Alternating Field Demagnetization Research Articles

A Non‐Magnetized Chondrite Parent Body Revealed by Paleomagnetic Investigation of LL6 Chondrite NWA 14180

AbstractMagnetic records from meteorites provide valuable information about the formation and evolution of the solar system and planets. The parent planetesimals of chondrites are typically considered to be undifferentiated based on their primary chemical composition and texture. However, recent paleomagnetic investigations of various chondrites indicate that they carry a primary remanence generated by a dynamo, suggesting partial differentiation of their parent planetesimals. The presence of a dynamo within the parent planetesimal of LL chondrites remains uncertain due to the ambiguous origin of the remanent magnetism. Here, we report petrographic, paleomagnetic, and rock magnetic properties for the novel LL6 chondrite NWA 14180. The high metamorphic temperature experienced by NWA 14180 could have removed the pre‐accretionary remanence. The fusion crust baked‐contact test suggests that NWA 14180 preserves primary magnetic information about its parent body. Alternating field demagnetization results from interior subsamples reveal distinct low‐ and medium‐coercivity components that may represent a viscous remanent magnetization acquired in the geomagnetic field. No natural remanent magnetization was unblocked in the high coercivity range, implying that NWA 14180 cooled in zero‐field conditions. Therefore, we suggest that the parent body of NWA 14180 did not have a dynamo. Furthermore, this result suggests that the LL chondrite parent planetesimal accreted later and was smaller in size than other chondrite classes.

High‐Resolution Paleomagnetic Secular Variation Since ∼13 ka From a Loess Section in Northwest China and a Regional Geomagnetic Directional Model for East Asia

AbstractPaleomagnetic secular variation (PSV) records provide important information for the dynamic processes of the Earth's geomagnetic field, and also can be used for regional stratigraphic correlation. We conducted a paleomagnetic study on a high sedimentation rate Holocene loess section (the Minle section) with precise 14C age constraints in Northwest China. Rock magnetic results indicate that single domain and pseudo‐single‐domain magnetites are the main magnetic carriers of the natural remanent magnetization. Combining the alternating field demagnetization and thermal demagnetization results, we obtained a high‐resolution PSV record with both absolute declination and inclination since ∼13 ka. Combining with previously published sedimentary records in East Asia, we developed an East Asian PSV reference curve over the past 14 ka (EASed14k) by relocating all the data to a common location (30ºN, 108ºE) and averaging them through Fisher statistics. We further generated a regional geomagnetic directional model spanning the past 14 ka for East Asia (SCHA.EAS14k), using the in situ PSV records and applying the revised spherical cap harmonic analysis in space and cubic B‐spline in time. The predicted directional curves from this model at the common location are consistent with the EASed14k curves, which represent the first‐order variation of the geomagnetic field in this area. The established reference curve and regional model for East Asia in this study will be used for regional and global comparison of the geomagnetic field.

On the early postclassic collapse of Plazuelas (Northern Border of Mesoamerica): Archaeomagnetic evidence based on full geomagnetic vectors

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.

Effects of Pressure and Temperature Changes on Shock Remanence Acquisition for Single‐Domain Titanomagnetite‐Bearing Basalt

AbstractKnowledge of the shock remanent magnetization (SRM) property is crucial for interpreting the spatial change in a magnetic anomaly observed over an impact crater. This study conducted two series of impact‐induced SRM acquisition experiments by varying the applied field intensity (0–400 μT) and impact conditions. Systematic remanence measurements of cube‐shaped subsamples cut from shocked basalt containing single‐domain titanomagnetite were conducted to investigate the effects of changes in pressure and temperature on the SRM acquisition. The peak pressure and temperature distributions in the shocked samples were estimated using shock‐physics modeling. SRM intensity was proportional to the applied field intensity of up to 400 μT. SRM intensity data for peak pressure and temperature of up to 8.0 GPa and 530 K, respectively, clearly show that it increases with increasing pressure and decreases with increasing temperature. The SRM has unblocking temperature components up to a Curie temperature of 510 K, and it easily demagnetizes with alternating field demagnetization. The observed SRM properties can be explained by the pressure‐induced microcoercivity reduction and temperature‐induced modification of the blocking curve. Although the remanence acquisition efficiency of the SRM is significantly lower than that of the thermoremanent magnetization (TRM), the magnetic anomaly originating from the SRM distribution in a broader region may show a contribution comparable to that of the impact‐induced TRM distribution in a narrow region.

