Geomagnetic field intensity behavior in South America between 400 AD and 1800 AD: First archeointensity results from Argentina

An absolute archeointensity study in Northwest Argentina provided 25 independent geomagnetic field lectures supported by 37 radiometric dates between AD 400 and 1800. The mean, cooling rate and remanence anisotropy corrected archeointensity values obtained in this study range from 36.5±2.6 to 63.1±8.7μT, with corresponding Virtual Axial Dipole Moments (VADMs) from 7.4±0.5 to 12.7±1.8 (1022Am2). Most of the data are concentrated between a relatively narrow interval from 1350 AD to 1550 AD. Three general features may be detected: the time intervals from about AD 1150 to 1350 and 1450 to 1600 are characterized by quite monotonic decay of geomagnetic intensity while some increase is observed from AD 1600 to 1700. The archeointensity decrease from about 17th century seems to be a general characteristic of global geomagnetic field because it was observed at different places worldwide. In the absence of reliable climate variation record for South America it is delicate to make any firm conclusions about the relationship between the Earth's magnetic field and multi-decadal climatic events. However, it is probably not a coincidence that persistent warm climate detected from Patagonia during AD 1200 to 1350 is consistent to geomagnetic intensity decrease revealed in this study.Fil: Goguitchaichvili, Avto. Universidad Nacional Autónoma de México; MéxicoFil: Greco Mainero, Mariano Catriel. Universidad de Buenos Aires. Facultad de Filosofía y Letras. Museo Etnográfico "Juan B. Ambrosetti"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Morales, Juan. Universidad Nacional Autónoma de México; Méxic

Early Cretaceous absolute geomagnetic paleointensities from Córdoba province (Argentina)

We present here new paleointensity and geochronology results from Early Cretaceous volcanic rocks of Sierra Chica de Cordoba (Argentina). The new K-Ar isotopic ages of 5 samples range from 136 to 122 Ma. Twenty five samples from 7 individual flows yielded acceptable paleointensity estimates. The mean paleointensity values per flow are ranging from 53.0±1.9 to 25.4±2.6 μT and the corresponding Virtual Dipole Moments (VDMs) are ranging from 9.3±1.3 to 4.6±0.5 (1022 Am2). This corresponds to the mean value of 7.3±1.7x1022 Am2, which is compatible to the present geomagnetic axial dipole. Currently available selected paleointensity data from 80 to 130 Ma suggest that geomagnetic field strength frequently fluctuated before and during the Cretaceous Normal Superchron while the magnetic polarity maintained stable. The mean paleointensities derived from Cordoba lavas agree remarkably well with those obtained from the Parana Magmatic Province (133-132 Ma). This reinforces the hypothesis about the unreliability of ‘Mesozoic Dipole Low'.Fil: Cejudo Ruiz, Ruben. Universidad Nacional Autónoma de México; MéxicoFil: Goguitchaichvili, Avto. Universidad Nacional Autónoma de México; MéxicoFil: Geuna, Silvana Evangelina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Alva-Valdivia, Luis M.. Universidad Nacional Autónoma de México; MéxicoFil: Solé, Jesus. Universidad Nacional Autónoma de México; MéxicoFil: Morales, Juan. Universidad Nacional Autónoma de México; Méxic

Paleomagnetic behavior of volcanic rocks from Isla Socorro, Mexico

The direction and magnitude of the geomagnetic field vary both spatially and temporally and undergo significant departures from that of a geocentric axial dipole. In order to properly characterize persistent behaviors, time-averaged field models must be based on the highest quality data. Here we present full-vector paleomagnetic data for volcanic units exposed in the southeast quadrant of the island of Socorro, Mexico. We carried out a joint expedition between the Scripps Institution of Oceanography and the Universidad Nacional Autónoma México to Isla Socorro in January of 2005 during which we collected oriented paleomagnetic samples from 21 sites, representing as many as 10 different volcanic units (the oldest of which is ∼540 ka). We subjected over 100 specimens to the most up-to-date paleointensity methods, and included the standard reliability checks. In an earlier study, Bohrson et al. (1996) proposed a series of widespread eruptive events, based on similarities of argon/argon dates. Paleointensity from specimens that conform to the strictest acceptance criteria are available from both the (unoriented) original sample collection and our fully oriented (but as yet undated) new collection. Correlation between the two collections is however problematic. The time-averaged direction from Socorro is consistent with that expected from a geocentric axial dipole, and the time-averaged intensity is 30.0±7.1 μT, equivalent to a virtual axial dipole moment (VADM) of 67.6±16.0 ZAm2

Reconstructing the geomagnetic field in west africa: first absolute intensity results from Burkina Faso

