Anhysteretic Remanent Magnetization Research Articles

Within-city spatial variations in PM2.5 magnetite nanoparticles and brain cancer incidence in Toronto and Montreal, Canada.

Magnetite nanoparticles are small, strongly magnetic iron oxide particles which are produced during high-temperature combustion and friction processes and form part of the outdoor air pollution mixture. These particles can translocate to the brain and have been found in human brain tissue. In this study, we estimated associations between within-city spatial variations in concentrations of magnetite nanoparticles in outdoor fine particulate matter (PM2.5) and brain cancer incidence. We performed a cohort study of 1.29 million participants in four cycles of the Canadian Census Health and Environment Cohort in Montreal and Toronto, Canada who were followed for malignant brain tumour (glioma) incidence. As a proxy for magnetite nanoparticle content, we measured the susceptibility of anhysteretic remanent magnetization (χARM) in PM2.5 samples (N = 124 in Montreal, N = 110 in Toronto), and values were assigned to residential locations. Stratified Cox proportional hazards models were used to estimate hazard ratios (per IQR change in volume-normalized χARM). ARM was not associated with brain tumour incidence (HR = 0.998, 95% CI 0.988, 1.009) after adjusting for relevant potential confounders. Although we found no evidence of an important relationship between within-city spatial variations in airborne magnetite nanoparticles and brain tumour incidence, further research is needed to evaluate this understudied exposure, and other measures of exposure to magnetite nanoparticles should be considered.

Multi-stage tectonic evolution of the Tatra Mts recorded in the para- and ferromagnetic fabrics

The Tatra Mts form the highest part of the Carpathian mountain chain; however, their tectonic and thermal evolution is still debatable. Previous magnetic fabric studies have primarily focused on the crystalline basement and its autochthonous cover. We investigate the magnetic fabrics of Cretaceous marly limestones from a Mesozoic nappe unit and post-thrusting Oligocene shales and mudstones to unravel the most recent tectonic evolution of the Tatra massif. In addition to standard petromagnetic measurements such as the acquisition of the Isothermal Remanent Magnetization or temperature-dependent susceptibility analyses, we investigated the paleotemperature of the Tatra region because high temperatures are known to significantly affect the magnetic mineralogy. The most common minerals in the studied units are paramagnetic phyllosilicates which govern the in-phase Anisotropy of Magnetic Susceptibility. The ferromagnetic fraction is represented by fine-grained magnetite with a minor contribution of hematite. Measured and counted vitrinite reflectances document an eastward increase in maturity, which is also reflected in the magnetite–hematite grain size ratios. Because the paleotemperatures recorded in the Cretaceous rocks follow the same increasing trend as the post-thrusting shales, it appears that both units were affected by a single major thermal event linked presumably to the Late Oligocene/Early Miocene burial. We propose that magnetic fabrics carried by phyllosilicates document the impact of crucial tectonic phases such as Miocene uplift and Cretaceous thrusting, whereas the out-of-phase Anisotropy of Magnetic Susceptibility and Anisotropy of Anhysteretic Remanent Magnetization fabrics most likely record the stress orientation during major burial episodes. Finally, the conspicuous vertical ferromagnetic lineation present in some Cretaceous sites documents the transpression-controlled tectonic regime in the Oligocene–Early Miocene.

A short-term warming interval during the apex of the Late Paleozoic Ice Age: Evidence from geochemical and magnetic records from South China

