Magnetic properties of marine sediments are dictated not only by the detrital mineralogy, but also by diagenetic processes that can start instantaneously after deposition and may proceed for a long time as sediments are buried. Early diagenesis encompass a range of biochemical reactions associated with bacterial respirations. These may include oxidation at the water sediment interface, iron reduction, sulfate reduction, and anaerobic oxidation of methane (AOM) if methane is present. To investigate the link between diagenesis, sedimentary magnetic properties and quality of paleomagnetic recording, we collected eight 6m-long piston-cores from the Holocene Eastern Mediterranean continental shelf in four locations. Two locations are characterized by high concentration of methane and detectable sulfate-methane transition zone (SMTZ) at depth of 1-4 m. In the other two locations, organoclastic sulfate reduction is dominant throughout the entire cores. Sedimentation rates in this region range between 1 - 5 mm/year. In all cores, concentrations of sulfate, methane and ferrous iron were measured from the pore water. The geochemical data were compared to the mineral magnetic profile that include a range of parameters calculated from IRM, ARM, low- and high- field susceptibility, hysteresis, and FORCs. Paleomagnetic time-series of declination and inclination were obtained from demagnetization experiments carried out in 2 cm resolution. Age models were constructed from radiocarbon dating of carefully collected foraminifera. The results show a consistent link between the diagenetic zones and the magnetic mineralogy: Increase of magnetic properties in the shallow ferruginous zone, decay of magnetic parameters in the sulfate reduction zones,  rapid decrease at the SMTZ and stabilization at the methanogenic zone. XHR-SEM analysis show multiple generations of greigite and pyrite framboids at all depths and unaltered detrital titanomagnetites. We find that except a short time interval below the SMTZ of one core, the paleomagnetic directions in these sediments do not represent the expected directions of the geomagnetic field. We conclude that continuous organoclastic sulfate reduction in marine sediments might have a profound effect on the quality of paleomagnetic recording, but AOM at the SMTZ may help stabilize the magnetic phase. 

How to cite: Zemach, Y., Shaar, R., Sivan, O., Herut, B., Katz, O., Hyams-Kaphzan, O., and Roberts, A.: The effect of early diagenesis on magnetic mineralogy and quality of paleomagnetic recording in marine sediments: case study from the NE Mediterranean shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12025, https://doi.org/10.5194/egusphere-egu23-12025, 2023.

In this work, we conduct inverse modelling of all opaque minerals in a thin section from the Black Hill Norite (BHN), South Australia. The BHN is a mafic intrusion and one of three plutons within the Black Hill Complex. The BHN is important as it has yielded an Early Ordovician paleomagnetic pole position for Australia. Paleomagnetic measurements from the BHN exhibit an intensity and direction different from the local magnetic inclination and declination, indicating a strong and stable NRM.

The total magnetic moment of the thin section was measured in a cryogenic magnetometer at Geological Survey of Norway and yielded D = 171°, I = 3°, and m = 4.5 x 10^-7 Am². An electron backscatter image was made of the entire thin section using a Phenom XL scanning electron microscope to map and identify the opaque minerals. The opaque minerals were ilmenite and magnetite. The thin section was then scanned using NTNU’s Scanning Magnetic Microscope in a near free field environment, therefore mapping the sample’s remanent anomalies. All opaque mineral grains in the thin section were modelled using a multistep parametric inversion approach. Each grain was modelled in two different 3-D geometries, as a single frustum volume and as a series of smaller tabular array volumes. This bulk and discretized modelling allowed for a multi-scale analysis of each grain. The calculated total moment of the frustum and tabular array modelling was D = 356°, I = 6.2°, m = 2.2x10^-7 Am², and D = 359°, I = 8.3°, m = 3.3x10^-7 Am² respectively.

Three modelled remanent magnetization populations were identified in this sample: discrete paramagnetic ilmenite, ilmenite with magnetite reduction-exsolution lamellae, and discrete magnetite with single-phase oxy-exsolved ilmenite. Although discrete ilmenite is paramagnetic, magnetic modelling of this thin section indicates that ilmenite grains without lamellae can be attributed inaccurate modelled magnetizations due to nearby magnetite grains.

How to cite: mcenroe, S., Lee, M., Pastore, Z., Church, N., and Schmidt, P.: Microscale inversion modelling of Black Hill Norite, South Australia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15529, https://doi.org/10.5194/egusphere-egu23-15529, 2023.

