This open session provides the opportunity for contributions that fall within the broad spectrum of Paleomagnetism but are not directly appropriate to any of the other proposed sessions. The session invites studies from all areas of paleomagnetism, rock and environmental magnetism which have an impact on climatic, stratigraphic, tectonic or environmental applications. This also includes new theoretical models or measurement techniques.
vPICO presentations: Wed, 28 Apr
Magnetic studies of methanic sediments focus mainly on magnetic iron sulfide (greigite, 3C pyrrhotite) formation and magnetic iron oxide (magnetite, titanomagnetite) dissolution, which mainly result from the release of hydrogen sulfide during sulfate-driven anaerobic oxidation of methane. In some instances, authigenic fine-grained magnetite within methanic environments is recognized from magnetic parameters, but the mechanisms for explaining its occurrence remain unclear. We report a novel authigenic nanoscale magnetite source in methanic marine sediments. The magnetite occurs in large concentrations in multiple horizons in a 230-m long sediment core with gas hydrate-bearing intervals. In contrast to typical biogenic magnetite produced by magnetotactic bacteria and dissimilatory iron-reducing bacteria, most particles have sizes of 200-800 nm and many are aligned in distinctive structures that resemble microbial precipitates. This new type of magnetite is interpreted to be a by-product of microbial iron reduction within methanic sediments. It will record younger paleomagnetic signals than surrounding sediments, which is important for paleomagnetic interpretations in methanic sediments.
How to cite: Lin, Z., Sun, X., Roberts, A., Strauss, H., Brunner, B., and Peckmann, J.: Authigenic nanoscale magnetite within methanic marine sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-460, https://doi.org/10.5194/egusphere-egu21-460, 2021.
Ti-rich titanomagnetite is a primary magnetic mineral in submarine basalts, as well as in some terrestrial volcanic rocks. On geological timescale, it undergoes a slow oxidation forming titanomaghemites. This natural process may be modeled to some extent by a prolonged annealing at moderately elevated temperatures. We test this by treating at 355°C for 4, 40, 110, and 375 hours a sample of submarine basalt containing titanomagnetite of approximate TM46 composition with Curie temperature of 205°C. To characterize the oxidation products emerged during annealing, we have carried out magnetic measurements between at cryogenic temperatures between 1.8 K and 300 K and at high temperatures up to 700°C.
Temperature dependences of magnetic susceptibility measured in an argon atmosphere reveal that annealing for 4 hours already leads to the formation of new magnetic phases (Phases 1 and 2 thereafter) with Curie temperatures of 420°C and 590°C, respectively. At the same time, a phase close to the initial titanomagnetite still remains in a noticeable amount, although its Curie point also shifts towards higher temperatures. Upon further annealing, the initial titanomagnetite completely disappears, the Curie temperature of Phase 1 increases, reaching 500°C after 375 hours, and the Curie temperature of Phase 2 remains practically unchanged. Phase 1 appears unstable to heating to 700°C in argon atmosphere. In samples annealed for up to 110 hours, Phase 1 disappears on cooling, and a phase with the same Curie temperature as the initial titanomagnetite reemerges. In the sample annealed for 375 hours, traces of Phase 1 are still visible in the cooling branch of the susceptibility vs. temperature curve, and the Curie temperature of the reemerged initial-like phase is 250°C. The newly formed Phase 2 remains stable when heated to 700°C in argon.
Effect of prolonged annealings is clearly seen in low-temperature magnetic properties. In the fresh sample, about one quarter of magnetization acquired at 1.8 K is demagnetized by 5 K. This feature holds for the annealed samples as well. The titanomagnetite phase in the fresh sample manifests itself in a magnetic transition at 58 K. Below this temperature, the FC and ZFC curves sharply diverge, as previously observed for titanomagnetites of intermediate composition. For the annealed samples, the shape of ZFC and FC curves and the ratio between them remain generally similar to those observed for the fresh sample, but there are also several differences. The magnetic transition temperature shifts to ~45 K, while the curves’ shape above the transition changes from concave-up to concave-down. RT-SIRM cycle to 1.8 K in zero field for the fresh sample has a characteristic convex shape and is almost reversible. Magnetization at 1.8 K is about 20% higher than the initial value at 300 K, and magnetization loss after the cycle is only 2-3%. The shape of RT-SIRM cycles changes progressively with increasing annealing time, the degree of irreversibility increasing to ~30% for the sample annealed for 375 hours.
This study is supported by Russian Foundation of the Basic Research, grants 19-05-00471 and 20-05-00573.
How to cite: Kosterov, A., Surovitskii, L., Maksimochkin, V., Yanson, S., and Smirnov, A.: Tracing Ti-rich titanomagnetite oxidation with low-temperature magnetic measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1980, https://doi.org/10.5194/egusphere-egu21-1980, 2021.
The fold test is inevitably used to determine whether a remanence is acquired prefold. The remanence declinations recorded in rocks may be affected by local rotation. Thus, it is difficult to restore the structure correction’s original orientation, leading to a possible incorrect conclusion of the fold test. However, the tilt correction of inclination is immune to the influence of local rotations. Therefore, we propose a more straightforward fold test based on the inclination-only mean. Examples are given to verify the validity of the inclination-only fold test, which can be applied to data affected by a possible local rotation. The inclination-only fold test should be used to determine the contribution of inclinations. The combined use of the 3-D and inclination-only fold tests is required to evaluate paleomagnetic results from the orogen or sampling sections with unrecognizable plunging folds or near the faults.
