Anhysteretic Remanent Research Articles

Remanent magnetization in fresh xenoliths derived from combined demagnetization experiments: Magnetic mineralogy, origin and implications for mantle sources of magnetic anomalies

Abstract Fresh mantle xenoliths represent an exceptional opportunity to directly access Earth's interior. In particular, magnetic signals carried by upper mantle xenoliths may provide the only opportunity to determine the depth of remanent magnetization below the Moho boundary. In recent years, the nature, intensity and magnetic properties of the magnetic signal have been characterized by measurements of the 0020 induced magnetization in hysteresis loops. The natural remanent magnetization (NRM) intensity, despite providing the first indication of the magnetic nature of magnetite inclusions, has not been studied in detail. This study focusses attention on the number of recorded NRM directions and characteristics of the anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) demagnetization spectra. A collection of 17 extremely fresh mantle xenoliths has been subjected to AF demagnetization, characterizing samples with one, two or three populations of remanence carriers, with differing coercivities, that are potential carriers of magnetization at mantle depths.

Cenozoic magnetostratigraphy and magnetic properties of the southern Issyk-Kul basin, Kyrgyzstan

We present paleomagnetic data from the northern flank of the Tianshan range, southeast of Lake Issyk-Kul (Kyrgyzstan). 613 cores were collected in two parallel sections with a total thickness of 960m (Chon Kyzylsuu, CK) and 990m (Jeti Oguz, JO), as well as 48 cores at six sites in a nearby anticline. Rock magnetic analyses identify both magnetite and hematite in the fluvial–lacustrine sediments. The concentration of both minerals, the magnetite:hematite ratio, and the average magnetite grain size increase upward in both sections. Anisotropy of anhysteretic remanent magnetization defines a tectonic fabric with sub-horizontal maximum axes that parallel the strike direction together with intermediate and minimum axes that streak out about a great circle orthogonal to the maximum axes suggestive of a tectonic fabric emplaced during folding. Stepwise thermal demagnetization isolates interpretable magnetization components in 284 samples that define 26 polarity chrons in CK and 19 in JO. A positive fold test, dual polarities and systematic changes in rock-magnetic parameters with depth suggest that the high temperature magnetization component was acquired coevally with deposition. An age model based on a visual magnetostratigraphic correlation of both sections with the geomagnetic polarity time scale defines absolute ages from 26.0 to 13.3Ma, with a fairly constant sedimentation rate of 9–10cm/ka. A correlation based on a numerical algorithm arrives at a slightly different conclusion, with deposition ages from 25.2 to 11.0Ma and sedimentation rates from 5 to 8cm/ka. In comparison with sedimentation rates found at other magnetostratigraphic sections in the Tianshan realm, we infer that the sedimentary record in this part of the Issyk-Kul Basin precedes the more rapid phase of uplift of the Kyrgyz Tianshan. The onset of deposition and concomitant erosion of the adjacent Terskey Range is in good agreement with independent assessments of the exhumation history of this mountain range, with erosion increasing at 25–20Ma and accelerating after 11–13Ma.

The effects of secondary mineral formation on Coe-type paleointensity determinations: Theory and simulation

Thellier-type experiments are the most widely applied approaches for determining the absolute paleointensities of Earth's magnetic field. One major problem, however, is that specimens are prone to thermal alteration due to the intensive thermal treatment during experiments. Linear Arai plots with acceptable partial thermal remanent magnetization (pTRM) checks have been considered as evidence for the absence of or negligible effects of thermal alteration and as reliable indicators of high-quality paleointensity estimates. However, by simulating the Coe variant of the Thellier method on assemblages of single domain (SD) magnetite particles, it is demonstrated that new magnetic minerals, which form during thermal treatments, can result in linear, concave-up, or concave-down Arai plots depending on the magnetic properties of both the primary and secondary magnetic phases. Among this range of behavior, pseudoideal Arai plots, which are linear with acceptable pTRM check statistics, would lead to paleointensity underestimates. It is further demonstrated that pTRM checks are proportional to the degree of underestimate with a magnetic granulometry dependency for SD particles. Due to the complexity of this dependency, pTRM check statistics are only comparable when specimens have similar magnetic properties. This suggests that a universal threshold for pTRM check statistics is not likely to be effective. Since the criteria of linearity and low pTRM check statistics are insufficient to guarantee the fidelity of the estimates auxiliary rock magnetic methods such as temperature-dependent hysteresis parameters and anhysteretic remanent magnetization are highly recommended to identify the presence of alteration.

