Magnetic Susceptibility Signal Research Articles

Bead magnetorelaxometry with an on-chip magnetoresistive sensor

Magnetorelaxometry measurements on suspensions of magnetic beads are demonstrated using a planar Hall effect sensor chip embedded in a microfluidic system. The alternating magnetic field used for magnetizing the beads is provided by the sensor bias current and the complex magnetic susceptibility spectra are recorded as the 2nd harmonic of the sensor response. The complex magnetic susceptibility signal appears when a magnetic bead suspension is injected, it scales with the bead concentration, and it follows the Cole-Cole expression for Brownian relaxation. The complex magnetic susceptibility signal resembles that from conventional magnetorelaxometry done on the same samples apart from an offset in Brownian relaxation frequency. The time dependence of the signal can be rationalized as originating from sedimented beads.

Cross-Basin Variations in Magnetic Susceptibility Influenced by Changing Sea Level, Paleogeography, and Paleoclimate: Upper Devonian, Western Canada Sedimentary Basin

Abstract Upper Devonian rocks in western Canada were deposited as carbonate-platform and surrounding slope and basinal rocks during a second-order sea-level cycle, characterized by long-term climatic warming, that ended with cooling associated with the Frasnian–Famennian mass-extinction event. In this contribution, we assess the hypothesis that high-resolution magnetic susceptibility (MS) data records cross-basin variations that are intimately tied to sea level and associated climatic change, the depositional environment, and the paleogeographic location of siliciclastic sediment sources. Four main trends are apparent in the MS data: (1) the magnitude of the MS signature increases from west to east across western Canada; (2) platform facies generally have low MS signatures compared with off-platform facies; (3) the overall MS signature increases and then decreases throughout the stratigraphic profile, largely in response to changes in depositional setting (i.e., item 2); (4) MS is generally high during third- and fourth-order sea-level lowstand and early transgression and lower during late transgression and highstand. During the late Middle (Givetian) to early Late (Frasnian) Devonian, siliciclastic sediments (silt and fine sand size) sourced from the west Alberta arch and Peace River arch, contributed to relatively high MS signatures in facies deposited proximal to these source areas. Other important detrital sediment sources were the Ellesmerian orogenic belt in northern Canada and the Laurussian continental landmass to the east. These latter sources supplied fine-grained sediments that were dispersed basinwide and resulted in a nearly order-of-magnitude increase in MS values to the east. This trend and fine grain size implies that some detrital sediment was delivered from the east as eolian dust from the “Old Red Continent.” By the early Frasnian, the west Alberta arch was submerged, and by middle to late Frasnian time the Peace River arch was largely covered. During the late middle Frasnian a pronounced rise in MS signatures can be tied directly to deposition of the clay- and locally silt-rich Mount Hawk Formation. During the late middle to late Frasnian, carbonate-ramp facies prograded from east to west, resulting in seaward migration of the shoreline and increased siliciclastic input to the ramp slope that influenced the pronounced increase in MS signatures to the east. By the late Frasnian, cross-basin MS profiles record uniformly low susceptibilities indicating that high MS siliciclastic detritus was either bypassed seaward, was swamped by carbonate input, or was dominated by diamagnetic quartz. Individual MS events can be accurately correlated across much of the basin at a resolution that is higher than single conodont zones despite the local influences on variability of the signature. Variations in the MS signature can thus be explained partly by depositional environment (platforms with low MS, basins with higher MS) and partly by third- and fourth-order sea-level and associated climatic changes. Fourth-order changes in sea level are routinely interpreted to result from variations in paleoclimate in the Milankovitch band and recent work indicates that third-order sea-level changes may be linked to longer-term modulation of Milankovitch-band orbital variations. The increase in MS during prolonged Frasnian warming thus appears to be directly linked to paleoclimatic change, and comparison of the MS signature with more direct measures, like oxygen isotopes, would serve as a test of its utility as a paleoclimate proxy.

