Jean-Pierre Valet started his CNRS research career in 1982 at the “Centre des Faibles Radioactivités” (Gif sur Yvette). His Doctorate studies, led by C. Laj, delt with the detailed study of the Earth’s magnetic field reversals recorded in the Mio-Pliocene marine sediments of Crète (Eastern Mediterranean sea). After his Doctorate thesis held in 1985, he spent one year in the paleomagnetism laboratory led by L. Tauxe at the Scripps Institution of Oceanography (California). From 1990 Jean-Pierre Valet joined the Geomagnetism and Paleomagnetism Team of the « Institut de Physique du Globe de Paris » as Director of research and joined coring and drillin campaigns on the R.V. Marion-Dufresne and ODP/IODP cruise of the Joide-Resolutin. Studying magnetization of marine sediment sequences, he and his team deeply improved the understanding of the dynamics of polarity changes and provided robust stacked records of the geomagnetic dipole moment variation over the last 2 Ma (e.g. Sint-2000). They revealed a long term decay of the dipole field intensity prior the reversals, followed by rapid return to strong dipole field intensity after the reversals, suggesting a progressive loss of the geodynamo energy followed by a strong and rapid gain of energy drawn from the reversal. In 2010, Jean-Pierre joined a project developed since 2000 at CEREGE (Nat. Lab. Cosmogenic nuclides) by N.Thouveny and Didier Bourlès (1955-2021), conceived to decipher geomagnetic dipole intensity lows and highs using both paleomagnetism and accelerator mass spectrometry for detection of the cosmogenic isotope Be-10. From 2015 to 2020, this project was led by Jean-Pierre with 2.5 M€ funding by the European Research council. The final results describe and critically analyze the 4 Ma proxy records of the geomagnetic dipole variations produced both by sediment magnetization paleointensity records and by cosmogenic Beryllium production records tuned by the magnetospheric screening of cosmic rays (e.g. Valet et al. 2024, Valet et al., submitted). Jean-Pierre Valet authored about 200 peer-reviewed articles and directed 12 PhD. CNRS Silver medal (2001), Fellow of the American Geophysical Union since 2003, Petrus Peregrinus Medal of the European Union of Geosciences (2010).
How to cite: Thouveny, N.: 4 Million Years of stable and unstable geomagnetic polarity states: a tribute to Jean-Pierre VALET, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3227, https://doi.org/10.5194/egusphere-egu25-3227, 2025.
A fundamental problem of relative paleointensity (RPI) estimations using marine sediments is that magnetic compositional variations induced by environmental changes may influence RPI estimations, which is known as climatic contamination or lithological contamination. Valet et al. (2011) demonstrated this problem by isolating a climatic contamination component from some RPI records using principal component analysis. In this presentation, we discuss the origin of the climatic contamination and a possible way to mitigate it. Recent RPI studies often show a negative correlation between RPI and magnetofossil abundance estimated from the ratio of ARM susceptibility to SIRM and FORC diagrams. This indicates that PRI recording efficiency of the magnetofossil component is lower than that of the detrital magnetic component. This implies that changing proportion of the magnetofossil component to the detrital component in magnetic mineral assemblages of sediments can be a source of the climatic contamination. When the coercivity distributions of the magnetofossil and detrital components are different, NRM-ARM demagnetization diagram (a variant of the pseudo-Thellier plot of Tauxe et al., 1995) shows curvature, and less contaminated RPI estimations may be possible by choosing a coercivity fraction reflecting the magnetofossil or detrital components on calculating a best-fitting slope. Linear NRM-ARM demagnetization diagram was sometimes used as a criterion for reliable RPI estimations, but largely overlapping coercivity distributions of the two components yield linear NRM-ARM demagnetization despite changing proportion of the two components results in contaminated RPI.
How to cite: Yamazaki, T., Li, J., Inoue, K., Shimono, T., and Kanamatsu, T.: On the climatic contamination of relative paleointensity estimations from sediments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5272, https://doi.org/10.5194/egusphere-egu25-5272, 2025.
In 1999, Jean-Pierre Valet and Vicente Soler published a study to explore the magnitude of deviations in direction and intensity of the ambient magnetic field induced by the crustal field and the possible consequences on paleomagnetic records. They measured, over more than two years, the total magnetic field above the surface of 12 lava flows in the islands of La Palma and Tenerife (Canary Islands, Spain) and observed that mean directions above the different flows showed values differing by 9° in declination and 6.5° in inclination. Also, virtual dipole moments differed by more than 10%. They concluded that paleomagnetic records could be significantly affected by this effect. Paleomagnetic measurements of a large number of samples or a spread-out sampling of lava flows would provide the best way to average out the contribution of anomalies.