Magnetic Properties of Coring Samples from Aceh Basin West Off Sumatera Island

Three oriented core samples was taken by Japanese MIRAI Research Vessel in Aceh Basin west of the Sumatera Island. 927 small box samples from 3 coring samples was measure. Core bottom ages of three cores are in the last glacial maximum (around 19 kyBP). We measured Natural Remanent Magnetization (NRM), NRM after Alternating Field Demagnetization up to 800 Oe. Measurement of NRM have done by 2G Enterprice Squid, Isothermal Remanent Magnetization (IRM) and Anhysteretic Remanent Magnetization (ARM) on Paleomagnetic Laboratory of Geological Survey of Japan. Result of NRM after Alternating Field Demagnetization shows that there are reverse polarizaty magnetizations. This reverse polarity seems not usual since in Geologic Time Scale 1989 showed that the first reversal polarity was between Brunches Normal Epoch and Jaramillo Epoch beginning at 0.75 Ma There are some possibility of reverse polarity events contained in the three cores mentioned above. The first possibility is the result of the tsunami so that the sediment that settles on the Aceh Basin was a sediment that remain turbid flows that enable magnetic recording on magnetic sediment did not follow directions when he formed so that it seems a polarity reverse polarity when only superficial. The second possibility is indeed occurs under reverse polarity at age 700.000 years. Research in Argentina, South America show that between 11 thousand to two thousand years ago there was an excursion from the magnetic polarity produces some reverse polarity at the time (Nami, 1999). The third possibility was circular variation magnetic.

Magnetic study of a late Alpine dike crosscutting the regional foliation

Dikes crosscutting existing folds or foliation are one of the classic time markers used in establishing relative geological chronology. We examine the paleomagnetic directions and magnetic fabric in a deformed gneiss from the Southern Steep Belt, which lies immediately north of the Insubric fault, and in an aplite dike that transects this foliation. No clear foliation or other evidence of internal deformation is discernible in the aplite dike in the field and it was originally interpreted as post-tectonic, with its intrusion age placing a young limit on the timing of deformation in the adjacent gneiss. The aplite dike does not carry a stable remanent magnetization and cannot be used to resolve the age of intrusion. The characteristic paleomagnetic remanence in the gneiss is compatible with Eurasian paleomagnetic directions during the last 20Ma. These results suggest that the timing of magnetization postdates dextral motion along the Insubric and Rhine–Rhone faults. The low-field anisotropy of magnetic susceptibility (AMS) in the gneiss is controlled by the regional schistosity. High-field AMS suggests that both paramagnetic and ferromagnetic minerals contribute to the magnetic fabric, whereby the ferromagnetic minerals show a stronger lineation. This is further confirmed by the anisotropy of anhysteretic remanent magnetization (AARM). Rock magnetic experiments show that multi-domain magnetite is the main ferromagnetic mineral in the gneiss. Unexpectedly, the AMS results from the aplite dike show that it also has a magnetic fabric, which is shown by high-field AMS, AARM and rock magnetic experiments to be carried by superparamagnetic magnetite. The presence of magnetic fabric in the dike demonstrates that regional deformation was still on-going in the Southern Steep Belt of the Central Alps at the time of its intrusion.

A comparative study of magnetic anisotropy measurement techniques in relation to rock-magnetic properties

Magnetic anisotropy measurements are becoming increasingly common to many studies within the different disciplines of geology, involving sedimentary, igneous and metamorphic rocks.A plethora of techniques exists for measuring magnetic anisotropy of rocks. Some are rapid and non-destructive while others are more labor-intensive or may result in alteration of the magnetic minerals. All, however, have the potential of revealing a wealth of information when measured and interpreted correctly. In broad terms, anisotropy techniques subdivide into measurements of susceptibility, remanence and torque; here we consider the first two of these.Anisotropy of magnetic susceptibility (AMS) is by far the most utilized, and measures composite fabrics. Magnetic susceptibilities in high fields and low temperatures, however, are being increasingly used to isolate the paramagnetic contribution to the fabrics.When distinguishing between fabrics carried by different ferromagnetic phases, or to separate these from the diamagnetic and paramagnetic contributions to the fabric, then remanence anisotropy techniques become necessary. Anisotropies of thermal remanence (ATRMs), of anhysteretic remanence (AARM) and of isothermal remanence (AIRM) are the most common examples. Remanence anisotropy may be measured over the full spectrum of magnetic coercivities or over a targeted range (e.g. partial or ApARM). Moreover, anisotropies may be calculated using only the resolved field-parallel component of the vector, in which case a minimum of six different orientations is necessary to obtain a complete symmetric tensor, or using the three components (full vector) of the measured magnetic vectors (e.g. AvARM), in which case three orthogonal applied magnetizations are the minimum requirement.In this study we utilize a variety of magnetic remanence room temperature techniques to measure remanence anisotropy of selected coarse and finer grained gneiss–granulitic specimens with well-pronounced fabrics. Results are compared to room temperature AMS and are interpreted in terms of the applicability of instrumentation/technique to specific rock-magnetic properties.

