The Earth's magnetic field provides protection against highly energetic particles from the Sun and outer space. Throughout geological epochs, the time-varying geomagnetic field exhibited periods of dramatic changes, both in intensity and direction. Recent data compilations of paleomagnetic records enable us to model the long-term, global evolution of the geomagnetic field and better understand the internal dynamics and underlying phenomena. Until now, global reconstructions of the geomagnetic field cover the past 100,000 years and the Matuyama-Brunhes reversal. A few models are available for the Laschamps excursion 41,000 years ago, a period marked by globally low intensity and a complex, multipolar field structure. These models enable the identification of robust characteristics of the geomagnetic field's behavior during this extreme event.

The spatial and temporal changes influence the shielding and cosmogenic nuclide production rates. In general, the higher the field intensity, the larger the shielding and the fewer cosmogenic nuclides are produced in the atmosphere. Variations in the production rates of cosmogenic radionuclides reconstructed from ice cores and sediments provide an independent proxy of paleointensity variations. On the one hand, models constrained by paleomagnetic data can be validated through comparison with actual measurements from ice and marine cores. On the other hand, the cosmogenic radionuclide data can be jointly inverted with the paleomagnetic data to build multi-proxy models of the geomagnetic field. Cosmogenic radionuclide records also serve as a proxy for solar variability and extreme solar events. Global geomagnetic field reconstructions are used to assess the space weather effects during an extreme solar storm event over the past millennia.

How to cite: Panovska, S.: Long-term changes of the geomagnetic field: recent progress, challenges and applications , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10977, https://doi.org/10.5194/egusphere-egu24-10977, 2024.

Paleomagnetic data enables the global reconstruction of the Earth's magnetic field, allowing the investigation of significant global events like polarity reversal. When compared to prior polarity reversals, the most recent one, the Matuyama-Brunhes (MB), is the best recorded reversal event in terms of the amount of available paleomagnetic data. Nevertheless, the majority of these data have poor age control, and they are not distributed equally over the world. Few global spherical harmonic (SH) models have been presented so far for the MB; the most recent is the GGFMB model. These models investigated the evolution of the dipole and non-dipole field components during the reversal process, as well as the morphology of the magnetic field at core-mantle boundary and at the Earth's surface. However, the accuracy of the models features is limited by the aforementioned issues associated with the MB data. In this study, a set of eight SH models were generated for the MB from sub-sets of data, which were classified according to their geographic distribution, timescale reliability, temporal resolution, and type. Comparing the model outputs will allow us to assess how robustly the MB's models can resolve characteristics of the reversal.

How to cite: Ahmed, A. N. M., Korte, M., and Panovska, S.: Robustness of new models of the Matuyama-Brunhes field reversal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9550, https://doi.org/10.5194/egusphere-egu24-9550, 2024.

The Porcupine geomagnetic excursion was firstly described at the IODP Site U1308 (North Atlantic). It is a reverse polarity record defined near the top of the Gauss chron (within C2An.1n) and chronostratigraphically constrained between the Kaena and the base of Matuyama reversed chrons. So far, it has been only reported in two different core sections from that site (U1308C-20H-5 and U1308F-20H-2; Channell et al., Q Sci. Rev. 2016). The Porcupine excursion displays a mean age of 2.737 Ma (with an undefined duration) and corresponds to MIS G6 and G7.

The almost 100 m-thick sedimentary infill of the Villarroya Basin (NE Spain), covers part of the Pliocene-Pleistocene period, from about 3.2 Ma to 2.6 Ma. It has been chronologically constrained by means of biostratigraphic and magnetostratigraphic analysis from the middle-upper part of Gauss (chron C2An.2n between Kaena and Mammoth reversed subchrons) to the lower-middle part of Matuyama chron (above the Feni normal event, formerly Réunion). In that work, a short reversal was identified in the upper part of the normal interval N2 of the local magnetostratigraphic sequence attributed to the top of the Gauss chron (C2An.1n around meter 17, figure 7 in Pueyo et al. IJES, 2016), which could possibly represent the first continental record of the Porcupine excursion (ca 2.7 Ma). Such identification was made possible by the occurrence of an outstanding lithological record; annually varved millimetric-scale lacustrine deposits at the base of the basin displaying unusually high sedimentation rates, which may have enabled the detection of such short magnetic excursion.