First Full‐Vector Archeomagnetic Data From Central Asia (3 BCE to 15 CE Centuries): Evidence for a Large Non‐Dipole Field Contribution Around the First Century BCE

AbstractUnraveling the short‐term behavior of the Earth's past geomagnetic field at regional scales is crucial for understanding its global behavior and, thus, the dynamics of the deep Earth. In this context, obtaining accurate full‐vector geomagnetic field records from regions where archeomagnetic data are absent becomes essential. Here, we present the first full‐vector archeomagnetic data from Central Asia, derived from the analysis of nine archeological kilns sampled in South Uzbekistan, dating back to the period between 200 BCE and 1429 CE. To obtain these new data, we conducted thermal and alternating field demagnetization procedures, along with Thellier‐Thellier paleointensity experiments, including partial thermoremanent magnetization checks, thermoremanent magnetization anisotropy and cooling rate corrections. The comparison between the new data, previous selected data from Central Asia, and available global models reveals important differences between approximately 400 BCE and 400 CE, especially concerning the geomagnetic field intensity element. In order to investigate this in detail, we have developed a regional update of the SHAWQ global models family by incorporating, for the first time, high‐quality data from Central Asia. The results suggest that this deviation is linked to non‐dipolar sources of the geomagnetic field in Central Asia reaching a maximum contribution around the first century BCE. According to the updated global paleoreconstruction, this non‐dipole feature, manifested at the Earth's surface as low intensities, is associated with the presence of a reversed flux patch at the core‐mantle boundary beneath this region.

Paleomagnetic and Rock Magnetic Study of the Gulerghat Area Amravati, Maharashtra

Magnetic Polarity and Stratigraphicstudies of the Narnala section from Akot and Amravatidistricts area(Lat. 21000'-21015'N; Long. 770–77015'E) are carried out. Total thousandsof oriented samples from fresh basalts were collected at close intervals. Detailed palaeomagnetic investigation where carried on 250 samples collected from 20 lava flows belonging to 17 selected traverses were subjected to progressive thermal demagnetizer at 15 different steps of temperatures from 50˚C to 600˚C. Traverse attaining a total thickness of 800m in the study area. Some rocks may fail to retain the magnetic record due to the modification subsequence to their formations by a number of natural phenomena like geotectonic disturbance, metamorphism and alteration, which can produce various degrees of secondary magnetisation in the rock. The secondary magnetisation acquired by the rock is unstable as compared to naturally remain magnetism which can be removed by cleaning techniques like stepwise Alternating field demagnetisation (AFD) and Thermal demagnetisation (TD). The palaeomagnetic results indicate that the most of the flows showed irregular natural remnant directions while a few specimen even from other flows for which a meaningful NRM could be deciphered also showed inconsistent directions. Only 35% of the samples showing irregular directions initially improved on alternating field and stepwise thermal demagnetization. High degree of alteration of the flows appears to have affected the stability of the palaeomagnetic directions. The mean palaeomagnetic directions indicate the presence of lower transitional directions at the bottom part of the sequence and upper reversed polarity in remaining part of the sequence with a few samples showing scattered behavior. The polarity transition has take place in the study area. Ancient latitude calculated for the study area is 30o S and that of Nagpur is 31o S indicating that India has drifted by 51o due N which is more than 5000 Kms. since the formation of this lavas.

