EMRP3.4

Volcanic rocks are believed to be reliable recorders of changes in the Earths magnetic field in the past. Paleomagnetic data from volcanic edifices is used to make reconstructions of the behavior of the Earth’s magnetic field through time. Recently, however, it became evident that volcanic rocks may not always record the ambient magnetic field accurately. Therefore, we set out to test the accuracy of paleomagnetic data recorded by Mt. Etna lavas by (1) directly measuring the magnetic field above the current topography, i.e. the field that would be recorded by a future flow in that location; and (2) by assessing the paleomagnetic information in historical (1850 – today) flows.

Mt. Etna is characterized by an irregular topography with ridges and gullies that may give rise to local magnetic anomalies that a new flow would record. We measured the ambient geomagnetic field on Mt. Etna with a three-component fluxgate magnetometer at five sites along the length of three paths with an irregular surface. Paths were walked perpendicular to ridges and gullies and measurements were made every meter at different heights above the ground. We found that the declination varies between -17.5 and 18°, and on average differs -3.3° from the expected geomagnetic field. The inclination has a range between 44.4 and 59.5° and is on average 52.3°, while 53.4° is expected. Lastly, the intensity varies between 37.2 and 50.4 µT and is on average 44 µT, with an expected value of 45.2 µT. The deviation with respect to the expected value decreases as function of height above the flow for the inclination and intensity, while the variation in declination does not improve. Most importantly, the variations in inclination and intensity correlate with topographic features: both inclination and intensity are higher above ridges and lower in gullies.

The second part of our study consisted of compiling an overview of literature data on historical Mt. Etna flows and combining this with newly measured paleomagnetic data from 12 sites from 9 different flows with ages between 1865 and 2002. We observed the same trends in this data compilation as in our field observations: the reported inclinations and intensities are often too low.

Our observations have consequences for paleomagnetic sampling strategies in rugged volcanic terrain. To avoid sampling a local magnetic anomaly, samples should be taken spread out over a larger area, preferably meters apart and from different parts of the flow. While this will lead to a higher degree in scatter in paleodirections and a lower precision parameter k, they will better represent the Earth’s magnetic field at the time of cooling. Paleodirections with a high k or paleointensities with a low uncertainty most likely sample a local magnetic anomaly arising from the underlying terrain.

How to cite: Meyer, R. and de Groot, L.: Local magnetic anomalies in rugged volcanic terrain explain bias in paleomagnetic data: consequences for sampling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1227, https://doi.org/10.5194/egusphere-egu22-1227, 2022.

Abstract

Recent micromagnetic simulations have found that particles in the size region of the single domain (SD) to single vortex (SV) transition zone are prone to poor thermal and field stabilities that could adversely affect the accuracy of interpretations of paleomagnetic recordings. In this study, we attempt to evaluate the internal magnetization characteristics of these magnetically unstable (MU) particles and the influence on paleomagnetic observations by simulating the magnetic behaviour of 68-104 nm truncated octahedral magnetite particles via the MERRILL modelling software. We found that: (i) The size region of the "MU zone" for grains of cubic octahedral shape is different with cubic octahedrons and spheres, indicating the zone may be controlled by the geometry and shape of particles; (ii) The MU zone has a range of 79-91 nm region, which is dominated by a hard-axis aligned single vortex (HSV); (iii) MU particles are unstable as a function of temperature. Finally, the numerical fitting of hysteresis parameters for experimental data suggests that the influence of such MU particles in samples cannot be ignored, especially for samples with fine-grained magnetic minerals as the primary magnetic recording carriers. This research has extended our understanding of the behaviour of the "MU zone" and its significance on paleomagnetic records.

How to cite: Wang, Y., Ge, K., and Williams, W.: Micromagnetic modeling of a magnetic unstable zone and its geological significances, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2185, https://doi.org/10.5194/egusphere-egu22-2185, 2022.