In a recent paleomagnetic and multimethod paleointensity study performed on rocks from a lava flow erupted on December 4th, 2021, in the island of La Palma, paleomagnetic results were obtained from a large number of samples, yielding a mean palaeomagnetic direction in excellent agreement with the actual IGRF-13 value. In addition, paleointensity determinations were carried out on 25 specimens, yielding results in agreement with the expected value in 60 per cent of the studied cases. The use of a relatively high number of specimens and the sampling performed across the whole thickness of the flow might have been useful to reduce the effect of local magnetic anomalies.
How to cite: Calvo-Rathert, M., Vernet, E., Soler, V., Parés, J. M., and Carrancho, Á.: Magnetic anomalies in lava fields and consequences on the reliability of the palaeomagnetic signal recorded in lava flows: results from the Canary Islands, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5983, https://doi.org/10.5194/egusphere-egu25-5983, 2025.
Reconstructing the past geomagnetic field variations is a key point for understanding the dynamo processes that take place in the Earth’s outer core and for deciphering the field’s behavior over centennial and millennial scales. For this purpose, well-dated paleomagnetic data from volcanic rocks and archeological artifacts is used, coming from different parts of the world and contributing to the calculation of regional and global geomagnetic field models. Japan, located in the eastern part of Asia and characterized by intense volcanic activity and long cultural heritage, represents a precious source of data for geomagnetic field reconstructions. This study aims to provide an updated overview of all the available paleo- and archeo-magnetic data from Japanese volcanos and archeological sites. Particular focus is given on the quality of the data, mainly regarding the paleointensity records, which present higher dispersion in respect to the directional data. A total of 303 directional and 135 intensity data are available for the last 3,000 years, most of them coming from archeological material such as kilns and pottery while 45 records come from volcanic rocks. Only 24 data offer full geomagnetic field determinations, including both direction and absolute intensity, most of them coming from in situ lava flows. The majority of the paleointensity data are obtained through the classical Thellier-Thellier method and its modifications, but in their great majority miss cooling rate and anisotropy corrections. The Japanese data are compared with data from nearby countries and geomagnetic field model predictions and are used to identify geomagnetic field characteristics such as abrupt changes and anomalies.
This study has been partially financed by the European Union’s Horizon 2020 Research and Innovation Programme Excellent Science, under the Marie Skłodowska-Curie RISE Action ‘BEYOND ARCHAEOLOGY’ (no. 823826).
How to cite: Tema, E.: Directional and intensity paleosecular variation data from Japan during the last three millennia: an overview, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6092, https://doi.org/10.5194/egusphere-egu25-6092, 2025.
The history of Earth’s magnetic field can provide insight into how life evolved, an area Jean-Pierre Valet studied in his insightful suggestion of a relationship between the Laschamp excursion and Neanderthal extinction. Here, I will discuss how recent findings show potential linkages on longer time scales between magnetism and the origin of life, animal evolution and habitability. The study of magnetism held by minute magnetic inclusions in zircons indicates that the geomagnetic field is at least 4200 million years old, which corresponds in time with genetic estimates for the age of the last universal common ancestor. The early establishment of the field would have provided shielding from solar and cosmic radiation, broadening the potential environments where life might have originated. At the end of the Precambrian, at ca. 591 to 565 million years ago, the magnetic field nearly collapsed, but growth of the inner core during earliest Cambrian times renewed the magnetic field and shielding, helping to prevent drying of the planet. Before this renewal, the ultra-weak magnetic shielding may have had an unexpected effect on evolution. The extremely weak field could have enhanced hydrogen escape to space, leading to increased oxygenation of the atmosphere and oceans. In this way, Earth’s magnetic field may have assisted the radiation of the macroscopic and mobile animals of the Ediacara Fauna. Whether the Ediacara Fauna are genetically related to modern life is a matter of debate, but if so the magnetospheric control on atmospheric composition may have led to an acceleration in evolution that ultimately resulted in the emergence of intelligent life.
How to cite: Tarduno, J.: Earth’s magnetic field and the evolution of life, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7304, https://doi.org/10.5194/egusphere-egu25-7304, 2025.
