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A detailed magnetic mineralogy and archaeomagnetic study was carried out on recently discovered domestic hearths and burned floors at the Chak Pet archaeological settlement (Tamaulipas, Mexico). The study aimed to obtain reliable absolute chronological constraints on the early development of Huastecs during the Formative period. Oriented hand samples corresponded to four domestic hearths and one burned floor. Continuous thermomagnetic curves revealed mostly irreversible behavior, while titanomagnetites, titanomaghemites, and goethites are assumed to carry the remanent magnetization. In total, 87 specimens were subjected to stepwise demagnetization of natural remanent magnetization using an alternating field procedure. Characteristic remanent magnetization directions were obtained for 29 samples of two hearths and one burned floor. No single, technically acceptable paleointensity determination was obtained. The new archaeomagnetic age intervals for Chak Pet allow locating the origin of this settlement at the Gulf of Mexico within the Middle Formative (900–600 BCE) continuing until the Late Formative period (350–100 BCE). New archaeomagnetic ages are in accordance with the diagnostic pottery analysis. Dated archaeological elements are associated with both ceramic types and different sets of burials, providing a reliable tool to calibrate their chronological and stratigraphic positions. [ABSTRACT FROM AUTHOR]

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We report a detailed rock-magnetic, archaeomagnetic and absolute intensity investigation on burned in situ remains of the Plazuelas archaeological site (Northern Border of Mesoamerica). Fifty-six specimens were subjected to step-wise alternating field demagnetizations to retrieve characteristic remanence directions. All samples yielded evidence of stable, single-component magnetization carried by magnetite or Ti-poor titanomagnetite. Sixty-four specimens were selected for paleointensity experiments following the Thellier double heating technique, including remanence anisotropy correction. The archaeomagnetic dating, based on full geomagnetic vectors, was performed using the global geomagnetic models and the regional Paleosecular Variation Curves available for Mesoamerica. The most probable age intervals obtained for the burned floor of Site 1 (Casas Tapadas North) are within the interval 972AD – 744 AD. In the case of the burned floor of Site 2 (Casas Tapadas West), the ages obtained fall within 971 AD - 752 AD. In the case of the burned floor of Site 3 (South Sector), the ages obtained fall within the interval 752 AD – 969 AD. The calculation of the most probable age intervals, carried out using the Bootstrap resampling method, provided the dates of 940 ± 6 AD, 896 ± 14 AD and 883 ± 14 AD, respectively. These ages agree well with various archaeological hypotheses on the sudden collapse of Plazuelas as a result of intentional destruction, mutilation and generalized firing due to the local war and/or social unrest. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The study of well-dated archeological potsherds and heated structures (i.e. kilns, floors, etc.) yields the opportunity for a better knowledge of the geomagnetic field variations at archeological timescales. We present the first archeointensity results obtained on Uruguayan ceramics prepared by indigenous societies on the coast of the Uruguay River. Titanomagnetite with low Ti content seem to be the responsible for the remanence in these ceramic fragments, as inferred from the results obtained from continuous thermomagnetic and isothermal remanent magnetization acquisitions curves. Fifteen (~100 specimens) out of the twenty- two analyzed fragments produced reliable archeointensity estimates. Archeointensity determinations were obtained using the Thellier-Coe method, while the corresponding archeointensity dates by using the dating arqueo_dating tool. Obtained archeointensities from the total of the analyzed samples vary between 23.3 ± 1.4 µT and 42.9 ± 2.4 µT, and correspond to ages between 1660 BC and 1660 AD. These ages are consistent with 14C dates previously reported in the area. [ABSTRACT FROM AUTHOR]

Highlights • Seven main distinctive zones through the entire core were identified. • A characteristic magnetization was retrieved from each sediment sample. • Comparison of inclination log obtained against the predicted by the global model. • An alternative sedimentation-rate independent age–depth model is proposed. • A correlation between magnetic mineralogy and paleoenvironments was stablished. Abstract We report detailed rock-magnetic and paleomagnetic results from a 27-m-long sedimentary core collected at depocenter of Lake Chapala, western Trans-Mexican Volcanic Belt. Rock-magnetic parameters point to PSD grain-size titanomagnetites as the dominant magnetic mineralogy. Observed variations, in both concentration and grain-size dependent magnetic parameters, reflect changes in the sediment supply through the studied depth, but mainly during the last half century. Stable characteristic remanent magnetization, representative of secular variation of the geomagnetic field, was retrieved after alternating field demagnetization of the natural remanent magnetization (NRM). The comparison of the obtained magnetic inclination and declination logs against the corresponding curves predicted by the Global Geomagnetic Field Model SHA.DIF.14k enables to establish an alternative sedimentation-rate independent age-depth model for the whole core, in reasonably good agreement with the one calculated using conventional radiocarbon dating. This approach works not only for Lake Chapala, but also for Lake Chalco. The correlation established between rock-magnetic parameters and paleoenvironments, based on diatom identification of representative samples within specific zones, can be considered as a novel proxy methodology. This approach opens a new perspective for new, more detailed rock-magnetic, paleomagnetic and paleoenvironmental investigations for better paleoclimatic reconstructions. [ABSTRACT FROM AUTHOR]