We present absolute geomagnetic intensities from iron smelting furnaces discovered at the metallurgical site of Korsimoro, Burkina Faso. Up to now, archaeologists recognized four different types of furnaces based on different construction methods, which were related to four subsequent time periods. Additionally, radiocarbon ages obtained from charcoal confine the studied furnaces to ages ranging from 700–1700 AD, in good agreement with the archaeologically determined time periods for each type of furnace. Archaeointensity results reveal three main groups of Arai diagrams. The first two groups contain specimens with either linear Arai diagrams, or slightly curved diagrams or two phases of magnetization. The third group encompasses specimens with strong zigzag or curvature in their Arai diagrams. Specimens of the first two groups were accepted after applying selection criteria to guarantee the high quality of the results. Our data compared to palaeosecular variation curves show a similar decreasing trend between 900–1500 AD. However, they reveal larger amplitudes at around 800 AD and 1650 AD than the reference curves and geomagnetic field models. Furthermore, they agree well with archaeomagnetic data from Mali and Senegal around 800 AD and with volcanic data around 1700 AD

The mechanism of self-reversal of thermoremanence in natural hemoilmenite crystals: new experimental data and model

17 pages, including 7 figuresNew magnetic and mineralogical findings on self-reversing hemoilmenite (Fe2-y Tiy O3) grains from Pinatubo lavas (1991 eruption) provide important clues regarding the acquisition process of reverse thermoremanent magnetization (rTRM) in this solid solution series. Magnetic Force Microscopy indicates the presence of multidomain magnetic structures in coexisting strongly and weakly magnetic crystallographic regions having compositions of y0.54 and y0.53, respectively. Yet, continuous thermal demagnetization of natural and laboratory TRM carried out on both whole rock samples and single hemoilmenite crystals shows that the magnitude of a normal TRM (nTRM) component, observed at temperatures above the Curie point of the strongly magnetic self-reversing regions, is much too large to be carried by a phase that is entirely cation-disordered. Consistent with this observation are findings using Transmission Electron Microscopy (TEM) which, in contrast to that what is commonly assumed, reveals the weakly magnetic regions to be magnetically heterogeneous. Specifically, these regions are found to contain tiny (20-40nm) domains that are cation-ordered and evidently ferrimagnetic dispersed within the cation-disordered, presumably spin-canted antiferromagnetic matrix. Given these findings, we argue that the so-called nTRM-carrying x-phase is itself partially cation-ordered, and thus ferrimagnetic, as postulated first by Ishikawa and Syono (1962). We propose a “nanophase” self-reversal model for the ilmenite-hematite solid solution series in which the rTRM and nTRM components are carried by the cores and margins, respectively, of the tiny, partially cation-ordered nano-sized domains observed by TEM. Due to the partial cation order, both the core and the margin of each domain are expected to behave in a ferrimagnetic fashion at temperatures below their respective Curie points. However, given the kinetics of the ordering process, their cation distributions need be antiphase, which causes their magnetic moments to be oppositely aligned. Since it is most reasonable to consider each margin to be slightly more Fe-rich than the inside core, upon cooling the margins acquire a magnetic remanence first (a nTRM). Then, upon further cooling, given that the intralayer and interlayer nearest-neighbor superexchange interactions are ferromagnetic and antiferromagnetic, respectively, the net magnetic moment of the core material need be oppositely aligned (producing a rTRM). The nano-sized regions would indeed behave in a superparamagnetic (SP) fashion if magnetically uncoupled to adjacent material; however, the spins in the margins (the x-phase) must be locked through superexchange to those of the surrounding disordered matrix, which we also claim to be locally enriched in iron. If so, then the magnetization of the x-phase can be both highly-coercive and thermally stable, as observed experimentally. Upon stepwise thermal demagnetization, the self-reversed remanence measured at room temperature is not destroyed until the unblocking temperature of the disordered Fe-enriched aureole (approximately 410°C) is reached. Mineralogical considerations and magnetic evidence from previous works suggest that this model is generally valid for self-reversed dacitic pumice, in particular the Mt. Haruna dacite and the 1985 Nevado del Ruiz dacitic andesite

Reproducibility of archaeointensity determinations with a multimethod approach on archaeological material reproductions

