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—Reconstructing the evolution of the geomagnetic field in the European part of Russia has been an extremely challenging problem due to the still limited data meeting the up-to-date quality criteria. The paper presents the results of archaeomagnetic studies for five groups of fragments of baked clay bricks from architectural monuments of the 18th to 19th centuries in the Yaroslavl region. Paleointensity of the samples was determined by the Triaxe method involving a series of continuous high-temperature measurements. The paleointensity determinations satisfying the up-to-date quality criteria are obtained from 71 samples of 19 fragments of five archaeomagnetic groups. The obtained paleointensity data are compared with the existing data for the European part of Russia and regional reference curves for Western and Eastern Europe.

This article offers a revision of the chronology and settlement history of Tula, Hidalgo, synthesizing information obtained from numerous investigations and 68 radiocarbon and seven archaeomagnetic dates. Tula Chico's earliest settlement appeared while the region was under the control of Teotihuacan as one of many hilltop Coyotlatelco settlements in the region. The monumental center at Tula Grande did not appear until after Tula grew to power, presumably with the consolidation of the other Coyotlatelco polities. Extensive exposure of residential structures in numerous localities have revealed a widespread pattern of barrios containing households exhibiting a wide range of social status that enjoyed access to a wide variety of luxury items including the first reported objects of gold. Tula Grande and the Tollan phase city appear to have already been abandoned and in ruins prior to the arrival of Aztec II peoples. The Late Aztec period occupation shows a preoccupation with Tula's ruins that parallels similar evidence from the Templo Mayor excavations suggesting it was indeed the place the Aztecs called Tollan.

Teotihuacan is one of the most studied archaeological sites in Mesoamerica because of its exceptional size and urban planning; however, its last years of occupation and abandonment are still under debate. We report a high-resolution chronology for the Xalla complex integrating archaeomagnetic dates, radiocarbon (14C) ages, and detailed archaeological information about sample type and context in a Bayesian model. The model includes 42 14C ages and 7 archaeomagnetic dates grouped in 6 phases, including samples from collapsed roofs with 14C ages earlier than expected, suggesting a problem of inbuilt age. The archaeomagnetic dates on lime plasters were classified in unburned samples, related to the time of construction, and burned samples, related to the Big Fire associated to the abandonment of Teotihuacan. The modeled 14C ages resulted in shorter intervals, with the possibility of differentiating the construction phases, confirming that big beams had inbuilt age. Further, combining the two dating methods and classifying lime plaster samples in burned and unburned, it was possible to date different events within the same archaeological context. It is concluded that by combining these two dating methods and understanding the moment that each sample is dating, it is possible to obtain solid and precise chronologies.

The objective of this study is to provide a well-dated point for a future palaeosecular variation (PSV) reference curve for western Russia. For this purpose archaeomagnetic and magnetic property analyses were carried out on a pottery kiln unearthed at the UNESCO World Heritage site of ancient Bolgar, having a rather precise age dating. The archaeological context provided an age between 1340 and 1360 C.E. The characteristic remanence vector was determined through alternating field demagnetisation and Thellier-Thellier palaeointensity experiments. Some innovations were introduced regarding palaeointensity. The check testing the equality of blocking and unblocking temperature was redefined. This allowed waiving the commonly used additional zero-field cooling steps during the Thellier-Thellier experiment. Another innovation concerns the calculation of archaeointensity at structure level. A Bayesian approach was introduced for averaging individual specimen archaeointensities using a prior probability distribution of unknown uncertainties. Next, an additional prior probability distribution was used to correct for cooling rate effects. This resulted in a lower uncertainty compared to common practice and in eluding time consuming cooling rate experiments. The complex magnetic mineralogy consists of maghaemite, multi-domain haematite and Al-substituted haematite. Some samples contained also some non-stoichiometric magnetite. The magnetic mineralogy was determined through hysteresis loops, backfield and remanence decay curves, measurements of the frequency dependence of magnetic susceptibility and through low temperature magnetisation curves. Accompanying high-temperature thermomagnetic analyses revealed an excellent thermo-chemical stability of the studied specimens. Directions obtained from alternating field demagnetisation and those extracted from archaeointensity experiments are congruent and have low uncertainties. The obtained archaeomagnetic results are fairly in agreement with global geomagnetic field models and contemporary PSV data of the wider area. The geomagnetic field vector obtained for ancient Bolgar is of high quality, deserving thus its inclusion in a future PSV reference curve for European Russia.

—The archaeomagnetic study of ceramic material from Veksa III archaeological site (φ = 59°17′ N, λ = 40°10′ E) yield the data on the intensity of geomagnetic field in the V–III millennia B.C. in the Russian Plain. The combined results from the material of the Veksa III and Sakhtysh I sites (φ = 56°48′ N, λ = 40°33′ E) suggest that in the studied time interval, the intensity of the geomagnetic field mainly varied within 30 to 50 μT. The gradual changes in the intensity are superimposed by a variation with a characteristic time of approximately 1000 years. The results on the intensity of the geomagnetic field determined for the Russian Plain for the time interval V–III millennia B.C. add substantially to the magnetic field data during this time interval, which promotes better understanding of the variations in the ancient geomagnetic field.

The geomagnetic field variations during the Iron Age (last 1200 years BC) are poorly constrained for Bulgarian territory due to the scarcity of archaeological sites with a well-established chronology. Additionally, the collected Iron Age materials frequently fail to provide reliable archaeointensity data. In an effort to expand the Bulgarian archaeomagnetic database for the last millennia BC and to find an explanation for the low success rate of Thellier experiment, 28 baked clay structures from 10 archaeological sites were archaeomagnetically studied. Magnetically soft minerals dominate in all collections despite the variety of the sampled structures (hearths, domestic ovens, ovens, pottery kilns, burnt dwelling remains, mud-bricks), which suggests differences in the ancient firing conditions. The main carriers are magnetite/titanomagnetite and/or maghemite/titanomaghemite. Verwey transition is well expressed in most samples. Usually more than one magnetic phase exists and noticeable mineralogical transformations occur during laboratory heating. Regardless of all the pre-selection criteria applied, the total success rate of intensity determination is quite low (21%), which seems to be typical for Bulgarian Iron Age materials. At this epoch that starts with a significant decline in the quality of material culture, combustion structures are usually poorly preserved, short-term used (frequently only once) and heated to temperatures around and below 600 °C. A clear relationship is observed between median destructive temperature and success of intensity determination, which suggests that this parameter have a potential as a pre-selection criterion. The new data double the Bulgarian archaeomagnetic dataset for the period in concern with 18 directional and 7 intensity results (6 full vectors). An inclination and intensity decrease is outlined at around 400 BC with assumed variation rates 0.10°/year and 0.32 μT/year, respectively. Considering the whole database, three intensity peaks are detected at circa 1050 BC (Fa ~ 78 μT), 550–500 BC (Fa ~ 96 μT) and 300 BC (Fa ~ 80 μT). The second peak is the highest over the entire eight-millennium long Bulgarian archaeomagnetic record. The first half of the Iron Age (1200–600 BC) is characterized with easterly declinations reaching ~18°E around 1150 BC, while in the last half it turns westerly with westernmost values around 400 BC. The current state of Bulgarian archaeomagnetic reference curves for the Early Iron Age is not sufficient for precise archaeomagnetic dating. Although the resulting dating interval for Ryahovets oven (the only structure with uncertain archaeological dating) coincides with the archaeological evidence, it spans more than 200 years. In such a case, correlation with other archaeomagnetically studied sites from the corresponding period can be especially helpful.

