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More than 350 oriented paleomagnetic cores were obtained for rock-magnetic and paleomagnetic analysis from radiometrically dated (40Ar–39Ar) magmatic rocks occurring in the southern segment (Jorullo and Tacambaro areas) of the Michoacan-Guanajuato Volcanic Field in the Trans-Mexican Volcanic Belt. Most of the lavas (37) stem from monogenetic volcanoes dated at less than 4 Ma. Two additional sites were sampled from the plutonic basement dated at 33–30 Ma. Primary remanences carried by low-Ti titanomagnetites allowed to determining 34 reliable site-mean directions of mostly normal (27) but also reversed (7) polarities. The mean directions of these two populations are antipodal, and suggest neither major vertical-axis rotations with respect to the North America craton nor tilting in the region for the last 4 Ma (rotation and flattening of the inclination parameters being less than −5.9 ± 3.8 and 0.1 ± 3.9, respectively). The corresponding paleomagnetic pole obtained for Pliocene–Pleistocene times is PLAT = 83.4°, PLON = 2.4° (N = 32, A95 = 2.7°). Virtual geomagnetic poles also contribute to the time averaged field global database and to the paleosecular variation (PSV) investigations at low latitudes from lavas for the last 5 Ma, showing a geomagnetic dispersion value that is in agreement with available PSV models. When comparing the magnetic polarities and corresponding radiometric ages of the studied sites with the Cenozoic geomagnetic polarity time scale (GPTS), a good correlation is observable. This finding underscores the suitability of data obtained on lavas in Central Mexico for contributing to the GPTS. Furthermore, the detection of short-lived geomagnetic features seems possible, since the possible evidence of Intra-Jaramillo geomagnetic excursion could be documented for the first time in these volcanic rocks.

"Los óxidos de hierro se encuentran en todos los suelos en cantidades diversas, son utilizados como indicadores de procesos pedo - genéticos y reflejan los ambientes de formación del suelo. Por otro lado, estos componentes afectan las propiedades y funciones de los suelos, porque influyen en el color, la agregación, la capacidad de intercambio de cationes y la retención de fósforo en el suelo. Una característica de los óxidos de hierro es la formación de minerales ferrimagnéticos; debido a esto, y mediante la utilización de técnicas magnéticas se pueden identificar algunos procesos pedogenéticos. El objetivo de este trabajo es la descripción de las propiedadesmag - néticas de los suelos y sus aplicaciones potenciales, como parámetros proxy , en la clasificación de suelos. Se mencionan los portadores magnéticos, las propiedades magnéticas, las formas de análisis y sus aplicaciones. Las propiedades magnéticas de los suelos permiten identificar y cuantificar los minerales magnéticos y relacionarlos con los procesos pedogenéticos. Las mediciones magnéticas son fáciles de realizar y son de bajo costo, lo cual permite realizar el análisis de un gran número de muestras, con lo que es posible la elaboración de mapas de suelos, la identificación de procesos pedogenéticos y el mejoramiento de la clasificación de suelos. En países con alta diversidad edáfica como México, es posible la generación de nuevos tipos de aplicaciones de las propiedades magnéticas de los suelos."

International audience; We report the results of a rock magnetic and archeointensity study on pottery fragments recovered at the archaeological site of Puertas de Rolón, Capacha culture (Colima), that is considered one of the most important pre-Classic cultural developments of western Mesoamerica, Mexico. Standard rock magnetic experiments were carried out in order to investigate the reliability of the ancient field strength recovered from studied materials. Six out of the seven analyzed fragments gave reliable intensity determinations. The mean archeointensity value obtained in this study is 35.3 ± 3.4 μT. It is in good agreement with another contemporaneous Mesoamerican mean archeointensity of 34.3 ± 1.1 μT obtained for El Opeño. Statistically similar rock-magnetic parameters between ceramics from El Opeño and Puertas de Rolón potsherds appear to be related to clay-sources with similar geological settings and weathering processes. Alternatively, ancient trade or interexchange between western (Capacha Culture) and eastern Mesoamerica locations, as it has been proposed earlier, seem to be supported by results obtained in this study.