A magnetic record of heavy metal pollution in the Yangtze River subaqueous delta

The rapid industrial development in the Yangtze River watershed over the last several decades has drawn great attention with respect to heavy metal pollution to the Yangtze River estuary and nearby coastal areas. In this study, a 236cm long sediment core was retrieved from the Yangtze River subaqueous delta (122°36′ E, 31°00′ N) in 2008 and analyzed for magnetic properties and geochemical compositions to investigate heavy metal pollution history. The activity of 137Cs peaked at depth 140cm, with a broad plateau between 120cm and 140cm, suggesting an average sedimentation rate of 3.11cmyr−1 for the upper 140cm layer. Magnetic susceptibility (χ), saturation isothermal remanent magnetization (SIRM), anhysteretic remanent magnetization (χARM) and heavy metal enrichment factors (EF) all showed an upward increase trend above depth 140cm, suggesting that increased ferrimagnetic mineral concentration was accompanied by heavy metal enrichment in the sediment. Geochemical and granolumetric analyses showed that sediment sources and particle sizes played minor roles in the variations of magnetic properties. The effect of diagenesis, which can lead to the selective removal of magnetic minerals, was noticeable in the lower part of the core (140–236cm). Co-variation between magnetic properties (χ, SIRM and χARM) and EF of Cu and Pb suggests that the elevated ferrimagnetic mineral concentration can be used as an indicator of heavy metal pollution in the reconstruction of environmental changes in estuarine and coastal settings.

Deformation in rocks from Itajaí basin, Southern Brazil, revealed by magnetic fabrics

Magnetic measurements were performed on sedimentary and volcanic rocks from the Itajaí basin. Rock magnetism analyses reveal that magnetite and hematite and paramagnetic minerals show different contributions to magnetic susceptibility. Magnetic fabrics were determined in 19 sites by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AARM). The analysis at the individual-site scale defines three AMS fabric types. The first type shows Kmin perpendicular to the bedding plane, while Kmax and Kint are scattered within the bedding plane itself. This fabric is usually interpreted as primary. The second type (77% of the sites) shows the three well-clustered AMS axes with Kmin still perpendicular to the bedding plane. The third type is characterized by well-clustered Kmax in the bedding plane, while Kmin and Kint are distributed along a girdle perpendicular to Kmax. The second and third fabric types are interpreted as combinations of sedimentary-compactional and tectonic contributions at the earliest, and at a slightly later stage of deformation, respectively. AMS represents the contribution of all the rock-forming minerals, while AARM isolates the contribution of remanence-bearing minerals with coercivity <100mT. For some sites, the AMS and AARM fabrics are non-coaxial, mainly with respect to the lineation orientations. The comparison of magnetic lineations and the regional structural data indicates that the rocks, with no mesoscopic or macroscopic deformation, were affected by the two deformation phases that occurred in the region. One of them results from compressional tectonics related to the collisional orogen (D1), and the other is extensional related to post-collisional orogen (D2). After the former tectonic event the volcanic rocks were emplaced at the top of the stratigraphic sequence and, after that the Itajaí basin as a whole was deformed by the extensional tectonic (D2) that occurred after 560Ma which is the age of the intrusion.

Monitoring of spatial variations of particulate matter (PM) pollution through bio-magnetic aspects of roadside plant leaves in an Indo-Burma hot spot region

