Partial Thermoremanent Magnetization Research Articles

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.

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.

Archaeointensity record of weak field recurrence in Japan: new data from Late Yayoi and Kofun ceramic artifacts

Summary We present new absolute archaeointensity data from six archaeological sites situated in the Okayama Prefecture, Japan. The materials studied are well-dated fragments from pottery, ceramic coffins and haniwa artifacts. Their ages range from 160 AD to 675 AD, covering the Late Yayoi and Kofun periods. Rock magnetic experiments suggest the presence of magnetite and/or Ti-magnetite as the main carrier of the remanence, with a possible minor contribution of higher coercivity minerals. After thermal demagnetization experiments, the most magnetically stable samples were selected for archaeointensity analysis performed following the double-heating method proposed by Thellier and modified by Coe. Partial thermoremanent magnetization (pTRM) checks and pTRM tail-checks were performed for monitoring possible chemical alterations during heating. All measurements were corrected for both anisotropy and cooling-rate effects. Successful archaeointensity determinations, following rigorous selection criteria, were obtained for samples from all the investigated archaeological sites. Compared with literature data from Japan, the new high-quality data show significantly lower intensity values. They also reveal possible fast secular variation changes during the Late Yayoi period and very weak geomagnetic intensity field around 630 AD. Such values offer evidence of a possible recurrence of weak intensity field in East Asia, suggesting an ancient recurrence of the West Pacific Anomaly. The new data might change the archaeomagnetic field models interpretations in the area, even though more data are still necessary to better understand the secular variation in Japan and the temporal evolution of the geomagnetic field's behavior in East Asia.

High‐Latitude Paleointensities During the Cretaceous Normal Superchron From the Okhotsk–Chukotka Volcanic Belt

AbstractWe conducted an absolute paleointensity survey on 74 lava flows from the Okhotsk–Chukotka Volcanic Belt (NE Russia), emplaced 90–83 Ma toward the end of the Cretaceous Normal Superchron (CNS, 121–84 Ma). Relying on preliminary results, we restricted our analysis to eight lava flows (140 Thellier–Coe experiments), two of which also yielded successful Wilson determinations. A detailed analysis of the magneto‐mineralogy—based on X‐ray structural analysis, reflected‐light microscopy, and thermomagnetic curves—indicates that the determinations from solely two lava flows can be fully trusted, with remanence carriers (a) dominated by low‐titanium titanomagnetite, (b) showing unambiguous traces of high‐temperature oxidation, and (c) yielding partial thermoremanent magnetization (pTRM) tails representative of pseudo‐single‐domain grains. Recalculated in terms of geomagnetic dipole strength, our two successful flow‐mean determinations yield virtual dipole moments of 4.76 ± 0.26 × 1022 Am2 (N = 7) and 9.07 ± 0.84 × 1022 Am2 (N = 8). Using an updated version of the paleointensity database for the Cretaceous epoch, we stress that determinations based on nonglassy whole rocks, submarine basaltic glasses, and single crystals are mutually inconsistent, suggesting a separate analysis of their distributions is more appropriate. Despite statistically indistinguishable estimates of dipole strength before and after the onset of the CNS—in accord with recent studies refuting the existence of a strict correlation between chron duration and dipole moment—we found that the distribution of dipole moments during the CNS is slightly bimodal with a leptokurtic dominant mode, thus more inclined to produce outliers and suggesting distinct geomagnetic field behavior during the CNS.

