Magnetotactic bacteria produce biogenic magnetite in aquatic environments with reduced oxygen (O2) and high Fe concentrations. Increased biogenic magnetite contents in geological archives have been associated with marine deoxygenation, Fe fertilization and productivity. However, these conditions, which depend on amplified nutrient supply to marine settings, enhance organic matter production and subsequent magnetic mineral dissolution due to reductive diagenesis. This suggests that the paleoenvironmental significance of biogenic magnetite content variability remains elusive, and more records are needed to clarify the mechanisms that control the abundance of magnetotactic bacteria biomineralization products in sedimentary records. Accelerated nutrient input from the continents to the oceans and reduced seawater O2 concentrations are recurrent during global warming events due to temperature controls on both O2 solubility and hydroclimate. This has motivated the generation of multiple early Eocene biogenic magnetite records, which based on rock magnetic and electron microscopy experiments, have related biogenic magnetite contents with productivity and seawater O2 variability. Here, we present new geochemical and rock magnetic data from the Contessa Road section (Gubbio, Italy), which records a series of early Eocene global warming events. Our new data reveal that biogenic magnetite content variability cannot be directly used as an index for increased productivity and/or marine deoxygenation; alternatively, it can be seen an index for Fe availability in marine settings. Our observations indicate that biogenic magnetite does not exclusively reveal a specific process, which suggests that its content variability may depend on different local paleoenvironmental conditions.
How to cite: Piedrahita, V., Roberts, A., Rohling, E., Heslop, D., Galeotti, S., Florindo, F., Yan, L., and Li, J.: Biogenic magnetite as an index for Fe availability , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-464, https://doi.org/10.5194/egusphere-egu25-464, 2025.
Néel's theory of single domain magnetizations has been widely applied in paleomagnetism since its conception in the 1940s. When applying this theory, paleomagnetists typically assume that all magnetic particles are composed of magnetite, and are shaped like highly elongate needles. Even in the SD size range, natural samples exhibit a much wider range of morphologies, causing a gap between theory and experiments. Although these assumptions were necessary in the 1940s, computing power today means they are no longer required.
To understand the behaviour of other types of SD particles, we have created a new software package called the Single Domain Comprehensive Calculator (SDCC). The SDCC can derive the magnetic properties of SD particles from first principles, without making the assumptions of Néel. The package provides a simple python scripting interface to simulate almost any paleomagnetic experiments, with assemblages of particles of different sizes, shapes and compositions.
Simulations with the SDCC show that single domain particles exhibit a much wider range of experimental behaviour than has previously been discussed. In particular, we show that the relationship between blocking temperature and relaxation time is strongly dependent on the shape and composition of the particle. This causes viscously acquired remanences to unblock over a wide range of temperatures on laboratory timescales. To demonstrate this, we ran a simulation where an assemblage of SD particles acquired a viscous overprint over a primary thermal remanence. On simulated thermal demagnetization, the two magnetizations exhibited overlapping unblocking spectra, which would lead to problematic behaviour in paleomagnetic experiments.
How to cite: Cych, B., Paterson, G., Nagy, L., and Williams, W.: PSD: Possibly Single Domain?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10133, https://doi.org/10.5194/egusphere-egu25-10133, 2025.
Toxic metal content testing, environmental magnetic monitoring and in vitro bioaccessibility experiments each have their own advantages and are often used independently for environmental monitoring, but there are few studies that combine the three to evaluate the hazards of toxic metals to humans. This paper investigated the total content, magnetic properties and bioaccessibility of nine potentially toxic metal elements (Zn, Sn, Pb, Cu, Fe, Ni, Cr, Sr, Mn) in dustfall from different functional zones in Shanghai, China, and systematically compared the related results. The results show that these nine metal elements have different degrees of contamination and enrichment in outdoor dustfall, and their content distribution shows the following trend: Zn > Sn > Pb > Cu > Fe > Ni > Cr > Sr > Mn. Magnetic characteristics χlf and SIRM are mostly positively correlated with the metal elements, indicating that the higher the content of magnetic minerals in the sample, the higher the concentration of metal elements. It was also found that χlf, SIRM, and χARM can well reflect the characteristics of dustfall pollution. The magnetic minerals have a certain degree of enrichment, and the particle size of the magnetic minerals is relatively coarse, mainly in the form of coarse multi-domain and pseudo-single-domain particles, which are largely derived from anthropogenic pollution. The χlf and PM10 concentrations in the precipitation show relatively similar spatial trends, so χlf, SIRM, and χARM can be used as air pollution indices to facilitate the evaluation of metal elements pollution in dustfall. The overall trend in gastric bioaccessibility is Pb > Zn > Mn > Cu > Cr. Due to the increase in the pH of digestive fluid, the bioavailability of toxic metals decreases significantly from the gastric stage to the intestinal stage. χlf, SIRM, and χARM/SIRM are all related to the bioaccessibility of toxic metals in the intestinal stage, so they can be used as toxicity indicators to evaluate the bioaccessibility of toxic metals in dustfall.
