Collaborative Research: Geomagnetic field strength and stability between 500 and 800 Ma: Constraining inner core growth

地球磁场保护地球不受太阳粒子的影响，否则太阳粒子会侵蚀大气层。因此，磁场被认为是确保行星长期宜居性的重要因素。今天，这个地磁场是由固体内核的增长提供动力的。但热模型表明，地球并不总是有一个坚实的内核；内核开始生长的时间从5亿年前到25亿年前不等。这代表了对这个星球的一个根本的未知。可以说，研究这个问题的最好方法是使用“古磁学”，即在岩石和晶体形成时被捕获的古代磁场的记录。这些数据推动了这样一种假设，即5.65亿年前，地磁场和大气层对太阳粒子的磁屏蔽几乎崩溃，但后来磁场慢慢恢复。这一事件可能记录了固体内核的诞生。这一假设将通过研究在澳大利亚、加拿大和美国发现的年龄从8亿到5亿年不等的岩石来验证。这项合作工作将由3个机构的5名科学家组成（包括一名未被充分代表的少数民族和女科学家），并将纳入每所大学的教育和外展工作，包括努力扩大第一代和历史上未被充分代表的个人的机会。地球内核成核（ICN）的时间是未知的，因此在我们对地球的理解中代表了一个一级问题。几十年来，人们一直认为地核有几十亿年的历史。然而，可行的岩心热导率值现在跨越了3倍，最高值与ICN发生在大约8亿到5亿年前相一致。最近的许多热演化模型都预测了这些起始年龄，但由于缺乏古磁场强度数据，无法确定是否存在年轻内核的迹象。最近的古地磁数据记录了5.65亿年前史无前例的低时间平均地磁场强度，比现在的地磁场强度低10倍以上。在其他15个最新前寒武纪-寒武纪火成岩和沉积单元中，超低磁场强度伴随着超高的反转频率和其他异常的磁场行为指标。这些观测结果和最近的模拟结果为ICN发生的前寒武纪晚期/早寒武纪地磁场接近崩溃（即磁能与动能之比小于1）的假设提供了依据。因此，内核可能很年轻。这一假设将通过对位于澳大利亚、美国和西北地区（加拿大）的4个火成岩省的研究进行验证，这些火成岩省位于大约5亿至8亿年前。最先进的古地磁方向和古强度数据，包括单硅酸盐晶体分析，以及U-Pb辐射年龄数据，将使我们对ICN最年轻预测年龄期间的地球动力学有一个概貌的认识。这项工作将涉及一个团队（3个机构的5名pi /共同pi），其中包括一名未被充分代表的少数民族和女科学家。这项工作将被纳入每所大学的本科和研究生教育和推广工作，包括努力扩大第一代和历史上代表性不足的个人的机会。学生团队将访问并在每个实验室进行分析，比较和对比技术。该项目将整合到大学特定的本科课程中，为实地和实验室调查做准备。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 protects the planet from solar particles that would otherwise erode the atmosphere. Thus, the magnetic field is thought to be an essential factor ensuring long-term planetary habitability. Today, this geomagnetic field is powered by growth of the solid inner core. But thermal models suggest Earth has not always had a solid inner core; the time of the onset of inner core growth has ranged from 500 million to more than 2.5 billion years ago. This represents a fundamental unknown about the planet. Arguably the best way to investigate this question is to use "paleomagnetism", the record of the ancient magnetic field trapped in rocks and crystals as they form. Such data have motivated the hypothesis that the geomagnetic field, and the magnetic shielding of the atmosphere from solar particles, almost collapsed 565 million years ago, but then the field slowly recovered. This event may record the birth of the solid inner core. This hypothesis will be tested through studies of rocks ranging in age from 800 to 500 million years old found in Australia, Canada and the United States. The collaborative work will involve a team of 5 scientists at 3 institutions (including an underrepresented minority and woman scientist), and will be integrated into education and outreach efforts at each university, including efforts to expand opportunities for first-generation and historically underrepresented individuals. The time of Earth's inner core nucleation (ICN) is unknown and thus represents a first-order problem in our understanding of the planet. For decades the inner core was assumed to be billions of years old. However, viable core thermal conductivity values now span a factor of 3, with the highest values compatible with ICN onset between approximately 800 and 500 million years ago. These onset ages are predicted by many recent thermal evolution models, but a paucity of paleofield strength data has thwarted efforts seeking to determine if there is a sign of a young inner core. Recent paleomagnetic data record an unprecedented low in time-averaged geomagnetic field strength 565 million years ago that is greater than 10 times lower than the strength of the present geomagnetic field. The ultra-low field intensity is accompanied by an ultra-high reversal frequency and other indicators of unusual field behavior in 15 other Latest Precambrian-Cambrian igneous and sedimentary units. These observations and recent modeling results are the basis for the hypothesis that the geomagnetic field approached collapse in late Precambrian/early Cambrian times (i.e., the ratio of the magnetic energy to kinetic energy is less than 1) coincident with the onset of ICN. Hence, the inner core may be young. This hypothesis will be tested through the study of 4 igneous provinces emplaced between about 500 and 800 million years ago, in Australia, the US and the Northwest Territories (Canada). State-of-the-art paleomagnetic directional and paleointensity data, including single silicate crystal analyses, and U-Pb radiometric age data will allow a synoptic view of the geodynamo during the youngest predicted ages of ICN. The work will involve a team (5 PIs/co-PIs at 3 institutions) including an underrepresented minority and woman scientist. The work will be integrated into undergraduate and graduate education and outreach efforts at each university, including efforts to expand opportunities for first-generation and historically underrepresented individuals. Student teams will visit and conduct analyses in each of the laboratories, comparing and contrasting techniques. The project will be integrated into university-specific undergraduate courses in preparation for field and laboratory investigations.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.