Figure S1: IRM acquisition curves reveal the presence of soft magnetic mineral saturated between 100 and 300 mT. A few samples, reveal a dominant high coercivity anti-ferromagnetic signal; whereas the remaining samples' IRM curves indicate the occurrence of a low coercivity mineral, perhaps magnetite and/or titano-magnetite.

An archaeomagnetic intensity study was conducted on nineteenth-century firebricks manufactured in Scotland and used in an iron foundry in Melbourne, Australia, between 1842 and 1864 CE. Archaeointensity results obtained from bricks with a single component of magnetization gave values of 61.45 + 0.89 and 61.92 + 6.84 μT. These values are in agreement with historical absolute intensity measurements taken at the Melbourne geomagnetic observatory between 1858 and 1863 CE (61.17 + 0.078 μT) and with the gufm1 model based on mariners’ data. A high-temperature vector component, presumably acquired at the time of manufacture in Scotland, was isolated in certain firebricks and an archaeointensity of 48.3 + 8.39 μT was obtained, which is consistent with the gufm1 model for Scotland at this time (48.79 μT). The dual archaeointensity record of the firebricks supports their geographical provenance, highlighting the potential for archaeointensity data to be used in archaeological artefact-sourcing studies, whilst anomalously high intensities recorded in one of the bricks highlight potential contamination issues from non-Earth magnetic fields in archaeometallurgical contexts. The new Melbourne archaeointensity data are the most precisely dated archaeomagnetic data produced so far for Australia.

The importance of the full understanding of the Holocene geomagnetic field spans from human history and archaeology, to palaeoclimatic changes and engineering, to geomagnetic field modelling and unravelling of the geomagnetic field characteristics and anomalies. Unfortunately, the dearth of data from large under-covered areas such as oceans, the African and South American continents, and the southern hemisphere (only 4–6% of the global datasets) dramatically limits our knowledge of the temporal and spatial evolution of the geomagnetic field and its application. Here, a review of all data from the African continent is presented in order to encourage and motivate a new generation of palaeomagnetic and archaeomagnetic studies. New data will sharpen the palaeomagnetism as a dating tool, improve our knowledge of local/global geomagnetic features, and will help to finally answer some of the fundamental questions in palaeomagnetism, like the temporal and spatial distribution of the palaeointensity peaks described, and the origin and evolution of the South Atlantic Anomaly. © 2020 The Author(s). Published by The Geological Society of London. All rights reserved.

Table S1. Summarized archaeomagnetic determinations of the studied collections. D – magnetic declination after sun compass correction; I – magnetic declination; Fa – archaeointensity value obtained with the corresponding standard deviation (σ) in a temperature interval Tmin – Tmax using NPI points; NRM fraction (f); gap factor (g); quality factor (q); maximum angle of deviation (MAD) are also given. In the sample name, the small letters indicate the specimens used for AI determination whereas each sample direction is calculated as an average of two or three specimen' directions after the corresponding cleaning (thermal or AF). Specimens treated in Rennes laboratory are marked in grey.

Apart from magnetostratigraphy, archaeomagnetism is rarely used in Middle and Late Pleistocene sites. Here we present detailed palaeomagnetic analyses of cemented hearths and burnt chert items from Amud Cave, Israel (68–55 ka) - two types of materials common in Levantine Middle Palaeolithic cave sites. Both materials are shown to be recorders of the geomagnetic field and were used to reconstruct either the ancient field direction (for cemented hearths) or intensity (palaeointensity) (for chert) at the time of the last burning or shortly after. We test the utility of palaeomagnetic data to further our understanding of temporal aspects of occupations in the cave by comparing the dispersion of the palaeomagnetic data to the known characteristics of geomagnetic secular variation in the Holocene. We show that divergent palaeointensities can help identify diachronic burning events, suggesting different activity patterns in two areas of the cave. Additionally, we used palaeomagnetic directional vectors to distinguish between a well-preserved hearth and one that had been mixed prior to cementation. Using rock magnetic investigations, we demonstrate that magnetic methods can be used as a relatively fast and inexpensive method to identify burning of cherts in antiquity above 500 °C. The palaeomagnetic results are in agreement with results of previous studies at Amud Cave, obtained by other independent methods. This study shows that palaeomagnetic methods can serve as a powerful tool in the study of Palaeolithic sites.

Se presentan los resultados arqueomagnéticos, de propiedades magnéticas y espectroscopia Raman obtenidos de analizar muestras de dos tipos de calizas calentadas experimentalmente bajo con-diciones controladas. Estas calizas han sido fre-cuentemente identificadas en los yacimientos de paleolítico medio de Cueva Descubierta y Abrigo de Navalmaíllo (Pinilla del Valle, Madrid, Es-paña). Mediante un sistema de termopares, se re-gistraron temperaturas máximas de calentamiento de 400-450 ºC durante una hora. El objetivo es caracterizar las variaciones en la mineralogía fe-rromagnética y los compuestos (in)orgánicos, así como estudiar el registro de la dirección del campo magnético terrestre en el momento del enfriamien-to. Esto permitirá obtener un patrón extrapolable a las calizas arqueológicas de cara a identificar estructuras de combustión en el yacimiento. Los experimentos de ambas técnicas fueron realizados en muestras representativas de ambas litologías tanto antes como después de la quema experimen-tal. Antes de la quema, ambos tipos de caliza es-taban dominados por dolomita y calcita y, como principales óxidos de hierro, goetita y magnetita. Tras la quema, la goetita original pervivió en las muestras calentadas, Here we report the archaeomagnetic, rock-magnetic and Raman spectroscopy results of sample analysis from two types of limestones that were experimentally heated under controlled conditions. These lithologies have been commonly identified at the middle-palaeolithic sites of Descubierta Cave and Navalmaillo rock-shelter (Pinilla del Valle, Madrid, Spain). By means of a thermocouple system, maximum heating temperatures of 400 − 450 ºC were recorded during one hour. The main goal was to characterize the variations in the magnetic mineralogy and (in)organic compounds, and to study the record of the Earth´s magnetic field direction at the time of cooling. This allowed us to obtain a pattern that is potentially applicable to archaeological limestones in order to identify combustion structures at the site. All experiments were carried out on representative samples both before and after experimental heating. Before heating, both types of limestones were dominated by dolomite and calcite and as main iron oxides, goethite and magnetite. After heating, the original goethite persisted in samples heated to low temperatures (, Junta de Castilla y León (projects BU066U16 and BU235P18) and the European Fund for Economic and Regional Development (FEDER)

