Displays

Néel theory (doi: 10.1080/0001873550010120 ) predicts that natural remanent magnetizations (NRMs) of thermal origin will be nearly linearly related to the magnetic field in which they are acquired for field strenghts as low as the Earth's. This makes it in principle possible to estimate the strength of ancient magnetic fields. In practice, however, recovering the ancient field strength is complicated. The simple theory only pertains to uniformly magnetized (single domain, SD particles). While SD theory predicts that a magnetization acquired at a temperature T should be demagnetized by zero-field reheating to T, yet failure of this “reciprocity” requirement has long been observed and the causes and consequences for grains with no domain walls are unknown. Recent experiments (Shaar and Tauxe, doi: 10.1073/pnas.1507986112 and Santos and Tauxe, doi:10.1029/2018GC007946) have demonstrated that, in contrast to the stability of SD remanences over time, the remanence in many paleomagnetic samples typically used in paleointensity experiments are unstable, exhibiting an "aging" effect in which the unblocking temperature spectrum changes over only a few years.  This behavior is completely unexpected from theory. Solving these mysteries is key to cracking the problem of paleointensity estimation. In this presentation we will demonstrate that it is a shift in unblocking temperatures observed over even relatively short time intervals (two years) in certain samples that leads to the failure of reciprocity which in turn limits the ability to acquire accurate and precise estimates of the ancient magnetic field. From rock magnetic experiments (xFORCs) it seems likely that magnetic grains larger than the highly stable single vortex state are the source of the non-ideal behavior. This non-ideal behavior which leads to differences between known and estimated fields that can be rather large (up to 10 μT) for individual specimens, does appear to lead to a bias in field estimates.  It is unclear how this behavior can be compensated for using the most common paleointensity estimation methods.   

How to cite: Tauxe, L., Santos, C., Zhao, X., and Roberts, A.: Transforming understanding of paleomagnetic recording: Insights from experimental observations of laboratory aged thermal remanences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1960, https://doi.org/10.5194/egusphere-egu2020-1960, 2020.

The arrival of the great Maori waka and the settlement of New Zealand some seven or eight hundred years ago are described in oral history, but details of exactly when and how colonisation occurred are undocumented. Radiocarbon dating of early archaeological sites is particularly problematic, due to the inbuilt age of datable materials, and non-linearity and ambiguity in the calibration of measurements to calendar dates. Hangi stones, used as heat retainers in traditional Maori earth ovens, hold thermoremanent records of Earth’s magnetic field at the time of their last cooling. Matching the directions of these magnetizations to established reference curves provides alternative, archaeomagnetic, estimates of age. Our results cover the past 700 years, with a cluster of dates between 1500 and 1600 AD, from both North and South Islands, but none earlier than 1300 AD, thus supporting a model of rapid coordinated migration around that time. Archaeointensity data have been obtained from sixteen distinct archaeological features, including twelve hangi from eight sites, and from them the first archaeointensity record for New Zealand has been constructed. To this has been added other archaeointensity and palaeointensity data from the SW Pacific region and virtual axial dipole moments (VADMs) have been plotted. This plot outlines steady VADM values of about 8 x10²² Am² from 1000-1300 AD, and 9.5 x10²² Am² from 1500 AD to the present, with an intervening sharp peak in the early 15^th century when the VADM reached about 13 x10²² Am². This peak bears many similarities to archaeomagnetic “jerks” and “spikes” in northern hemisphere records from the first millennia BC and AD. However, it is the first such feature to be found in the southern hemisphere at this date, suggesting, in accordance with recent modelling, that it may be a feature of the non-dipole field, associated with rapid growth and decay of an intense flux patch on the core-mantle boundary.

How to cite: Turner, G. M., Kinger, R., McFadgen, B., and Gevers, M.: Archaeomagnetic directions and intensities from New Zealand: evidence for a fifteenth century AD archaeomagnetic “spike” in the SW Pacific?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13317, https://doi.org/10.5194/egusphere-egu2020-13317, 2020.

The period encompassing the past 40 ka is crucial to constrain the characteristic time of the axial dipole, which is computed so far from the historical period and still fails to be tested against long-term field changes. The past 7 kyr of geomagnetic history are primarily documented from archeological artefacts, yet the last 4 kyr remain relatively poorly constrained. Beyond this period, we are dealing with long-term changes of the dipole field that are relatively poorly documented by sedimentary records or by volcanic lava flows. Many measurements of absolute paleointensity do not incorporate directional information, while it is crucial to document the entire field vector and consequently can only be analyzed in terms of virtual axial dipole moments (VADM). In summary, no high resolution dataset covers the field changes which followed the Laschamp event and therefore we have poor knowledge of the pattern of fluctuations and the rate of the changes that were associated with the field recovery after the Laschamp. We have selected a set of marine sedimentary cores based on the quality of their oxygen isotope records. Their deposition rates are comprised between 10 and 20 cm/ka and therefore offer a great potential to constrain the filed intensity changes with a resolution of the order of 100 ka. during this period. We will present the results obtained from 7 marine core records and investigate their common and their discrepant features in order to identify the true paleointensity signal.

How to cite: Valet, J.-P., Bassinot, F., Egli, R., and Thevarasan, A.: Field Intensity changes during the past 40 ka, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11300, https://doi.org/10.5194/egusphere-egu2020-11300, 2020.

Geomagnetic dipole moment variations, for example associated with polarity reversals and excursions, are linked to changes in cosmogenic radionuclide production rates. Therefore, it is possible to reconstruct past changes in the dipole moment based on cosmogenic radionuclide records from natural archives such as ice cores. Here we present a geomagnetic dipole moment reconstruction based on ¹⁰Be and ³⁶Cl data from two Greenland ice cores over the period from 11.7 ka to 108 ka BP (before present AD 1950). We find significant correlations between the cosmogenic radionuclides and climate proxies which may be due to the common transport and deposition processes of these species. In an attempt to minimize climate-related variations in our dipole moment reconstruction, we apply a multi-linear correction method by removing common variability between ¹⁰Be and ³⁶Cl and climate parameters (accumulation, δ¹⁸O and aerosol data) from the radionuclide records. The comparison of the resulting cosmogenic radionuclide-based dipole reconstruction with independent geomagnetic field records shows good agreement. This validates the use of cosmogenic radionuclides in ice cores to reconstruct past geomagnetic dipole moment variations after correction for the climate effect.

How to cite: Zheng, M., Sturevik-Storm, A., Nilsson, A., Adolphi, F., Aldahan, A., Possnert, G., and Muscheler, R.: Reconstruction of geomagnetic dipole moment variations for the last glacial period based on cosmogenic radionuclides from Greenland ice cores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1652, https://doi.org/10.5194/egusphere-egu2020-1652, 2020.

Understanding the pre-Paleoarchean geodynamo is arguably the greatest technical challenge for paleomagnetism: only silicate crystals bearing magnetic inclusions now found in younger sedimentary units may have escaped the metamorphism that otherwise excludes extant Paleoarchean to Hadean whole rocks from consideration. The recent optical and electron microscope documentation of primary magnetite inclusions in Jack Hills zircons (Tarduno et al., PNAS, 2020), previously predicted by paleomagnetic unblocking temperatures, together with microconglomerate test results, Pb-Pb radiometric age data and Li-diffusion constraints, support a geodynamo as old as 4.2 billion-years-old. While the available record is to first-order consistent with a continuous geodynamo since the Hadean, there are several 50-100 m.y. gaps in the record. Herein we examine these gaps and further test the paleointensity history derived from Jack Hills zircons through study of Paleoarchean and older detrital zircons of the Singhbum craton of eastern India. Preliminary paleomagnetic and paleointensity data suggest the presence of a primary magnetism, magnetite inclusion carriers and field strengths similar to those of the Jack Hills record.