The Late Paleozoic Ice Age (LPIA) was the longest-lasting icehouse in the Phanerozoic. Previous studies indicated a climate warming interval in the earliest Permian—an apex of the LPIA. To better understand hydroclimatic changes and carbon cycling within this time interval, we conducted a series of high-resolution geochemical and rock magnetic analyses, including magnetic measurements (e.g., magnetic susceptibility, anhysteretic remanent magnetization, hysteresis loops, temperature-dependent susceptibility), ratio of iron to potassium (Fe/K), and carbonate carbon isotope (δ13Ccarb) from a deep-marine section in South China. Calibrated to the published astronomical time scale, a negative δ13Ccarb excursion (−1‰) was identified during 298.4–297.9 Ma, and it was time-equivalent with the warming interval, recognized by a decrease in conodont apatite oxygen isotopes in South China, low-latitude sea-level transgression, and high-latitude deglaciation. The negative δ13Ccarb excursion also coincided with a magmatic pulse of the Skagerrak-Centered Large Igneous Province (LIP), indicating that the perturbation in carbon cycling is likely associated with the release of volcanic CO2. Furthermore, cyclostratigraphic analysis on multiple paleoclimatic proxies, including ferrimagnetic susceptibility, gamma-ray, and Fe/K, suggested that the negative excursion of δ13Ccarb also overlapped with the maximum of the eccentricity cycle. It implied that orbital configurations may amplify the feedback of the LIP, causing this short-term warming interval.

Geomagnetic relative paleointensity and direction during the last 40,000 years obtained from a sediment core in the Nankai Trough

Construction of regional geomagnetic secular variation curves for the last several tens of thousands of years is important for understanding the behavior of non-dipole fields and applications to geochronology. Around Japan, secular variation records of older than 10 ka was scarce, in particular for relative paleointensity (RPI). Here, we conducted a paleomagnetic study of a sediment core covering the last ~ 40 kyr taken from a small basin in the Nankai Trough. The core consists of homogenous hemipelagic sediments except for turbidites and volcanic ashes. The age model was constructed based on seven 14C datings and two volcanic ashes. Turbidites and volcanic ashes were excluded from the construction of secular variation curves because of geologically instantaneous deposition. It was revealed that the magnetization of this core is carried largely by detrital magnetic minerals, although magnetofossils are also contained. Bulk magnetic properties show some temporal changes in magnetic concentration and grain size, but still homogeneous enough for reliable RPI estimations except for turbidites and volcanic ashes. The resultant RPI shows no correlation with the normalizer, anhysteretic remanent magnetization, of the RPI estimations or with a proxy for a magnetic grain size and/or the proportion of magnetofossils to detrital magnetic minerals. The obtained RPI record shows a long-term increasing trend since ~ 40 ka, which coincides with global stack curves. On the other hand, there are some differences in shorter timescale variations, which may reflect non-dipole fields. This study demonstrated that hemipelagic sediments in the Nankai Trough have potential for recovering high-quality RPI records when turbidites and volcanic ashes were excluded and are useful for accumulating records to construct a regional master curve.Graphical abstract

Emplacement of the paleoproterozoic Itareru tonalite (NE São Francisco Craton, Brazil) during arc-continent collision revealed by magnetic fabrics

The Itareru tonalite is one of the Paleoproterozoic plutonic rock units that intruded the Serrinha block, an Archean-Paleoproterozoic crustal fragment of the São Francisco Craton (Bahia, Brazil). It is characterized by a ca. 80 km long, NW-SE elongated leaf-shaped body, mainly comprised of tonalite-diorite to granodiorite rocks with numerous dioritic enclaves. Magnetic studies were performed on 26 sites of the Itareru pluton (including country rocks). Anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) are coaxial, indicating that all minerals have the same orientation, partially controlled by regional strain. Rock-magnetism measurements reveal that paramagnetic–matrix minerals contributed significantly to the magnetic susceptibility and AMS while magnetite grains in the range of 2–5 μm are responsible for AARM. Magnetic fabrics in the central region of the pluton are probably primary (magmatic), which was acquired when the rocks were crystallized, reflecting magma flow linked to a magma feeder zone (magma chamber). The moderated magnetic lineation plunges in the northern part of the pluton suggest that this region was close to the feeder zone. In contrast, the southern region should be far from it, since the magnetic lineations are horizontal to sub-horizontal. The Itareru tonalite emplacement was also controlled by regional strains related to the collision of the Paleoproterozoic Rio Itapicuru greenstone Belt with the Archean basement.