Particulate matter (PM) is an important air pollutant because of its adverse impacts towards human health. The existing and conventional methods of PMs monitoring are found to be inadequate in feasibility, which paved the way of magnetic biomonitoring approach. The magnetic measurement carried through the plant leaves is useful means in assessing the PM pollution. Plant species are found to be an effective biomonitors and may act as natural filters by trapping and retaining the PM on their leaf surfaces. Therefore, the aim of this communication is to demonstrate the magnetic properties [magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM)] of two roadside plant leaves (Hibiscus rosa-sinensis and Mangifera indica) at four spatially distant sites. The study measures and compares the capabilities of these plants to accumulate and retain the PMs. The study also assesses the PM pollution at selected sites and establishes the relationship between magnetic properties and PM in the city of an Indo-Burma hot spot region. The results indicated a significant correlation between the concentration of ambient PM and magnetic measurement (χ, ARM and SIRM) of both the roadside plant leaves. Similarly, reasonably good correlations are obtained between magnetic parameters (χ, ARM and SIRM) and Fe content in PMs. Present study is, perhaps, a novel contribution in the area of bio-magnetic monitoring studied with several magnetic parameters viz., χ, ARM and SIRM. Results indicated that the bio-magnetic monitoring is applied for environmental geomagnetism which act as proxy for ambient PM pollution and further employed as an eco-sustainable tool for environmental management in urban and peri-urban regions.

Quantitative relationships between magnetic enhancement of modern soils and climatic variables over the Chinese Loess Plateau

Environmental magnetism has been widely employed to reconstruct past climate changes on the Chinese Loess Plateau (CLP), and several climofunctions based on the magnetic properties of loess have been developed. However, systematic investigation of the quantitative relationship between topsoil magnetic enhancement and modern climate remains uncommon. In this study, we obtained surface soil samples from 257 sites over the CLP and adjacent regions. From this set, we used 180 samples from sites unaffected by potential contamination to investigate the relationship between the commonly measured magnetic properties of magnetic susceptibility and magnetic remanence and modern climatic variables. The spatial distribution of the results demonstrates a strong NW–SE gradient of the magnetic enhancement of surface soils. The results of more detailed magnetic parameters indicate that pedogenic viscous superparamagnetic and stable single-domain particles are mainly responsible for the magnetic enhancement of the soils in the studied region; and that the magnetic grain-size distribution of the ferrimagnetic components of pedogenic origin remains almost constant, independent of pedogenic intensity. The uniform mechanism of magnetic enhancement, mainly linked with the concentration of the pedogenic components rather than with variations in magnetic grain-size, decreases the level of ambiguity in climate reconstructions based on magnetic measurements of Chinese loess. Statistical analyses, including correlation, Principal Component Analysis (PCA) and multiple regression analysis, suggest that annual rainfall rather than temperature exerts the dominant effect on soil magnetic enhancement. Finally, we used the results to develop several transfer functions to reconstruct mean annual precipitation (MAP). Transfer functions based on frequency-dependent susceptibility (χfd) and anhysteretic remanent magnetization (χARM) provide the most reliable estimates of MAP. This study significantly improves the understanding of the relationship between soil magnetic properties and climate.

Mineral magnetism of atmospheric dust over southwest coast of India: Impact of anthropogenic activities and implications to public health

Abstract We have used rock magnetic techniques in this study to assess atmospheric pollution at five stations in and around Mangalore city on the southwestern coast of India. Samples of dust were collected from two suburban areas (Thokkottu and Pumpwell located respectively ~ 10 km and 3 km from the city center), the city center itself (Milagres) and industrial/port areas (Panambur and Mangalore Refinery and Petrochemicals Limited (MRPL)). Low-frequency magnetic susceptibility (χ lf ), frequency-dependent susceptibility (χ fd ), susceptibility of anhysteretic remanent magnetization (χ ARM ) and isothermal remanent magnetization (IRM 20 to 1000 mT) were determined on 23 dust samples and inter-parametric ratios calculated. Results show that samples from suburban areas (particularly Thokkottu) are characterized by low χ lf ( − 8 m 3 kg − 1 ) and up to 6% χ fd , suggesting low levels of pollution and the presence of pedogenic magnetite possibly derived from soils by wind erosion. However, the average χ lf of Milagres, Panambur and MRPL dust samples is high by factors of 9.2, 3.3 and 2.6 compared to that of the Thokkottu sample. The Milagres sample contains magnetically “soft” minerals like magnetite, possibly indicating its derivation from motor vehicle exhaust. In contrast, the Panambur dust sample is characterized by magnetically “hard” minerals such as hematite and goethite as it has an 8-fold higher HIRM value compared to the Thokkottu sample. This magnetic signature is perhaps the result of dust particles derived from the grinding of hematite-rich iron ore by the Kudremukh Iron Ore Company Limited (KIOCL) at Panambur and its storage and export through the nearby New Mangalore Port. However, the dust sample from MRPL has magnetically “soft” minerals like magnetite. This magnetic mineral may have originated from petroleum refining processes at MRPL. Particulate pollution from industrial activities and motor vehicle exhaust is a threat to human health and is known to cause cardiovascular and respiratory ailments. Therefore, the pollution levels brought out by this study warrant a comprehensive epidemiological study in the area of study.