espanolEn el presente trabajo se aplico el metodo de fechamiento arqueomagnetico a rocas provenientes de cinco flujos de lava que pertenecen al campo volcanico Ceboruco en Jalisco, en el occidente de Mexico, utilizando el modelo de prediccion de campo geomagnetico SHADIF.14K a partir de las inclinaciones, declinaciones e intensidades absolutas. Se estudiaron cinco flujos de lava de cinco eventos eruptivos, los cuales tienen edad establecida por metodos radiometricos de C14. En dos sitios se obtuvo un fechamiento similar al obtenido mediante el metodo radiometrico; un sitio presento discrepancias con la datacion radiometrica, dado que se trata de eventos volcanicos distintos (explosivo vs. efusivo) y las rocas usadas en este estudio fueron emplazadas en diferente tiempo. En dos sitios el fechamiento no se pudo hacer debido a que las rocas pudieron haber sufrido alguna rotacion o desplazamiento posterior al emplazamiento no advertido durante el muestreo. EnglishIn this paper we present the magnetic dating results of five independent lava flows belonging to the Ceboruco volcanic field in Jalisco, western Mexico, using the SHADIF14K geomagnetic field prediction model. Five lava flows corresponding to well-documented eruptive events were studied. The ages of these flows have already been established by radiometric methods (C14). In two sites the archaeomagnetic ages coincide with C14 ages previously reported. In one site the archaeomagnetic age is considerably different from the radiocarbon age, possibly due to a discrepancy between one or both of the ages and the actual age of emplacement. For the two remaining lava flows, the paleomagnetic directions were found to be scattered and thus no characteristic remanence was determined. These sites probably underwent some rotation or a later displacement that was unnoticed during the sampling.

Changes of the Earth's magnetic field have been observed with varying spatial and temporal resolution since the middle ages by means of compasses, and later by refined mechanical inclination and intensity instruments, until the modern use of quantum mechanical devices and satellite observations. Ancient records, on the other hand, rely on the natural magnetization of archaeological artifacts and other materials, such as rocks and sediments, being therefore indirect, less accurate and sparser. The combination of such heterogeneous records into a single, self-consistent, global model of the geomagnetic field is very challenging: the highly uneven data coverage, both in space and time, requires a careful handling of data uncertainties and error correlations. Previous models dealt with this problem using separated treatments of instrumental data and indirect records, respectively, as well as a different handling of the dipole field component, with respect to non-dipole terms. Here we present a global geomagnetic field model based, for the first time, on the simultaneous inversion of historical, archaeomagnetic, and volcanic records, using a Bayesian approach with minimal-committing time regularization that minimizes the energy of secular variation. A detailed assessment of data uncertainty and error correlation is used to minimize artifacts generated by the appearance of an overwhelming number of incomplete, mostly declination-only records, associated with shipboard measurements in the early colonial period. Our model yields lower dipole energies, which better match modern values, as well as higher non-dipole energies and a stronger secular variation of the dipole moment. Some model artifacts associated with extremely heterogeneous distributions of records in time and space are not completely eliminable and might be caused by incorrect a-priori uncertainty assessments. Alternatively, they might represent an intrinsic limit that can be overcome only by adding new records at critical locations and times.

The archaeomagnetic studies conducted at the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (IPE RAS) have made a significant contribution to the international studies of the main magnetic field of the Earth for the past few thousand years. These studies have yielded extensive data on the intensity of the geomagnetic field in the past 8000 years. Four of the most representative and long time series of the data have been constructed for Eurasia: for the Iberian Peninsula, the Caucasus, Middle Asia, and Siberia. Unique studies of rapid variations of the geomagnetic field intensity (with the characteristic times starting from several tens of years) have been carried out which have no analogues in international research. The analysis of the international data on the ancient geomagnetic field made it possible to obtain a spectrum of the variations of the geomagnetic field intensity in the range from decades to millennia and to determine the characteristics of the variations whose superposition can describe the pattern of the changes of the geomagnetic field intensity. It was established that variations with different characteristic times have differently directed the drift and the “main oscillation” (with a characteristic time of 8000 years) has an eastern drift.

An integrated magnetic survey was carried out on key places within the La Quemada archaeological complex, built over a hill on the northern frontier of Classic period Mesoamerica. Samples were collected from the Plaza of Sacrifices and Hall of Columns in an attempt to precisely determine the age intervals corresponding to the decline and abandonment of the site, apparently due to the intentional firing as a closure ritual. Well-defined characteristic remanent magnetizations were retrieved from 23 of 32 samples. Moreover, twelve samples belonging to two areas yielded reliable determinations under criteria curtailed in this study. In any case, a primary remanent magnetization is carried by pseudo-single-domain Ti-poor titanomagnetite, as evidenced by hysteresis and continuous thermomagnetic curves. The archaeomagnetic dating was performed using full geomagnetic vector (directions and absolute intensity) using the last SHADIF14k model. The Plaza of Sacrifices seems to been burned between a time interval from 854 to 968 CE, while a late interval from 1018 to 1163 CE is assigned to the samples collected in the Hall of Columns, which suggest the gradual abandonment of the site. The first abandonment stage is almost synchronous to the same phenomena observed for other sites at the Bajio area (central and western Mesoamerica) region. Under these circumstances, it cannot be discarded that La Quemada was burned in its entirety during the first stage of abandonment, but the Hall of Columns was again burned later.

We report comprehensive rock-magnetic and archaeointensity investigations from 21 well-constrained pottery fragments from the Catamarca province of northwest Argentina. The absolute ages of the studied sites are ascertained by several high-quality radiometric ages and range between 1940 to 114014C yr BP. Magnetic mineralogy experiments indicates that the remanence is carried by thermally stable Ti-poor titanomagnetites. Forty-seven samples belonging to 11 out of 98 studied potsherds yielded reliable absolute intensity determinations judging from the quality parameters associated with the Thellier double-heating experiments. Moreover, we analyzed the available absolute geomagnetic intensities associated with the radiometric ages to construct the first intensity paleosecular variation curve (PSVC) for South America using thermoremanent magnetization carried by burned archaeological artifacts obtained in the present investigation and 79 other selected archaeointensities (out of 213 published in the literature). The dataset is used to build the PSVC reference curve by combined bootstrap and temporal P-spline methods. The variation curve shows significant differences with the global prediction model SHA.DIF.14k mainly based on the GEOMAGIA database. This intensity PSVC curve shows reasonably good agreement with paleosecular variation curves for Europe between 850 through 1150 BC and for Asia between 1000 and 1500 BC. This regional curve may be used as most reliable archaeomagnetic dating tool for the major part of South America (Peru, Brazil, Argentina, Chile, and Bolivia) for the last two millennia. Fil: Goguitchaichvili, Avto. Universidad Nacional Autónoma de México. Instituto de Geofísica; México Fil: Greco Mainero, Mariano Catriel. Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: García Ruíz, Rafael. Universidad Nacional Autónoma de México. Instituto de Geofísica; México Fil: Pereyra Domingorena, Lucas. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Saavedra 15. Instituto de las Culturas. Universidad de Buenos Aires. Instituto de las Culturas; Argentina Fil: Cejudo, Ruben. Universidad Nacional Autónoma de México. Instituto de Geofísica; México Fil: Morales, Juan. Laboratorio Interinstitucional de Magnetismo Natural; México Fil: Gogorza, Claudia Susana. Universidad Nacional del Centro de la Provincia de Buenos Aires. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; Argentina Fil: Scattolin, Maria Cristina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Saavedra 15. Instituto de las Culturas. Universidad de Buenos Aires. Instituto de las Culturas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Tarrago, Myriam Noemi. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Saavedra 15. Instituto de las Culturas. Universidad de Buenos Aires. Instituto de las Culturas; Argentina