San Jacinto archaeological site in Northern Colombia hosts America's oldest pottery, radiometrically dated between 5940 ± 60 BP and 5190 ± 40 BP. Eleven ceramic fragments were subjected to detailed archaeomagnetic and archaeointensity investigation. Alternating field and thermal demagnetization, as well as magnetic mineralogy experiments, were carried out before double heating paleointensity determination to select the most suitable samples. Magnetic susceptibility vs. temperature curves revealed that magnetite and hematite grains are the main carriers of the thermoremanent magnetization. However, the contribution of hematite in total remanence seems to be negligible. Thirty-six samples corresponding to six pottery fragments were selected for Thellier-type paleointensity determinations. Mean intensity values range from 50.7 ± 4.7 to 65.3 ± 11.9 μT while virtual Axial Dipole Moments vary from 12.6 ± 1.2 to 16.2 ± 2.9 × 1022 Am2, indicating high geomagnetic field strength compared to values predicted by global geomagnetic models. This represents the first evidence of relatively high archaeointesities before 1000 BC. Similar or even higher geomagnetic strengths are observed in the Levant around 1100–800 BC and SW Pacific Region at 1450 AD. The actual cause of such anomalously high intensity, often attributed to the processes in the vicinity of the Core-Mantle Boundary, is still not fully understood. [Display omitted] • San Jacinto archaeological site in Northern Colombia probably hosts America's oldest pottery. • Available radiocarbon ages range from 5940 ± 60 BP to 5190 ± 40 BP. • Six out of 11 ceramic fragments were subjected to magnetic mineralogy and absolute intensity experiments. • Virtual Axial Dipole Moments indicate high intensity compared to values predicated by global geomagnetic models. • First strong evidence of high geomagnetic intensity before 1000 BC. [ABSTRACT FROM AUTHOR]

The ability of baked clay-elaborated objects to record the temporal variations of both direction and intensity of the geomagnetic field at the time of their elaboration or last use has been widely used during the last decades to fill the gaps left by available time-discrete volcanic-rock logs for a better knowledge of the evolution of the geomagnetic field. Further refinement of the secular variation curves has motivated the search for alternative non-conventional materials as attractive targets for archeomagnetic studies. Both clay and copper have been used since pre-Hispanic times for the elaboration of a wide variety of ornamental and utilitarian objects in Mesoamerica. Previous studies carried out in Europe and Israel have shown the usefulness of copper slags as one of these non-conventional materials. We present the results of an integrated study carried out on ten copper slags from seven ancient metallurgical sites of Michoacán. The analysis included a series of rock magnetism experiments, scanning electron microscope analysis, archeointensity determinations following the Thellier-Coe method and their corresponding dating. Based on the results obtained the slag-set analyzed can be divided in two main groups: low Fe concentrations (associated with high Cu concentrations) for slags comprising the first group, and high Fe concentrations (associated with low Cu concentrations) for slags from second group. Slags from the first group are characterized by almost reversible thermomagnetic curves, while those corresponding to the second group show a quite irreversible behavior. A crystalline (vitreous) structure within the massive surface of the slags is observed for those coming from the second (first) group. These striking features could reflect significant differences in the metallurgical process followed at these sites, which according to archeological and historical evidences correspond to Colonial and pre-Hispanic usage metallurgical sites; first and second group, respectively. The good experimental behavior exhibited by these materials demonstrates the suitability of copper slag to obtaining absolute archeointensity data which can be used to improve the existing Mesoamerican archeomagnetic record. This in turn, can be used both for geomagnetic field modelling purposes and archeomagnetic dating. [ABSTRACT FROM AUTHOR]

The Cretaceous Normal Superchron is a period of great interest to investigate global scale variations of the geomagnetic field. Long periods of single polarity are still a matter of debate: up to now there are two contradicting theories, which try to relate geomagnetic field intensity and reversal rate. We aim to shed light on the geomagnetic field strength during the Cretaceous Normal Superchron because data are still scarce and of dissimilar quality. To obtain reliable, absolute paleointensity determinations we investigate volcanic rocks from the Western Cordillera of Colombia. Several age determinations allow relating the samples to an age of about 92.5 Ma. To characterize the samples, we investigate rock magnetic properties and determine the characteristic remanent magnetization behavior. To determine paleointensities, we use a multimethod approach: first, we apply the classic Thellier-Coe protocol, and then, the relatively new multispecimen method. Rock magnetic measurements indicate magnetite as the main ferrimagnetic mineral, a stable magnetization revealed by reversible and nearly reversible thermomagnetic curves, and grain sizes that are either in the pseudosingle domain range or a mixture of single and multidomain grains. Alternating field and thermal demagnetization are rather complex, although we observe a few vector diagrams with a single, essentially uni-vectorial component with a small viscous overprint. Paleointensity determination with the Thellier-Coe protocol was unsuccessful, while with the multispecimen protocol we obtained four successful determinations out of 20. The failure of the Thellier-Coe protocol can be attributed to multidomain grains, which were observed during demagnetization and in rock magnetic experiments, and to the inhomogeneity of the volcanic rocks. Our multispecimen paleointensity determinations support low field strength at around 90 Ma during the Cretaceous Normal Superchron. [ABSTRACT FROM AUTHOR]

