Very long timescale variations in the palaeomagnetic record and the evolution of the Earth's deep interior

Lead Research Organisation: University of Liverpool
Department Name: Earth, Ocean and Ecological Sciences

Abstract

The Earth's magnetic field is produced by the geodynamo in the molten outer core of the planet and varies dramatically in time, becoming stronger and weaker and occasionally reversing its polarity altogether. The field plays a crucial role, not only in providing a means of navigation for humans and other species, but also in protecting the Earth's surface and atmosphere from the solar wind. It is important that we study these variations because if the magnetic field were to weaken substantially in the future, artificial satellites and power stations would be more prone to being damaged by this stream of charged particles, and rates of skin cancer among humans could also increase. The study of variations of the magnetic field can also tell us about the deep interior of the planet, where the field is generated, but which we are not able to access directly. Scientists study short-term variations of the magnetic field for this purpose using data obtained by magnetic observatories and satellites. However, we are also fortunate that many rocks 'lock in' the direction and / or strength of the magnetic field from the time they form. 'Palaeomagnetic' measurements from these rocks (together with estimates of their age) allow us to study the variations of the field over much longer periods than do satellite or observatory data: thousands, millions, or even billions of years. A recent study led by the principle investigator of the proposed project used palaeomagnetic data to study the intrinsic variation of the Earth's magnetic field around two and a half billion years ago and concluded that it was significantly different, on average, from that of the more recent field. Specifically, the directional variability of the field as viewed at low latitudes appears to have been reduced relative to more recent times, possibly indicating that the field was in a more stable state in which it reversed its polarity less often. The proposed project aims to find out when, how, and why it changed from this one state to the other. We will constrain the timing and nature of the transition by undertaking new analyses of palaeomagnetic data (both directional and intensity) from rocks which are younger than 2.5 billion years. We will also use artificial data taken from mathematical models of the geodynamo to try and understand why these changes occurred. Finally, in a related PhD project, we will undertake a palaeomagnetic study of rocks from South Africa which are nearly 3.5 billion years old to extend our knowledge of the behaviour of the Earth's magnetic field back even further. Knowledge of how the Earth's magnetic field has varied over billions of years has the potential to tell us a great deal about the evolution of the planet's deep interior. Changes that might have affected the geodynamo over these very long timescales include the growth of the underlying solid inner core and the convection of the overlying mantle. Recent studies have used numerical models to try and understand how these changes might have affected the geodynamo. We propose to tie in the observations we make from the palaeomagnetic record with the results of these models so that we can interpret them in the most realistic way.
 
Description A major outcome was a multi-disciplinary investigation into how the Earth's solid mantle influences the generation of the magnetic field in the underlying liquid outer core. We found evidence to support new links between two mantle processes and changes in the magnetic field behaviour over tens of millions of years. Both potential links need to be looked at carefully in the future as they might provide significant insights into the evolution of the planetary interior.

We have developed new tools for analysing records of the ancient magnetic field derived from igneous rocks.

One of these tools measures the amount of similarity (correlation) between lava flows that formed on top of one another in major volcanic events. It showed that similar directions did not necessarily indicate that lavas formed immediately after one another as others have claimed. The finding that directions can correlate over much longer time periods than people thought is a surprising one.

Another tool is a new set of criteria for grading measurements of the ancient strength of the magnetic field. This is a controversial area and it is hoped that the new criteria will enable the community as a whole to make progress at a higher rate than previously possible.

The final tool again relates to measurements of the ancient direction of the magnetic field from rapidly cooled igneous rocks. For decades, we have used the scatter of such directions as an indicator of the stability of magnetic field. We have developed a new distribution that is, in many cases, a better fit to real data and which therefore describes this scatter more reliably than current methods. This new technique also showed a new transition in magnetic field behaviour around ten million years ago
Exploitation Route The core-mantle links we proposed will be tested by better datasets and modelling results in the future.

The three techniques developed will all likely be applied in future palaeomagnetic studies.
Sectors Environment

 
Description Our 2012 paper was picked up by ScienceDaily.com http://www.sciencedaily.com/releases/2012/07/120729142156.htm
First Year Of Impact 2012
Sector Education
Impact Types Cultural

 
Description Leverhulme Research Leadership Award
Amount £999,592 (GBP)
Funding ID RL-2016-080 
Organisation The Leverhulme Trust 
Sector Academic/University
Country United Kingdom
Start 07/2017 
End 07/2022
 
Description NERC Standard Grant
Amount £436,183 (GBP)
Funding ID NE/P00170X/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 12/2016 
End 08/2020
 
Description NERC Strategic Capital Grant
Amount £384,814 (GBP)
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 12/2013 
End 12/2015
 
Description Royal Society International Exchanges
Amount £3,000 (GBP)
Funding ID IE131187 
Organisation The Royal Society 
Sector Academic/University
Country United Kingdom
Start 03/2014 
End 06/2014
 
Description Royal Society International Exchanges RFBR costshare
Amount £12,000 (GBP)
Funding ID IE150283 
Organisation The Royal Society 
Sector Academic/University
Country United Kingdom
Start 09/2015 
End 08/2017
 
Title PINT database 
Description A global open-access database of absolute palaeointensity has been continually updated, analysed, and improved. 
Type Of Material Database/Collection of data 
Provided To Others? Yes  
Impact Three papers have been published describing updates to the database: Biggin et al. (2009a,b) Biggin & Paterson (2014) These have so far been cited more than 50 times. Additionally, it formed the basis for a working group at a recent international workshop in Oslo. 
URL http://earth.liv.ac.uk/pint/
 
Description Bernhard Steinberger 
Organisation Helmholtz Association of German Research Centres
Department German Research Centre for Geosciences
Country Germany 
Sector Private 
PI Contribution Provided expertise on long-term geomagnetic field behaviour. Lead-authored a joint publication
Collaborator Contribution Provided expertise on and results from mantle flow modelling
Impact Biggin et al. (2012)
Start Year 2010
 
Description Greig Paterson 
Organisation Chinese Academy of Sciences
Country China 
Sector Public 
PI Contribution Provided specialist palaeointensity expertise and measurement data.
Collaborator Contribution Provided computer modelling results and written up research
Impact Paterson et al. (2012) Paterson et al. (2014) Biggin et al. (2014)
Start Year 2011
 
Description Julien Aubert 
Organisation Paris Institute of Earth Physics
Country France 
Sector Academic/University 
PI Contribution Provided expertise of long term geomagnetic field behaviour. Ran dynamo models.
Collaborator Contribution Provided expertise of geodynamo modelling results and computer simulation.
Impact Biggin et al. (2012) Suttie et al. (submitted)
Start Year 2010
 
Description Geomagnetism.org Blog 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Numerous responses to blog posts.

Researchers outside of immediate team have proactively offered to write blog pieces, widening the appeal of the website.
Year(s) Of Engagement Activity 2011,2012,2013,2014
URL http://www.geomagnetism.org/
 
Description Media surrounding 2015 inner core paper 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact A press release was made following the publication of Biggin et al. (2015) in Nature. This was picked up well by media organisations internationally (e.g. BBC, Mail online, El Pais, ABC, etc) and involved a radio interview broadcast on the BBC World Service.
Year(s) Of Engagement Activity 2015
URL http://www.bbc.co.uk/news/science-environment-34475464