Rapid Intensity Decrease During the Second Half of the First Millennium BCE in Central Asia and Global Implications

AbstractRecent paleomagnetic studies have shown that important short‐lived intensity fluctuations occurred during the first millennium BCE. However, the knowledge of the spatial and temporal extension of these features is still limited by the scarce availability of robust data. In this study we focus on the study of the intensity decrease that took place in Central Asia during the second half of the 1st millennium BCE after the high intensities that characterized the Levantine Iron Age Anomaly. Since previous archeointensities available for this period and region were obtained without accomplishing modern standards of quality, we present here new archeointensities that are derived from classical Thellier and Thellier experiments, including partial thermoremanent magnetization (pTRM) checks, thermoremanent magnetization (TRM) anisotropy and cooling rate corrections at the specimen level. The new 51 archeointensities, together with previous archeointensities, have been used to present a new local paleosecular variation curve for Central Asia. The results confirm the existence of an important geomagnetic field intensity decrease in South Uzbekistan from the 4th century BCE to the end of the 1st century BCE associated with rates of changes up to −15 μT/century. A critical analysis of the archeointensity global database indicates that this feature was present at continental scale, from Western Europe to Central Asia. However, this trend is not identified in other regions such as Japan or Mexico. Finally, the comparison with the dipole moment derived from recent global geomagnetic field reconstructions suggests a strong influence of non‐dipolar sources upon this continental intensity feature.

RESET: A Method to Monitor Thermoremanent Alteration in Thellier‐Series Paleointensity Experiments

AbstractAccurately estimating the paleomagnetic field intensity recorded in terrestrial and planetary materials is the key to understanding dynamo processes. Thellier‐series stepwise‐heating methods with partial thermoremanent magnetization (pTRM) checks for alteration are considered the most reliable technique even though pTRM checks are shown to be incapable of detecting the entirety of thermal alteration. We utilize a recently developed multidomain‐correction experiment (or RESET) to monitor thermally induced magnetic property alteration that may happen to the very specimen used in Thellier‐series experiments. We also use rock magnetic property changes to track the physicochemical alteration of fresh companion specimens of Galapagos lavas that were previously used for paleointensity determinations. Our results show that laboratory heating induced thermal alteration of a typical Galapagos lava sample starts to occur at around 500°C. It escaped detection by pTRM checks but was caught by our RESET method.

Single Phase Oxidation of Ferrimagnetic Grains as a Cause of L-Shaped Arai–Nagata Diagrams

Abstract—The experiments on the determination of paleointensity Banc of the Earth’s magnetic field by Thellier method on basalts of the Berd (Jurassic) and Paravakar (Cretaceous) collections sampled in the northeastern Armenia in 2006 revealed an anomalous behavior of the Arai–Nagata diagrams. The anomaly manifests itself by a sharp drop in the intensity of natural remanent magnetization (NRM) under heating of samples to about 400°C, which is accompanied by a very weak acquisition of partial thermoremanent magnetizations (pTRMs) in this temperature interval. The further increase in temperature leads to the opposite phenomenon—an unexpectedly sharp rise in pTRMs intensity with almost no decrease in NRM, which creates the L-shape of the Arai–Nagata diagram. A similar phenomenon of a steep decline during thermal demagnetization was also established for remanent saturation magnetization Mrs(T). We carried out a number of magnetomineralogical experiments from which we conclude that, consistent with the hypothesis suggested by (Kosterov and Prevot, 1998), the sharp drop in Mrs(T) and NRM(T) curves is caused by the transitions of the domain state from a metastable configuration to a more stable one. These transitions are initiated by the processes of single- and heterophase oxidations of primary titanomagnetites under the laboratory heating of samples to moderate temperatures.

Determination of the Formation Field of Artificial CRM and pTRM by the Thellier Method at Different Oxidation Stages of Natural Titanomagnetite