How to cite: Wang, G., Yang, F., Wang, Y., Ren, F., Hou, Y., Su, S., and Li, W.: Magnetic response and bioaccessibility of heavy metal pollution inoutdoor dustfall in Shanghai, China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10426, https://doi.org/10.5194/egusphere-egu25-10426, 2025.
Marine magnetic anomalies provide fundamental information on plate tectonics, seafloor spreading, and geomagnetic field variations over time. It has long been debated whether the long-term variations of the marine magnetic anomalies are related to records of paleointensity variations or the progressive off-axis hydrothermal oxidation of magnetic minerals in mid-ocean-ridge basalts (MORBs). However, these processes are highly dependent on rock type and textures. Moreover, the style and extent of off-axis low-temperature alteration in MORBs varies and can be pervasive through the rock matrix or localized in vein halos, leading to significant heterogeneities in the upper crust. Studying the magnetic properties of both fresh and altered MORBs from transects covering a range of crustal ages is essential to assess and constrain the factors affecting the origin and evolution of magnetic anomalies.
In this study, we combined rock magnetic investigation with petrography and geochemical data to characterize the magnetic behaviour of basalts formed between ~7 and 61 Ma across the western flank of the Mid-Atlantic Ridge, recovered during the South Atlantic Transect (SAT) ocean drilling expeditions. This transect provides the opportunity to test the influence of primary and secondary factors on the magnetic signature of MORBs.
Our results show the strong influence of primary igneous features on the intensity of the remanent magnetization of MORBs. For example, the grain sizes and concentrations of magnetic minerals change according to different emplacement styles, with finer and coarser-grained magnetite in basaltic pillows and massive lava flows, respectively. Moreover, the combination of rock magnetic and major elements analyses reveals variation in primary and alteration mineral assemblages (clay minerals and goethite), composition (e.g., changes in Ti-content) and grain-size across the alteration halos, which in turn affects the intensity of the magnetic signature.
Overall, the variation of magnetic properties across the ridge flank testifies to a complex interplay of factors such as the nature of volcanic units and the style and evolution of hydrothermal alteration on the nature and long-term variation of the marine magnetic anomalies.
How to cite: Robustelli Test, C., Amadori, C., Harris, M., Belgrano, T., Jonnalagadda, M., Evans, A., Grant, L., Albers, E., Coggon, R., Teagle, D., and Zanella, E. and the The South Atlantic Transect IODP Expedition 390 & 393 Scientists: Unveiling factors affecting the magnetic signature of MORBs and their long-term variation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10493, https://doi.org/10.5194/egusphere-egu25-10493, 2025.
Understanding the nanoscale composition and exsolution behaviour of iron oxides is crucial for interpreting paleomagnetic signals and the thermochemical history of volcanic rocks. This study employs Atom Probe Tomography (APT) and Transmission Electron Microscopy (TEM) to investigate the nanoscale features of exsolved iron oxides within a volcanic basalt sample.
We investigate an intergrowth of ilmenite, titanohematite, and rutile, using 3D chemical mapping to precisely determine exsolution boundaries and quantify compositional transitions between the minerals with atomic-scale precision. TEM analyses complement these findings, offering structural and morphological context to the chemical variations. Preliminary results reveal distinct exsolution zones characterized by well-defined compositional gradients and interfacial features. These findings offer novel perspectives on the nanoscale architecture of exsolved iron oxides, providing a foundation for more accurate interpretations of paleomagnetic data.
While this study currently focuses on ilmenite-titanohematite-rutile systems, ongoing research will incorporate data from a magnetite-ilmenite assemblage, expanding our understanding of the broader mineralogical and magnetic structure of the sample. This research will be particularly valuable for researchers who may not be extensively familiar with the capabilities of APT in resolving nanoscale compositions and mineral phase relationships within geological samples.
How to cite: Gergov, H., Muxworthy, A., J. Harrison, R., and Williams, W.: Compositional insights into exsolved mineral phases: Atom Probe Tomography at the exsolution boundary., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19138, https://doi.org/10.5194/egusphere-egu25-19138, 2025.
Understanding how human activities mitigate the negative impacts of climate change and promote sustainable development is a critical scientific question (e.g., Lamb and Steinberger, 2017). Throughout history, humans have made many attempts to adapt to climate change, particularly through their adoption of settlement environments and the evolution of building materials (e.g., Olsson, 2024). The controlled use of fire, a hallmark of human civilization, has been instrumental in developing ceramic building materials since the Neolithic period (e.g., Jones, 2021). While numerous burned clay artifacts from the Neolithic period have been found worldwide (e.g., Pérez-Monserrat et al., 2022), little is known about their detailed characteristics and the potential influence of climate change on artificial fire usage. This study focuses on a Neolithic site in eastern Fujian, South China. By magnetic and geochemical analysis, we aim to reconstruct the firing temperatures and, consequently, the technological characteristics of artificial fire used in Neolithic house construction in the context of climate change. Magnetic results suggest an average ancient firing temperature of approximately 620°C, consistent with findings from other Eurasian Neolithic sites (e.g., Jordanova et al., 2020). The magnetic properties of burnt clay may reflect the characteristics of the in-situ clay source, offering insights into the geological background. However, the artificially controlled fires result in differences in the structure, geochemical characteristics, and color of the various layers of the burned clay. Furthermore, a potential link between temporal variations in ancient firing temperatures and surrounding paleoenvironmental changes is suggested, potentially influenced by feedback mechanisms between temperature-moisture conditions and human activities. In addition, this study would potentially contribute to further studies on artificial fire usage evolution in cultural relics from an interdisciplinary perspective under specific environmental conditions.