How to cite: Ma, Y., Yang, T., Wang, H., Hu, S., Wang, M., and Zou, D.: An inclination-only fold test, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4094, https://doi.org/10.5194/egusphere-egu21-4094, 2021.
Palaeomagnetic poles form the building blocks of apparent polar wander paths and are used as primary input for quantitative palaeogeographic reconstructions. The calculation of such poles requires that the short-term, palaeosecular variation (PSV) of the geomagnetic field is adequately sampled and averaged by a palaeomagnetic dataset. Assessing to what extent PSV is recorded is relatively straightforward for rocks that are known to provide spot readings of the geomagnetic field, such as lavas. But it is unknown whether and when palaeomagnetic directions derived from sedimentary rocks represent spot readings of the geomagnetic field and sediments are moreover suffering from inclination shallowing, making it challenging to assess the reliability of poles derived from these rocks. Here, we explore whether a widely used technique to correct for inclination shallowing, known as the elongation-inclination method (E/I), allows us to formulate a set of quality criteria for (inclination shallowing-corrected) palaeomagnetic poles from sedimentary rocks. The E/I method explicitly assumes that a sediment-derived dataset provides, besides flattening, an accurate representation of PSV. We evaluate the effect of perceived pitfalls for this assumption using a recently published dataset of 1275 individual palaeomagnetic directions of a >3 km-thick succession of ~69-41.5 Ma red beds from the Gonjo Basin (eastern Tibet), as well as synthetic data generated with the TK03.GAD field model. The inclinations derived from the uncorrected dataset are significantly lower than previous estimates for the basin, obtained using coeval lavas, by correcting inclination shallowing using anisotropy-based techniques, and by predictions from tectonic reconstructions. We find that the E/I correction successfully restores the inclination to values predicted by these independent datasets if the following conditions are met: the number of directions N is at least 100, the A95 cone of confidence falls within a previously defined A95min-max reliability envelope, no negative reversal test is obtained and vertical-axis rotation differences within the dataset do not exceed 15°. We propose a classification of three levels (A, B, and C) that should be applied after commonly applied quality criteria for paleomagnetic poles are met. For poles with classification ‘A’, we find no reasons to assume insufficient quality for tectonic interpretation. Poles with classification ‘B’ could be useful, but have to be carefully assessed, and poles with classification ‘C’ provide unreliable paleolatitudes. We show that application of these criteria for datasets of other sedimentary rock types classifies datasets whose reliability is independently confirmed as ‘A’ or ‘B’, and that demonstrably unreliable datasets are classified as ‘C’, confirming that our criteria are useful, and conservative. The implication of our analysis is that sediment-based datasets of quality ‘A’ may be considered statistically equivalent to datasets of site-mean directions from rapidly cooled igneous rocks like lavas and provide high-quality palaeomagnetic poles.
How to cite: Vaes, B., Li, S., Langereis, C., and van Hinsbergen, D.: Reliability of palaeomagnetic poles from sedimentary rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5351, https://doi.org/10.5194/egusphere-egu21-5351, 2021.
This work presents preliminary paleomagnetic results from the Cadí basin, one of the Pyrenean Late-Carboniferous-Permian basins.
These basins are the consequence of a complex tectonic evolution. At the outset, they were controlled by an extensional or transtensional regime during the progressive dismantling of the Variscan chain that gradually changed to a strike-slip fault regime. Afterwards, during Pyrenean compression (Late Cretaceous to Cenozoic) most of these basins were inverted and transported southwards as a part of the basement units. All of them are characterized by a similar subsidence pattern interpreted as graben or half-graben continental troughs. In a close relationship with its genesis, during Late-Carboniferous-Permian times, several magmatic events took place. Because of that, fluvio-lacustrine sediments present lateral changes to volcanic and volcanoclastic deposits and materials are sometimes cut by intrusions.
The studied zone, the Cadí basin, is located at the eastern Pyrenees, south of the Axial Zone, and shows a high variability of Late-Carboniferous-Permian volcanic products in E-W continuous outcrops that reach several hundred meters of thickness with a good preservation and exposition. The Late Carboniferous-Permian evolution of the Cadí basin was strongly controlled by tectonics and volcanism. In order to characterize this stage and its volcanic rocks, we applied in a previous work the anisotropy of magnetic susceptibility (AMS) technique to determine the primary magnetic fabric of lava flows and volcanoclastic materials. A dominant WNW-ESE direction of the magnetic lineation was recognized related to the paleoflow direction. Paramagnetic minerals and magnetite were recognized as the main carriers of the AMS.
In this work we present the paleomagnetic results from 15 sites (about 150 specimens) in volcanic, volcanoclastic and intrusive materials sampled along four N-S cross sections. The obtained results indicate that the carrier of the magnetization are both magnetite and haematite. The thermal demagnetization of samples shows a paleomagnetic component with unblocking temperatures from 480ºC with reversed polarity and very low inclination after bedding correction. In some sites a normal polarity component is also recognized. These results seem to be coherent with a magnetization coetaneous with the emission and deposition of these materials during Permian times. In cases where the samples record a normal component is important to take into account the complex structural situation of these outcrops.
How to cite: Simon-Muzas, A., Casas-Sainz, A. M., Soto, R., Beamud, E., Oliva-Urcia, B., Pueyo, E. L., and Gisbert, J.: Paleomagnetic study in lava and volcanoclastic materials of the Late-Carboniferous-Permian Cadí basin (Central-Eastern Pyrenees), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7593, https://doi.org/10.5194/egusphere-egu21-7593, 2021.
