained in the Ruiz-Tolima volcanic massif of the Central Cordillera (Thouret et al.,
1996) and in the currently ice-free ranges south of Bogotá in the Eastern Cordillera (Helmens et al., 1997). Although the advances are constrained by rather broad age ranges, it is apparent that the maximum MIS 2 advance occurred between ca. 23,500 and 19,500 14C B.P. The glaciers reached more extensive limits earlier in the last glacial cycle, prior to MIS 2 (Helmens et al., 1996). Farther south in Ecuador, an outlet glacier from ice fields covering the Eastern Cordillera close to the equator advanced eastward down the Papallacta Pass on two different occasions at some time after ca. 31,000 14C B.P., but before ca. 13,200 14C B.P., leaving moraines at altitudes of 3000 and —3250 m. Last glacial maximum moraines in the Andes of north-central Peru (Cordillera Blanca) were deposited before ca. 19,700 14C B.P. (Rod-
bell, 1993), but in southeast Peru the maximum extent of glaciation occurred before ca. 41,000 14C B.P. (Mercer and Palacios, 1977; Goodman, 1999). The next most extensive advance was not until shortly after ca. 13,900 14C B.P. in southeastern Peru and reached less than half the distance of the earlier event. These data imply that glaciers in this part of the Andes did not advance very far during the CLIMAP LGM. One site in the Eastern Cordillera of Bolivia does suggest that the last glacial maximum was attained prior to 20,000 14C B.P. (Seltzer, 1994). However, it appears that glaciers were small in many areas of the Eastern Cordillera of Bolivia during the LGM and may not have advanced to their maximum MIS 2 limits until the interval of ca. 14,000-12,000 14C B.P. (Seltzer, 1992; Clapperton et al., 1997). This inference is compatible with evidence from the Cerro Azanaque massif, which rises to 5200 m above the salt basins of Poopo and Uyuni in the southern Altiplano, showing that the maximum MIS 2 advance impacted peat beds at about 13,000 14C B.P. Meltwater from this advance discharged sediment to form Gilbert-type deltas in a high stand of paleolake Tauca (Clayton and Clapperton, 1997).
Although small glaciers may have formed in the currently arid region between latitudes 18° and 29°S, where modern glaciers do not exist (Messerli et al., 1993; Jenny and Kammer, 1996), Quaternary glaciers seem to have been absent south of here to about latitude 32°S because of arid conditions. South of 33°S, however, sharp-crested moraines are present in most drainages all the way to Tierra del Fuego, but the only well-dated sequences are in the Chilean Lake District (and adjacent Isla de Chiloe) and along the central Strait of Magellan (Porter, 1981; Porter et al., 1992). The remarkably detailed morphological, stratigraphical, and chronological investigation of last glacial deposits around Lago Llanquihue and the marine embayment of Seno Relon-cavi has revealed that outlet glaciers from the Andean ice cap advanced at least four times during MIS 2 (Lowell et al., 1995; Denton et al., 1999). More than 450 radiocarbon dates underpin the current conclusion that these advances culminated at ca. 29,400, 26,800, 22,400, and 14,600 14C B.P., all reaching limits within the ca. 5-to 10-km wide Llanquihue moraine belt. Whereas the Llanquihue ice lobe was most extensive at ca. 22,400 14C B.P., the Reloncavi lobe farther south, which advanced onto Isla de Chiloe, reached its maximum limit later at ca. 14,600 14C B.P., a pattern reminiscent of that in the South Canadian Rockies-Puget lowlands area of North America.
The southernmost part of the conterminous Pata-gonian ice cap drained to the east and north as large lo-bate outlet glaciers (Caldenius, 1932). The lobe that flowed along the central Strait of Magellan terminated its maximum Stage 2 advance at Peninsula Juan Mazia, some 180 km from the ice divide (Porter et al., 1992; Clapperton et al., 1995; Anderson and Archer, 1999). The radiocarbon ages of marine shells in deformation till associated with the outermost moraine are in the range of ca. 27,700-44,000 14C B.P. This range indicates a long marine interval in the Strait of Magellan during MIS 3 and implies extensive deglaciation early in MIS 2. A minimum age for the maximum advance is given by AMS radiocarbon dates of ca. 25,200 and 23,600 14C B.P. on organic matter from the base of an interdrum-lin hollow within the moraine limit. Following a brief interval of recession, during which ice-marginal and proglacial lacustrine and outwash sediments accumulated, the Magellan glacier re-advanced over these deposits to limits approximately 2-3 km short of the maximum advance. This event is constrained by a minimum radiocarbon age of ca. 17,700 14C B.P. from basal organic matter within the moraine limit. The Magellan glacier then receded an unknown distance from this re-advance limit, but not far enough south to allow penetration of Pacific seawater into the strait. A subsequent advance formed a complex of bouldery moraines along the very edge of both sides of the central Strait of Magellan and terminated about 25 km short of the earlier limits. This event is constrained by minimum radiocarbon dates of ca. 14,000-13,500 14C B.P. from the base of kettle holes inside the moraine limit (see McCulloch et al., 2000).
It is evident that at least three advances occurred between ca. 27,000 and 4500 14C B.P., reaching almost to the same limits. Advances in Colombia and along the Strait of Magellan are bracketed by radiocarbon dates that imply they may have occurred coevally with advances dated in the Chilean Lake District to ca. 26,800, 22,400, and 14,800 14C B.P. Evidence for additional advances has not yet been observed, other than that dated to ca. 29,400 14C B.P. in the Chilean Lake District. Notable exceptions to this pattern of MIS 2 fluctuations are in southern Peru and Bolivia, where the maximum MIS 2 advance peaked between ca. 14,000 and 12,000 14C B.P.
Was this article helpful?