References
Jones, R., 2021. The Decoration and Firing of Ancient Greek Pottery: A Review of Recent Investigations. Advances in Archaeomaterials. 2, 67–127.
Jordanova, N., Jordanova, D., Lesigyarski, D., et al., 2020. Imprints of paleo-environmental conditions and human activities in mineral magnetic properties of fired clay remains from Neolithic houses. Journal of Archaeological Science: Reports. 33,102473.
Lamb, W. F., Steinberger, J. K., 2017. Human well‐being and climate change mitigation. Wiley Interdisciplinary Reviews: Climate Change. 8(6): e485.
Olsson, O. 2024. Human Capital Evolution in a Cooling Climate[M]//Paleoeconomics: Climate Change and Economic Development in Prehistory. Cham: Springer Nature Switzerland. 39-58.
Pérez-Monserrat, E. M., Causarano, M., Maritan, L., et al., 2022. Roman brick production technologies in Padua (Northern Italy) along the Late Antiquity and Medieval Times: Durable bricks on high humid environs. Journal of Cultural Heritage. 54,12-20.
How to cite: Li, G., Chen, G., Cheng, Z., Huang, Y., Fang, J., Ge, W., and Zhan, C.: Magnetic Constraints on the Role of Artificial Fire in Neolithic Burnt Clay in Southern China under Climatic Impacts, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5355, https://doi.org/10.5194/egusphere-egu25-5355, 2025.
Under certain conditions, buried rocks rich in organic matter can undergo spontaneous combustion if their temperature is raised to a certain ignition threshold and oxygen is supplied. Although spontaneous combustion is not very common, it has been described by several authors (Alastuey et al., 1993; Svensen et al., 2003; Abad et al., 2019). In the geological record, these phenomena can be observed due to the pyrometamorphic processes they generate, characterised by very high temperature and low pressure. When the altered rocks are clays, the result are natural clinkers. In this work we carry out a palaeomagnetic and rock magnetism study of these very rare materials. Specifically, we study stratiform bodies of baked clays by spontaneous combustion from 1) the Lower Cretaceous Escucha Formation near Utrillas (Iberian Ranges) and 2) lake sediments from the upper Miocene Molinicos Basin (Betic Cordillera).
Rock magnetism analysis indicates that natural baked clays are characterised by the presence of a very rare mineral phase of high coercivity, low unlocking temperature and thermal stability so-called HCSLT. This phase has only been described in controlled laboratory conditions (Petersen et al., 1987) and in some well-heated archaeological materials (McIntosh 2007). Recently, it has been shown that HCSLT is carried by ε-Fe2O3, a rare metastable polymorph of Fe2O3 with ferrimagnetic behaviour at room temperature (López-Sánchez et al., 2019; Dejoie et al., 2014). In the samples analysed, this phase coexists with magnetite, hematite or maghemite. The palaeomagnetic study indicates that the NRM is carried by the HCSLT phase with the same palaeomagnetic direction as the other high-temperature magnetic phases. The analysis of the directions of the characteristic component provides information about the timing of the spontaneous combustion that generated the carrier minerals and their magnetization.
References:
Abad, I., Sánchez-Gómez, M., Reolid, M., Sánchez-Vizcaíno, V.L. (2019). Minerals, 2019, 9(12), 748
Alastuey, A., Bastida, J., Fernández-Turiel, J.L., Querol, X., Signes, M. (1993). Cuadernos de Geología Ibérica, 17, 171–184.
Dejoie, C., P. Sciau, W. Li, L. Noé, A. Mehta, K. Chen, H. Luo, M. Kunz, N. Tamura, and Z. Liu (2014). Sci. Rep., 4, 4941
López-Sánchez, J., McIntosh, G., Osete, M.L., del Campo, A., Villalaín, J.J., Pérez, L., Kovacheva, M., Rodríguez de la Fuente, O. (2017). Geochemistry, Geophysics, Geosystems, 18 (7), pp. 2646-2656.
McIntosh, G., M. Kovacheva, G. Catanzariti, M. L. Osete, and L. Casas (2007). Geophys. Res. Lett., 34, L21302.
Petersen, N., N. Schembera, E. Schmidbauer, and H. Vali (1987). Phys. Chem. Miner., 14, 118–121.
Svensen, H.; Dysthe, D.G.; Bandlien, E.H.; Sacko, S.; Coulibaly, H.; Planke, S. (2003). Geology, 31, 581–584.Díaz García, F. (1993). Revista de la Sociedad Geológica de España 6, 105-114.