Artificial chemical remanent magnetization (CRM) created on a basalt rock from the Red Sea rift zone, originally containing titanomagnetite (TM) with a Curie temperature of 203–208°С, is studied at various oxidation stages. CRM was formed by annealing the samples at temperature T = 355°C during 4.5–350 h in the magnetic field with induction of 100 μT. The thermomagnetic analysis has shown that with the increase of the annealing time, a single-phase oxidation process takes place, the degree of the oxidation increases and reaches its maximum after annealing during 110 hours. It is established that the subsequent heating of the samples to 600°C in argon leads to the homogenization of the single-phase-oxidized titanomaghemite. When the annealing time increases to 350 h, oxi-exsolution of titanomaghemite begins. CRM as well as the sum of CRM and partial thermoremanent magnetization (pTRM) have been investigated by the Thellier–Coe procedure. The magnetic field Bcalc calculated in the temperature interval 355–475°C from chemical magnetization formed at the annealing times of 4.5, 16.5, and 40.5 h (CRM4.5, CRM16.5 and CRM40.5) proved to be underestimated by 55, 48–52, and 16–34%, respectively. The difference between the calculated and true field decreases with the growth in the degree of single-phase oxidation. The Bcalc values calculated in the temperature interval 475–580°С from CRM40.5 and CRM110 corresponding to the high degree of single-phase oxidation turned out to be underestimated by 69–72 and 62–63%. The last result is caused by the mineral transformations of titanomaghemite at heatings in the Thellier–Coe experiments. During paleointensity determination, it is proposed to control mineral transformations not only using the check-point procedure but also based on the difference in the removal of remanent magnetization in the Thellier experiments and by thermal demagnetization.

Study of Similarity between Blocking Temperature Spectra of Chemical and Thermoremanent Magnetizations by Computer Simulation

Monte Carlo numerical simulation of the formation of chemical remanent magnetization (CRM) by the mechanism of the growth of volumes from superparamagnetic (SPM) to single-domain (SD) in the ensembles of magnetostatically interacting particles, thermoremanent magnetization (TRM), and experiments on determining paleointensity by the Thellier–Coe and Wilson–Burakov methods are carried out. The obtained results agree with the Smirnov–Tarduno hypothesis (Smirnov and Tarduno, 2005) that the similarity of the spectra of blocking temperatures Tb of CRM and TRM can be due to the narrowness of the interval of the blocking temperatures Tb of CRM and TRM, with the reservation that the similarity can be observed on part of the total interval of the Tb spectrum provided that this interval accommodates a significant fraction of the total CRM intensity. The analytical examination of the case of non-interacting particles (sparse concentration of grains) shows that in this case, the (quasi-)linearity of the Arai–Nagata diagrams is due to the presence of a plateau on the curves of the derivative dCRM/dpTRM (pTRM is partial thermoremanent magnetization) which emerges in a narrow spectrum of blocking temperatures. The results of the numerical experiments suggest that at a particle concentration of above 0.2% magnetostatic interaction leads to the practically full linearity of the Arai–Nagata diagram over a significant interval of the total spectrum of blocking temperatures for CRM. At the same time, on the remaining temperature interval, both the Arai–Nagata diagrams and the CRM(TRM) curves are substantially concave, indicating the lack of similarity between the spectra of blocking temperatures for CRM and TRM. The analysis of the empirical data revealed their resemblance, up to a certain point, to the results of simulation but at the same time clearly demonstrated the noticeable distinction between them, associated with the significant differences in the conditions of the numerical and laboratory experiments.

An experimental approach to the preservation potential of magnetic signatures in anthropogenic fires.

Archaeomagnetic and rock-magnetic methods are of great value in the identification of archaeological fire, especially in Palaeolithic sites where evidence is usually scarce, ambiguous or poorly preserved. Although taphonomic processes can significantly modify Palaeolithic combustion structures, the extent to which such processes affect the magnetic record remains unknown. Here we report the results of an archaeomagnetic study involving five, two-to-five-year-old experimental combustion structures in open-air and cave settings. Some of these combustion structures involved post-combustion human actions such as trampling and relighting. Our results show pseudo-single domain (PSD) magnetite as the main magnetic carrier. Wood ash layers of combustion structures are the most magnetic facies followed by thermally altered sediments constituting the combustion substrates. A decreasing magnetic concentration pattern in depth was observed as a function of temperature. Positive correlation was found between good-quality directional data and macroscopically well-preserved combustion structures. Partial thermoremanent magnetization (pTRM) was the main magnetization mechanism identified in the combustion substrate facies. These data coupled with partial thermomagnetic curve experiments show the potential of these methods to estimate maximum temperatures of the last combustion event. Relightings show very good directional results, but they cannot be identified because the time between them is not enough to statistically distinguish directional variations of the local Earth´s magnetic field. The substrate sediment of an intensively trampled combustion structure yielded reliable archaeomagnetic directions. The results are discussed in terms of magnetization preservation potential and the effects of taphonomic processes on the archaeomagnetic record.