We present a detailed rock-magnetic and paleomagnetic survey from Autlan volcanic succession in western Mexico. The principal aim of this study is to extend paleomagnetic data from Autlan lavas in order to confirm vertical-axis rotation observed in reconnaissance study and to evaluate long-term variation of the geomagnetic field strength based on existing and global data. The mean inclination (44.7°) is in agreement with the expected inclination for 60 and 70 Ma, as derived from available reference poles for the North American craton. The declination (333.6°), however, is significantly different from those expected, which suggests a statistically significant counterclockwise tectonic rotation ranging between 10° ± 6° and 14° ± 7°. As a measure of paleosecular variation (PSV), we obtained a geomagnetic field dispersion of 9.6° (upper and lower limits: 7.2°-11.9°) in perfect agreement with the previously published PSV compilation of selected Cretaceous data from lavas. The mean virtual dipole moments available for Autlan lavas are about 65% of the present geomagnetic axial dipole but are in reasonably good agreement with other comparable quality determinations between 5 and 90 Ma. This reinforces the hypothesis that low geomagnetic field strengths persisted for the entire Jurassic extending into the Upper Cretaceous. [ABSTRACT FROM AUTHOR]

Goguitchaichvili et al. (Physics of the Earth and Planetary Interiors, doi.org/10.1016/j.pepi.2020.106641, 2021) reported a detailed rock-magnetic and paleointensity study of a Lesser Caucasus lava sequence comprised between 1.93 ± 0.09 and 1.78 ± 0.11 Ma. Reverse polarity magnetization determined for 20 consecutive lava flows permitted to suggest that the Olduvai subchron is probably disrupted by a short reverse-polarity episode. No paleointensity was obtained because of thermally unstable samples. Here, we report a detailed rock-magnetic, paleomagnetic, and multispecimen parallel differential pTRM absolute paleointensities performed on the nearby parallel Khertvisi lava succession. New isotopic age determinations indicate that the lavas of the Khertvisi section erupted in the Early Pleistocene, at the boundary of Gelasian and Calabrian ages in a time span of 1.88 ± 0.10 to 1.71 ± 0.12 Ma. All lava flows yielded well-defined reverse polarity magnetization with site mean directions Inc. = −56.1°, Dec = 189.5°, N = 20 (lava flows), α 95 = 2.3°, k = 187. These directions are close to the Toloshi mean paleodirections, and both slightly deviate clockwise from the geocentric axial dipole and expected Plio-Quaternary directions. The values of the virtual geomagnetic pole (VGP) scatter parameters indicate that the studied sequence was formed during a very short time, most probably insufficient to average paleosecular variation. The multispecimen methodology provided a relatively low paleointensity for most analyzed lava flows, comparable to the transitional field intensity in Georgia during the Bruhnes and Matuyama chrons. These results definitively confirm a rather unstable, reverse polarity geomagnetic field regime across the Olduvai normal superchron. Thus, we propose to consider the Khertvisi section as the firm, volcanic evidence of Olduvai's short-lived geomagnetic event. Goguitchaichvili et al. (Physics of the Earth and Planetary Interiors, doi.org/10.1016/j.pepi.2020.106641 , 2021) reported a detailed rock-magnetic and paleointensity study of the Lesser Caucasus lava sequence comprised between 1.93 ± 0.09 and 1.78 ± 0.11 Ma. Reverse polarity magnetization determined for 20 consecutive lava permitted to suggest that the Olduvai subchron is probably disrupted by a short reversepolarity episode. No paleointensity was obtained because of thermally unstable samples. Here, we report a detailed rock-magnetic, paleomagnetic, and multispecimen parallel differential pTRM absolute paleointensities performed on nearby parallel Khertvisi lava succession. The new isotopic age determinations indicate that the lavas of the Khertvisi section erupted in the Early Pleistocene, at the boundary of Gelasian and Calabrian ages in a time span of 1.88 ± 0.10 to 1.71 ± 0.12 Ma. All lava flows showed well-defined reverse polarity magnetization with site mean directions Inc. = −56.1°, Dec = 189.5°, N = 20 (lavas), α95 = 2.3°, k = 187. These directions are close to the Toloshi mean paleodirections, and both slightly deviate clockwise from the geocentric axial dipole direction and expected Plio-Quaternary directions. The values of the virtual geomagnetic pole (VGP) scatter parameters indicate that the studied sequence was probably formed during a very short time, most probably insufficient to average paleosecular variation. The multispecimen methodology provided a substantially reduced intensity for 5 out of 6 analyzed lava flows, comparable to transitional field intensity. These results definitively confirm a rather unstable, reverse polarity geomagnetic field regime within the Olduvai normal superchron. Thus, we propose to consider the Khertvisi section as the firm, volcanic evidence of Olduvai's short-lived geomagnetic event. [Display omitted] • A detailed rock-magnetic and paleointensity study of the Lesser Caucasus lava sequence comprised between 1.88 ± 0.10 and 1.71 ± 0.12 Ma. • All lava flows yielded well-defined reverse polarity magnetization. • The sequence was formed during a very short time, insufficient to average paleosecular variation. • The multispecimen methodology provided a relatively low paleointensities. • Unstable, reverse polarity geomagnetic field regime across the Olduvai normal subchron. [ABSTRACT FROM AUTHOR]