At the moment, air quality is a major concern for human and environmental health. The challenge is to produce a robust air pollution mapping for different metrological contexts. In optical physics, the development of particulate matters counters is in full expansion, but the calibration is time-consuming and is difficult to achieve. To answer this problem, many alternatives have been developed as environmental magnetism applied to dust deposition on accumulative surfaces (plant leaves, barks, paper filters,…). In this context, our study focuses on the characterization of the influence of road traffic on air quality along the western bypass of the city of Montpellier (south of France). This work was requested by the Vinci ASF company, in charge of the requalification. 

We chose to work with a mixed approach to be able to free ourselves from the influence of environmental variables. We used local vegetation by selecting species with a good spatial distribution and a capacity of capture of particulates matter (PM) recognized in the literature. Our choice was Pinus pinea, Pinus halepensis, and Arundo donax. In order to support the measurements obtained from these plants, we also used passive cellulose filters (Cao et al. method, 2015). The idea was to plot the relative concentrations of the pollutants (Letaïef et al. 2020) measured for each type of sensor on a map, and to compare them to each other. These concentrations were obtained with saturation isothermal magnetization (sIRM, 1000 mT). Finally, to complete this field analysis we plan to use low-cost PM counters (Alphasens model, prototyped by Thierry Poidras). The results show that the highest concentrations are most often associated with partitioned areas and heterogeneous traffic (acceleration and braking). Another interesting result shows that vehicles emit more PM at the exit of a roundabout than at the entrance of it. To characterize the source of the pollutants, we are currently doing a series of complementary analyses. These series include mineralogy analyses on SEM images, hysteresis cycles, and KT curves. Finally, as plant leaf characteristics (macro and micromorphology) influence PM sorption, we also plan to compare the uptake efficiency of our selected species through an experiment within our wind tunnel, the ZephyrLAB (Saint-Aunès, south of France).

References

Cao, Liwan, Erwin Appel, Shouyun Hu, et Mingming Ma. 2015. « An Economic Passive Sampling Method to Detect Particulate Pollutants Using Magnetic Measurements ». Environmental Pollution 205 (octobre): 97‑102. https://doi.org/10.1016/j.envpol.2015.05.019.

Letaïef, Sarah, Pierre Camps, Thierry Poidras, Patrick Nicol, Delphine Bosch, et Romane Pradeau. 2020. « Biomagnetic Monitoring vs. CFD Modeling: A Real Case Study of Near-Source Depositions of Traffic-Related Particulate Matter along a Motorway ». Atmosphere 11 (12): 1285. https://doi.org/10.3390/atmos11121285.

How to cite: Lefèvre, M., Nicol, P., Poidras, T., Dosias-Perla, D., and Camps, P.: Air quality biomonitoring: application of environmental magnetism to provide air pollution cartographies in a road requalification context, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12660, https://doi.org/10.5194/egusphere-egu23-12660, 2023.

Speleothems are potential candidates for paleomagnetic and paleoclimatic reconstructions. Recent studies suggest that the abundance of magnetic minerals trapped in the calcite laminae of stalagmites is controlled by paleoclimatic and paleoenvironmental parameters including precipitation, vegetation and soil erosion. Here we test this hypothesis by studying speleothems from the Cerâmica cave in Penela, Portugal. We conducted carbon and oxygen isotopic analysis, together with the measurement of concentration-dependent magnetic proxies, including natural remanent magnetization, anhysteretic remanent magnetization, isothermal remanent magnetization and mass specific magnetic susceptibility. Results show a moderate but significant correlation between carbon and oxygen isotopic composition and the content in magnetite. An increase in magnetite content is correlated to increased precipitation and increased contribution of organic material. These results corroborate previous observations that rock magnetic properties of speleothems can provide useful information for paleoclimate reconstructions.

Acknowledgments: this work was funded by the Foundation of Sciences and Technology of Portugal (refs. PTDC/CTA-GEO/0125/2021; MIT-EXPL/ACC/0023/2021) and IDL (ref. FCT/UIDB/50019/2020).

How to cite: Brás, A. R., Font, E., Reboleira, A. S., Melo, R., E. Spangenberg, J., and Fonseca, P.: Paleoprecipitations recorded by rock magnetic properties of stalagmites from the Gruta da Ceramica, Portugal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9558, https://doi.org/10.5194/egusphere-egu23-9558, 2023.