How to cite: Tarduno, J., Cottrell, R., and Hofmann, A.: Testing continuity of the Hadean-Eoarchean geodynamo with zircon paleomagnetism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11157, https://doi.org/10.5194/egusphere-egu2020-11157, 2020.

How to cite: Nowaczyk, N., Liu, J., and Arz, H.: Fast geomagnetic field variations recorded in glacial Black Sea sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4406, https://doi.org/10.5194/egusphere-egu2020-4406, 2020.

How to cite: Maffei, S., Livermore, P., Greenwood, S., and Mound, J.: Modelling fast geomagnetic reversals , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10488, https://doi.org/10.5194/egusphere-egu2020-10488, 2020.

The thorough understanding of magnetic mineralogy is a prerequisite of any successful palaeomagnetic, and in particular, archaeomagnetic study. Magnetic minerals in archaeological ceramics and baked clay may be inherited from the parent material, or, more frequently, formed during the firing process. The resulting magnetic mineralogy may be complex, including ferrimagnetic phases not commonly encountered in rocks. Towards this end, we carried out a detailed rock magnetic study on a representative collection of archaeological ceramics (baked clay from combustion structures and bricks) from Bulgaria and Russia. Experiments included measurement of isothermal remanence acquisition and demagnetization as a function of temperature between 20°C and >600°C, and a variant of Lowrie 3-axis IRM test with measurements performed at elevated temperatures. For selected samples, low-temperature measurements of saturation remanence and initial magnetic susceptibility between 1.8 K and 300 K have been carried out.
All studied samples contain a magnetically soft mineral identified as maghemite probably substituted by Al and/or Ti. Stoichiometric magnetite has never been observed, as evidenced by the absence of the Verwey phase transition. In addition, one or two magnetically hard mineral phases have been detected, differing sharply in their respective unblocking temperatures. One of these unblocking between 540°C and 620°C is believed to be substituted hematite. Another phase unblocks at much lower temperatures, between 140°C and 240°C, and its magnetic properties correspond to an enigmatic high coercivity, stable?, low unblocking temperature (HCSLT) phase of McIntosh et al. [McIntosh, G., M. Kovacheva, G. Catanzariti, M. L. Osete, and L. Casas (2007), Geophys. Res. Lett., 34, L21302, doi: 10.1029/2007GL031168]. In a few samples high- and low-unblocking temperature magnetically hard phases appear to coexist, in the others the HCSLT phase is the only magnetically hard mineral present. We finally compare the samples performance in archaeointensity experiments with their respective magnetic mineralogy.
This study is supported by Russian Foundation of the Basic Research, grant 19-55-18006, and Bulgarian National Science Fund, grant KP-06-Russia-10.

How to cite: Kosterov, A., Kovacheva, M., Kostadinova-Avramova, M., Minaev, P., Sal'naya, N., Surovitckii, L., Yanson, S., and Sergienko, E.: High-coercivity magnetic minerals in archaeological ceramics: new insights from remanence acquisition and demagnetization measurements at elevated temperatures , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10130, https://doi.org/10.5194/egusphere-egu2020-10130, 2020.

Archaeomagnetism deals with baked clay materials carrying a thermoremanent magnetization acquired in the Earth`s magnetic field, which determines its importance for two scientific fields – geophysics and archaeology. It is well known that the success of an archaeomagnetic study is closely related to the magnetic properties of the materials used. In turn, the magnetic properties depend on the initial clay mineralogy, firing conditions and burial history. In order to get more information about the influence of the firing process, samples prepared of raw clays (taken from six different sources) were subjected to the successive experimental baking in three experimental combustion structures: open-hearth, single-chamber round furnace and double-chamber rectangular kiln. Heating and cooling temperatures in the various parts of the structures were constantly monitored. Rock magnetic measurements and analyses were carried out prior to, after the first and after the fourth experimental firing.

The heating/cooling cycle in the single-chamber furnace was the most prolonged. The temperatures achieved vary from 400 to 540°C displaying very uneven distribution after 400 °C. Maximum temperatures of about 850 – 900°C were reached in the hearth and in the double-chamber kiln but they were retained for a relatively short time (5 – 10 min) whether or not extra fuel was added. The heating and especially the cooling were the most homogeneous in the double-chamber kiln, where the cooling temperatures in its different parts varied within 50°C. In contrast, these temperatures differ by about 250°C in the single-chamber furnace and almost 400°C in the hearth.

X-ray diffraction analyses classify the chosen six clays as calcareous (all grayish clays) and non-calcareous (all brownish clays).  Magnetic susceptibility behaviour monitored during stepwise heating and the shape of alternative field demagnetization curves of laboratory induced isothermal magnetization divided clays into three groups. Remanence and magnetic susceptibility measured after the first experimental firing are quite variable according to the clay type, structure and samples position, but some trends are obvious. The lowest magnetic properties generally correspond to the samples heated in the single-chamber furnace where the lowest firing temperatures developed. However, in many cases the measurements for samples baked in the hearth and/or in the kiln are very close. The highest magnetic enhancement was always achieved in the double-chamber kiln but only in the parts farthest from the entrance. The reheating increases (except for one clay) and homogenizes the magnetic properties of the kiln samples but this pattern is not systematically observed for the hearth. Magnetically soft minerals dominate. Presence of a high-coercivity carrier (probably hematite) is supposed for three clays single-baked in the hearth and the single-chamber furnace (but only when the samples were placed in the parts with the most oxygen access). During the multiple experimental firing, some samples disintegrated in different extent.

This study is funded by the grant KP-06-N30/2 from the Bulgarian National Science Fund. The support by Russian Foundation of the Basic Research grant 19-55-18006 is also acknowledged.

How to cite: Dimitrov, P., Kostadinova-Avramova, M., Kosterov, A., and Lesigyarski, D.: Influence of firing conditions on the rock magnetic properties. Preliminary results from experimental heating experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7814, https://doi.org/10.5194/egusphere-egu2020-7814, 2020.

Bulgarian archaeomagnetic database is the longest local geomagnetic field record covering almost completely the last 8000 years. However, very poorly constrained periods in need of elucidation still exist. Among the most problematic are the last 1200 years BC corresponding to the Iron Age in Bulgarian lands. In contrast to the relatively well studied Neolithic, Eneolithic and Bronze Age settlements, the Iron Age sites (especially in the Early Iron Age phase) are not sufficiently investigated from the archaeological point of view. The lack of precise stratigraphic frames significantly hampers the specification of sites chronology, and as a result, baked clay features that could serve as reliable reference points are rather scarce.

The most recent compilation of the Bulgarian archaeomagnetic database contains 18 intensity and 16 directional reference points belonging to the Iron Age, which have very uneven temporal distribution. To extend the coverage, 26 baked clay structures from nine different archaeological sites were sampled and archaeomagnetically studied producing ten new directional, but only five intensity data. It seems quite often materials from Iron Age combustion structures to possess magnetic properties unfavorable for archaeomagnetism, generally reflected in non-linear and concave Arai plots. Hereby, the lowest success rate of archaeointensity determination experiments is registered for this period within the whole database. Usually it is much easier to determine the past geomagnetic field direction compared to the intensity and the failure of the Thellier experiment is not surprising. Nevertheless, more than 90% of the investigated features belonging to all the other epochs except Iron Age normally display success rates over 50%, as only for 1-2% the archaeointensity determination experiments fail completely. In contrast, in the Iron Age the successful features comprise only 56% of the total number studied with 28% failure. A possible explanation for this observation was sought in some specific inappropriate conditions during firing/burial time that affect magnetic properties of baked clay materials during and/or after their thermoremanence acquisition.