Shipborne ex-situ oxygen measurements in mid-ocean ridge flank sediment cores from the eastern low-latitude North Pacific (Clarion-Clipperton Zone) revealed a downward increase of pore-water oxygen above the sediment-crust interface (Mewes et al., 2016, Kuhn et al., 2017). This inverse redox zonation is caused by an upward diffusion of oxygen (and other solutes) from fluids circulating through the underlying 20 Mio. Year old and still cooling ocean crust. In consequence, these sediments experience a cyclic change in redox-conditions from oxic seafloor conditions at the top through mostly suboxic conditions throughout the sediment column back to oxygen-rich pore water in the last few sediment meters above the rock basement.
We studied paleomagnetic records and bulk magnetic properties of three gravity cores from such settings that were collected during RV Sonne expedition SO-240 in 2015 and obtained high-quality magnetostratigraphic records covering the past 3.2 Ma. The generally very good preservation and interpretability of our reversal and RPI records, however, conflicts with a well-defined, but irregular ‘ghost event’ of normal polarity within the upper Gilbert reversed C2Ar section. This magnetic polarity and intensity artifact cannot be explained by sediment tectonics, but coincides with the present depth of the lower suboxic-to-oxic redox boundary. Although chemical overprinting could be considered as an obvious explanation of such findings, bulk magnetic analyses (FORCs, thermomagnetics) infer no diagenetic alteration of the magnetic minerals. Over the entire paleomagnetic record, bacterial magnetite appears to be the predominant NRM carrier. We therefore introduce a novel conceptual model of secondary biogenic magnetite formation at crustal depth, hypothesizing that microaerophilic magnetotactic bacteria live and biomineralize not only in the shallow subsurface, but also near the deep oxygen above the sediment-crust interface.
Mewes, K., Mogollón, J.M., Picard, A., Rühlemann, C., Eisenhauer, A., Kuhn, T., Ziebis, W., Kasten, S., 2016. Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion-Clipperton Fracture Zone. Earth and Planetary Science Letters 433, 215-225.
Kuhn, T., Versteegh, G.J.M., Villinger, H., Dohrmann, I., Heller, C., Koschinsky, A., Kaul, N., Ritter, S., Wegorzewski, A.V., Kasten, S., 2017. Widespread seawater circulation in 18-22 Ma oceanic crust: Impact on heat flow and sediment geochemistry. Geology 45, 799-802.
How to cite: Höfken, A., von Dobeneck, T., and Kasten, S.: Magnetostratigraphic effects and artifacts of an inverse redox zonation in bottom-up oxygenated East Pacific mid-ocean ridge flank sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9458, https://doi.org/10.5194/egusphere-egu21-9458, 2021.
The paleogeographic reconstructions of the Iberian plate during the opening of the Atlantic Ocean is still poorly constrained. Major limitations include the paucity of high quality paleomagnetic poles and geochronological constrains, the occurrence of widespread remagnetization events, and controversial seafloor magnetic anomalies. Recent studies provided new high quality paleomagnetic poles from intrusive rocks (sills) dated at 88 and 94 Ma, which contributed to improve the calibration of the apparent polar wander path of Iberia at this time interval. These intrusive rocks are part of the Cretaceous Alkaline Magmatic Pulse that occurred between 72 and 94 Ma, and that is expressed by sills and lava flows cropping out in the Lusitanian Basin of Portugal. Here we provided new paleomagnetic, rock magnetic and anisotropy of magnetic susceptibility (AMS) data of two sills apparently contemporaneous of the Cretaceous Magmatic Alkaline Pulse, namely the Anços sill in the city of Mafra and the Lomba dos Piano sill in the vicinity of the city of Sintra. Rock magnetic experiments consisted in the acquisition and unmixing of isothermal remanent magnetization curves, thermomagnetic analyses, and hysteresis curves, complemented by petrographic analyses. Results indicate that the main magnetic carrier is a mixture of SD to MD titanomagnetite. The magnetic fabric of the Anços sill is oblate in both sills and sub-horizontal k3 eigenvectors indicate that no major titling occurred after the intrusion of the rocks. The Lomba do Piano sill shows more scattered eigenvector directions with a mixture of oblate and prolate fabrics. After alternating field demagnetization, all samples show high-quality and reliable magnetic vectors, with a mean characteristic remanent magnetization orientated Dec=346.88º, I=42.66º (n/N=219/228; k=78.19; a95=1.8º) for Anços and Dec=351.12º, I=48.90º (n/N=142/143; k=94.03; a95=1.23º) for Lomba dos Pianos. All magnetic vectors show a normal (positive) polarity, characteristic of the Cretaceous Normal Polarity Superchron. The corresponding virtual geomagnetic poles (VGP) are Plong=212.62º and Plat=72.03º (N=219, K=98.81, A95=0.96º) for the Anços sill and Plong=212.12º and Plat=78.35º (N=142, K=74.22, A95=1.38º) for the Lomba dos Pianos sill. The Anços VGP plots close to the poles of the Paços d’Ilhas (PI, 88 Ma) and Foz da Fonte (FF; 94 Ma) sills previously published. However, the VGP of the Lomba dos Pianos has a distinct and lower paleolatitude, questioning the contribution of paleosecular variations (PSV). We applied the method of the A95 envelope and find that PSV has not been minimized in the studied sills, which can be explained by the rapid cooling of this kind of rocks. We compiled paleomagnetic data of all sills to provide a more robust paleomagnetic pole for the interval of 88-94 Ma.
