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

• Main Author: Lifton, Nathaniel Aaron, 1963-
• Other Authors: Bull, William B.
• Language: en_US
• Published: The University of Arizona. 1997
• Subjects: Geochemistry.
• Online Access: http://hdl.handle.net/10150/289000

The potential utility of in situ cosmogenic ¹⁴C (in situ ¹⁴C) in surficial process studies is widely recognized, yet a reliable means of isolating it has proved difficult to develop. Here we present a new method for isolating in situ ¹⁴C in quartz that yields more precise estimates of production rates than achieved by previously published extraction techniques. The new technique involves resistance heating of samples, and collection of all evolved carbon as CO₂ between 500°C and 1500°C. Our improved technique holds three 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, and (3) background ¹⁴C levels are consistently low. To develop our new procedures, we identified and sampled wave-cut quartzite benches associated with Lake Bonneville's two highest shorelines, as well as basalts that erupted into the late Pleistocene lake at Tabernacle Hill. Comparison of ¹⁴C thermal release patterns from the shoreline quartzites to well-shielded quartzite samples showed that contaminant ¹⁴C is released at ≤ 500°C, and that ¹⁴C released from 500 to 1500°C is essentially all in situ-produced. Two replicate analyses yield a sampling site production rate of 59.8 ± 4.6 (¹⁴C atoms/g SiO₂)/yr. Uncertainties in altitude and latitude scaling factors yield a 2σ range of sea-level, >60° geomagnetic latitude ¹⁴C production rate estimates consistent with an independent estimate based on an experimental ¹⁴C/¹⁰Be production ratio (Reedy et al., 1994) and ¹⁰Be production rate estimates from similar Lake Bonneville shoreline sites (Gosse and Klein, 1996). Our preferred production rate estimate is thus 18.2 ± 2.0 (¹⁴C atoms/g SiO₂)/yr. This estimate is also consistent with revised in situ ¹⁴C production rate estimates based on our previously published data, but is lower and more precise--indicating that we have successfully reduced contaminant ¹⁴C and other sources of variability in our data. After we have replicated these Lake Bonneville results, our new extraction procedures should bring in situ ¹⁴C into the mainstream of process-oriented cosmogenic nuclide surface studies.