First Steps Towards a New High Resolution Proxy for Paleomagnetic Field Instabilities: Tritiogenic 3He Archived in Speleothems

地球磁场对地球上的生命至关重要，因为它可以保护地球免受有害的宇宙辐射。它还可以保护技术基础设施免受太阳风暴的破坏。地球磁场是由地球外核（地球发电机）中液态铁的有组织运动产生的，其过程尚不完全清楚。可用于为地球发电机模型提供信息的关键可观测特征包括磁场强度和方向，两者都会随地质时间而变化。磁场强度随时间变化的现有记录不完整，并且可能受到二次过程的影响。潜在的新档案源于该领域的保护性质。宇宙射线不断撞击大气层，但它们的通量受到磁场的调制。较强的磁场使来自地球的宇宙射线产生更大的偏转，而较弱的磁场则提供较小的偏转。宇宙射线的通量反过来控制着宇宙射线与大气中的气体碰撞时产生的稀有同位素——宇宙发生核素的产生速率。一些宇宙成因核素档案已被用来建立磁场强度的历史。该项目将采取探索性措施，开发一个新的高时间分辨率档案：不同年龄的洞穴沉积物中的流体包裹体中富含氚（一种半衰期为 12 年的宇宙成因核素）。作为氢的同位素，氚的后期产生行为将受到大气水循环的控制，并且不会遭受与先前测量的磁场强度的宇宙成因核素档案相同的复杂性。氚（3He）的衰变产物将在石笋生长时捕获的水囊中进行测量。工作假设是，这些水夹杂物吸收的氚与大气中宇宙射线通过将当地雨水输送到洞穴系统产生的氚成比例。捕获后，氚衰变成 3He，随后保留在水包裹体中直至分析。石笋通常有年带，可以使用 U/Th 地质年代学准确测定年代。总而言之，这些想法表明石笋提供了地磁场变化的潜在记录，其时间细节比其他方法提供的记录更好，并且具有独立的潜在误差源。该项目将通过开发必要的分析方法来迈出创建此类记录的第一步，然后通过分析捕获 20 世纪下半叶核武器试验产生的大气氚的石笋，以确认假定的档案按预期工作。由于炸弹测试产生的大气氚有详细记录，因此档案将受到非常严格的测试。拟议工作结束时寻求的两种产品是a）一种能够成功释放和测量洞穴中氚质3He的方法，b）对该提案的关键假设的确认或反驳：保存在洞穴中水包裹体中的氚质3He可以被测量，并且会随着当地降水中氚浓度的变化而变化。

The terrestrial magnetic field is critical to life on Earth because it shields the planet from harmful cosmic radiation. It also protects technological infrastructure from damage from solar storms. The Earth's magnetic field is generated by organized motions of liquid iron in the Earth's outer core - the geodynamo - by processes that are not fully understood. Key observable features that can be used to inform models of the geodynamo include magnetic field strength and orientation, both of which change over geologic time. Existing records of the temporal variation of magnetic field strength are incomplete and potentially compromised by secondary processes. A potential new archive originates from the protective nature of the field. Cosmic rays continually strike the atmosphere, but their flux is modulated by the magnetic field. Stronger magnetic fields provide greater deflection of cosmic rays from Earth, and weaker fields provide less deflection. The flux of cosmic rays in turn controls the production rate of cosmogenic nuclides - rare isotopes produced when cosmic rays collide with gases in the atmosphere. Several cosmogenic nuclide archives have already been used to establish the history of magnetic field strength. This project will take exploratory steps towards development of a new high temporal resolution archive: the abundance of tritium - a cosmogenic nuclide with a half-life of 12 years - in fluid inclusions trapped in cave deposits of various ages. As an isotope of hydrogen, the post-production behavior of tritium will be controlled by the atmospheric water cycle, and will not suffer the same complications as previously measured cosmogenic nuclide archives of magnetic field strength.The decay product of tritium (3He) will be measured in water pockets that are trapped by stalagmites as they grow. The working hypothesis is that these water inclusions take in tritium in proportion to its production by cosmic rays in the atmosphere via the delivery of local rainwater into the cave system. After entrapment, tritium decays to 3He, which is thereafter retained in the water inclusion until analysis. Stalagmites often have annual bands, and can be dated accurately using U/Th geochronology. Taken together, these ideas suggest stalagmites offer a potential record of geomagnetic field changes with better temporal detail than those provided by alternative methods and with independent sources of potential error. This project will take the first steps for creating such a record by developing the necessary analytical methods and then by analyzing stalagmites that captured atmospheric tritium created by nuclear weapons testing in the latter half of the 20th century to confirm that the putative archive works as expected. Because the atmospheric tritium production from bomb testing is well-documented, the archive will be subjected to a very rigorous test. The two products sought by the end of the proposed work are a) a methodology that can successfully liberate and measure the tritiogenic 3He in speleothems, and b) a confirmation or refutation of the key hypothesis of this proposal: Tritiogenic 3He preserved in water inclusions in speleothems can be measured and will change as a function of the concentration of tritium in local precipitation.