Over the past five years, J.-P. Valet actively collaborated with the authors on studying long-term climate variations recorded in Chinese red clay deposits. Notably, our joint research identified the 1.2 Myr band of Earth-Mars obliquity modulation in the Late Miocene. Building on this foundation, we extend our investigation to red clay sequences in Tibet, employing magnetostratigraphy and rock-magnetic analyses to date the sections and extract paleoclimate signals influenced by orbital parameters.
The Eocene epoch witnessed significant global climate cooling and aridification, particularly near the Tibetan Plateau. Previously, the aridification was attributed primarily to the plateau’s uplift. However, recent studies suggest that global climate trends played a more substantial role. Here, we present new paleoclimate data from the Altun Shan red clay sequence, deposited between 40 and 50 Ma at the northeastern edge of the Tibetan Plateau. By constructing an age model using a synthesis of magnetostratigraphy and cyclostratigraphy, we show that variations in magnetic susceptibility in the Altun Shan sequence are linked to eccentricity cycles. The ~100-kyr short eccentricity cycle can be compared to marine climate proxy records, with 405-kyr eccentricity and 173-kyr obliquity cycles modulating the record amplitudes. The amplitude of the short eccentricity cycles decreased starting at the onset of global cooling at 49.1 Ma. Furthermore, we demonstrate that the aridification event in Altun Shan aligns with several key climate records: a cooling interval in the global oxygen isotope record, a sea surface temperature drop on the east Tasmanian Plateau, and increased aridity in Central Asian sedimentary basins. The middle Eocene aridity and cooling peaked between 45.5 and 44 Ma, marking a critical phase in the interplay between orbital forcing and global climate trends.
How to cite: Kravchinsky, V., Zhang, R., Pan, F., and Qin, J.: Discovery of One-and-a-Half Million Yearlong Aridity in the Tibetan Plateau Eocene Red Clays, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7511, https://doi.org/10.5194/egusphere-egu25-7511, 2025.
New paleomagnetic measurements, coupled with 40Ar/39Ar dating of terrestrial lava sequences, are revolutionizing our understanding of the geodynamo by providing high resolution records of the paleomagnetic field. As part of an investigation of the short-term behavior of the geomagnetic field, we performed detailed sampling of ten cooling units of the Kaupo vent belonging to the Honolulu Volcanic series of the Koolau Volcano, Oahu Hawaii. At least eight samples, collected from each of 10 successive cooling units at Kaupo, were stepwise demagnetized by both alternating field (5 mT to 100 mT) and thermal (from 28°C to 575–650°C) methods. Mean directions were obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin of vector demagnetization diagrams based on seven or more demagnetization steps, with thermal and AF results differing insignificantly. Low-field susceptibility vs. temperature (k–T) analysis conducted on individual lava flows indicated approximately half with reversible curves. Curie point determinations from these analyses revealed a temperature close to or equal to 580°C, indicative of almost pure magnetite ranging from single domain (SD) to pseudo-single domain (PSD) grain sizes for most of the flows. The mean directions of magnetization of the entire section sampled indicate a normal polarity, with ∼10 m of the section characterized by excursional directions (~6 cooling units). Paleomagnetic investigations revealed a series of excursional directions. Absolute paleointensity determinations were performed by means of the modified Thellier-Coe protocol, the most salient results indicate absolute P.I. as low as ~26.9m-Teslas (VADM 5.887x1022 Am2) (i.e. 22° Lat. North) and as high as ~87.2m-Teslas (VADM 19.082x1022 Am2) at high latitudes (i.e. 87° Lat. North). The results of both the directional results (i.e. Declination, Inclination and VGPs) of the 10 cooling units in question in general terms correlate well to the GAD (i.e. +38°). The Kaupo flow VGPs are located over the eastern part of Asia (i.e. over Japan and Korea) 40Ar/39Ar ages from multiple flows give a weighted mean of 64.2+/-2.7 ka, which correlates well with the Norwegian-Greenland Sea excursion recorded in sediments.
How to cite: Herrero-Bervera, E. and JIcha, B.: First terrestrial geomagnetic record of the Norwegian-Greenland Sea excursion in the Kaupo flow, Koolau Volcano, Oahu, Hawaii: Insights from 40Ar/39Ar, NRM and absolute paleointensity determinations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13972, https://doi.org/10.5194/egusphere-egu25-13972, 2025.
