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

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.

Non-academic beneficiaries of the proposed research include:

1. Members of the Hydrocarbon Sector (conventional and shale gas)
2. Suppliers of specialist stratigraphic services to the Hydrocarbon Sector
3. Government and state geological surveys
4. Educational bases
5. The general public

Means by which they will benefit:
We are proposing to undertake magnetostratigraphic studies within the time period 320 to 370 million years ago which will ultimately result in gains for beneficiaries 1-4. Because of the globally synchronous and rapid nature of geomagnetic reversals, magnetostratigraphy can be used as a powerful correlation tool across geoscientific sectors.

Magnetostratigraphy is being increasingly used by oil companies (beneficiary 1), to supplement other means for high resolution stratigraphic correlation and chronostratigraphic dating which is then built into basin and reservoir layer models (e.g. Playton et al. 2013, West Australian Basins Symposium 2013, Perth, 18-21 Aug; http://www.searchanddiscovery.com/abstracts/html/2013/90163ace/ abstracts/rat.htm;). This data contribute to investigating exploration targets, and aids in reservoir development models. Such information is supplied through the hydrocarbon service sector (beneficiary 2; UK has many strong companies operating globally in this area) which utilises a geomagnetic polarity timescale. However, for reservoirs older than 250 million years ago, such a timescale does not currently exist. Beneficiaries 1 and 2 will be targeted through the attendence of the Lancaster Co-I at a major petroleum industry conference (AAPG 2017, see Pathways to Impact for more information).

This project will lay the foundation for future work to complete a geomagnetic polarity timescale in the interval 320-370 Myr and subsequently, across the whole Palaeozoic (250-540 Myr). Once the final timescale is developed (which may take 20-30 years), it will improve the chronostratigraphy of the studied successions and so help mapping and sub-surface models such as those developed by various government and state geological surveys (beneficiary 3). In sections which are widely used for teaching (beneficiary 4; e.g. some of the UK and Spitsbergen sections), the long-term improved age control will likely improve the utility of the studied sections for teaching, since external-basinal controls will be clearer, and therefore be able to fit into a more global framework. Beneficiary 3 will be targeted via a broad-interest article planned for publication in a widely-read Earth Science news publication (GSA Today, see Pathways to Impact for more information).

Beneficaries 4 and 5 would also benefit from the fundamental advance in our understanding of the two main layers of our planet: (core and mantle) that we are aiming to make. Teachers, students, and the public at large are excited by fundamental unsolved problems such as the influence that processes, hidden deep within the Earth, can have on our daily lives. We are proposing to shed light on the processes responsible for the magnetic field that shields us from harmful solar wind radiation and mantle convection which dictates the forms of continents and the distribution of natural resources. If these findings can be properly communicated, society would gain from increased interest and engagement with the Earth sciences. Beneficiaries 4 and 5 will be targeted through a via of outreach and public engagement mechanisms (see Pathways to Impact for more information).