Northeastern Section - 42nd Annual Meeting (12–14 March 2007)

Paper No. 2
Presentation Time: 8:40 AM


DOUGLASS, Daniel C., Earth and Environmental Science, Northeastern University, Boston, MA 02115, SINGER, Bradley S., Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, ACKERT, Robert P., Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, STONE, John O., Department of Earth and Space Sciences, Univ of Washington, Seattle, WA 98195, KAPLAN, Michael R., Geochemistry, LDEO Columbia University, Palisades, NY 10964 and CAFFEE, Marc, Physics, Purdue University, W. Lafayette, IN 47906,

The glacial record near Lago Buenos Aires, Argentina (71.0°W, 46.5°S) spans the last ~1 Ma, and provides a unique opportunity to investigate the relationships between the extent of an outlet glacier of the Patagonian Ice Cap in the southern mid-latitudes and the global ice volume record, which is dominated by the growth and decay of the Northern Hemisphere continental ice sheets. We use cosmogenic nuclide surface-exposure dating of erratic boulders to refine the ages of three Middle Pleistocene moraines. Successful application of the surface-exposure method to deposits older than 200 ka requires excellent preservation of glacial deposits and precise measurements of boulder erosion rates. Nine 10Be/36Cl paired analyses constrain both exposure age and boulder erosion rate. The median boulder erosion rate is 0.2 mm kyr-1 (range = 0.0 to 4.6), comparable to erosion rates estimated for other hyper-arid and semi-arid regions. This erosion rate is applied to an additional nine 10Be or 10Be/26Al analyses. Together, the 18 exposure ages on the three moraines range from 104 to 370 ka. The variability is greater than the analytical uncertainties associated with the methods, implying that boulders from the same moraine were exposed at different times. We conclude that the boulders with younger exposure ages were initially buried within the moraine, shielding them from cosmogenic radiation, but were later exhumed by the erosion of the overlying moraine matrix. We assume that the moraine matrix erodes faster than the boulders. Based on this interpretation, we view the oldest exposure ages as minimum estimates for the depositional age of the moraine. More than one boulder has an exposure age of ~300 ka on each of the three moraines. Based on our work with younger moraines at Lago Buenos Aires, it is highly unlikely that all of these ~300 ka boulders contain inherited isotopes and we conclude that the moraines are at least that old. Even though the erosion rates for these boulders are very low, the calculated exposure ages of some boulders are infinite at the upper confidence limit of the erosion rate parameter, i.e. erosion rate plus one sigma uncertainty. However, these moraines are stratigraphically younger than an underlying 760 ka basalt flow. Therefore, we conclude that these moraines were deposited between MIS-8 and MIS-18.