Targeted palaeomagnetic studies and geomagnetic field modelling to probe dynamics of the geodynamo

点击翻译按钮获取中文摘要

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.