How to cite: Villalaín, J. J., Casas-Sainz, A. M., Sánchez-Gómez, M., Calvín, P., Yenes, L., Marcén, M., and Pérez-Valera, L. A.: Paleomagnetic and rock magnetic study of baked clays by spontaneous combustion, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18109, https://doi.org/10.5194/egusphere-egu25-18109, 2025.
The Quantum Diamond Microscope holds the potential to revolutionize our field of research. It enables measurements on micrometer scale, going beyond the measurements on bulk rocks that we as a field of research have been using so far. An inherent limitation of QDM observations, however, is that it measures magnetic flux in one direction, usually perpendicular to the surface of a sample. A QDM image is therefore a set of one-dimensional measurements, characterizing the magnetic flux in a plane very close to a polished surface of a sample. Following the potential theory of Kellogg, this geometry precludes making reliable three-dimensional interpretations of these magnetic measurements unless additional information is added.
The required additional constraints can be in the form of imposed boundary conditions, or additional data. If the magnetic sources are assumed to be well separated and of dipolar nature, it is possible to determine the magnetic moment and location of the magnetic sources. This works well in samples with a very low concentration of very small magnetic particles that behave single-domain-like, such as speleothems. In the vast majority of geological materials, however, the sources are not sufficiently well isolated, and complex magnetic anomalies as measured from especially the larger particles are expressions of multidomain states in the grains. Such samples require additional data to reliably determine the magnetic moments of the magnetic grains in them.
Micromagnetic Tomography combines QDM data with information on the location of the magnetic sources in the sample material that is obtained by NanoCT scans. NanoCT measurements can identify iron-oxide bearing grains by their attenuation contrast that is much higher than the other minerals present in geological materials. Currently it is possible to routinely acquire NanoCT scans with a resolution down to 350 nm, enabling the identification of potentially magnetic grains well into the range of vortex domain states.
In this contribution we will present the potential and pitfalls of the interpretation of QDM data, the current state-of-the-art of Micromagnetic Tomography, and their practical applications for paleomagnetic studies on various types of geological samples.
How to cite: de Groot, L., Out, F., de Boer, R., Kosters, M., Bian, G., Cortés-Ortuño, D., and Fabian, K.: The Potential of the Quantum Diamond Microscope and Micromagnetic Tomography for Paleomagnetic Measurements, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17536, https://doi.org/10.5194/egusphere-egu25-17536, 2025.
The behaviour of the geomagnetic field through time provides insight into the formation and evolution of Earth’s interior. However, for certain geological periods the Earth’s magnetic field behaviour is poorly understood. For instance, the Devonian period remains enigmatic as palaeomagnetic records of this time are ambiguous. Devonian bulk rock samples often yield scattered magnetic directions, even when ideal magnetic carriers are present. This may be explained by partial remagnetisation or by complex, non-dipolar magnetic field behaviour.
We conducted a palaeomagnetic study on Middle Devonian pillow basalts from the Rhenish Massif, Germany and encountered inconclusive bulk magnetic behaviour. Directions obtained through alternating field (AF) demagnetisation are scattered and do not cluster around a common mean. While some of these results can be attributed to partial overprints, not all findings can be explained with this mechanism. To investigate this further, we analysed the individual magnetic grain behaviour in the samples using Micromagnetic Tomography (de Groot et al., 2018; 2021).
With MMT, we measured the natural remanent magnetisation (NRM) of the samples, and we conducted an AF demagnetisation sequence. With this, we studied the individual grains in the samples across five demagnetisation states. This approach aimed to identify the magnetically stable grains in the samples that are capable of preserving interpretable geomagnetic signals.
Our findings may improve the understanding of Middle Devonian geomagnetic field behaviour and enable the use of previously uninterpretable samples in reconstructing long-term geomagnetic field behaviour.
How to cite: de Boer, R. and de Groot, L.: Deciphering Middle Devonian geomagnetic field behaviour through analysis of individual remanence carriers., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19482, https://doi.org/10.5194/egusphere-egu25-19482, 2025.
The 2024 Ma Vredefort (South Africa) impact structure comprises a 40-50 km central region of Archean basement rocks surrounded by a 15-20 km wide collar of late Archaean to early Proterozoic Witwatersrand Supergroup sedimentary and volcanic rocks. The collar is characterized by a ring of prominent, negative (<-5,500 nT) aeromagnetic anomalies surrounding much of the structure where the strata dip near-vertical to overturned. To better understand the origin of this magnetic feature, we undertook a ground magnetic survey throughout the Vredefort structure using a three-axis fluxgate magnetometer mounted on a mountain bicycle. The upward continuation of our profiles to 150 m matches the aeromagnetic data in shape and amplitude. The near-ground magnetic measurements allowed us to pinpoint the rocks responsible for the intensely negative anomalies. Field observations and microfabric analyses of six outcrops confirmed that the magnetic signal correlates with specific metamorphosed banded iron formations (BIFs) at the base of the supergroup, 10 to 100 m thick, as the main producer of the strongly negative magnetic anomalies. Paleomagnetic samples collected from the rocks at the surface that produce the most intense anomalies (up to -22,000 nT) have extremely high natural remanent magnetization intensities (up to >1000 A/m) likely arising from lightning strikes. Stepwise demagnetization and rock magnetic experiments distinguish one site as likely having escaped remagnetization from lightning that possesses the established 2.02 Ga paleodirection at Vredefort. Thermoremanent magnetizations (TRM) imparted on 41 samples using a 40 μT field yielded an average intensity of 25 A/m. Using the results of TRM experiments and the paleodirection enabled us to successfully model the prominent negative anomalies in the metasediments only when accounting for the post-impact orientation of the BIFs. We interpret the strongly negative magnetic anomaly in the collar region as being formed directly after crater exhumation and uplift of the rocks. This interpretation implies that Bushveld-related metamorphism created the up to mm-sized magnetite and garnet crystals in the BIFs, which resided at temperatures higher than the Curie temperature of magnetite (580°C) until the impact rapidly brought the BIFs close to the surface where magnetite cooled to acquire a thermal remanence in the 2.02 Ga field.