This work was supported by the project FCT/UIDB/50019/2020 – IDL funded by FCT
How to cite: Dinis, R., Font, E., Neres, M., Ganerød, M., Gomes, E., Terrinha, P., and Sanchez-Moreno, E.: Paleomagnetism of the Cretaceous Alkaline Magmatism of the Lusitanian Basin, Portugal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10336, https://doi.org/10.5194/egusphere-egu21-10336, 2021.
Skewness analysis of marine magnetic anomalies is the most misunderstood methodology in paleomagnetism. Such analysis has several advantages. First, marine magnetic anomalies innately average secular variation. Second, paleomagnetic poles determined by analysis of their skewness are not biased by overprints. Third, skewness analysis can determine high precision paleomagnetic poles. Specifically, skewness analysis of magnetic anomalies recording Late Cretaceous and early to mid-Cenozoic seafloor spreading between the Pacific and Farallon plates, because of their geometry with respect to the paleo-spin axis, results in high-precision paleomagnetic poles. These anomalies in many cases span ~140° of effective remanent inclination over a span of ~40° of latitude, reducing uncertainty by a factor of ~0.3 when mapping from direction space to pole space (Zheng et al. 2018).
Paleomagnetic poles have been previously determined from skewness analysis for six Pacific plate anomalies: C32n (74-71 Ma), C31n-C27r (60-63 Ma), C26r (62-59 Ma), C25r (59-58 Ma), C24r (57-54 Ma), C20r (46-43 Ma), and C12r (33-31 Ma). The younger group, C20r and C12r, together with independent paleo-spin axis estimates from the paleo-distribution of sediment accumulation rates from 12-46 Ma, define an approximately stationary paleo-spin axis location relative to the Pacific hotspots but offset from the current spin axis by 3°. The older group, 74-54 Ma, also shows that the Pacific hotspots remained approximately stationary relative to an additional paleo-spin axis location separated by 8° from the 12-46-Ma paleo-spin axis, implying an episode of reorientation of the entire solid earth – i.e., true polar wander (TPW) – of ~8° over at most 8 Ma between 54 and 46 Ma, or a rate of TPW of ~1°/Ma or more.
To constrain the timing and rate of reorientation, we analyze anomaly C21n (47-46 Ma), the youngest anomaly inside the 54-46-Ma interval. We incorporate 33 total-intensity ship- and 11 vector aero-magnetic track lines and find a well-constrained paleomagnetic pole near 77N, 23E in the fixed-Pacific plate reference frame.
Our new paleomagnetic pole is consistent with a prior, more uncertain, 48-Ma paleo-spin axis location from the paleo-distribution of sediment accumulation rates. When reconstructed into the Pacific hotspot reference frame, our new paleomagnetic pole lies close to the younger 46 to 12-Ma TPW stillstand location, indicating that true polar wander was completed by 47 Ma, if not earlier. Thus the ~8° shift occurred in, at most, 6.0 Ma at a rate of at least ~1.3°/Ma, and potentially even faster. The lower bound of ~1.3°/Ma of TPW indicate that Early Eocene TPW is comparable to the rate of present-day TPW (~1.1°/Ma extrapolated from geodetic data (Argus and Gross, 2004)). This new pole bounds the Early Eocene TPW episode between approximately the old and young ends of the Early Eocene Climatic Optimum (EECO; 53.2-49.1 Ma (Westerhold et al. 2018)). Thus, there may be a link between Early Eocene TPW and important climate events, such as the frequency of hyperthermals and the onset of Eocene cooling. In addition, TPW was likely complete before the 47.4-Ma age of the bends in Pacific plate hotspot chains (Gaastra & Gordon, this meeting).
How to cite: Woodworth, D., Gordon, R., and Gaastra, K.: Skewness Pole from Magnetic Anomaly C21n Implies Rapid Early Eocene True Polar Wander, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13644, https://doi.org/10.5194/egusphere-egu21-13644, 2021.
Widespread carbonate rocks from the Tibetan plateau have been extensively used to constrain terrane paleolatitudes involved in the India-Asia collision. However, their reliability in preserving a primary magnetization has been recently put into question. A transformation of pyrite to magnetite has been recently proposed as a cause for late re-magnetizations in Paleocene Tethyan Himalaya carbonates (1) and late Triassic carbonates from the Qiantang (2), thus discarding such Characteristic Remanent Magnetizations (ChRM) for tectonic purposes. We have re-examined the paleomagnetic data obtained on late Triassic carbonate rocks from the Qiantang. Our SEM observations indicate pristine pyrite in non-weathered carbonate rocks. Optical microscope observations in reflected light demonstrate that pyrite, when it is weathered, is transformed to iron hydroxides minerals but not to magnetite. This is at odds with previously proposed pyrite to magnetite transformation hypothesis mainly based on interpretations of Scanning Electron Microscope data (SEM/EDS). We thus interpret the ChRM more likely related to an early diagenetic magnetization of Late Triassic age. Knowing that the arguments put forward for a remagnetization of Triassic carbonates are the same as those proposed for the remagnetization of Paleocene carbonates, the ChRM in some Paleocene carbonates could also be of early diagenetic origin. However, there is also a growing number of studies where remagnetization is obvious in the Tethyan Himalaya and undetected remagnetizations (3) are likely the cause of the large differences in the estimation of the size of Greater India. These examples show the urgent need to publish the complete demagnetization dataset in an open database like MAGIC or the FAIR data initiative from (4) in order to reassess previous interpretations if we want to solve problems like the size of Greater India and hypothesis like the Greater India basin.