Tracking the migration of our Chinese ancestors using molecular science

The Bible tells the story of Noah, who, under the direction of God, constructed a giant boat (Noah’s Ark) to save his family and all the world’s animals from the Great flood. In China, the hero Dayu (∼4000 years ago) spent 13 years managing and fighting floods to save human lives. Yet, the wisest thing our ancestors did when facing floods was moving to higher grounds.The pattern of migration has thus been of great interest to archaeologists. However, the link between human migration and hydrological changes in ancienthistoryhasbeendifficult tofind because of the lack of valid proxies. Xie et al. [1] demonstrated that the combination of two independent proxies (the hopanoid biomarkers from aerobic bacteria living in peat bogs and the fine magnetic particles deposited in cave stalagmite) reasonably explained the events of floods associated with human migration in central China between 1000 and 13 000 years ago. Organic biomarkers are commonly used to reconstruct paleotemperatures. They appear, however, also to be sensitive to hydrological changes in the terrestrial environment. Hopanoids can be produced by a variety of microorganisms (primarily bacteria) including both aerobic and anaerobic species [2]. Linking hopanoids to ancient hydrology is based on the idea that hopanoid-producing aerobes in peatlands will be more abundant when the water table is low and the airsaturated surface zone is thicker. Thus, periods of high hopanoids abundance should reflect less rainfall and a dry climate, whereas periods of low hopanoids abundance should reflect greater rainfall and flooding. Indeed, Xie et al. observed patterns of high and low hopanoids at time scales consistent with the dry and wet periods’ characteristics of the Asian monsoon system in the past 13 000 years in the Dajiuhu peatland, 600 km northwest of Wuhan. Furthermore, Xie et al. surveyed extensively the archaeological sites in central China and found fewer archaeological sites in the lowlands in periods of low hopanoids, indicating human migration to higher ground during wet periods. Biomarkers are effective proxies for paleoenvironmental and paleoclimatic studies [3]. Xie et al. further showed that hopanoids correlated poorly with temperature inferred by othermethods.They also prudently validated the hopanoid abundance—oxic water table hypothesis using molecular DNA containing the specific gene sqhC that is responsible for the biosynthesis of hopanoids. The results indicate that 90%–94% of the sqhC sequences were attributed to aerobic organisms. One assumption of this claim is that the DNA of hopanoid producing organisms has been well preserved and the organisms have not continued to grow after burial. The predominance of aerobic organisms in the tested sqhC sequences indicates that this might be the case. However, caution needs to be taken when interpreting DNA recovered from ancient deposits. To validate the hopanoid proxy, Xie et al. employed an inorganic proxy. This proxy is based on the relative abundance of fine-grained magnetic minerals in total ferromagnetic particles collected from a stalagmite in the Heshang Cave in central China, ∼200 km south of the Dajiuhu peatland. The term anhysteretic remanent magnetization (ARM) is commonly used to characterize the magnetic property of fine-grained particles having a single domain; the term saturation isothermal remanentmagnetization (SIRM) characterizes the total magnetic property in a sample containing not only single-domain particles but also pseudo-single-domain and multidomain grains that are commonly coarser than the single-domain particles. In soil environments, fine-grained magnetic particles (e.g. magnetite and/or greigite) can be produced by microorganisms, such as magnetotactic bacteria and ironreducing bacteria [4] or by pedogenic processes.Coarser-grainedmagnetic particles (magnetite,maghemite, titanomagnetite or hematite), in contrast, are commonly derived from weathering of igneous or metamorphic rocks, so-called detrital sources. Xie et al. were unable to identify any of these minerals from the stalagmite because of the difficulty of isolating them without chemical damage. However, the authors cleverly linked the ratio of ARM/SIRM of the stalagmite in the sub-ground to the hydrological dynamics of the soil above the ground, with low ARM/SIRM ratios indicating a high water level in the soil resulting from greater rainfall. One caveat of the ARM/SIRM proxy is that the fine magnetic particles formed in the soil can go through transformation or degradation [5] during

Sea‐ice conditions in the Okhotsk Sea during the last 550 kyr deduced from environmental magnetism