How to cite: Sleptsova, I., Gilder, S., Le Goff, M., Dellefant, F., Trepmann, C., Lhuillier, F., and Webb, S.: Explaining the Intensely Negative Magnetic Anomalies in the Vredefort Impact Structure, South Africa, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4372, https://doi.org/10.5194/egusphere-egu25-4372, 2025.
Posters on site: Thu, 1 May, 10:45–12:30 | Hall X3
Magnetic particles trapped into speleothems usually originate from the soils capping the cave and are transported into the cave by dripwaters. However, authigenic magnetic particles may also precipitate under conditions likely to prevail during speleothem growth. Here we investigate the magnetic mineralogy of a stalagmite from the Cerâmica Cave, Central Portugal, characterized by brown to red calcite laminations. We also analyzed the host carbonate, the cave sediments, and the soils capping the cave. We measured concentration- and grain size-dependent magnetic proxies, including natural remanent magnetization, anhysteretic remanent magnetization, isothermal remanent magnetization, mass-specific magnetic susceptibility, FORC, and hysteresis curves. Results show that magnetic and hematite are the main magnetic carriers in all samples. A gradual enrichment of hematite relative to magnetite is observed following the transportation path from the soils to the cave sediments up to the stalagmite. The higher contribution of hematite relative to magnetite in the speleothem may reflect the precipitation of authentic hematite during speleothem growth or the selective transport of finer particles from the soil to the cave.
This project is funded by Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), and PTDC/CTA-GEO/0125/2021, and is part of the Ph.D. of Ana Raquel Bras (2024.03482.BD).
How to cite: Brás, A. R., Font, E., Feinberg, J., Dinis, R., Reboleira, A. S., and Adatte, T.: Origin of hematite in a stalagmite from the Cerâmica Cave, Central Portugal, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20296, https://doi.org/10.5194/egusphere-egu25-20296, 2025.
Speleothems can provide high-resolution records to reconstruct the ancient Earth's magnetic field. However, little is known about the influence of speleothem morphologies on the natural remanent magnetisation record. A previous study by Ponte et al. (2017) showed that the magnetic inclination recorded in stalagmites decreases according to the slope of the calcite laminae. Magnetic inclination is 5º lower at the bottom of the stalagmite than at the top. The authors suggest that magnetic particles are re-orientated due to particle rolling during the deposition onto the flank of the stalagmite, resulting in magnetic inclination shallowing. In this scenario, magnetic inclinations are expected to be deeper on the other flank of the stalagmite. Still only half of the stalagmite was available for the study of Ponte et al. (2017). Here, we present new data on a cone-shaped stalagmite from Central Portugal, labelled LM1. We use alternating field demagnetisation, anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetisation (AARM), and X-ray microtomography to investigate the orientation of the calcite and magnetic fabrics along the flanks of three detrital-rich calcite layers and their relationship with paleomagnetic directions. We show that magnetic inclination varies up to more than 10° depending on the slope of the calcite laminae. In contrast, magnetic declination is almost constant, corroborating previous results by Ponte et al. (2017). Orientation of the k1 of the calcite fabric determined by AMS shows a striking correlation with the k1 of the magnetic fabric obtained by AARM, suggesting that the orientation of the magnetic particles is mainly controlled by the growth direction of the calcite crystals, which is perpendicular to the surface of the stalagmite. However, magnetic inclinations are not symmetrical between both flanks, suggesting that the particle rolling hypothesis is invalid in this case. In addition, the fact that the maximum variations of the remanent magnetic inclination do not exceed 10º indicates that not all the magnetic carriers are reoriented according to the calcite fabric and that the mechanisms responsible for the acquisition of the detrital remanent magnetisation are more complex than previously thought.
Acknowledgments: PTDC/CTA-GEO/0125/2021 Foundation of Sciences and Technology (Portugal) and BU037P23 Junta de Castilla y León (Spain). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101005611 for Transnational Access conducted at DMEX-UPPA-FRANCE.