The causes of the systematic decay of the Earth’s Magnetic Field strength since eighteen century have been a matter of debate during the last decade. It is also well known that such variations may have completely different expressions under an area characterized with strong magnetic anomalies, such as the South Atlantic Magnetic Anomaly. To fully understand these atypical phenomena, it is crucial to retrieve the past evolution of Earth’s magnetic field beyond the observatory records. We report on detailed rock-magnetic and archeointensity investigations from some well-studied historical buildings of Buenos Aires city, located at the heart of the South Atlantic Magnetic Anomaly. Samples consist of bricks, tiles, fireplaces and pottery, which are considered as highly suitable materials for archaeointensity studies. The dating is ascertained by historical documents complemented by archeological constraints. Eighteen out of 26 analyzed samples yield reliable absolute intensity determinations. The site-mean archaeointensity values obtained in this study range from 28.5 to 43.5 μT, with corresponding virtual axial dipole moments (VADMs) ranging from 5.3 to 8.04 × 10 22 Am 2 . Most determinations obtained in the present study are in remarkable agreement with the values predicted by the time varying field model CALS10k.1b (Korte et al., 2011). For the older periods the recently available SHA.DIF.14 model (Pavon-Carrasco et al., 2014) seems to have greater resolution. South American archaeointensity database now includes absolute intensities from 400 to 1930 AD based on 63 selected archaeointensity determinations. The data set reveals several distinct periods of quite large fluctuations of intensity. However, most data are concentrated into a relatively narrow interval from AD 1250 to AD 1450. At the beginning of the record, values between 400 AD and 830 AD match well with ARCH3k.1 model. Some general features may be detected: the time intervals from about AD 400 to 950 and 1150 to 1280 are characterized by a quite monotonic decrease of geomagnetic intensity, while some increase is observed from AD 950 to AD 1250. In contrast, a systematic intensity decay is detected from 1550 to 1930 in excellent agreement with the model prediction. No firm evidence of correlation between the climate changes over multi-decadal time scales and geomagnetic intensity was found for South America. [ABSTRACT FROM AUTHOR]

Revealing the evolution of the past geomagnetic field is of dual interest. It is crucial to estimate the conditions of the Earth's deep interiors, while it offers a reliable absolute dating tool of burned archaeological artifacts. Although a substantial effort was paid during the last decade to construct a reference archaeomagnetic curve for Mesoamerica, mostly absolute intensity variations are revealed for the previous three millennia. Here, we report an updated directional (inclination and declination) reference secular variation curve based on volcanic lava flows and archaeological artifacts carrying full thermoremanent magnetization. A bootstrap method with penalized cubic b-splines was used to develop the directional curve. Relocation error exercise and strict selection criteria allowed to use 82 well-defined palaeodirections. Both data distribution density and the firm relationship between the ages and samples, ensure the reliability of the curve for the dating purposes. The combination of directional data with available intensity variation highlights some sudden geomagnetic field fluctuations. [ABSTRACT FROM AUTHOR]

