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

机译：石英中原位宇宙成因碳14的新提取技术和生产率估算

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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.
机译：原位宇宙成因的13 C（原位14 C）在表面过程研究中的潜在效用已得到广泛认可，但事实证明，很难开发出一种可靠的分离方法。在这里，我们提出了一种在石英中原位分离C的新方法，该方法产生的生产率估算值比以前公布的提取技术要精确得多。新技术涉及样品的电阻加热，以及在500°C至1500°C之间收集所有释放出的碳（作为CO 2）。与其他提取方法相比，我们改进后的技术具有三个明显的优势：（1）我们可以识别并定量消除大气/有机¹C污染，（2）可以毫无疑问地识别岩石中的¹CC成分，而无需假设岩石中的¹CO/¹CO2产生比例或萃取中的平衡，并且（3）背景C水平始终较低。为了开发我们的新程序，我们确定并取样了与波纳维尔湖两个最高海岸线以及喷入帐幕山的晚更新世湖中的玄武岩相关的切波石英岩长椅。从海岸线石英岩到屏蔽良好的石英岩样品的14 C热释放模式的比较表明，污染物≤C在≤500°C时被释放，而从500到1500°C释放的¹C基本上全部在原地产生。两次重复分析得出的采样点生产率为59.8±4.6（14 C原子/ g SiO 2）/年。高度和纬度比例因子的不确定性会导致海平面> 2°的2σ范围，地磁纬度≥1C的生产率估算，与基于实验¹C/¹Be的生产比率（Reedy等，1994）和¹Be的生产量得出的独立估计一致。从类似的邦纳维尔湖沿岸站点的流量估算（Gosse和Klein，1996）。因此，我们优选的生产率估算值是18.2±2.0（14 C原子/ g SiO 2）/ yr。此估算值也与根据我们先前发布的数据修订的现场¹C生产率估算值相一致，但是更低且更精确-表示我们已成功降低了污染物¹C和数据中其他可变性来源。在我们复制了邦纳维尔湖的这些结果之后，我们的新提取程序应将原位13 C纳入面向过程的宇宙成因核素表面研究的主流。

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