Impact Objectives • Construct the first composite curve of the geomagnetic field intensity variations over the past 5 million years by integrating measurements of cosmogenic beryllium-10 (10Be) production with records of relative paleointensity derived from the natural remanent magnetization of deep-sea sediments • Investigate the field changes during geomagnetic reversals • Test predictions of numerical geodynamo simulations against the data
Insights into the Earth’s magnetic field changes Jean-Pierre Valet from the Institut de Physique du Globe de Paris discusses his latest work determining with accuracy the variations of the geomagnetic field intensity during the past million years Could you explain a little more about the structure of the Earth’s magnetosphere? The magnetosphere defines the extent of the Earth’s magnetic field in space which extends to distances larger than about five times the radius of the Earth. The field is strongly modified by the interaction with the solar wind that compresses the field on the day side. The magnetic field reduces the cosmic ray flux at low latitudes because the particles impinge at right angles to the horizontal field lines and are therefore deflected. At high latitudes cosmic rays of all energies can reach the Earth because the magnetic lines extend far out into space and guide the particles at the top of the atmosphere. Therefore changes in magnetic dipole intensity should have no effect at high latitudes and be quite significant at low latitudes, but large atmospheric mixing homogenises the amount of 10Be at all latitudes. Could you explain a little more about how changes in magnetic dipole occur? The Earth’s magnetic field can be described
by a relatively stable dipole that represents 80-90 per cent of the field that is usually aligned closely with the Earth’s rotation axis and a rapidly time varying 10-20 per cent non-dipole field. Since the dipole dominates, the magnetic poles are close to the geographic poles, which is why a compass works. In fact, the orientation of the dipole changes slightly with time. However when this is averaged over a few thousand years the dipole axis is perfectly aligned with the rotation axis. In other words, the magnetization of sediment that was deposited relatively slowly will point north or south depending on the field polarity. Occasionally the dipole reverses completely, such as switches its orientation from north to south or south to north defining a polarity reversal. The 180° change in orientation during reversals is accompanied by a significant drop of field intensity which can be reduced to 5-10 per cent of the present field value. What impact do these changes in the geodynamo have on the planet? The magnetosphere surrounding the planet is usually regarded as a shield which protects us against penetration of highly energetic cosmic and solar particles. Several
studies focused on apparent correlations between evolutions of species, particularly radiolarians and field reversals. The correlations were disputed and disregarded on statistical grounds, as well as because the atmosphere is much more efficient than the geomagnetic field to protect us against harmful radiations. There is no reason why the atmosphere would have been affected during relatively short periods of weak geomagnetic field. We know that field reversals and short events are always associated with very low field strength and a complex geometry which is likely multipolar. In such conditions the entire atmosphere becomes more accessible to relatively low energetic solar protons. Ionization in the middle atmosphere initiates a chain of chemical reactions that yield production of nitric oxide, which is a well-known ozone depleting chemical. Simulations have shown that in the absence of field the total ozone low can reach 40 per cent at mediumhigh latitudes particularly in the northern hemisphere. The resulting UV-B flux increases by up to 20 per cent which is three to five times as large as the present annual southern hemisphere ozone hole. www.impact.pub 35
Defining mechanisms that drive magnetic field reversals The EDIFICE project is mapping the intensity variation of the Earth’s geomagnetic field over the last five million years, ultimately identifying the processes that are associated with reversals of the magnetic field The Earth’s magnetic field is driven by electrical currents of a conducting fluid within the Earth’s core and could be regarded as a stable field. However, it is well known that there have been periods, even in fairly recent history, when that magnetic field has experienced large changes, right up to a complete reversal of poles. The five-year project EDIFICE (Changes in the geomagnetic dipole (Earth Dipole Field Intensity from Cosmogenic Elements) funded by EU Seventh Framework Programme is attempting to unravel some important features behind the variations of the geomagnetic field. It is hoped they can answer four key questions: What controls the moment strength and why does it fluctuate? What causes polarity reversals? Why does the field successfully reverse and fail in some other cases? What is the ratio of aborted versus successful reversals? EDIFICE Principal Investigator Jean-Pierre Valet is a Research Director at the Centre National pour la Recherche Scientifique (CNRS) and has been working in this field for many years. He explains that the
present-day Earth’s magnetic field can be regarded as a dipole field similarly to the magnetic distribution of a bar magnet, that would be tilted by about 10° with respect to the Earth’s rotation axis: ‘Data has shown that the orientation of Earth’s magnetic field has experienced multiple reversals, with geomagnetic north becoming geomagnetic south and vice versa – known as a geomagnetic reversal.’ But, in fact, the field attempted many more times to reverse without being successful. Whether successful and failed reversals have different signatures, it is important to understand the processes that are responsible for field generation. The Earth’s magnetic field is believed to have alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field switched. ‘The history of reversals has been deciphered using the magnetisation of lava flows or from sequences of sediments which contain small magnetized particles that are oriented by the field and preserve their orientation when the rock consolidates,’ Valet outlines.
