he in situ cosmogenic 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. Current extraction equipment and procedures reduce blank levels to (2.4 0.1) x 105 14C atoms, and allow reliable in situ 14C extraction at lower temperatures (< 1200C) and in less time than the previous method. Experiments have shown that this new procedure can effectively isolate the in situ 14C fraction with replicate analytical precision approaching 2% (n = 5), while maintaining consistency with earlier results. This is a level of precision and accuracy comparable to or exceeding those currently obtainable with in situ cosmogenic 10Be, 26Al, 3He, 21Ne, and 36Cl. The precision on current measurements will allow measurement of in situ 14C in a 5 g quartz sample after about 400 years of exposure (sea level, high latitude). Having achieved a viable extraction technique with NSF funding, this proposal focuses study of in situ 14C on key applications to which it is uniquely well suited. The proposed investigation, incorporating an extensive field and laboratory analytical program, will build on advances in in situ 14C research by (1) constraining the relative magnitudes of in situ 14C production mechanisms at sea level and high latitude by high-energy neutron spallation (spallogenic 14C) and slow, negative muon capture (muogenic 14C), (2) independently estimating shallow subsurface (< 20 m depth) and atmospheric attenuation lengths for spallogenic and muogenic in situ 14C production, and (3) empirically testing theoretical models of spallogenic and muogenic production rate variation with altitude and latitude. Such production rate variations can be measured with in situ 14C to a much greater extent than is possible with other currently used cosmogenic nuclides, by taking advantage of its rapid (~20 kyr) attainment of secular equilibrium and insensitivity to low and moderate (< 1 cm/kyr) erosion rates. These advances will allow other researchers to use in situ 14C routinely to determine production rates for a range of cosmogenic nuclides on a site-by-site basis, potentially making the use of scaling models unnecessary. Samples used in this study will come from altitude and latitude transects in the western U.S., Chile, Australia, and Antarctica. Finally, results of this research will be fully published to enable widespread application of in situ 14C to surficial process studies.
