COSMOGENIC DATING OF OLD AND YOUNG MORAINES IN SOUTHERN SOUTH AMERICA AND PALEOCLIMATE IMPLICATIONS

Dateable glacial moraine sequences spanning several glacial cycles are rare. Lago Buenos Aires, 46° S, in southern Argentina, contains one of the oldest most complete moraine records on Earth, and provides unique information to help understand low to high latitude ice-age climates and associated driving factors such as the tropics. ⁴⁰Ar/³⁹Ar and K-Ar dated lava flows interbedded with the glacial deposits provide broad chronological constraints, for the 1 Ma to 15 ka moraine and outwash sequence, and calibration sites for production rates of in situ produced cosmogenic isotopes. ¹⁰Be, ²⁶Al, ³He, ⁴⁰Ar/³⁹Ar, and radiocarbon data constrain the timing of glacial events spanning at least 2 glacial cycles. The distribution and average of cosmogenic nuclide ages for boulders indicate that penultimate glacial moraine complexes formed between ca. 185 and 110 ka and ca. 140 and 125 ka, respectively; these ages are derived with independently determined production and erosion rates using the field area’s ⁴⁰Ar/³⁹Ar and K-Ar dated lava flows. Alternatively, if previously published production rates (Stone, 2000) are used, the distribution and average of cosmogenic ages for the same boulders are between ca. 210 and 110 ka and ca. 155 and 140 ka, respectively. Even considering uncertainties, these ages strongly suggest that the southern South American penultimate glaciation occurred during marine isotope stage 6 (i.e., 190-130 ka). There is no evidence of a stage 4 glacial event around 75 ka. The last major glaciation occurred from ca. 24 to 16 ka, during stage 2. The last two recorded periods of glacial expansion in southern South America coincided with the existence of large Northern Hemisphere ice sheets, despite differences in solar insolation received. Thus, at least on Milankovitch time scales, global atmospheric cooling must override differences in solar insolation and synchronize periods of snowline depression in the middle latitudes of both hemispheres. Ongoing refinements to erosion and production rates will improve further the accuracy of cosmogenic ages and allow additional chronological constraints on Patagonian ice fluctuations, and inferences regarding interhemispheric paleoclimate change, over the last 1 million years.