Magnetic fabric data on partially molten granitic rocks from the Florianópolis Batholith, Southern Brazil

Contact thermal aureoles around mafic intrusions provide information about the emplacement and segregation of melts. The Garopaba-Silveira region is characterised by intrusions with large heat input responsible for the formation of a thermal aureole in the granitoids of the Paulo Lopes Suite (PLS), which extends along the coast to more distant locations in this region. Such melting processes are observed from meso-to microscale, along the granitoid-mafic dike interface and in areas as distant as 500 m from the mafic dikes. It is suggested that these melting processes have been aided by the breakdown of hydrated minerals, such as biotite, through dehydration-melting processes, as commonly found in migmatites and partially molten rocks. On the other hand, the contacts between the granitoids and the mafic dikes are predominantly sharp and straight, as well as, in some dikes, they are sinuous, diffuse, and present back veining, attesting to mutual intrusion. Given the complexity of these melting processes along the granitoid-mafic dike interface, this study applies magnetic susceptibility anisotropy (AMS), anisotropy of anhysteretic remanent magnetisation (AARM) and rock magnetism techniques to melting processes in a thermal aureole context. The magnetic mineralogy of the mafic dikes comprises ferrimagnetic minerals, magnetite, and Ti-poor titanomagnetite. In the granitoids, the ferrimagnetic contribution is dominant, although there are paramagnetic contributions, allowing for direct lateral variations of size of the magnetic grains at different distances (profiles), which may indicate crystallisation time as well as thermal effects in these rocks due to the mafic dike intrusions.

Inverse magnetic fabric of remagnetized limestones in the Zaduo area, Eastern Qiangtang Terrane: Implications for oroclinal bending in the Eastern Himalayan Syntaxis

Magnetic fabric analysis is a common technique to assess the strain regime during mountain building processes. Here, we use this approach to evaluate the tectonic evolution of the Tibetan Plateau and the Eastern Himalayan Syntaxis by analyzing the limestones of the Jurassic Buqu Formation in the Zaduo area, Eastern Qiangtang Terrane (China). These limestones were chemically remagnetized during the Cenozoic. For a proper assessment, it is relevant to understand how the mineralogy of the remagnetized limestones affects their magnetic fabric and how the magnetic fabric can improve our understanding of the tectonic strain and regional deformation. The role of the authigenic magnetite in the development of the magnetic fabric should thus be explored. Comparison of the bulk susceptibility (Km) with various natural and laboratory rock magnetic properties (Km versus natural remanent magnetization, Km versus saturation isothermal remanent magnetization, and Km versus saturation magnetization) indicates that susceptibility and remanences are both carried by authigenic magnetite. Most of the magnetite grains show axial ratios <1.3:1 according to the Néel diagram, and fall within the single-domain range based on the mass magnetic susceptibility (χ) and DC field-normalized anhysteretic remanent magnetization (χARM) ratio, giving rise to the inverse magnetic fabric observed. Twelve sites (120 specimens) are divided into four groups based on the magnetic fabric and rock magnetic behaviors. Overall, there is a clear trend of decreasing Km, natural remanent magnetization, saturation isothermal remanent magnetization, ferromagnetic percentage and shape parameter from Group I to IV. The K1 axis of all four groups documents a NNE-SSW oriented compression during remagnetization, contrasting with the Eocene NE-SW compression in the Gongjue area farther east. This different compressional regime likely resulted in different rotations and structural trends surrounding the Eastern Himalayan Syntaxis.