Results from the Sept-Îles intrusive suite suggest that the Earth’s magnetic field reached ultra-low intensities at 565 Ma (~3 μT, Bono et al., 2019) during the Ediacaran Period. Additional evidence from this study suggests abnormally high paleosecular variation (S= ~26˚). Other studies of Ediacaran rocks indicate very high reversal frequencies. Based on this abnormal geomagnetic field behavior, Bono et al. (2019) suggested inner core nucleation at about 565 Ma. Therefore, we conducted a detailed paleomagnetic study of the early Cambrian (²⁰⁶Pb/²³⁸U age of 532.49±0.12 Ma, Wall et al., 2020) Glen Mountains Layered Complex (GMLC) of southwestern Oklahoma to assess behavior of the geomagnetic field 30 myr later. One hundred ninety independently oriented cores were drilled from 17 sites collected from the anorthosites of the GMLC. Primary paleomagnetic directions were isolated by thermal and alternating field demagnetization. Magnetic susceptibility versus temperature and isothermal remanence acquisition experiments both indicate that low-Ti magnetite is the dominant remanence carrier, although minor pyrrhotite is seen in some samples. Nearly antipodal directions, collected from different sites, pass a simple reversals test, confirming that geomagnetic field polarity reversals are recorded (Roggenthen et al., 1981) and that the GMLC anorthosites carry a primary remanence. Paleosecular variation recorded by the GMLC (S=10.9˚) for a paleolatitude of 10.3˚ is in agreement with Smirnov et al.’s (2011) tabulation of paleosecular variation for 1.0-2.2 Ga intrusive and extrusive igneous rocks. A reversed polarity paleopole for the complex is located at 26.1°E, 25.3 °N (A₉₅ = 7.4°). Our results suggest that the geomagnetic field returned to stable, dipole-dominated behavior by about 530 Ma.

Bono, et al. (2019). Nature Geoscience, 12(2), 143-147.

Smirnov, et al. (2011). PEPI, 187(3-4), 225-231.

Roggenthen et al., (1981) GRL, 8, 133-136.

Wall et al. 2020, Geology, 49, 268-272.

How to cite: Kodama, K., Tetto, F., and Tarduno, J.: Paleomagnetism of the Glen Mountains Layered Complex: Dipolar field behavior at ~530 Ma, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3175, https://doi.org/10.5194/egusphere-egu22-3175, 2022.

Paleomagnetic results obtained from rocks of Ediacaran age in several localities in Siberia display co-existence of two magnetization components, one shallowly component and the other one is steeply inclined. Both components pass criteria for a primary magnetization. The conventional interpretation of paleomagnetic results in such cases have been rejected as dynamically implausible. In search of the reasons for the irregular behavior of the geomagnetic field in the Ediacaran, the study compares the key Ediacaran sections of the Yenisei Ridge and the Olenek Uplift, which are close in age.

The first paleomagnetic data for the Vorogovka Group were obtained in the northwest of the Yenisei Ridge. Age of deposits, according to different authors, from Cryogenian to Ediacaran. However, recent data indicate that the age of sedimentation within the Vorogovka basin is less than 585 million years. In the southwestern margin of the Siberian platform, the Taseeva Group is of considerable interest for paleomagnetic studies. New data on the age make it possible to significantly narrow the formation interval of the series to the Late Ediacaran, and also to refine the paleomagnetic pole calculated for the upper and lower formations of the Taseeva Group. In the Vorogovka and Taseeva groups, the shallowly inclined component of magnetization is more often present, while the steeply inclined component is less common.

Over the past several years, we have carried out paleomagnetic studies of the Precambrian rocks of the Olenek Uplift, including the Late Ediacaran Maastakh and Khatyspyt formations, as well as intrusions of the Tas-Yuryakh volcanic complex that break through them. In both sedimentary and igneous rocks of the Olenek Uplift, only a steeply inclined component of magnetization is present.

The results of the studies of paleointensity in basalts of the Tas-Yuryakh volcanic complex indicate an ultra-low intensity of the Earth's magnetic field at the end of the Ediacaran. These results confirm that the explanation for the disparate paleomagnetic data for the Ediacaran period must be sought in the behavior of the geomagnetic field, and not in tectonic reasons.

This work was financially supported by the Russian Science Foundation grant no. 21-17-00052.

