Middle Pleistocene glaciation in Patagonia dated by cosmogenic-nuclide measurements on outwash gravels

The well-preserved glacial record in Argentine Patagonia offers a similar to 1 Ma archive of terrestrial climate extremes in southern South America. These glacial deposits remain largely undated beyond the range of radiocarbon dating at ca. 40 ka. Dating old glacial deposits (> several 10(5) a) by cosmogenic surface exposure methods is problematic because of the uncertainty in moraine degradation and boulder erosion rates. Here, we show that cobbles on outwash terraces can reliably date 'old' glacial deposits in the Lago Pueyrredon valley, 47.5 degrees S. Argentina. Favorable environmental conditions (e.g., aridity and strong winds) have enabled continuous surface exposure of cobbles and preservation of outwash terraces. The data demonstrate that nuclide inheritance is negligible and we therefore use the oldest surface cobbles to date the deposit. Be-10 concentrations in outwash cobbles reveal a major glacial advance at ca. 260 ka, concurrent with Marine Isotope Stage 8 (MIS 8) and dust peaks in Antarctic ice cores. A Be-10 concentration depth-profile in the outwash terrace supports the age and suggests a low terrace erosion rate of ca. 0.5 mm ka(-1). We compare these data to exposure ages obtained from associated moraines and find that surface boulders underestimate the age of the glaciation by similar to 100 ka; thus the oldest boulders in this area do not date closely moraine deposition. The Be-10 concentration in moraine cobbles help to constrain moraine degradation rates. These data together with constraints from measured Al-26/Be-10 ratios suggest that all moraine boulders were likely exhumed after original deposition. We determine the local Last Glacial Maximum (LGM) occurred at similar to 27-25 ka, consistent with the maximum LGM in other parts of Patagonia. (C) 2009 Elsevier B.V. All rights reserved.