This study is supported by the grant KP-06-Russia-10 from the Bulgarian National Science Fund and Russian Foundation of the Basic Research grant 19-55-18006.

How to cite: Kostadinova-Avramova, M., Kosterov, A., Jordanova, N., Dimitrov, P., and Kovacheva, M.: Geomagnetic field variations and low success rate of archaeointensity determination experiments for Iron Age sites in Bulgaria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7717, https://doi.org/10.5194/egusphere-egu2020-7717, 2020.

Japan is a country with very rich cultural heritage and with many archaeological sites that can offer precious information about the geomagnetic field secular variation in the past. However, even though archaeomagnetic research in Japan started more than 60 years ago, with numerous studies focused on archaeodirection determinations of in situ archaeological structures, the available up to now archaeointensity data are still scarce. Most of the absolute intensity records come from archaeomagnetic studies carried in 60’s, 70’s and 80’s, mainly obtained with the original Thellier-Thellier method and/or its modifications. In none of these data, cooling rate and anisotropy corrections were applied. During the last 20 years, only two more archaeointensity studies have been published, applying the Tsunakawa-Shaw palaeointensity method on baked clays from Japanese kilns. This current status of archaeointensity studies in Japan makes evident the need of new high-quality reference data in order to reconstruct the geomagnetic intensity secular variation path in Japan. In this perspective, in the frame of the “Be-Archaeo” MSCA-RISE project, we have collected a total of 56 fragments of archaeological artifacts from the archaeological sites of Sada Higashizuka, Sada Nishizuka, Tatezaka, Tenguyama, Tatetskuki and Nima Ohtsuka, situated at the Okayama prefecture. The baked clays studied belong to ancient coffins, haniwa artifacts and pottery and their ages range from 100 AD to 675 AD. Preliminary rock magnetic and archaeomagnetic analysis including magnetic susceptibility, Q-ratio, isothermal remanent magnetization (IRM) curves, thermal demagnetization of a composite IRM component as well as stepwise thermal and alternating field (AF) demagnetizations were performed to investigate the magnetic mineralogy of the samples and their suitability for archaeointensity experiments. The results show the presence of a magnetic mineral with Curie temperature ranging from 480 to 560 ^oC, most probably magnetite and/or Ti-magnetite. IRM curves and AF demagnetization suggest also the presence of a high coercivity component in some samples, as saturation is not reached at 1 T and samples are not completely demagnetized at 180 mT. Demagnetization diagrams reveal a stable single component of magnetization for most of the samples. However, some samples demonstrate disturbed Zijderveld diagrams and/or two components of magnetization; no correlation between the quality of the results and the material studied (pottery, haniwa or coffin fragments) was found. These preliminary results were used to select promising samples for archaeointensity experiments, aiming to obtain new high-quality archaeointensity records for the Late Yayoi and Kofun periods.

How to cite: Hatakeyama, T., Tema, E., and Matsumoto, N.: Archaeointensity data from Japan: current status and future perspectives, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4419, https://doi.org/10.5194/egusphere-egu2020-4419, 2020.

The bricks are one of the best material for archeomagnetic studies. They usually contain a very stable and intense remanent magnetization and their backing technique (i.e. horizontal location in the furnace) allow to determine the value of inclination of geomagnetic field.  The technique were not changing since the middle ages up to the half of the XIX century, when a machine production have started. Preliminary archeomagnetic studies of the brick samples from Poland that providing a general information about paleoinclination changes in Gdańsk since 1080 AD indicated that this material is suitable for determination of ancient geomagnetic field parameters. However, in spite of the presence of many brick objects, as well as early and great tradition of brick building in Poland, this region of Europe is still “Tabula Rasa”  on the map of current archeomagnetic investigations. The archeomagnetic curves for this part of  Central Europe will be constructed almost from foundations. Well defined curves with secular variations of geomagnetic field during last 2500 year were constructed for example in neighboring Germany. The bricks for our archeomagnetic study were selected from churches, castles and palaces of well-known age. The age uncertainty in each case was less than 25 years. In order to check historical ages, a comparative TL dating of selected bricks was also conducted. The paleointensity of geomagnetic field  was determined using the IZZI-Thellier-Thellier protocol. About 300 cylindrical  specimens from the bricks located in more than 50 historical objects were examined. Studies of magnetic carriers and studies of anisotropy of magnetic susceptibility and anisotropy of isothermal remanent magnetization were also performed. The obtained paleosecular curves were compared with the coeval data from other regions of Europe.

How to cite: Nawrocki, J., Standzikowski, K., Rosowiecka, O., Wójcik, K., Werner, T., Łanczont, M., Gancarski, J., and Wiewióra, M.: Geomagnetic paleointensity and paleoinclination between 1200 and 1700 AD derived from brick buildings in northern and south-eastern Poland. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5529, https://doi.org/10.5194/egusphere-egu2020-5529, 2020.

With the goal to obtain new data of geomagnetic field intensity in the Bronze Age in the Eastern Europe the arheomagnetic study of fired ceramic samples from the settlements Sakhtysh-I and Sakhtysh - II were done. The settlements Sakhtysh-I and Sakhtysh -II are placed in Teikovo district of the Ivanovo region of Russia (56^о48′ N, 40^о33′ E). Archeological excavations of ancient ceramics were carried out by the Upper Volga Archeological Expedition of the Institute of Archeology RAS. The studied collection of pottery fragments belongs to three cultures: the Fatyanovo, the Fatyanoid (or the Fatyanivo-like) and the Textile ceramics culture.  The composition of the ferromagnetic fraction presented in the studied archaeological samples have been performed by the complex of standards petromagnetic methods. The thermomagnetic analysis (TMA) in dependence of the saturation magnetic moment on temperature and determination of the Curie points were carry out. Thus based on TMA one can conclude that the main carrier of the magnetisation 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.  Based on the carbon-isotope dating the age of pottery fragments corresponds to the ~ 2000-700 years BC, and we can construct a curve of paleointensity variations of the geomagnetic field from the age.  The data obtained for this period can provide new information about variations of the geomagnetic field intensity during the Bronze Age, which will make it possible to specify the character of changes in geomagnetic field. Earlier for the time interval II millennium BC a certain amount of the geomagnetic field intensity data were obtained in the Russian Plain region. Due to the uncertainty of the dating, these data allowed us to evaluate only the general features of geomagnetic  field intensity variations.  Rapid sharp changes in field intensity occurred with an increase in the average level of the field intensity compared with the level in the previous two millennia. 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.

How to cite: Pilipenko, O., Nachasova, I., Tsetlin, Y., and Filina, E.: Archeomagnetic studies of fired ceramics from Sakhtysh-I and Sakhtysh -II settlements (Ivanovo Region, Russia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5627, https://doi.org/10.5194/egusphere-egu2020-5627, 2020.

Over the past years several groups have made efforts to calibrate the ‘pseudo-Thellier’ technique to obtain paleointensities from materials that acquired their natural remanent magnetizations thermally, while avoiding heating the samples during the experiments. These calibrations revolve around mapping laboratory induced Anhysteretic Remanent Magnetizations (ARMs) to thermally acquired Natural Remanent Magnetizations (NRMs).

