IMPROVING IN-SITU COSMOGENIC CHRONOMETERS

New radiocarbon ages for Sierra Nevada deglaciation, the first Be-10 m easurements from the Laurentide terminal moraine, and calculations bas ed on paleomagnetic field strength have the potential to substantially improve the accuracy of cosmogenic age estimates. Specifically, three new constraints apply to the interpretation of measured abundances of ill situ produced cosmogenic Be-10 and Al-26: (1) A suite Of minimum- limiting radiocarbon dates indicates that the Sierra Nevada was deglac iated at least several thousand years earlier than assumed when Nishii zuni et al. (1989) first calibrated Be-10 and Al-26 production rates b ased on polished bedrock surfaces in the range, with retreat beginning by 18,000 cal yr B.P. and completed by 13,000 cal yr B.P. (2) Concent rations of Be-10 in moraine boulders and glacier-polished bedrock in N ew Jersey show little variance (10%, 1 sigma) and can be used to calcu late a preliminary Be-10 production rate (integrated over the past 21, 000-22,000 cal yr B.P. at 41 degrees, 200-300 m altitude) that is abou t 20% lower than currently accepted. (3) Calculations of the effect of past geomagnetic field-strength variations on production rates sugges t that the use of temporally averaged production rates may generate ag e errors of >20%; however, cosmogenic exposure ages can be corrected f or this effect, although the corrections currently are imprecise. Many previously reported late-Pleistocene Be-10 and Al-26 exposure ages ar e probably too young and are less accurate and less precise than impli ed by reported uncertainties, The discrepancy between accepted product ion rates and those calculated from Laurentide exposures, when conside red together with the Sierran deglacial chronology and the model resul ts, suggest that correlations between cosmogenic and other numerical a ges, especially for brief events like the Younger Dryas and Heinrich e vents, will not be robust until temporal variations and the altitude/l atitude scaling of production rates are fully understood and quantifie d at levels comparable to current analytic uncertainties (-3%). (C) 19 95 University of Washington.