Anisotropy of magnetic remanence (AMR) holds promise to quantify relative paleointensity records from sedimentary rocks. A proof of concept was established in applied fields that exceeded the intensity of the geomagnetic field. Under Earth-like fields, large uncertainties necessitate the development of a method to improve the estimation of the anisotropy tensor. To this aim, we designed a numeral method that demonstrates the applicability and resolution needed to optimize the experimental protocol. We then implemented a 30-position procedure for AMR measurements and compared the numerical and experimental data with a typically-used, 12-position procedure. Redeposition experiments with sediments rich in single domain magnetite were carried out in fields with intensities of 0, 10, 50 and 100 µT; 25 individual samples were redeposited and measured at each field condition. The 30-position protocol facilitates the isolation and resolution of the field-aligned prolate fabric (1-2% of the total) from the oblate sedimentary fabric, and more so when applying tensor subtraction of the fabric obtained in a null field. Scatter of the anisotropy parameters is inversely proportional to field strength, which together with the high-resolution protocol and tensor subtraction method, should lead to robust relative paleointensity corrections and/or estimates.
How to cite: Ostermeier, F., Gilder, S., Wack, M., Ježek, J., and Finn, D.: Determining magnetic field intensity from anisotropy of magnetic remanence in sediments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15177, https://doi.org/10.5194/egusphere-egu25-15177, 2025.
Despite over 70 years of research on sedimentary paleomagnetism, fully quantitative models describing the timing and efficiency of the recording mechanism are not yet available. Here, we present new insights for bioturbated sediments, based on cosmogenic 10Be records of the global field intensity, relative paleointensity, and the depth distribution of microtektites. The effect of bioturbation on cosmogenic 10Be and on microtektites is described by a solid diffusion model, with microtektites serving as calibration for the impulse response associated with the diffusion process. The acquisition of a post-depositional remanent magnetization, on the other hand, is governed by a biased rotational diffusion process with bias proportional to the intensity of the magnetic field. The dependence of diffusion on depth yields the relative offset between the impulse responses of 10Be and relative paleointensity, and thus also the time delay of the latter. The delay expected from this model compares favorably with 10Be and relative paleointensity records of sediment cores from the Indian and Pacific Oceans. This work was made possible by Jean-Pierre’s ERC Grant EDIFICE.
How to cite: Egli, R., Savranskaia, T., and Valet, J.-P.: Field recording mechanism in bioturbated sediments: new insights from 10Be, relative paleointensity, and microtektites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17222, https://doi.org/10.5194/egusphere-egu25-17222, 2025.
Dyke swarms are a widespread geological feature in Precambrian craton blocks. Such large magmatic plumbing system serve as exceptional recorders of the Earth’s magnetic field, offering valuable insights into paleogeography of these early geological periods and constrains on the intensity of the ancient magnetic field, as contributed to demonstrated in past work with J.P. Valet. But by using rock magnetism, especially the anisotropy of magnetic susceptibility (AMS), dykes are also excellent witnesses to provide a comprehensive picture of magmatic systems and the tectonic evolution of an area. While it has recently become easy to measure the out-of-phase susceptibility, its use for studying dyke’s emplacement mechanisms is relatively modest. In this study, we analyze the AMS in Ediacaran dykes from Jordan to investigate the potential of such out-of-phase signal on magma flow patterns. A total of 315 samples from 42 sites, including dolerites and rhyolites, were analyzed. Our results show that 47% of the doleritic dykes exhibit a clear difference between in-phase AMS (ipAMS) and out-of-phase AMS (opAMS) fabrics. This may reflect the presence of sub-fabrics, potentially linked to distinct grain populations crystallized under varying stress conditions (Hrouda et al., 2019). Magnetic mineralogy data (susceptibility as a function of temperatures and FORC measurements) suggests the presence of SD (single-domain) magnetite grains in dykes with differing fabrics, and MD (multi-domain) grains in cases with consistent fabrics. This distinction allows us to identify zones where magma flowed freely versus those dominated by forceful intrusion. Our findings emphasize the potential of opAMS measurements in deciphering emplacement processes in mafic dykes.
How to cite: Antonio, P. Y. J., Macouin, M., Rousse, S., Al-Hseinat, M., Jarrar, G., Ghanem, H., Dawaï, D., and Demory, F.: In-phase- and out-of-phase magnetic susceptibility fabrics in Ediacaran dykes from Jordan: insights into magma flow and emplacement processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20308, https://doi.org/10.5194/egusphere-egu25-20308, 2025.
