The timing and magnitude of mountain glaciation in the tropical Andes

摘要

The Andes of Ecuador, Peru and Bolivia host the majority of the world's tropical glaciers. In the tropical Andes, glaciers accumulate during the wet season (austral summer) and ablate year-round. Precipitation is delivered mainly by easterlies, and decreases both N-S and E-W. Chronological control for the timing of glacial advances in the tropical Andes varies. In Ecuador, six to seven advances have been identified; dating is based on radiocarbon ages. Timing of the local Last Glacial Maximum (LGM) and the existence of Younger Dryas advances remain controversial. in Peru, local variability in glaciation patterns is apparent. Surface exposure dating in the Cordillera Blanca and Junin Plain suggests that the local LGM may have been early (similar to 30 ka), although uncertainties in age calculations remain; the local LGM was followed by a Lateglacial readvance/stillstand and preceded by larger glaciations. In contrast, preliminary data from an intervening massif indicate that the largest moraines are Lateglacial. Chronologies from Bolivia also suggest local variability. In leeward Milluni and San Francisco Valleys, local LGM moraines descend to similar to 4300 m above sea level (a.s.l.), whereas in windward Zongo Valley Lateglacial moraines reach similar to 3400 m a.s.l. Atlantic and Pacific sea surface temperatures, El Nino-Southern Oscillation and insolation changes all likely play roles in mediating tropical Andean glacial cycles.