Given these encouraging results, these deposits were specifically targeted for a subsequent high-resolution palaeomagnetic study focused on ca. 6 m thick subsection where 55 new stratigraphic levels (mean spacing of 10 cm) were studied in detail. In total, more than 120 new specimens were analyzed. Both, stepwise AF and TH demagnetizations of samples were carried out in the paleomagnetic laboratories of the University of Burgos and the National Research Centre on Human Evolution (CENIEH) using 2-G cryogenic superconducting magnetometers. ChRM directions unblocking temperatures between 250-550°C yield the best results and document a complex reverse polarity record. A preliminary and cautious estimation of the time span of this excursion based on the number of annually varved sediments gives a duration of less than 0.5 ka if the sedimentation rate is assumed to be constant and significant sedimentary hiatuses are ruled out.

Channell, J. E. T., Hodell, D. A., & Curtis, J. H. (2016). Relative paleointensity (RPI) and oxygen isotope stratigraphy at IODP Site U1308: North Atlantic RPI stack for 1.2–2.2 Ma (NARPI-2200) and age of the Olduvai Subchron. Quaternary Science Reviews, 131, 1-19.

Pueyo, E. L., Muñoz, A., Laplana, C., & Parés, J. M. (2016). The last appearance datum of Hipparion in Western Europe: magnetostratigraphy along the Pliocene–Pleistocene boundary in the Villarroya basin (Northern Spain). International Journal of Earth Sciences, 105, 2203-2220.

How to cite: Pueyo, E. L., Otaño, L., Silva-Casal, R., Calvín, P., Laplana, C., Galindo-Pellicena, M. A., Duval, M., Mata, M. P., Parés, J. M., and Muñoz, A.: The first continental geomagnetic record of an elusive Porcupine?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8153, https://doi.org/10.5194/egusphere-egu24-8153, 2024.

Paleomagnetism and modern geomagnetic measurements indicate that SAA is undergoing rapid changes, with rapid expansion of area and rapid decreasing of intensity. Even in recent years SAA has expanded the minor minima seen in southern Africa. This has led to speculation that the expanding SAA may be an indicator of an upcoming geomagnetic reversal. However, how to know the future evolution direction of SAA is still debatable.

Apparently, the study of the changes of other SAA-like structure anomaly regions in history has important implications for the study of SAA evolution. Here, we focus on the evolution and disappearance of the paleo West Pacific Anomaly (WPA). Utilizing the gufm1 model, it is found that WPA occurred between 1620 and 1810 CE. Over its duration, WPA underwent phases of rapid expansion, drift, and split, eventually the primary part vanished, while a new segment persisted and expanded. The striking similarity in evolutionary behavior between WPA and the rapid expansion, drift, and recent splitting in the SAA over the past century is noteworthy. This similarity can be attributed to the fact that WPA, like the SAA, is controlled by the reversal flux patch in the CMB on the northern hemisphere. Therefore, we posit WPA as a significant magnetic anomaly in the Northern Hemisphere, and propose that its evolutionary patterns can serve as predictive indicators for the future evolution of the SAA. Studying the evolution of WPA is an important means to understand the global magnetic field and the evolution of Southeast Asia magnetic field. It is of great significance for us to understand the nature of SAA and predict the evolution trend of SAA.

How to cite: Yue, Y., Gao, J., Wei, Y., He, F., Cai, S., Wang, H., Wang, Y., and Rong, Z.: Evolution and Disappearance of the paleo-West Pacific Anomaly: implications to the South Atlantic Anomaly, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13711, https://doi.org/10.5194/egusphere-egu24-13711, 2024.