We report a detailed palaeomagnetic investigation of 28 lava flows (221 standard palaeomagnetic cores) collected from the Paraguayan part of the Paraná flood basalts (the Alto Paraguay Formation). The initial aims of our study were to (i) document variability of the Earth's magnetic field during the time interval sampled, (ii) obtain a new Early Cretaceous palaeomagnetic pole (PP) for stable South America, and (iii) estimate the extrusion rate of the Paraná magma. We precisely determined the palaeofield direction for 26 sites for which the characteristic remanent magnetizations exhibit small within-site dispersion and high directional stability. No palaeodirections were determined for two sites because of a very complex and erratic behaviour of the remanence during the palaeomagnetic treatments. Nine sites display normal polarity magnetization, whereas nine others are reversely magnetized and the remaining eight sites yield intermediate palaeodirections. The mean palaeomagnetic direction of normal polarity sites giveI = −41.8°,D = 4.9°,k = 112, and α95 = 4.9°, whereas reversely magnetized sites giveI = 37.1°,D = 181.4°,k = 23, and α95 = 11.1°. The reversal test as defined by McFadden and McElhinny (1990; Classification of the reversal test in paleomagnetism: Geophysical Journal International, v. 103, p. 725–729) is positive, corresponding to Type B with γ = 8.7° and γ0 = 3.7°. This ensures that the palaeomagnetic treatment successfully removed the secondary natural remanent magnetization and that the sampling adequately averaged the palaeosecular variation (PSV). The mean PP position obtained from 18 sites is palaeolongitude (Plong) = 359.2° and palaeolatitude (Plat) = 86.2° S. We show by means of probability plots and formal testing procedures that a Fisher distribution with a concentration parameterK = 65 satisfactorily fits the distribution of virtual geomagnetic poles (VGPs). The PP obtained in this study agrees reasonably well with coeval pole positions, in particular with those obtained from the Central Paraná Magmatic Province in Brazil, Los Adobes, and Misiones in Argentina. However, some other similar age PPs show significant departure that may be attributed to local tectonic rotations or insufficient sampling needed to overcome the PSV. This new PP differs slightly from the reference poles at 135 Ma for South America given by Besse and Courtillot (2002; Apparent and true polar wander and the geometry of the magnetic field in the last 200 million years: Journal of Geophysical Research, v. 107, no. B11, p. 2300). The PSV parameters are in agreement with those obtained from selected data reported for the Cretaceous Normal Superchron (CNS). In contrast, VGP angular dispersion found here is lower with respect to the Jurassic and Plio–Pleistocene data. The intermediate VGPs determined in the present geographical frame show a northern hemisphere cluster of seven VGPs located east of India, whereas one other VGP is located in the vicinity of Australia. [ABSTRACT FROM AUTHOR]

Abstract: New Thellier-Coe archeointensity determinations have been measured on 15 potsherds from the Oaxaca Valley belonging to three of the four Formative Periods (Pre-Classical) of Mesoamerica, spanning 1400–700 BC. Seven of these are considered to be reliable and indicate a geomagnetic field strength of about 30μT. This value is some 75% of the present geomagnetic field strength but is in agreement with the absolute intensities predicted from global models for this time and location, and consistent with coeval published determinations. These data thus provide significant evidence for the geomagnetic field strength in an area and for a time that was previously poorly constrained, thus providing an important contribution towards establishing a local master curve for the last 3500yr. When established, such a curve would be a useful dating tool and also enable establishing for field strength correlations with climatic events and civilization evolutions in a region that is particularly strong in archeological and geological features. Such potential is examined for aridity events, although such observations can only be considered tentative at this stage. [Copyright &y& Elsevier]

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

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

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

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). [Copyright &y& Elsevier]

We carried out a detailed rock-magnetic and paleointensity study of the ∼187-Ma volcanic succession from northern Chile. A total of 32 consecutive lava flows (about 280 oriented standard paleomagnetic cores) were collected at the Tocopilla locality. Only 26 samples with apparently preserved primary magnetic mineralogy and without secondary magnetization components were pre-selected for Thellier paleointensity determination. Eleven samples coming from four lava flows yielded reliable paleointensity estimates. The flow-mean virtual dipole moments range from 3.7±0.9 to 7.1±0.5 (1022 A m2). This corresponds to a mean value of (5.0±1.8)×1022 A m2, which is in reasonably good agreement with other comparable quality paleointensity determinations from the Middle Jurassic. Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003). [Copyright &y& Elsevier]

Dating analysis of potsherds from three of the six units recently excavated in the Tingambato archaeological site, central-western Mexico, was carried out employing two regional intensity Paleosecular Variation Curves (PSVC) and the global model SHA.DIF.14k. A complete evaluation of the ceramic fragments' magnetic characteristics was done to determine the samples' magnetic carriers and their capacity to provide reliable archaeointensity data. The presence of two magnetization components in five analyzed potsherds provides two snapshots of the Earth's magnetic field intensity behavior at different temporalities. Intensity values were obtained following strict quality criteria, including cooling rate and anisotropy corrections. The principal causes of failure in archaeointensity experiments are associated with the alteration of the magnetic mineralogy during the heating of the specimens and with the presence of multidomain grains. Intensities within the established quality criteria were obtained for four out of the eleven ceramic fragments analyzed. The dating exercise shows a better correspondence of the intensity values with the regional curves than with the global model, especially for the temporal range between AD 400–1200, for which both regional curves are in good agreement. The obtained archaeomagnetic ages had a good correspondence with the radiocarbon ages reported in previous studies. By determining two archaeointensity values in one of the studied potsherds: one associated whit its fabrication and the other related to the fire (after which the site is supposed to be abandoned), archaeomagnetic ages of the probable end of Tingambato occupation were estimated. • Age comparison exercise favors the use of regional models over global models. • Two proposals for the end occupation of Tingambato are presented. • The period AD 400–1200 has the best dating results for the regional models. [ABSTRACT FROM AUTHOR]