Rock-magnetic investigations of environmental samples, such as soils, dust and leaves, can provide useful information on environmental stress. Aim of this paper is to report on the first magnetic survey carried out on environmental samples from Bogota, Colombia, which is the fifth most populated city of Latin America. We studied three kinds of samples in order to evaluate their potential as pollution markers: soils, urban dust and leaves of the Sauco (Sambucus nigra) tree. Standard magnetic methods were used to analyze the three groups of samples, with special emphasis on leaves investigated here for the first time for environmental purposes. Samples were collected along two perpendicular transects in an area of the city presumably unpolluted, with two different systems of public transportation. Thermomagnetic curves obtained from soils and atmospheric dust show clearly defined ferrimagnetic phase (probably magnetite) as the main magnetic carrier. Curves of acquisition of isothermal remanent magnetization confirm the ferrimagnetic character of magnetic particles. Frequency-dependent magnetic susceptibility was found between 0 and 10%, suggesting less-important contribution of ultrafine super-paramagnetic magnetite. The apparent contradictory results between soils and the rest of samples may reflect different accumulation period for magnetic particles, being shorter for leaves and urban dust than for soils.

A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40 Ar/ 39 Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude  = 81.1, longitude  = 94.3) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed

The Pacific coast of Mexico has repeatedly been exposed to destructive tsunamis. Recent studies have shown that rock magnetic methods can be a promising approach for identification of tsunami- or storm-induced deposits. We present new rock magnetic and anisotropy of magnetic susceptibility (AMS) results in order to distinguish tsunami deposits in the Ixtapa–Zihuatanejo area. The sampled, 80 cm-deep sequence is characterized by the presence of two anomalous sand beds within fine-grained coastal deposits. The lower bed is probably associated with the 14 March 1979 Petatlan earthquake (M W = 7.6), whereas the second one formed during the 21 September 1985 Mexico earthquake (M W = 8.1). Rock magnetic experiments discovered significant variations within the analysed sequence. Thermomagnetic curves reveal two types of behaviour: one in the upper part of the sequence, after the occurrence of the first tsunami, and the other in the lower part of the sequence, during that event and below. Analysis of hysteresis pa...

We report a detailed rock-magnetic and archaeointensity analysis of four pottery fragments and a burned floor recovered from the Tzintzuntzan archaeological site in western Mexico. Results from rock-magnetic experiments (x-T curves and first-order reversal curves [FORC] diagrams) indicate the suitability of most of these materials as faithful geomagnetic field recorders. Potsherds were archaeomagnetically dated by comparing their mean intensity values against the paleosecular variation curve CALS3k, suggesting A.D. 600–941 as the most probable age range. This is younger than 14C-dated charcoal from the same burned floor (A.D. 1294–1426). More precise age estimates will require the use of the full geomagnetic vector (declination, inclination, and intensity). Multiple reheatings of the ceramic pieces, evidenced as secondary components in Zijderveld plots, could reflect multiple heating of these objects, perhaps from use as incense burners. Our study demonstrates the potential of archaeomagnetic analysis to both date burned ceramics recovered in situ and provide insight into their use-history.

We present a diagnostic study to evaluate the suitability of Ficus benjamina tree leaves as a captor of heavy metal particles from atmospheric dusts in urban areas. Leaf samples were taken at 16 localities within three areas in Morelia, state of Michoacan (Mexico’s medium size city, 830000 inhabitants). Measurements of magnetic susceptibility were conducted to determine the magnetic enhancement using samples from green, relatively unpolluted area, as a reference. The samples collected at areas with heavy traffic (main avenues) yielded values almost ten times higher than the values obtained for the unpolluted reference. Isothermal Remanent Magnetization curves are proportional to the degree of pollution. Associations of almost pure magnetite with heavy metals were revealed by scanning electron microscope.