(1) doi:10.1002/2016JB013662 ; (2) doi:10.1016/j.epsl.2019.06.035 ; (3) doi:10.1016/j.epsl.2020.116330; (4) doi:10.1029/2019GC008838.
How to cite: Roperch, P. and Dupont-Nivet, G.: Are carbonates from the India-Asia collision remagnetized ? , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15842, https://doi.org/10.5194/egusphere-egu21-15842, 2021.
Even though multidisciplinary approaches applied to the investigation of archaeological findings are widely used, the use of rock magnetic properties is still poorly exploited in the determination of the use of ancient artefacts. In this study, we present the results of a combined archaeological, morphological and magnetic analyses applied on the ring-shape clay artefacts found at the archaeological site of Villa del Foro, in Northern Italy. The materials studied are dated between the sixth and the first half of fifth century BC and are found in large quantities in different trenches of the archaeological excavation. To investigate their thermal history and to exploit their possible use as kiln supports, cooking stands, or loom weights, we have investigated their natural remanent magnetization (NRM) and the magnetic mineralogy changes occurred during laboratory heating. Magnetic analysis used for the determination of the firing temperatures show thermal stability up to 500-600 o C, while further laboratory heating at 700 o C introduces magnetic alteration. Thermal demagnetization of the samples generally shows a strong and stable thermal remanent magnetization. In few cases, a clear secondary component is present, suggesting partial re-heating or displacement at temperatures ranging from 200 oC to 450 °C. Such secondary magnetic component can be indicative of a secondary heating or of a displacement of the rings from their initial firing position while still hot. Even though the studied rings belong to casually different morphological typologies, no connection among type and magnetic behavior was observed, suggesting that the ring’s morphology does not define neither their production conditions nor the final use of the artefacts. The estimated firing temperatures of around 600-700 oC are compatible with the heating of the rings during their manufacture rather than related to cooking activities. In combination with the archaeological evidence and the morphological analysis it is thus suggested that the rings were used as weight looms and baked only during their production procedures. Such a pilot study can be used as reference for the identification of similar objects found in Italy and Europe during the Iron Age and confirms the great potential of rock magnetic analysis in the investigation of the technology and use of ancient baked clays.
How to cite: Tema, E., Ferrara, E., Zamboni, L., Venturino, M., Reboldi, M., Egea Guevara, A., and Casas, L.: Rock magnetism as a tool for investigating the use of archaeological artefacts from baked clay, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3158, https://doi.org/10.5194/egusphere-egu21-3158, 2021.
Human behavior and especially the use of fire increasingly influence our environment during the Anthropocene epoch. Balkan Peninsula is on the road of the ancient human dispersal during the Neolithic period. Burnt Neolithic remains are often related to ancient houses which ended their existence as a result of extensive fire. Materials from burnt clay remains from house destructions originating from 18 Neolithic sites from Bulgaria were studied using rock magnetism. Mineral magnetic studies and equivalent firing temperature estimates were carried out. The aim of the study was to explore the magnetic signature of fired clay materials in relation to the most important environmental factors. The main magnetic minerals identified were magnetite, maghemite and hematite, in several cases also epsilon-Fe2O3. Magnetic susceptibility enhancement is dependent on the raw clay mineralogy and the firing intensity, being higher for sites developed on loess materials. Sites located in river valleys from South Bulgaria show lower susceptibility enhancement. Magnetic susceptibility and percent frequency dependent magnetic susceptibility at site level were considered in relation to the climatic conditions during the Neolithic as revealed by anthracological studies already published for the study region. Firing temperature estimates, comprising 198 single determinations in total, vary in the range 580 – 1050°C across the sites. Estimated average firing temperatures at site’s level showed higher values in Early Neolithic sites (Tfire average=815°C) as compared to Late Neolithic ones (Tfire average = 746°C and 713°C). Several possible hypotheses for the trend observed are considered: difference in climate regimes across the territory leading to different “fire weather”; difference in the vegetation fuel used in house construction; and intentional burning of Early Neolithic houses. This study is financially supported by the project KP-06-COST/2, funded by the Bulgarian National Science Fund.
How to cite: Jordanova, N., Jordanova, D., Lesigyarski, D., and Kostadinova-Avramova, M.: Fired clay from Neolithic houses – the role of past environment revealed in mineral magnetic properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9845, https://doi.org/10.5194/egusphere-egu21-9845, 2021.
A multidisciplinary approach, including compositional, spectroscopic and microscopic methodologies, is often used for the analysis and identification of pigments in Fine Arts. Although a large part of widely used natural and synthetic pigments contain Fe-oxides and hydroxides, their magnetic characterization is still poorly explored. The application of rock magnetism analyses through fast, cheap and non-destructive measurements, can be instead useful for the identification and discrimination of pigments through their distinctive magnetic properties.
In this preliminary study, the magnetic properties of several iron-based commercial pigments together with paintings models and supports, were analyzed.
In order to investigate the compositional differences of pigments by means of their magnetic behavior, the magnetic susceptibility, the hysteresis properties and the magnetic susceptibility variation at low and high temperature were measured on selected samples.
All the pigments showed different magnetic properties, mainly related to variable proportions of magnetite, hematite and maghemite as the main magnetic carriers.
Further studies will be addressed to define a protocol for applying the magnetic techniques to the characterization of pigments, including tests on samples produced by different brands and different periods, with the final aim of integrating the magnetic measurements with the different spectroscopic techniques commonly employed for the preservation and the analysis of cultural heritage.
