A NEW EXTRACTION TECHNIQUE AND PRODUCTION RATE ESTIMATE FOR IN SITU COSMOGENIC 14C IN QUARTZ

Abstract

The potential of in situ cosmogenic 14C (in situ 14C) for 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 14C from quartz that overcomes difficulties encountered with earlier techniques, yielding more reliable production rate estimates. Comparison of 14C thermal release patterns from surficial and deeply shielded quartz samples (Lifton, 1997) demonstrated that contaminant 14C is released at or below 500°C, and that 14C released between 500 and 1500°C 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°C and 1100 to 1200°C. Our improved method has four distinct advantages over other extraction methods: (1) we can identify and quantitatively eliminate atmospheric/organic 14C contamination; (2) we can identify the in situ 14C component unambiguously without assumptions of 14CO/14CO2 production proportions within the rock or equilibria on extraction; (3) in situ 14C is reliably extracted from quartz at lower temperatures and in less time than earlier methods and (4) blank 14C levels are consistently low ((2.3 +/- 0.1) x 105 14C atoms (1σ)). Our new extraction procedures should thus enable researchers to use in situ 14C 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 Provo (16.8 +/- 0.3 cal ky) shorelines of Pleistocene Lake Bonneville, Utah, and from underlying deeply shielded locations. In situ 14C 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 14C fraction with replicate analytical precision better than 2% (1σ, 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 10Be, 26Al, 3He, 21Ne, and 36Cl. Resulting weighted mean in situ 14C site production rates for the Bonneville and Provo shorelines are 52.9 +/- 1.7 (14C atoms/g SiO2)/y and 48.7 +/- 2.8 (14C atoms/g SiO2)/y (1σ), respectively-consistent with earlier production rate estimates.Current and previously published in situ 14C 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 14C data yield integrated late Quaternary production rate estimates at sea level and high latitude of 15.1 +/- 0.5 (14C atoms/g SiO2)/y using the Lal (1991) model, and 15.8 +/- 0.5 (14C 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 14C.