A chronology of the Little Ice Age in the tropical Andes of Bolivia (16 degrees S) and its implications for climate reconstruction

摘要

Dating moraines by lichenometry enabled us to reconstruct glacier recession in the Bolivian Andes since the Little Ice Age maximum. On the 15 proglacial margins studied, we identified a system of tell principal moraines that marks the successive positions of glaciers over the last four Centuries. Moraines were dated by perl'onning statistical analysis of lichen rneaSUrenients based on the extrerne values theory. Like glaciers ill many mid-latitude mountain areas, Bolivian glaciers reached their maximal extent during the second half of the 17th century. This glacier maximum coincides with the Maunder minimum of solar irradiance. By reconstructing the equilibrium-line altitude and changes in mass-balance, we think the glacier maximum may be due to a 20 to 30% increase in precipitation and a 1.1 to 1.2 degrees C decrease in temperature compared with present conditions. In the early 18th century, glaciers started to retreat at varying rates until the late 19th to early 20th century; this trend was generally associated with decreasing accumulation rates. By contrast, glacier recession in the 20th century was mainly the consequence of an increase in temperature and humidity. These results are consistent with observations made ill the study region based on other proxies. (C) 2008 University of Washington. All rights reserved.