Palaeomagnetic field behaviour in the Palaeozoic and the hunt for inner core birth

通过描述和解释3.3亿至6亿年前地球磁场的变化，该项目旨在解决我们对地球深层内部的知识中最深刻的突出差距之一：固体内核的年龄。地球的磁场延伸到遥远的太空，保护地球表面的生命和大部分大气层免受有害太阳风的影响。它产生于地球的液态外核，但底层内核的增长提供了今天驱动它的大部分能量。我们的星球在其45亿年历史的大部分时间里都有磁场，但在这段时间里，固体内核并不存在。理论预测，地球中心固体铁的冻结开始与之前较弱的长期平均磁场强度的急剧增加有关。我们正在寻找这个地球内核诞生的标志，它已经变得几乎和月球一样大。如果我们能确定地球历史上的这一时刻，那么它的时间将为整个地球的热演化提供一个主要的限制。我们最近的工作强调了描述和解释磁场行为所需的下一步，这样我们就可以探测到内核的诞生。我们的测量已经确定，在6 - 5.4亿年前和3.3 - 4.2亿年前的时期，磁场与最近的时代相比异常微弱。我们现在需要知道它在这些时期之间是如何演变的。如果它在这段时间（4.2 - 5.4亿年前）保持微弱，那么这将表明内核形成日期要晚得多，对太阳的磁屏蔽也比之前认为的要少得多。除了确定在我们目前没有数据的时期内的平均强度外，我们还需要建立一个更完整的图像，了解在3.3亿到6亿年前的整个时期内，全球磁场的形状和稳定性是如何变化的。目前地球磁场的结构（类似于一个几乎与行星旋转轴对齐的条形磁铁）是一种有效的结构，可以偏转近地空间周围的太阳辐射，避开大气层和生物圈。我们需要知道这种情况是否一直存在，这样我们才能限制磁屏蔽模型和地核发电机的模拟。一旦我们对长期磁场行为有了很好的描述，我们就需要知道如何通过模拟地核中液态铁的流动来重现它。有许多可能的内核生长速率和覆盖地幔的强迫模式；我们需要看看这些因素的哪一种组合产生的磁场行为最符合观测结果。我们将在团队先前工作的基础上，采用多学科方法进行这项研究。场强的演变将通过严格的古地磁测量来确定，这些古地磁测量是用我们最先进的设备和多功能的新仪器对精心挑选的岩石样本进行的。该领域的全球统计特征将使用一套新的标准，由新的和现有数据集的详尽汇编提供信息。最后，我们将利用我们自己的一套核心-地幔演化模型，以及我们的合作者的模型，对地球核心发电机的数值模拟进行迭代，在受外部影响的情况下，最佳地再现项目其他部分提供的观测结果。通过实验室实验、数据分析和数值模拟的结合，我们的目标是了解磁场在这一时期是如何变化的，从而获得有关地球深部结构演变的重要信息，以及早期多细胞生命免受有害太阳风辐射的保护水平。

Through describing and explaining how Earth's magnetic field changed between 330 and 600 million years ago, this project aims to tackle one of the most profound outstanding gaps in our knowledge of the Earth's deep interior: the age of the solid inner core.Earth's magnetic field extends far out into space and shields life on the surface and much of Earth's atmosphere from harmful solar wind. It is generated in the liquid outer core of the planet but the growth of the underlying inner core provides much of the energy to drive it today. Our planet has had a magnetic field for most of its 4.5 billion year history but the solid inner core did not exist for all of this time. Theory predicts that the onset of freezing of solid iron at Earth's centre will have been associated with a sharp increase in the long-term average magnetic field strength from a weak preceding state. We are hunting for this signature of the birth of Earth's inner core which has since grown to be nearly the size of the Moon. If we could definitively identify this moment in Earth's history, then its timing would provide a major constraint on the thermal evolution of the entire planet.Our recent works highlight the next steps required to describe and explain magnetic field behaviour such that we can detect inner core birth. Our measurements have established that, during the time periods 600-540 million years ago and 330-420 million years ago, the magnetic field was anomalously weak compared to more recent times. We now need to know how it evolved between those time periods. If it remained weak through this interval (420-540 million years ago) then this would suggest a much later inner core formation date, and far less magnetic shielding from the sun, than previously thought. As well as determining its average strength during the period for which we currently have no data, we also need to build a more complete picture of how the shape and stability of the global magnetic field changed over the complete period 330 to 600 million years ago. The current configuration of Earth's magnetic field (resembling a bar magnet nearly aligned with the planetary rotation axis) is an effective one for deflecting solar radiation around near-Earth space and avoiding the atmosphere and biosphere. We need to know if this was always the case so that we can constrain models of magnetic shielding and simulations of Earth's core dynamo. Once we have a good description of the long-term magnetic field behaviour, we then need to know how we can reproduce it using simulations of the flow of liquid iron in Earth's core. There are many possible rates of inner core growth and patterns of forcing from the overriding mantle; we need to see which combination of these produces magnetic field behaviour that best matches the observations.We will undertake this research using a multidisciplinary approach building on previous work undertaken by the team. The evolution of field strength will be determined by rigorous palaeomagnetic measurements performed on carefully selected rock samples using our state-of-the-art facilities augmented by a versatile new instrument. The global statistical characterisation of the field will be produced using a novel set of criteria informed by an exhaustive compilation of new and extant datasets. Finally, we will draw on our own suite of core-mantle evolution models, and those of our collaborators, to iterate towards numerical simulations of the Earth's core dynamo that, subject to external influence, optimally reproduce the observations delivered in the other parts of the project. Through using out combination of laboratory experiments, data analyses and numerical simulations, we aim to understand how the magnetic field changed during this period yielding vital information about the evolution of deep Earth's structure and also about the level of protection that early multicellular life had from harmful solar wind radiation.