Geomagnetic intensity variations during the past 2Myr. Black and white bars indicate field polarity. 36 www.impact.pub
MAPPING POLARITY CHANGES While there is much speculation about processes and mechanisms that lead to geomagnetic reversal, there is a distinct lack of hard evidence. Understanding which processes govern the geomagnetic field requires a detailed knowledge of field evolution in the past, specifically its intensity. Since the start of the EDIFICE project in 2014 the team has focused on understanding the phenomenon and constructing a composite curve of geomagnetic field intensity variations, which they hope will go a long way to answering both of those questions. Their first goal is to gather data and construct the field intensity curve covering the past five million years. The originality and the challenge of the programme is to combine two independent indicators of the field variations that are recorded in worldwide marine sedimentary sequences. They also plan to investigate the variations of the magnetic vector (direction and intensity) across a few field reversals that have been recorded in a number of sequences of overlying lava flows. Finally, the researchers will then be able to test the
Geomagnetic intensity during the last reversal from magnetic (red) and beryllium ten (green) measurements.
10 Be measurements and records of relative paleointensity derived from the magnetization of the same sediments. ‘By integrating measurements of cosmogenic 10 Be radioisotope production with records of relative paleointensity of multiple deep sea sediments, the hope is to construct a curve that will be a unique reference for the evolution of geomagnetic intensity for the five million year period under review.’
We hope to construct a reference curve that will also act as a stratigraphic reference for the five million year period under review predictions of numerical dynamo models against the record of field evolution and the dynamical structure of the reversing field. The geomagnetic field strength and its geometry impose the shape of the magnetosphere surrounding the planet. This is usually regarded as a shield that modulates the amount of cosmic radiation (protons, Helium and heavier nuclei and electrons) that penetrate the atmosphere. When the Earth’s magnetic field weakens such as during reversals, energetic cosmic particles can progress deep in the atmosphere. ‘They collide with more atmospheric constituents such as nitrogen and oxygen and these interactions produce cosmonuclides like carbon-14 and beryllium-10 (10Be). Therefore, the evolution of field intensity in the past can be documented by measuring the amount of cosmonuclides that were incorporated over the years within deep-sea sediments,’ Valet observes. Due to its 1.36 Ma long half-life, 10 Be production is supposed to document the field intensity over several million years. ‘The beryllium measurements are performed using the ASTER mass accelarator of Cerege (Aix en Provence). This work is being done in collaboration with Professor Nicolas Thouveny, Professor Didier Bourlès and Dr Quentin Simon. In parallel, datation of the sediments is conducted at Laboratoire des Sciences du Climat et de l’Environnement (Gif-sur-Yvette) by Dr Franck Bassinot by measuring oxygen isotope ratios that recorded the succession of the climatic cycles,’ he says. CONSTRUCTING A REFERENCE CURVE The main tool for the EDIFICE team’s investigation is to combine
So far, knowledge of the changes in geomagnetic intensity over long time periods has been mostly acquired by measuring the magnetization intensity of sediments. This technique requires homogeneous sedimentary sequences and is affected by uncertainties. The best option to improve data quality is to compare two independent indicators. Integration of cosmogenic 10Be production with relative paleointensity will provide a unique and highly reliable reference curve of the geomagnetic field variations for the past 5 million years. The researchers are also focusing on direction and intensity variations during the period of transition when the field flips between the two polarities. Because these transitions are very short (a few thousand years at most), it is appropriate to turn towards lava flows as their magnetization is locked rapidly during cooling. ‘The counterpart is that volcanic eruptions tend to be irregular and relatively short-lived when they do occur,’ Valet says. The team has selected volcanic sequences from the Cape Verde and Canary Islands. They have just completed a preliminary field trip and identified several sequences of potential interest, and are now checking whether they successfully recorded a reversal. VALUABLE PROGRESS Paleomagnetic records have proved accurate in determining the dipole field changes that have occurred over the last 2 million years. Valet points out that data which is older tends to require a good deal of adjustment: ‘As an added problem, there have been very few detailed investigations into the generation and distribution of 10Be, so part of the investigation is associated with determining the precision and reliability of the data that will also yield interesting information about how 10Be is incorporated in the sedimentary archives.’
place, the actual effects of a dipole reversal can be investigated. The final results will constrain the models of the geodynamo and address questions such as the influence of orbital variations of the Earth’s rotation axis (e.g. the 41 thousand year periodicity of the obliquity). With the suggestion that geomagnetic events align sometimes with significant climate occurrences, it is important that the team can establish whether there is an actual link.
Project Insights FUNDING This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement number 339899 CONTACT Jean-Pierre Valet Principal Investigator T: +33 183957503 E:
[email protected] W: http://cordis.europa.eu/project/ rcn/188497_en.html PRINCIPAL INVESTIGATOR BIO Jean-Pierre Valet is Research Director at CNRS at the Institut de Physique du Globe de Paris. He has authored and co-authored more than 130 publications and supervised 12 PhD students. His scientific activity is primarily focused on the geomagnetic field variations in the past, with a special interest for the evolution of field intensity, polarity reversals and excursions. Valet is also interested by the mechanisms involved in magnetisation acquisition and other applications of magnetism such as biomagnetism and paleoenvironmental studies. Valet has been awarded the CNRS silver medal and the European Geosciences Union Petrus Peregrinus medals, and is a fellow of the American Geophysical Union.
With the project now just over the half way mark the EDIFICE investigation still has much work to complete, but from Valet’s perspective being able to make a correlation between instances of 10Be and geomagnetic events is a major achievement: ‘With the principle of assessing a link in www.impact.pub 37