How to cite: Vinogradov, E., Metelkin, D., Zakharov, S., Abashev, V., Pakhomova, K., and Eliseev, A.: Paleomagnetic data from Siberian Ediacaran rocks (Yenisei Ridge and Olenek Uplift), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9876, https://doi.org/10.5194/egusphere-egu22-9876, 2022.

For decades, scientists have tried to obtain paleomagnetic data from Devonian rocks, but acquiring accurate data from this time period remains problematic. The lack of data has traditionally been interpreted as caused by overprinting during the Kiaman reverse superchron, but recent studies have suggested that the field could have had a non-dipolar configuration. Either way, overprinting is a common problem, although an overprinting mechanism is sometimes lacking. In other cases, data is not interpretable due to large scatter, and a clear explanation has not been found to date.

The lack of paleomagnetic data from the Devonian significantly hampers the fundamental understanding of long-term (>100 Myr) magnetic field behaviour and the creation of geodynamo models. However, paleointensities can sometimes be successfully determined, and show that the field was weak to extremely weak during the Late Devonian, with values similar to the Ediacaran.

To further constrain the onset of the weak field period, we sampled a succession of Middle Devonian pillow lavas that outcrop in the Philippstein Quarry near Braunfels, Germany. These lavas are subject to some faulting, but are fresh looking and consist of unaltered, unmetamorphosed igneous rock. Pillow lavas cool quickly, which leads to stable magnetic behavior, making them well-suited for magnetic analyses.

Here we present our results of traditional paleomagnetic analyses to determine the direction and intensity of the Devonian paleomagnetic field.

How to cite: de Boer, R. A., van der Boon, A., Königshof, P., and de Groot, L. V.: Paleomagnetism of Middle Devonian pillow lavas from Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1499, https://doi.org/10.5194/egusphere-egu22-1499, 2022.

Holocene geomagnetic data from large areas such as the oceans, the African and South American continents, constitute only the 4–6% of the global datasets, limiting our understanding of the geomagnetic field features and evolution. So far, 48 studies from Africa sensu latu are available, the 14.5% of studies from Southern Africa, the 23% from Central Africa and 62.5% from the Norther Africa. About half of them are from archaeomagnetic data, and the rest are from volcanic and sedimentary records. Here, after selecting the available volcanic and archaeomagnetic data following a set of 3 FAIR (findability, accessibility, interoperability and reproducibility) principle-based criteria, we build a first regional geomagnetic model for Africa covering the last 4000 years, based on the revised version of the spherical cap harmonic analysis in 2 dimensions. The new regional model helps understanding the most important feature of the modern geomagnetic field anomaly, the South Atlantic Anomaly. The model shows, at the Earth’s surface, the westward migration of the SAA from the Indian Ocean over Africa since 1100 AD. Finally, we test the new model as a paleomagnetic dating tool by re-dating previous archaeomagnetic data from Africa, confirming that, despite being based on a still sparse database, it can be used to date other African archeological sites and the many active and dormant volcanoes of the East African System.

How to cite: Pavón-Carrasco, F. J. and Di Chiara, A.: A first regional model of the past Earth’s magnetic field from Africa for the last 4,000 years, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8535, https://doi.org/10.5194/egusphere-egu22-8535, 2022.

International Ocean Drilling Program (IODP) Expedition 341 in the Gulf of Alaska recovered a 112-meter-long sedimentary record from the continental slope at Site U1419. At this site, an exceptionally expanded late Pleistocene sequence (sedimentation rates >100 cm/kyr) combined with a high-resolution radiocarbon chronology (Walczak et al., 2020), provide an opportunity to study Paleomagnetic Secular Variations (PSV) on centennial to millennial timescales over the past ~43,000 years.
Natural and laboratory-induced magnetic remanence were measured on u-channels using the stepwise AF demagnetization procedure. In addition to continuous magnetic susceptibility measurements, hysteresis parameters were obtained on 95 discrete samples, and IRM acquisition curves on 9 discrete samples to obtain additional information on the magnetic mineralogy of the sediment. Due to the influence of lithology, magnetic mineralogy, depositional and post-depositional processes, Site U1419 is not suitable for paleointensity studies. However, with removal of intervals influenced by the environmental signal and/or coring deformation, the high sedimentation rates at this site have helped to preserve a reliable record of inclination. Because of signal to noise issues, inclination as measured after the 20 mT AF demagnetization step provides the most accurate estimate. This is demonstrated by comparing the U1419 inclination to a stack of the shipboard inclination at Site U1418 on a new age model developed from 19 radiocarbon dates on U1418 and 18 magnetic susceptibility-based tie-points to site survey core EW0408-87JC (Praetorius et al., 2015). This independently replicated inclination record verifies centennial to millennial scale variations in the Gulf of Alaska that can now be compared with other northeast Pacific and western North American records to begin deciphering geomagnetic variability and provide a new stratigraphic correlation tool for 15 and 30 cal kyr BP interval in this region.