Magnetic multi-fabrics as tools for understanding ignimbrite emplacement processes: An example from late-Variscan Tharandter Wald Caldera, Bohemian Massif

Felsic collapse calderas pose a hazard to our society, thus detailed knowledge of collapse mechanisms and behavior of eruptive products could help us to better understand the active calderas. We present a model of the eruption and collapse of Late Carboniferous Tharandter Wald Caldera (Variscan Bohemian Massif) based on field mapping and a magnetic multi-fabric approach. We investigate intra-caldera rhyolitic ignimbrites using a combination of conventional in-phase and relatively new out-of-phase anisotropy of magnetic susceptibility, along with the anisotropy of anhysteretic remanent magnetization. A two-stage caldera evolution model is proposed. (1) The high-energetic and high-temperature ignimbrites filled the paleo-valleys to form a thick ignimbrite sheet with welded to rheomorphosed central portion. Concurrently, eruptions emptied the source magma chamber triggering the piston caldera collapse. (2) The subsidence caused a monoclinal ductile bending of the still-hot and ignimbrite sheet around the caldera rim towards its center. Caldera activity was terminated by the emplacement of subvolcanic ring and radial dikes associated with hydrothermal activity. The latter likely induced abundant magneto-mineralogical alteration and formed composite fabrics. Hence, the application of the multi-fabric approach may serve as a useful tool to extrapolate reliable geodynamic proxies from the magnetic fabric of old, eroded, or altered caldera ignimbrites.

An end-member modelling approach to pseudo-Thellier palaeointensity data

SUMMARY Absolute palaeointensities are notoriously hard to obtain, because conventional thermal Thellier palaeointensity experiments often have low success rates for volcanic samples. The thermal treatments necessary for these experiments potentially induce (magnetic) alteration in the samples, preventing a reliable palaeointensity estimate. These heating steps can be avoided by pseudo-Thellier measurements, where samples are demagnetized and remagnetized with alternating fields. However, pseudo-Thellier experiments intrinsically produce relative palaeointensities. Over the past years, attempts were made to calibrate pseudo-Thellier results into absolute palaeointensities for lavas by mapping laboratory induced anhysteretic remanent magnetizations (ARMs) to the thermally acquired natural remanent magnetizations (NRMs). Naturally occurring volcanic rocks, however, are assemblages of minerals differing in grain size, shape and chemistry. These different minerals all have their own characteristic mapping between ARMs and thermal NRMs. Here, we show that it is possible to find these characteristic mappings by unmixing the NRM demagnetization and the ARM acquisition curves into end-members, with an iterative method of non-negative matrix factorization. In turn, this end-member modelling approach (EMMA) allows for the calculation of absolute palaeointensities from pseudo-Thellier measurements. We tested our EMMA using a noise-free numerical data set, yielding a perfect reconstruction of the palaeointensities. When adding noise up to levels beyond what is expected in natural samples, the end-member model still produces the known palaeointensities well. In addition, we made a synthetic data set with natural volcanic samples from different volcanic edifices that were given a magnetization by heating and cooling them in a controlled magnetic field in the lab. The applied fields ranged between 10 and 70 µT. The average absolute difference between the calculated palaeointensity and the known lab field is around 10 µT for the models with 2–4 end-members, while the palaeointensity of almost all flows can be retrieved within a deviation of ±20 µT. The deviations between the palaeointensities and the known lab fields are almost Gaussian distributed around the expected values. Although the two data sets in our study show that there is potential for using this end-member modelling technique for finding absolute palaeointensities from pseudo-Thellier data, these synthetic data sets cannot be directly related to natural samples. Therefore, it is necessary to compile a data set of known palaeointensities from different volcanic sites that recently cooled in a known magnetic field to find the universal end-members in future studies.