First archaeointensity reference paleosecular variation curve for South America and its implications for geomagnetism and archaeology - Corrigendum Figure 6 was printed incorrectly in the original publication of Goguitchaichvili et al. (2019). First archaeointensity reference paleosecular variation curve for South America and its implications for geomagnetism and archaeology. [Extracted from the article]

Three lava flows (Buenavista, Xalitzintla and Nealtican) and pyroclastic density currents (Lorenzo and Pink Pumice) from two Popocatepetl Plinian eruptions were sampled for paleomagnetic dating. A detailed rock-magnetic characterization of the lavas, scoria clasts and pottery shards intercalated between the volcanic deposits was also carried out. Reliable results, both in direction and in intensity, were obtained for the Nealtican lava flow, which enabled its full-vector paleomagnetic dating using the archaeo_dating tool together with the global paleosecular variation model SHA.DIF.14 k, obtaining an age interval between 1040 AD and 1140 AD (95% probability confidence level), in good agreement with its associated 14C age. The well-grouped paleomagnetic direction of the seven specimens from two different scoria clasts of the Lorenzo Pumice pyroclastic density current suggests that clasts were emplaced hot, at a temperature that seems to have almost completely erased the original remanent magnetization of the clasts. This fact is supported by the reheating of the underlying pottery shards, evidenced as a clear secondary low-temperature range (room temperature to 350 °C) component at the orthogonal vector plots. Similarly, the three mean clusters directions obtained for site PO-2 (Pink Pumice)—roughly concentrated around the present geomagnetic field—suggest also a high emplacement temperature. Also, the first archeointensity dating of a pottery shard within the pyroclastic density current is reported. Finally, results of the rock-magnetic and paleomagnetic dating of the last Plinian eruptions from the Popocatepetl volcano, applied to different volcanic materials (lava and pyroclastic density currents), show the usefulness of these nonconventional and alternative techniques in the study of the eruptive activity of volcanoes.. [ABSTRACT FROM AUTHOR]

Study of the paleosecular variation of the Earth's Magnetic Field is an almost unique way to estimate the critical conditions at the core-mantle boundary. The continuous and quasi-periodic changes in the magnitude and directions (declination and inclination) could be detected from direct instrumental measurements or through the remanent magnetization of volcanic and sedimentary rocks. Burned archaeological artifacts are another reliable source of geomagnetic information. Besides the great efforts in Central and North America to retrieve the fine characteristics of the geomagnetic field during the last centuries, no homogeneous data distribution has still been achieved in Mexico and the surrounding areas. To improve the analysis of the geomagnetic full vector in Mexico, we compiled and updated available data for the last 200 years exclusively based on full geomagnetic vectors obtained from direct historical measurements, magnetic repeat stations, geomagnetic observatories and maritime exploratory campaigns. A relocation error procedure allowed the selection of 658 reliable data, discarding those belonging to the sites located in the most northern part of the analyzed area. The spectral analysis through the second derivatives reveals relatively fast fluctuations in both intensity and directions between 1890 and 1960. Moreover, a marked difference is observed between the Mexican record and global geomagnetic models between 1840 and 1940. • Updated available data for the last 200 years exclusively based on full geomagnetic vectors. • A relocation error procedure allowed the selection of 658 reliable data. • Homogeneously distribution across Mexico and the southern United States. • Marked differences with previously obtained records and main geomagnetic global models. • Extreme variation episodes between 1840 and 1940, some of which were observed in Europe. [ABSTRACT FROM AUTHOR]

Highlights • Geomagnetic field variation may precisely date red color mural paintings. • Comprehensive magnetic survey on two major murals of Great Teotihuacan. • The Pictorial Remanent Magnetization carried by both hematite and magnetite grains. • Mural paintings may be considered as an invaluable source of magnetic information. Abstract 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. [ABSTRACT FROM AUTHOR]