How to cite: Anselmi, C., Sgamellotti, A., Vagnini, M., and Winkler, A.: The magnetic properties of natural pigments: preliminary analyses for their identification in Fine Arts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3142, https://doi.org/10.5194/egusphere-egu21-3142, 2021.
Cyclical changes in the magnetic mineral assemblages have been observed in numerous sedimentary records confirming the relationship between rock magnetism and past global change. Several studies have shown that the magnetic susceptibility data of cave sediments reflect both long- and short-term climatic oscillations. These magnetic susceptibility variations are attributed to changes in climate-controlled pedogenesis which influence the production of low coercivity magnetic mineral phases, magnetite, and maghemite outside the cave. These soils with climate-dependent magnetic properties are then washed, blown, or tracked into the cave where they accumulate, creating the changes observed in rock magnetic data. We present a rockmagnetism study of the sediments from the Urșilor cave and the soils above the cave. Our focus is the detailed characterization of the ferromagnetic mineralogy preserved in the cave sediments and its links with potential soil sources. In the cave, we sampled four sections (2-3 m high) consisting mainly of silts and clays, with some sand layers. The age of the sediments is older than 40 ka. At the surface, we sampled various types of soils from 9 sites. For all samples, we measured: variation of magnetic susceptibility with frequency (976 and 15616 Hz), the anisotropy of magnetic susceptibility, isothermal remanent magnetization, and anhysteretic remanent magnetization. Because soils are characterized by the presence of superparamagnetic magnetite produced by pedogenesis which can be detected by the frequency dependence of magnetic susceptibility, we also measured the frequency dependence of soils and selected cave sediment samples at 13 frequencies (between 128 and 512000 Hz). Multi-frequencies measurements of the magnetic susceptibility of recent soils show that all the sampled soils have a strong frequency dependence indicating the presence of superparamagnetic particles produced by pedogenesis. Most of the sediment samples have an important frequency dependence similar to the one observed in the recent soils. As a preliminary conclusion, we can state that most of the fine cave sediments contain superparamagnetic particles, which can be probably attributed to soils transported into the cave by erosion. These results suggest that during the deposition of high magnetic susceptibility sediments it was a climate favorable for intense pedogenesis. The interpretation of the intervals with lower values of magnetic susceptibility is still under investigation to decide if represents a climatic signal or a change in the dynamics of sediment transport. Acknowledgment: The research leading to these results has received funding from the EEA Grants 2014-2021, under Project contract no. EEA-RO-NO-2018-0126.
How to cite: Panaiotu, C. G., Necula, C., Roban, R. D., Petculescu, A., Mirea, I.-C., Faur, L., and Constantin, S.: Linking cave sediments and soil magnetism in the Urșilor cave (Romania), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2973, https://doi.org/10.5194/egusphere-egu21-2973, 2021.
Climate affects the mineralogy and grain size of sediments deposited in lakes. These properties are reflected in the sediment magnetic properties and can be characterized using magnetic methods. As part of the Cadagno-Project, which recovered several gravity and piston cores spanning the entire lake history from the deglacial to the present from the deepest part of permanently stratified Lake Cadagno, which is due to its peculiar water column chemistry considered an early Earth ocean analogue, our study aims to define changes in climate conditions during sedimentation. Here, we present a rock magnetic dataset (low-field magnetic susceptibility and its temperature dependence, anhysteretic and isothermal remanent magnetization (ARM, IRM), acquired in various fields, AF demagnetization, and hysteresis loops) that helps characterize the concentration, mineralogy, and grain size of magnetic carriers, and their variability with depth. Susceptibility, ARM, and IRM were measured on core sediments down to a depth of 886 cm below the lake bottom, providing a high-resolution record of the sedimentary environment of Lake Cadagno over the last 11,000 years. In addition to these depth profiles, detailed rock magnetic experiments were conducted at specific depths. The cores consist of pelagic sediments, flood turbidites, and late glacial sediments. In order to determine the characteristics of the background sedimentation, only turbidite-free intervals were included in this study. The depth profiles of susceptibility, ARM and IRM have approximately similar variations with depth. They show distinct peaks at the upper parts of the pelagic sediments (156-158 cm below the lake bottom, ̴1280-1320 cal. Yr Bp) and of the late glacial sediments (826-844 cm below the lake bottom), which can be interpreted as increased concentration of ferromagnetic minerals or as a change in the magnetic mineralogy, in addition to decreasing trend in the background. Several intervals within the pelagic sediments are dominated by low-coercivity minerals (<10 mT), while higher coercivity grains (10–100 mT) contribute significantly at (150-170, 418-448 and 719-735 cm below the lake bottom). Magnetic grain size was analyzed using a Day plot, and shows that single domain magnetite dominates at (844 cm) below the lake bottom, indicating the presence of magnetotactic bacteria, which are believed to dwell mainly in the oxic–anoxic interface where chemical gradients are high. These results provide important constraints on the environmental conditions and climate change recorded by the magnetic minerals in Lake Cadagno.
How to cite: Ali, A., Biedermann, A., Berg, J., Lever, M., and Vogel, H.: Environmental magnetism study of Lake Cadagno, Switzerland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16462, https://doi.org/10.5194/egusphere-egu21-16462, 2021.