Preferred Orientations of Magnetic Minerals Inferred from Magnetic Fabrics of Hantangang Quaternary Basalts

This paper presents a comprehensive analysis of the anisotropy of magnetic susceptibility (AMS) and paleomagnetic data from Quaternary basalt outcrops along the Hantangang River, Korea. A total of 554 samples were collected from 20 sites, representing three distinct units, Unit I, Unit II, and Unit III. Paleomagnetic data reveal a difference in the timing of eruptions between Units I and II, suggesting distinct periods by volcanic episodes. The mineral magnetic analysis identified titanomagnetite as the dominant magnetic carrier in the samples. AMS results indicated weak anisotropy and scattered AMS directions, indicating a low degree of preferred orientation of grains within the basalt rocks. The inverse AMS fabrics observed at specific sites are attributed to single-domain (SD) grains. Comparing the AMS data with the anisotropy of anhysteretic remanent magnetization (AARM) data, three distinct types of magnetic fabrics (normal, intermediate, and inverse) were discerned. The magnetic fabric was utilized to ascertain the flow direction based on the findings obtained from the AMS results. The findings suggest that the Quaternary basalts in this study’s area were primarily confined to the Hantangang River channel and its immediate vicinity during lava flow. However, distinct flow patterns are observed in the southwestern region, implying the presence of unknown volcanic sources.

Magnetoclimatological Record of Late Pleistocene Loess in the Southern Hunshandake Sandy Land, Inner Mongolia: A Threshold Response to the East Asian Summer Monsoon Variations

AbstractWe report the first complete loess–paleosol record spanning the last 130 kyr from the southern extremity of the Hunshandake Sandy Land (HSL) in central‐eastern Mongolia. Our combined mineral magnetic and geochemical results demonstrate that during the last interglacial, the front of the East Asian summer monsoon (EASM) extended to central‐eastern Inner Mongolia, consistent with modern observations of climate change. However, during the last glacial, typical magnetic parameters (e.g., magnetic susceptibility and anhysteretic remanent magnetization and their ratios), which have been successfully adopted to denote the EASM variations on the Chinese Loess Plateau (CLP), show only muted temporal variations and cannot be readily correlated with equivalent records from the main body of the CLP. Remarkably, the Zr/Rb ratio, a robust geochemical indictor of the winter monsoon, is positively correlated with saturation isothermal remanent magnetization and saturation magnetization during the last glacial, demonstrating the strong control of wind vigor on high‐field magnetization signals dominated by lithogenic ferrimagnetic minerals. In contrast to the variable response of magnetic parameters to regional paleoclimatic fluctuations, the Rb/Sr and Ba/Sr ratios (two summer monsoon indicators) faithfully track weak chemical weathering processes and fine‐scale monsoon oscillations, especially during the last glacial. Additionally, we found that higher proportions of pedogenic high‐coercivity magnetic minerals were formed during the last interglacial, which may reflect relatively low rainfall but higher evapotranspiration in the southern extremity of the HSL compared with the CLP. We therefore attribute this complex magnetic record to a threshold response to the EASM variations in eastern Inner Mongolia.

Mineral magnetism records shift in climate during the Kannur regolith formation, North Kerala, India

In this study, we conducted mineral magnetic analyses on three borehole core samples (∼40 m each) from the Kannur midland of northern Kerala, India to investigate iron oxide variations. Magnetic susceptibility (χlf) and its frequency dependency (χfd), Anhysteretic Remanence Magnetization (ARM), Isothermal Remanence Magnetization (IRM), Saturation IRM (SIRM), Hard IRM (HIRM), Soft IRM, and Munsell-Redness Rating were utilized to identify and estimate the iron oxide variations in these samples. We observed a notable shift from reductive to oxidative environments between 20 and 30 m depths among the studied cores, indicating a significant climatic or tectonic event in the region. This abrupt changeover is consistent with the lithologic change marked by a brief peat interval followed by lateritic sediments, which reflect the anoxic to oxic conditions within the closed system of the Kannur regolith basin. The basin-fill, characterized by the rapid hinterland erosion of upland laterites without the return of peat environments, suggests monsoonal intensification. The widespread occurrence of this interval within the basin may indicate a significant change in the climate. We observed an enrichment of authigenic magnetites in peat and lagoonal clays, which are succeeded by hematite and goethite-dominant estuarine to fluvial conditions inferred for monsoonal intensification during stabilized sea level and tectonic stability in the mid-Holocene. The goethite/hematite magnetic mineral ratio revealed the variation within warm-humid to warm-arid conditions, elaborating the monsoonal variability through the majority of the Mid to Late Holocene in the absence of chronology. We propose that this changeover in authigenic reductive to oxidative detrital magnetic mineralogy can serve as a stratigraphic benchmark for climate change during the Mid to Late Holocene in the Kannur basin of Kerala.

