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 χ_lf 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 χ_lf, SIRM, and χ_ARM 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 χ_lf and PM10 concentrations in the precipitation show relatively similar spatial trends, so χ_lf, SIRM, and χ_ARM 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. χ_lf, SIRM, and χ_ARM/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.