The South Atlantic Anomaly (SAA) is a dynamic feature of the geomagnetic field. Systematic observations have revealed an expansion of the SAA in terms of its geographical extent and a decrease in field strength over the last decades. Its evolution can be reconstructed further back in time by means of archeo- and paleomagnetic investigations conducted on suitable recorders of the ancient geomagnetic field. Historical geomagnetic field models indicate a westward movement of the SAA from Africa during the last centuries, while its emergence is still under debate, mainly due to the lack of ancient field records in the Southern hemisphere. However, even on much longer timescales, paleomagnetic studies on volcanic rocks from the South Atlantic have suggested a persistence or recurrence of the anomaly.
Here we present ancient geomagnetic field records from the island of Saint Helena from two different periods. Archeomagnetic directions and intensity were obtained from a historical lime kiln that operated until the middle of the 19th century. The reconstructed geomagnetic field components agree well with available instrumental measurements from the island. Geological timescales are covered by paleomagnetic examinations of lava flow profiles from Prosperous Bay and Munden’s Hill. A geomagnetic polarity transition was recorded by some flows at Properous Bay. Several other flows characterized by stable field directions are associated with weak intensities below 20 µT. These results suggest a prolonged anomalous field behavior in the South Atlantic region as indicated by previous studies.

How to cite: Arneitz, P., Schnepp, E., and Leonhardt, R.: Archeo- and paleomagnetic records from Saint Helena, South Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7439, https://doi.org/10.5194/egusphere-egu24-7439, 2024.

The recovery of the secular variation of the Earth's magnetic field in places of low latitude and in volcanic oceanic islands is crucial for the development of more precise models. Fogo Island (Lat. N14º57’; Long. W24º20’) in the Cabo-Verde Archipelago (Atlantic Ocean) is formed by a major conical and asymmetrical Quaternary strato-volcano with a summit depression (Chã das Caldeiras). It presents itself as a good object of study given its geographical position and frequency of eruptive events, 28 since 1460 A.D.. Accordingly, around Chã das Caldeiras and the east flank of the island, were sampled for paleomagnetic purposes 40 historical and prehistorical lava flows, representing 55 sampling sites. Rock magnetic and petrographic analyses reveal as main magnetic carriers several phases of the titanomagnetite solid solution without visible effects of alteration, as expected for such young basaltic rocks and semi-arid climate. The anisotropy of magnetic susceptibility ellipsoid is low and no effect in the paleomagnetic directions is observed. Thermal and alternating field demagnetizations were successful, retrieving well clustered mean characteristic remanent magnetizations (ChRM) directions. In a first instance twelve ChRM of lava flows previously mapped as historical were compared with known models of secular variation, revealing that nine are in close agreement with the suspected ages while the remaining three failed that concordance, corresponding to prehistoric eruptions. Moreover, it is possible to verify that ChRM of prehistoric lava flows closely follow known secular variation curves showing a small and consistent angular difference. Therefore, such results contribute for a better constraint of the volcano stratigraphy of this island and supply new paleomagnetic directions needed to improve the accuracy of secular variation models.

This research has been funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – (https://doi.org/10.54499/UIDB/50019/2020), UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020). This work is a contribution to projects REGENA (Ref. PTDC /GEO-FIQ/3648/2012) and GEMMA (Ref. PTDC/CTA-GEO/2083/2021).

How to cite: Silva, P., Ramalho, R., Madeira, J., Moreira, M., Mata, J., Brum Silveira, A., and Foiada, S.: Archeomagnetic study at low latitude - North Atlantic Ocean (Fogo Island, Cabo Verde Archipelago), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13316, https://doi.org/10.5194/egusphere-egu24-13316, 2024.