Geophysical identification of unmarked historic graves

AbstractDown‐hole magnetic susceptibility techniques were explored as a means of improving near‐surface geophysical surveys in historic grave detection. These techniques were used to document distinctive magnetic characteristics of grave shafts at three historic cemeteries first surveyed using various near‐surface geophysical methods. Tests revealed a low magnetic susceptibility signature that soil magnetic studies indicated was largely related to differential soil compaction associated with the excavation and refilling of grave shafts. Most apparent at depths beyond those reached by soil penetrometers, this magnetic signature offers a way to assess anomalies identified by near‐surface techniques that potentially represent graves. At one cemetery, magnetic studies of the interments themselves suggested spatially patterned, magnetically enhanced zones that might also aid in burial identification in certain contexts. While down‐hole techniques will not be foolproof, they do provide another geophysical tool that can be used to improve grave detection. © 2010 Wiley Periodicals, Inc.

On the room temperature multiferroic BiFeO3: magnetic, dielectric and thermal properties

Magnetic dc susceptibility between 1.5 and 800 K, ac susceptibility and magnetization, thermodynamic properties, temperature dependence of radio and audio-wave dielectric constants and conductivity, contact-free dielectric constants at mm-wavelengths, as well as ferroelectric polarization are reported for single crystalline BiFeO3. A well developed anomaly in the magnetic susceptibility signals the onset of antiferromagnetic order close to 635 K. Beside this anomaly no further indications of phase or glass transitions are indicated in the magnetic dc and ac susceptibilities down to the lowest temperatures. The heat capacity has been measured from 2 K up to room temperature and significant contributions from magnon excitations have been detected. From the low-temperature heat capacity an anisotropy gap of the magnon modes of the order of 6 meV has been determined. The dielectric constants measured in standard two-point configuration are dominated by Maxwell-Wagner like effects for temperatures T > 300 K and frequencies below 1 MHz. At lower temperatures the temperature dependence of the dielectric constant and loss reveals no anomalies outside the experimental errors, indicating neither phase transitions nor strong spin phonon coupling. The temperature dependence of the dielectric constant was measured contact free at microwave frequencies. At room temperature the dielectric constant has an intrinsic value of 53. The loss is substantial and strongly frequency dependent indicating the predominance of hopping conductivity. Finally, in small thin samples we were able to measure the ferroelectric polarization between 10 and 200 K. The saturation polarization is of the order of 40 microC/cm^2, comparable to reports in literature.

Orbitally forced climate and sea-level changes in the Paleoceanic Tethyan domain (marl–limestone alternations, Lower Kimmeridgian, SE France)

High-resolution analysis of multiple climatic proxies was carried out on a ∼ 8.5-m Lower Kimmeridgian interval of a pelagic marl–limestone succession at Châteauneuf-d'Oze (Vocontian Basin, southeastern France). The aim of the study was to characterize the orbitally controlled sedimentary cyclicity and to decipher paleoclimatic and paleoceanographic changes. All analysed proxies (magnetic susceptibility, carbonate content, manganese content, and bulk carbon and oxygen stable isotopes ( δ 13C and δ 18O) respond in synchrony to orbitally forced climate change. Precession index cycles modulated by short (∼ 100 kyr) eccentricity cycles are strongly expressed in magnetic susceptibility, carbonate and Mn contents but less so in δ 18O. Obliquity cycles are expressed in manganese and particularly in δ 13C. The orbital forcing was conferred to these rhythmic pelagic sediments via paleoclimatic and paleoceanographic changes as follows. Detrital input and marine carbonate production are recorded by the magnetic susceptibility and carbonate signals with strong precession cyclicity. Calcareous nannofossil analysis shows the omnipresence of coccoliths and debris of coccoliths in the marls and limestones, suggesting that the carbonate production was in largest part in situ. Precession index may exert oscillations in the antagonist marine surface productivity and detrital flux processes via solar radiation change. Oxygenation of the seafloor leading to redox cycles is detected in the manganese signal and supported by the δ 13C record, with a strong expression of the obliquity. Finally, paleotemperature variations were deduced from the δ 18O record, although this latter has been in part perturbed by diagenesis. In sum, the variations in the studied proxies suggest a depositional model of combined climate and sea-level cycles forced in concert by Earth's orbital parameters. In particular, the limestones represent warmer climates and higher sea-levels and the marls colder climates and lower sea-levels. This depositional model concurs with a previous depositional model of deep-water carbonates of the German Kimmeridgian, while is in opposition to a previous one of the same section in the Vocontian Basin.

