Rozprawy doktorskie na temat „Geomagnetic field variations”

This thesis comprises two parts. The main part is involved with laboratory studies of the palaeosecular variation of the geomagnetic field as recorded in lake sediments. The natural remanent magnetization of the sediments cored from the two Italian maar lakes, Lago di Monticchio and Lago di Martignano, has been studied. Further studies were carried out on the sediments of Lago di Martignano to determine the cause of large variations in the magnetic intensity of the sediments with an age of ˜ 6000 years BP and it was concluded that this was caused by the arrival of Neolithic man and the advent of agriculture in the catchment area. The directional record for this lake was also compared to the established record for north west Europe. Several declination and inclination features could be correlated between the two records. The record from Lago di Martignano can be accepted as a regional palaeomagnetic reference curve for central Italy. Cores up to 50m long were taken from Lago di Monticchio. Whilst not yet firmly dated, it is agreed that this record spans the last 250,000 years. A relative palaeointensity record has been calculated and spectral analysis has been carried out. It is concluded provisionally that the palaeointensity recorded in the sediments was effected by the astronomical frequencies associated with precession of the earth, the eccentricity and the obliquity of the Earth's orbit. The second part of this thesis is concerned with modelling the palaeosecular variations as recorded in sediments around the world through the Holocene, i.e. the last 10,000 years. The properties of sequential secular variation records from sediments are compared with palaeosecular variation scatter determined from sets of lava flows. It is concluded that a comparison between PSV recorded in lava flows and lake sediments is valid.

A description of the temporal variations of the main geomagnetic field (i. E. , the secular variation, or SV) is crucial to the understanding of core dynamo generation. It is known with high accuracy at observatory locations, which are globally unevenly located, hampering the determination of a global pattern of these variations. Satellites have allowed global surveys of the field and its SV. Their data has been used by global spherical harmonic models using data selection criteria to reduce external contributions. SV small spatial scales may not be well described by these models, and can show significant errors compared to ground measurements. This study attempts to extract temporal variation time series from satellite measurements as it is done at observatory locations. We follow a Virtual Observatories (VO) approach, defining a global mesh of VOs at satellite altitude. We apply an Equivalent Source Dipole (ESD) technique. For each VO and a given time interval all measurements are reduced to a unique location, leading to time series similar to those available at the ground. Synthetic data is first used to validate the approach. We then apply our scheme to Swarm mission measurements. We locally compare the VO-ESD derived time series to ground observations and to satellite-based model predictions. The approach is able to describe field's time variations at local scales. The global mesh of VO time series is used to derive global spherical harmonic models. For a simple parametrization the model well describes the trend of the magnetic field both at satellite altitude and at the surface. Nevertheless more complex modelling can be made to properly profit of VO-ESD time series
La description des variations temporelles du champ géomagnétique (variation séculaire ou SV) est cruciale pour la compréhension de la dynamo. La SV est connue avec une grande précision dans les observatoires magnétiques, qui ont une répartition spatiale inégale. Les satellites donnent des observations globales du champ et de sa SV. Leurs données sont utilisées par les modèles globaux en harmoniques sphériques. Les petites échelles spatiales de la SV décrites par ces modèles peuvent montrer des erreurs par rapport aux mesures des observatoires. Dans cette étude je tente d'extraire des séries temporelles avec des mesures satellitaires comme dans les observatoires. L'approche des observatoires virtuels (VO) est suivie. Un maillage global de volumes à l'altitude du satellite est défini. Pour cela, la technique des Equivalent Source Dipoles (ESD) est appliquée. Pour chaque VO et intervalle de temps donné, toutes les mesures sont réduites à un endroit unique, menant à des séries temporelles similaires à celles disponibles dans les observatoires à la surface. L’approche est validée avec des donnes synthétiques et puis appliquée aux mesures de la mission Swarm. Les séries temporelles VO-ESD sont comparées à celles à la surface et aux prédictions par un modèle. L'approche décrit correctement les variations temporelles du champ à l'échelle locale. Un maillage global de VO est construit et utilisé pour obtenir des modèles globaux. Les modèles sont capables de décrire l'évolution du champ magnétique à la fois à l'altitude du satellite et à la surface. Toutefois des modélisations plus complexes pourront être faites pour profiter des séries temporelles VO-ESD

