SNOWLINE DEPRESSION IN THE TROPICAL ANDES AT THE LOCAL LAST GLACIAL MAXIMUM

Our study of deglaciated valleys in the eastern cordillera of the Peruvian and Bolivian Andes confirms the presence of moraines from the local last glacial maximum and from several older glaciations. Cosmogenic dating (¹⁰Be and ²⁶Al) of boulders on moraines in valleys bordering the Junin Plain in the central Peruvian Andes (ca. 11° S, 76° W, altitude 4100 m) and in the Cordillera Real of the Bolivian Andes (ca. 16° 16' S, 68° 9' W, altitude 4600 m) indicates that the local last glacial maximum likely occurred during oxygen isotope stage 2, but prior to the inferred peak of Northern Hemisphere continental glaciation (approximately 21,000 calendar years BP). The moraines deposited by older glaciations in the study areas are substantially larger and extend farther downvalley than those of the local last glacial maximum. We have been studying snowline depression at the last glacial maximum in tropical South America. Modern snowline in the region has been estimated from remaining small ice caps and geomorphic evidence at approximately 4800-5000 m. Valleys in the study area bordering the Junin Plain are relatively shallow, having a maximum difference in altitude between headwall and mouth of about 700 m (Alcacocha Valley). This shallow profile constrains snowline depression. Our estimate of the snowline at the local last glacial maximum in Alcacocha Valley is approximately 4600 m, indicating a relatively small snowline depression of about 200-400 m. Estimates of snowline at the last glacial maximum elsewhere in tropical South America range from about 300 m to more than 1000 m, but the related deposits typically have not been dated directly. Few, if any, moraines in tropical South America have been tightly constrained in age to the last glacial maximum of the Northern Hemisphere, or even to a local last glacial maximum. Our results, which include direct dating of moraines from the local last glacial maximum and from previous glaciations, make a fundamental contribution to paleoclimate reconstruction efforts in the tropics.