EARLY LAST GLACIAL MAXIMUM IN THE TROPICAL ANDES

Unresolved issues related to the last ice age in the tropics include the magnitude of changes in terrestrial temperatures, the relative importance of temperature versus effective moisture and radiation balance in promoting ice accumulation, and the timing of the last glacial maximum (LGM).  The question of timing underpins all paleoclimate reconstructions for the tropics.  A definitive chronology for the LGM has been elusive, in large part due to the limitations of radiocarbon dating and the scarcity of datable material in high-altitude glacial environments.  We present an absolute chronology for the LGM in the tropical Andes based on cosmogenic dating (¹⁰Be) of glacial erratics on moraines.  The LGM in the Junin region of the central Peruvian Andes (11°S 76°W) and in the Cordillera Real of Bolivia  (16°S 68°W) occurred ~34,000 to 22,000 ¹⁰Be years before present (BP) and was less extensive than older glaciations.  Recession from LGM moraines ~21,000 ¹⁰Be years BP was followed closely by stillstand until ~15,000 ¹⁰Be years BP, when final deglaciation began.  Our terrestrial glacial chronology is consistent with lacustrine paleoclimate records from nearby lakes: Lake Junin (11°S 76°W) on the Junin Plain of Peru and Lake Titicaca (16°S 69°W) on the Peruvian-Bolivian Altiplano.  LGM snowline depression was 300-600 m, except where local factors such as shading and debris cover produced glacier expansion to lower elevations.  Regional evidence for a wet LGM in the tropical Andes suggests that limited temperature depression, rather than lack of moisture, constrained the size of LGM glaciers.  Asynchrony between the LGM in the Northern Hemisphere (~21,000 years BP) and the tropical Andes (~34,000-22,000 years BP) implies decoupling of climate forcing mechanisms and the need to reexamine climate models based on assumptions of a globally synchronous LGM.