Parmi les méthodes de reconstitution de l'histoire du champ géomagnétique, l'étude des variations de la production atmosphérique d'isotopes cosmogéniques s'est récemment développé. Cette production est modulée au premier ordre et aux échelles multimillénaires par l'intensité du champ géomagnétique. Son enregistrement dans les archives de l'environnement terrestre en apporte une lecture indépendante, donc complémentaire des méthodes paléomagnétiques. Ce travail vise à retracer les changements de taux de production de 10Be enregistrés dans les sédiments marins, afin de restituer les variations du moment géomagnétique depuis environ 800 000 ans. Les rapports 10Be/9Be authigéniques mesurés par Spectrométrie de Masse par Accélérateur le long de trois séquences de différentes latitudes, permettent de caractériser la production globale de 10Be sur deux intervalles de temps. (1) Lors de la baisse du moment dipolaire associée à l'excursion Laschamp (~41 000 ans BP), la surproduction de 10Be à 38°N et 2°S, confirmée par des mesures de 10Be/230Thxs, est identique à celle restituée dans les glaces du Groenland. (2) L'étude menée sur une carotte prélevée à l'équateur couvrant l'intervalle 800 000 – 250 000 ans BP (époque Brunhes), révèle les phases successives de surproduction globale de 10Be déclenchées par les chutes de moment dipolaires liées à l'inversion Brunhes-Matuyama d'une part et d'autre part à une dizaine d'excursions documentées. La calibration de ces enregistrements avec des valeurs absolues disponibles dans la littérature permet la quantification des moments dipolaires
Among the methods for reconstructing the geomagnetic field history, studying the variations in cosmogenic isotopes production in the atmosphere has recently developed. At multi millennial scales, this production is mainly modulated by the geomagnetic field intensity. Its record in terrestrial archives provides an independent reading to complement paleomagnetic methods. This work aims at tracing the changes in 10Be production rates recorded in marine sediments, in order to reproduce the geomagnetic variations for the past 800,000 years. Authigenic 10Be/9Be ratios measured using Accelerator Mass Spectrometry along three sequences from different latitudes, characterize the 10Be global production during two time intervals. (1) During the dipole moment low associated with the Laschamp excursion (~ 41,000 years BP), the 10Be overproduction at 38°N and 2°S, confirmed by measurements of 10Be/230Thxs, is identical to that recorded in the Greenland ice sheet. (2) Studying a core collected near the equator and covering the interval 800,000 – 250,000 years BP (Brunhes epoch) reveals the successive phases of global 10Be overproductions triggered by dipole moment lows associated to the Brunhes-Matuyama reversal and also to several other documented excursions. Calibrating these records with absolute values available in the literature allows quantifying dipole moments. These are then compared to paleomagnetic reference reconstructions over the same time series. Bearing out the number and extent of these dipole field lows allows considering to refine their chronology before using their features to get a better understanding of the geodynamo rhythms throughout the last Million years

Knowledge of the intensity of the Earth's magnetic field throughout geological time can deliver a wealth of information regarding the development of the planet. The nature of the geomagnetic field is dependent on processes that occur deep in the Earth's core. By analysing long period changes in geomagnetic field intensity inferences can be made about conditions in the Earth's interior far back into Earth history. The microwave palaeointensity technique is a relatively recent addition to palaeomagnetic investigation. High-frequency microwaves, which are resonant with the constituent magnetic system of a rock, are used to isolate and progressively remove the magnetisation of samples acquired at the time of formation in a controlled manner. By exciting the magnetic system directly, thermal-type experiments can be conducted, whilst minimising the risk of chemical alteration, which is a major cause of experimental failure.

