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

Lead Research Organisation: University of Liverpool

Abstract

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.

Publications

10 25 50