Constraining secular variation of the Earth's magnetic field strength in the past is fundamental to understanding short-term processes of the geodynamo. Such records also constitute a powerful and independent dating tool for archaeological sites and geological formations. In this study, we present 11 robust archaeointensity results from Pre-Pottery to Pottery Neolithic Jordan that are based on both clay and flint (chert) artifacts. Two of these results constitute the oldest archaeointensity data for the entire Levant, ancient Egypt, Turkey, and Mesopotamia, extending the archaeomagnetic reference curve for the Holocene. Virtual Axial Dipole Moments (VADMs) show that the Earth's magnetic field in the Southern Levant was weak (about two-thirds the present field) at around 7600 BCE, recovering its strength to greater than the present field around 7000 BCE, and gradually weakening again around 5200 BCE. In addition, successful results obtained from burnt flint demonstrate the potential of this very common, and yet rarely used, material in archaeomagnetic research, in particular for prehistoric periods from the first use of fire to the invention of pottery.

Beginning in 1964, an academic lineage of Robert DuBois and his students, Daniel Wolfman and Jeffrey Eighmy, developed dedicated United States-based archaeomagnetic research programs. Collectively,...

Beginning in 1964, an academic lineage of Robert DuBois and his students, Daniel Wolfman and Jeffrey Eighmy, developed dedicated United States-based archaeomagnetic research programs. Collectively,...

Constraining the secular variations of the Earth’s magnetic field strength in the past is fundamental to understanding short term processes of the geodynamo. Such records constitute a powerful and independent dating tool for archaeological sites and geological formations. In this study, we present 10 new and robust archaeointensity results from Pre-Pottery to Pottery Neolithic and, for one of the first times, flint (burnt chert) from Jordan. Two of these results constitute the oldest archaeointensity data for the entire Levant, ancient Egypt and Mesopotamia extending the archaeomagnetic dating reference for the Holocene. Virtual Axial Dipole Moments (VADM)s show that the Earth’s magnetic field in the Southern Levant was weak (about half the present field) at around 7,600 years BCE, recovering its strength to greater than the present field around 7,100 BCE and gradually weakening again around 5,200 years BCE. In addition, successful results obtained from burnt flint demonstrate the potential of this rarely used material in archaeomagnetic research, in particular for prehistoric periods from the first use of fire to the invention of pottery.

SUMMARYThe thorough understanding of magnetic mineralogy is a prerequisite of any successful palaeomagnetic or archaeomagnetic study. Magnetic minerals in archaeological ceramics and baked clay may be inherited from the parent material or, more frequently, formed during the firing process. The resulting magnetic mineralogy may be complex, including ferrimagnetic phases not commonly encountered in rocks. Towards this end, we carried out a detailed rock magnetic study on a representative collection of archaeological ceramics (baked clay from combustion structures and bricks) from Bulgaria and Russia. Experiments included measurement of isothermal remanence acquisition and demagnetization as a function of temperature between 20 and >600 °C. For selected samples, low-temperature measurements of saturation remanence and initial magnetic susceptibility between 1.8 and 300 K have been carried out. All studied samples contain a magnetically soft mineral identified as maghemite probably substituted by Ti, Mn and/or Al. Stoichiometric magnetite has never been observed, as evidenced by the absence of the Verwey phase transition. In addition, one or two magnetically hard mineral phases have been detected, differing sharply in their respective unblocking temperatures. One of these unblocking between 540 and 620 °C is believed to be substituted hematite. Another phase unblocks at much lower temperatures, between 140 and 240 °C, and its magnetic properties correspond to an enigmatic high coercivity, stable, low-unblocking temperature (HCSLT) phase reported earlier. In a few samples, high- and low unblocking temperature, magnetically hard phases appear to coexist; in the others, the HCSLT phase is the only magnetically hard mineral present.

Paleomagnetic analysis of archaeological materials is crucial for understanding the behavior of the geomagnetic field in the past. As it is often difficult to accurately date the acquisition of magnetic information recorded in archaeological materials, large age uncertainties and discrepancies are common in archaeomagnetic datasets, limiting the ability to use these data for geomagnetic modeling and archaeomagnetic dating. Here we present an accurately dated reconstruction of the intensity and direction of the field in Jerusalem in August, 586 BCE, the date of the city's destruction by fire by the Babylonian army, which marks the end of the Iron Age in the Levant. We analyzed 54 floor segments, of unprecedented construction quality, unearthed within a large monumental structure that had served as an elite or public building and collapsed during the conflagration. From the reconstructed paleomagnetic directions, we conclude that the tilted floor segments had originally been part of the floor of the second story of the building and cooled after they had collapsed. This firmly connects the time of the magnetic acquisition to the date of the destruction. The relatively high field intensity, corresponding to virtual axial dipole moment (VADM) of 148.9 ± 3.9 ZAm2, accompanied by a geocentric axial dipole (GAD) inclination and a positive declination of 8.3°, suggests instability of the field during the 6th century BCE and redefines the duration of the Levantine Iron Age Anomaly. The narrow dating of the geomagnetic reconstruction enabled us to constrain the age of other Iron Age finds and resolve a long archaeological and historical discussion regarding the role and dating of royal Judean stamped jar handles. This demonstrates how archaeomagnetic data derived from historically-dated destructions can serve as an anchor for archaeomagnetic dating and its particular potency for periods in which radiocarbon is not adequate for high resolution dating.