Systematic studies of numerical dynamo simulations reveal that the transition from dipole-dominated non-reversing fields to models that exhibit reversals occurs when inertial effects become strong enough. However, the inertial force is expected to play a secondary role in the force balance in Earth’s outer core. Here we show that reversals in numerical dynamo models with heterogeneous outer boundary heat flux inferred from lower mantle seismic anomalies appear when the amplitude of heat flux heterogeneity is increased. The reversals are triggered at regions of large heat flux in which strong small-scale inertial forces are produced, while elsewhere inertial forces are substantially smaller. When the amplitude of heat flux heterogeneity is further increased so that in some regions sub-adiabatic conditions are reached, regional skin effects suppress small-scale magnetic fields and the tendency to reverse decreases. Our results reconcile the need for inertia for reversals with the theoretical expectation that the inertial force remains secondary in the force balance. Moreover, our results highlight a non-trivial non-monotonic behavior of the geodynamo in response to changes in the amplitude of the core-mantle boundary heat flux heterogeneity.
How to cite: Terra-Nova, F. and Amit, H.: Regionally-triggered geomagnetic reversals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6693, https://doi.org/10.5194/egusphere-egu25-6693, 2025.
Paleomagnetic field reconstructions provide us with the access to the long-term evolution of geodynamo mechanisms. The longest continuous paleomagnetic records originate from measurements of thermoremanent magnetization of ocean crust, although the sources of magnetization and the process governing changes in these records are still debated. Reconstructions based on magnetic signal in marine sediments are another way to obtain continuous paleomagnetic field variations spanning several million years in case of relative paleointensity (RPI) and cosmogenic 10Be records. These two independent proxies appear to align particularly well during periods of low dipole strength, such as geomagnetic excursions and reversals. This agreement is especially compelling because previous joint analyses of environmental proxies, RPI, and cosmogenic 10Be in ice and marine sedimentary cores indicate that both records can be influenced by non-geomagnetic modulations. Stacking a large number of RPI records from different depositional environments has been proposed to eliminate the site-related effects. However, this approach does not allow for a fair estimation of the relative climate versus geomagentic contributions to the overall paleomagnetic record. Moreover, when global climatic controls are embedded in individual records, stacking alone fails to remove these artefacts.
In the present study, we use Principal Component Analysis (PCA) to evaluate the extent to which geomagnetic, climate and environmental modulations influence both RPI and 10Be signals. The effectiveness of this method in separating geomagnetic and climatic contributions was recently demonstrated on a 10Be signal from the North Atlantic site MD95-2016. Recognising that PCA performance strongly depends on the number of available records, we have extended this analysis to a globally distributed 30 RPI and 4 10Be records spanning the past 380 ka. This global dataset enables us to compare the efficiency of each recording mechanisms during 8 geomagnetic excursions and 11 marine isotopic stages. We then compare independently derived geomagentic components from both 10Be and RPI records against previously elaborated RPI and Be - based VADM curves. Meanwhile, other contributions are interpreted in terms of global and regional climatic variations.
How to cite: Savranskaia, T., Egli, R., Korte, M., and Panovska, S.: Understanding Geomagnetic Dipole Behaviour and Climatic Fluctuations over the Past 380,000 Years Using Cosmogenic 10Be and Paleointensity Records, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4216, https://doi.org/10.5194/egusphere-egu25-4216, 2025.
A good knowledge of the long-term variations of geomagnetic paleofield intensity is essential for a complete description of the field history. However, we lack a complete description of the geomagnetic field over many time scales, especially beyond the last million years, making older measurements of great value. IODP Expeditions 391 and 397T cored igneous rocks from the Tristan-Gough-Walvis submarine seamount chain. Cores from Sites U1575, U1576 and U1577, drilled on Valdivia Bank, recovered late-Cretaceous basalts and related rocks while cores from Sites U1578 and U1585, drilled on Tristan and Center seamount chains, recovered more recent Paleocene and related rocks. We selected 95 samples from these sites, mostly from massive lava flows and from some pillow lava flows, for paleointensity measurements. Hysteresis and FORC diagram measurements indicate that selected samples are single-domain, with the exception of those from Hole U1585A, which are pseudo-single-domain. Susceptibility vs. temperature curves indicate a wide variety of magnetic mineralogy in these rocks, ranging from reversible titanomagnetite with Curie temperatures varying between 150 and 550°C, to strongly irreversible assemblages of magnetic minerals. Thellier-Thellier paleointensity experiments gave a success rate of 20/95. The most reliable results come from the high-Ti titanomagnetite samples. Corresponding VDM values averaged by cooling unit are generally low, ranging from 3.1 to 4.3×1022 Am2, except for Hole U1577A, where the VDM is close to 7×1022 Am2. These values are consistent with the few, low VDMs measured on whole rocks in the 62-85 Ma time interval, a period of low reversal frequency–less than 2/Myr. Our results do not seem to support an inverse relationship between field strength and reversal frequency for this period.
