9614366 Jull The in situ-produced 14C (in situ 14C) research group at the University of Arizona, after a number of years developing reliable extraction techniques and equipment, is now prepared to bring in situ 14C into the mainstream of cosmogenic nuclide surficial process studies, alongside 10Be, 26Al, 36Cl, 3He, and 21Ne. New extraction and graphitization lines have been constructed in the last two years which, in concert with improved procedures, have produced a breakthrough in the isolation of in situ 14C at blank levels of approximately 1.1 x 106 atoms. Using these systems, quartz separates from a wave-cut quartzite bench from the well-dated (17.4 + 0.2 cal ky) Bonneville shoreline of Pleistocene Lake Bonneville, Utah, and from underlying deeply shielded locations were analyzed recently. Results demonstrate that these new systems and procedures can effectively isolate the in situ 14C fraction, with a late Quaternary production rate estimate of 20.4 + 1.7 14C atom/gram/year (sea level, >60(N)3/4indistinguishable from earlier estimates using older, less precise techniques. The precision on current measurements will allow measurement of in situ 14C in quartz after about 500 years of exposure (sea level, >60(N). Research described in this proposal will continue to lay the groundwork for using in situ 14C in surficial process studies by (1) further testing the accuracy of production rate estimates, (2) assessing the variability of 14C thermal release patterns with varying grain size and quartz type (i.e., platonic, volcanic, metamorphic), (3) assessing the feasibility of whole-rock analyses by comparison to those of quartz separates, (4) independently estimating attenuation mean free paths for 14C, and (5) empirically testing theoretical models of production rate variation with altitude and latitude, to a much greater extent than possible with other currently used cosmogenic nuclides, by taking advantage of 14C's rapid (~20 ka) attainment of secular equilibrium and insensitivity to low and moderate (< 1 cm/ka) erosion rates. Samples used in this study will come from Tabernacle Hill, the Bonneville and Provo Shorelines and other features of the Lake Bonneville system, and from altitude and latitude transects in the western U.S., Australia, and Antarctica.