In absence of organic remains suitable for radiocarbon dating, archaeomagnetic dating can contribute significantly to the better understanding and rescue of our past and cultural heritage. Nowadays, in the archaeological research is fundamental to obtain as much information as possible, including precise dates of the archaeological sites; indeed, the rapid expansion of urban areas implies the destruction of archaeological structures, causing the loss of knowledge about our past. In this effort, we report an archaeomagnetic and rock-magnetic study on burnt archaeological samples from two kilns discovered at the monastery of San Pelayo del Cerrato (Cevico Navero, Palencia, Spain). This was a rescue excavation where arose the possibility of studying archaeomagnetically two ovens for the manufacture of tiles and bells before their destruction due to the restoration works of the monastery. Given their suitable conditions and the lack of independent absolute dating, the aim of the study is to archaeomagnetically date their last use. The main magnetic carrier is pseudo-single domain (PSD) Ti-poor titanomagnetite, indicating that the magnetic signal is stable. The comparison of mean full-vector values (mean direction and absolute archaeointensity determinations) with the SCHA.DIF.14k model determined their last use between 1301–1391 CE (bells´ kiln) and 1295–1404 CE (tiles´ kiln) at 95% confidence level, respectively. These dates agree well with the archaeological evidence indicating that the last use of both kilns occurred almost simultaneously or closely confined in time (14th century AD). The dating resolution obtained is comparable to the radiocarbon with the benefit that archaeomagnetism dates the last use (abandonment) of the structures and no material associated to it which might be slightly younger.

Tsunamis are high energy events capable of leaving their imprint in the geological record. The western Mexican Pacific coast, located in the Mexican Subduction Zone, is prone to large earthquakes (M ≥ 8.0) where tsunami hazard has been underrated. The geologic archive obtained in a sheltered coastal lagoon was analyzed applying a multi-proxy approach to reveal the occurrence of historical earthquakes in the Jalisco coast. Foraminifera and Ostracoda aid identifying earthquake and tsunami geologic evidence in this section of the Mexican Pacific coast, where tropical conditions prevail. We present the evidence of changes in stratigraphy, grain size, biostratigraphy supported by archaeomagnetic dating. They were complemented with published 210Pb dating, magnetic properties, and historical data. We identified three events: the tsunami deposit left by the June 3rd, 1932 tsunami, produced by the largest earthquake (M = 8.2) recorded in instrumental history in Mexico and the coseismic deformation resulting from the 1932 event, also a probable tsunami deposit likely produced by an historical earthquake in 1900, and the coseismic deformation associated to this event, and the coseismic deformation resulting produced by the 1563 earthquake. The site geomorphology, with a marginal lagoon closed by a little barrier 7 m ± 1 m above sea level, precludes tropical storms as responsible for marine incursions. This work contributes to strengthening the earthquake and associated tsunamis chronology, aiming to improve coastal hazard assessment for the area.

The arrival of the great Maori waka and the settlement of New Zealand some seven or eight hundred years ago are described in oral history, but details of exactly when and how colonisation occurred are undocumented. Radiocarbon dating of early archaeological sites is particularly problematic, due to the inbuilt age of datable materials, and non-linearity and ambiguity in the calibration of measurements to calendar dates. Hangi stones, used as heat retainers in traditional Maori earth ovens, hold thermoremanent records of Earth’s magnetic field at the time of their last cooling. Matching the directions of these magnetizations to established reference curves provides alternative, archaeomagnetic, estimates of age. Our results cover the past 700 years, with a cluster of dates between 1500 and 1600 AD, from both North and South Islands, but none earlier than 1300 AD, thus supporting a model of rapid coordinated migration around that time. Archaeointensity data have been obtained from sixteen distinct archaeological features, including twelve hangi from eight sites, and from them the first archaeointensity record for New Zealand has been constructed. To this has been added other archaeointensity and palaeointensity data from the SW Pacific region and virtual axial dipole moments (VADMs) have been plotted. This plot outlines steady VADM values of about 8 x1022 Am2 from 1000-1300 AD, and 9.5 x1022 Am2 from 1500 AD to the present, with an intervening sharp peak in the early 15th century when the VADM reached about 13 x1022 Am2. This peak bears many similarities to archaeomagnetic “jerks” and “spikes” in northern hemisphere records from the first millennia BC and AD. However, it is the first such feature to be found in the southern hemisphere at this date, suggesting, in accordance with recent modelling, that it may be a feature of the non-dipole field, associated with rapid growth and decay of an intense flux patch on the core-mantle boundary.

The first directional archaeomagnetic results from Cyprus are presented, based on the study of seven baked clay structures excavated at the copper smelting site of Agia Varvara-Almyras, situated some 20 km south of Nicosia. This ancient copper working site represents a unique example of complete primary production of the copper metal in Cyprus. In total five furnaces, one ore roasting pit and an oven were sampled in situ for an archaeomagnetic investigation. All the studied structures are well dated, based on pottery finds and radiocarbon dates, with ages ranging from 600 BCE to 50 BCE. Several rock magnetic experiments were performed to determine the main magnetic carrier and to investigate the thermal stability of the studied material. The direction of the Characteristic Remanent Magnetization (ChRM) for each structure has been determined by demagnetization of 7–15 specimens in alternating magnetic fields up to 100–120 mT. Mean directions were calculated assuming a Fisherian distribution and are well defined for five out of the seven structures studied. The new directions are compared with the directional data and the secular variation curves available from the surrounding area, and with the predictions of global geomagnetic field models. They are an important contribution to improving our knowledge of the geomagnetic field variations in the Eastern Mediterranean and offer new insights about fast geomagnetic directional changes during the first millennium BCE that were not previously identified by the available geomagnetic field models’ predictions in this area.

Teotihuacan, the most important city of the Americas during the Classic period, developed a deep and complex civilization without any written histories. This millennial culture mostly used pictorial forms of visual communication expressed in numerous mural paintings. Thus, the knowledge of the absolute chronology of some of the most important mural arts is of great importance. Here, we report a comprehensive magnetic survey on two major murals of Palace of Quetzalcoatl, which represent an initial stage of the Teotihuacan archaeology. The magnetic mineralogy of sampled murals is dominated by both magnetite and hematite grains. The contribution of hematite, however, seems to be minor in the Pictorial Remanent Magnetization. Characteristic remanence was successfully isolated for 11 out of 16 analyzed samples belonging to both paintings. Archaeomagnetic dating was achieved using magnetic inclination and declination, yielding an interval between 312 CE and 409 CE as the best estimate of the time of painting of the murals. This study reinforces the early hypothesis that, red color mural paintings may be considered a complementary source of information about the secular variation of the Earth’s Magnetic Field.

