Phanerozoic palaeomagnetic variations and their implications for the Earth's deep interior

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This project will describe and model long term variations in Earth's magnetic field through the last 500 million years in order to address three major linked controversies regarding the dynamics and evolution of Earth's deep interior: the mobility of large seismically anomalous structures in Earth's lowermost mantle, the age of Earth's inner core, and the nature and causes long-term palaeomagnetic variations. The geomagnetic field, generated in Earth's outer core by the geodynamo process, is a fundamental property of the planet. It shields the atmosphere and life from harmful solar wind radiation and provides a window to the deep interior of the planet which, through palaeomagnetic records, has the unique geophysical capability of being extended back through geological time. Long term variations in Earth's magnetic field (more than 10 million years) are widely thought to reflect the influence of core-mantle evolution on the geodynamo and therefore could provide badly-needed observational constraints on poorly understood deep Earth processes. In particular, they could help resolve a major outstanding controversy concerning the mobility of large structures in the Earth's lower mantle which are argued by some to be static long-lived "thermochemical piles" that dictate the nature of plate tectonics at Earth's surface, but by others to be transient "superplumes" forming in response to surface-dominated processes. They could also help to resolve a major outstanding controversy concerning the age of Earth's inner core; a topic on which members of the assembled research team have recently published conflicting results (differing by approximately 1 billion years) in the leading multidisciplinary journal, Nature. Previous work by our team has focused on palaeomagnetic variations and core-mantle evolution in the Precambrian (older than 540 million years). Now we are in a position to directly address both of the above controversies and the broader question of what, precisely, controls the long term magnetic variations, by focusing on the last 500 million years. In order to do this, we need to answer some fundamental questions:1. Are palaeomagnetic variations cyclic (with a period of ca. 200 million years) as current records hint?2. Do all aspects of palaeomagnetic field behaviour (polarity reversal frequency, field strength, and short-term directional variability) vary simultaneously in a predictable way? 3. How would we expect different degrees of mobility of the large lower mantle structures to affect palaeomagnetic behaviour (E.G. In a cyclic way way and/or with all the different aspects of palaeomagnetic field behaviour changing simultaneously?)?4. How would we expect different inner core ages to manifest themselves in palaeomagnetic behaviour over the last 500 million years?In this project, we will firstly use new measurements and recently published analysis techniques to answer questions 1 and 2 and build a description of palaeomagnetic variations in the last 500 million years that is unprecedented in its quality and usefulness. To answer questions 3 and 4, we will also produce synthetic records of palaeomagnetic variations from numerical models of the geodynamo subject to different core-mantle evolution scenarios. By comparing and contrasting the new observational and synthetic records, we will then be in a position to determine which scenarios are the most realistic (static thermochemical piles or mobile superplumes? Young or old inner core?), to address the outstanding controversies, and to move towards a fully integrated model of core-mantle evolution incorporating the Earth's magnetic field.