Reconstructing past sea‐ice conditions in the Okhotsk Sea is important because sea‐ice conditions vary in response to global climate changes, which in turn may affect global ocean circulation through intermediate water mass formation. We conducted an environmental magnetic study of six cores from three stations in the central Okhotsk Sea to better understand temporal and spatial sea‐ice variations. Intercore correlations and age estimations are based mainly on geomagnetic paleointensity; an oxygen‐isotope stratigraphy is available for one station. Magnetic susceptibility (MS) minima are accompanied by maxima in color b*, the ratio of the anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (kARM/SIRM), and the S‐ratio, which indicates a higher proportion of biogenic to terrigenous magnetic components. This reflects enhanced ocean productivity. First‐order reversal curve diagrams and IRM component analyses support the dominance of biogenic magnetite at MS minima. In contrast, color b*, kARM/SIRM, and S‐ratio values are low when MS is high, which indicates an increased proportion of the terrigenous component that was probably transported as ice‐rafted debris (IRD). For the southern two stations, IRD accumulation increased in glacial and deglacial periods, which implies mobile sea‐ice conditions even in full glacials. This was succeeded by extremely enhanced ocean productivity in early interglacials, which suggests nearly ice‐free conditions. For the northernmost station, on the other hand, IRD accumulation was low in glacials and increased in early interglacials, which indicates perennial sea‐ice coverage with little mobility in glacials. Succeeding ocean‐productivity enhancement was delayed compared to the southern stations.

Origin of magnetic anomalies in the large Ebersbrunn diatreme, W Saxony, Germany

The Ebersbrunn diatreme is a deeply eroded (>1 km) diatreme structure in western Saxony, Germany. At current erosion levels, this ultramafic to carbonatitic diatreme is about 2 × 1.5 km in map view, which makes it a large one. Based on shallow drill cores, the diatreme contains coarse unbedded volcaniclastic rocks with up to 80 % country rock fragments. The diatreme is characterised by positive and negative magnetic anomalies, which are controlled mostly by the changing proportions of magnetic minerals in the rocks. The magnetic minerals are themselves contained in the juvenile fraction. Rock magnetic studies on three drill cores, one from the magnetic low and two from magnetic highs, including bulk susceptibility and its anisotropy, temperature dependent susceptibility, various remanence measurements (natural remanent magnetisation, anhysteretic remanence and isothermal remanent magnetisation) and alternating field demagnetisation have been performed. Additionally scanning electron microscope imaging and energy-dispersive X-ray spectroscopy were performed to better characterise the magnetic minerals. Magnetisation is caused by titanomagnetite with slightly varying Ti contents within all cores. Samples from the positive magnetic anomalies have a relatively high degree of anisotropy, but no preferred orientation of one of the principal axes can be seen. The magnetic highs are caused by non-bedded volcaniclastic rocks comparatively rich in juvenile particles. The negative magnetic anomaly is associated with a compaction-like magnetic fabric but without macroscopically visible bedding. Hence, the magnetic low is caused by rocks with a lower content of juvenile material. To the authors’ knowledge, this is the first time rock magnetic methods have been applied to diatreme rocks.

The fidelity of paleomagnetic records carried by magnetosome chains

Magnetotactic bacteria (MTB), which use aligned chains of magnetosomes (magnetic crystals) as a navigation tool, are found in a wide range of modern day marine, river and lacustrine environments and their fossilized remains are being increasing recognized in geological records. Despite an increasing realization that biogenically derived magnetic particles may play a key role in sedimentary magnetizations, little is known about the influence that they may have on the fidelity of paleomagnetic recordings. Using cultured Magnetospirillum magneticum strain AMB-1, we have conducted simple 2D (i.e., zero-inclination) deposition experiments to assess the efficiency with which magnetosome chains align along magnetic field lines and the implications that this has for paleomagnetic records. Our results indicate that the natural remanent magnetization (NRM) acquired by deposited MTB is near linear with applied field (0–120 μT), but that NRM acquisition does not perfectly follow the assumed linear trend and over a six-fold increase in applied field only a factor ∼5.5 increase in NRM intensity is observed (i.e., ∼9% lower than the assumed trend). Both anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) normalized relative paleointensities (RPIs) can successfully recover field strength variations under identical situations. When the MTB concentration of the initial solution is varied (up to a factor 8), NRM carried by MTB responds in the expected fashion (i.e., a doubling of bacteria produces a doubling of NRM intensity). Both ARM and SIRM, however, do not respond to as expected: A doubling of concentration corresponds to an increase of ∼1.76 in SIRM intensity and an increase of only ∼1.63 in ARM intensity. Both are influenced by magnetostatic interactions that arise from the close packing of bacterial cells or subchain interactions within a single bacterial cell. First-order reversal curve (FORC) diagrams, however, are too insensitive to indicate the presence of paleomagnetically relevant magnetic interactions. Remanence based methods (i.e., the ARM ratio) are more appropriate to detect such interactions.