This article presents the first palaeomagnetic results from 13 independent cooling units in the Chiapanecan Volcanic Arc (ChVA). Six sites were directly dated by Ar–Ar or K–Ar methods: their dates range from 2.14 to 0.23 Ma. We isolated the characteristic palaeodirections for all 13 lavas. Eleven non-transitional directions yield a mean direction with inclination, I = 30.7°, declination, D = 4.1°, and precision parameters k = 63 and α95 = 5.8°. The corresponding mean palaeopole position is Plat = 83.3°, Plong = 203.8°, K = 227, A 95 = 5.1°. The mean inclination is in good agreement with the expected value for the last 5 million years, as derived from the synthetic North American polar wander path [Besse and Courtillot 2002, Apparent and true polar wander and the geometry of the magnetic field in the last 200 million years: Journal of Geophysical Research, v. 107, no. B11, p. 2300], but a measured rotation of the palaeodeclination of about 8° with respect to the expected direction suggests the possibility ...

This paper focuses on the study of the correlation between magnetic parameters with the level of contamination by heavy metals in urban soils. We report a magnetic investigation of urban soil samples from Merida, state of Yucatan, Southern Mexico. It appears that most of our samples contain ferrimagnetic minerals as the magnetic carriers, probably coming from the titanomagnetites/titanomaghemites series. This is inferred by the acquisition of isothermal remanent magnetization, which shows that most of samples are almost completely saturated at about 200 mT. The S−200 value (factor characterizing stability of remanent magnetization) is between 0.8 and 1.0, characteristic of ferrimagnetic minerals. The susceptibility vs. temperature measurements also point to some titanomaghemites and titanomagnetites as probable responsible for magnetization. On the other hand, measurements of magnetic susceptibility at high and low frequencies helped us to determine the high content of superparamagnetic grains in the majority of the samples, although not all of these showed high values of magnetic susceptibility. We found that the most contaminated samples by Pb and Zn showed the higher saturation isothermal remanent magnetization values, whereas the higher values in magnetic susceptibility correspond to samples contaminated by Cr. Finally, we noted that a high level in Sr decreases the magnetic susceptibility.

The paleosecular variation (PSV) and polarity transitions are two major features of the Earth’s magnetic field. Both PSV and reversal studies are limited when age of studied units is poorly constrained. This is a case of Central and western Mexico volcanics. Although many studies have been devoted to these crucial problems and more than 200 paleomagnetic directions are available for the last 5 Ma, only few sites were dated directly. This paper presents new paleomagnetic results from seventeen independent cooling units in the Michoacan-Guanajuato Volcanic Field (MGVF) in western Mexico. Twelve sites are directly dated by 40Ar/39Ar or K-Ar methods and span from 2.78 to 0.56 Ma. The characteristic paleodirections are successfully isolated for 15 lava flows. The mean paleodirection (inclination I and declination D) obtained in this study is I = 28.8°, D = 354.9°, and Fisherian statistical parameters are k = 28, α95 = 7.3°, N=15, which corresponds to the mean paleomagnetic pole position Plat = 83.9°, Plong = 321.6°, K = 34, A95 = 6.6°. The paleodirections obtained in present study compiled with those, previously reported from the MGVF, are practically undistinguishable from the expected Plio-Quaternary paleodirections. The paleosecular variation is estimated through the study of the scatter of the virtual geomagnetic poles giving SF = 15.9 with SU =21.0 and SL = 12.7 (upper and lower limits respectively). These values agree reasonably well with the recent statistical Models. The oldest sites analyzed (the Santa Teresa and Cerro Alto) yield normal polarity magnetizations as expected for the cooling units belonging to the Gauss geomagnetic Chron. The interesting feature of the record comes from lava flows dated at about 2.35 Ma with clearly defined normal directions. This may point out the possible existence of a normal polarity magnetization in the Matuyama reversed Chron older than the Reunion and may be correlated to Halawa event interpreted as the Cryptochron C2r.2r-1. Another important feature of the geomagnetic record obtained from the MGVF is the evidence of fully reversed geomagnetic field within Bruhnes Chron, at about 0.56 Ma corresponding to the relative paleointensity minimum of global extent found in marine sediments at about 590 ka.