The first shallow hydrate sample off southwest Taiwan has been collected at core site MD18-3542 in cruise MD214. Attractive features are present at ~5.5 m core depth where an unconformity lies, and sediment properties are distinctively different above and below the layer. In the work, we perform rock magnetic analyses to study the core magnetic behavior, including magnetic susceptibility (MS), hysteresis parameters, and X-ray diffraction spectra. Combining the core features and rock magnetic results with the shallow hydrate formation off southwest Taiwan, we consider that variation of the core magnetic property could relate to the pyritization of magnetic minerals caused by the dissociated gas, and summarize the gas distribution at the shallow strata: Abundant methane with hydrogen sulfide trapped below the unconformity could form an anoxic setting and activate the pyritization. Dissociated gas may have also leaked to its above layer, featuring the MS anomaly at ~3.5 m core depth. Magnetite appears in addition to iron sulfides at the depth between ~6.5 and ~10.0 m, indicating a transition layer and implying the insufficiency of the dissociated gas to complete the pyritization. Iron sulfides become dominant again at the core bottom, revealing that the dissociated gas mainly migrates from deep strata.

How to cite: Huang, Y.-S., Horng, C.-S., Su, C.-C., Hsu, S.-K., and Lin, J.-Y.: Rock magnetic signatures reveal the dissociated gas distribution in the gas hydrate reservoir off SW Taiwan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6523, https://doi.org/10.5194/egusphere-egu23-6523, 2023.

A new paleomagnetic study is carried out on the ca. 1150 Ma mafic dykes from the North China Craton (NCC). After stepwise thermal demagnetization, most samples yielded two components. The low-temperature component directs north and down with intermediate inclination, which is interpreted as a viscous remanent magnetization of the present magnetic field. The high-temperature component (HC) directs east and down with steep inclination. Positive baked-contact tests and a reversal test suggest the HC is primary. Combined with the relevant paleomagnetic data previously published (Hou et al., 2009; Wang et al., 2020), a new pole is calculated from more than thirty high-quality virtual geomagnetic poles. The pole has averaged out the paleo-secular variation and is different from any younger paleomagnetic poles of the NCC. The pole places the NCC in a mid-latitude region. Combined with the reported 1230-780 Ma paleomagnetic poles from NCC and Laurentia, the new results suggest the two continents assembled during ca. 1150–1110 Ma at high-latitude region. They then together moved to low-latitude region and merged with Amazonia, Baltica, Kalahari to become a part of Rodinia.

How to cite: Ding, J., Zhang, S., Evans, D., Zhao, H., Li, H., Yang, T., Wu, H., and Shi, M.: New ca.1150 Ma paleomagnetic results of North China Craton (NCC) its paleogeographic and tectonic implications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4774, https://doi.org/10.5194/egusphere-egu23-4774, 2023.

We present new rock magnetic and paleomagnetic results from late Mesoproterozoic carbonate successions of the Jingeryu Formation (Fm) in the northern North China craton (NCC). Rock magnetic experiments, including thermomagnetic susceptibility curves, isothermal remanent magnetization (IRM) and thermal demagnetization of three-axis IRM, carried out for representative samples from the Jingeryu Fm indicate that both hematite and magnetite are abundant in the purplish-red limestone in the lower part of strata and the magnetic carriers of the pale blue limestone in the upper part are magnetite grains. For most samples subjected to stepwise thermal demagnetization, a high-temperature component (HTC) was isolated after removing a low-temperature component of viscous magnetic remanence acquired in recent geomagnetic fields. The HTC is characterized by steep inclinations, whose primary nature is supported by the presence of geomagnetic reversals. The similarities in paleomagnetic results and the correlatable lithologic sequence support that the Jingeryu Fm, the Nanfen Fm in the eastern Liaoning region in northeastern NCC and the Xinxing Fm in the Xuzhou region in southeastern NCC are correlative strata deposited in the polar region. Recently published radiogenic isotopic results constrain the depositional age of the Nanfen and Xinxing formations around ca. 1.1 Ga. Those high-latitude carbonate rocks widely distributed in the NCC predict an extraordinary climate condition around ca. 1.1 Ga.

How to cite: Zhao, H., Zhang, S., Qu, J., Yang, K., Li, H., Yang, T., and Wu, H.: New paleomagnetic results from the Jingeryu Formation in the North China craton and their implications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6238, https://doi.org/10.5194/egusphere-egu23-6238, 2023.