Numerical relationships between magnetic parameters measured in Quaternary sediments and global paleoclimatic proxies

The complexity of most geological and geophysical problems prompts sometimes the use of non linear mathematical methods to handle them. An adaptive neuro fuzzy inference system (ANFIS) that combines fuzzy logic with neural networks, is applied here to study a paleoclimate section from the Quaternary sedimentary fill of the Lake Mucubaji (western Venezuela). The purpose of this work is to find a set of numerical relationships that could predict the possible connections between oxygen isotope (δ18O) values from two different locations in the northern hemisphere (Ammersee in southern Germany and an ice core from the Greenland Ice Core Project — GRIP) and rock-magnetic parameters measured in Mucubaji samples (i.e. mass-specific magnetic susceptibility — χ, magnetic remanence S-ratio, mass-specific saturation isothermal remanent magnetization — SIRM and anhysteretic remanent magnetization — ARM). The best inferences in terms of coefficient of determionation R2 and the Root Mean-Square Error (RMSE) are obtained using those magnetic data as input that include information about magnetite grain size distributions, e.g., SIRM and ARM in FIS structures [1χ, 4ARM] and [4ARM, 1SIRM]. A comparison between Ammersee and GRIP actual data, as well as their corresponding inferences for the FIS structure [4ARM, 1SIRM], reveals a reasonable good inference of global trends for both records, overlooking the regional and/or local paleoclimate forcings that might have affected Ammersee. A better correlation between global isotope paleoclimate records and magnetic proxies, is perhaps prevented by the role played by local and regional paleoclimate and tectonism in Mucubaji. We also argue that the ratio of ARM over SIRM appears to be related in a complex way to the onset and to the end of the Younger Dryas. Our novel approach to the assessment of a specific paleoclimate case study shows the potential of the ANFIS technique in solving problems where traditional univariate and multivariate linear regression methods could prove inadequate.

Dating of a Last Glacial loess sequence by relative geomagnetic palaeointensity: A case study from the Middle Danube Basin (Süttő, Hungary)

A thorough rock- and palaeomagnetic investigation of an almost 13-m-thick Upper Pleistocene loess–palaeosol sequence (LPSS) outcropping near the village of Süttő in northern Hungary was conducted to obtain rock magnetic and palaeomagnetic-based chronostratigraphies. The results of our rock magnetic investigations indicate that the classical “Chinese” model of magnetic enhancement, showing increasing concentration-dependent rock magnetic parameters in the course of pedogenesis, is also valid for the northern Carpathian Basin. We constructed a relative geomagnetic palaeointensity (RPI) record for the central part of the section by using the natural remanent magnetisation (NRM) after demagnetisation in alternating fields of 15 mT and by employing anhysteretic remanent magnetisation (ARM), low-field susceptibility (Klf) and “saturation” remanent magnetisation (“S”IRM) as normalisers. The three obtained RPI data sets show almost identical trends and are therefore regarded as reliable recorders of the palaeointensity of the Earth's magnetic field. The obtained RPI record allows for indirect dating of the LPSS at Süttő by correlation with the GLOPIS and the PISO stack. The derived age model is in agreement with the OSL-based chronostratigraphy, but contradicts the chronology based on the correlation between the susceptibility record and marine isotope stratigraphy (MIS), which gives rise to the hypothesis of a possible decoupling of the palaeoclimate in the Carpathian Basin and the marine realm during early MIS 3.

Soil formation and mineralogy of a Rhodic Luvisol — insights from magnetic and geochemical studies

