Paleointensity Values Research Articles

КОРРЕЛЯЦИЯ ТОРФЯНЫХ РАЗРЕЗОВ НИЖНЕГО ПРИАМУРЬЯ ПО РЕЗУЛЬТАТАМ РАСЧЕТА ОТНОСИТЕЛЬНОЙ ПАЛЕОНАПРЯЖЕННОСТИ

The paper considers the possibility of determining the age and correlating peat sections by solving the inverse problem of magnetostratigraphy based on the findings of comprehensive (palaeomagnetic, petromagnetic, microprobe, and radiocarbon) studies on peats of the «Tyapka» and «Chlya» peat sections in the Lower Amur region. For this purpose, we used the values of relative paleointensity calculated following the technique described by Bagina-Petrova. There is no difference in the rate of magnetization fixation among studied peat samples, and the age of magnetization is comparable to the age of the peat deposits itself. The solution of the inverse problem of magnetostratigraphy using the values of relative paleointensity can be used to correlate peat sections among themselves and to estimate the rate of peat accumulation in different periods of the Holocene, even in the absence of radiocarbon dating for one of the correlated sections.

Relative paleointensity of geomagnetic field during the last 9000 years estimated by the pseudo Thellier method from the bottom sediments of Lake Shira, Northern Khakassia

The results of rock magnetic studies and determination of relative paleointensity from sediments of Lake Shira, Khakassia, are presented. The NRMcarrier minerals were determined from the hysteresis parameters, thermomagnetic and X-ray diffraction (XRD) analyses. The age of the sediments was determined by radiocarbon dating. According to these measurements, the column spans about 9100 years. The qualitative determinations of relative paleointensity were obtained from linear segments of the pseudo-Arai–Nagata diagrams. The quality of the determinations was evaluated by the following criteria: number of points used to calculate the slope; quality criterion (q), fraction ofNRMdestroyed in the paleointensity determination interval, and relative paleointensity determination error (σ). According to rock magnetic studies and XRD analysis, the magnetization carriers are represented mainly by single-domain and pseudo-single-domain magnetite and hematite. The comparison of the obtained series of relative paleointensity data with both the model paleointensity values calculated for Shira coordinates from various models (CALS10K.1b [Korte et al., 2011], PFM9k.1 [Nilsson et al., 2014], HFM.OL1.AL1, CALS10k.2 ARCH10k.1 [Constable et al., 2016]) and with absolute paleointensity, as well as the aggregate results of the studies on sedimentary and igneous rocks and on archaeomagnetic objects has shown that these data are in good agreement with each other and have common trends. This provides a rationale for using this methodology to determine paleointensity from sediments of modern lakes using the pseudo-Thellier method.

Full vector paleomagnetic estimation from the Paraná-Etendeka Large Igneous Province, southern Brazil: Implications on the onset of Cretaceous Normal Superchron

Comprehensive paleomagnetic study from the Paraná-Etendeka Igneous Province (PEIP), Serra Geral Formation, southern Brazil (∼ 135 Ma) adds to the scarce southern hemisphere paleomagnetic database. Twelve paleomagnetic sites from the exposed stratigraphy of the PEIP were recovered from basaltic lava flows in northern Rio Grande do Sul state. These volcanic rocks erupted between 131.5 and 135.6 Ma. The reliability of the rock magnetism data was confirmed before being accepted for paleo-intensity analysis. The Thellier-Coe paleointensity protocol was applied to 109 collected specimens, and 26 individual samples from 9 sites were selected to test for consistency of the paleointensity determinations. The new high-quality mean paleointensity value of 30.6 ± 7.2μT corresponds to a virtual dipole moment (VDM) of 5.75 ± 0.49 × 1022 Am2. These values correspond to approximately 72% of the present-day Earth's magnetic field and suggest that the Earth's magnetic field strength was slightly elevated compared to the long-term average geomagnetic field that prevailed during the Cretaceous Normal Superchron. The calculated paleomagnetic pole (Plong = −27.5° and Plat = −78.6°, A95 = 3.5°) presents an angular difference of 22° with respect to the expected pole declination with an inclination of 5°. This difference may correspond to secular variations considering the confidence cones. Our results, when compared with previous paleomagnetic data, approach the accuracy of recent models for the Cretaceous interval.

