Continuing Studies of in Situ Cosmogenic 14C in Terrestrial Materials

亚利桑那大学的原位宇宙生成14C（原位14C）研究小组经过多年开发可靠的提取技术和设备，现在准备将原位14C与10Be、26Al、36Cl、3He和21Ne一起纳入宇宙生成核素表面过程研究的主流。目前的提取设备和程序减少空白水平到（2.4 0.1）× 105 14C原子，并允许可靠的原位14C提取在较低的温度（1200℃）和在更短的时间比以前的方法。实验表明，这种新方法可以有效地分离原位14C组分，重复分析精度接近2% (n = 5)，同时与先前的结果保持一致。这是一种精度和准确度水平，可与目前在原位宇宙生成的10Be， 26Al, 3He， 21Ne和36Cl相媲美或超过。当前测量的精度将允许在暴露约400年后（海平面，高纬度）原位测量5克石英样品中的14C。在国家科学基金会的资助下实现了可行的提取技术，本提案将重点研究原位14C的关键应用，这是它唯一适合的。拟议的调查，包括广泛的实地和实验室分析计划，将建立在原位14C研究进展的基础上：(1)通过高能中子散裂（spallogenic 14C）和缓慢的负介子捕获（muogenic 14C），限制海平面和高纬度的原位14C产生机制的相对大小；(2)独立估算地表浅层（20 m深度）和大气衰减长度，(3)经验验证空间成因和空间成因14C产量随海拔和纬度变化的理论模型。利用原位14C快速（~ 20kyr）达到长期平衡以及对低和中等（1cm /kyr）侵蚀速率不敏感的优势，这种生产速率变化可以比目前使用的其他宇宙生成核素在更大程度上测量。这些进步将使其他研究人员能够常规地使用原位14C来逐点确定一系列宇宙核素的生产速率，从而可能使比例模型的使用变得不必要。本研究中使用的样本将来自美国西部、智利、澳大利亚和南极洲的海拔和纬度样带。最后，本研究结果将全面发表，使原位14C广泛应用于地表过程研究。

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.