For long time the western-central Mexico has been affected by oblique subduction caused by Farallon plate beneath North America. As result, smaller plates (e.g. Cocos Plate), several fault systems outlining crustal blocks (e.g. Michoacan block) and magmatic arcs (e.g. Paleocene-Early Oligocene magmatism and the Trans-Mexican Volcanic Belt) were developed. Still, no paleomagnetic data are available for Oligocene and Miocene. The principal aim of this study is to evaluate whether the tectonic rotations and relative motions of these blocks occurred before the Miocene. Here, we report a detailed rock-magnetic and paleomagnetic results from Tecalitlan area, located in the Michoacan block. Sixteen sites (about 150 oriented samples) were collected including one radiometrically dated diabase dike (35.0 ± 1.8 Ma). Rock-magnetic experiments permitted identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Reliable paleomagnetic directions were obtained for 12 sites. Inclination I and declination D of the mean paleomagnetic direction obtained in this study are I = 33.1°, D = 345.0°, and Fisherian statistical parameters are k = 25, α95 = 8.9°. The corresponding mean paleomagnetic pole position is Plat = 75.7°, Plong = 166.6°, K = 31, A95 = 8.0°. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Magnetic declination is not significantly different from that expected which is in disagreement with a counterclockwise tectonic rotation of about 20° previously reported for the studied area. Based on paleomagnetic results obtained in this study compiled with those currently available from the Michoacan Block, we propose a simple model suggesting that sometime in Eocene epoch the convergence vector of the Farallon plate relative to North America plate was normal to the trench before reaching an actual oblique convergence.

Despite of the impressive cultural heritage and abundant archaeological sites, absolute geomagnetic intensity data from Mesoamerica are still sparse. Archeointensity determinations using the Coe variant of the Thellier and Thellier method have been carried out on some selected pottery fragments from the El Openo archeological site which has the earliest funeral architecture known in western Mesoamerica. The El Openo chronology is supported by six C 14 datings performed on carbon-bearing materials. Detailed rock magnetic experiments including susceptibility vs. temperature curves, hysteresis cycles and thermal demagnetization procedures were carried out in order to estimate the magnetic carriers and their stability. Cooling rate and anisotropy remanence corrected intensity values range from 25.0 ± 2.3 to 40.2 ± 3.0 µT and corresponding virtual axial dipole moments (VADM) range from 5.6 ± 0.5 to 8.9 ± 0.7  10 22 Am 2 . In addition, we present here a new compilation and analysis of existing absolute intensity data in order to try to estimate the variation of the Earth’s magnetic field over the past three millennia. The mean archeointensity obtained in the present study agrees reasonably well with the predicted absolute intensities retrieved from the CALS7K main field model. Other available Mesoamerican data, however, differ from this model. Most of available archeointensity data from Mesoamerica agree reasonably well with the ARCH3K main field model prediction. The broad peak defined at about 50 A.D. by our data is also predicted by the CALS3k.3 main field model but slightly displaced to the right