Mass specific magnetic susceptibility variations with depth along soil profiles developed on loess parent material is one of the most frequently used physical parameters in local, regional and global correlations of loess deposits. It is also utilized as a paleo-precipitation proxy, defined either as absolute difference between susceptibilities of the enhanced B-horizon and parent loess, or as relative enhancement using ratios of magnetic parameters. These different approaches in the application of magnetic susceptibility as paleoclimate proxy lead us to perform a comparative study on a number of Holocene soil profiles developed on loess from European loess area and the Chinese Loess Plateau (CLP). We made a compilation of data including 20 profiles from North Bulgaria, 28 profiles from Eastern and Central Europe; and 26 profiles from the CLP. Minimum magnetic susceptibilities of the last glacial loess (Xmin) from the compiled data base for European and Chinese profiles show climate related variability, revealing multi linear relationship with both present day MAP and MAT values for the corresponding locations. Strong deviations of Xmin from this dependence display sites located at low elevation river terraces, Black sea coast and possessing large content of coarse silt and sand fractions. Pedogenic magnetic susceptibility (Xpedo) defined as (Xmax - Xmin) with Xmax determined from the youngest part (last 1500 – 2000 years B.P.) of the Holocene magnetic susceptibility records of Chinese sections and absolute Xmax of the European sites show systematic dependence on modern MAP and MAT values. This dependence is uniform for all sites with steppe vegetation, while higher scatter and steeper regression trends are observed for sites under mixed (steppe – forest) and forest vegetation. The study is financially supported by project No KP-06-N34/2 funded by the Bulgarian National Science Fund.
How to cite: Jordanova, D. and Jordanova, N.: Reconsidering the paleoclimate implications of magnetic susceptibility records of Holocene loessic soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10914, https://doi.org/10.5194/egusphere-egu21-10914, 2021.
Palaeoclimatic reconstructions including rock magnetic, magnetostratigraphic, palaeopedological and pollen studies were performed on two reference Ukrainian loess-palaeosol sequences at Stari Kaydaky and Vyazivok, located within the Middle Dnieper River Basin. The results have shown that rock magnetic palaeoenvironmental proxies, primarily magnetic susceptibility (MS), are clearly correlated with the marine isotope record (MIS), being enhanced in the palaeosols compared to the loesses. The background susceptibilities of the Stari Kaydaky and Vyazivok sections are both in the range of 7×10-8 to 10×10-8 m3kg-1, which is a half of those at the Danube Basin loess sites. The good correspondence of the MS curve with changes in the marine δ18O signal, provides a strong evidence for correlating the Pryluky/Kaydaky (S1) pedocomplex at Stari Kaydaky and Vyazivok to MIS 5, and the welded Zavadivka (S3/S4) pedocomplex to MIS 9–11. In the Upper Pleistocene deposits, the well developed Luvisol of the Kaydaky unit has a pollen succession of the last interglacial, with high percentages of broadleaf trees’ pollen indicating the early and late temperate phases of the interglacial. Relatively high pollen percentages of broadleaf trees are observed in the Greyzem of Lower Pryluky subunit (the correlative of MIS 5c) and indicate the south-boreal, transitional to temperate, climate. The Upper Pryluky Cambisols and Chernozems (MIS 5a) formed under boreal forest-steppe and steppe, as well as the Vytachiv soils (L1S1, MIS 3). Pollen assemblages from the Uday (L1L2) and Bug (L1L1) loesses reflect tundrasteppe in the north of the studied area and dry periglacial steppe in its southern part. The Late Middle Pleistocene is represented by three interglacial pedocomplexes (correlatives of MIS 11, 9 and 7), with the larger proportions of broad-leaved trees’ pollen in the lower forest soils. Judging from these indices, the climates were warmer than during the last interglacial. Only the upper soils of the MIS 7 formed in the cooler climate. Indicative Pterocarya pollen occur in the MIS 11 soil. During the Early Middle Pleistocene, the temperate climate was typical for the pedocomplex correlated with MIS 13, whereas the older red-brown and dark-brown clayey soils formed in warm-temperate climate (their palynospectra include pollen of thermophilic Neogene relics). At Vyazivok, the Matuyama/Brunhes (M/B) boundary has been detected in the lower part of the Shyrokyne (S7?) soil unit, providing an ultimate chronological benchmark of 780 ka. The Kryzhanivka pedocomplex, located below the M/B boundary, formed in subtropical climate. Preliminary investigations at Stari Kaydaky did not detect the M/B boundary within the studied upper part of the Shyrokyne unit. Pseudo-single-domain/multi-domain magnetite and hematite particles are the main magnetic carriers of the loess and palaeosol ChRMs. The reconstructions demonstrate a correspondence between the pollen and soil sequences and other palaeoenvironmental records including rock magnetic data.
The research was supported by the National Research Foundation of Ukraine grant 2020.02/0406 "Magnetic proxies of palaeoclimatic changes in the loess-palaeosol sequences of Ukraine".
How to cite: Hlavatskyi, D., Gerasimenko, N., Bakhmutov, V., Bonchkovskyi, O., Poliachenko, I., Shpyra, V., Kravchuk, I., and Cherkes, S.: Reconstruction of the Pleistocene climate change in the Middle Dnieper area on the basis of rock magnetic, palaeopedological and pollen studies of two reference loess-palaeosol sequences, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1758, https://doi.org/10.5194/egusphere-egu21-1758, 2021.
The finer fraction of the particulate matter (PM) is the most harmful health wise, as it has more capacity to reach deeper parts of the respiratory system. Among other constituents, PM also contains iron oxides, allowing for the use of magnetic methods in its investigation as proxies for the whole of PM. Those methods present advantages in comparison to traditional ones, being quick, cost effective and sensible to investigate iron oxides among PM.