Magnetic polarity & stratigraphy of the Bundelkund area Amravati, Maharashtra

Magnetic Polarity and Stratigraphic studies of the Bundelkund area Amravati districts (Lat. 21000'-21015'N; Long. 770–77015'E) are carried out. Total thousands of oriented samples from fresh basalts were collected at close intervals. Detailed palaeomagnetic investigation where carried on 250 samples collected from 20 lava flows belonging to 03 selected traverses were subjected to progressive thermal demagnetizer at 15 different steps of temperatures from 50˚C to 600˚C. Traverse attaining a total thickness of 800m in the study area. Some rocks may fail to retain the magnetic record due to the modification subsequence to their formations by a number of natural phenomena like geotectonic disturbance, metamorphism and alteration, which can produce various degrees of secondary magnetisation in the rock. The secondary magnetisation acquired by the rock is unstable as compared to naturally remain magnetism which can be removed by cleaning techniques like stepwise Alternating field demagnetisation (AFD) and Thermal demagnetisation (TD). The palaeomagnetic results indicate that the most of the flows showed irregular natural remnant directions while a few specimen even from other flows for which a meaningful NRM could be deciphered also showed inconsistent directions. Only 35% of the samples showing irregular directions initially improved on alternating field and stepwise thermal demagnetization. High degree of alteration of the flows appears to have affected the stability of the palaeomagnetic directions. The mean palaeomagnetic directions indicate the presence of lower transitional directions at the bottom part of the sequence and upper reversed polarity in remaining part of the sequence with a few samples showing scattered behavior. The polarity transition has take place in the study area. Ancient latitude calculated for the study area is 30o S and that of Nagpur is 31o S indicating that India has drifted by 51o due N which is more than 5000 Kms. since the formation of this lavas.

Exothermic events in a fossil seismogenic fault acquiring thermoviscous remanent magnetization in an exhumed accretionary complex

Detecting exothermic events within fault rocks is key to understanding fault behavior. While paleo-temperature studies of fault rocks have been conducted via vitrinite reflectance or magnetic analysis, secondary remanent magnetization caused by exothermic events within fault rocks has not been examined directly. Direct detection of the secondary remanence has the advantage of revealing both paleo-temperature constraints and the paleomagnetic direction, which can help assess the relationship between secondary remanence and deformation. We conducted thermal and alternating-field demagnetization analyses and other rock–magnetic experiments on a fossil seismogenic fault zone and the surrounding host rocks in an exhumed accretionary complex. The fault zone comprises a few cataclastic zones, including thin-slip zones. The blocking temperature in the secondary magnetization site was detected at 300–360 °C only in cataclasite with magnetite or pyrrhotite as carriers. The paleomagnetic direction of the secondary magnetization is characteristic in the cataclasite, and the blocking temperature was higher than the paleo-maximum temperature of the host rocks. Therefore, the blocking temperature records a local exothermic event in the cataclasite. Thermoviscous remanent magnetization (TVRM) were suggested as candidate mechanisms for the blocking temperature.

Paleomagnetism and age of the Leucite Hills Volcanic complex, Wyoming: Implications for eruptive history, landscape evolution, and the geomagnetic instability timescale (GITS)