Properties of Red Sea Pillow-Basalt Titanomagnetite at Different Distances from the Crust

The magnetic properties and mineralogy of titanomagnetite in the Red Sea pillow-basalt at different distances from the contact surface with water were studied. It was revealed that the composition of titanomagnetite in the pillow basalt changes from the crust deep into the pillow: the titanium content in titanomagnetite increases, while the magnesium and aluminum content decreases. Titanomagnetite in the surface layer of pillow basalt (0–2 cm) is single-phase oxidized, with a maximum oxidation degree of 0.13, and at more than 3 cm from the crust has stoichiometric composition. According to the increase in magnetic susceptibility from 0.2 up to 1.8 × 10–2 SI, saturation magnetization from 0.026 to 0.895 A m2/kg and residual saturation magnetization from 0.014 to 0.296 A m2/kg, decrease in coercivity (Hc from 90 to 15 mT, Hcr from 153 to 20 mT), as well as by observations of electron and magnetic force microscopy, it is shown that the concentration of titanomagnetite in basalt and the grain size from single-domain state to pseudo-single-domain state increase with distance from the crust. The magnetic hardness of titanomagnetite grains was found to correlate with the maximum cooling rate of basalt. The NRM of basalt varies non-monotonically with distance from the pillow surface: up to a depth of about 3 cm its growth caused by the growth of titanomagnetite concentration was revealed, then a decrease caused by a decrease in the magnetic hardness of the grains. In spite of a rather strong variation of the magnetic properties in the 0–7 cm layer of pillow basalt, the experiments by the Thellier-Coe method for all layers showed close values of the geomagnetic field paleointensity (62.5–66.0 μT) with a high value of quality coefficient q (11–45). No regularity in the value of paleointensity with distance from the cushion surface was detected.

Rock Magnetic Characterization of Returned Samples From Asteroid (162173) Ryugu: Implications for Paleomagnetic Interpretation and Paleointensity Estimation

AbstractIn this study, systematic rock magnetic measurements and saturation isothermal remanent magnetization (SIRM) paleointensity calibration experiments were conducted for the returned samples from C‐type asteroid (162173) Ryugu and two carbonaceous chondrites (Orgueil and Tagish Lake) to evaluate the remanence carriers of the Ryugu sample and its ability as a paleomagnetic recorder. Our magnetic measurements show that Ryugu samples exhibit signatures for framboidal magnetite, coarse‐grained magnetite, and pyrrhotite, and that framboidal magnetite is the dominant remanence carrier of Ryugu samples in the middle‐coercivity range. The SIRM paleointensity constant was obtained for two Ryugu samples, and the median value was 3,318 ± 1,038 μT, which is close to the literature's value based on the average among magnetite, titanomagnetite, pyrrhotite, and FeNi alloys and is widely used for SIRM paleointensity experiments. The paleointensity values estimated using the obtained SIRM paleointensity constant indicate a strong magnetic field of the protoplanetary disk, suggesting that Sun's protoplanetary disk existed at the disk location of Ryugu's parent planetesimal when framboidal magnetite precipitated from the aqueous fluid.

Reassessing the paleointensities of three Quaternary volcanic structures of the -Michoacán-Guanajuato Volcanic Field (Mexico) through a multimethodological analysis