Relict terra rossa soil from the most south-eastern part of Bulgaria, characterized by transitional Mediterranean climate, has been comprehensively studied by integrating magnetic, geochemical and spectroscopic methods to reveal the origin, pedogenic processes and phases in soil development of this particular soil type. The red colored Rhodic Luvisol is developed on metamorphosed Triassic limestones. Magnetic methods, which include thermomagnetic analysis of susceptibility, isothermal remanent magnetization (IRM) acquisition and thermal demagnetization, IRM component analysis, hysteresis measurements, low-temperature (down to 10K) IRM behavior, anhysteretic remanence and frequency dependent susceptibility, indicate the presence of three major magnetic phases — maghemite, hematite (Hm) and goethite (Gt). Hematite and goethite are identified also by diffuse reflectance spectroscopy (DRS). Depth variations of the ratio Hm/(Hm+Gt), deduced from the DRS spectra show higher hematite content in the upper soil horizons (A+B), while goethite's contribution is enriched in the lowermost part of the profile. A similar ratio, based on the established magnetic proxies for hematite and goethite, was constructed and its variations were compared with the DRS data. The magnetic proxy for Hm/(Hm+Gt) reflects the variations in the remanence-carrying mineral fraction of hematite and goethite and the obtained difference with the DRS data are ascribed to the presence of the paramagnetic (or superparamagnetic) goethite in the A and illuvial Bt1 and Bt2 horizons. The low ratio Feo/Fed between dithionite (Fed)- and oxalate (Feo)-extractable iron, and the large proportion of extractable iron with respect to total iron (Fed/Fetot) indicate an advanced degree of weathering. Depth variations of magnetic parameters and ratios (χlf, χfd, S-ratio) suggest magnetic enhancement with SP–SD maghemite grains, accompanied by magnetically stable magnetic carriers in the soil, while the parent material is magnetically depleted. Based on grain-size variations of the pedogenic maghemite phase, deduced from a χARM vs. χfd plot, and variations in the content of hematite and goethite in depth, in the uppermost part of the profile, aging of the pedogenic iron oxides is assumed (grain growth from SP to SD and larger), while in the bottom part of Bt1 and the Bt2, changes only in the concentration of maghemite are inferred. The presence of goethite in different parts of the profile and the mineralogy of Fe–Mn nodules are linked to more recent (Holocene) pedogenic changes in the profile. Analyses of trace and rare earth element content and magnetic data suggest possible contributions of aeolian dust flux from Sahara during the soil formation.

The Middle Palaeolithic site of Birzgane (Tebessa, Algeria): Rock magnetic property characterisation and past rainfall reconstruction

A new Middle Palaeolithic site was discovered at Birzgane, south of the city of Tebessa in eastern Algeria. The archaeological study revealed richness in archaeological artefacts, belonging probably to the Aterian culture, but an absence of pedunculates. The Aterian archaeological layer was assigned to the end of the Middle Pleistocene by uranium–thorium dating. The environmental conditions at the time were assessed by means of magnetic proxy data such as hysteresis parameters, magnetic susceptibility and remanent magnetisation.The magnetic fraction of the alluvial sediment contains mainly oxidised magnetite and smaller amounts of haematite and goethite; indicated by thermal demagnetisation curves of isothermal remanent magnetisation. Magnetic properties such as enhanced susceptibility, anhysteretic remanent magnetisation and frequency dependent susceptibility indicate that the upper part of the section was subjected to pedogenesis after deposition. A conceptual magnetic enhancement model was applied to reconstruct past rainfall quantities in the upper part, taking into account present day temperatures and rainfall, water saturation, soil moisture and magnetic susceptibility. A precipitation quantity of 560 ± 40 mm was estimated for the transition period from MIS 5 to 4, corresponding to the Saharan Pluvial, an extended humid climate period in northern Africa between 100,000 and 70,000 years before present.

Magnetic properties and heavy metal pollution of soils in the vicinity of a cement plant, Xuzhou (China)

Abstract This study characterized the magnetic property and levels of heavy metals of the topsoils near a cement plant. The concentrations of five selected heavy metals (Pb, Cu, Zn and Cd) were measured on 32 topsoil samples (0–20 cm) collected near a cement plant via inductively coupled plasma/mass spectroscopy (ICP-MS). The orders of enrichment factors (EF), on average, were Cd (7.3) > Cu (3) > Zn (2.9) > Pb (2.1), respectively. A self-organizing map (SOM) was applied to the concentrations of heavy metals for “correlation hunting”. Mineral magnetic concentration parameters, such as the specific magnetic susceptibility (χ), susceptibility of anhysteretic remanent magnetization (χ ARM ), saturation isothermal remanent magnetization (SIRM), together with interparametric ratios (such as IRM − 100mT /SIRM, SIRM/χ, χ ARM /SIRM) show that ferrimagnetic, superparamagnetic (SP) and multi-domain (MD) minerals dominated the soils. The results of correlation analysis indicate that copper showed a significant correlation with χ, χ ARM and SIRM but such a relationship with χ, χ ARM and SIRM was only weakly identified for Zn, Cd and Pb.