One approach has been to plot pseudo-Thellier slopes against paleointensities that are either known (for very young lavas) or result from different paleointensity techniques. Although the obtained calibration relation is linear and closely follows the data, the relation worryingly misses the origin, i.e. a pseudo-Thellier slope of 0 leads to a paleointensity of up to 14.7 µT. Currently, there is no satisfying explanation for this non-zero axis intercept. Another approach has been to calibrate the mapping between the TRM and ARM by giving (thermally stable) samples a remanent magnetization and force the calibration through the origin. Although to the current state of our knowledge this is theoretically correct, the mismatch between the calibration relation and the data introduced by this approach is evident. So far, neither of these approaches yielded a generically applicable and theoretically acceptable mapping between ARMs and TRMs.

Naturally occurring basalts, however, are assemblages of magnetic minerals differing in grain size, shape, and chemistry. Here we take a new approach to the interpretation of pseudo-Thellier data by trying to find end-members for the ARMs, through nonnegative matrix factorization, that represent these different magnetic minerals in the samples. With the idea that the quantity of these end-members in the different ARMs are related to the original NRMs intensities. We use a set of 580 samples from different volcanic edifices (Hawaii, Mt. Etna, Tenerife, Gran Canaria, and Iceland) that recently cooled in the Earth’s magnetic field, so in known field strengths. The first results that we will present are encouraging and address the current challenges with obtaining absolute paleointensities from lavas with a pseudo-Thellier approach.

How to cite: van Grinsven, L., van Leeuwen, T., and de Groot, L.: End-Member Modeling Analyses (EMMA) of pseudo-Thellier style experiments to derive absolute paleointensities from lavas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19991, https://doi.org/10.5194/egusphere-egu2020-19991, 2020.

Many pyroclastic flows are distributed around Japan. They usually involve volcanic-glass grains. These grains are considered to form at the timing of volcanic eruptions and are expected to have magnetic inclusions consisting of tiny single (titano)magnetites with recording the paleomagnetic field. We have extracted single volcanic-glass grains of pumice-type with a diameter of 0.60-0.84 mm from an unwelded part of the Ito pyroclastic flow deposits (A-Ito, 26-29 ka; Machida and Arai, 2003), Kyusyu, Japan. A series of rock- and paleomagnetic measurements have been made on the grains.

Sixty-seven out of 88 grains had detectable intensities of natural remanent magnetization. Some of such grains were further investigated. Results of low-temperature magnetometry exhibited inflection points at 105-120 K, suggesting magnetite as a main remenence carrier. Stepwise alternating field demagnetization revealed an existence of stable characteristic remanence (ChRM) which was interpreted to be a primary component. 

Tsunakawa-Shaw method (Tsunakawa and Shaw, 1994; Yamamoto et al., 2003), one of the latest absolute paleointensity (API) techniques to date, was applied to selected grains having stable ChRMs. On the application we newly included measurements related to an isothermal remanent magnetization (IRM). Four successful results were obtained by an adoption of IRM corrections, giving an average API value of about 25 μT. This corresponds to a virtual axial dipole moment (VADM) of about 50 ZAm², which is consistent with the contemporaneous VADM of the sedimentary record (PISO-1500; Channell et al., 2009). 

How to cite: Yamamoto, Y., Takeda, H., Sato, M., and Kawabata, H.: Preliminary absolute paleointensity estimation from a single volcanic-glass grain extracted from an unwelded pyroclastic flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3242, https://doi.org/10.5194/egusphere-egu2020-3242, 2020.

Rangitoto is an island volcano situated outside the city of Auckland, New Zealand. The volcano is the youngest and largest volcano in the monogenetic Auckland Volcanic Field (AVF), with the last eruption occurring about 550-500 calibrated years BP, a date determined from studying historical records. The eruption history of Rangitoto is unknown, however all other volcanoes in the AVF have a brief eruption history. In February 2014 a core spanning 127 metres in length was recovered, consisting of 53 lava flows varying in thickness from 1 to 15 metres. Radiocarbon dates taken from marine sediments found at the bottom of the core, underneath the Rangitoto’s lava flows, suggest that there was early activity as far back as 6000BP, after which Rangitoto may have been dormant until the main shield building phase at around 600BP. Magnetic mineralogy analysis has also shown that much of the core is a reliable recorder of the past geomagnetic field. 156 samples have been analysed for palaeodirectional data and 21 acceptable palaeointensity estimates have been accrued from a range of depths throughout the core length. The collected palaeomagnetic data are used to reconstruct variations in the geomagnetic field, which in turn are used to constrain the eruption rate. Preliminary results suggest that the palaeomagnetic data are incompatible with a short eruption duration of the shield building phase implied by the radiocarbon data (under 100 years) and more compatible with a longer duration of shield building for Rangitoto Island. We discuss alternative explanations for this discrepancy and potential implications of our results in regard to improving hazard planning in Auckland.

How to cite: Allington, M., Nilsson, A., Hill, M., Suttie, N., Hjorth, I., Aulin, L., and Augustinus, P.: Constraining the Eruption History of the Rangitoto Volcano using Palaeomagnetic Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13712, https://doi.org/10.5194/egusphere-egu2020-13712, 2020.

This work is devoted to paleomagnetic studies of lava samples from three volcanoes of Kamchatka in order to define the age of lava flows and to obtain data of paleosecular variations of the geomagnetic field for the Kamchatka region. We studied 53 samples from 7 sites from lava flows of the Avachinsky, the Gorely and the Tolbachik volcanoes. The study of paleosecular variations recorded in the magnetization of the lava flows of volcanoes makes it possible to create a magnetochronological scale for epochs of the same polarity.

According to the data of electron microprobe and thermomagnetic analyzes, the magnetic properties of samples from the lava flows of the Avachinsky volcano are mainly determined by titanic magnetite with a Curie temperature Tc = (540-580) °С. The study of magnetic mineral grains using electron and magnetic force microscopy showed the presence of decay structures in grains, indicating the high-temperature oxidation of titanomagnetite. Ferrimagnetic grains of samples from the Gorely and Tolbachik volcanoes are represented by titanomagnetite with a Curie temperature Tc = (200–300) °C. According to the hysteresis characteristics, the magnetic structure of the grains corresponds to a single-domain and pseudo-single-domain state. Thermal and magnetic cleanings showed the predominance of one component in the NRM. The geomagnetic field intensity was determined by the Thellier method in the Coe modification.

It was found that the paleointensity value H_anc = 55±3 μT, determined from the NRM of samples of the 2012 eruption from the Tolbachik volcano, differs from the modern magnetic field in the area of this volcano by the IGRF-12 model by only 4% (Н_IGRF = 53 μT). This indicates the reliability of our methodology for determining paleointensity from the most stable part of the NRM of igneous rocks.

A comparison of the coordinates of the paleomagnetic pole (N 66º±4º, E 266º±5º) and the virtual dipole magnetic moment of the Earth (VDM = 8.3±0.9*10²² A*m²) with data on variations of the geomagnetic field over the past 10,000 years [Burlatskaya, 2007; McElhinny, 1982] allows us to conclude that the investigated lava flow belongs to the historical eruptions of 1827. The coordinates of the virtual geomagnetic pole (N 83º±3º, E 254º±21º) and the value of VDM = 8.0±0.3*10²² A*m² determined from the samples belonging to the second lava flow of the Avachinsky volcano indicate that rocks are formed in the result of the eruption, which occurred 5-5.5 thousand years ago.

