The archaeointensity of the earth's magnetic field retrieved from Pampean ceramics (South America)

Absolute intensity determinations using the Coe variant of the Thellier method have been carried out on some selected pottery fragments collected in the wetlands of the lower Paraná (Pampean region, Argentina) in order to construct the first archaeointensity master curve for South America. Associated radiometric ages range between 1640±70 and 730±70 bp. Twenty-one samples (five fragments) out of 46 studied (eight fragments) provided successful absolute intensity determinations. The fragment-mean archaeointensity values obtained in this study range from 21.9±2.3 to 42.6±5.4 μT, with corresponding virtual axial dipole moments (VADMs) ranging from 4.0±0.5 to 8.1±1.0 x 10 22Am 2. This corresponds to a mean VADM of (6.4±1.8)×10 22 Am 2. The synthetic record retrieved from southern Argentina and Brazil consists of 17 mean archaeointensities distributed between approximately ad 700 and ad 1700. The data set shows several distinct periods of fluctuations of quite large intensity. However, most data are concentrated into a relatively narrow time period from ad 950 to ad 1300. Three general features may be detected: the time intervals from about ad 950 to 1130 and 1350 to 1480 are characterized by quite monotonic increases of geomagnetic intensity, while some decrease is observed from ad 1150 to 1280. These variations may be speculatively correlated to climate changes over multi-decadal time scales. Important differences are observed between the data and the geomagnetic field predictions derived from recently reported global models, which reinforces the importance of regional reference curves for dating purposes.Fil: Goguitchaichvili, A.. Universidad Nacional Autónoma de México; MéxicoFil: Loponte, Daniel Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Secretaría de Cultura de la Nación. Dirección Nacional de Cultura y Museos. Instituto Nacional de Antropología y Pensamiento Latinoamericano; ArgentinaFil: Morales, J.. Universidad Nacional Autónoma de México; MéxicoFil: Acosta, Alejandro Alberto. Secretaría de Cultura de la Nación. Dirección Nacional de Cultura y Museos. Instituto Nacional de Antropología y Pensamiento Latinoamericano; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

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

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

New archaeointensity data from Italy and geomagnetic field intensity variation in the Italian Peninsula

We present new archaeointensity results from three Italian kilns situated at Ascoli Satriano, Vagnari and Fontanetto Po obtained with the Thellier modified by Coe double heating method. These data complement the directional results previously published. All sites are dated on the basis of archaeological information and/or thermoluminescence dating. The results are corrected for the anisotropy of the thermoremanent magnetization and the cooling rate effects. The new data are compared with previously published archaeointensity data from Italy and nearby countries within 900 km radius from Viterbo. An initial data set including archaeointesity data mainly coming from Italy, France, Switzerland, Czech Republic, Slovakia, Hungary, Greece and Bulgaria has been compiled. After the application of strict selection criteria, the most reliable data have been used for the calculation of a preliminary Italian intensity secular variation (SV) curve for the last 3000 yr. The new curve covers the 300 BC–400 AD and 1200–1900 AD periods. It is established by means of sliding windows of 200 yr shifted by 100 yr. The lack of reliable data for the 1000–200 BC and 400–1200 AD time intervals does not permit the calculation of a continuous curve. Clearly, more high-quality archaeointensity data from Italy and Europe are still needed to draw a robust intensity SV curve for the Italian Peninsula that could be used for archaeomagnetic dating in combination with the directional data

Estudio arqueomagnético de cerámicas del Holoceno elaboradas por sociedades indígenas del Uruguay

Las variaciones del campo geomagnético en escalas de tiempo arqueológico pueden obtenerse a partir del estudio de estructuras quemadas y cerámica arqueológica fechadas. Presentamos los primeros resultados de arqueointensidad obtenidos de cerámicas del Uruguay elaboradas por sociedades indígenas que ocuparon diversos espacios del territorio uruguayo. Los datos obtenidos de curvas termomagnéticas continuas y de adquisición de magnetización isotermal remanante indican que la titanomagnetita pobre en titanio es la responsable de la remanencia en estos fragmentos cerámicos. Quince fragmentos (~100 especímenes) de los veintidós analizados produjeron estimaciones confiables de intensidad. Las determinaciones de arqueointensidad se obtuvieron empleando el método de Thellier-Coe, mientras que los fechamientos correspondientes utilizando la herramienta de datación arqueo_dating. Las arqueointensidades obtenidas del total de las muestras analizadas varían entre (23.3 ± 1.4) μT y (42.9 ± 2.4) μT, y corresponden a edades entre 1660 AEC y 1660 EC. Dichas edades son consistentes con las cronologías obtenidas por 14C reportadas para los sitios arqueológicos de donde proviene la cerámica