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，否则这些粒子会侵蚀大气层。因此，磁场被认为是确保行星长期宜居的重要因素。今天，这个地磁场是由固体内核的增长提供动力的。但热学模型表明，地球并不总是有一个坚实的内核；内核开始增长的时间从5亿年前到25亿多年前不等。这代表了关于这颗行星的一个基本未知。可以说，研究这个问题的最好方法是使用“古地磁”，即记录岩石和晶体形成时被困住的古老磁场。这样的数据激发了一种假设，即5.65亿年前，地球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坚固内核的诞生。这一假说将通过对澳大利亚、加拿大和美国发现的年龄在8亿到5亿年之间的岩石的研究来验证。这项协作工作将由3个机构的5名科学家组成(包括一名任职人数偏低的少数群体和女科学家)，并将纳入每所大学的教育和外联工作，包括努力扩大第一代和历史上任职人数偏低的个人的机会。地球内核成核(ICN)的时间是未知的，因此在我们对这颗行星的理解中，这是一个一级问题。几十年来，内核被认为有数十亿年的历史。然而，可行的岩心热传导值现在的范围是3倍，最高值与大约8亿到5亿年前的ICN开始兼容。最近的许多热演化模型都预测了这些开始年龄，但古场强度数据的匮乏阻碍了试图确定是否有年轻内核的迹象的努力。最近的古地磁数据记录到5.65亿年前的时间平均地磁场强度是前所未有的低，比现在的地磁场强度低10倍以上。超低的场强伴随着超高的反转频率和其他标志着其他15个最新的前寒武纪-寒武纪火成岩和沉积单元的异常场行为。这些观测结果和最近的模拟结果支持了这样的假设，即地球磁场在晚前寒武纪/早寒武世(即磁能与动能之比小于1)接近崩溃时，与ICN的开始相吻合。因此，核心可能是年轻的。这一假设将通过对大约5亿至8亿年前澳大利亚、美国和西北地区(加拿大)4个火成岩省份的研究来验证。最先进的古地磁方向和古地磁强度数据，包括单硅酸盐晶体分析和U-铅辐射年龄数据，将使我们能够对ICN最年轻的预测年龄期间的地球发电机进行概貌观察。这项工作将涉及一个小组(3个机构的5名私人助理/共同私人助理)，其中包括一名代表不足的少数群体科学家和女科学家。这项工作将纳入每所大学的本科生和研究生教育和外联工作，包括努力扩大第一代和历史上任职人数不足的个人的机会。学生小组将参观并在每个实验室进行分析，比较和对比技术。该项目将被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NSF的法定使命，并通过使用基金会的智力优势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.