It was revealed that the magnitude (H_anc =65±5μT) and the direction of paleointensity determined by the NRM of the samples from Gorely volcano significantly differ from the characteristics of the modern magnetic field. The assumption is made that the studied samples belong to the outpouring of lava, which occurred about 2.7 thousand years ago, during the "Sterno-Etrussia" geomagnetic excursion.

This work was supported by the Russian Foundation for Basic Research, project 20-05-00573.

How to cite: Sleptsova, I. and Maksimochkin, V.: Paleointensity derived from igneous rocks of Kamchatka volcanoes of the Late Pleistocene-Holocene epoch , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7954, https://doi.org/10.5194/egusphere-egu2020-7954, 2020.

A new full-vector palaeosecular variation curve for Italy is presented based on a selection of high-quality data from sites within a 1000 km radius around Viterbo. The intensity and direction curves were calculated separately, using an updated compilation of Italian archaeomagnetic data from both archaeological material and volcanic rocks. The quality of the data was carefully evaluated, with particular attention on the reliability of the dating of the volcanic rocks and on the quality of the archaeointensity determinations. Only data from volcanic rocks of undisputable age have been considered. The new curves were calculated using Bayesian statistics and cover the last three millennia. The directional curve is very well constrained whilst the intensity curve is characterized by a larger error envelope, highlighting the need for new high-quality intensity data from Italy. Despite the limited number of reference data, the Italian intensity curve confirms periods with high intensity values of around 80 μT at 800-700 BC and 700-800 AD, in accordance with the geomagnetic intensity spikes previously identified in Middle East and Western Europe. Thanks to the privileged geographical position of the Italian peninsula, situated almost in the center of the Mediterranean, the Italian secular variation (SV) curves were used to analyze the evolution of the geomagnetic field in Europe by comparing them with other recently published SV curves for Western and Eastern Europe and with geomagnetic field models. The new curves can be used for archaeomagnetic dating not only in Italy but also in other countries of Europe such as Croatia, Slovakia and Serbia where no local SV curves are available so far.

How to cite: Tema, E. and Lanos, P.: The first full vector palaeosecular variation curve for Italy based on revised data from archaeological material and volcanic rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8261, https://doi.org/10.5194/egusphere-egu2020-8261, 2020.

Recent advances in geomagnetic field modelling of palaeomagnetic data have led to significant improvements of our understanding of the geomagnetic field and how it varies on millennial timescales. Among other things, palaeomagnetic field reconstructions have shown that large-scale non-dipolar field anomalies, similar to or even larger than the present day South Atlantic Anomaly (SAA), have occurred several times in the past, implying that such structures are not necessarily associated with polarity reversals or excursions. A recent study even suggests that such large-scale field asymmetries could be periodically recurrent features of the field. Here we present results from a new Holocene geomagnetic field model, pfm9k.2, constructed using a novel Bayesian approach to account for chronologic uncertainties and address problems with potential smoothing induced by post-depositional remanent magnetisations. We focus our attention on a particular time period, around 700BC, where our new model shows a large-scale field asymmetry in the northern hemisphere with a weak field anomaly in the North Pacific accompanied with strong field intensities in Europe. The field evolution predicted by the model during this time period shares many similarities with the present day field, including a rapid decay of the dipole moment, suggesting that there may be common driving processes. We discuss the physical implications of these results.

How to cite: Nilsson, A. and Suttie, N.: A North Pacific Anomaly around 700BC: a potential analogy to the present day geomagnetic field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21424, https://doi.org/10.5194/egusphere-egu2020-21424, 2020.

Temporal changes in the main geomagnetic field, the so-called secular variation, can range from decades to millennia without showing any clear periodicity. A better knowledge of the secular variation behaviour is important to determine the mechanisms that maintain the magnetic field and can help to establish constraints in dynamo theories. Considering that the magnetic dipole contributes to around 90 % of the total main field, we have searched for periodicities in this component over the last 10,000 years using four global paleomagnetic field reconstructions (SHA.DIF.14k, CALS10k2, BIGMUDI4 and SHAWQ2k). We have applied three techniques commonly used in signal analysis: a) the Fourier transform to identify the characteristic frequencies of the dipole field; b) the Empirical Mode Decomposition to separate the secular variation of the dipole into short and long wavelength signals; and c) the wavelet analysis to know how the characteristic periods are distributed over the time studied. Results show that for short-wavelength terms we find a recurrent periodicity of 700 – 800 years, present throughout most of the last 10,000 years with small variations. Focusing on long-wavelength terms for SHA.DIF.14k and CALS10k2, we observed a drop in the dipole field, controlled by the axial dipole, starting around 7000 BC. We have fitted it as an exponential decay obtaining a relaxation time of 8,000 – 10,000 years, which well agrees with the theoretical diffusion time of the geomagnetic field. The dipole field starts to increase around 4,500 BC for nearly 4,000 years. If we consider that this increase is comparable to the charge of a capacitor, it would give a characteristic time of 15,000 years. However, the theoretical maximum value obtained for the axial dipole field is never reached and the charge stops at 40 % around the year 100 AD. At that time, the dipole impulse ended and the present large trend dipole decrease seems to start, with a relaxation time of 13,000 years.

How to cite: González-López, A., Campuzano, S. A., Molina-Cardín, A., Pavón-Carrasco, F. J., De Santis, A., and Osete, M. L.: Pulses and decay of the dipolar field during the Holocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-449, https://doi.org/10.5194/egusphere-egu2020-449, 2020.

For the global time stationary geomagnetic core field, a new modeling concept for Holocene archeomagnetic data is presented. Major challenges consist of the uneven data distribution, missing vector field components and non-linear relations between observations and the geomagnetic potential. Instead of a truncated spherical harmonics approach, we propose a fully Bayesian, Gaussian process based model. Inherently, the Bayesian approach provides location dependent uncertainties.

The geomagnetic potential is assumed to be a Gaussian process whose covariance structure is given by an explicit kernel function, including several hyperparameters. For this kind of non-parametric models, the full Bayesian posterior is numerically intractable. Instead, we propose an approximate computation using a Bayesian update system. In a first step, the full vector records are used to obtain, within Laplace approximation, a rough field estimate. This estimate serves as a point of linearization for the non-linear observations. The approximate posterior is then given by a Gaussian mixture. Marginals for all relevant parameters and the field itself can be computed. We are able to quantify the impact of data coverage on uncertainty reduction.

How to cite: Schanner, M. A., Mauerberger, S., Korte, M., and Holschneider, M.: Correlation based snapshot models of the archeomagnetic field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6808, https://doi.org/10.5194/egusphere-egu2020-6808, 2020.

High-resolution palaeomagnetic records preserved in sediments (especially those that are well dated) provide valuable continuous information on past changes in Earth’s magnetic field. These data are essential for us to develop better understanding on the dynamics and causes of geomagnetic changes at various time scales. In this study, we conducted palaeomagnetic analyses on continuous u-channel samples collected from well-dated late Quaternary sediment sequences cored in the west Iberian Margin during Integrated Ocean Drilling Program (IODP) Expedition 339. Natural remanent magnetisations (NRM) as well as a suite of laboratory-induced magnetisations of the samples were measured at 1-cm interval resolution on a superconducting rock magnetometer before and after stepwise alternating field (AF) demagnetisation. NRM demagnetization data of the samples reveal a very stable and well-defined primary magnetisation component. Chronology of the studied cores is well constrained and tied to the polar ice cores as well as the absolutely dated Asian speleothem records. Average sedimentation rates of the studied cores range between ~10 cm/kyr to over 70 cm/kyr. Relative palaeointensity (RPI) records reconstructed from these sediments, when placed on the acquired age models, correlate well with other global and regional RPI records on time scales of ~10 kyr or longer. RPI features recorded at higher sedimentation rate sites appear slightly younger (a few hundreds to a couple of thousand years), possibly due to effects of the sediment magnetisation lock-in process. These Iberian Margin RPI records also show common millennial to multi-millennial scale variabilities, especially after deconvolution and correction of the lock-in induced age offset.