How to cite: Carvallo, C., Gaastra, K., Thoram, S., Tikoo, S., Sager, W., and Heaton, D.: Paleointensity determinations on late Cretaceous – early Paleocene basalts from Walvis Ridge (IODP Exp. 391 and 397T), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6670, https://doi.org/10.5194/egusphere-egu25-6670, 2025.
Geomagnetic field models constructed from direct observations extend back some 400 years and the magnitude of decadal to centennial variations seen is often taken to be typical of the secular variation over longer timescales. Here we present archaeomagnetic directions from layered Neolithic hearths in Orkney that challenge this assumption. Combining stratigraphic controls with radiocarbon dates allows for a precise chronology, which, in turn, implies directional change of more than 12°/century lasting for 200 years, far in excess of anything seen globally at this latitude over the era of historical observations. These archaeomagnetic data are complemented by extreme inclinations recorded in two sediment cores from Kälksjön, Sweden. As well as raising the possibility of using archaeomagnetic dating within this important archaeological period, the new data pose questions regarding our understanding of the secular variation, the limitations of the historic field as a proxy for the past, and the underlying dynamical processes within the core.
How to cite: Suttie, N., Harris, S., Batt, C., Nilsson, A., Snowball, I., Card, N., and Outram, Z.: Exceptional secular variation recorded in Neolithic hearths, Orkney., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6849, https://doi.org/10.5194/egusphere-egu25-6849, 2025.
Trees in undisturbed natural environments are known to contain ferromagnetic particles. The application of conventional paleomagnetic techniques to both living and fossil tree trunks has revealed the presence of low coercivity minerals and measurable remanences, with intermediate (300°C) unblocking temperatures and some archaeomagnetic records that are often magnetically viscous (Krs et al., 1994). However, many questions remain about the origin of the magnetization, the locking mechanisms and its stability.
There are several long-lived tree species in the Biosphere, with ages up to 35005000 years, distributed mainly in temperate (Fitzroya cupressoides) and dry mountain (Pinus longaeva) forests. Most living conifers (sequoias, pines, firs, cypresses, cedars, junipers, larches, spruces, yews) and hardwood tree species (beech species, oaks, chestnuts, etc.) have maximum ages ranging 200-1000 years. In addition, the dendrochronological record and field evidence have demonstrated the ability of certain trees to withstand multiple fires and/or direct and repeated lightning strikes. However, to the best of our knowledge, there are no studies on magnetic remanence in trees affected by fire or lightning compared to undisturbed wild trees, soils or rocks, which are better documented. Given the ages and distribution of certain tree species, the geomagnetic recording source has the potential to enhance our understanding of the secular variation of the Earth's magnetic field over the past 500-1000 years and beyond.
In this contribution, we will present the first results derived from the following working hypothesis: "Wood structures associated with lightning strikes or wild fires (including those generated internally to facilitate healing and survival) may have reliably recorded the Earth's magnetic field during the immediate aftermath of the fire (TRM and/or CRM) or in the subsequent period following the lightning strike (CRM, acquired during the healing phase)”. In order to validate or falsify this working hypothesis, several trees affected by lightning or fire were sampled in the Southern Pyrenees (External Sierras, Sto. Domingo range) between 2014 and 2019. The wood samples were analyzed in the paleomagnetic laboratories of the universities of Leoben, Burgos and Barcelona. The analyses performed included AF demagnetization of the NRM, IRM acquisition curves (and AF demagnetization). Magnetization measurements were obtained with 2G superconducting magnetometers in all cases. Additionally, preliminary observations were conducted using a JEOL 6010 Plus scanning electron microscope (located at the IGME headquarters in Tres Cantos, Madrid), operating in low vacuum mode with a backscattered electron detector and EDS microanalysis.
Krs M, Krsová M, Pruner P, Čápová J, Parés JM (1994) Magnetism of subfossil and fresh wood: initial reports. In: Růžičková E, Zeman A (eds) Holocene flood plain of the Labe River. Geol Inst Acad Sci, Praha, pp 51-65
How to cite: Pueyo, E. L., Egi, R., Camarero, J. J., Sánchez-Moreno, E. M., Calvín, P., Scholger, R., Mata, M. P., Beamud, E., and Compaired, F.: Geomagnetic record in trees affected by fires or lightning?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6866, https://doi.org/10.5194/egusphere-egu25-6866, 2025.