A magnetic mineralogy and absolute geomagnetic intensity study was carried out on prehispanic ceramic fragments from the Southern Andes region (Northwest Argentina). Magnetic susceptibility vs. temperature curves and alternating field demagnetization treatments were applied to select the most suitable samples for the absolute archaeointensity experiments. A total of 66 samples were analyzed following the Thellier modified by Coe protocol, including cooling rate corrections and potential mitigation of the magnetic anisotropy effect. Thirty-eight samples (out of 66) belonging to nine pottery fragments yielded reliable absolute intensity determinations The new data, accompanied by associated radiocarbon ages, were integrated into previously published data in order to construct an improved paleosecular variation curve for South America. The highest quality data covers only the last 1000 years. The general variation patterns show good agreement with global geomagnetic prediction models. This updated regional curve may be used as the reference curve for archaeomagnetic dating in the major part of South America (Brazil, Argentina, Chile, Uruguay and Bolivia), even though caution should be paid for the areas and time periods poorly covered by data. • A detailed magnetic, mineralogy and absolute geomagnetic intensity study was carried out from Northwest Argentina. • Thirty-Eight specimens yielded technically acceptable paleointensity determinations based on their quality factors. • These data are distributed between 1606 BP and 416 BP. • The regional curve may be used as the most reliable archaeomagnetic dating tool for the major part of South America. [ABSTRACT FROM AUTHOR]

A detailed magnetic mineralogy and archaeomagnetic investigation was performed on thirty-two red mural painting samples belonging to the colonial Santo Domingo Convent in Oaxaca (Southern Mesoamerica). Systematic magnetic experiments included continuous thermomagnetic curves, hysteresis cycles, acquisition of isothermal remanence and step-wise alternating field demagnetization of the pictorial remanent magnetization (PiRM). Titanium-poor titanomagnetite seems to act as responsible for major part of pictorial remanent magnetization while heating and cooling curves show marked irreversibility and high thermal instability. Although all samples were demagnetized by means of alternating fields up to 90 mT, only seven of them provided reasonably well-grouped paleodirections yielding following mean values: Inclination = 35.2°, Declination = 2.3°, α 95 = 4.8° and k = 161. According to the probability density function obtained by means of the global geomagnetic model SHA.DIF.14 K, the possible elaboration date of the painting layer under study is estimated between 1684 CE and 1744 in accordance with historical documents. Despite low success rate, the described methodology deserves to be considered for dating of wide range of red mural paintings worldwide. [Display omitted] • A detailed magnetic mineralogy and archaeomagnetic investigation was performed on thirty-two red mural painting samples. • Titanium-poor titanomagnetite seems to act as responsible for major part of pictorial remanent magnetization. • The possible elaboration date of the painting layer under study is estimated between AD 1684 and 1744. • Despite low success rate, the described methodology deserves to be considered for dating of wide range of wall paintings. [ABSTRACT FROM AUTHOR]

We have sampled a sequence of 45 late Miocene consecutive lava flows in the Tepic area (Nayarit State, Mexico). The age of the volcanic units lies between 8 and 9 million years (Ma) according to available radiometric data. All samples were stepwise demagnetized, partly with alternating field (AF), partly thermally with very similar results. Most of the rocks exhibited well-defined one component remanent magnetisation with high unblocking temperatures (mostly above 525 °C) and high median destructive fields (MDF) (40–50 mT). Rock-magnetic experiments combined with microscopy show that, in most cases, the main magnetic mineral is Ti-poor titanomagnetite associated with exsoluted ilmenite. Continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases nearly reversible curves with Curie points close to that of magnetite and pseudo-single-domain characteristics. Characteristic remanent magnetisations (ChRM) isolated after the first steps of demagnetisation are all normal polarity. According to the dispersion of virtual geomagnetic pole (VGP) directions, paleosecular variation was abnormally lower than the one observed in general during Miocene. Considering our paleomagnetic results together with available radiometric data, it seems that the volcanic units have been emplaced during a very short time span of about 0.08 million years. The mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle Miocene. Thirty-one samples from eight individual flows yielded acceptable paleointensity estimates. The site mean paleointensities range from 27.8±0.9 to 42.0±7.9 μT. The virtual dipole moments (VDM) range from 5.9 to 9.5×1022 Am2. This corresponds to a mean value of 7.6±1.4×1022 Am2, which is higher than the average VDM value for late Miocene. Altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations. These results support the theoretical suggestion about an inverse relationship between secular variation and local field strength as result of electromagnetic coupling between the solid inner core and liquid outer core, with the inner core tending to stabilise core convection, and hence the field, when intensity is high. Some fluctuation of absolute intensity was detected within the same directional group (DG) indicating that the intensity of the geomagnetic field varies faster than its direction. [Copyright &y& Elsevier]

