REFINED CHRONOLOGIES OF YELLOWSTONE ICE CAP AND TETON RANGE OUTLET GLACIER FLUCTUATIONS DURING THE LATE PLEISTOCENE

We have obtained new cosmogenic ¹⁰Be surface exposure ages from boulders on moraines deposited by the southern Yellowstone Ice Cap and a prominent outlet glacier on the eastern front of the Teton Range. One boulder at the southern terminus of the Munger glaciation, an event that filled all of Jackson Hole with ice, yields a preliminary minimum age of ~140 ka. This age is consistent with the prior interpretation of the Munger glaciation as an extensive advance of the southern Yellowstone Ice Cap during oxygen isotope stage 6 (i.e., a Bull Lake-age glaciation). Two boulders on the innermost set of Pinedale end moraines enclosing Jenny Lake on the eastern Teton front yield preliminary ages of 11.8 ± 0.3 and 12.1 ± 0.3 ka, and one boulder on an outer set of Jenny Lake end moraines yields an age of 12.5 ± 0.3 ka. These ages would suggest a major advance of an outlet glacier in the Teton Range during the Younger Dryas. This result differs with that expected from cosmogenic dating of the northern Yellowstone outlet glacier (recession before 14 ka) and relations of outwash from Yellowstone Plateau ice to the Jenny Lake moraines. Our new findings, when viewed in the context of other well-dated glacial records in the western U.S., suggest spatial and temporal patterns of alpine glacier responses during the late Pleistocene may be more complex than previously thought. Additional exposure ages forthcoming from glacial deposits in the greater Yellowstone - Grand Teton region are expected to reveal complexities of the glacial chronologies of these ranges and enable a more detailed assessment of their significance to regional paleoclimate.