How to cite: Xuan, C., Nichols, M., Stoner, J., Richter, C., and Acton, G.: Global and regional geomagnetic variabilities recorded in late Quaternary sediments from the west Iberian Margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11984, https://doi.org/10.5194/egusphere-egu2020-11984, 2020.

In this paper we present the results of paleomagnetic investigations of the Lake Pleshcheevo sediments (Yaroslavl region, Russia). Sediments of modern lakes are a unique record's archive changes of environment, climate, geomagnetic field over the past millennia. From lake were selected 4 cores up to 6.3 meters. From the core using a special sampler was selected undeformed samples for petromagnetic and paleomagnetic investigations. Magnetic susceptibility, NRM (modulus and direction), demagnetization by an alternating magnetic field were made for all samples. The absolute age of the sediments was determined using the radiocarbon dating. To establish the absolute age of magnetization in sediments, it is necessary to use information from observational, archaeomagnetic, and other data. After correlation of data from Lake Pleshcheevo sediments with archaeomagnetic and other corrected limnomagnetic records, we were able to construct an adequate time scale for recording geomagnetic variations. The obtained changes of direction of the characteristic NRM component of sediments were compared with archaeomagnetic data and records of geomagnetic field variations reconstructed from studies of lake sediments in Western and Eastern Europe. There is observed a very good agreement of all these data. It testifies the high quality of the magnetic record in the sediments of the Pleshcheevo Lake and the need for special studies and obtaining a high-quality master curve of the geomagnetic field variations over the last 12 thousands years for the European part of Russia.

This work was funded by the Russian Science Foundation under grant № 18-17-00251. Authors acknowledge measurements at the resource Center "GEOMODEL", Scientific Park of Saint-Petersburg State University.

How to cite: Nurgaliev, D., Dilyara, K., Lina, K., Nuriia, N., Pavel, K., Anatoly, B., and Damir, K.: High resolution paleosecular variations recorded in Lake Pleshcheevo sediments (Yaroslavl region, Russia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20101, https://doi.org/10.5194/egusphere-egu2020-20101, 2020.

Paleomagnetic records reconstructed from globally distributed marine sediments have greatly improved our understanding of long-term paleosecular variations and geomagnetic excursions. Nevertheless, questions regarding to the development of the geomagnetic field anomaly in the Southern Atlantic Ocean and the asymmetric geomagnetic field between Northern and Southern Hemispheres are not yet satisfactorily resolved. Paleomagnetic data, particularly from the Southern Hemisphere, is needed to better define the global geomagnetic field configurations spanning paleosecular variations and excursions. In this study, three sediment cores (PS97-085, PS97-84, PS97-079) recovered from the Drake Passage, Southern Ocean were subjected to detailed rock magnetic and paleomagnetic investigations. Preliminary age models were obtained by correlating their magnetic susceptibility to the ẟ¹⁸O master record from Dome C, Antarctica. In addition, rock magnetic records of the studied PS97 cores were further correlated to that of core PS67/197-1 with AMS ¹⁴C age constraints. The results from PS97 cores are thus continuously covering the past about 110 ka. Rock magnetic results indicate titanomagnetite is the dominant magnetic carrier in the studied PS97 cores. Relative paleointensities (RPI) derived from these PS97 cores are comparable with the regional relative paleointensity records and the South Atlantic paleointensity stack (SAPIS). Additionally, anomalous inclinations at about 41 ka and 35 ka, observed in core PS97-085, are coeval with the Laschamps and the Mono Lake excursions, respectively. This study provides new paleomagnetic records from the Southern Ocean, though further age constrains are needed to consolidate the paleomagnetic interpretations. The up to now obtained paleomagnetic records, together with previous studies from the Southern Ocean, are aiming to clarify the asymmetric pattern of non-dipole geomagnetic field between Northern and Southern Hemispheres.

How to cite: Liu, J., Nowaczyk, N., Zheng, X., Liu, Q., and Arz, H.: Preliminary paleomagnetic results from PS97 cores from the Drake Passage for the past 110 ka, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3292, https://doi.org/10.5194/egusphere-egu2020-3292, 2020.

A series of paleomagnetic works relying on the ocean sediments present some significant astronomic periods, such as a 100 kyr quasi-period and 41 kyr obliquity signal. These studies provide the new insights unscrambling what and how the earth magnetic field changed in despite of the ongoing debating. Numerical studies of recent years also reveal the possibility of the precession drive the dynamos and influence the magnetic field. However, the less of reliable high-resolution paleomagnetic records besides of relative paleointensity reduce its credibility. Here, we present some detailed rock magnetic and paleomagnetic studies on the continuous 40-m-thick sediments in two parallel cores retrieved from Tianyang Maar lake, southern China. The new results would contribute to discuss the correlation of paleomagnetic field with the astronomical factors.

Tianyang Maar lake  is located in the southern part of the Leizhou Peninsula.  The maar lake has a surface area of ~ 7.3 km² surrounded by a 40 - 60 m high crater rim composed of basaltic breccia and tuff . Two new parallel cores, TY08 and TY15 (~ 10 m apart), were extracted from center of the crater in 2008 and 2015, respectively, using a rotary borer consisting of a stainless steel outer tube and a plastic inner tube to minimize sediment disturbances and contamination. The sediments of two cores can divided into three zones: about upper 15.59 m was composed of varying colors clay and the middle part (15.59-21.94 m), was dominated by the grey and greyish-brown fine to coarse sand with occasional gravels, embedded a thick grey clay layer; the lower part (21.94-40.0 m) shown as the dark grey and black organic-rich clay.

The paleomagnetic results show that the natural remanent magnetization (NRM) of the sediments is mainly contributed by magnetically soft minerals, and the sediments have fairly documented geomagnetic field variations. A chronology is constructed using multiple methods, including radiocarbon dating, optically stimulated luminescence dating and terrestrial-marine pollen correlation. The 340-kyr paleomagnetic inclination record displays patterns similar to those seen in regional records over a large spatial scale (> 3000 km), implying that these records may reflect large-scale core dynamics on timescales of 10⁴ - 10⁵ years in this low-latitude region. The Tianyang inclination record exhibits a negligible inclination anomaly (∆I = -0.08°) and features six anomalous inclination events, which are assigned to the Laschamp, Blake, Fram Strait II/6α, Iceland Basin, Mamaku and 9α excursions respectively. The spectral and singular spectrum analysis (SSA) exhibit that the inclination does not show the significant signal of 100-kyr periodicity, however, the closed precession period is obvious in the third components of inclination (PC3). PC3 component shows nearly synchronous variations with the precession parameter while the opposite correlation appeared under the condition of eccentricity minima strong. This corresponding pattern hint us that astronomical parameters have the essential influence to the earth magnetic field, however, the different moving may forc or constrain the earth magnetic behavior.

How to cite: Yang, X., Chen, C., and Zheng, Z.: High-resolution Geomagnetic Field Records Decipher the Possible Precession links since about 340-kyr in a maar lake sediment sequence in tropical Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3785, https://doi.org/10.5194/egusphere-egu2020-3785, 2020.