Understanding how paleomagnetic and rock magnetic properties vary as a function of time and space is important for acquiring reliable information about the past Earth’s magnetic field. In basalts the magnetic properties are controlled by Fe-Ti oxides precipitating from the magma. The fraction, type, and mineral and magnetic grain size and shape are determined by a complicated multiphase process through the history of the lava or tephra. The deep magmatic conditions with melt composition, temperature, pressure, and oxygen fugacity are the starting point influencing the initial magnetic mineralogy and the matrix surrounding it. The final stage of crystallization is controlled also by syn-eruptive and emplacement conditions such as viscosity, cooling rate, incorporation of water, eruption style and atmospheric oxidation. Lastly, regardless of the natural environment the magnetic mineralogy is bound to further suffer from some degree of chemical alteration and low temperature oxidation. Therefore, the magnetic properties vary considerably even within the same cooling unit, and where one samples can affect the successfulness of paleomagnetic or paleointensity study. Historical eruptions like Fagradalsfjall 2021 and 2022 provide ideal locations for case studies since we know exactly not only the direction and strength of the magnetic field, but also the geochemical properties of the lavas due to near real time analysis of the eruption products. Here we present the magnetic properties of the Fagradalsfjall 2021 and 2022 eruption products. We collected samples during the eruptions as a function of time from the fast cooled materials, such as samples from the naturally fast cooled flow tops, manually quenched samples from the molten lava, and tephra. After eruptions seized and it was safe to work on the lava, we drilled over 100 lava samples from various locations and morphologies. These samples were subjected to a battery of measurements possible at University of Iceland Paleomagnetism Laboratory slightly varying depending on the type of sample such as bulk susceptibility, low field susceptibility vs temperature, hysteresis properties, backfield coercivity distribution, First Order Reversal Curves, alternating field, and thermal demagnetization. With our preliminary results we show how the magnetic properties vary a) between different types of volcanic products, b) across the lava fields and their multiple morphologies, c) as a function of depth, and d) as a function of time from the beginning of the eruption for the rapidly quenched materials.

How to cite: Piispa, E., Marshall, E., and Mandon, C.: 4D paleomagnetic and rock magnetic properties of the 2021 and 2022 Fagradalsfjall eruption products, Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14480, https://doi.org/10.5194/egusphere-egu23-14480, 2023.

Our Earth’s magnetic field reverses its polarity non-periodically. Despite the large number of reversals in the magnetographic record, the geometry of the Earth’s magnetic field and the mechanisms driving reversals still remain obscure. One reason for this is that the majority of reversal datasets are sedimentary, which lack temporal resolution for recording the rapid field changes during reversals. Another reason is that most of the available volcanic reversal datasets -that may have the necessary temporal resolution- describe a reversal in discrete steps and therefore often have insufficient transitional directions to decipher the geometry of the changing field. Leonhardt et al. (2002) and Leonhardt and Soffel (2002) discovered and described a detailed volcanic record of a ~14Ma reversal on Gran Canaria that formed during a highly eruptive shield-building phase of the island. This paleomagnetic record has sufficient temporal resolution to show the dynamic behavior of the field before and after the reversal. It could therefore illustrate the non-dipolar behavior of the Earth’s magnetic field during a reversal. We resampled the section and extended it by 600 m below Leonhardt et al.’s (2002) section to a total of 110 flows. These flows were measured extensively for both paleodirections and paleointensities. A total of 922 paleodirection measurements were performed, 239 thermal and 683 alternating field demagnetization measurements, which resulted in a paleodirection for 109 flows. A total of 994 paleointensity measurements were performed, 307 IZZI-Thellier and 687 pseudo-Thellier measurements. The IZZI-Thellier measurements produced an absolute paleointensity for 29 flows, based on five sets of selection criteria. The Pseudo-Thellier measurements produced a relative paleointensity for 108 flows, based on one set of selection criteria. Absolute paleointensities from pseudo-Thellier measurements were obtained by scaling their results to IZZI-Thellier results. The resulting paleomagnetic record shows new and interesting features. First of all, during this single reversal the declination reverses only once whilst the inclination reverses its polarity at least five times. In addition, the magnetic field intensity appears to pulsate coinciding with the pattern of inclination reversals, which is observed in the IZZI-Thellier results as well as the pseudo-Thellier results. To analyze this behavior we used a simplified spherical harmonic model with stringent boundary conditions. We show that both these observations can be explained by interaction of the higher order poles during this reversal.

How to cite: van Grinsven, L., Béguin, A., Kuiper, K. F., and de Groot, L. V.: Detailed full-vector record of a mid-Miocene geomagnetic reversal from lava flows of Gran Canaria, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7181, https://doi.org/10.5194/egusphere-egu23-7181, 2023.

Documenting the Earth’s magnetic field variations through time is fundamental for several disciplines. However, current geomagnetic models heavily rely on datasets biased towards mid- and high northern latitudes, whereas data from Africa and surrounding oceans are particularly underrepresented. In this study, we present a new record of paleo-secular variations (PSV) in geomagnetic field inclinations during the last 23 ka from Lake Chala, situated at 3° S near Mt Kilimanjaro in eastern equatorial Africa. This deep groundwater-fed crater lake is characterized by a high sedimentation rate (ca. 1 cm/10 years) and a particularly well-constrained age model based on ²¹⁰Pb and ¹⁴C dating; and rock magnetic analyses have demonstrated the magnetic stability of the sediments. The Chala dataset is linked to PSV records from Lake Malawi and Lake Victoria using a sequence slotting technique to generate a composite PSV record for East Africa. The Lake Chala PSV record not only represents an important contribution toward a better understanding of local and global features of the Earth’s magnetic field, but also expands the utility of paleomagnetism as a key tool for dating of, and cross-correlation between, lake-based paleoenvironmental records and archeological sites throughout eastern tropical Africa.

