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31.
Juan Morales Avto Goguitchaichvili Bertha A. Aguilar‐Reyes Modesto Pineda Claire Carvallo Laura Beramendi‐Orosco Galia Gonzlez‐Hernndez Arturo Oliveros 《Geoarchaeology》2012,27(6):521-537
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. 相似文献
32.
Manuel Calvo-Rathert Bertha Aguilar Reyes Avto Goguitchaichvili José Rosas Elguera Héctor Franco Juan Morales Ruth Soto Ángel Carrancho Hugo Delgado 《Studia Geophysica et Geodaetica》2013,57(2):309-331
A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40Ar/39Ar 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 λ = 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 polarity observed in the other site (220 ± 36 ka) very likely provides new evidence for the Pringle Falls excursion or the event recorded in the Mamaku ignimbrite. 相似文献
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34.
Juan MoralesAvto Goguitchaichvili Jaime Urrutia-Fucugauchi 《Physics of the Earth and Planetary Interiors》2003,138(1):1-10
Absolute paleointensity experiments were carried out using Shaw’s method [Geophys. J. Res. Astr. Soc. 39 (1974) 133] and its modifications [Geophys. J. Res. Astr. Soc. 54 (1978) 241; Geophys. J. Int. 80 (1985) 773; Geophys. J. Int. 118 (1994) 781] on 49 samples belonging to six Late Quaternary basaltic flows from central Mexico. Samples were selected from a large collection because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. Moreover, they previously yielded high quality Thellier paleointensity results, which makes them good candidates to assess the reliability of Shaw’s paleointensity method. Only 13 samples yielded acceptable results using Shaw’s original method (ARM2/ARM1 ratio varies from 0.95 to 1.05 for accepted determinations) although 6 samples do not pass the validity test proposed by Tsunakawa and Shaw [Geophys. J. Int. 118 (1994) 781] and thus should be rejected for paleointensity analyses. Rolph and Shaw’s [Geophys. J. Int. 80 (1985) 773] method gives reliable determination only in one case and no single determination was obtained by Kono’s [Geophys. J. Res. Astr. Soc. 54 (1978) 241] modification. Our results indicate an extremely low success rate of Shaw’s paleointensity method, which may be due to magneto-chemical changes that occurred during heating of samples above their Curie temperatures. 相似文献
35.
Juan Morales Avto Goguitchaichvili Edgardo Cañon-Tapia Raquel Negrete 《Comptes Rendus Geoscience》2003,335(14):995-1004
From a large collection (more than 300 oriented cores) of Baja California Mio-Pliocene volcanic units, sampled for magnetostratigraphy and tectonics, 46 samples were selected for Thellier paleointensity experiments because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. 19 samples, coming from 4 individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.6 to 6.2 ×1022 A m2. Our results, although not numerous, are of high technical quality and comparable to other paleointensity data recently obtained on younger lava flows. The NRM fractions used for paleointensity determination range from 38 to 79% and the quality factors vary between 4.8 and 16.7, being normally greater than 5. The combination of Baja California data with the available comparable quality Plio-Plesitocene paleointensity results yields a mean VDM of 6.3 ×1022 A m2, which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and of dissimilar quality, which makes it hard to draw any firm conclusions regarding the Pliocene and Early/Middle Pleistocene evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003). 相似文献
36.
Bertha Aguilar Reyes Rubén Cejudo Ruiz Juan Martínez-Cruz Francisco Bautista Avto Goguitchaichvili Claire Carvallo Juan Morales 《Studia Geophysica et Geodaetica》2012,56(3):879-887
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 Michoacán (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. 相似文献
37.