New archeointensity data from South Brazil and the influence of the South Atlantic Anomaly in South America

We obtained six new high-quality archeointensity results for the Pelotas city region, in South Brazil with ages ranging from 1790 to 1943 CE. Archeointensity measurements were performed with the double heating technique, including partial thermoremanent magnetization (pTRM) checks and pTRM tail-checks. Measurements were corrected from anisotropy of TRM and cooling-rate effects. Archeological sites providing successful results comprised: Charqueada Santa Bárbara (1790–1838 CE; 36.4 ± 0.6 μT), Chácara da Brigada Militar (1830–1851 CE; 32.6 ± 2.9 μT), Casa Número 08 (1878–1879 CE; 32.1 ± 3.1 μT), Colégio da Família de Eliseu Maciel (1881–1883 CE; 28.7 ± 2.7 μT), Fundação Simon Bolívar (1884–1889 CE; 30.1 ± 2.0 μT), Campus Porto UFPel (1942–1943 CE; 27.8 ± 2.0 μT). Two other sites did not provide meaningful results. The new data complement previous results obtained in South and Southeast Brazil, Argentina and Chile and provide additional evidence for the rapid decay of the field in the region from about 1500 to 2000 CE. In addition, they show a gradual increase in the contribution of non-dipolar components in the geomagnetic field of South America since 1800 CE, likely indicating a strengthening of the South Atlantic Anomaly over the continent from this time onward.

New archeointensity data from NW Argentina (1300–1500 CE)

A good characterization of the geomagnetic field strength at centennial to millennial time scales in the Southern Hemisphere is particularly crucial to disentangle the long-term evolution of the South Atlantic Anomaly (SAA), an intriguing geomagnetic feature currently observed at the Earth’s surface. Here we present 59 new archeointensities obtained from four well-dated groups of potteries with ages ranging between 1300 and 1500 CE and collected in Northwest Argentina. The new data were obtained in accordance with the Thellier paleointensity method including partial thermoremanent magnetization (pTRM) checks and TRM anisotropy and cooling rate corrections. We have also performed a comparative study of the efficiency of magnetic susceptibility, ARM and TRM anisotropy tensors to correct the TRM anisotropy effect upon intensity estimates. Our results suggest that the magnetic susceptibility tensor systematically underestimates the TRM anisotropy effect by 10–30%. Our new data, together with selected archeointensities already published, confirm that the decrease of the geomagnetic field intensity in South America started around 1600–1650 CE, due to the arrival of the SAA.

Thermoremanent Behavior in Synthetic Samples Containing Natural Oxyexsolved Titanomagnetite

