Targeted palaeomagnetic studies and geomagnetic field modelling to probe dynamics of the geodynamo
Lead Research Organisation:
University of Liverpool
Department Name: Earth, Ocean and Ecological Sciences
Abstract
The Earth's magnetic field is generated by fluid motions in the outer core (the geodynamo). Whilst the magnetic field we measure at the Earth's surface can be approximated to that of a bar magnet at the centre of the Earth (roughly aligned with the geographic North and South Pole) it is in fact much more complicated than this. The field varies on many different timescales (e.g. years, decades, centuries, millennia). By studying variations in the geomagnetic field this tells us about the deep Earth where the field is generated. Thus surface measurements, and models derived from them, provide a window in to the deep Earth. In this project we propose to investigate dynamics of the core and the influence of core mantle interactions that affect the geomagnetic field on the thousand to million year timescale. Whilst the present day field is very well known from satellite and direct measurements, in order to see variations in the past we have to look at the record contained in burnt archaeological and geological material - the study of palaeomagnetism. The problem is that palaeomagnetic data are not as numerous or as well distributed over the globe as direct observations. In fact, most palaeomagnetic data are from the northern hemisphere and there are far more directional than field strength (palaeointensity) data which makes modelling and geophysical inferences from the models more challenging. Consistent features of the geomagnetic field seen over time have been interpreted as mantle control on the geodynamo. Two features clearly seen in field models of the present day and over the last 400 years are: four high intensity patches of flux at high latitudes symmetric about the equator seen in models at the core mantle boundary and; secondly, low secular variation seen in the Pacific (as opposed to the Atlantic) Hemisphere at the Earth's surface. In this project we will use a combined approach of targeted acquisition of palaeomagnetic data and geomagnetic modelling methods in order to determine: are flux lobes symmetric about the equator? How dynamic and long lived are they? How persistent and what is the extent of low secular variation in the Pacific Hemisphere? This will allow determination of whether these features are temporary or fundamental characteristics of the geomagnetic field. Flux lobes are assumed to be signatures of whole core processes rather than localised at the core surface; by determining symmetry between northern and southern hemispheres this will be tested. We will generate high quality palaeomagnetic data (directions and intensity if possible; otherwise, concentrating on palaeointensity) from the SW Pacific region. The SW Pacific has been targeted as it is away from Hawaii where most Pacific data currently stems, is directly influenced by one of the flux lobes and is currently a region of very high non dipole field. We will study ceramics from the SW Pacific Islands, volcanic rocks from New Zealand and Australia concentrating on well dated material from the last 10,000 years but also including less well dated material up to 5Ma. Detailed studies will be carried out to determine the suitability of samples for palaeointensity experiments as whilst the basis behind the method is simple in practise there are many factors that can ruin or bias experiments. An associated project student will investigate different palaeointensity methods including using single crystals as opposed to the usual whole rock samples. Combining our new palaeomagnetic data with archived data, geomagnetic field models will be constructed. We will test whether the palaeomagnetic data allow, and a model can cope with, some additional constraint such as synthetic data which does have symmetric flux lobes. The generated palaeomagnetic data and the new field models will be of use not only to those interested in the Earth's magnetic field and deep Earth processes but also geophysicists, climatologists and archaeologists.