The Leucite Hills Volcanic Field, southwest Wyoming comprises two dozen volcanic features including necks, flows, dikes, and plugs. It has been the focus of many petrologic studies as its volcanic and shallow intrusive rocks are one of the only surficial manifestations of ultrapotassic lamproite. We build on paleomagnetic findings of Sheriff and Shive (1980) by providing further paleomagnetic data from the Boars Tusk dike and Black Rock flows. We also characterize the magnetic mineral assemblage of these lamproites. Principal component analysis of alternating field (AF) and thermal demagnetization data indicate that the dike and breccias of Boars Tusk record a reversed magnetic polarity and the Black Rock lava records a normal polarity, both consistent with previous findings. This recording is typically carried by minerals with coercivities >15 mT and susceptibility measurements indicate magnetite, maghemite, and titanomagnetite as likely magnetic carriers. AF and thermal demagnetization experiments evince secondary magnetizations held by lower coercivity grains, likely caused by lightning strikes. 40Ar/39Ar incremental heating experiments from Boars Tusk and Black Rock give plateau ages of ∼2500 ka and ∼900 ka, respectively. Recent advances in the chronology of geomagnetic field reversals and excursions during the Quaternary permit integration of the Boars Tusk dike into the lower Matuyama chron, whereas the Black Rock lavas most probably record the Kamikatsura excursion. Notably, Black Rock records high inclinations that suggest the short-lived excursion achieved a full geomagnetic reversal, something not observed at other localities recording the Kamikatsura excursion. The Leucite Hills offer further opportunities to refine the Quaternary geomagnetic instability time scale (GITS), and to improve understanding of the eruptive and geomorphic evolution of this unusual volcanism.

PEAT DEPOSITS AS A NEW SOURCE OF PALEOMAGNETIC RECORDS IN THE HOLOCENE AS EXEMPLIFIED BY THE ETRUSSIA EXCURSION

The paper presents the results of microprobe, paleomagnetic and rock magnetic studies on peats from the Tyapka peat section (Khabarovsk region) formed 4 thousand years. Alternating field demagnetization of peats yielded a horizon of 57–109 cm (1833–3083 cal. BP) with negative inclination values of the magnetization vector, which corresponds to the geomagnetic Etrussia excursion. Obtaining relative paleointensity estimates for peats was pioneered, which are consistent with the literature archeomagnetic data for the western part of Eurasia. It was found that the Etrussia excursion took place against the background of increased values of the Earth’s magnetic field, and its duration was ~ 1200 years.

Anisotropy of magnetic susceptibility impressed during rock magnetic procedures (AF, IRM) and information on the domain state of the magnetic carriers

We present and interpret anisotropy of magnetic susceptibility (AMS) fabrics in various rocks, focusing on the effects of Alternating Field (AF) demagnetization and Isothermal Remanent Magnetization (IRM). Our findings reveal that AMS in samples from intrusive rocks with large multidomain magnetite grains is minimally affected by IRM or static AF demagnetization. In nearly isotropic volcanic rocks with titanomagnetite pseudo single domain (PSD) carriers, AMS fabrics caused by static AF demagnetization are easily identifiable, with the most prominent effect being a well-defined AMS lineation (up to 1.04) in the direction of the applied AF demagnetization. Conversely, in samples from rapidly cooled volcanic rocks with titanomagnetite of smaller magnetic grain size, an AMS foliation (∼1.02) is observed orthogonal to the direction of the applied AF field, instead of a lineation. In such samples, an IRM produces a much larger AMS foliation up to 1.3 orthogonal to the IRM. The IRM-impressed AMS is also particularly strong in metamorphic rocks in the greenschist facies with either titano-hematite or pyrrhotite magnetic carriers. Samples with the largest IRM-impressed fabric have very high Mrs/Ms ratio (>∼0.4). Mrs/Ms ratios above 0.5 may indicate the contribution of SD magnetic grains with multiaxial anisotropy. As the apparent multiaxial anisotropy is especially observed in volcanic rocks with micron size dendrites of titanomagnetites, the complex shape of the magnetic particles and their chemical composition likely play a key role in IRM-impressed AMS. AMS fabric in volcanic rocks should not be measured after static AF demagnetization. Tumbling AF demagnetization does not alter significantly the initial magnetic fabric and could be safely used in rocks with strong magnetization related to lightning possibly recording an IRM impressed AMS.

Magnetic characterization of the Daule chondrite (Ecuador's first meteorite fall): The case of elusive tetrataenite?