The Eocene-Oligocene Transition (EOT) marks one of the most profound climatic shifts in Earth’s history, as recorded by deep-sea isotope data. This interval is characterized by global cooling and the onset of modern icehouse conditions, making it a focal point of paleoclimate research. Understanding the expression of the EOT globally as well as its regional variability requires the correlation of disparate records over the globe. To achieve this magnetostratigraphic data is key to provide the necessary independent age constrains.
Detailed magnetostratigraphic records embracing the EOT have at times revealed the presence of very short magnetostratigraphic intervals within chron C13r. Some have interpreted these small-scale features as true geomagnetic reversals that could correspond to cryptochrons or even subchrons. Worth noting is the record in Leg 73, Site 522 (Tauxe et al., 1984), where a subchron was identified in the uppermost part of C13r, at the approximate location of the E/O boundary. Other sections around the globe have yielded records of lower resolution that could record this same event (Miller et al, 1993). On the other hand, several studies of varying resolution in the Rupelian GSSP at Massignano (Italy), yielded contrasting results. While the early study of Bice and Montanary (1988) revealed a single-sample normal polarity interval close to the E/O boundary, the higher resolution study of Lanci et al. (1996) did not reveal such an event.
Here we present a review of earlier magnetostratigraphic records and new revisited ones embracing the EOT, from marine and continental sedimentary successions and around the globe (Tramoy et al, 2016; Valero et al., 2015; Huber et al., 2019). They show compelling evidence for the occurrence of a normal subchron, namely C13r.1n, at very short distance to the E/O boundary, and that is worth included in the Geomagnetic Polarity Time Scale. Recognition of C13r.1n provides a new anchor point to calibrate records of the EOT at finer resolution.
References
Bice, D.M. and Montanari A., 1988. Magnetic Stratigraphy of the Massignano section across the Eocene-Oligocene boundary. In: Premoli Silva, I., et al. (Eds.), The Eocene-Oligocene Boundary in the March-Umbria Basin (Italy). IUGS International Commission on Stratigraphy, Subcommission on Paleogene Stratigraphy, pp. 111-117.
Huber, B.T et al., (2019). Site U1514. In Hobbs, R.W., Huber, B.T., Bogus, K.A., and the Expedition 369 Scientists, Australia Cretaceous Climate and Tectonics. Proceedings of the International Ocean Discovery Program, 369: College Station, Texas.
Lanci, L., et al. (1996). Magnetostratigraphy of the Eocene/Oligocene boundary in a short drill-core. Earth and Planetary Science Letters, 143(1–4), 37–48.
Miller, K. G., et al. (1993). Integrated late Eocene-Oligocene Stratigraphy of the Alabama coastal plain: Correlation of hiatuses and stratal surfaces to glacioeustatic lowerings. Paleoceanography, 8(2), 313–331.
Tauxe, L., et al. (1984). Magnetostratigraphy of Leg 73 sediments. Initial Reports DSDP, 73, 609–621.
Tramoy, R., et al. (2016). Stepwise palaeoclimate change across the Eocene-Oligocene transition recorded in continental NW Europe by mineralogical assemblages and δ15Norg (Rennes Basin, France). Terra Nova, 28(3), 212–220.
Valero, L. et al. (2015). Linking sedimentation rates and large-scale architecture for facies prediction in non-marine basins. (Paleogene, Almazán Basin, Spain). Basin Research, 1–20.
How to cite: Garcés, M., Li, J., Valero, L., Beamud, E., López-Blanco, M., Sáez, A., and Cabrera, L.: The elusive subchron C13r.1n: a tie point for the Eocene-Oligocene Transition., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13811, https://doi.org/10.5194/egusphere-egu25-13811, 2025.
The monogenetic Quaternary La Garrotxa Volcanic Field is located in the North-East of the Iberian Peninsula and was active from the Middle Pleistocene to the Early Holocene, thus, being the youngest known volcanic activity in the Iberian Peninsula. It forms part of the Catalan Volcanic Zone (Girona, NE Iberian Peninsula), one of the alkaline volcanic provinces of the European rift system.