Dans ce mémoire nous présentons une série d'études dont le double objectif est de mieux comprendre les phénomènes qui gouvernent l'évolution du champ magnétique principal de la Terre, et d'améliorer notre capacité actuelle à en prédire l'évolution à court terme. Pour cela, nous réalisons d'abord quelques inversions de la Variation Séculaire (VS) pour plusieurs époques et calculons une première estimation de l'accélération de l'écoulement à partir d'écoulements successifs. Ces calculs suggèrent que les écoulements qui génèrent la VS ne sont pas stationnaires. Nous montrons ensuite comment calculer simultanément ces écoulements et leur accélération pour une époque donnée, si le champ ainsi que sa première et seconde dérivées temporelles (la VS et l'accélération séculaire(AS)) sont connues. Ceci nous permet de montrer sans ambiguïté qu'un écoulement stationnaire ne peut pas rendre compte de la AS observée. Nous considérons ensuite la possibilité d'améliorer la prévision à court terme du champ, actuellement basée sur une extrapolation linéaire, dans le cadre du modèle "International Geomagnetic Reference Field" (IGRF), en utilisant une méthode basée sur un écoulement uniformément accélérée. Nous considérons aussi la possibilité d'augmenter la résolution spatiale de la VS prédictive actuellement utilisée par l'IGRF. De ces études, nous concluons que le principal obstacle à la prédiction à court terme du champ vient des changements brusques de la AS (jerks). Nous étudions la nature des changements d'accélération rendant compte du `jerk' de 2003. Nous montrons que ce changement n'est hélas pas compatible avec un mécanisme simple tel que les oscillations de torsion
This thesis reports on a series of investigations directed towards the dual goal of improving our understanding of what drives the time evolution of the Earth's main magnetic field, and of improving our current ability to predict its short-term behavior. For that we first perform core flow inversions for several epochs and compute first estimates of the flow acceleration by computing differences between successive flows. These first results suggest that the flow generating the Secular Variation (SV) is not stationary. Next, we show how both the core flow and core flow acceleration can be computed simultaneously at any given epoch, if the field and its first and second time derivatives (the SV and the Secular Acceleration (SA)) are known. This allows us to unambiguously show that no stationary flow can account for the observed SA. Based on those results, we consider the possibility of improving on the short-term prediction of the field based on a linear extrapolation currently used by the International Geomagnetic Reference Field (IGRF) by using a method based on a steadily accelerated flow. Then we independently consider the possibility of increasing the spatial resolution of the predictive SV model currently used by IGRF. From all of the above, we conclude that the main limitation to shortterm predictions of the field is the occurrence of sudden changes in the SA (geomagnetic jerks). We thus investigate the type of core ow acceleration changes that are needed to account for those jerks and focus on the 2003 ge

O campo magnético da Terra varia em diferentes escalas de tempo, de milissegundos a bilhões de anos. Os dados de observatórios magnéticos e satélites obtidos nos últimos 150 anos indicam que o momento do dipolo magnético terrestre está diminuindo continuamente. Essa queda está associada à presença de fontes não-dipolares do campo em uma extensa região que abrange todo o Atlântico Sul e uma porção da América do Sul, sendo que no Brasil a contribuição dessas fontes varia fortemente com a latitude. Em escala de tempo arqueomagnética (~1.000-10.000 anos) a evolução do campo magnético terrestre não é tão bem estabelecida, principalmente em função da escassez de dados no hemisfério Sul, que contribui com apenas 5% dos dados de intensidade obtidos para os últimos 4.000 anos. A América do Sul, com alguns poucos resultados no Peru, Equador e Bolívia, pode ser considerada a terra incógnita da arqueointensidade. Nesta tese são apresentados os primeiros resultados arqueomagnéticos para o território brasileiro. Foram escolhidas duas regiões de estudo, o Nordeste e o Sudeste do Brasil, situadas em diferentes faixas de latitude de modo a investigar diferentes contribuições de componentes não-dipolares do campo. No Nordeste, as amostras foram coletadas na cidade de Salvador (BA), a primeira capital do