A new extraction technique and production rate estimate for in situ cosmogenic 14C in quartz

The potential of in situ cosmogenic C-14 (in situ C-14) f0r surficial process studies is widely recognized, yet a reliable means of isolating it has proved difficult to develop. We present here a new method for extracting in situ C-14 from quartz that overcomes difficulties encountered with earlier techniques, yielding more reliable production rate estimates. Comparison of C-14 thermal release patterns from surficial and deeply shielded quartz samples (Lifton, 1997) demonstrated that contaminant C-14 is released at or below 500 degreesC, and that C-14 released between 500 and 1500 degreesC is essentially all in situ-produced. The new technique builds on this key result, using resistance heating of samples in the presence of a lithium metaborate (LIBO2) flux, and collection of all evolved carbon as CO2 between 500 degreesC and 1100 to 1200 degreesC. Our improved method has four distinct advantages over other extraction methods: (1) we can identify and quantitatively eliminate atmospheric/organic C-14 contamination; (2) we can identify the in situ C-14 component unambiguously without assumptions of (CO)-C-14/(CO2)-C-14 production proportions within the rock or equilibria on extraction; (3) in situ C-14 is reliably extracted from quartz at lower temperatures and in less time than earlier methods and (4) blank C-14 levels are consistently low ((2.3 +/- 0.1) X 10(5) C-14 atoms (1 sigma)). Our new extraction procedures should thus enable researchers to use in situ C-14 in diverse applications without reservation. We developed our new procedures using samples of wave-cut quartzite benches from the well-dated Bonneville (17.4 +/- 0.3 cal ky) and Prove (16.8 +/- 0.3 cal ky) shorelines of Pleistocene Lake Bonneville, Utah, and from underlying deeply shielded locations. In situ C-14 was extracted from quartz separated from 2 Bonneville shoreline samples (6 aliquots) and 1 Provo shoreline sample (2 aliquots). Results demonstrate that our new procedures can effectively isolate the in situ C-14 fraction with replicate analytical precision better than 2% (1 sigma, n = 5), while remaining consistent with earlier results. This level of precision and accuracy is comparable to or exceeds those currently obtainable with in situ cosmogenic Be-10, Al-26, He-3, Ne-21, and Cl-36. Resulting weighted mean in situ C-14 Site production rates for the Bonneville and Prove shorelines are 52.9 +/- 1.7 (C-14 atoms/g SiO2)/y and 48.7 +/- 2.8 (C-14 atoms/g SiO2)/y (1 sigma), respectively-consistent with earlier production rate estimates. Current and previously published in situ C-14 site production rate estimates were then scaled to sea level and high geomagnetic latitude using the latitude-altitude scaling models of Lal (1991) and Dunai (2000). Results indicate that both models yield sea level, high-latitude production rates consistent with independent estimates. Our new in situ C-14 data yield integrated late Quaternary production rate estimates at sea level and high latitude of 15.1 +/- 0.5 (C-14 atoms/g SiO2)/y using the Lal (1991) model, and 15.8 +/- 0.5 (C-14 atoms/g SiO2)/y with that of Dunai (2000). Until significant uncertainties in these models are addressed, however, we prefer the value from the widely-used Lal (1991) model as our best estimate of the integrated late Quaternary production rate for in situ C-14. Copyright (C) 2001 Elsevier Science Ltd.