An approach that has acquired popularity in the last decade to ensure the quality of paleointensity determinations is the use of multimethodological analysis, especially when comparing intensity values obtained by methods with different physical bases such as the conventional Thellier-type experiments against the Shaw-family methods and the most recent multispecimen protocol. This research presents the reevaluation of the intensities of three monogenetic volcanic structures: the Rancho Seco and Jabalí cones and lavas, and the Malpaís Prieto fissure lava flow, all located in the Michoacán-Guanajuato volcanic field (central Mexico). This analysis is accompanied by an exhaustive study of rock-magnetic properties to obtain a complete characterization of the magnetization carriers and a better understanding of the intensity results. The three volcanic structures have previously published intensity data; one of them was obtained with the original Shaw method that nowadays is considered unreliable due to the absence of monitoring of thermal alteration during the experiment. The quality of the previously published data is discussed to be compared to our intensity values, which were obtained employing Thellier-type methods and the multispecimen protocol (MSP). A good concordance was found for the data obtained by more than one methodology for samples from the Jabalí volcano and the Malpaís Prieto lava flow, obtaining average intensity values of 43.7 ± 5.1 μT and 47.8 ± 3.1 μT, respectively. However, a difference of almost 30 μT was found between our MSP results (50.9 ± 2.8 μT) and previously published data using the IZZI method (20.0 ± 1.4 μT) for Rancho Seco volcano. An attempt was made to clarify this controversial result with the support of stepwise susceptibility against temperature curves analysis and the results obtained from a set of samples treated with the Tsunakawa-Shaw paleointensity method. The results obtained indicate that the low paleointensity values are associated with the thermal alteration of the magnetic mineralogy of the samples.

New criteria for selecting reliable Thellier-type paleointensity results from the 1960 Kilauea lava flows, Hawaii

Thellier-type paleointensity experiments associated with partial thermal remanent magnetization checks have been widely used to determine paleointensity values from volcanic and archaeological media. However, previous studies have revealed that a substantial portion of paleointensity results with positive checks for historical lava samples largely fails to predict known Earth magnetic field intensity values. To determine the fidelity of paleointensity values, conventional Thellier-type paleointensity experiments were performed on Kilauea lava flows that erupted in 1960. The positive partial thermal remanent magnetization checks for our results range from 30.28 ± 1.38 µT to 52.94 ± 1.89 µT. This strongly indicates that conventional paleointensity checks cannot guarantee the fidelity of paleointensity results, especially when the unblocking temperatures for the newly formed magnetic particles are higher than the treatment temperature. Therefore, in this study, to check for thermal alteration during heating, the temperature dependence of the hysteresis parameter measured at room temperature for the thermally treated samples was also measured. Our new results show that nearly all biased paleointensity values correspond to a ratio of the coercivity of remanence to the magnetic coercivity of > 3 and a chemical alteration index > ~ 10%, which indicates the strong effect of the domain state and thermal alteration on the fidelity of the paleointensity results. Our study provides feasible criteria to further improve the fidelity of paleointensity estimations.

Low Geomagnetic Field Paleointensity on the Permian–Triassic Boundary from Study of the Kuznetsk Basin Traps (Southern Siberia)

Abstract ––Here we report the first data on the absolute intensity value of the geomagnetic field on the Permian–Triassic boundary from basalts of the Kuznetsk basin. The latter are considered as one of the manifestations of the initial stage of trap magmatism during the formation of the Siberian large igneous province. The good preservation of information on the ancient geomagnetic field in the Mal’tsevskaya Formation basalts is due to the presence of small single domain and pseudo-single domain grains of primary magmatic titanomagnetite in the groundmass. The paleointensity values obtained following the Thellier-Coe method correspond to the generally accepted criteria of reliability and indicate that the geomagnetic field intensity during the formation of the Kuznetsk basin traps on the Permian–Triassic boundary was almost an order of magnitude lower than the present-day one. Moreover, the mean values of the virtual dipole moment for the Kozhukhta and the Vlasov units in the lower and middle Mal’tsevskaya Formation ((1.9 ± 0.6) ⋅ 1022 A ⋅ m2 and (1.1 ± 0.7) ⋅ 1022 A ⋅ m2, respectively) are in good alignment with determinations of the paleointensity during the accumulation of the Ivakinsky Formation of the Norilsk Region in the Siberian province, which confirms the accuracy of traditional regional correlations.