Brasil, fundada em 1549 AD. Na região Sudeste a amostragem foi efetuada nas cidades de Anchieta (ES), Rio de Janeiro (RJ), Niterói (RJ), Iperó (SP), Piracicaba (SP) e Botucatu (SP). Nas duas regiões, a paleointensidade do campo magnético terrestre foi obtida em materiais construtivos (tijolos e alguns fragmentos de telhas) datados entre 1550 AD e 1920 AD. As idades desses materiais foram estabelecidas com base em estudos arqueológicos e registros históricos das construções, fornecendo incertezas inferiores a 30 anos para a grande maioria das amostras. As paleointensidades foram estimadas utilizando-se dois métodos: (a) duplo aquecimento com medidas em temperatura ambiente, pelo protocolo de Thellier modificado por Coe; (b) duplo aquecimento com medidas contínuas em alta temperatura, pelo protocolo Triaxe. Após as medidas e correções magnéticas, todas as amostras foram analisadas com base em rigorosos critérios de seleção, que resultaram em 23 novas determinações de intensidade de alta qualidade (correspondendo a um total de 584 espécimes analisados, com uma taxa de sucesso de 57%). A partir desses resultados foram traçadas duas curvas de variação da intensidade do campo magnético para cada uma das regiões estudadas, abrangendo os últimos 500 anos. Essas curvas revelam uma oscilação do momento de dipolo nos últimos cinco séculos, que não foi prevista nos modelos de campo disponíveis atualmente, trazendo implicações importantes no entendimento da evolução dos campos dipolar e não-dipolar nessa escala de tempo. As variações rápidas descritas nessas curvas permitem aplicar o arqueomagnetismo como ferramenta de datação arqueológica, como exemplificado pela datação de uma casa do Pelourinho em Salvador.
The Earth\'s magnetic field varies in different timescales, from milliseconds to billions of years. Magnetic data from observatories and satellites indicate that the dipole moment has continuously been decreasing for the past 150 years. This decay is associated to the presence of non-dipole sources covering a wide region that encompasses the South Atlantic and part of South America; in Brazil, the contribution of the non-dipole fields varies strongly with latitude. In the archeomagnetic timescale (~1,000-10,000 years), the evolution of the Earth\'s magnetic field is not well established, mainly due to the scarcity of data from southern hemisphere, which contributes with only 5% of the intensity data for the past 4,000 years. South America is the terra incognita of archeointensity, counting only a handful of results from Peru, Ecuador and Bolivia. This thesis presents the first archeomagnetic results from Brazil. In order to investigate different contributions of non-dipolar sources, we concentrated our sampling in two regions located in different latitudes the Northeast and Southeast regions of Brazil. In the Northeast region, all samples were collected in the city of Salvador (BA), the first Brazilian capital settled in 1549 AD. In the Southeast region, sampling was conducted in the cities of Anchieta (ES), Rio de Janeiro (RJ), Niterói (RJ), Iperó (SP), Piracicaba (SP) and Botucatu (SP). All paleointensity data was obtained from architectural fragments (bricks and some tiles) dated between 1550 AD and 1920 AD. The age of bricks and tiles was established on the basis of archeological studies and the historical record of the buildings, providing age uncertainties of less than 30 years for most of the samples. Paleointensity estimates were obtained by two methods: (a) double-heating with measurements in room temperature, using the modified version of the Thellier protocol; (b) double-heating with measurements in high temperatures, using the Triaxe protocol. After measurements and magnetic corrections, all samples were screened using strict selection criteria resulting in 23 high-quality new site-mean intensity values (from 584 analyzed specimens, with a success rate of 57%). These results were integrated into two curves of geomagnetic intensity variation for each studied region over the past five centuries. These curves reveal an oscillating dipole moment for the past five centuries, a behavior not predicted in currently available geomagnetic field models, thus providing key information on the dipole and non-dipole field evolutions in this timescale. The rapid intensity changes described in these curves permit the application of archeointensity techniques as an archeological dating tool, as exemplified by the dating of a house from the Pelourinho area, in Salvador city.

