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，因为它保护地球免受有害的宇宙辐射。它还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中液态铁的有组织运动--地球发电机--通过尚未完全了解的过程产生的。可以用来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，这两者都会随着地质时间的变化而变化。现有的磁场强度随时间变化的记录是不完整的，可能会受到二次过程的影响。潜在的新档案源于该领域的保护性。宇宙射线不断地撞击大气层，但它们的流量受到磁场的调制。较强的磁场提供更大的宇宙射线偏离地球，而较弱的磁场提供较小的偏转。宇宙射线的流量反过来又控制着宇宙成因核素的产生速度--宇宙射线与大气中的气体相撞时产生的稀有同位素。一些宇宙成因的核素档案已经被用来建立磁场强度的历史。该项目将采取探索性步骤，开发一种新的高时间分辨率档案：捕获在不同年龄洞穴沉积中的流体包裹体中的氚丰度--一种半衰期为12年的宇宙核素。作为氢的一种同位素，氚的后期产生行为将受到大气水循环的控制，不会像之前测量的磁场强度的宇宙源核素档案那样出现复杂情况。氚(3He)的衰变产物将在生长过程中被石笋捕获的水袋中进行测量。工作假设是，这些水包裹体吸收氚的比例与大气中宇宙射线通过将当地雨水输送到洞穴系统而产生的氚成比例。捕获后，氚衰变到3He，然后保留在水包裹体中，直到分析。石笋通常有年带，利用U/Th年代学可以准确地测定其年龄。总而言之，这些想法表明，石笋提供了一种潜在的地磁场变化记录，比其他方法提供的记录具有更好的时间细节，并且具有独立的潜在误差源。该项目将采取建立这种记录的第一步，开发必要的分析方法，然后分析捕获20世纪后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。由于核弹试验产生的大气氚是有据可查的，档案将受到非常严格的测试。拟议工作结束时寻求的两个产品是：a)一种能够成功地释放和测量洞穴中的氚成因3He的方法，以及b)对这一提议的关键假设的证实或反驳：保存在洞穴中水包裹体中的氚成因3He是可以测量的，并且将随着局部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.