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

Effect of land use conversion on heavy metals and magnetic minerals on water reservoir riparian soils

Spatial hydrological alterations seem to influence the variability of soil-bound magnetic particles and heavy metals in water reservoir riparian wetlands (RW). To date, applying geochemical analysis with magnetic techniques to assess heavy metal pollution is rarely practiced in RW soils. We studied the magnetic properties and heavy metals, including Cu, Cr, and Zn, of topsoils in RW and the adjacent upland regions (UR, as control) in the Three Gorges Reservoir, China. Potentially elevated low-frequency mass magnetic susceptibility (χLF), anhysteretic remanent magnetization susceptibility (χARM), isothermal remanent magnetism, and all selected heavy metals were found in RW. The grain size of the magnetic minerals was coarser in RW than that in UR. The pollution load index (PLI) of the studied samples was 1.18 ± 0.12 and 1.04 ± 0.21 in RW and UR, respectively. PLI and concentrations of Cu, Cr, and Zn were positively correlated with χLF, χARM, and isothermal remanent magnetism in RW, whereas no clear linkages were observed between PLI and isothermal remanent magnetism in UR. This finding reveals that hydrological alterations increased the magnetic enhancement and heavy metal enrichment in RW. We find that magnetic proxies of soils could trace the concentration of selected anthropogenic heavy metals and their pollution level in RW.

A magnetic measurement technique for small rare samples: Pre-study for Chang’e-5 returned and other extraterrestrial samples

Standard-size samples are usually used for traditional paleomagnetic studies. In some cases, small irregular samples are employed considering their amount and magnetic strength as well as experiment efficiency. With the development of deep space exploration, the demand for studying small and magnetic weak samples is increasing. In this study, we established a magnetic measurement technique for small rare samples. We designed low-background, sample-nondestructive and high-flexibility holders with various materials for different measurements of small samples. With the customized holders, we estimated uncertainty and reliability of magnetic measurements of small specimens from a terrestrial basalt. The results indicate that susceptibility and remanence of small specimens (e.g., 2-mm cubes) are more affected by measurement position than larger specimens (≥5 mm), which reminds us to pay special attention to consistency of the small specimen position during series of measurements. Moreover, susceptibility and remanence of small sister specimens (2-mm and 5-mm cubes) are more scattered than those of large ones (cube length ≥1 cm), mainly due to inhomogeneity of magnetic mineral abundance in the sample. Nevertheless, the average magnetic results of different-size specimens are generally consistent within error, indicating the inhomogeneity of small specimens can be averaged out and demonstrating the reliability of small samples during magnetic measurements. Finally, we tested our measurement system with a lunar meteorite, which mimics the Chang’e-5 returned samples in both size and mineralogy. The main minerals of the meteorite are plagioclase, pyroxene, troilite and kamacite. The isothermal remanent magnetization normalized paleointensity method recovered the expected laboratory field while the anhysteretic remanent magnetization corrected paleointensities are about 2-3 times the expected field, where the latter can be explained by inapplicability of the empirical f′ to the meteorite specimens. The anhysteretic remanent magnetization and IRM paleointensity calibration factor f′ and a of the lunar meteorite were calculated to be 3.89 and 3138 μT, respectively, which provides new constraints for the empirical values. The magnetic results of the lunar meteorite demonstrate the utility of our measurement system for small weak samples. This study provides technical support for paleomagnetic study of the Chang’e-5 and other extraterrestrial samples in the future.