The global production rate of the cosmogenic isotope ¹⁰Be by cosmic ray spallation is modulated by the activity of the sun and the intensity of the far-reaching component of the Earth magnetic field, which is in turn dominated by the dipolar term. Therefore, sedimentary ¹⁰Be records can be used to reconstruct past variations of the geomagnetic dipole moment. However, several environmental factors affect the transfer of ¹⁰Be atoms from the high atmosphere and soils, where it is produced, to the sediment, introducing a significant climatic modulation that can, in worst cases, completely obscure the paleomagnetic signal. These factors include variations of the continental runoff, oceanic circulation, sediment fluxes, and sediment scavenging effi­ciency. The latter is largely removed by normalizing the ¹⁰Be record with the concentration of authigenic ⁹Be, which is accumulated by sediment particles in the same manner as the cosmo­genic isotope. Even with this correction in place, individual ¹⁰Be/⁹Be records are significantly influenced by climate, to the point that only major geomagnetic events, such as the MB reversal, can be recognized. We present a model, which, for the first time, enables to deconvolve, at least partially, the climatic and magnetic components of ¹⁰Be/⁹Be records on a set of cores from the Atlantic, Indian, and Pacific Ocean. The climatic modulation is composed of an additive term, which reflects Be recycling through diagenetic release from sediments, and a multiplicative term, which is dominated by oceanic current patterns. Knowledge of these terms enables to remove, at least partially, site-specific environmental effects, obtaining a corrected ¹⁰Be/⁹Be stack that can be inverted to reconstruct variations of the dipole moment during the last geomagnetic reversal.

How to cite: Egli, R., Savranskaia, T., Valet, J.-P., Bassinot, F., Meynadier, L., Simon, Q., Bourlès, D., and Thouveny, N.: Dipole strength during the Matuyama-Brunhes reversal reconstructed from the deconvolution of magnetic and climatic modulation of 10Be/9Be records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19562, https://doi.org/10.5194/egusphere-egu2020-19562, 2020.

The knowledge of the geomagnetic field intensity during the Cretaceous Normal Superchron, a long term of forty million years without polarity reversals, may have a large impact on our understanding of the dynamo process occurring in Earth’s outer core. How, it is difficult to get the geomagnetic field behavior during the Cretaceous Normal Superchron resulting from the inadequate sampling or data of variable qualities from igneous rocks and sedimentary. Here we examine 20 magnetic anomaly profiles across the Cretaceous magnetic quiet zone of the Central Atlantic Ocean in the African flank extracted from the EMAG2v3, and calculate a synthetical magnetization profile based on the forward modeling method. We suggest that this profile records the high strength of geomagnetic field at the beginning of ~30 million years and low signal during the late period, which could be correlated with the low-resolution relative paleointensity record from the sediment samples at the Falkland Plateau, and which also could be found the VDMs/VADMs averaged by a 7-Ma sliding window from the absolute intensity records mostly from the MagIC database. Our results support the hypothesis that the distribution of heat flow along the core-mantle boundary is positively correlative to the intensity of the dipole field.

How to cite: Li, Y. and Liu, Q.: Geomagnetic Field Paleointensity during the Cretaceous Normal Superchron from Marine Magnetic Anomalies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12152, https://doi.org/10.5194/egusphere-egu2020-12152, 2020.

There has been an increasing effort toward the constraint of the average and long-term variability of the magnetic field strength, fundamental to better understand the characteristics and behaviour of the geomagnetic dipole field. Nonetheless, open questions remain about the value of the average dipole field, the relation between dipole strength and excursion reversal. Indeed, depending on the criteria adopted to analyse the current database, different long-term average values can be found, leading to different answers. The reason for the open debate can explained with the limited amount of data from key time intervals and geographical areas, due to both to complexities behind the method to obtain absolute paleointensities (several methods and experimental designs, selection criteria, high failure rate, etc..) and suitable materials.

Here, we focus on the Cretaceous Normal Superchron, a long period, from approximately 121 to 83 Ma, when the magnetic field was characterised by a stable polarity. Yet, few paleointensity data were available so far. In this study, we present new results from 48 Submarine Basaltic Glass sites from pillow lava margins, sampled on the upper crust sequence of the Costa Rica Ophiolite. Ar/Ar ages along with biostratigraphic age constraints from previous studies indicate ages ranging from from 139 to 94 Ma. After 473 samples were measured using the IZZI-Thellier protocol and analysed using strict selection criteria, 13 sites between 109 and 133 Ma gave reliable and robust results. Our new results from Costa Rica suggest that the strength of the Earth Magnetic field during CNS, 70.2 ± 21 ZAm²  are slightly lower than the pre-CNS and also lower than, for instance, at Troodos Ophiolite (81 ± 43 ZAm²; Tauxe and Staudigel 2004), consistent with the observations by Tauxe (2006) of an average dipole moment being substantially less than the present day value.

How to cite: Di Chiara, A., Tauxe, L., Staudigel, H., Florindo, F., Yu, Y., Protti Quesada, M., and Hoernle, K.: The strength of the Earth Magnetic field around the Cretaceous Normal Superchron: new data from Costa Rica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18463, https://doi.org/10.5194/egusphere-egu2020-18463, 2020.

We present the first definition of paleointesity of the Earth’s magnetic field that were obtained in the Early Cretaceous igneous rocks from the Franz Josef Land archipelago (Hooker and Scott Kelty Islands). The age of magmatism was determined by U-Pb method as the Early Cretaceous, about 125 Ma. A mean paleomagnetic direction for these rocks was calculated as D=40.2 deg, I=75.5 deg, a95=2.1 deg, k=89.3, N=52. A corresponding paleomagnetic pole is now located at Plat=69.0 deg; Plon=180.3 deg, A95=3.7 deg. An assessment of the domain structure of ferrimagnets using the Day plot diagram shows that the carriers of the natural remanent magnetization are pseudo-single-domain grains of titanomagnetites with varying Ti-content. Magnetic remanence was unblocked in temperatures of 350-400 °C. Some samples are characterized by unblocking temperatures of 560 °C. The determinations of the absolute values of paleointensity were obtained by the Thellier-Coe method with the implementation of the procedure "check-points". The values of B_anc vary within 8.4–16 µT, which is noticeably lower than the current magnetic field at the sampling point ≈55 µT. The corresponding VDMs of 1.13–2.25 × 10²² Am², with the current value of VDM ≈8 × 10²² Am². Numerous basalt flows are well studied by paleomagnetic and rockmagnetic methods, together with a large number of geochronological definitions, this makes basalts from the Franz Josef Land promising for obtaining new qualitative determinations of paleointensity in the Early Cretaceous time.

This work was supported by the RSF (project no. 19-17-00091) and the RFBR (project nos. 18-35-00273, 18-05-70035).

How to cite: Vinogradov, E., Eliseev, A., Metelkin, D., Abashev, V., Vernikovsky, V., and Mikhaltsov, N.: The first definition of paleointensity in the Early Cretaceous basalts from the Franz Josef Land, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13546, https://doi.org/10.5194/egusphere-egu2020-13546, 2020.

The Precambrian period occupies ≈ 85% of the Earth’s geological history and accommodates all the main formation stages of the Earth as a planet, including the emergence of its magnetic field. Variations in the time-averaged geomagnetic dipole moment have the potential to learn about the long-term development of the geodynamo and its response to mantle forcing and thermal evolution of the core. But determinations of paleointensity (Banc) of the geomagnetic field during this period are sparse and of limited reliability. Here we report detailed palaeomagnetic and paleointensity studies combined with comprehensive investigations of magnetic properties of Proterozoic volcanic rocks from the Ukrainian Shield.