We investigated the magnetic properties of Ecuador's first reported meteorite fall (March 23, 2008), the Daule ordinary chondrite (L5, S4, W0) using thermomagnetic analyses at high and cryogenic temperatures, analyses of magnetic hysteresis and first-order reversal curves, and thermal and alternating field (AF) demagnetization of natural remanent magnetization (NRM). The mineralogical and chemical composition of Daule was examined using scanning electron microscopy with energy-dispersive x-ray spectroscopy. Most methods indicate that the magnetic properties of Daule are dominated by multidomain FeNi alloys (kamacite) with Ni content varying between ∼4% and ∼17%. However, backfield demagnetization (BFD) analyses revealed the presence of high-coercivity tetrataenite that survived shock metamorphism. The differential survival of tetrataenite at the millimeter scale indicates heterogeneity of the impact-related temperature and pressure fields within the Daule meteorite. BFD curves may serve as an efficient tool for identifying minor amounts of tetrataenite that otherwise cannot be discerned from the signal from magnetically-soft FeNi mineral phases by methods based on induced magnetization. Thermal demagnetization experiments unveiled the presence of a well-defined characteristic component of NRM, which remains resistant to AF demagnetization. We interpret this component as a pre-impact thermochemical remanence carried by tetrataenite and acquired during the thermal metamorphism of the parent body. At cryogenic temperatures, the magnetic properties of Daule are dominated by low-Mg magnesiochromite with the Curie temperature at 60–70 K.

Thermal Resolution of Unblocking Temperatures (TROUT): A Method for “Unmixing” Multi‐Component Magnetizations

AbstractSome rocks contain multiple remanence “components,” each of which preserves a record of a different magnetic field. The temperature ranges over which these remanence components unblock can overlap, making it difficult to determine their directions. We present a data analysis tool called Thermal Resolution Of Unblocking Temperatures (TROUT) that treats the process of thermal demagnetization as a function of temperature (or alternating field demagnetization as a function of coercivity). TROUT models the unblocking temperature/coercivity distributions of components in a demagnetization experiment, allowing these distributions to overlap. TROUT can be used to find the temperatures/coercivities over which paleomagnetic directions change and when two directional components overlap resulting in curved demagnetization trajectories. When applied to specimens given multi‐component Thermoremanent Magnetizations (TRMs) in the laboratory, the TROUT method estimates the temperature at which the partial TRMs were acquired to within one temperature step, even for specimens with significant overlap. TROUT has numerous applications: knowing the temperature at which the direction changes is useful for experiments in which the thermal history of a specimen is of interest (e.g., emplacement temperature of pyroclastic deposits, re‐heating of archaeological artifacts, reconstruction of cooling rates of igneous bodies). The ability to determine whether a single component or multiple components are demagnetizing at a given temperature is useful for choosing appropriate ranges of temperatures to use in paleodirection/intensity experiments. Finally, the width of the range of temperature overlap may be useful for inferring the composition, grain size and domain state of magnetic mineral assemblages.

Evolution of the Earth's magnetic field strength in northwestern Argentina during the last two millennia: Towards the improvement of south American geomagnetic paleosecular variation curve

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.

Monte Alban periphery (Oaxaca, middle Mesoamerica): New evidences on the early occupation trough archaeomagnetic study of pottery kilns

Aimed to obtain absolute chronology data for the early stage of Monte Alban, magnetic mineralogy and archaeomagnetic investigation were carried out of recently discovered pottery kilns. Oriented samples belong to four rectangular kilns, both single and double chambers, and one fire pit at the piedmont peripherical area. Susceptibility vs. temperature curves show market thermal instability while large pseudo-single domain grains seem to carry the major part of remanence for the kiln sample. Fire pit samples, in contrast, seem to be dominated by antiferromagnetic hematite. Alternating field demagnetization allowed to reach primary, characteristic magnetization components with reasonably well-defined mean archaeodirections. Possible archaeomagnetic age intervals were obtained considering currently available global models and local curves. The dates are consistent with the most important periods of occupation at the site, although the results may indicate that there was also a significant cultural presence in the Early Classic Period. Recently discovered kilns indicate constant manufacture of pottery. Although we cannot currently define the intensity of production, we believe that pottery could be a specialized activity in peripherical areas of Monte Alban to exchange ceramics or for its local consumption. Consequently, we propose that the piedmont area was an economically autonomous community whose inhabitants dedicated themselves to various tasks throughout their occupation.