Publications
Biggin A
(2013)
The effect of cooling rate on the intensity of thermoremanent magnetization (TRM) acquired by assemblages of pseudo-single domain, multidomain and interacting single-domain grains
in Geophysical Journal International
Brown M
(2015)
GEOMAGIA50.v3: 1. general structure and modifications to the archeological and volcanic database
in Earth, Planets and Space
Brown M
(2015)
GEOMAGIA50.v3: 2. A new paleomagnetic database for lake and marine sediments
in Earth, Planets and Space
Ertepinar P
(2020)
Extreme geomagnetic field variability indicated by Eastern Mediterranean full-vector archaeomagnetic records
in Earth and Planetary Science Letters
Ferré E
(2014)
Coseismic magnetization of fault pseudotachylytes: 1. Thermal demagnetization experiments
in Journal of Geophysical Research: Solid Earth
Fu C
(2015)
Occurrence of greigite in the P liocene sediments of L ake Q inghai, C hina, and its paleoenvironmental and paleomagnetic implications
in Geochemistry, Geophysics, Geosystems
Grappone J
(2020)
Comparison of Thermal and Microwave Paleointensity Estimates in Specimens Displaying Non-Ideal Behavior in Thellier-Style Paleointensity Experiments
in Journal of Geophysical Research: Solid Earth
Greve A
(2017)
The geomagnetic field intensity in New Zealand: palaeointensities from Holocene lava flows of the Tongariro Volcanic Centre
in Geophysical Journal International
Hammond M
(2016)
An Archaeomagnetic Study of a Roman Bath in Southern France
in Archaeometry
Hill M
(2020)
Persistent westward drift of the geomagnetic field at the core-mantle boundary linked to recurrent high-latitude weak/reverse flux patches
in Geophysical Journal International
Hodgson E
(2018)
Thermoremanent Behavior in Synthetic Samples Containing Natural Oxyexsolved Titanomagnetite
in Geochemistry, Geophysics, Geosystems
Lougheed B
(2014)
A deglacial palaeomagnetic master curve for Fennoscandia - Providing a dating template and supporting millennial-scale geomagnetic field patterns for the past 14 ka
in Quaternary Science Reviews
Mellström A
(2015)
Post-depositional remanent magnetization lock-in depth in precisely dated varved sediments assessed by archaeomagnetic field models
in Earth and Planetary Science Letters
Nilsson A
(2014)
Reconstructing Holocene geomagnetic field variation: new methods, models and implications
in Geophysical Journal International
Nilsson A
(2018)
Short-Term Magnetic Field Variations From the Post-depositional Remanence of Lake Sediments
in Frontiers in Earth Science
Paterson G
(2015)
Thellier-type paleointensity data from multidomain specimens
in Physics of the Earth and Planetary Interiors
Poletti W
(2013)
The cooling-rate effect on microwave archeointensity estimates
in Geophysical Research Letters
Snowball I
(2013)
Geomagnetic and mineral magnetic characterization of the Anthropocene
in Geological Society, London, Special Publications
Snowball I
(2013)
An estimate of post-depositional remanent magnetization lock-in depth in organic rich varved lake sediments
in Global and Planetary Change
Description | By using a novel approach to address dating uncertainties in sedimentary data a new geomagnetic field model covering the last 9000 years pfm9k.1 was developed. This model demonstrates evidence for dominantly westward drift of northern high latitude high intensity flux patches at the core mantle boundary for the last 4000 years. Intermittent occurrence of reversed flux at the edge of, or inside the inner core tangent cylinder is also observed which may possibly originate from the equator. By constructing a SW Pacific archaeointensity database we have found that over the last 3000 years there were periods of low geomagnetic field strength in the SW Pacific around 1000 BC, 250 BC and 1000 - 1400 AD and that the most prominent peak in field strength occurs at around 0 AD. This shares similarities to the record found for China. When the new dataset is added to our geomagnetic field model and pushed to fit the data, these new models show interesting features such as growth of reverse flux patches beneath the SW Pacific, reminiscent and possibly analogous to the present day field and the South Atlantic Anomaly. The associated studentship has characterised the partial thermoremanent magnetisation (pTRM) behaviour of synthetic samples containing natural oxyexsolved titanomagnetite and compared them to that of synthetic samples containing multi domain magnetite grains. We have shown that the samples containing the closely packed single domain grains of oxyexsolved titanomagnetite are subject to many of the same sources of non ideal behaviour during palaeointensity experiments as those of homogeneous magnetite. This has significant implications for the interpretation of future palaeointensity results performed on natural samples with similar magnetomineralogy. |
