Cosmogenic age constraints on the last deglaciation in Southern Patagonia (49 - 50°S)

The glacial-to-interglacial transition since the Last Glacial Maximum (LGM) reflects climate system dynamics and the mechanisms that force climate change. Numerous studies of glacier behavior, mainly in the Northern Hemisphere, have sought to delineate the LGM termination as well as post-LGM climate reversals. We know far less about the Southern Hemisphere, where the extent and timing of climate shifts, and the degree of linkage to changes in the Northern Hemisphere, remain uncertain. Quantifying the temperature and precipitation changes across southern South America can help assess the migration of precipitation patterns tied to the position of the Southern Westerlies and the Antarctic Polar Front. This data can also be used to detect the extent of temperature reversals across and between continents, such as the Antarctic Cold Reversal, if present. The testing of hypotheses regarding changes in global atmospheric and oceanic circulation since the LGM is partly dependent on the deconvolution of these two parameters. Glacial activity in the Southern Patagonian Icefield is highly sensitive to changes in temperature and precipitation, while the adjacent Andean rainshadow desert preserves a remarkably complete set of glacial deposits that span a wide transect of the southern mid-latitudes. These deposits are ideal for surface exposure dating, which in turn can provide a proxy for glacial activity and for changes in the factors that govern glacial mass balance. We use a new beryllium-10 production rate calibrated to the southern mid-latitudes to recalculate published surface exposure chronologies from Patagonia (46 to 53°S). The revised temporal framework indicates the presence of expansive ice from ~30 ka until regional retreat 17-18 ka, with a significant late-glacial re-advance or still-stand ending ~13 ka. This latitudinal transect is notable for a gap in glacial records between 49 and 50°S. New surface exposure age determinations from the Santa Teresita moraine along the western shores of the large outlet valley of Lago Viedma, and from several moraines within the narrow pre-Cordilleran Rio Guanaco mountain valley begin to fill this gap. Initial results from a moraine in Rio Guanaco valley indicate ice retreat at ~18 ka, correlative with final LGM moraines at Lago Buenos Aires and Lago Pueyrredon to the north (46 - 47°S) and to the south near Torres del Paine and the Straits of Magellan (51 - 53°S). Exposure ages for the Santa Teresita moraine indicate ice retreat at ~13 ka, coincident with the well-dated Puerto Banderas moraine (Ackert et al., 2008, Science) in the Lago Argentino outlet valley to the south. These surface exposure chronologies are consistent with other regional records. Moreover, the emerging chronology is consistent with results of a fully-coupled transient general circulation model that predicts minimal deglacial precipitation change with significant temperature and equilibrium line altitude lowering during the last deglaciation at 50°S near 14 ka, followed by resumed warming from ~13.5 ka into the Holocene. These findings suggest that unlike more northerly portions of Patagonia, the Antarctic Cold Reversal impacted the Andes as far north as 50°S.