PRECISE COSMOGENIC BE-10 MEASUREMENTS IN WESTERN NORTH-AMERICA - SUPPORT FOR A GLOBAL YOUNGER DRYAS COOLING EVENT

The Inner Titcomb Lakes moraine in the Wind River Mountains, Wyoming, has been dated with a precision of 5% using accelerator mass spectrometric measurements of cosmogenic Be-10 in surface samples from nine boulders resting on its crest. The age of this moraine is 11.4 +/- 0.5 to 13.8 +/- 0.6 ka; the range in age is due to the uncertainty in the Be-10 production rate and uncertainties in rock erosion rate and shielding of cosmic rays by snow. Additional exposure ages on bedrock and boulder surfaces that are stratigraphically younger and older than the Inner Titcomb Lakes moraine conform with this age. The exposure ages of the Inner Titcomb Lakes moraine overlap the North Atlantic Younger Dryas event, 12.8 +/- 0.2 to 11.5 +/- 0.3 ka.