Stratigraphic significance and resolution of spectral reflectance logs in Lower Devonian carbonates of the Barrandian area, Czech Republic; a correlation with magnetic susceptibility and gamma-ray logs

Spectral reflectance (SR) is a relatively novel technique which is used as a proxy for organic carbon and siliciclastic input in carbonate sediments, in particular in core research. This study summarizes results from high-resolution outcrop SR logging of a Lower Devonian distal ramp carbonate section of the Barrandian area, Czech Republic. The SR data were correlated with parallel petrophysical and geochemical data with an aim to improving their cyclostratigraphic and event stratigraphic interpretation potential and the gaps SR data can fill. Transgressive–regressive trends, indicated by facies stacking patterns, CaCO 3 data and spectral gamma-ray (GRS) logs, are well correlated in the regional context and point to third-order eustatic sea-level fluctuations. Peak regressions of the TR cycles are sensitively marked by CIE L* a* b* parameters and percent reflectance in colour bands. The SR and magnetic susceptibility (MS) data are markedly cyclic on a more detailed scale. Their inferred periods vary between 49.2 and 205 kyr indicating that they may have been forced by orbital cycles in the Milankovitch band. The SR and MS cycles are either in phase or out of phase. The latter cycles point to a strong diagenetic forcing of the colour cyclicity. Hematite concentration cycles, inferred from SR signal in the Pragian red hemipelagic carbonates, are interpreted as reflecting changing bottom oxygenation and subsurface redox gradients during early diagenesis. The CIE L* parameter was found to be related to the presence of diagenetic hematite, which indicates that cyclostratigraphic analysis of simple greyscale data can be affected by considerable error due to chromacity. Although the CIE L* parameter is partly correlated with CaCO 3 concentrations, the MS data are not, which suggests that the MS signal can be significantly influenced by diagenetic minerals and its cyclostratigraphic interpretation can be potentially misleading. The SR data, backed up by GRS, MS and CaCO 3 data indicate that the red colour in the Pragian hemipelagic limestones originated from early diagenetic hematite precipitation under the conditions of an oxygenated ocean floor. This suggests that deep-sea bottom oxygentation may have been in operation during the early Devonian, which is traditionally perceived as a typical greenhouse period. The SR data can be obscured in several m thick zones of elevated CIE L* and other colour changes (“leaching zones”) related to postdepositional alteration along subvertical faults, however, this fault-related telogenic alteration has little influence on the MS signal. The combined use of SR and MS has a synergic effect and can significantly improve the interpretation of their respective cyclic signals, particularly in relation to the diagenetic overprint and possible carriers of the MS.

Contribution of rock magnetism to stratigraphy and palaeoenvironment of the Karaïn cave infill, Antalya, Turkey

The magnetic properties of the infill of the Karain cave in Turkey were compared to the results of a sedimentological study on three longitudinal sections. The sediments of this site, namely cavity E, cover a period of ca. 500 ka and correspond to a large part of the Middle and Upper Pleistocene. The evolution observed for the various magnetic parameters is mainly related to climatic changes which followed one another during the infilling history of the cave. Two preconditions are necessary to understand the environmental magnetism, namely both the identification and the quantification of different magnetic phases (components) in the sediments, expressed by the ratio of the various magnetic parameters and the analysis of the magnetization curves. The results obtained through the study of the different magnetic parameters (low-field initial bulk (κlf) and mass (χlf) magnetic susceptibility, frequency dependence of the magnetic susceptibility (χfd), anhysteretic remanent magnetization (ARM), hysteresis parameters, anhysteretic susceptibility, ...) on 171 samples allowed us: 1 — to follow the nature, size and source of the grains at each level, and 2 — to identify six alternating levels, having different magnetic susceptibility signatures during warm, damp periods, in which the contribution of prehistorical hominidae (Homo Sapiens or Homo Neandertalensis) and animals, could have modified the original sedimentary structures.