We report a detailed palaeomagnetic investigation of 28 lava flows (221 standard palaeomagnetic cores) collected from the Paraguayan part of the Paraná flood basalts (the Alto Paraguay Formation). The initial aims of our study were to (i) document variability of the Earth's magnetic field during the time interval sampled, (ii) obtain a new Early Cretaceous palaeomagnetic pole (PP) for stable South America, and (iii) estimate the extrusion rate of the Paraná magma. We precisely determined the palaeofield direction for 26 sites for which the characteristic remanent magnetizations exhibit small within-site dispersion and high directional stability. No palaeodirections were determined for two sites because of a very complex and erratic behaviour of the remanence during the palaeomagnetic treatments. Nine sites display normal polarity magnetization, whereas nine others are reversely magnetized and the remaining eight sites yield intermediate palaeodirections. The mean palaeomagnetic direction of normal polarity sites give I = -41.8°, D = 4.9°, k = 112, and α95 = 4.9°, whereas reversely magnetized sites give I = 37.1°, D = 181.4°, k = 23, and α95 = 11.1°. The reversal test as defined by McFadden and McElhinny (1990; Classification of the reversal test in paleomagnetism: Geophysical Journal International, v. 103, p. 725-729) is positive, corresponding to Type B with γ = 8.7° and γ0 = 3.7°. This ensures that the palaeomagnetic treatment successfully removed the secondary natural remanent magnetization and that the sampling adequately averaged the palaeosecular variation (PSV). The mean PP position obtained from 18 sites is palaeolongitude (Plong) = 359.2° and palaeolatitude (Plat) = 86.2° S. We show by means of probability plots and formal testing procedures that a Fisher distribution with a concentration parameter K = 65 satisfactorily fits the distribution of virtual geomagnetic poles (VGPs). The PP obtained in this study agrees reasonably well with coeval pole positions, in particular with those obtained from the Central Paraná Magmatic Province in Brazil, Los Adobes, and Misiones in Argentina. However, some other similar age PPs show significant departure that may be attributed to local tectonic rotations or insufficient sampling needed to overcome the PSV. This new PP differs slightly from the reference poles at 135 Ma for South America given by Besse and Courtillot (2002; Apparent and true polar wander and the geometry of the magnetic field in the last 200 million years: Journal of Geophysical Research, v. 107, no. B11, p. 2300). The PSV parameters are in agreement with those obtained from selected data reported for the Cretaceous Normal Superchron (CNS). In contrast, VGP angular dispersion found here is lower with respect to the Jurassic and Plio-Pleistocene data. The intermediate VGPs determined in the present geographical frame show a northern hemisphere cluster of seven VGPs located east of India, whereas one other VGP is located in the vicinity of Australia. Fil: Goguitchaichvili, Avto. Universidad Nacional Autónoma de México. Instituto de Geofísica; México Fil: Cervantes Solano, Miguel. Universidad Nacional Autónoma de México. Instituto de Geofísica; México Fil: Camps, Pierre. Université Montpellier II; Francia Fil: Bettucci Sanchez, Leda. Universidad de la República; Uruguay Fil: Mena, Mabel. 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; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología. Instituto de Geofísica "Daniel Valencio"; Argentina Fil: Trindade, Ricardo. Universidade de Sao Paulo; Brasil Fil: Aguilar Reyes, Bertha. Universidad Nacional Autónoma de México. Instituto de Geofísica; México Fil: Morales, Juan. Universidad Nacional Autónoma de México. Instituto de Geofísica; México Fil: López Loera, Héctor. Instituto Potosino de Investigación Científica y Tecnológica A.C. ; México