How to cite: Sánchez-Moreno, E. M., Font, E., Calvín, P., Faucher, S., Sénéchal, P., and Dimuccio, L. A.: New insights into the origin of magnetic inclination shallowing in stalagmites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18301, https://doi.org/10.5194/egusphere-egu25-18301, 2025.
The Moinhos Velhos-Contenda Cave, located in the Estremenho Karst Massif, Central Portugal, is the second-largest and the most visited cave of Portugal. Here we provide new paleomagnetic, rock magnetic and mineralogical data from a stalagmite (GMA-III) collected in Moinhos Velhos Cave, at about ~60m depth. Besides, cave sediments, surface soils and another (clean/whitish) stalagmite (GMA-IV) was collected. This speleothem is mushroom-shaped with dark-reddish laminations, hypothesized to be related to flooding periods. However, how the flooding events are recorded in the calcite laminae of the stalagmite and how the respective conditions affected the quality of the paleomagnetic signal is poorly constrained. After stepwise alternating field demagnetization, the viscous remanent magnetization was cleaned below 10mT, followed by a natural remanent magnetization pointing to the origin. After 100mT, more than 90% of the magnetization was cleaned, suggesting low to medium coercive magnetic minerals as main magnetic carriers of the natural remanence. Sample-based mean directions show an average declination of ~353º and a mean inclination of 38.5º. Isothermal Remanent Magnetization (IRM) curves shows the presence of two components – the first one with a mean coercivity of ~30mT and a DP of ~0.26, typical of pedogenic/detrital magnetite, contributing to ~94% of the total remanence; and the second one with a mean coercivity of ~245mT and a DP of ~0.30, interpreted as being hematite, contributing to ~6% of the total remanence. Plot of kARM/IRM versus the mean demagnetization field (MDF) show three distinct clusters, corresponding to the GMA-III and GMA-IV stalagmites, while the third cluster corresponds to the soils and cave sediments. All clusters are close to the pedogenic-detrital zone of the diagram. Notably, the GMA-III stalagmite has significantly lower values of kARM/IRM than the GMA-IV stalagmite. Cisowski test on GMA-III samples shows an interaction degree close to 0.5, suggesting the presence of non-interacting single-domain magnetite. Under binocular microscope, the reddish horizons show the presence of opaque materials and dissolution features, suggesting the occurrence of hiatus of precipitation. Analysis of these horizons under scanning electron microscopy (SEM) coupled to energy dispersive spectra show the presence of large (~50-100 micrometers) crystals of Ti-bearing iron oxides, quartz, and carbon-rich particles. Sylvite is also observed in the form of small (~2 micrometer) crystals and filling fractures, suggesting evaporitic conditions. Polarized light observations indicate that Ti-iron oxides correspond to titano-hematite, probably with a detrital origin. Oil immersion objective microscopy observation of carbon-rich particles classifies them as fossil organic matter particles, with grain size between 60 to 300 micrometers. The association of hematite, quartz grains and organic matter particles deposited onto dissolution surfaces confirm a detrital origin, presumably during flooding periods. Although magnetite was not observed under SEM and optical microscope, it carries the natural remanent magnetization, suggesting that hematite deposited by flood events has no or few implications in the quality of the paleomagnetic directions.
This project is funded by Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), and 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., P. S. Reboleira, A. S., Ribeiro, J., Gomes, E., and Vilhena, L.: Mineralogical and magnetic evidence of flooding events recorded in a stalagmite from the Moinhos Velhos-Contenda Cave, Portugal, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13770, https://doi.org/10.5194/egusphere-egu25-13770, 2025.
Reconstructing the North Atlantic Deep Water (NADW) formation and strength over time is fundamental to understanding North Atlantic and global ocean circulation and climate evolution. A multi-proxy approach on sedimentary core records can provide the temporal resolution needed to investigate NADW history for the last 12 Ma (from Upper Miocene to Pleistocene).
Here, we focus on the Björn Drift, located in the eastern flank of the Reykjanes Ridge, where the accumulation rate and provenance of sediment in the Iceland-Scotland Overflow Water (ISOW) path are sensitive to variations in the strength of NADW production as well as sediment production processes. IODP expeditions 384, 395C, and 395 drilled site U1554 (holes A-H) on the Björn Drift (60°7.5'N, 26°42.1'W, 1870 mbsl). A preliminary biostratigraphic and magnetostratigraphic age model defines the age of the basement-sediment interface at approx. 12 Ma and a maximum ≥10 cm/kyr accumulation rate. Preliminary rock magnetic analyses on selected discrete samples consist of bulk magnetic susceptibility, Anysteretic remanent Magnetization (ARM), and Isothermal Remanent Magnetization (IRM). These are compared with Nd-Sr isotopes (measured on the same samples) and used as proxies to examine sediment provenance in the Björn Drift. The aim is to determine changes to the endmember components before and after drift development. Correlations of these components with previous studies targeting IODP and ODP cores from the North Atlantic suggest that terrigenous sediment has a dominance of Icelandic source contribution in interglacial intervals and a dominance of European continental source contribution during glacial intervals.
