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: 1723105
• 负责人: Andrea Balbas
• 金额: $ 26.5万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-01 至 2018-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1723105&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.

陆地磁场对地球上的生命至关重要，因为它屏蔽了行星免受有害宇宙辐射的影响。它还保护技术基础设施免受太阳风暴的损害。地球的磁场是由液体铁在地球外核中的有组织运动-Geodynamo-通过未完全理解的过程产生的。可用于告知地毛的模型的关键可观察特征包括磁场强度和方向，这两者在地质时间内都会改变。磁场强度的时间变化的现有记录是不完整的，并且可能因次要过程而损害。潜在的新存档源自该领域的保护性。宇宙射线不断击中大气，但它们的通量是由磁场调节的。更强的磁场可提供更大的宇宙射线偏转，而较弱的磁场提供的挠度较小。宇宙射线的通量反过来控制着宇宙核素的产量 - 当宇宙射线与气体在大气中相撞时产生的罕见同位素。已经使用了几个宇宙核素档案来确定磁场强度的历史。该项目将采取探索性的步骤，以开发新的高时间分辨率档案库：trip的丰度 - 一种宇宙核素，半衰期为12年 - 在被困在各个年龄段的洞穴沉积物中的流体夹杂物中。作为氢的同位素，tri虫的后期产生行为将由大气水周期控制，并且不会像以前测量的磁场强度的宇宙基因核素档案一样遭受相同的并发症。tri虫（3HE）的衰减产物（3HE）将在水袋中测量与它们生长的水袋一样。工作假设是，这些水夹杂物通过将局部雨水输送到洞穴系统中，与宇宙射线的生产​​成比例。陷入困境后，trium衰变至3HE，此后将其保留在水包含中，直到分析为止。 Stalagmites通常具有年度频段，并且可以使用U/TH地质学准确地进行日期。综上所述，这些想法表明，与其他方法相比，Stalagmites提供的地磁场变化具有更好的时间细节，并具有更好的时间细节，并具有独立的潜在错误来源。该项目将通过开发必要的分析方法，然后分析捕获20世纪后半叶核武器测试创建的大气tri虫来确认假定的档案工作是否如预期的作品来确认，该项目将采取第一步来创建这种记录。由于炸弹测试的大气trium生产已得到充分记录，因此该档案将经过非常严格的测试。在拟议的工作结束时寻求的两种产品是a）一种方法，可以成功地释放并测量speleothems中的三构生成3E，b）b）确认或反驳该提议的关键假设：三核3HE在闪闪发光的水置换中保存在闪存中的水置液中可以衡量和降临的限制。