AbstractUnderstanding Earth's geodynamo provides us a window into the evolution of the Earth's core, which requires accurate data about how its strength varies with time. Classic Thellier‐style paleointensity experiments assume that studied specimens contain only noninteracting single‐domain (SD) magnetic particles. Interacting grains commonly occur in volcanic rocks but are generally assumed to behave like equivalently sized SD grains. Multidomain (MD) grains can cause erroneous PI estimates or cause Thellier‐style experiments to fail entirely. Synthetic specimens containing naturally formed magnetite with MD grains and oxyexsolved titanomagnetite (closely packed SD grains) were subjected to various partial thermoremanent magnetization (pTRM) experiments, which tested nonideal behavior as a function of pTRM acquisition and loss inequality, thermal history, and repeated heating steps. For all grain sizes and domain states, pTRMc (heating and cooling in a nonzero field) gives larger values, compared to pTRMb (heating in a zero field and cooling in a nonzero field), by ∼5.5%. Oxyexsolved grains appear prone to the same concave‐up, nonideal Arai plots commonly observed in MD specimens, which also has potential implications for the multiple‐specimen, domain‐state corrected protocol. Repeated heatings cause additive deviations from ideality with relatively small impacts on Arai plot curvature for both grain types. Experiments with higher initial demagnetization temperatures had lower curvatures, with the most SD‐like behavior occurring in the uppermost 20 C of the (un)blocking temperature range. Samples containing mixtures of magnetic domain sizes are likely to behave less ideally at lower temperatures but become more ideal with increasing temperature as the nonideal grains unblock.

Test determinations of paleointensity in historical lavas of Kamchatka

The reliability of the Thellier method for determining the paleointensity of a geomagnetic field is explored on recent igneous rocks of Kamchatka. The main magnetic mineral in the studied rocks is titanomagnetite with different degree of oxidation. It is obtained that the reliability of the results can be assessed based on the deviations of the check points of the partial thermoremanent magnetization (pTRM) during the Thellier experiment. Besides, for different rocks, it is found that the stability of titanomagnetites to heating during the experiments can be insufficient for validating the reliability of the results of paleointensity determination; however, at the same time, the reliability may depend on the initial (oxidation) state of the magnetic minerals of the studied rocks.

Microwave paleointensities indicate a low paleomagnetic dipole moment at the Permo-Triassic boundary

The quantity of igneous material comprising the Siberian Traps provides a uniquely excellent opportunity to constrain Earth’s paleomagnetic field intensity at the Permo-Triassic boundary. There remains however, a contradiction about the strength of the magnetic field that is exacerbated by the limited number of measurement data. To clarify the geomagnetic field behavior during this time period, for the first time, a microwave paleointensity study has been carried out on the Permo-Triassic flood basalts in order to complement existing datasets obtained using conventional thermal techniques. Samples, which have been dated at ∼250Ma, of the Permo-Triassic trap basalts from the northern extrusive (Maymecha-Kotuy region) and the southeastern intrusive (areas of the Sytikanskaya and Yubileinaya kimberlite pipes) localities on the Siberian platform are investigated. These units have already demonstrated reliable paleomagnetic directions consistent with the retention of a primary remanence. Furthermore, Scanning Electron Microscope analysis confirms the presence of iron oxides likely of primary origin. Microwave Thellier-type paleointensity experiments (IZZI protocol with partial thermoremanent magnetization checks) are performed on 50 samples from 11 sites, of which, 28 samples from 7 sites provide satisfactory paleointensity data. The samples display corresponding distinct directional components, positive pTRM checks and little or no zig-zagging of the Arai or Zijderveld plot, providing evidence to support that the samples are not influenced by lab-induced alteration or multi-domain behavior. The accepted microwave paleointensity results from this study are combined with thermal Thellier-type results from previously published studies to obtain overall estimates for different regions of the Siberian Traps. The mean geomagnetic field intensity obtained from the samples of the northern part is 13.4±12.7μT (Maymecha-Kotuy region), whereas from the southeastern part is 17.3±16.5μT (Sytikanskaya kimberlite pipe) and 48.5±7.3μT (Yubileinaya kimberlite pipe), suggesting that the regional discrepancy is probably due to the insufficient sampling of geomagnetic secular variation, and thus, multiple localities need to be considered to obtain an accurate paleomagnetic dipole moment for this time period. It demonstrates that the overall mean paleointensity of the Siberian Traps is 19.5±13.0μT which corresponds to a mean virtual dipole moment of 3.2±1.8×1022Am2. Results indicate that the average magnetic field intensity during Permo-Triassic boundary is significantly lower (by approximately 50%) than the present geomagnetic field intensity, and thus, it implies that the Mesozoic dipole low might extend 50Myr further back in time than previously recognized.