How to cite: Di Chiara, A., Maher, B. A., Karloukovski, V., Hounslow, M., Blaauw, M., Van Daele, M., and Verschuren, D.: Geomagnetic secular variations over the last 23 ka recorded in Lake Chala, eastern equatorial Africa, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15278, https://doi.org/10.5194/egusphere-egu23-15278, 2023.

The South Atlantic Anomaly (SAA) is an ongoing regional minimum of the Earth’s magnetic field, located beneath South America. The field intensity there is substantially weaker than is expected from the current geocentric axial dipole moment. Currently, the SAA is observed to be expanding and gradually moving westwards. Some geomagnetic field models suggest that the SAA developed underneath the Indian Ocean and has been moving westwards since, while others propose that the anomaly originated under Southern Africa. The current field models that track the onset and evolution of the anomaly, however, are hampered by a lack of paleomagnetic data from the Southern Hemisphere.

Here we present new paleomagnetic directions and paleointensity estimates from Piton de la Fournaise, an active shield volcano on Réunion Island. Réunion Island is located in the Western Indian Ocean (21°S, 55°E) and therefore ideally located to assess whether the SAA once moved from the Indian Ocean westwards, or whether it originated underneath the African continent. A recent study reporting data from Réunion Island revealed a low field intensity of around 30µT for a lava flow dated at ~1600CE. This suggests that the SAA has had an influence in the region at that time. Unfortunately, there are very few datapoints between 1400-1600CE. In our study, we sampled an additional 18 lava flows with ages between 573 and 2019CE, of which seven were deposited between 1400-1600CE. Samples are subjected to rock-magnetic analyses, thermal demagnetization and AF demagnetization methods. Obtained paleointensities with the IZZI-Thellier technique illustrate a high geomagnetic field around 1400CE, after which the field rapidly decreased until 1600CE. This behavior deviates significantly from current geomagnetic models. We discuss the implications these preliminary results have for the evolution of the SAA in the South African region.

How to cite: Meyer, R., van Grinsven, L., and de Groot, L.: Paleomagnetic data from recent volcanics of Piton de La Fournaise, Réunion Island: constraints on the evolution of the South Atlantic Anomaly., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3088, https://doi.org/10.5194/egusphere-egu23-3088, 2023.

Sedimentary records of the Earth’s magnetic field often contain unwanted climatic overprints, distorting the reconstruction of paleomagnetic field intensity variations.  In case of field reconstructions based on the cosmogenic isotope ¹⁰Be, whose production is modulated by the solar activity and the dipolar component of the geomagnetic field, environmental overprints arise from climatic modulations of the source distribution, transport, and sediment scavenging efficiencies. Although the lithological dependence of the scavenging efficiency is supposed to be removed by normalizing ¹⁰Be with the stable isotope ⁹Be, this normalization can introduce its own environmental effects, caused by changes in source, distribution and transport of two isotopes. These processes lead to inter-sites differences observed between ¹⁰Be/⁹Be and corresponding relative paleointensity records, limiting use for global magnetic field models constructions. Principal component analysis (PCA) and independent component analysis (ICA) of four ¹⁰Be/⁹Be records from West Pacific and North Atlantic Ocean sites, characterized by different environmental settings, allowed us to extract the common pattern controlled by the evolution of the dipole field. These observations are made on records covering the last 380 ka, including seven geomagnetic excursions.While the first component of cosmogenic ¹⁰Be records clearly reflects geomagnetic dipole changes, it seems that the second and third components are dominated by 100- and 23-ka periodic oscillations respectively, corresponding to Earth’s orbital forcing. PCA and ICA methods are shown to be a powerful tool for disentangling and assessing different components of cosmogenic beryllium records. The geomagnetic component can serve to better understand the long-term geomagnetic field evolution, thus improving our knowledge of driving mechanisms sustaining the geodynamo.

How to cite: Savranskaia, T., Egli, R., Panovska, S., and Korte, M.: Extracting Magnetic Dipole field variations from cosmogenic 10Be records, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4435, https://doi.org/10.5194/egusphere-egu23-4435, 2023.