Late Glacial and Holocene glacial history of Monte San Lorenzo, Central Patagonia, Chile.

Abstract

Many glaciated areas around the world underwent periods of renewed glacial activity during the Last Glacial-Interglacial Transition (LGIT), and later, during the Holocene. Even though important efforts have been made, the causes and mechanisms underlying these glacial fluctuations are still being debated. In Southern South America most of the record and understanding of former glacial fluctuations comes from studies of the outlet glaciers from the Patagonian Icefields. This ongoing research will reconstruct Late Glacial and Holocene history of the Monte San Lorenzo area (47°35’S; 72°19’W), an isolated granitic massif that supports several small glaciers that were independent of outlet glaciers from the two major icefields. This study focuses on Río Tranquilo glacier, located on the northern flank of Monte San Lorenzo. Geomorphic evidence indicates that during times of major glacial expansion, Río Tranquilo glacier flowed more than 8 km downvalley, depositing a series of moraine complexes at former maximum positions. We identify 4 distinctive groups of moraines, which together contain at least 10 moraine complexes. Several techniques have been used to develop a preliminary chronology of glacier fluctuations of Rio Tranquilo glacier spanning at least the last ~11 kyr. Detailed dendrochronological dating indicates that the innermost moraine complex was deposited during the last 400 years. Almost 3 and 4 km downvalley, preliminary basal radiocarbon dates obtained from pits dug in intermorainal depressions yield minimum ages of ~6500 14C yr BP (2 dates) and ~9.200 14C yr BP (2 ages) for the deposition of the fifth and ninth moraine complexes, respectively. Cosmogenic samples, currently being processed will provide additional dating control for these and several other moraine complexes. We expect to develop a complete chronology of the Late Glacial and Holocene glacier history in this valley and one of the most detailed chronologies available for Southern South America.