How to cite: Velle, J. H., Walczak, M., Reilly, B., St-Onge, G., Stoner, J., Fallon, S., Mix, A., Belanger, C., and Forwick, M.: High-resolution inclination records from Sites U1418 and U1419 in the Gulf of Alaska (IODP Expedition 341), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4862, https://doi.org/10.5194/egusphere-egu22-4862, 2022.

The geomagnetic field is created by the motion of molten iron inside the liquid outer core of the Earth. It has undergone a number of drastic changes over geological time, the most notable of which are field reversals, in which the magnetic north and south poles change locations. Because sediments can retain information about past magnetic field directional and intensity changes through depositional or post-depositional remanent magnetization acquisition mechanisms, oceanic sediment cores can be used to track reversals. Lavas also document reversals by spot readings of thermal remanent magnetization. The Matuyama-Brunhes reversal (MBR) was the last time the Earth’s poles reversed, and it is documented by the largest number of sediment records compared to earlier reversals. Investigating the MBR globally therefore can help to understand the physical processes that occur in the Earth’s core. A few global spherical harmonic (SH) models have previously been proposed for the MBR. However, the number, distribution, and timescale reliability of the input data are limitations of these models, the last one of which has been published more than 10 years ago. In this study, we take advantage of new sediments and lava data for the MBR that have been published since these models were developed, and often have better age control and higher temporal resolution than data used in previous SH models. Smoothing splines were used to examine the temporal resolution of all sediment records in our new global compilation, and the results show a median smoothing time of 350 years (±200 years). We present a new global SH geomagnetic field model for the MBR, constructed from 67 sediment cores and 93 lava sites that span the last 900-700 ka and have a reasonable geographical distribution. In addition, we investigate the robustness of model features by deriving models from sub-sets of data, e.g., using only well-dated, high-resolution sediment data that are consistent with surrounding records (if any exist). The model’s featured will be discussed, including (1) field morphology at the CMB and at the Earth’s surface; (2) axial dipole (AD) and non-axial dipole (NAD) power at the CMB; and (3) magnetic energy of AD and NAD fields at the Earth’s surface. Furthermore, we will compare the current models to the previous MBR models.

How to cite: Mahgoub, A. N., Korte, M., and Panovska, S.: New spherical harmonic global geomagnetic field models for the Matuyama-Brunhes reversal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1812, https://doi.org/10.5194/egusphere-egu22-1812, 2022.

The goal of our research is to obtain new data of geomagnetic field intensity in the Eastern Europe in the Bronze Age.  The arhaeomagnetic study of fired ceramic from the Grishinsky Istok III settlement was done. The settlement Grishinsky Istok III is situated in the Oka region of Ryazan district of Russia (54^о41′, 40^о57′). The studied collection of pottery fragments of that archaeological site pertains to the “textile” ceramics of Bronze Age.  The age of pottery fragments corresponds to the ~ 1500-1300 years B.C. The composition of the ferromagnetic fraction of the studied samples has been examined by the thermo-magnetic analysis.   The dependence of the saturation magnetic moment on temperature in the magnetic field and determination of the Curie points were carry out with an analyzer of ferromagnetic fraction. Thus based on the thermo-magnetic analysis one can conclude that the main carrier of the thermo remanent magnetization of the samples is relatively resistant to heat maghemite. The size of grains lies in a pseudo single domain area. The determination of the ancient magnetic field intensity was carried out by modified Thellier method. Fifteen geomagnetic field intensity determinations were obtained. The geomagnetic field intensity varies between 37 and 66 µT with an average value of about 50 µT.  Acquisition of new data about the Earth’s magnetic field during the Bronze Age makes it possible to advance the studies of geomagnetic variations. This work was supported by the Russian Foundation for Basic Research, project no. 19-55-18006 and the State task of the Schmidt Institute of Physics of the Earth RAS no. 0144-2019-0006.