The record of the Earth’s magnetic field intensity during the past (paleointensity) carries important information about the geodynamo and the state of the Earth’s interior that is not contained in the record of its paleodirection. To determine what the critical factors in obtaining reliable estimate of paleointensity are, we present new results of a paleointensity study of the 1955 and 1960 Kilauea volcano lava flows, from the Big Island of Hawaii. Rock magnetic measurements on representative hand samples from each flow in conjunction with reflected light microscopy observations show the primary carriers of remanence to be pseudo-single domain titanomagnetite with various titanium contents. Paleointensity samples (small fragments previously embedded into salt pellets) were subjected to the Thellier-Coe experimental procedure. Fourteen temperature steps were distributed over the entire temperature range used (ambient temperature to 570°C). Control heating steps (commonly referred to as partial thermo-remanent magnetization — pTRM checks) were also conducted each third double heating step. Mean field intensity value (36.6 ± 0.7 μT) retrieved from 3 reliable site mean determinations reproduces the expected value within 1.1%. With the knowledge of the rock magnetic characteristics of the samples and the strength of the geomagnetic field during cooling of the lava, our investigation suggests that the Hawaiian lavas can faithfully record the local geomagnetic field and confirms that the Thellier-Coe type techniques are suitable on historical lava flows to yield reliable absolute paleointensity determinations. The variations in direct field measurements and in lab paleofield determinations may reflect local heterogeneities of the lava or influence of very local field anomalies due to the volcanic underlying terrain. These results underscore the importance that a better understanding of intensity results of historical lava flows is still required if reliable paleointensity determinations of older periods, for which we do not know the answer, are sought. 相似文献
38.
Miguel Cervantes Solano Avto Goguitchaichvili Leda Sánchez Bettucci Ruben Cejudo Ruiz Manuel Calvo-Rathert Vicente Carlos Ruiz-Martinez Ruth Soto Luis M. Alva-Valdivia 《Studia Geophysica et Geodaetica》2010,54(4):533-546
We report detailed rock-magnetic and paleomagnetic directional data from 35 lava flows (302 standard paleomagnetic cores)
sampled in the Central-Northern region of Uruguay in order to contribute to the study of the paleosecular variation of the
Earth’s magnetic field during early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South
America. The average unit direction is rather precisely determined from 29 out of 35 sites. All A95 confidence angles are less than 8°, which points to small within-site dispersion and high directional stability. Normal polarity
magnetizations are revealed for 19 sites and 10 are reversely magnetized. Two other sites yield well defined intermediate
polarities. The mean direction, supported by a positive reversal test is in reasonably good agreement with the expected paleodirection
for Early Cretaceous stable South America and in disagreement with a 10° clockwise rotation found in the previous studies.
On the other hand, paleomagnetic poles are significantly different from the pole position suggested by hotspot reconstructions,
which may be due to true polar wander or the hotspot motion. Our data suggest a different style of secular variation during
(and just before) the Cretaceous Normal Superchron and the last 5 Ma, supporting a link between paleosecular variation and
reversal frequency. 相似文献
39.
Modesto Pineda Duran Avto Goguitchaichvili Juan Morales Bertha Aguilar Reyes Luis M. Alva Valdivia Arturo Oliveros-Morales Manuel Calvo-Rathert Tomas Gonzalez Moran Jasinto Robles-Camacho 《Studia Geophysica et Geodaetica》2010,54(4):575-593
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 Opeño archeological site which has the earliest funeral architecture known in western Mesoamerica. The El Opeño chronology is supported by six C14 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 × 1022 Am2. 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 while two smaller peaks are observed on the ARCH3K curve for the same time interval. The intensity value obtained at about 200 B.C. is a clear outlier and thus its geomagnetic significance should be confirmed by further investigations. 相似文献
40.
The microwave palaeointensity technique has been used to determine palaeointensity during the R3–N3 geomagnetic field reversal, using lavas from the Esja region of southwestern Iceland. The resulting intensity determinations have been compared to two previous studies which used the Shaw and Thellier techniques. Both reported low field intensities during the reversal except for four flows that produced high values (20.4–36.8 μT) using the Shaw technique compared to a single maximum intermediate value of 15.9 μT using the Thellier method. In this paper, an average microwave transitional palaeointensity of 6.95 ± 2.07 μT is found for samples used in the Shaw technique study, and of 7.80 ± 1.61 μT for samples used in the Thellier study, demonstrating that there is no evidence for strong fields during this reversal. 相似文献