The Ukrainian Shield comprises the crust of the Palaeoproterozoic protocraton Volgo-Sarmatia, which together with the Fennoscandian crustal segment constitutes the East European Craton (Baltica). The different megablocks of Ukrainian Shield can be treated as a coherent unit since 1.77 Ga.  Our studies has been performed on gabbro-anorthosite complexes from Ingul megablock within the Korsun-Novomigorodsky Pluton (ages 1.75-1.72 Ga) and North-Western megablock within the Korosten Pluton (age ca 1.76 Ga). The high-temperature stable ChRM component was isolated in the interval of blocking temperatures of 500-580°C by more than 300 samples from 7 sites. The presence of dual-polarity high-temperature component, lack of signs of metamorphism and good agreement of the mean palaeomagnetic pole position obtained from the Ingul block with age ca.1.75 Ga (Φ=22.5º, Λ=167.3º, dp/dm=4.0/7.7) with previous studies of anorthosites (Elming et al., 2001) of similar age suggests a primary origin of ChRM.

Comprehensive investigations of magnetic properties of rocks, the electron microscopic images of thin sections and X-ray diffractograms were performed. Rocks demonstrate thermally stable successive Msi(T) curves with clearly pronounced near-magnetite Tc. The carriers of remanent magnetization are fine magnetite isolated needle-like and/or lamellar ferromagnetic particles dispersed in plagioclas. According to the thermomagnetic criterion, high-temperature pTRMs show typical SD-PSD behavior. Palaeointensity determinations were successful on samples from 5 sites carrying well-identified ChRM components using the Thellier-Coe method with pTRM checks and the Wilson protocols. Reliable Banc values give generally low palaeofield (3.7-6.6 µT) with corresponding VDM values in the range (0.93-1.6)×10²² Am². These findings agree with our previous results for Proterozoic rocks of Kola Peninsula (age 1.86 GA) and with the data reported in the World paleointensity databases (http://wwwbrk.adm.yar.ru/palmag/index_e.html and others data), which also provide a noticeably low paleofield intensity with mean VDM = 3.2×10²² Am² for the Paleo-Proterozoic period. Thus, our new data support the Proterozoic dipole low hypothesize by Biggin et al., 2009. The work was supported by the state assignment 17-05-00259 and the RFBR grant 19-05-00433.

How to cite: Shcherbakova, V., Bakhmutov, V., Shcherbakov, V., and Zhidkov, G.: New 1.72-1.76 GA paleointensity data obtained on Proterozoic volcanic rocks from the Ukrainian Shield, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5776, https://doi.org/10.5194/egusphere-egu2020-5776, 2020.

We present absolute paleointensity results obtained from a collection of samples from ~250 Ma Kuznetsk Traps (Kuznetsk Depression, Southern Siberia). In addition to similar age these rocks display geochemical signatures similar to those reported for basalts of Siberan Traps and represent the southernmost affinity of the latter.

The primary nature of magnetic remanence in studied rocks was established by previous paleomagnetic studies Rock magnetic analysis indicates that the main magnetic mineral is titanomagnetite in a predominantly single-domain state with Curie temperatures between ~275 and 350⁰C. Scanning electron microscopy showed that titanomagnetite grains range in size from 0.5 to 1 μm. Individual grains are separated from each other and “sealed” within silicate matrix, which largely predetermined perfect preservation of primary mineral textures.

Paleointensity estimates were obtained using the Coe-version of Thellier-Thellier double-heating protocol with partial TRM checks. 36 samples (5 sites taken along the Tom River) yielded straight Arai-Nagata diagrams within temperature interval between 100 to 275⁰C. The average paleointensity value obtained from these samples was calculated at 12.7 ± 1 µT (with a factor q of about 11) with corresponding VDM=2.1 ± 0.2 × 10²² Am².

Arai-Nagata diagrams for 20 samples from two other sites (collected in quarries on the Karakansky ridge) display more complex behavior. Straight linear segment of Arai plots between ~100 and 300⁰C yielded an average paleointensity value of 44 ± 1 μT, which corresponds to a VDM=7.0 ± 0.1 × 10²² Am². However on higher temperatures, NRM vs. TRM data have a trend of flattening that increase in artificial TRM is accompanied by no loss of NRM. We interpret this observation as a result of laboratory-induced thermochemical alteration, namely, unmixing of homogeneous Ti-magnetites into Ti-rich and Fe-rich phases, with the latter phase responsible for such NRM-lost vs. TRM-gained behavior. Thermomagnetic analyses on these samples indicated mineralogical changes that set approximately at 300⁰C, supporting our interpretation. However, reversible thermomagnetic curves and p-TRM checks within 10% from initial pTRM, below ~300⁰C suggest that paleointensities determined between ~100 and 300 ⁰C of Arai plots are trustworthy.

We will discuss our results and reasons for such radical differences between paleointensity estimates obtained from the same suite of rocks sampled at different locations.

The study was supported by the Russian Foundation for Basic Research, grant No. 18-05-00234 and the Russian Science Foundation, grant No. 19-17-00091.

How to cite: Eliseev, A., Mikhaltsov, N., and Kulakov, E.: New absolute paleointensity results from ~250 Ma Kuznetsk basalts. Weak versus strong geomagnetic field at the P-T boundary., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21196, https://doi.org/10.5194/egusphere-egu2020-21196, 2020.

Palaeomagnetic investigations from the Ediacaran period (635-541 Ma) give anomalous results, which might indicate unusual behaviour of Earth’s magnetic field. In contrast to the conventional geomagnetic dipole field, geocentric and aligned with Earth’s rotational axis, records of the palaeomagnetic field from several locations in Laurentia and Baltica indicate that the Ediacaran geomagnetic field might have been exceptionally weak and spent extended periods of time with its poles close to the geographic equator. Multi-method palaeointensity determinations have been performed on rocks from the Grenville Dykes (Canada, 584-598 Ma), Skinner Cove volcanics (Newfoundland, 550.5 Ma) and the Volyn Traps (Ukraine, 560-580Ma), confirming that the field was exceptionally weak, with VDM values between 4 and 15 ZAm². These values could correspond to considerably lower VDM strengths predicted by geodynamo simulations for fields with low dipolarity before the onset of nucleation of the solid inner core. In contrast, preliminary evaluation of published directional data indicates that palaeosecular variation in the Ediacaran might not be distinguishable from palaeosecular variation predicted from palaeomagnetic data of the last ten million years.

These new palaeointensity results contribute to the elimination of data gaps in the Neoproterozoic palaeomagnetic record and will be used in combination with qualitatively assessed published directional and intensity data to capture the behaviour of the geomagnetic field of this time period in statistical field models. Comparisons of time averaged palaeosecular variation and VDM distribution with predictions of the geomagnetic field from geodynamo simulations will help to verify and improve models of deep Earth structures and dynamics.

How to cite: Thallner, D., Biggin, A., Hill, M., Halls, H., McCausland, P. J. A., Shcherbakova, V. V., Shcherbakov, V. P., and Bakhmutov, V. G.: Evaluating the anomalous palaeomagnetic field behaviour in the Ediacaran with new palaeointensity data from Laurentia and Baltica., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9121, https://doi.org/10.5194/egusphere-egu2020-9121, 2020.