In the framework of a gradual global warming, which is one of the topic of major interest in the recent years and which importance is resumed in the Intergovernmental Panel on Climate Change (IPCC), it is important the study of the variability of the Earth’s system at the high latitudes i.e., in Artic and Antarctic areas, because these are the regions more sensitive to climatic changes. The possibility to study marine sedimentary sequences from Antarctica thus represented an important opportunity to investigate such climatic variability. Cold water mass formation in the Southern Ocean is involved in the global thermohaline circulation (THC) through the convection and inter-ocean exchanges of surface, intermediate and bottom waters. This work focus on the study of marine sequences cores from mid-high latitudes from strategic locations far and near the continental margin of Antarctica: (1) ANTA 95-157 (62°05.95'S) and ANTA 96-16 (66°20.09'S) cores are a long transect from New Zealand and Ross Sea slightly south of the present day Polar Front and the Pacific Antarctic Ridge, respectively; (2) MD97-2114 (42°22.32'S) core is on the northern side of Chatham Rise, east of New Zealand; (3) ODP Site 1166 (67°41.77'S) and 1167 (66°24.01'S) are in Prydz Bay continental shelf and slope, respectively. It has been provided a detailed magnetostratigraphy of the sequences and have been investigated magnetic proxies, which reflect variations in mineralogy, grain-size, and concentration of the magnetic fraction. Environmental magnetic data, along with the chronology derived by the magnetostratigraphy, provided an important contribution to our understanding of environmental and climate changes during the time intervals here examined. The cores ANTA and MD97-2114 span the shift from predominant 41-ka to predominant 100-ka glaciation cycles at around 1.0–0.9 Ma, (the Mid-Pleistocene Climate Transition (MPT)) that was centered at 922±12 ka and lasted about 40±9 ka. The amplitude of the 100-ka cycle abruptly increased much later on, at 641±9 ka. The transition was accompanied by an increase in 18O and decrease in 13C but the causes may be many and further information about changes in global ice volume during this period needed. In this core it has been found evidence of orbital influence on sedimentary processes. The core MD97-2114 records long-term variation of the upper Circumpolar Deep Water component of the Deep Western Boundary Current and manifests a stepwise modification of the THC during the transition. ODP Site 1166 and 1167 provide a record of the process involved in the story of the glacial expansion and retreat of the ice at the margin of the continent in Prydz Bay during the Plio-Pleistocene. Preliminary results from SEDANO cores shows downcore variations in concentration, mineralogy, and grain-size of magnetic minerals. In particular, there is a relative increase of moderate coercivity respect to low-coercivity minerals (magnetite) during glacials and a millennial scale variability of the magnetic grain size characterizes the last glacial (core SED 12 and 13) and it may be related to changes in the bottom current velocity.
INGV
Unpublished
2.2. Laboratorio di paleomagnetismo
reserved

yes
Archaeomagnetic dating offers a valuable chronological tool for archaeological investigations, particularly for dating fired material. The method depends on the establishment of a dated record of secular variation of the Earth's magnetic field and this paper presents new and updated archaeomagnetic directional data from the UK and geomagnetic secular variation curves arising from them. The data are taken from publications from the 1950's to the present day; 422 dated entries derived from existing archaeo and geomagnetic databases are re-evaluated and 487 new directions added, resulting in 909 entries with corresponding dates, the largest collection of dated archaeomagnetic directions from a single country. An approach to improving the largest source of uncertainty, the independent dating, is proposed and applied to the British Iron Age, resulting in 145 directions from currently available databases being updated with revised ages and/or uncertainties, and a large scale reassessment of age assignments prior to inclusion into the Magnetic Moments of the Past and GEOMAGIA50 databases. From the significantly improved dataset a new archaeomagnetic dating curve for the UK is derived through the development of a temporally continuous geomagnetic field model, and is compared with previous UK archaeomagnetic dating curves and global field models. The new model, ARCH-UK.1 allows model predictions for any location in the UK with associated uncertainties. It is shown to improve precision and accuracy in archaeomagnetic dating, and to provide new insight into past geomagnetic field changes.
Arts and Humanities Research Council