Rockmagnetic Characterization of the Quaternary Fluviolacustrine Sediments of the Thimi Section, Kathmandu Valley, Nepal

The fine-grained silt/clay sediments sampled at 38 levels of a 21 m thick segment at the Thimi outcrop in the Kathmandu valley have been studied for their rockmagnetic properties. These sediments have yielded consistently normal magnetic polarity and 14C ages determined by various workers between 20 and 45 kyrs or even more. They were measured for a set of rock magnetic parameters: low-field magnetic susceptibility, parameters derived from isothermal remanent magnetization (IRM) acquisition and demagnetisation (IRM0.3T, IRM3T, MDFIRM), anhysteretic remanent magnetization (ARM) (ARMtot, ARM(30 mT)/ARMtot, χARM), hysteresis parameters, etc. Several relationships between these parameters have been suggested for the Kathmandu valley sediments. Four rockmagnetic zones, based on the minor variations in magnetic mineral types and grain size parameters, are proposed and their possible relationship to the published pollen data has been discussed.

Interpretation of Anhysteretic Remanent Magnetization Carriers in Magnetofossil‐Rich Marine Sediments

AbstractThe anhysteretic remanent magnetization (ARM) is an important magnetic parameter for quantifying the concentration of “fine”, mostly sub‐micron ferrimagnetic particles in rocks and sediments. A sound understanding of ARM carriers is needed to interpret magnetic and environmental information in sedimentary archives. This is often not achieved for marine sediments, which routinely contain several magnetic mineral components. Here, we analyze marine sediment records over different timescales for four sediment cores from the Eastern Pacific Ocean (Hole 1218A), the Antarctic margin (core P4‐1), the Arctic Ocean (core ARC5‐ICE4), and the South China Sea (core L07), using coercivity spectra analyses of ARM, first‐order reversal curve principal component analysis (FORC‐PCA), transmission electron microscopy (TEM), and unmixing of isothermal remanent magnetization (IRM) curves. By combining rock magnetic and TEM results, we find that the ARM is mainly carried by non‐interacting single domain (SD) biogenic magnetite (magnetofossils) in the studied sediments over all timescales. Low‐coercivity magnetic particles (detrital and fine‐grained extracellular magnetite) also contribute to the bulk ARM. Variable magnetofossil chain structures are detected and have a significant influence on ARM data interpretation. Quantitative FORC‐PCA endmember analyses based on quantile contours and coercivity distributions provide valuable information on the nature of endmembers and improve ARM data interpretation. We provide an integrated strategy to help reduce interpretational ambiguities related to ARM in future studies.

Magnetic fabric of the early-Ediacaran Itapetim monzogranitic pluton: magma flow during oblique extension along strike-slip shear zones (Eastern Brazil)

The Itapetim pluton is a sigmoid-shaped intrusion bounded by the NE-SW, sinistral Itapetim and Taperoá shear zones. The Anisotropy of Magnetic Susceptibility (AMS) and Anhysteretic Remanent Magnetization (AARM) techniques and microstructural analysis were employed in order to characterize the pluton's emplacement mechanism and its relationships with regional tectonics. Microstructures are mainly sub-magmatic/incipient solid-state. Magnetic susceptibilities are typically paramagnetic (k ≤ 0.5 mSI). Thermomagnetic curves and hysteresis loops further indicate that negligible ferrimagnetic susceptibilities may be the result of fine-grained, oxidized magnetite inclusions. AARM fabrics do not show direct correlations with AMS fabrics. A WNW to E-W trending, shallowly-plunging magnetic lineation is associated with an intermediate-to steep-dipping magnetic foliation. The magnetic lineation locally displays intermediate plunges that may represent possible feeder zones for magma ascent during the initial stages of emplacement. The proposed emplacement model for the Itapetim pluton combines bulk NNW-SSE shortening with NE-SW trending simple shear along the Itapetim and Taperoá shear zones, and a WNW-ESE/E-W dextral stretching in the splay between both shear zones. The crystallizing magma mush flowed laterally along the splay during bulk NNW-SSE transpression, which is consistent with continental collision and subsequent extrusion of the Central Domain of the Borborema Province during the Neoproterozoic.