How to cite: Di Chiara, A., Dweyer, D., Friedman, S., Satolli, S., Bonilla-Alba, R., Hemming, S. R., Rasbury, T., Karatsolis, B. T., Sinnesael, M., Parnell-Turner, R., Briais, A., Levay, L., and 395Scientists, E.: Rock magnetic proxies to infer terrigenous provenance variation on Björn Drift IODP Site U1554, preliminary results., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21031, https://doi.org/10.5194/egusphere-egu25-21031, 2025.
We investigated the rock magnetic and geochemical properties of a collection of pumice samples gathered from the shores of Japan and France. These pumices exhibit a vitreous matrix with a foam-like texture, resulting in low density that allows them to float and drift far from their point of origin to distant coastlines.
A range of analytical techniques was employed, including major and trace element geochemical analyses, SEM and FEG-SEM imaging, EDS-X analyses, and rock magnetic experiments such as thermomagnetic analyses, hysteresis loops, coercivity measurements, and FORC diagrams. The collected pumices display contrasting geochemical signatures and magnetic properties.
The origin of the Japanese pumices was unambiguously traced to the 2021 Fukutoku-Okanoba (FOB) eruption. In contrast, the origin of the pumices found along the French coastline remained uncertain prior to this study. Their geochemical and magnetic properties place most of them slightly outside the range of natural volcanic pumices. We therefore favor an anthropogenic origin.
The concentrations of elements such as arsenic, mercury, and nickel were analyzed to assess potential biohazard risks associated with these pumices. While some metal levels, such as barium and nickel, were found to exceed European hazardous limits, they remain at trace levels. To further explore possible sources, we used oceanic current modeling, which helped us narrow down likely points of origin for these anthropogenic pumices. This finding raises questions about the drift of floating materials across oceans and their impact on coastal environments.
How to cite: Carlut, J., Isambert, A., Garcia da Fonseca, G., Carvallo, C., Fluteau, F., Besson, P., and Libier, Y.: The Journey of Pumice: Rock Magnetic and Geochemical Insights from Coastal Stranding Samples, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17203, https://doi.org/10.5194/egusphere-egu25-17203, 2025.
Paleomagnetic signals from bulk rock samples are often affected by the magnetic properties of individual minerals and the limitations of laboratory methods. Focusing on understanding single magnetic grains enhances the reliability of these signals and refines their interpretation. Advances in micromagnetometry have significantly improved magnetic field imaging capabilities, enabling detailed analyses at the grain level. In this study, quantum scanning microscopy (QDM) and magnetic force microscopy (MFM) were employed to measure magnetic stray fields and domain states of a ~4.5 µm3 large single magnetite grain. Micromagnetic simulations using MERRILL (Conbhuí et al. 2018) were conducted to investigate the grain’s magnetic configurations under varying external fields. The aim was to understand the grain's magnetic behavior in response to these fields and to establish correlations between experimental observations and simulation results.
Four hundred simulations revealed that the magnetite grain, due to its size, symmetry and crystallographic orientation, developed multi-vortex structures with magnetization concentrated near its boundaries. The vortex cores exhibited intricate configurations and were not aligned with specific crystallographic axes. Mean magnetization varied by approximately 38% after exposing to the external field ranged from 100 to 150 mT. The average dipole moment orientations shifted by up to 40° across this field range. After exposure to an external field of 700 mT, the dipole moment orientations became highly stable.
Surface scans showed the evolution of magnetic domains under different external field strengths, particularly at scan heights below 400 nm. At 300 nm, the stray field intensity reached 5 mT, with Bz extremes localized at the grain corners. At scan heights exceeding 500 nm, the stray field patterns transitioned into dipole configurations, obscuring domain-level details. Despite variations in domain structures, dipoles exhibited consistent shapes at higher scan heights. These findings highlight the magnetite grain's response to external fields, with simulations providing valuable insights into its magnetic domain state developments. Systematic analyses with experiements and simulations will enable the differentiation of magnetically stable and unstable grains based on their shape, size, and composition, improving the assessment of magnetic grains as reliable paleomagnetic recorders.
Reference: Ó Conbhuí et al. (2018). Geochemistry, Geophysics, Geosystems, 19, 1080–1106.
How to cite: Bian, G., Williams, W., de Boer, R., Fabian, K., and de Groot, L.: Magnetic Behavior of Single Magnetite Grains: Insights from Micromagnetics and Micromagnetometry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17992, https://doi.org/10.5194/egusphere-egu25-17992, 2025.
Magnetic hysteresis measurements are widely used in Earth and planetary sciences with the aim of identifying geologically meaningful magnetic recorders, and to study variations in present and past environments. The interpretation of hysteresis data in terms of domain state and paleomagnetic stability are major motivations behind undertaking these measurements, but are fraught with challenges and ambiguities. We have undertaken a systematic micromagnetic study to quantify the magnetic hysteresis behavior of room-temperature magnetite as a function of particle size (45–195 nm; equivalent spherical volume diameter) and shape (oblate, prolate and equant); our models span uniformly magnetized single domain (SD) to non-uniformly magnetized single vortex (SV) states. Within our models the reduced magnetization associated with SV particles marks a clear boundary between SD (≥0.5) and SV (<0.5) magnetite. We further identify particle sizes and shapes with unexpectedly low coercivity and coercivity of remanence. These low coercivity regions correspond to magnetite particles that typically have multiple possible magnetic domain state configurations, which have been previously linked to a zone of unstable magnetic recorders. Of all the hysteresis parameters investigated, transient hysteresis is most sensitive to particles that exhibit such domain state multiplicity. When experimental transient hysteresis is compared to paleointensity behavior, we show that increasing transience corresponds to more curved Arai plots and less accurate paleointensity results. We therefore strongly suggest that transient behavior should be more routinely measured during rock magnetic investigations.