Speleothems magnetism is considered an excellent recorder of short-lived variations of the Earth’s magnetic field. Here we provide new geochronological, petrographic, paleomagnetic, and rock magnetic data of a stalagmite from the Estremenho Limestone Massif (from Central Portugal). The stalagmite was dated at 114.7 (±1.5) k.y. B.P. to 106.3 (±1.6) k.y. B.P. based on the U-Th series. This time interval is coeval with the age of the Blake Geomagnetic Excursion (~116.5 ± 0.7 k.y. B.P. and 112.0 ± 1.9 k.y. B.P.). Petrographic observations under a binocular stereo microscope of two thin sections located at the lower part of the stalagmite and scanning electron microscopic observation of rock fragments show the alternance of porous and compact columnar calcite and dendritic calcite, which are typical features of primary calcite precipitation. No significant gap in the calcite growth was identified along the two thin sections. After alternating field demagnetization, a viscous secondary magnetization was cleaned below 10 mT, followed by a primary magnetization pointing to the origin. Magnetic vectors have an average declination of 60.3° with a mean inclination of 20.1°. No geomagnetic excursion or reversal was identified along the stalagmite. Analysis of Isothermal Remanent Magnetization curves shows the presence of two components. Component 1 has a mean coercivity of ~35 mT and DP ~0,20-0.25, typical of pedogenic/detrital magnetite, and contributed to 75% of the total remanence. Component 2 has a mean coercivity of ~95 mT and DP ~0.30, interpreted as being hematite, and contributing to 25% of the total remanence. We also calculated relative paleointensity (RPI) based on the Pseudo-Thellier method. The RPI curve exhibits a consistent and gradual increase from the base to the top of the stalagmite, with a pattern of variation comparable to the reference curves of marine sediments from the Portuguese margin, in the same time interval. In summary, petrographic and magnetic data point to a primary origin for the calcite precipitation and the corresponding remanent magnetization. The absence of the record of the Blake excursion in this stalagmite still remains to be resolved.

This work was supported by the SAMEPA project funded by FCT (ref. PTDC/CTA-GEO/0125/2021) and is part of the Ph.D. of Rafael Dinis (FCT-10216.2022.BD).

How to cite: Dinis, R., Font, E., Parés, J., Carmo, J., Trindade, R., Vilhena, L., Reboleira, A. S., Hasözbek, A., Jimenez-Barredo, F., and Carvalho, E.: Paleomagnetism and Relative Paleointensity of Estremenho Limestone Massif stalagmites, from Portugal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17017, https://doi.org/10.5194/egusphere-egu24-17017, 2024.

Retrieving precise information about the ancient geomagnetic field strength (paleointensity) is crucial for understanding the Earth's interior evolution. The Shaw-type method is one of the major protocols used to estimate paleointensity, characterized by relatively expeditious processes and high success rates. However, albeit employing strict selection criteria, Shaw-type paleointensity results occasionally exhibit bias, necessitating further methodological development. Despite the significance of identifying the causes of these biased estimates, there has been limited detailed discussion on this issue.

To clarify the culprit of bias in Shaw-type paleointensity, we conducted a pseudo-Tsunakawa-Shaw experiment using Permian basalts from the Tarim large igneous province (TLIP). In this experiment, the "NRMs (natural remanent magnetizations)" of the samples were acquired in a controlled lab field. We then monitored changes in various rock magnetic parameters, such as squareness (M_rs/M_s), coercivity (B_c), ARM, amtLTD (amount of remanence demagnetized by low-temperature treatment), MDF (median destructive field), and R (the ratio of thermal remanent magnetization (TRM) to anhysteretic remanent magnetization (ARM)), before and after heating, and compared them to the recovered paleointensities.