How to cite: Pilipenko, O., Nachasova, I., and Azarov, E.: Archeomagnetic investigation of textile ceramics from Grishinsky Istok III  settlement  (Oka  Region, Ryazan district, Russia), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9718, https://doi.org/10.5194/egusphere-egu22-9718, 2022.

The study of the geomagnetic field evolution on minor timescales, in particular of such significant events as geomagnetic reversals and excursions, has acquired particular relevance nowadays due to the increased attention of mankind to the environment. The question of how exactly abrupt changes in the characteristics of the geomagnetic field affect the climate and biosphere remains largely debatable; the idea of ​​the speed and dynamics of these changes is also very vague. "Classical" geological objects and existing methods provide limited opportunities for highly detailed reconstructions of geomagnetic field variations; therefore, paleomagnetologists are looking for new objects and approaches to solve this problem. The research that we have begun involves the use of speleothems to study secular variations of the geomagnetic field.

This study presents paleomagnetic records of two drill-cores from the flowstone from Vorontsovskaya cave, located on the western flank of the Caucasus Mountains in the valley of the river Kudepsta. Preliminary results indicate the presence of a geomagnetic excursion record in both drill-cores. Further study of the samples from Vorontsovskaya cave will make it possible to compare the discovered event with known excursions, as well as to clarify its age, duration, and dynamics.

How to cite: Gavrushkin, D., Pasenko, A., Veselovskiy, R., and Rudko, D.: Geomagnetic Excursion Record Preserved In The Speleothem From Western Caucasus: First Data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6552, https://doi.org/10.5194/egusphere-egu22-6552, 2022.

The Earth's magnetic field varies on a range of spatial and temporal scales. Recent models, spanning the past 100 ka, greatly improved our knowledge of the long-term changes of the geomagnetic field, and geomagnetic excursions - events associated with strong directional deviations and low field intensities. However, these models are limited by the spatial and temporal data distribution, and magnetic and age uncertainties of underlying data. Variations in the production of cosmogenic radionuclides, such as ¹⁰Be from ice cores and sediments, provide an independent proxy of paleointensity variations for a range of timescales. This study demonstrates the potential of a joint inversion of paleomagnetic data with cosmogenic nuclide production rates to reconstruct the geomagnetic field evolution over the past 70 ka. Here, we present a global compilation of ¹⁰Be records, converted to magnetic field intensity, and compare them to paleomagnetic sediment records. General trends of the virtual axial dipole moment (VADM) stacks derived from the two data sets agree well. Two models have been constructed: GGFSS70-10Be where the converted ¹⁰Be records are used similarly to the paleomagnetic records, and GGFSS70-10Be-DIP where the converted ¹⁰Be data have been considered to be a function of the dipole only (the first three Gauss coefficients). These models are compared to the previously published GGFSS70 model based on paleomagnetic data only. Geomagnetic excursions are reconstructed in the multi-proxy models: the most pronounced Laschamps event (41 ka) and a few other excursional signatures at the times of the Norwegian-Greenland Sea and Mono Lake/Auckland excursions. Model predictions and global field maps show that the cosmogenic isotope records have regional effects on the reconstructed variations (Greenland, South/Southern Ocean, east-equatorial Pacific), especially those with high resolution (Greenland ice cores). The ¹⁰Be records not only provide an independent source of information on the geomagnetic field, thus confirming the reliability of the paleomagnetic records, they also improve the global geomagnetic field reconstructions over long timescales.

How to cite: Panovska, S., Korte, M., Adolphi, F., Nowaczyk, N., and Scherler, D.: Joint inversion of paleomagnetic and cosmogenic isotope data for modeling the global geomagnetic field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9209, https://doi.org/10.5194/egusphere-egu22-9209, 2022.