The radiometric data available in the Trans-Mexican Volcanic Belt enables a quasi-continuous record of the Earth's magnetic field over the last million years to be obtained. Here, we analyze 149 well-defined paleomagnetic directions supported by an equal number of isotope ages. Eleven new paleomagnetic directions from the Tacambaro-Puruaran area (the Michoacán Guanajuato Volcanic Field) are incorporated into the database. Global mean paleomagnetic directions are calculated for one Ma after discarding transitional lavas. The mean direction agrees with the expected paleomagnetic directions obtained from the Pliocene-Quaternary reference poles within the range of uncertainty. The secular variation rate estimated through the scattering of the calculated virtual geomagnetic poles, and the inclination anomaly, both agree with recently reported latitudinal distribution models. An interesting feature of the paleomagnetic direction compilation is the possible evidence of relatively short geomagnetic events or excursions within Matuyama and Brunhes chrons, which may help to refine the geomagnetic instability time scale (GITS). • 149 well-defined paleomagnetic directions supported by an equal number of isotope ages. • The mean direction agrees with the expected paleomagnetic directions retrieved from the Pliocene-Quaternary reference poles. • Analysis of the paleosecular variation for the last 1 Ma. • Possible evidence of relatively short geomagnetic events or excursions within Matuyama and Brunhes chrons. [ABSTRACT FROM AUTHOR]

We report a detailed aeromagnetic, field magnetometry and paleomagnetic survey from the most recent volcano on the American continent that emerged in February of 1943. Paleomagnetic measurements were performed to determine the characteristic remanent magnetization directions, considering that the Geomagnetic Field observed in nearby existing observatories at the time of extrusions is well known for the time of eruption of this volcano. The paleomagnetic records of the Paricutin flows showed lower values compared to the Teoloyucan Geomagnetic Observatory and those estimated with the International Geomagnetic Reference Field model (IGRF). Also, an aeromagnetic data modeling provided by the Mexican Geological Service together with field magnetometry along two profiles in perpendicular directions were carried out. The main objective was to model two levels of the internal structure, one that has to do with deep sources (not exclusively associated with the volcano itself), and another one of a more superficial area directly related to recent eruptions and the immediate geological basement. • Detailed magnetic survey from the most recent volcano on the American continent. • Paleomagnetic directions are deflected compared to nearby observatory data. • An aeromagnetic data modeling performed along two profiles. • Direct geomagnetic measurements show little deviations probably due to the depth sources. [ABSTRACT FROM AUTHOR]

Earth Magnetic Field is characterized by continuous variations in terms of direction and intensity. These fluctuations occur with periodicities of the order of years to tens of thousands of years and constitute the principal characteristic of the geomagnetic field. Revealing such variations at a regional scale has a significant impact in geophysics, but also in archaeology and thus represents a clear example of interdisciplinary research. Due to the importance of non-dipole field components in the secular variation, reference curves can only be constructed for limited areas, commonly within less than 1200 km radius. Three recently reported paleosecular variation curves for most Mesoamerica show contradictory trends, probably due to inappropriate selection of data and the criteria applied to individual intensity determinations. The use of unpublished data, essentially reported in thesis, may lead the discrepancy over the reported curves. Here, we report a critical revaluation of existing published and unpublished data with particular emphasis on the sample-radiometric age relationship for the last 3600 years. • Reliable paleosecular variation curve throughout the last 3,600 years in Mexico. • Similar archaeointensity trends are observed between central Mexico and Southern United States. • The present curve shows significant difference with previously reported trends. [ABSTRACT FROM AUTHOR]

Well-exposed lava sequences with available geochronological information may provide invaluable information on the fine characteristics of the Earth's magnetic field variation. A paleomagnetic study has been performed on twenty Pleistocene age, sub-horizontal lava flows in the Javakheti Highland (Lesser Caucasus). The sequence is formed by calc-alkaline basaltic lavas and is divided by a thin sedimentary layer in its upper part. We obtained two new radiometric ages: an age of 1.93±0.09 Ma for the flows below the sedimentary layer and 1.78±0.11 Ma for the upper part of the sequence. Thermomagnetic susceptibility vs. temperature curves indicate that remanence is carried by ferrimagnetic minerals like magnetite, titanomagnetite or titanomaghemite. Reliable paleomagnetic results could be obtained from all 20 analyzed flows, yielding the following reverse-polarity mean direction for the whole sequence: inclination I =-61.5°and declination D =200.1° (N = 20; α 95 =3.1°; k=114). This direction does not agree with the expected one. The discrepancy can be explained by an apparent clockwise 17.1 ± 6.1° vertical-axis rotation. Alternatively, such moderate deviation may be due to some unstable geomagnetic regime or undersampled paleosecular variation. Isotope ages obtained in the present study indicate that the Toloshi sequence was emplaced during the normal-polarity Olduvai subchron, but palaeomagentic results indicate a reverse-polarity magnetization. At the same time, the analysis of virtual geomagnetic pole (VGP) scatter indicates that the Toloshi sequence was probably formed during a short time, insufficient to average paleosecular variation. These results suggest that the Olduvai subchron is probably disrupted by a short reverse-polarity episode. • A detailed paleomagnetic study has been performed on Pleistocene lava succession in southern Georgia. • Isotope age determinations range between 1.95 and 1.75. • The evidence of a moderate clockwise vertical-axis rotation is detected. • he results suggest that the Olduvai subchron is not really uniform but is disrupted by a short reverse-polarity episode. [ABSTRACT FROM AUTHOR]