Regional synthesis of last glacial maximum snowlines in the tropical Andes, South America

The modern glaciers of the tropical Andes are a small remnant of the ice that occupied the mountain chain during past glacial periods. Estimates of local Last Glacial Maximum (LGM) snowline depression range from low (e.g., 200-300 m in the Junin region, Peru), through intermediate (600 m at Laguna Kollpa Kkota in Bolivia), to high (e.g., 1100-1350 m in the Cordillera Oriental, Peru). Although a considerable body of work on paleosnowlines exists for the tropical Andes, absolute dating is lacking for most sites. Moraines that have been reliably dated to ∼21 cal kyr BP have been identified at few locations in the tropical Andes. More commonly, but still rarely, moraines can be bracketed between about 10 14C kyr (∼11.5 cal kyr BP) and 30 14C kyr BP. Typically, only minimum-limiting ages for glacial retreat are available. Cosmogenic dating of erratics on moraines may be able to provide absolute dating with sufficient accuracy to identify deposits of the local LGM. Ongoing work using cosmogenic 10Be and 26Al in Peru and Bolivia suggests that the local LGM may have occurred prior to 21 cal kyr BP.