Continuing Studies of in Situ Cosmogenic 14C in Terrestrial Materials

亚利桑那大学的原位14C研究小组经过多年的研究，开发了可靠的提取技术和设备，现在准备将14C与10Be、26Al、36Cl、3He和21Ne一起纳入宇宙成因核素表面过程研究的主流。目前的提取设备和程序将空白水平降低到(2.40.1)×10514C原子，并允许在更低的温度(1200℃)下可靠地原位提取14C，而且比以前的方法需要更少的时间。实验表明，该新方法能有效地分离14C组分，重复性分析精度接近2%(n=5)，同时与早期结果保持一致。这一水平的精密度和准确度可与或超过目前用原位宇宙成因10Be、26Al、3He、21Ne和36Cl获得的水平相媲美。目前测量的精度将允许在暴露约400年(海平面，高纬度)后对5克石英样品中的14C进行原位测量。在NSF资助下实现了一种可行的提取技术后，该建议将14C的研究重点放在它唯一非常适合的关键应用上。拟议的调查结合了广泛的野外和实验室分析计划，将建立在现场14C研究进展的基础上，方法是：(1)通过高能中子散裂(散裂14C)和缓慢的负Muon捕获(Mugenic 14C)来限制海平面和高纬度现场14C产生机制的相对大小，(2)独立估计浅地表(20米深)和大气衰减长度，以产生散裂和致突变的就地14C，以及(3)经验性地测试自爆和致突变产生速率随海拔和纬度变化的理论模型。通过利用14C快速(约20KYR)达到长期平衡以及对低和中等(1厘米/KYR)侵蚀速率不敏感的优势，14C可以在原位测量到比目前使用的其他宇宙成因核素更大的程度上的这种生产速率变化。这些进展将使其他研究人员能够常规地使用现场14C来逐个地点确定一系列宇宙成因核素的产生率，这可能使使用比例模型变得不必要。这项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横断面。最后，这项研究的结果将被全面公布，以使原位14C在地表过程研究中得到广泛应用。

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.