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 223110

A detailed rock-magnetic and archeointensity study was carried out on materials baked by a western Mexican artisan following traditional techniques to produce faithful reproductions of archeological pieces of the Michoacan region (Western Mesoamerica). The field strength at the site (41.0 ± 0.5 lT) was measured with a fluxgate magnetometer and the temperature of the furnace during the baking process was monitored continually by means of a thermocouple placed in the middle of the baking cavity. Rock-magnetic experiments performed on the raw material (clay and paste) and on in situ prepared baked ceramics and bricks included measurement of thermomagnetic curves (susceptibility and strong-field magnetization versus temperature), first-order reversal curves (FORC), anisotropy of magnetic susceptibility (AMS) and anisotropy of thermoremanent magnetization (A-TRM). Magnetite and probably hematite are present in the samples as carriers of the remanence. Hysteresis ratios suggest that the samples fall in the pseudo-single-domain grain size region, which may indicate a mixture of multidomain and a significant amount of single-domain grains. Ceramic pieces and brick fragments were subjected to the Thellier-Coe archeointensity method and to an alternative paleointensity experiment, with a TRIAXE magnetometer, in order to check whether they are faithful recorders of the local geomagnetic field strength. Mean raw-intensity of sample M1 (pottery) overestimates a 7% the expected site intensity, while those corresponding to the brick samples (LQ1 and LQ2) underestimate it 15%. Brick sample LNQ shows a slightly lower intensity (7%), but agrees with the expected site intensity within the experimental uncertainty. The intensity retrieved from the volcanic fragment also included closely reproduces the expected intensity. After A-TRM and cooling-rate corrections, all mean raw values move closer to the expected intensity. Measurement of temperatures at different parts inside the kiln (bottom and upper parts of both central and peripheral parts) revealed the existence of significant thermal gradients, similar to those observed in ovens from other localities. Different cooling rates are then expected in a single oven. The scatter in the intensity determinations observed in this study, retrieved from pieces elaborated together in the same oven, could arise from this differentiated cooling rate within the oven and thus, to an inappropriate cooling rate correction in the archeointensity protocol. As this situation was probably reproduced in the baking of ancient ceramic artifacts, a better knowledge of the temperature distribution inside these types of kiln would be desirable in order to choose the appropriate cooling rate correction. 2011 Elsevier B.V. All rights reserved.

We present rock-magnetic and paleomagnetic results obtained on samples belonging to a Neogene sequence of 11 successive lava flows and a dyke from La Gomera (Canary Islands, Spain). Analysis of thermomagnetic curves allows to distinguish three types of samples: (i) Type H samples with low-Ti titanomagnetite as the only carrier of remanence; (ii) type M samples with a main intermediate Curie-temperature phase (TC = 450°C) and low-Ti titanomagnetite; (iii) type L curves with a low Curie-temperature phase (TC = 120 to 200°C) and an intermediate Curie-temperature phase (TC = 400°C). Analysis of hysteresis parameters suggests that the grain size of most studied samples corresponds to pseudo single-domain particles, which can be also interpreted as a mixture of single-domain and multi-domain particles. Paleomagnetic experiments reveal only a single paleomagnetic component. Characteristic remanence of all studied lava flows and the dyke shows reverse polarity. The mean direction of the whole sequence is D = 188.2°, I =-35.4° (k = 46.9; α95 = 6.4°) and the calculated paleomagnetic pole yields a longitude λ = 150.7° and a latitude φ = 78.8° (k = 59.4; A95 = 5.7°). Secular variation is analysed through the scatter of virtual geomagnetic poles (VGP). A VGP angular scatter SB = 5.9 with an upper confidence limit Sup = 8.0 and a lower confidence limit Slow = 4.6 are obtained. This scatter is clearly smaller than the average for this latitude obtained for the last 5 Ma. The studied lava flows were probably emitted in a relatively short time interval.