Paleointensity Results From Pliocene Lavas of the Lesser Caucasus Obtained Using the Multispecimen Parallel Differential pTRM Method: A Comparison With Thellier‐Thellier and IZZI Data

AbstractWe report paleointensity results obtained with the multispecimen method (MSP) over the Pliocene sequence of Apnia (Georgia) which records a polarity reversal. Paleointensity determinations with the multispecimen technique were performed on 12 flows with the original (MSP‐DB) and the domain‐state corrected (MSP‐DSC) protocol. Eight MSP‐DSC determinations passed the proposed quality criteria. To obtain highly reliable data through the agreement between intensity values from different methods, MSP results were combined with paleointensities from a previous study with Thellier‐type methods and especially strict selection criteria (RCRIT) on same flows (Sánchez‐Moreno et al., 2020). Application of this multimethod procedure resulted in three new paleointensities including both MSP and Thellier‐type results and an additional one obtained with two different Thellier‐type methods, yielding one paleointensity of 36.9 µT in the normal‐polarity, and three paleointensities between 19.2 and 24.1 µT in the reverse‐polarity section. Additionally, Thellier‐type data have been reinterpreted in this study with more flexible criteria (TTP) and the results combined with the MSP data. As a result, four flows yield paleointensities including MSP and Thellier‐type determinations and seven include paleointensities obtained with two different Thellier‐type methods. Results range from 37.2 and 44.3 μT in the normal‐polarity and from 12.5 to 24.6 μT in the reverse‐polarity section. Comparison of results from the four flows yielding multimethod determinations applying RCRIT criteria with those from the same flows under TTP criteria yields no significant difference in paleointensity values and their experimental uncertainty. Thus, application of a multimethod approach supports the possibility of using TTP criteria.

New archeomagnetic secular variation data from Central Europe, II: Intensities

Variations of the geomagnetic field during the past millennia can be obtained by investigating archeological material. The spatial distribution of the available data across Europe is rather uneven and only about 15% of the sites provide a full vector record of the geomagnetic field. Here we report 39 new reliable paleointensity data for sites from Austria, Germany and Switzerland, with ages ranging between 1500 BCE and 1900 CE. For 31 of them, directional information is available, too. Paleointensities were determined with Thellier type techniques, with the multiple-specimen domain-state corrected paleointensity protocol or with both techniques. Corrections for anisotropy of thermoremanent magnetization were carried out for the structures, showing small variations of only a few per cent in most cases. Cooling rate dependence was tested for 16 structures. Often strong alteration during the experiments was observed. Seven successful cooling rate corrections were applied, which lowered the paleointensity values by about 10%. Values obtained from the multiple-specimen technique are generally somewhat lower than those from the Thellier experiments. Rock magnetic measurements revealed magnetite-type minerals mostly in pseudo single domain range as main magnetic carrier, but also high coercive mineral components like hematite or epsilon iron oxide are present. The new data are mostly in good agreement with published paleointensities. They support the presence of strong intensity variations around 800 CE and in the millennium BCE. The new full vector data reveal that the deep paleointensity minimum observed from German data around 850 BCE is accompanied by large variations in direction with declinations up to 50°.

A paleomagnetic record of the early Matuyama chron including the R\xe9union subchron and\xa0the onset Olduvai boundary: High-resolution magnetostratigraphy and insights from transitional geomagnetic fields

