The First Billion Years of the Geodynamo

在液态铁核中产生的地球磁场保护大气层不受来自太阳的粒子流的影响，这些粒子流被称为“太阳风”。如果没有这个保护层，大气层就会被侵蚀，水就会流失，尤其是在地球年轻、太阳风强烈的时候。因此，了解地球最早的磁场可以为我们了解地球核心和大气层的演变提供独特的视角。因为正如我们所知，水对生命至关重要，这一知识也与地球可居住性的起源有关。唯一能准确记录这一早期磁性历史的已知物质是矿物锆石，现在在较年轻的沉积岩中发现了微小的颗粒。其中一些锆石非常引人注目，因为它们含有更小的内含物，这些内含物可以保存数十亿年前古代磁场的信号。但是因为这些锆石非常小——只有人类头发的几倍厚——测量它们的磁性是极具挑战性的。研究人员已经开发并应用了新技术来表征锆石中磁性包裹体的性质，并使用一系列高灵敏度仪器来检索它们的磁性信号。他们的测量表明，至少42亿年前存在磁场。这些数据表明当时有大气屏蔽。它们还对地核的物理条件施加了限制，从而产生了强磁场。研究人员将填补磁场历史上的空白，从西澳大利亚、印度和非洲南部的新锆石位置生成数据。他们将与跨越6个国家的国际合作者团队合作，开展多学科实验室研究。然后，研究小组将使用这些新数据来测试磁历史的保真度，并进一步探索磁屏蔽和地球演化的含义。这项工作将支持研究生和本科生，他们将在该领域和多学科分析方面接受广泛的培训，并将通过为教师和学生举办的项目向罗切斯特中学推广。了解地磁场的早期历史可以为我们理解地核、大气层和地球的可居住性的演变提供关键的见解。唯一已知的能够准确测定年代并能够在数亿年的时间尺度上记录这段历史的材料是太古代到冥古宙的锆石，它们含有微小的磁性包裹体，现在在更年轻的沉积单元中被发现。然而，锆石的磁性测量，以及对其磁化强度的询问，以确定它们是否保留了原始信号，是一项艰巨的技术挑战，需要多学科的方法。研究人员最近公布了新的古地磁、电子显微镜、地球化学和古强度数据，表明西澳大利亚杰克山的部分锆石中存在原生磁铁矿包裹体。这些新数据进一步支持了杰克山锆石记录的地球发电机的主要磁信号，其记录可以追溯到约42亿年前。因此，这些分析表明，在地球历史的早期，磁层对大气的屏蔽就已经存在了。新的分析还表明，大约40亿年前冥古宙晚期有一个强大的磁场，这可能是地核化学沉淀为地球发电机提供动力的信号。该团队将使用古磁学方法进一步测试和填补最近在全球其他地点发现的太古宙至冥古宙碎屑锆石的古强度记录的空白。具体来说，他们将从西澳大利亚、印度和非洲南部的新地区产生跨越数亿年的记录。他们将与跨越6个国家的国际合作者合作，进行多学科实验室研究[包括光学和扫描电子显微镜，聚焦离子束切片和观察和提升，透射电子显微镜，磁光克尔效应测量，超灵敏三分量直流超导量子干涉装置（SQUID）磁强计，扫描SQUID显微镜磁强计，灵敏的高分辨率离子微探针（SHRIMP）分析和二次离子质谱（SIMS）]。该团队将使用这些新数据来测试每个记录的内部保真度，测试记录之间的一致性，并进一步探索行星磁屏蔽和地核化学演化的含义。这项工作将支持研究生和本科生接受多学科分析方面的广泛训练。这项工作将有助于博士和硕士论文。调查人员将通过为教师和学生举办项目，继续与罗切斯特中学进行接触。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Earth's magnetic field, generated in the liquid iron core, shields the atmosphere from particles streaming from the Sun, known as the “solar wind". Without this shield, the atmosphere would have been eroded, and water would have been lost, especially when Earth was young and solar winds were intense. Hence, knowledge of Earth's earliest magnetic field can provide unique insight into the evolution of our planet's core and atmosphere. Because water is essential to life as we know it, this knowledge also bears on the origin of Earth's habitability. The only known material that can accurately record this early magnetic history is the mineral zircon, now found as tiny grains in younger sedimentary rocks. Some of these zircons are remarkable because they contain even smaller inclusions that can preserve signals of the ancient magnetic field from billions of years ago. But because these zircons are so small - only a few times thicker than a human hair - the measurement of their magnetism is extremely challenging. The investigators have developed and applied new techniques to characterize the nature of magnetic inclusions in zircons and to retrieve their magnetic signals, using a host of highly sensitive instruments. Their measurements indicate the presence of a magnetic field at least 4.2 billion years ago. These data indicate atmospheric shielding was in place. They also place constraints on the physical conditions in the core that resulted in the generation of a strong magnetic field. The investigators will fill gaps in the magnetic field history, generating data from new zircon localities in Western Australia, India and southern Africa. They will work with a team of international collaborators spanning 6 countries in multidisciplinary laboratory studies. The team will then use these new data to test the fidelity of the magnetic history, and to further explore the implications for magnetic shielding and Earth evolution. The work will support graduate and undergraduate students who will receive broad training in the field and in multidisciplinary analyses, and will include outreach to Rochester secondary schools through programs hosted for instructors and students.Knowledge of the earliest history of the geomagnetic field can provide key insight into our understanding of the evolution of the core, atmosphere and Earth's habitability. The only known materials that can be accurately dated and that are able to record this history on the multi-hundred-of-million-year time scales required to advance our understanding of these fundamental issues of Earth evolution are Eoarchean to Hadean zircons bearing minute magnetic inclusions that are now found in younger sedimentary units. However, the magnetic measurement of zircons, and interrogations of their magnetizations to determine if they preserve primary signals, are formidable technical challenges requiring a multidisciplinary approach. The investigators have recently presented new paleomagnetic, electron microscope, geochemical, and paleointensity data that indicate the presence of primary magnetite inclusions in select zircons from the Jack Hills of Western Australia. These new data further support that select Jack Hills zircons record primary magnetic signals of the geodynamo, with a record extending back in time to ~4.2 billion years ago. These analyses thus indicate that shielding of the atmosphere by the magnetosphere was in place very early in Earth history. The new analyses also suggest a strong magnetic field in the Late Hadean at ~4 billion years ago which could be a signal that chemical precipitation in the core was powering the geodynamo. The team will use a paleomagnetic approach to further test and fill gaps in the paleointensity record building on recent discoveries of Eoarchean to Hadean detrital zircons at other global sites. Specifically, they will generate records spanning hundreds-of-millions of years from new localities in Western Australia, India and southern Africa. They will work with a team of international collaborators spanning 6 countries in multidisciplinary laboratory studies [including light and scanning electron microscopy, focused ion beam slice and view and lift-outs, transmission electron microscopy, magneto-optical Kerr effect measurements, ultra-sensitive 3-component direct current superconducting quantum interference device (SQUID) magnetometry, scanning SQUID microscope magnetometry, sensitive high-resolution ion microprobe (SHRIMP) analyses, and secondary-ion mass spectrometry (SIMS)]. The team will use these new data to test the internal fidelity of each record, test consistency between records, and further explore the implications for planetary magnetic shielding and chemical evolution of the core. The work will support graduate and undergraduate students who will receive broad training in multidisciplinary analyses. The work will contribute to Ph.D. and M.S. theses. The investigators will continue outreach to Rochester secondary schools by hosting programs for instructors and students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.