Rotational remanent magnetization as a magnetic mineral diagnostic tool at low rotation rates

SUMMARY Prior work on rotational remanent magnetization (RRM) and rotational anhysteretic remanent magnetization (ARMROT) has demonstrated promise for magnetic mineral identification in earth materials. One challenge has been to calibrate the measurements to magnetic mineral types and microstructural controls, since previous studies have used differing spin rates, alternating field (AF) intensities and decay times, which hinders a comparison of data sets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25–3 Hz [rps] which allows a wide range of RRM and ARMROT characteristics to be utilized (at 100 mT AF field, 100 μT bias field). Sets of magnetic mineral extracts from sediments, and well characterized rock samples that contain the key magnetic minerals magnetite, pyrrhotite and greigite are used for a calibration of the RRM-ARMROT behaviour. Detrital pyrrhotite and pyrrhotite-bearing phyllites have largely small positive effective field (Bg) values (up to 6 μT), with differences in Bg and ARMROT ratios at 0.5 and 2.5 Hz [rps] allowing grain size discrimination. The positive Bg values, and changes in RRM and ARMROT with rotation rates allow distinction of pyrrhotite from magnetite and diagenetic greigite. Diagenetic greigite has Bg values of –83 to –109 μT (at 0.5 Hz [rps]) and unusual RRM variation at low rotation rates caused by anisotropy affects. In contrast to previous work, based on crushed and sized natural magnetite at high spin rates, Bg for single domain magnetite from intact bacterial magnetofossils from Upper Cretaceous Chalk has some of the lowest Bg (0–1 μT) and displays a steep decline in ARMROT with increasing rotation rates. A simple tool for particle size characterization of magnetite may be the ratio of ARMROT at spin rates 2.5 and 0.5 Hz [rps]. Stability of RRM is better studied using RRM acquisition with increasing AF field intensity, since static demagnetization imparts a nuisance gyroremanence along the field axis. Mineral microstructure, dislocations and particle interactions are likely additional effects on RRM behaviour that need more investigation.

Abundance and characteristics of sediment-bound magnetic minerals and trace elements in karst ditch wetland: A case study from Guizhou Province, China

This study investigated sediment-bound magnetic properties and selected trace elements level in the karst ditch wetland, Caohai National Nature Reserve, Guizhou Province, China. Sediment-bound magnetic signals were quantified using low-frequency mass magnetic susceptibility (χLF), anhysteretic remanent magnetization susceptibility (χARM), saturation isothermal remanent magnetization (SIRM), and percentage frequency-dependent susceptibility (χfd%). Concentrations of Cd, Cr, Sb and Zn were determined using inductively coupled plasma mass spectrometry. Sediment χLF, χARM, SIRM, and χfd% were higher than those of bedrocks and mainly altered by the pedogenic processes. The estimated χfd% ranged from 6.15 % to 14.62 % and reflected the magnetic grain sizes were largely concentrated in the range of superparamagnetic particles. The elevated concentrations of sediment-bound Cd, Cr, Sb, and Zn supported the significant contribution of the anthropogenic sources in the karst ditch wetlands. The weak relationship between magnetic signals and selected trace elements (p < 0.05) suggested the occurrence of few sediment-bound iron-containing minerals associated with selected trace elements. These results indicated that a minor contribution of anthropogenic sources of selected trace elements to the elevated sediment magnetic signals in the karst ditch wetlands.