We present a novel paleomagnetic record for the lower Matuyama chronozone, which includes the Réunion subchronozone and the lower Olduvai polarity reversal, from a continuous section of a 168-m-thick on-land marine succession in the southernmost part of the Boso Peninsula, central Japan. In this section, the Réunion subchronozone and the lower Olduvai reversal are observed at 38.6–44.6 m and 142.0 m, respectively. The average sedimentation rates between the lower and upper Réunion boundaries and between the upper Réunion boundary and lower Olduvai boundary are calculated as 25 cm/ky and 57 cm/ky, respectively. The virtual geomagnetic pole (VGP), observed in the Boso Peninsula, at both the upper and lower Réunion boundaries passed across the equator within a similar longitudinal band over Africa. Immediately below the upper boundary, between 43.0 and 43.5 m, the VGP settled in a cluster area around China. Relative paleointensity (RPI) values for the entire Réunion interval are generally lower than the average for the entire interval from the Réunion to the lower Olduvai subchronozone. Conversely, the VGP for the lower Olduvai reversal boundary did not pass across the equator within a narrow longitudinal band but settled in several cluster areas; i.e., the southern Indian Ocean, North America, and the southern South Pacific Ocean off South America. The VGP then moved rapidly between the clusters. The locations of VGP cluster areas in the lower Olduvai reversal seem to coincide with areas where a vertical component of the present geomagnetic non-axial dipole (NAD) field is dominant. During the reversal, the RPI declined rapidly and recovered slowly as the VGP moved rapidly between cluster areas. Our new paleomagnetic data are one of the most detailed records for those geomagnetic reversals from marine sediments, and will, therefore, help to understand the dynamics of the geomagnetic reversals.

Paleomagnetism, paleointensity and geochronology of a Proterozoic dolerite dyke from southern West Greenland

Archean to Paleoproterozoic rocks potentially record the evolution of the geodynamo and the tectonic mode of the early Earth. The paleomagnetic intensity and direction data provide important information on the Earth’s core–mantle revolution. Herein, we report the results of paleomagnetic and geochronological studies of a Proterozoic dolerite dyke from southern West Greenland. Clinopyroxene grains from the dyke yielded Ar–Ar plateau ages from 1808 to 1887 Ma (1816.0 ± 14.6 Ma; 2σ). The paleomagnetic direction of the dyke (D = 243.6°, I = 66.3°, α95 = 3.9°) yielded a virtual geomagnetic pole (VGP) of 33.5 °N and 96.4 °W. This 1.8 Ga pole falls in a limited area where Paleoproterozoic poles between 2.5 Ga and 1.7 Ga for southern West Greenland are distributed. Comparison of the Paleoproterozoic poles of southern West Greenland with those of North America suggests that the North Atlantic Craton of southern Greenland could have been an independent stagnant tectonic block, different from the drifting Superior and Slave Cratons in the Early Proterozoic. Thellier experiments on 13 specimens yielded a mean paleointensity value of 14.8 ± 2.3 μT, indicating a virtual dipole moment of 2.88 ± 0.46 × 1022 Am2. This value is approximately one-third of the present-day Earth's field intensity, and is consistent with the value for the period between 1400 Ma and 2400 Ma. This small paleointensity value of the Proterozoic rocks is due to a gradual dipole moment change over a long period (∼1 Gyrs) since 4000 Ma.

Archeointensity of the Four Corners Region of the American Southwest

AbstractNew paleointensity experiments were conducted using the IZZI protocol (a variation on the Thellier‐Thellier method) on 289 specimens from 54 baked pottery fragments collected from four archeological sites in the American Southwest. Anisotropy experiments were conducted to correct for anisotropy of remanence, a common problem in archeological material. Additionally, the effect of cooling rate was evaluated and the calculated paleointensity values were adjusted accordingly. Using the Thellier graphical user interface program, the specimen results were analyzed, averaged by sample (i.e., pottery fragment), filtered for the highest quality, and converted to Virtual Axial Dipole Moments (VADMs). Stylistic evidence, historical documentation, and dendrochronology analyses provide age constraints with up to decade resolution for the VADM results. We compared these new results with the highest quality previously published paleointensity values from the Four Corners region of the American Southwest—defined here as the four states of New Mexico, Arizona, Utah, and Colorado. None of the previously published data were corrected for cooling rate and only some were corrected for anisotropy, resulting in a systematic bias between data sets. To accommodate this difference, an estimated cooling rate correction was applied to all the previously published data. No correction can be made for anisotropy as this is specimen specific. Our estimated cooling rate correction and the empirical correction applied to the new data both require an estimation of the historical cooling rate of the pottery. Experimental pottery firings using one ancient technique and outfitted with thermocouples were conducted to determine the historical cooling rate.

Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of Fine‐Particle Grain‐Size Variations

AbstractFine‐grained, Ti‐poor titanomagnetite in the ~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grain‐size variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellier‐type paleointensity estimates from the lowermost ~7.0 m of the flow. Magnetic hysteresis, frequency‐dependent susceptibility, and thermomagnetic data show that sample grain‐size distribution is dominated by SP in the lower ~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 μT (VADM of 51.3 ZAm2) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional quality‐control tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermal‐chemical remanent magnetization if microcrystal growth continued at T < Tc and/or the section is affected by post‐emplacement vapor‐phase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction.

Reassessment of the eruptive chronology of El Metate shield volcano (central-western Mexico) based on a comprehensive rock-magnetic, paleomagnetic and multi-approach paleointensity survey

Abstract We study seven out of the thirteen lava flows associated to El Metate shield volcano using paleomagnetic, absolute intensity and rock-magnetic experiments. Characteristic paleomagnetic directions were successfully determined from most of the flows. A combined mean direction of D = 346.4° and I = 32.1° (N = 34, R = 33.73, k = 121.74, α95 = 2.2) was obtained for the early flows (Flow 1 − Flow 3). Flow 6a (a flow from the late emplacement stage) yielded a statistically distinguishable mean paleomagnetic direction of D = 359.2° and I = 29.6° (N = 9, R = 8.97, k = 294.6, α95 = 3.0). Furthermore, the significant between-flow variation of absolute paleointensity values obtained using two different methodologies, which resembles the trend followed by that of the global model SHA. DIF 14k intensity curve for the period 1000 AD 1600 AD, disagrees with previous hypothesis supporting a short emplacement period between 50 and 100 years for the whole sequence. Additionally, paleodirectional and paleointensity dating of the studied flows allowed to refine the eruptive chronology indicating that early flows were produced over a short time period, whereas at least two of the later flows were erupted 200 years later; the activity ending just prior to the arrival of the Spaniards in ∼1520. Under these circumstances, and in light of the presented high quality paleomagnetic and paleointensity data, the assumed monogenetic nature of El Metate volcano seems to be questionable.

Relative paleointensity estimates from magnetic anisotropy: Proof of concept

Relative paleointensity data from sedimentary rocks play an important role to decipher the workings of the geodynamo and to correct for atmospheric cosmogenic radionucleide production, so it is important to understand how sediments acquire remanent magnetizations and to better assess the quality of relative paleointensity data. We present experimental results from sediments deposited in controlled magnetic fields to observe the changes in magnetic anisotropy as a function of applied field strength going from near Earth-like values to almost full saturation. Relative paleointensity values followed a very well defined power law through the entire range of applied field intensities. Magnetic remanence fabrics evolved from oblate with maximum anisotropy axes in the sedimentary plane at low field strengths to prolate with maximum anisotropy axes parallel to the applied field direction at high fields. Anisotropy of magnetic susceptibility also evolved with field strength, but in a much less coherent manner than anisotropy of magnetic remanence. The experiments used well-characterized, natural sediments containing single domain magnetite, which made it possible to numerically model the data. The model matches the field dependency of both relative paleointensity and magnetic fabric development using a simple assumption that a large proportion (∼80%) of the remanence carriers in the sediments are unable to align with the magnetic field while a small fraction are free to align. Anisotropy of magnetic remanence thus holds promise to improve and assess relative paleointensity estimates and helps improve theoretical treatment of magnetic recording in sediments.

Paleomagnetism and age constraints of historical lava flows from the El Jorullo volcano, Michoacán, Mexico