How to cite: Paterson, G., Moreno, R., Muxworthy, A., Nagy, L., Williams, W., and Tauxe, L.: Magnetic Hysteresis Properties of Magnetite: Trends With Particle Size and Shape, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18553, https://doi.org/10.5194/egusphere-egu25-18553, 2025.
Nihewan Basin is one of a series of well-developed East Asian Cenozoic basins in Hebei Province, North China, which are rich sources of mammalian faunas and Paleolithic sites. During the past decades, detailed magnetostratigraphic dating was conducted on the Nihewan Formation and associated mammalian faunas and Paleolithic sites, and the results have contributed significantly to our understanding of the chronostratigraphy of the Nihewan Basin. However, the information about the local regional environments in the Nihewan Basin is limited. In this study, 456 oriented samples were collected from the fluvio-lacustrine sequences of Xiashagou section (thickness 260 m) for detailed rock magnetic and magnetic fabric studies. We try to rebuild the environmental evolution process of Xiashagou area and then discuss the evolution of the paleolake, and provide some new instructions for the environmental changes in Nihewan Basin. We selected 12 typical samples to measure hysteresis loops，isothermal remanence magnetization acquisition and its back-field demagnetization characteristics. The results show that the magnetic minerals in the fluvio-lacustrine sequences are mainly Pseudo-Single Domain (PSD) magnetite particles and a small amount of hematite. The results of magnetic fabric are typical for a primary sedimentary magnetic fabric, which show that the original sedimentary fabric has been preserved in the Xiashagou deposits.
The magnetic foliation (F) of sediments was found to be larger than the magnetic lineation (L). F and anisotropy degree (Pj) and the flow velocity function (Fs) in the magnetic fabric parameters are more obviously sensitive to environmental changes. There is no long-term stable flow direction in Xiashagou area. According to magnetostratigraphic age framework of fluvio-lacustrine sequences in the Xiashagou section and the top age of fluviol-acustrine sediments in the Haojiatai section, the age of the magnetic fabric parameters and magnetic susceptibility change curves in the Xiashagou section is from 2.7 Ma to 0.3 Ma. The analysis of the lithology and the periodic changes of the parameters of the Xiashagou section showed that the environment of the lake has undergone several significant changes during the period of 2.7～0.3 Ma，which occurred at 2.4 Ma，1.9 Ma，1.3 Ma and 0.9 Ma. So the environmental evolutions of the Nihewan paleolake are divided into five stages.
How to cite: Liu, P., Li, J., and Gao, X.: Charcteristics analysis of magnetic fabric in Xiashagou section and the environmental changes of the Nihewan Lake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16484, https://doi.org/10.5194/egusphere-egu25-16484, 2025.
Marine anoxia events, as indicated by geochemical proxies, have been widely studied as a possible mechanism driving the end-Permian mass extinction, which also inhibited the subsequent biotic recovery. However, redox changes in the aftermath of extinction and their further ecological effects are still controversial. Here, we report the characteristics of rock magnetic changes in several sections of the Permian-Triassic transition in South China and reinterpret the changes in redox conditions during this period. Our study shows that the magnetic mineral assemblages and magnetic characteristics considerably altered in the aftermath of the extinction, implying significant environmental changes. In microbialite sections developed in the shallow-water carbonate platform after the extinction, the variation of magnetic mineral assemblages records the fluctuated redox conditions under oxic and euxinic conditions, which explains the cleavage between continuous oxic or euxinic conditions suggested by paleontological and pyrite framboid evidence in earlier work, respectively. Due to the sensitivity of magnetite and hematite to the redox change, their preservation and the relative change between these two minerals have the potential to reflect the redox state. Hence, we propose a new magnetic proxy to reflect the redox conditions in the ancient ocean. This proxy correlates well with other geochemical redox proxies and the abundance changes of aerobic fossils, supporting its considerable reliability. In all, our study indicates that the microbialites were affected by frequent anoxia events while the photosynthetic oxygen production of cyanobacteria in the microbialites may be the key factor for buffer effect. Under the rapid increase in ocean temperature and an overall seawater oxygen depletion during the Permian-Triassic transition, microbialite and the microbes within it thus provide a more favorable habitat for the surviving metazoans.
How to cite: xiong, W. and wang, T.: Fluctuated redox condition in Lower Triassic microbialite: evidence from magnetic mineral assamblage, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-805, https://doi.org/10.5194/egusphere-egu25-805, 2025.