For centuries, and until a few years ago, it was considered that the distillation process had been brought to the new world by the Spaniards, who in turn learned it from the Arabs. For this reason, it was believed that the only alcoholic beverage of the Mesoamerican societies was pulque - a ferment of maguey. However, recent archaeological investigations revealed that the alcohol distillation was known in Mesoamerica long before the arrival of Europeans, for at least 25 centuries. The direct evidence comes from the ceremonial and administrative center of Xochitecatl-Cacaxtla (state of Tlaxcala) with several ovens where cooked maguey remains were discowered. The corresponding archaeological context was radiometrically dated from 600 to 400 BCE. Here, we report a detailed archaeomagnetic study on burned archaeological artifacts found in these cooking ovens. 35 specimens belonging to two pottery fragments, one burned rock and two burned soil samples were pre-selected for archaeointensity experiments. Pottery samples exhibited essentially reversible behavior during thermomagnetic experiments pointing to Ti-poor titanomagnetite (almost magnetite phase) as main magnetic carrier while two ferromagnetic phase seems to co-exist in burned soils. In contrast, burned rock samples exhibited some instabilities during the heating at high temperatures and indirect evidence of the presence of antiferromagnetic hematite grains. In total, 29 specimens allowed the estimation of absolute geomagnetic intensity recorded during the last use of the furnace. Archaeomagnetic dating yielded two possible time intervals between 878 to 693 BCE and 557 to 487 BCE. These new data reinforce the initial hypothesis and corroborate the temporality of these pre-Hispanic kilns.

In 1972, two ovens from dwellings of the Volyntseve culture were studied with archaeomagnetic method by G. F. Zagnii. Proposed archaeomagnetic dating for the oven from Khodosivka dwelling 2 («6th century AD») was different from the archaeological date for two centuries, thence both archaeomagnetic dates seemed unreliable and they were never used in archaeological research as a chronological marker for the Volyntseve culture. The task of this study was to determine whether it is possible to verify old archaeomagnetic data with the help of modern analysis tools and to compare it with recent archaeological datings of the same complexes. Finds from Hodosivka dwellings 1 and 2 include iron buckle, belt ornament, earring, glass beads of the Saltiv cultural circle and wheel-made pottery of the Saltiv technology, that allows using detailed chronological scale of the Saltiv culture and limiting the chronological framework for dwellings 1 to 790—835 AD and for the stratigraphically earlier dwellings 2 to 740—790 AD. Analysis of archaeomagnetic data was made with Matlab tool for archaeomagnetic dating software and three global models: ARCH3K.1, SHA.DIF.14K and CALS3K.3. Two versions of the archaeomagnetic data were examined. First version of data (1977) presented only declination and inclination values, calculated with larger number of measured samples accepted; later version (1986) reflects data with fewer samples accepted after the procedure for excluding extreme values and also field intensity values. Comparison showed that the later version of the data is much better consistent with archaeological dates and must be used as basic. The results of 95 % probability archaeomagnetic dating of the dwelling № 1 in all three used models ARCH3K.1, SHA.DIF.14K and CALS3K.3 are in good agreement with archaeological date: respectively 752—845; 758—855; 678—858 AD. Agreement is less precise in archaeomagnetic dating of the dwelling № 2: respectively 677—784; 702—802; 622—745, 770—814 AD, where only the second part of the time range corresponds to the archaeological dating. However, both new results are acceptable, which means the presence of a chronological error in the local archaeomagnetic curve of G. F. Zagnii and O. M. Rusakov, created for the territory of Ukraine and Moldova. This conclusion means that all Early Mediaeval chronological schemes in archaeology based on this archaeomagnetic curve needs verification of archaeomagnetic data using modern databases and global models.

Archaeomagnetic dating is a firmly established dating technique applicable to a wide variety of heat-treated anthropological materials and is advantageous for sites that lack materials suitable for radiocarbon dating. To correct recent misinterpretations of the method, we provide examples of how archaeomagnetic dating curves are calibrated and show how, in some instances, the technique can provide superior results. We emphasize that no single dating technique is capable of resolving the challenging chronology controversies in the Levant, and instead argue that multiple dating methods must be integrated in order to achieve the highest possible temporal resolution.

Earth’s Magnetic Field variation strength may provide crucial information to understand the geodynamo mechanism and elucidate the conditions on the physics of the Earth’s deep interiors. Aimed to reveal the fine characteristics of the geomagnetic field during the last three millennia in Mesoamerica, we analyzed the available absolute geomagnetic intensities associated to absolute radiometric dating as well some ages provided by historical documents. This analysis is achieved using thermoremanent magnetization carried by volcanic lava flows and burned archaeological artefacts. A total of 106 selected intensities from Mesoamerica and other 100 from the southern part of the United States represent the main core of the dataset to construct the variation curve using both combined bootstrap method and temporal penalized B-spline methods. The obtained intensity paleosecular variation curve for Mesoamerica generally disagrees with the values predicted by the global geomagnetic field models. There is rather firm evidence of eastward drift when compared to similar reference curves in Western Europe, Asia and Pacific Ocean. The recent hypothesis about the relationship between the geomagnetic field strength and paleoclimate is also critically analyzed in the light of this new data compilation., This work was supported by CONACYT project n° 252149 and UNAM-DGAPA-PAPIIT IN101717.

International audience; We propose two ensembles of geomagnetic field models spanning the last three millennia: COV-ARCH is calculated using all available archeological artefacts and volcanic lava flows; lake and marine sediment records are added to this data set to build COV-LAKE. Given the sparse distribution of archeomagnetic observations and their associated large uncertainties, the recovery of magnetic field changes from such data is an ill-posed inverse problem that requires assuming some prior information. This is usually performed by imposing arbitrary regularizations in space and time. Instead, we construct the prior knowledge entering the objective function to be minimized from spatial and temporal statistics of the geomagnetic field, as available from satellites, ground-based observatories and paleomagnetic measurements, and validated by numerical geodynamo simulations. Our approach relies on the projection of model coefficients onto temporal cross-covariance functions. We show with synthetic experiments that the dispersion within the ensemble of solutions provides a coherent measure of the model uncertainties. Gauss coefficients inverted from geophysical records compare satisfactorily with those deduced from the independent database built upon historical and observatory records. A posteriori model errors are reduced when incorporating sediment records; they logically increase towards decreasing length-scales, indicating that a partial information is available up to a spherical degree 4-5. Such models and their associated uncertainties are suited to be used as observations in geomagnetic data assimilation studies. Our results advocate for an approximately constant dipole decay since ≈1700 AD, preceded by an era (going back to 1000 AD) where the dipole trend is weak, possibly slightly positive. We observe in both hemispheres, at both low- and high-latitude, persistent patches over the past 3000 yr. We also confirm a westward drift of flux patches at the core-mantle boundary at a speed of about 0.20 to 0.25° yr-1. Despite the sparse data distribution in the southern hemisphere, the South Atlantic Anomaly appears in both ensembles of models around 1800 AD. A similar low-intensity event seems to have appeared below the Indian Ocean over 600-1400 AD. Both global models are in general good agreement with regional master curves, though filtering out some of the centennial oscillations.