To better understand the risks related to PM exposition in the domestic context, the assessment of magnetic parameters may be used in outdoor and indoor environments, giving us information on the concentration of iron oxides (and consequently, PM) and its dispersion from one environment to the other.
We developed a citizen sciences experiment in the city of Toulouse, France. Tree barks were used as bio-collectors. Garlands composed of tree bark pieces were distributed to the population in May-2019, and placed in both indoors and outdoors of flats and homes to capture PM. They were retrieved after one year. Measurement of magnetic susceptibility, ARM, SIRM, S -ratio and estimation of superparamagnetic concentration were performed. A total of 86 bio-collectors kits were successfully analyzed. The preliminary results indicate a higher concentration of iron oxides outdoors, with a mean difference between outdoor and indoor measurements of 6.58x10-9m3/kg and 1.38x10-5Am2/kg in susceptibility and SIRM respectively. The concentration of the SP fraction also follows this trend of higher outdoor values. The magnetic mineralogy is mostly dominated by low coercivity magnetite-like carriers.
How to cite: Da Silva Leite, A., Macouin, M., Rousse, S., Leon, J.-F., Drigo, L., and Ivan Ferreira da Trindade, R.: Magnetic properties of PM in indoor/outdoor domestic environments , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14416, https://doi.org/10.5194/egusphere-egu21-14416, 2021.
Numerous studies have already shown the possibility of tracing the sources, the
compositions, and the concentration of atmospheric pollutants deposited on plant
leaves. In environmental geochemistry, inter-element and isotope ratios from
chemical element assays have been used for these purposes. Alternatively,
environmental magnetism represents a quick and inexpensive asset that is
increasingly used as a relative indicator for concentrations of air pollutant on bio
accumulator surfaces such as plants. However, a fundamental issue is still pending:
Do plants in urban areas represent a sink for fine particles that is sufficiently effective
to improve air quality? This is a very topical issue because some studies have shown
that the foliage can trap fine particles by different dry deposition processes, while
other studies based on CFD models indicate that plant hedges in cities can hinder
the atmospheric dispersion of pollutants and therefore increase pollution at the level of
emission sources such as traffic. To date, no consensus was made because several
factors not necessary well known must be taken into account, such as, PM
concentration and size, prevailing wind, surface structures, epicuticular wax, to
mention just a few examples. A first step toward the understanding of the impact of
urban greens on air quality is the precise determination of the deposition velocity (Vd)
parameter. This latter is specific for each species and it is most of the time
underestimated in modeling-based studies by taking standard values.
In that perspective, we built a wind tunnel (6 m long, 86 cm wide and 86 cm high) to
perform analogical experiments on different endemic species. All parameters are
controlled, i.e, the wind speed, the nature and the injection time of pollutants (Gasoline
or Diesel exhausts, brakes or tires dust, etc…). We can provide the PM concentrations
upwind and downwind of natural reconstituted hedges by two dustmeters (LOACs -
MétéoModem). Beforehand, parameters such as the hedge resistance (%) or the leaf
area index (LAI) have been estimated for each studied specie to allow comparability
between plants removal potential. The interest would ultimately combine PM
concentration measured by size bins from the LOACs with magnetic measurements
(ARM, IRM100mT, IRM300mT and SIRM) of plant leaves. The idea is to check whether it
would be possible to precisely determine in situ the dust removal rate by urban greens
with environmental magnetism measurements. Up to now, we have carried out on
different endemic species such as Elaeagnus x ebbingei leaves and Mediterranean
pine needles, the results of which will be presented.
How to cite: Letaïef, S., Camps, P., Poidras, T., Nicol, P., Bosch, D., and Bruguier, O.: To what extent does the environmental magnetism technique acts as a way of assessing the uptake of atmospheric particles by plants?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8263, https://doi.org/10.5194/egusphere-egu21-8263, 2021.
An extensive survey of the magnetic properties of PM filters from selected air monitoring stations in Rome and other localities in Latium Region (Sacco Valley, Civitavecchia, Fiumicino) was conducted for outlining the impact of the lockdown measures on air quality.
The magnetic measurements highlighted a relevant content of magnetic minerals, mostly attributable to traffic related sources, on the filters from two stations in Rome and two stations from the urban areas of Civitavecchia and Fiumicino.
The PM filters from the Sacco Valley showed reduced concentrations of magnetic minerals, compared to Rome, however higher than the Castel Di Guido and Civitavecchia Sant'Agostino control stations.
The daily PM concentration data did not generally correlate with the mass susceptibility data, indicating that PM was often dominated by non-ferromagnetic contents, presumably due to wind-driven natural dusts, as stressed by the frequent anticorrelation between mass magnetic susceptibility and PM concentration.
In Magnagrecia air quality station, Rome, the average values of the concentration depending magnetic parameters resulted about a half of those measured in 2005 on the filters from the same station.
From the Day plot, the filters with higher magnetic susceptibility values showed relatively coarse magnetite-like particles as the main magnetic minerals, ascribable to non-exhaust PM emissions from brakes.
This study confirmed that the interpretation of PM concentration during the lockdown is not straightforward and depends on many factors, such as natural inputs, resuspension and local conditions; anyway, magnetic analyses confirmed to be a valuable tool in PM source apportionment and concentration data interpretation.
How to cite: Winkler, A., Amoroso, A., Di Giosa, A., and Marchegiani, G.: Magnetic monitoring of PM filters from Air Monitoring Stations during the COVID-19 lockdown in Rome, Italy (March 10th - May 18th, 2020), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3817, https://doi.org/10.5194/egusphere-egu21-3817, 2021.
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