New Absolute Chronological Constraints to La Playa (Sonoran Desert) Archaeology between the American Southwest and Mesoamerica—From Long Period Human Resilience to Apparent Abandonment

Sonoran Desert archaeological settlement is one of the most representative sites in Northwestern Mexico/Southwestern United States of the Early Agriculture period because of various cultural processes involved, such as the introduction of the first cultigens and the construction of Pit Houses. These early desert village settlements used geomorphological features of the local landscape to facilitate their sophisticated form of agriculture. Most of the features and artifacts at the site are associated with the Early Agricultural period of 3150-1900 cal B.P., while most occupation dates are in the Cienega phase (2800-1900 cal B.P.). Later stages are poorly documented because of the apparent reduction in population, less marked archaeological features, and extreme erosion processes. Systematic archaeological excavation revealed evidence of completely burned Pit Houses. We analyzed 56 samples belonging to four Pit Houses and one different combustion feature (Kiln or Horno, as they are locally known) in different areas of the settlement. The experimental procedure included continuous susceptibility vs. temperature measurements and step-wise alternating field demagnetizations. Only 36 samples yielded technically acceptable determinations that allowed the determination of archaeomagnetic directions. Statistically indistinguishable results were obtained from all five studied features. This finding reinforces archaeological evidence of ritual-related paraphernalia and/or apparent abandonment or, at least, migration.

Paleomagnetic constraint of the age and duration of the Taupō Eruption, New Zealand

We have carried out paleomagnetic analyses of tephras from the Taupō eruption, one of the most violent eruptions on Earth in the past 5000 years. Pyroclastic deposits were collected with 7 cm3 cubes pushed into each unit of the Taupō eruption sequence, consisting of airfall units and overlying ignimbrite. Where possible, we targeted fine-ash layers and matrix, as lapilli sized material can significantly affect the quality of the analysis. The samples were oriented using a collection device specially designed to maximize accuracy. All samples were subjected to alternating field demagnetization, while samples from Taupō ignimbrite (Y7)—the only unit deposited hot were also subjected to thermal demagnetization. The characteristic remanent magnetizations (ChRMs) for specimens from unit Y1, the lower and upper parts of unit Y4, and unit Y7 are well determined (α95 < 3.3°). The declinations and inclinations of site-mean ChRMs range from 3.0° to 7.1° and − 53.4° to − 56.2°, respectively, in close agreement with published results from lithic fragments of the Taupō ignimbrite (Y7). The mean ChRM of unit Y3 does not fit within 95% confidence limits of the ChRM of other units. We presume this is a consequence of unit Y3 samples containing relatively coarse grains and of probable secondary process of the deposit. This outlier aside, our results show consistency between thermoremanent magnetizations of ignimbrite and detrital remanences of co-eval ashfalls, thus validating our method for further tephra research. Both geological observations and paleomagnetic estimation using angular difference suggest that the duration of the Taupō eruption sequence was less than a few tens of years. Furthermore, matching the overall mean ChRM direction (Dec = 4.3°, Inc = − 55.3°, α95 = 1.3°, N = 38 specimens) to the New Zealand paleosecular variation record using the MATLAB dating tool, most likely supports a younger age (ca. 310 CE) than the reported wiggle match eruption age of 232 ± 10 CE.Graphical

An In Situ Detection System for Obtaining IRM Acquisition Curves and Alternating Field Demagnetization Remanence Curves of Rocks

An In Situ Detection System for Obtaining IRM Acquisition Curves and Alternating Field Demagnetization Remanence Curves of Rocks

New archaeomagnetic directions from Late Neolithic sites in Shandong province, China