Since the establishment of direct estimations of the Earth's magnetic field intensity in the first half of the nineteenth century, a continuous decay of the axial dipole component has been observed and variously speculated to be linked to an imminent reversal of the geomagnetic field. Furthermore, indirect estimations from anthropologically made materials and volcanic derivatives suggest that this decrease began significantly earlier than direct measurements have been available. Here, we carefully reassess the available archaeointensity dataset for the last two millennia, and show a good correspondence between direct (observatory/satellite) and indirect (archaeomagnetic) estimates of the axial dipole moment creating, in effect, a proxy to expand our analysis back in time. Our results suggest a continuous linear decay as the most parsimonious long-term description of the axial dipole variation for the last millennium. We thus suggest that a break in the symmetry of axial dipole moment advective sources occurred approximately 1100 years earlier than previously described. In addition, based on the observed dipole secular variation timescale, we speculate that the weakening of the axial dipole may end soon.

We present a new curve of the directional secular variation of the geomagnetic field in Western Europe between 1500 bce and 200 ce. Its computation relies on a Bayesian framework. The fast secular variation during the Late Bronze and Early Iron Ages makes archaeomagnetic dating efficient with a respective precision of 150–200 and 60–100 years during these periods. The Bayesian method also provides posterior date distributions that refine the dating of reference data, especially during the period of the Hallstattian radiocarbon plateau. Archaeomagnetism becomes a valuable alternative to radiocarbon and will help to improve the archaeological chronologies.

This study presents new archaeomagnetic results from 33 combustion structures (kilns and hearths) from the archaeological sites of Castelinho, Crestelos, Olival Poço da Barca and Fonte do Milho in NE Portugal. The age of the investigated structures ranges from 1210 BC to 200 AD according to calibrated radiocarbon dating, thermoluminescence dating and archaeological constraints. Stepwise thermal and alternating field demagnetization isolate a single, stable, characteristic remanence component with very well defined directions. Rock magnetic analyses suggest low-Ti titanomagnetite/maghemite as the main magnetic carrier of the remanence. Mean directions are well grouped in most structures. The effect of thermoremanent anisotropy on mean directions has been evaluated and was found to be important. Inclination increases of between 2° and 13° after applying the anisotropy correction at specimen level. This highlights the requirement of evaluating this effect on the directions of small and flattened thin kilns and hearths. The 31 new directional data improve both the temporal and spatial distribution of the Iberian archaeomagnetic dataset from Late Bronze Age to Roman Times. Finally, a new directional palaeosecular variation curve for Iberia for the last twelve centuries BC is proposed. The curve has been computed using the bootstrap method and includes data coming from sites within 900 km of Madrid. The new palaeodirectional secular variation curve for Iberia is consistent with the Western European palaeosecular variation curve and with the prediction of regional European models., The authors are Grateful to the Spanish Research Project CGL2014-54112-R which has supported the field and laboratory costs, as well as the post-doctoral contract of APO.

A detailed rock-magnetic and archaeomagnetic investigation were carried out on seven pottery fragments (49 specimens) found within the burial of an individual with the most body/dental modifications and prestige elements found so far in the ancient Teotihuacan city. The skull of this 35 to 40 years old woman (at the time of death) shows an intentional cephalic modification with frontal-occipital compression. This is very uncommon within the Teotihuacan population and indicating the southern provenance (Oaxaca or Maya area). The age estimations, based on relative datings, placed the archaeological context between 350 and 400 CE. The magnetic mineralogy experiments were characterized by a single remanence component, occasionally accompanied by a soft overprint, probably of viscous origin and Curie temperatures between 540 °C and 565 °C. Only twelve specimens, belonging to two distinct pottery fragments, yielded reliable determinations meeting very strict selection criteria. The values retrieved from both fragments are statistically undistinguishable. The absolute age estimation of the Tlailotlacan Woman archaeological context obtained in this study contradicts the early estimations and indicates a late time interval between 650 and 800 CE corresponding to the decline of Teotihuacan, where the social differences of their inhabitants became much more evident.

The understanding of the paleosecular variations (PSV) of the geomagnetic field in South America is still biased by the scarcity of data. Especially, the recent geomagnetic PSV is characterized by the large growth of the South Atlantic Magnetic Anomaly (SAMA) during the last centuries, first documented by the geomagnetic model gufm1 (Jackson et al., 2000). A large amount of data is required to understand the time and geographic distribution of this primary feature, and the Andean Pleistocene and Holocene volcanoes are an excellent recorder of instant local changes in SV. Here we present a preliminary study from 18 paleomagnetic samples collected during 2015 on what it was supposed to be the 1750 or the 1957–58 AD lava flow on the Llaima Volcano (38.692° S; 71.729° W), one of the most active centers of the Chilean Andes, in the Southern Volcanic Zone. A detailed paleomagnetic study was performed in order to recover the Declination and Inclination of the geomagnetic field, obtain the paleointensity and define the magnetic mineralogy. AF demagnetization until 1 T yielded an average vector at Dec/Inc 2.3°/-33.1° with α95 of 2.4°. This direction is carried by titanomagnetite grains with 40–45% ulvospinel as revealed by thermomagnetic curves. Paleointensity estimates were obtained following the IZZI-Thellier protocol. Seven specimens from 5 samples provided reliable results (success rate of 35%), giving an average paleointensity for these specimens of 30.88 ± 2.39 μT. The full magnetic vector obtained here was compared to archaeomagnetic reference curves and the IGRF suggest that the lava flow has the age of 1957–58 AD.

Many decades of archaeological research in the Mimbres region have resulted in hundreds of chronometric dates obtained from contexts dating prior to A.D. 1450, but until now these data have not been compiled into a single database. Using dendrochronological, radiocarbon, and archaeomagnetic dates, we analyze each of the chronometric data sets to identify patterns and biases that help us better understand the tempo and duration of fundamental transitions in the Mimbres archaeological record. We also identify critical gaps in our knowledge of the chronometric record that provide new research opportunities.

Landscape geophysical survey around the small upland ‘henge’ at Yarnbury, Grassington, North Yorkshire revealed few anthropogenic features around the enclosure but did identify a small rectangular structure in the same field. Sample trenching of this feature, radiocarbon and archaeomagnetic dating proved it to be an earlier Neolithic post and wattle structure of a type that is being increasingly recognised in Ireland and the west of Britain. It is the first to be recognised in the Yorkshire Dales and it is argued that the Dales may have been pivotal in the Neolithic for east–west trade as well as pastoral upland agriculture.