Our analysis uncover a proportional relationship between changes in R and the bias in paleointensity, likely attributed to variations in grain sizes. Combining with microscopic observations, we further reveal samples with particular magnetic grain sizes (~200 nm) are more prone to alteration in the integral TRM/ARM efficiency during heating. Building on these findings, we propose an innovative computational method exploiting the linear regression of R with various cut-off coercivity intervals for Shaw-type paleointensities (LoRA-Shaw), which yield accurate results in both laboratory-tested and modern lava samples. We also invoke curve fits for samples with “folding” phenomenon which are not suitable for linear regression. The LoRA-Shaw method combined with curve fits may help in mitigating bias from the thermal alteration and multi-domain effect in paleointensity study, and enhance the success rate in paleointensity determinations for constraining geomagnetic field evolution.

How to cite: Qi, K., Cai, S., Qin, H., Deng, C., Pan, Y., Yamamoto, Y., Cheng, X., Wu, H., and Zhu, R.: The Culprit of Bias in the Shaw-type Method of Estimating Geomagnetic Paleointensity and an Innovative Computational Method for Enhanced Reliability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9314, https://doi.org/10.5194/egusphere-egu24-9314, 2024.

This work, based on cartographic, morphologic, structural, paleo-magnetic, stratigraphic, and petro-chemical data, depict a new scenario of the Monte Somma growth and the volcanic evolution of this portion of Somma-Vesuvius volcanic complex. The volcanic growth is dominated by the migration of the volcanic source along three main tectonic trends (N040-050°, N070°, and N320°) that governed the formation of the poly-phased Somma caldera. Stratigraphic evidences, stressed by the UBSU use and by age clustering, reveal the occurrence of important climatic changes during Monte Somma formation, affecting its morphology and producing huge volcaniclastic glacial deposits. Magneto-stratigraphic data confirm the presence of an old edifice (Proto Somma Super-Synthem, 25,000 – 39,000 a BP) in the Cognoli di Trocchia area, beheaded by the Pomici di Base flank collapse about 22 ka ago. After this cataclysmic event, volcanic activity produced news volcanic morphologies (Somma Super-Synthem, 5,600 – 25,000 a BP). This second portion of the reconstructed and dated stratigraphic series is subdivided on the four main units: Cognoli di Santa Anastasia, Cognoli di Ottaviano, Cognoli di Levante, and Mercato Synthems. 

How to cite: Principe, C., Brocchini, D., Arrighi, S., Malfatti, J., Gogichaishvili, A., Cerbai, I., Devidze, M., Crocetti, S., Paolillo, A., La Felice, S., and Giordano, D.: Monte Somma magneto-stratigraphy and growth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21570, https://doi.org/10.5194/egusphere-egu24-21570, 2024.

Historical records of lunisolar eclipses and star occultations provide us useful information on the variation of the length of day (LOD) over the last 3,000 years. Besides a steadily increasing trend caused by tidal friction effects, there is evidence for a 1300-year oscillation in the LOD. An adequate explanation for this phenomenon requires considering an interaction between the Earth's mantle and core. Several mechanisms in which the geomagnetic field is directly or indirectly involved have been suggested, including electromagnetic coupling or angular momentum exchange. This study explores the potential connection between fluctuations in the length of day and the energy of the Earth's magnetic field. Through a frequency domain analysis and correlation tests, we examined non-tidal observations in LOD and geomagnetic field energy, using the latest archaeoreconstructions of the global magnetic field spanning the last 3,000 years. As a result, we have found a shared 1,300-year period in the LOD and energy of the non-axial terms of the Earth's magnetic field. These findings are interesting and open a possibility for further research in this field to validate and enhance our understanding of the nature and underlying causes of this potential connection between the Earth's magnetic field and the temporal variability in the LOD

How to cite: Puente-Borque, M., Pavón-Carrasco, F. J., Campuzano, S. A., Osete, M. L., González-López, A., and Folgueira, M.: Temporal variability in Earth's length of day and its connection with the geomagnetic field energy over the last 3,000 years, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19446, https://doi.org/10.5194/egusphere-egu24-19446, 2024.