The main goal of this work was to obtain a reliable full-vector record of the Earth’s Magnetic Field (EMF) recorded by 247 kya to 16 kya lava-flows in La Garrotxa Volcanic Field. With this purpose, fifteen basaltic lava-flows with known absolute and/or relative ages from previous studies, were selected and sampled. Paleomagnetic and paleointensity experiments were performed on these samples at the University of Burgos and the CCiTUB - Geo3BCN-CSIC (Spain). For paleomagnetic experiments, thermal and alternating field progressive demagnetizations were applied to obtain the Characteristic Remanent Magnetization direction of each lava-flow. Paleointensity values were obtained by applying the Thellier-Coe method, including in some cases a low temperature demagnetization pre-treatment to compare the outcome with and without pre-treatment. Also, rock-magnetic experiments have been carried out with a Variable Field Translation Balance to assess the suitability of the obtained determinations. This study represents the most continuous record of the EMF features evolution in La Garrotxa Volcanic Field so far.
How to cite: Vernet Tarragó, E., Calvo Rathert, M., Carrancho Alonso, Á., Beamud Amorós, E., Planagumà Guàrdia, L., Bógalo Román, M. F., and Martí Molist, J.: Earth’s Magnetic Field record from the Middle to Upper Pleistocene lava-flows of La Garrotxa Volcanic Field (NE Iberian Peninsula) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16895, https://doi.org/10.5194/egusphere-egu25-16895, 2025.
The magnetic parameter variations of core MD01-2385 revealed a climatic signal recorded over the last 100 ka. This core was retrieved on the northwest margin of Papua-New Guinea, in the western equatorial Pacific Ocean. This area is located in the Western Pacific Warm Pool (WPWP), which is a major source of heat and moisture to the atmosphere and plays an important role on global climate. The climate of this region is complex, being affected by the El Nino-Southern Oscillation (ENSO) and the Australian-Indonesian monsoon.
Core MD01-2385 was dated by correlating the planktonic oxygen isotope record with a global stack for the 100-40 ka interval and using 14C for the last 40 ka. The mean sedimentation rate is ~20 cm/ka over the last 100 ka. A sample was taken every 2 cm, which corresponds to a time resolution of about 100 years. The saturation magnetization (Ms) is interpreted as tracing the proportion of magnetite in the detrital fraction. At orbital time scale, Ms variations correlate with the New Guinea physical erosion proxy estimated by Yu et al. (2023) on the same core, reflecting past variations in precipitation intensity in the region. During the Holocene, long scale Ms variations correlate quite well with september local insolation variations controlled by precession, which suggest a sensitivity to ENSO oscillation regime. However, during the last glacial period, the correlation between long scale MS variability and precession dominated insolation is less obvious. The difficulty seems to be due to the high amplitude of centennial to millennial variability superimposed on the long-period signal, distorting the precessional component. A first estimation of the suborbital variability suggests a periodicity of ~4 ka. An interval of low concentration in magnetite is synchronous with the Heinrich 1 event, which we interpret as resulting from a decrease in precipitation, in good agreement with the New Guinea physical erosion proxy from Yu et al. (2023).
How to cite: Dauchy-Tric, L., Bassinot, F., and Carlut, J.: Magnetic parameters derived from a sediment core from the western equatorial Pacific over the last ~ 100 ka: sedimentary inputs and climatic variations. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17422, https://doi.org/10.5194/egusphere-egu25-17422, 2025.
The geomagnetic field presents continuous changes over time, a phenomenon that provides a valuable tool for dating archaeological materials, particularly those exposed to high temperatures, such as kilns and ceramics. This study focuses on a Iron Age kiln used for salt production in the site of Piscina Torta (Rome), with four oriented hand samples collected and analyzed. Alternating field demagnetization experiments were conducted to determine the paleodirection of the geomagnetic field recorded in the kiln during the last heating event, as well as rock magnetic experiments to determine the magnetic behavior of the samples, which included thermomagnetic curves and hysteresis loops. Using the inclination and declination obtained, the software ‘ArcheoDating’ was employed to estimate the kiln's last heating event. By comparing the direction obtained with the European regional geomagnetic model, the software determined the most probable date range of the kiln studied. The archeomagnetic age is consistent with preliminary radiocarbon and archeological observations.
How to cite: Bonilla Alba, R., Di Chiara, A., Alessandri, L., Cusimano, L., Della Sala, G. A., Fiorillo, A., Gianni, V., Rossi, C., Sotgia, A., and Florindo, F.: Archaeomagnetic Dating of Roman Iron Age Kilns, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19529, https://doi.org/10.5194/egusphere-egu25-19529, 2025.