New constraints on the most significant paleointensity change in Western Europe over the last two millennia. A non-dipolar origin?

Over the last years new evidences of several short-lived regional maxima of the geomagnetic field intensity at various times and locations have been defined. These features have important implications both for geomagnetic field modeling and for Earth's dynamo simulations. However, the nature, extent and underlying causes of these variations are still poorly understood. The major constraint for an adequate description of these important features is the absence of continuous detailed records and the current limited availability of precisely dated geomagnetic field recorders. In this context, archeological materials from superimposed strata covering long sequences of occupation provide a powerful tool to investigate the temporal variability of geomagnetic field strength at decadal and centennial time scales. In this work we report the archeomagnetic study of 79 potteries from Southeastern Spain collected in 14 different stratigraphic units. The chronological framework of the studied collection, ranging from the 9th to the 12th centuries, is based on historical/archeological data such as written sources and well-established typological and archeometric documentation on ceramics found on reference contexts in the area. Additionally, two radiocarbon dates obtained from two different stratigraphic units confirm the proposed chronological intervals. From classical Thellier and Thellier experiments including partial thermoremanent magnetization (pTRM) checks and TRM anisotropy and cooling rate corrections, height new high-quality mean intensities were derived. The new data provide an improved description of the sharp abrupt intensity decay that took place in Western Europe after the 800 AD intensity maximum, the most significant geomagnetic field intensity feature observed in Europe over the last two millennia. The new results confirm that several rapid intensity changes (with rates higher than 10 μT/century) took place in Western Europe during the recent history of the Earth. The comparison between the regional curve of Western Europe and the SHA.DIF.14k global field model predictions suggests that the 800 AD event observed in Europe is probably controlled by non-dipolar geomagnetic sources.

MSP-Tool: A VBA-Based Software Tool for the Analysis of Multispecimen Paleointensity Data

The multispecimen protocol (MSP) is a method to estimate the Earth’s magnetic field’s past strength from volcanic rocks or archeological materials. By reducing the amount of heating steps and aligning the specimens parallel to the applied field, thermochemical alteration and multi-domain effects are minimized. We present a new software tool, written for Microsoft Excel 2010 in Visual Basic for Applications (VBA), that evaluates paleointensity data acquired using this protocol. In addition to the three ratios (standard, fraction-corrected and domain-state-corrected) calculated following Dekkers and Bohnel (2006) and Fabian and Leonhardt (2010) and a number of other parameters proposed by Fabian and Leonhardt (2010), it also provides several reliability criteria. These include an alteration criterion, whether or not the linear regression intersects the y axis within the theoretically prescribed range, and two directional checks. Overprints and misalignment are detected by isolating the remaining natural remanent magnetization (NRM) and the partial thermoremanent magnetization (pTRM) gained and comparing their declinations and inclinations. The NRM remaining and pTRM gained are then used to calculate alignment-corrected multispecimen plots. Data are analyzed using bootstrap statistics. The program was tested on lava samples that were given a full TRM and that acquired their pTRMs at angles of 0, 15, 30 and 90° with respect to their NRMs. MSP-Tool adequately detected and largely corrected these artificial alignment errors.