The present study employs petrology, rock magnetics, and paleointensity on four lava flows (seven sites) of the El Jorullo volcano; a historical (1759–1766 AD) cinder cone in the Michoacán-Guanajuato Volcanic Field. El Jorullo lava flows cover an extensive area of ∼10.8 Km2, and a volume ∼0.35 Km3. Rock-magnetic experiments and microscopic observations reveal pseudo-single domain size Ti-poor titanomagnetite as the main carrier of magnetization in all sampled flows. Notably the calculated mean direction from all the sites show a slight westwards bias when compared against the secular variation global models. The mean paleo-direction for the El Jorullo historical eruption is: Dec = 2.5°, Inc = 37.5°, kappa = 214, α95 = 1.8, N = 31; the Virtual Geomagnetic Pole has Plat = 87° N, Plong = 307° E, Kappa = 260, A95 = 1.6; paleointensity data: 45.4 ± 3.6 μT; and Virtual Dipole Moment: 10.3 ± 1 × 1022 Am2. The paleodirections and the paleointensity values from these lava flows are well-constrained and reliable. These results allow to improve the accuracy and reliability of the global and regional secular variation models available for the last centuries.

Paleointensity Behavior and Intervals Between Geomagnetic Reversals in the Last 167 Ma

The results of comparative analysis of the behavior of paleointensity and polarity (intervals between reversals) of the geomagnetic field for the last 167 Ma are presented. Similarities and differences in the behavior of these characteristics of the geomagnetic field are discussed. It is shown that bursts of paleointensity and long intervals between reversals occurred at high mean values of paleointensity in the Cretaceous and Paleogene. However, there are differences between the paleointensity behavior and the reversal regime: (1) the characteristic times of paleointensity variations are less than the characteristic times of the frequency of geomagnetic reversals, (2) the achievement of maximum values of paleointensity at the Cretaceous–Paleogene boundary and the termination of paleointensity bursts after the boundary of 45–40 Ma are not marked by explicit features in the geomagnetic polarity behavior.

Paleointensity determination on Neoarchaean dikes within the Vodlozerskii terrane of the Karelian craton

The results of paleomagnetic studies and paleointensity determinations from two Neoarchaean Shala dikes with an age of ~2504 Ma, located within the Vodlozerskii terrane of the Karelian craton, are presented. The characteristic components of primary magnetization with shallow inclinations I = −5.7 and 1.9 are revealed; the reliability of the determinations is supported by two contact tests. High paleointensity values are obtained by the Thellier–Coe and Wilson techniques. The calculated values of the virtual dipole moment (11.5 and 13.8) × 1022 A m2 are noticeably higher than the present value of 7.8 × 1022 A m2. Our results, in combination with the previous data presented in the world database, support the hypothesized existence of a period of high paleointensity in the Late Archaean–Early Proterozoic.

Nonheating methods for absolute paleointensity determination: Comparison and calibration using synthetic and natural magnetite‐bearing samples

AbstractNonheating paleointensity methods utilize an anhysteretic remanent magnetization (ARM) or a saturation isothermal remanent magnetization to model the natural thermal remanent magnetization (TRM) to avoid heating‐induced alteration. We report the results of paleointensity experiments using the ARM, pseudo‐Thellier, and ratio of equivalent magnetization (REM) methods conducted to investigate their relative efficiency in recovering the true paleofield strength and to provide additional estimates of their calibration factors. The experiments on synthetic magnetite‐bearing samples representing single‐domain (SD) and pseudo‐single‐domain (PSD) magnetic states indicated that the correction factors for the ARM‐based methods depend on the magnetic grain size/domain state changing from ~6.3 (for SD grains) to ~4.1 (for ~1.5 µm PSD grains). The pseudo‐Thellier method yielded correct absolute paleointensity values when normalization by the TRM/ARM demagnetization slope was used. When applied to samples of lava flows and dikes from the ~32 kyr Lemptégy volcano (France), both the ARM and pseudo‐Thellier methods produced similar paleointensity estimates (28.0 ± 5.1 μT and 26.9 ± 4.7 μT, respectively) consistent with the available Thellier data for the 31–33 kyr time interval. The correction factors estimated from our synthetic samples for the REM (~3000) and for REMc (~1500) and REM′ (~1500) variants are consistent with the previously published estimates. However, all REM variants yielded unrealistically high estimates (>110 μT) of the paleofield strength from our natural samples. Our experimental results support ARM as a better proxy of TRM and suggest that the ARM‐based methods currently represent the best alternative to heating‐based absolute paleointensity method.