Exploitation Route | The model is already being used by other researchers and researchers are waiting for the finalised palaeomagnetic dataset to use in their own work. Data and model are likely to be used for the development of secular variation dating curves for the SW Pacific region for use by archaeologists. The study using synthetic samples containing natural oxyexsolved titanomagnetite will aid future studies on palaeointensity methodology - both in interpretation and also how best to tailor experiments to maximise reliability. |
Sectors | Environment,Culture, Heritage, Museums and Collections |
Description | Institute of Rock Magnetism, Minnesota, Rock Magnetism Summer School |
Amount | £300 (GBP) |
Organisation | University of Minnesota |
Department | Institute of Rock Magnetism |
Sector | Academic/University |
Country | United States |
Start | 06/2013 |
End | 07/2013 |
Title | SW Pacific Archaeointensity Database |
Description | A new archaeointenisty dataset ranging in age from 1200 BC - 1600 AD for the SW Pacific region has been obtained. |
Type Of Material | Database/Collection of data |
Provided To Others? | No |
Impact | This has been a major output from our research. We are using it along with geomagnetic models developed to investigate features of the geomagnetic field. Other researchers are waiting for the finalised dataset in order to use it for their own work. Invited talks at international conferences |
Title | Thermoremanent Behavior in Synthetic Samples Containing Natural Oxyexsolved Titanomagnetite |
Description | Within the National Geoscience Data Centre (NGDC) is held the raw experimental data generated and used for the G cubed publication 'Thermoremanent Behavior in Synthetic Samples Containing Natural Oxyexsolved Titanomagnetite'. It consists of rock magnetic data (SEM, forc data and data from a Variable Field Translation Balance) along with all the partial thermoremanent experimental data. |
Type Of Material | Database/Collection of data |
Year Produced | 2018 |
Provided To Others? | Yes |
Impact | This has been a major output from our research and has been presented at international conferences. |
URL | http://www.bgs.ac.uk/services/ngdc/citedData/catalogue/8e0f07ce-d8ef-4286-92bd-9019875df5cb.html |
Title | pfm9k.1 |
Description | A new geomagnetic field model covering the last 9000 years pfm9k.1 has been published. The model utilises a novel approach to address dating uncertainties in sedimentary data. Individual timescales of each record were adjusted whilst preserving stratigraphic order, to find the optimal fit to a preliminary model prediction. Using this approach, combined with new ways of weighting the data, inconsistencies within the database were removed, thus improving the temporal resolution of the model. |
Type Of Material | Computer model/algorithm |
Year Produced | 2014 |
Provided To Others? | Yes |
Impact | This has been a major output from our research. We are using it along with the palaeomagnetic data produced to investigate features of the geomagnetic field. The model has been incorporated into the geomagia database and its outputs are being used by other researchers. Invited talks at international conferences Model files are deposited in the National Geoscience Data Centre. |
URL | http://earthref.org/ERDA/1951/ |
Description | Auckland |
Organisation | University of Auckland |
Country | New Zealand |
Sector | Academic/University |
PI Contribution | Archaeomagnetic Research |
Collaborator Contribution | Supply of dated ceramics from SW Pacific and archaeological information |
Impact | Multi-disciplinary archaeology and geophysics. Majority of samples analysed. Joint conference presentations. |
Start Year | 2007 |
Description | Australian samples |
Organisation | La Trobe University |
Country | Australia |
Sector | Academic/University |
PI Contribution | Determination of archaeomagnetic information from supplied Australian material. |
Collaborator Contribution | Supplied with dated Australian archaeological material. |
Impact | Archaeomagnetic data obtained forms part of SW Pacific dataset to be used for geomagnetic modelling. Results also aid archaeological understanding of interest to the collaborators. Collaboration is multi-disiplinary:- archaeology and geophysics |
Start Year | 2008 |
Description | GFZ |
Organisation | Helmholtz Association of German Research Centres |
