COSMOGENIC DATING OF GLACIATION IN THE PERUVIAN ANDES: >400 10BE KA TO LAST GLACIAL MAXIMUM

In glacial valleys bordering Lake Junin in the central Peruvian Andes, moraines deposited during the last glacial maximum (LGM) are smaller and much farther up valley than well-preserved lateral moraines deposited during previous glaciations. Cosmogenic dating (¹⁰Be) of four moraines in the Alcacocha Valley (S 11^o 03', W 75^o 58', elev. 4300 m, length ~14 km) indicates that the most extensive moraines are at least 400,000 years old. In contrast, the LGM is represented mainly by a low, broad terminal moraine located approximately 7 km up valley and 200-250 m higher than the limit of glaciation in the Alcacocha Valley. Within the LGM limit, a recessional moraine dams Laguna Alcacocha. The median ¹⁰Be date on the recessional moraine is 17+/-0.4 ka, which is consistent with an end to the LGM in the region at 21 k cal yr BP based on the sedimentary record in Lake Junin. Down valley of the LGM limit, larger lateral moraines date from approximately 240 to more than 400 ¹⁰Be ka. An overall picture emerges of a relatively minor glaciation in the tropical Andes during the LGM, following at least three previous larger glaciations. The pattern is repeated in neighboring valleys. This relationship between the LGM moraines and older moraines is in contrast to the SPECMAP oxygen isotope record, which indicates that global ice buildup during the LGM was similar in magnitude to that of previous glacial periods. These findings invoke a mechanism for maintaining a relatively reduced ice extent in the tropics while allowing significant ice build-up in the high latitudes during the LGM. One possible mechanism might be the disruption of ocean circulation patterns such that the transfer of warmer ocean water from the tropics to the poles was impeded, preventing the tropics from cooling to the same extent as higher latitude regions.