| Summary: | The relict moraines in Argentine Patagonia archive major expansions of the Patagonian Ice Sheet throughout the Quaternary, and are one of the few terrestrial climate proxies in the middle latitudes of the Southern Hemisphere that extends beyond the last glacial cycle. Determining their individual ages has proved challenging but has important implications for our understanding of terrestrial climate change in southern South America over the duration of the Quaternary. Here, for the first time, we demonstrate that sediment on outwash terraces can be directly dated to determine the timing of early–middle Quaternary glacial advances in southern South America. Cosmogenic , 10 Be and 26 Al surface exposure ages were obtained from outwash gravels associated with two of the oldest glacial sequences in the Lago Pueyrredón valley (47.5°S), Argentina. The outermost Gorra de Poivre glacial sequence marks the greatest extent of the Patagonian Ice Sheet. A cobble from this surface gives 10 Be and 26 Al surface exposure ages of ca. 1.2 Ma, consistent with bracketing 40 Ar/ 39 Ar age constraints obtained elsewhere in Patagonia. This is the first time early Pleistocene glacial surfaces have been directly dated in Patagonia. Cobbles on a younger Cañadon de Caracoles outwash terrace give exposure ages of ca. 600 ka, while 4 of 5 boulders on an associated moraine give exposure ages that are significantly younger. If the demonstrated stability of outwash terraces in the valley is common throughout the region, it will be possible to extend Patagonian glacial chronologies as far back as the early Pleistocene.
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