Archaeointensity determinations on burnt archaeological material are complex and reliable data scarce, although this kind of material can be of great interest in archaeological investigations. With the goal of analysing the reliability of archaeointensity determinations, an interlaboratory comparison study has been performed combining different experimental protocols on present-day reproductions of Pre-Columbian Mesoamerican archaeological artefacts and two brick samples. Samples were baked in an original kiln from an artisan workshop in western Mexico. The ambient magnetic field at the site during the experiment was measured and continuous temperature data were recorded at four different positions in the kiln during the heating–cooling procedure. Archaeointensity determinations were carried out with four different methods at four different palaeomagnetic laboratories: Thellier–Coe (Burgos, Spain), microwave (Liverpool, UK), multispecimen (Morelia, Mexico) and multispecimen with the extended protocols for fraction and domain-state correction (Montpellier, France). 26 conventional resistive heating determinations with the Thellier–Coe protocol yielded a 100 per cent success rate, while 7 out of 8 microwave-heating determinations with the Thellier–Coe protocol also provided successful results. Also, two multispecimen determinations performed with both multispecimen methods provided statistically reliable results. In all cases, a good agreement between the determined archaeointensities and the ambient field at the production site could be observed. Highly reversible magnetization-versus-temperature curves yielded slightly Al, Mg or Ti-substituted magnetite as the main ferromagnetic (s.l.) phase. In addition, in several samples, a thermally stable low Curie-temperature phase displaying a high coercivity behaviour could be observed in thermomagnetic curves and by thermal demagnetization of saturation isothermal remanent magnetization. This phase is interpreted as ε-Fe2O3. To our knowledge, its occurrence has never been reported through the experimental recreation of burnt archaeological materials. No correlation could be observed between the proxies of domain-state behaviour and deviation of palaeointensity determinations from the expected result. Results obtained on clay samples heated in this type of ancient kiln can be considered a good source for determining the geomagnetic field strength variation in the past. Matching palaeointensity results obtained with different methods based on different principles can be taken as a quality criterion for result reliability and consistency.Projects BU0066U16 and BU235P18 (Junta de Castilla y León, Spain) and the European Regional Development Fund (ERDF). AG is grateful for financial support of CONACyT 252149 and UNAM-PAPIIT project 101717. The Géosciences Montpellier survey was supported by a grant from the CNRS-PNP. The FUReMAG rapid furnace construction was supported by the French National Agency for Research (ANR-12-BS06–0015)

Rock-magnetic properties of topsoils and urban dust from Morelia (>800,000 inhabitants), Mexico: Implications for anthropogenic pollution monitoring in Mexico’s medium size cities

En el presente trabajo, investigamos la correlación entre algunos parámetros magnéticos y el nivel de contaminación por metales pesados en suelos urbanos de la ciudad de Morelia, en el occidente de México. El estudio magnético fue llevado a cabo en 98 muestras urbanas provenientes de diferentes tipos de uso de suelo. La mayoría de las muestras contienen minerales ferrimagnéticos como responsables de la magnetización, pertenecientes probablemente a las soluciones sólidas de las titanomagnetitas/ titanomaghemitas. Esto es inferido a partir de las mediciones de susceptibilidad en función de la temperatura y de los experimentos de magnetización remanente isotérmica (MRI). Estas mediciones indican además, que la mayoría de las muestras se saturan casi completamente antes de los 300 mT. Adicionalmente, los valores S-200 (S-200 = IRM-200/SIRM, donde IRM-200= magnetización a campo inverso de 200 mT después de la saturación magnética) se encuentran entre 0.7 y 1.0, característicos de minerales de baja coercitividad magnética. Las curvas promedio de magnetización remanente isotérmica de saturación (SIRM) pueden ser usadas como un indicador del nivel de contaminación, ya que estas curvas muestran diferentes valores de saturación de acuerdo al nivel de contaminación por metales pesados: Cu, Ni, Cr y Sr. Estas asociaciones de (titano)magnetitas con metales pesados fueron observadas bajo el Microscopio Electrónico de Barrido, revelando algunos agregados complejos en lugar de las esférulas detectadas comúnmente.In this work, we investigate the correlation between some magnetic parameters and the level of contamination by heavy metals in urban soils from Morelia city, western Mexico. The magnetic study was carried out on 98 urban soils samples belonging to distinct land uses. Most of analyzed samples contain ferrimagnetic minerals as the responsible for magnetization, most probably corresponding to the titanomagnetites/titanomaghemites solid solutions. This is inferred from the susceptibility vs. temperature measurements and the isothermal remanent magnetization (IRM) experiments. These measurements also indicate that most of samples are almost completely saturated before 300 mT. Additionally, the S-200 values (S-200 = IRM-200/ SIRM, where IRM-200= Back-field of 200 mT after magnetic saturation) are between 0.7 and 1.0, characteristic of low coercivity magnetic minerals. The averaged saturation isothermal remanent magnetization (SIRM) curves can be used as an indicator of pollution level, as these curves show different saturation values according to the level of contamination by heavy metals: Cu, Ni, Cr and Sr. These associations of (titano)magnetite with heavy metals were observed by Scanning Electron Microscope revealing some complex aggregates rather than commonly detected spherules.Universidad Nacional Autónoma de México through the projects CONACYT 118971 and PAPIIT IN 22311