Lateglacial and Holocene cosmogenic surface exposure age glacial chronology and geomorphological evidence for the presence of cold-based glaciers at Nevado Sajama, Bolivia

摘要

Comparisons of palaeo-equilibrium line, altitudes between the Western and Eastern Cordilleras in the Central Andes are commonly based on the assumption that the tall outermost moraines visible in remotely sensed images of the Western Cordillera date to the Last Glacial Maximum (LGM). However, field investigation and geomorphic mapping at Nevado Sajama, Bolivia, indicates the tall moraines are relic features with shorter moraines overlying and in some cases extending beyond them. Cl-36 exposure ages from the shorter moraines suggest that they date to Lateglacial times ca. 16.9-10.2 ka. Although Lateglacial deposits have been found throughout the Central Andes, the extent of these, deposits relative to LGM deposits varies both between the Western and Eastern Cordilleras and north-to-south along the Western Cordillera. In the Western Cordillera in the zone of easterly winds, the Lateglacial appears to be the most extensive glacial advance of the last glacial cycle. Geomorphic evidence also suggests that some Lateglacial moraines were deposited by cold-based ice, a previously unreported finding in the tropical Andes. Retreat from other glacial features occurred at about 7.0-4.4 ka and 4.7-3.3 ka. These are the first directly dated Holocene glacial deposits in the Western Cordillera of Bolivia, and their presence suggests that the mid Holocene may not have been as warm and dry as previously thought. Copyright