[1] We report a detailed archeomagnetic and micro–Raman spectroscopy investigation on pre-Columbian pottery fragments from Cuanalan (a formative village in the valley of Teotihuacan, central Mexico). Available radiocarbon ages range from 2320 ± 80 to 2060 ± 90 B.P. Continuous low-field susceptibility versus temperature curves performed in air indicate Ti-poor titanomagnetites as magnetization carriers. Few samples, however, show two ferrimagnetic phases with Curie temperatures compatible with both Ti-poor and Ti-rich titanomagnetites. Hysteresis parameter ratios fall essentially in the pseudosingle-domain region, which may indicate a mixture of multidomain and a significant amount of single-domain grains. Mineralogical composition of the Teotihuacan ceramics has been investigated using micro–Raman spectroscopy. Samples are characterized by highly heterogeneous body matrix mineralogy due to the presence of a large variety of minerals with several mineralogical phases. Observed titanomagnetite and magnetite bands shift toward higher wave numbers, confirming a reducing atmosphere and a relatively high temperature (about 800–900°C) during the ceramic production. This definitively indicates the thermoremanent origin of magnetic magnetization. Archeointensity values have been determined from 7 pottery fragments (47 samples) out of 10 (70 samples) analyzed. Anisotropy of thermoremanent magnetization and the cooling rate effect upon thermoremanent magnetization intensity acquisition have been investigated in all the samples. The mean archeointensity values obtained in this study range from 24.2 ± 3.2 to 40.0 ± 1.7 μT, and corresponding virtual axial dipole moments range from 4.8 ± 0.6 to 8.0 ± 0.4 (1022 A m2). This corresponds to a mean virtual dipole moment value of 5.9 ± 1.1 × 1022 A m2, which is lower than the present-day field strength and the predicted values by global models and the latest data compilation. However, our data agree well with currently available absolute intensity values from Mesoamerica. The archeointensity values uncorrected for cooling rate and anisotropy are systematically higher than the corrected values. These uncorrected values agree with the CALS7K model, which may be biased by the fact that such corrections were not applied to most of the previous data.

[1] Thellier paleointensity experiments are carried out on the Early Cretaceous Arapey Formation, which belongs to the Parana Magmatic Province (PMP) in northern Uruguay. Twenty-seven samples out of 225 analyzed yield reliable absolute intensity determinations. For these samples the NRM fraction f used for determination ranges between 0.33 to 0.83 and the quality factor q ranges from 4.5 to 14.1, being generally greater than 5. The flow-mean paleointensity values obtained in this study range from 16.3 ± 2.3 to 41.5 ± 3.8 μT, with corresponding virtual dipole moments (VDMs) from 3.4 ± 0.6 to 8.2 ± 0.8 (1022 Am2). This corresponds to mean VDM value of 6.4 ± 2.3 × 1022 Am2. 40Ar/39Ar dating on the PMP sequence by Feraud et al. (1999) gave a mean age of 132.2 ± 0.5 Ma. Our results in conjunction with previous studies (Kosterov et al., 1998; Goguitchaichvili et al., 2002) suggest that the paleostrength during the early Cretaceous might be as variable and strong as Brunhes data and not anomalously low as previously suggested. The scatter observed in our study is similar to recent findings by Granot et al. (2007) for the Troodos Ophiolite.

We report a detailed archeomagnetic investigation on pre-Columbian potteries from Quiahuiztlan, Veracruz, from ~900 AD to 1521 AD. Archeological samples analyzed by modern magnetic techniques carry a stable univectorial thermoremanent magnetization under alternating field treatment. Continuous low-field susceptibility vs. temperature curves performed in air indicates Ti-poor titanomagnetites as magnetization carriers. Few samples, however, show two ferrimagnetic phases with Curie temperatures compatible with both Ti-poor and Ti-rich titanomagnetites. Hysteresis parameter ratios fall essentially in the pseudo-single-domain region, which may indicate a mixture of multi-domain and a significant amount of single domain grains. Early geomagnetic field intensity determinations using the Coe variant of Thellier method were performed on 90 selected samples. Cooling rate corrected intensity values range from 34.0 ± 1.2 to 62.2 ± 0.2 ?T. Corresponding virtual axial dipole moments (VADM) range from 5.7 to 10.7 x1022 Am2. Absolute intensity curves for Mesoamerica present limited spa-tial/temporal resolution. For tentative dating purposes, we used global archeointensity curves reduced to central and eastern Mexico that permit preliminary dating of Quiahuiztlan potteries. Comparison of our data against those reported in ArcheoInt database allow to identify two periods 900- 1000 and 1400-1600 AD as the most probable manufacturing ranges for the pot- teries studied.