The progressive southward reoccupation of territories of the Iberian Peninsula by the Christian kingdom against the Muslims from the eighth century AD onwards is a well-known process. However, there are few well-dated sites of this period, especially in the northern plateau of Spain. Here we report the full vector archaeomagnetic dating of two hearths from the archaeological site of La Pudia I (Caleruega, Castile-and-Leon, Spain). Both hearths were archaeomagnetically investigated in order to date their last use linked to the abandonment of the site. The archaeomagnetic direction was analysed through thermal (TH) and stepwise alternating field (AF) demagnetization of the natural remanent magnetization (NRM). Pseudo-single domain slightly substituted magnetite was identified as the main magnetic carrier. Thellier–Coe type absolute archaeointensity determinations were carried out on 48 samples from both hearths. The mean directions obtained were independently analysed both at sample and at specimen levels yielding very similar results but statistically distinguishable at 95% confidence level. The archaeomagnetic dating was carried out by comparing the mean directions and archaeointensity values of both hearths with the SHA.DIF.14k geomagnetic field model. The results obtained are in agreement with the archaeological context, suggesting that the abandonment of the archaeological site took place between the end of ninth century and the first half of the eleventh century AD. These results provide one of the first evidences of independently well-dated sites of the Christian conquest in the Iberian northern plateau at the onset of the Early–High Middle Ages.

Archaeomagnetic directions of 141 archaeological structures have been studied from 21 sites in Austria, 31 sites in Germany and one site in Switzerland. Characteristic remanent magnetization directions obtained from alternating field and thermal demagnetizations provided 82 and 78 new or updated (12 and 10 per cent) directions of Austria and Germany, respectively. Nine of the directions are not reliable for certain reasons (e.g. displacement) while three of the features are not well dated. Apart from this some updated age information for the published databases is provided. Rock magnetic experiments revealed magnetite as main magnetic carrier of the remanences. The new data agree well with existing secular variation reference curves. The extended data set covers now the past 3500 yr and a lot of progress were made to cover times BC with data. Here enhanced secular variation is observed manifested in declinations with values up to 70��. The new data will allow for recalculation of archaeomagnetic calibration curves for Central Europe from mid Bronze Age until today. �� 2019 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society.

Temporal variations of Earth\'s magnetic field provide a great range of geophysical information about the dynamics at different layers of the Earth. Since it is a planetary field, regional and global aspects can be explored, depending on the timescale of variations. In this thesis, the geomagnetic field variations for the last two millennia were investigated. For that, some improvement on the methods to recover the ancient magnetic field intensity from archeological material were done, new data was acquired and a critical assessment of the global archaeomagnetic database was performed. Two methodological advances are reported, comprising: i) the correction for microwave method of the cooling rate effect, which is associated to the difference between the cooling times during the manufactory of the material and that of the heating steps during the archaeointensity experiment; (ii) a test for thermoremanent anisotropy correction from the arithmetic mean of six orthogonal samples. The temporal variation of the magnetic intensity for South America was investigated from nine new data, three from ruins of the Guaraní Jesuit Missions and six from archaeological sites associated with jerky beef farms, both located in Rio Grande do Sul, Brazil, with ages covering the last 400 years. These data combined with the regional archaeointensity database, demonstrates that the influence of significant non-dipole components in South America started at ~1800 CE. Finally, from a reassessment of the global archaeointensity database, a new interpretation was proposed about the geomagnetic axial dipole evolution, where this component falls constantly since ~700 CE associated to the breaking of the symmetry of the advective sources operating in the outer core. Variações temporais do campo magnético da Terra fornecem uma grande diversidade de informações geofísicas sobre a dinâmica das diferentes camadas da Terra. Por ser um campo planetário, aspectos regionais e globais podem ser explorados, dependendo da escala de tempo das variações. Nesta tese, foram investigadas as variações do campo geomagnético para os dois últimos milênios. Para isso, aprimoramentos nos métodos de aquisição da intensidade geomagnética registrada em materiais arqueológicos foram realizados, bem como a aquisição de novos dados e uma avaliação crítica da base de dados arqueomagnética global. Dois novos avanços metodológicos são aqui propostos, sendo eles: i) correção para o método de micro-ondas do efeito da taxa de resfriamento, que está associada à diferença entre os tempos de resfriamento durante a manufatura do material e o das etapas de aquecimento durante o experimento de arqueointensidade; (ii) teste para correção da anisotropia termorremanente a partir da média aritmética de seis amostras posicionadas ortogonalmente umas às outras durante o experimento de arqueointensidade. A variação temporal da intensidade magnética para a América do Sul foi investigada a partir de nove dados inéditos, sendo três provenientes das ruínas das Missões Jesuíticas Guaraníticas e seis de sítios arqueológicos associados a fazendas de charque, ambos localizados no Rio Grande do Sul, Brasil, com idades que cobrem os últimos 400 anos. Esses dados, combinados com o banco de dados regionais de arqueointensidade, demonstram que a influência significativa de componentes não-dipolares do campo magnético na América do Sul começou em ~1800 CE. Finalmente, a partir de uma reavaliação do banco de dados globais de arqueointensidade uma nova interpretação foi proposta a respeito da evolução do dipolo axial geomagnético, sugerindo que essa componente está decrescendo constantemente desde ~700 CE devido à quebra da simetria das fontes advectivas que operam no núcleo externo.

Holocene geomagnetic field models are used to study processes in Earth's core, millennial scale variations in solar activity and to relatively date geological and archaeological archives. Constructing models from magnetic field directions and intensities stored in archaeological artefacts, igneous rocks and sediment records is challenging due to the sparse data distribution and the large chronological and magnetic uncertainties associated with the data. In addition, data from sediment records are further complicated by the effects of post-depositional remanent magnetisations (pDRMs) that lead to a magnetic signal which is smoothed and offset in time. Here we present a new probabilistic modelling approach that addresses these problems by co-estimating the geomagnetic field and the age of the data as well as accounting for the effects of pDRM and systematic directional errors in the sedimentary data. The use of a flexible Student's t-distribution in the likelihood function removes the need for outlier rejection which may otherwise lead to excessive smoothing of the model when applied to data with uncertain ages. A final ensemble of models is obtained through Hamiltonian Monte Carlo simulation of the posterior distribution. The modelling approach is tested and evaluated using synthetic data generated from the prior with an equivalent spatial and temporal distribution to the real data and corrupted with realistic noise.

We report new results from frontier research that relates the methodology and knowledge of geophysics (geomagnetism) with archeology and the data will represent a valuable contribution to both disciplines. A detailed magnetic survey was performed on limekilns from the northern Yucatan peninsula. We used Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) Spectroscopy to determine the temperature of calcination in order to select limekiln samples that were heated above the Curie temperature for common magnetic minerals. Using rock-magnetism, we identified the main carrier of remanent magnetization to be multi-domain or large pseudo-single domain low titanium titanomagnetite. Alternating magnetic field demagnetizations delivered reliable archaeomagnetic directions. Our summary of 12 limekiln ages shows a large span from 707 to 1900 yr AD. Such a wide time range signifies that Maya used limekilns in the Yucatan peninsula area in the Late Classic period (600 AD), and that some of these limekilns were reused in the Colonial period. From a geophysical point of view, information can be used about the behavior of the Earth's magnetic field in the recent historical past in Mesoamerica. In this context, the hundreds of Mayan limekilns which are distributed in large areas, open great opportunities to contribute to both archeology and geomagnetic field fluctuations during the past two millennia.