CORRELATIONS OF MORAINES BY EXPOSURE AGES (AND OTHER BEST LAID PLANS)

Cosmogenic 10Be ages on boulders measured in 1992-1994 (n=9 + 1 outlier) from the inner Titcomb Lakes moraine, Wind River Mountains, Wyoming yield a weighted mean of 12.1 +/- 1.1 (2s) (10Be) ka. The mean age increases and uncertainty significantly improves to 12.5 +/- 0.8 (2s) (10Be) ka when the estimated effects of boulder geometry are taken into consideration (Masarik et al., 2000, NIM-B 786-789). This Titcomb Lakes moraine age is supported by plant macrofossils collected from sediments in the lakes above and below the moraine (Davis, P.T. et al., this meeting), and by additional exposure ages on boulders and bedrock surfaces below and above the Titcomb Lakes moraine. Similar precisions for similar exposure durations have been attained by others (e.g. the Swiss Alps, Ivy-Ochs et al., 1999, Geografiska Ann., 313-323). Palynology on a core from the lower Titcomb Lake suggests that climate was drier and perhaps cooler prior to 11.4 +/- 1.1 cal ka. We have concluded that the inner Titcomb Lakes moraine was formed synchronously with the Younger Dryas chron as dated from the Greenland ice cores and many European records. Based on the exposure ages alone, we cannot preclude the possibility that the inner Titcomb Lakes moraine was deposited during the shorter-lived Older Dryas event. This is also true for late glacial boulders in Labrador, Canada exposure dated at 12.9 +/- 2.1 2s (10Be) ka and a single age on a moraine in the Rio Atuel, Argentina. We can confidently distinguish these glacial records from pre-Older Dryas events, and after the decade of improvements since 1994 we may be able to resolve ages with 1800 to 2000 year time lags (e.g. as hypothesised between the North Atlantic and high latitude southern hemisphere). We have recently achieved similar precisions on boulders near Peggy’s Cove, Nova Scotia which date to 15.9 +/- 1.3 2s (10Be) ka. Unfortunately these precisions worsen with exposure duration due to boulder weathering, and other factors such as the duration of moraine deposition (e.g. ca. 5 kyr for the Pinedale terminal).