Modern and last glacial maximum snowlines in the Peruvian-Bolivian Andes

Andrew G. Klein and Bryan Isacks, Dept. Geological Sciences, Cornell University, Ithaca, New York, 14853 USA; and Geoffrey O. Seltzer, Dept. Earth Sciences, Syracuse University, Syracuse, New York, 13244 USA.

The magnitude and pattern of the Last Glacial Maxium (LGM) snowline depression in the Peruvian and Bolivian Andes was mapped using remote sensing and geographic information systems technologies. LGM snowlines in the central Andes were 500-1200 meters lower than at present. Both modern and LGM snowlines rise from the northeast to southwest in response to a regional decrease in precipitation. The modern and LGM snowlines trends are parallel, implying that general atmospheric circulation patterns during the late Pleistocene were similar to those at present. The few available dates constraining the age of the LGM in the central Andes indicate the maximum extent of glaciation occurred prior to 20,000 ¹⁴C yr BP. However, lack of maximum limiting ages on LGM moraines adds considerable uncertainty to the timing.

Snowline modeling demonstrates that snowlines in the eastern and western cordilleras of the Andes exhibit significantly different responses to temperature and precipitation changes. In the eastern cordillera, snowline elevations are presently near the level of the annual 0° C isotherm and melting can occur throughout the year. The long melt period makes snowlines more sensitive to temperature changes than to accumulation changes. In the western cordillera, snowlines are 1000 m higher than in the east due to increasing aridity. At these elevations, the melt period is shortened significantly and consequently snowlines exhibit a stronger sensitivity to accumulation changes than in the eastern cordillera.

The prevalent 1200 m snowline depression along the eastern cordillera can be modeled by a 4-8° C cooling, which is inconsistent with the small (<2° C) cooling in tropical sea surface temperature suggested by CLIMAP. Poor constraints on the timing of the LGM glacial advance, however, do not allow snowline depressions to unequivocally refute CLIMAP sea surface temperature estimates. The 800-1000 m snowline depression observed in the western cordillera cannot be accounted for solely by cooling, but requires that precipitation during the LGM must have been greater than at present.

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