The influence of the chemical magnetization of oceanic basalts on determining the geomagnetic field paleointensity by the Thellier method

Dredged samples of basalts from the Mid-Atlantic Ridge (MAR) and the Red Sea Rift that are 0.2 to 1 Ma in age are used to show the possibility of separating the thermoremanent and chemical remanent magnetization components in oceanic basalts by the conventional Thellier–Coe method. It is found that the decay rate of chemical remanent magnetization during exposure of basalt samples with Curie temperatures of Tc = 145 and 240°C at temperatures of 340–380°C in the air was less than the formation of partial thermoremanent magnetization during the experiments using the Thellier–Coe method by a factor of 4.5–5; in contrast, for thermoremanent magnetization these rates almost coincide. Using this fact, we estimated the contributions from chemical remanent and thermoremanent magnetization components into the natural magnetization of basalts and, based on the thermoremanent magnetization component, defined the paleointensity of the geomagnetic field for the epoch when these basalts formed. In basalts from the southern MAR that are 1 Ma in age, the contribution of chemical remanent magnetization to the natural remanent magnetization is 60% to 80%; in those of 0.35 Ma it was less than 50%; in those of 0.2 Ma it was less than 10%. The calculated values of the virtual magnetic dipole moment of the Earth were close to those that have been published in the literature.

Thellier-type paleointensity data from multidomain specimens

Identifying and excluding inaccurate paleointensity estimates that can arise from the effects of multidomain (MD) particles remains a key challenge for paleointensity studies. Here we approach this problem by incorporating realistic experimental noise into a phenomenological MD model, systematically exploring the parameter space of paleointensity experiments (the choice of protocol, laboratory field strengths and directions), applying optimization algorithms to the model predictions, and testing these against experimental datasets. The results allow us to conclude that: (1) In some cases, applying the lab field in a direction ≲60° from the NRM direction and with a magnitude ∼2 times larger than the ancient field may help to produce more linear and accurate Arai plots by allowing partial thermoremanent magnetization (pTRM) tails to compensate from excess natural remanent magnetization (NRM) loss, the mechanism behind typical MD Arai plot curvature. (2) The recently proposed FRAC criterion is consistently better at isolating accurate results than the traditionally used f parameter and should ideally have minimum values of 0.65, 0.45, 0.65, and 0.55 for the Thellier, Coe, Aitken, and IZZI protocols, respectively. (3) Newly developed sets of selection criteria (MC-CRIT) should maximize the likelihood of accepting accurate estimates from a suite of specimens influenced by MD behavior. The models, methods, and data presented here provide the means to objectively define data selection and demonstrate the effectiveness of a given set of criteria. Such approaches are vital, if we wish to remove the arbitrariness that hinders the identification of reliable paleointensity data.

Archeointensity study on baked clay samples taken from the reconstructed ancient kiln: implication for validity of the Tsunakawa-Shaw paleointensity method

In 1972, a reconstruction experiment of a kiln had been done to reproduce an excavated kiln of the seventh century in Japan. Baked clay samples were taken from the floor surface and −20 cm level, and they have been stored after determinations of the paleomagnetic directions by partial alternating field demagnetizations. We recently applied the Tsunakawa-Shaw method to the samples to assess how reliable archeointensity results are obtained from the samples. A suite of the rock magnetic experiments and the scanning electron microscope observations elucidate that dominant magnetic carriers of the floor surface samples are Ti-poor titanomagnetite grains in approximately 10 nm size with single-domain and/or super-paramagnetic states, whereas contributions of multi-domain grains seem to be relatively large for the −20-cm level samples. From the floor surface samples, six out of the eight successful results were obtained and they give an average of 47.3 μT with a standard deviation of 2.2 μT. This is fairly consistent with the in situ geomagnetic field of 46.4 μT at the time of the reconstruction. They are obtained with a built-in anisotropy correction using anhysteretic remanent magnetization and without any cooling rate corrections. In contrast, only one out of four was successful from the −20-cm level samples. It yields an archeointensity of 31.6 μT, which is inconsistent with the in situ geomagnetic field. Considering from the in situ temperature record during the firing of the kiln and the unblocking temperature spectra of the samples, the floor surface samples acquired full thermoremanent magnetizations (TRMs) as their natural remanent magnetizations whereas the −20-cm level samples only acquired partial TRMs, and these differences probably cause the difference in the archeointensity results between the two sample groups. For archeointensity researches, baked clay samples from a kiln floor are considered to be ideal materials.