CONICET | Buscador de Institutos y Recursos Humanos

Glaciar Perito Moreno (hereafter GPM; 50°30´S and 72°50´W) is a major outlet glacier fromthe South Patagonia Icefield. In contrast to most Patagonian glaciers, GPM has been advancing or remained stable during the 20th and 21 st centuries. The advances periodically block the Canal de los Tempanos creating an ice dam that impounds the Brazo Sur-Rico arms of Lago Argentino, raising their elevation, until the water is released in a major outburst flood. Historical and documentary evidence indicates that GPM reached the coast of Península de Magallanes for the first time in 1917. However, the first major damming and rupture event occurred in 1936, when the flooding of the shorelines of Brazo Sur-Rico killed thousands of Nothofagustrees, some still standing dead today. Naturalists who visited the area before 1936 described dense forests extending downslope to the lake shoreline, confirming the evidence of photographs from 1899-1928 displaying no standing dead trees by the lake shores. Estimates of the water level in Brazo Sur-Rico during each glacier-damming vary, but indicate values of 10-12 m for the 1936-1948 events and maxima of 23.5 m for the 1954-56 ice dams. There is evidence for 21 ice dams between 1936 and 2013 with an average interval of ca. 4 years from 1936-1988 and ca. 2 years between 2004-2012. However, ring counts from standing dead trees along the formerly flooded shorelines indicate that the oldest trees killed by drowning were at least 250 years old providing evidence of no damming and rupture events from ca 1650 to 1936. These data indicate that damming and outburst events by GPM, world-renowned processes, did not occur for a long-term period before 1936. Unlike many other glaciers in Patagonia, GPM has continued advancing or remain stable during the 20 th and 21 st centuries, and was less extensive in the 1700s and 1800s than it is today.