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

迈向古磁场不稳定性新高分辨率代理的第一步：三成因 3He 存档于 Speleothems

• 批准号: 2129791
• 负责人: Andrea Balbas
• 金额: $ 26.5万
• 依托单位: California State University-Long Beach Foundation
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-10-01 至 2023-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2129791&HistoricalAwards=false
• 关键词: First; Steps; Towards; New; Resolution

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.

地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（地球发电机）中液态铁的有组织运动产生的，其过程尚未完全了解。可用于为地球发电机模型提供信息的关键可观察特征包括磁场强度和方向，这两者都随着地质时间而变化。现有的磁场强度随时间变化的记录是不完整的，并可能受到二次过程的影响。一个潜在的新档案来源于该领域的保护性质。宇宙射线不断地撞击大气层，但它们的通量受到磁场的调制。较强的磁场提供来自地球的宇宙射线的较大偏转，而较弱的磁场提供较小偏转。宇宙射线的通量反过来控制宇宙成因核素的产生率-宇宙射线与大气中的气体碰撞时产生的稀有同位素。几个宇宙成因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，开发新的高时间分辨率档案：不同年龄洞穴沉积物中捕获的流体包裹体中氚（半衰期为12年的宇宙成因核素）的丰度。氚（3 He）是氢的一种同位素，其生成后的行为将受到大气水循环的控制，不会像以前测量的宇宙成因核素磁场强度档案那样复杂。氚（3 He）的衰变产物将在石笋生长过程中捕获的水囊中测量。目前的假设是，这些水包裹体吸收的氚与大气中宇宙射线通过当地雨水输送到洞穴系统中产生的氚成比例。在捕获后，氚衰变为3 He，然后保留在水包裹体中直到分析。石笋通常具有年带，可以用U/Th年代学精确定年。总之，这些想法表明，石笋提供了一个潜在的记录地磁场的变化与更好的时间细节比其他方法提供的潜在误差的独立来源。该项目将采取第一步，通过开发必要的分析方法，然后通过分析捕获世纪后半叶核武器试验产生的大气氚的石笋，以确认推定的档案馆如预期的那样工作，来创建这样一个记录。由于原子弹试验产生的大气氚有详细的记录，档案将接受非常严格的测试。在建议的工作结束时寻求的两个产品是：a）一种方法，可以成功地释放和测量的氚3 He在洞穴沉积物，和B）确认或反驳这一建议的关键假设：保存在洞穴沉积物中的水包裹体中的氚3 He可以测量，并会改变作为当地降水中的氚浓度的函数。