Department | German Research Centre for Geosciences |
Country | Germany |
Sector | Private |
PI Contribution | Collaboration of geomagnetic modelling and updating of the geomagia database |
Collaborator Contribution | Research visits. Collaboration of geomagnetic modelling and updating of the geomagia database |
Impact | Joint publication and conference presentations. |
Description | New Zealand |
Organisation | Victoria University of Wellington |
Country | New Zealand |
Sector | Academic/University |
PI Contribution | Archaeomagnetic expertise Laboratory facilities and measurements Hosting visits by collaborators Associate Investigator on Royal Society of New Zealand Marsden Fund grant held by collaborators. |
Collaborator Contribution | Assisted in field collection of New Zealand lavas Helped in sample selection and delivery of ceramic samples. Secured Royal Society of New Zealand Marsden Fund grant which in part funded collaboration (laboratory costs and travel for NZ personnel to visit Liverpool). |
Impact | Joint conference presentations. Ongoing research |
Start Year | 2011 |
Description | Geomagnetic Field Modelling and Palaeo/Archaeomagnetic Data Workshop |
Form Of Engagement Activity | Participation in an activity, workshop or similar |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Other academic audiences (collaborators, peers etc.) |
Results and Impact | As part of impact plan workshop organised to bring together geomagnetic data gatherers, modellers, users and interested members of the public to share information, stimulate discussion and to determine future strategies to further understanding of geomagnetic field behaviour. Much lively discussion was had sparked by key note talks and led discussione themes. The amount of discussion, the positive comments from all attendees and the concencus that a smiliar event should be held in the future. |
Year(s) Of Engagement Activity | 2014 |
URL | http://pcwww.liv.ac.uk/~nilsson/workshop2014.html |
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 | Contributing to blog Blog instigated by Liverpool colleague to share geomagnetic research with the general public now has wider community contributing |
Year(s) Of Engagement Activity | 2012,2013,2014 |
URL | http://www.geomagnetism.org |
Description | IAGA Teachers Workshop Mexico |
Form Of Engagement Activity | Participation in an activity, workshop or similar |
Part Of Official Scheme? | Yes |
Geographic Reach | International |
Primary Audience | Schools |
Results and Impact | Talk and demonstration on how to make a simple magnetomter for school teachers. Questions and disucssions afterwards Contacted by American High School Teacher who now has students making magnetometers for projects. |
Year(s) Of Engagement Activity | 2013 |
Description | Isle of Man Astronomical Society |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | Regional |
Primary Audience | Public/other audiences |
Results and Impact | 60 people attended talk on planetary magnetism, but a question about reversals allowed an additional preprepared talk segment on magnetic reversals. Significant in raising understanding of geophysical science among a related, scientifically interested, audience. |
Year(s) Of Engagement Activity | 2014 |
Description | Liverpool Geological Society |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | Regional |
Primary Audience | Public/other audiences |
Results and Impact | Many positive responses recieved after talk Many positive responses recieved after talk so helped in maintaining public understanding of geological science. |
Year(s) Of Engagement Activity | 2014 |
Description | Radio Sweden (SR) program Vetandets värld interview |
Form Of Engagement Activity | A press release, press conference or response to a media enquiry/interview |
Part Of Official Scheme? | No |
Geographic Reach | National |
Primary Audience | Public/other audiences |
Results and Impact | Interviewed about magnetic reversals. Apparently pulled out excerpt of interview for use in advertising the show. Approached for a followup interview in another radio show The programme was aired again 2014 |
Year(s) Of Engagement Activity | 2013 |
URL | http://sverigesradio.se/sida/avsnitt/246034?programid=412 |
Description | Scarisbrick Hall School talk |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | Local |
Primary Audience | Schools |
Results and Impact | Talk to school on how to make a simple magnetometer. Talk sparked questions and discussion afterwards After talk, further requests were made for similar talks |
Year(s) Of Engagement Activity | 2013 |