SUMMARY Archaeomagnetism provides important constraints to help us understand the past behaviour of the geomagnetic field. For archaeologists, archaeomagnetic dating has become a potential supplement to traditional dating methods (e.g. radiocarbon dating). Although China has a long history with numerous archaeological discoveries, the collection of archaeomagnetic data remains scarce. In this paper, we provide new archaeomagnetic directions from four late Neolithic (c. 2000 BC) sites in Shandong province, China. After a careful characterization of magnetic mineralogy and a detailed alternating-field demagnetization of the oriented samples, a total of nine archaeomagnetic directions (each with both declinations and inclinations) were obtained, which fill the large gap at c. 2000 BC in the Chinese palaeomagnetic secular variation (PSV) curve. Combining these new results with previously published data, we updated the Chinese PSV curve for the last 7 kyr. We have compared the updated curve with several global geomagnetic models (e.g. pfm9k.1a, ARCH10k.1, CALS10k.1b). Comparisons show that the CALS10k.1b model does not yield a reasonable fit of the data, and the fit becomes worse for older intervals. This poor fit could be explained by the fact that the CALS10k.1b model consists of a large amount of sedimentary data. Therefore, the PSV pattern is difficult to determine due to the strong aliasing effect. On the contrary, the ARCH10k.1 model gives a much better fit than other models because its data are mostly from archaeological materials and the data are mainly from the Northern Hemisphere. The field intensity and PSV are potentially correlated, with a weak field corresponding to an enhanced PSV. However, due to the lack of data for certain time intervals, the proposed correlations need to be further tested. To explore if PSV exhibits longitudinal symmetric or latitudinal antisymmetric patterns like those of the geocentric-axial-dipole (GAD) model, we compiled and compared data from three East Asia countries (China, Korea and Japan) and from four areas (East Asia, North America, Europe and the Middle East, and Australia and New Zealand) of which the latitudes are between 30° and 40°. In the East Asia region, the PSV patterns shown in each data set are consistent because of the geographic proximity of these three countries. However, when comparing the PSV curves from the four global areas, we suggest a potential declinational minimum between 0 AD and 2000 AD. Although further confirmation and investigation are needed, this declination minimum could be diachronous, sweeping from East Asia to Australia and New Zealand, and then North America. Future studies should focus on adding more reliable and precisely dated data to better delineate the PSV trends. Archaeomagnetic dating is promising when a PSV curve can be continuously reconstructed.

Late Holocene paleosecular variation and relative paleointensity records from Lagoa dos Patos (southern Brazil)

Lake and lagoon sediments are important recorders of the Earth's magnetic field variations. However, the Southern Hemisphere, particularly the South American continent, contributes only a small fraction of the global paleosecular variation (PSV) and relative paleointensity data, which hinders a better understanding of the global PSV. Moreover, the scarcity of information on the geomagnetic field in South Brazil for the past few millennia impedes, for example, a detailed analysis of the evolution of the South Atlantic Magnetic Anomaly (SAMA), which encompasses the weakest geomagnetic field on the Earth's surface. Here, we present high-resolution paleomagnetic and rock magnetic data of two cores collected at the lagoon of Lagoa dos Patos, Rio Grande do Sul State, Brazil. Sediment cores from Lagoa dos Patos represent the period from ∼4540 to 3320 cal years BP. Rock magnetic results show the remanent magnetization resides in pseudo single-domain (PSD) magnetite and/or titanomagnetite. Magnetization inclinations and declinations were isolated after alternating field demagnetization (AFD) and principal component analysis (PCA). Mean inclinations are −39,6° and − 38,4° for cores PT-04 and PT-06, respectively. Relative paleointensity results are compatible with geomagnetic field models, implying very promising results in the reconstruction of a reference curve for the region. As there is no PSV and relative paleointensity data for this region in this period, this study helps to elucidate the past field and the presence of SAMA in South America.