Magnetic susceptibility, phonons and dielectric constant of single crystalline BiFeO3

The magnetic susceptibility between 1.5 and 800 K, the infrared reflectivity at room temperature and the temperature dependence of the dielectric constant at mm-wavelengths are reported for single crystalline BiFeO3. A well developed anomaly in the magnetic susceptibility signals the onset of antiferromagnetic order close to 640 K. Beside this anomaly, no further indications of phase or spin-glass transitions are found in the susceptibility down to the lowest temperatures. From infrared reflectivity we were able to identify all 9 phonon modes which are expected to be infrared active within the ab plane of the crystal. The temperature dependence of the dielectric constant was measured contact free in the 100 GHz range yielding ϵ′ = 54 at room temperature. The loss is substantial and strongly frequency dependent indicating the predominance of hopping conductivity.

Drastic climatic shift at ∼2.8 Ma as recorded in eolian deposits of China and its implications for redefining the Pliocene-Pleistocene boundary

There has been a long-running debate over the position of the Pliocene-Pleistocene boundary, i.e. 1.8 Ma versus 2.6 Ma. This study presents litho- and pedo-stratigraphic characteristics, grain size, magnetic susceptibility, and dust sedimentation rates for ‘red clay’-loess sequences over northern China, in order to assess the drastic climatic event(s) recorded in the quasi-continuous eolian deposits, and to readdress the position of the Pliocene-Pleistocene boundary. In complete Chinese loess sequences, 33 soil-loess couplets have been identified and labeled with S i-L i system. Traditionally, the base of Chinese loess has been designated at the bottom of loess unit L33. However, a typical loess layer within the uppermost ‘red clay’ is identified and formally designated here as L34 and the soil unit between L33 and L34 as S33, which lowers the basal age of Chinese loess from 2.6 Ma to ∼2.8 Ma. Although the Neogene ‘red clay’ and the overlying Quaternary loess are both of windblown origin, they have quite different pedogenic characteristics. The loess consists of alternating reddish soil and yellowish loess layers, while the ‘red clay’ is composed of soils with much redder colour and carbonate nodule layers. In general, the “red clay” has been subjected to stronger pedogenic processes than the overlying loess, and can be regarded as an extremely thick soil complex. The alternation of loess and soils reflects large-scale oscillations between glacial and interglacial conditions, whereas the well-developed ‘red clay’ deposition indicates a relatively stable climate condition. At ∼2.8 Ma, shortly before the Gauss/Matuyama boundary, the eolian sequences show a sedimentary transition from the well-weathered ‘red clay’ to the alternating loess and paleosol deposits, accompanied by coarser particle size, increased dust accumulation rates, and change of magnetic susceptibility signature. These indicate a large climatic shift from long-lasting warm-humid conditions to large-amplitude cold-dry and warm-humid fluctuations around 2.8 Ma, and a dramatic increase in aridity over the dust source region. In contrast, such a drastic event is not identified around the top of the Olduvai event or at any other time of the past 7.7 Ma. As the Gauss/Matuyama boundary offers a sound foundation for global time correlation and chronostratigraphic classification, these results still support the proposal of ICS and INQUA, that the conventional Pliocene-Pleistocene boundary should be lowered from 1.8 Ma to 2.6 Ma.

A late Miocene–early Pliocene Antarctic deepwater record of repeated iron reduction events

In this study we present a late Miocene–early Pliocene record of sixty-four zones with prominent losses in the magnetic susceptibility signal, taken on a sediment drift (ODP Site 1095) on the Pacific continental rise of the West Antarctic Peninsula. The zones are comparable in shape and magnitude and occur commonly at glacial-to-interglacial transitions. High resolution records of organic matter, magnetic susceptibility and clay mineral composition from early Pliocene intervals demonstrate that neither dilution effects nor provenance changes of the sediments have caused the magnetic susceptibility losses. Instead, reductive dissolution of magnetite under suboxic conditions seems to be the most likely explanation. We propose that during the deglaciation exceptionally high organic fluxes in combination with weak bottom water currents and prominent sediment draping diatom ooze layers produced temporary suboxic conditions in the uppermost sediments. It is remarkable that synsedimentary suboxic conditions can be observed in one of the best ventilated open ocean regions of the World.

Influence of sedimentary setting on the use of magnetic susceptibility: examples from the Devonian of Belgium

AbstractBulk magnetic susceptibility measurements on sedimentological samples from all geological periods have been used widely in the last two decades for correlations and as a proxy for sea‐level variations. This paper explores the link between magnetic susceptibility, depositional setting and environmental parameters. These environmental parameters include distal–proximal transects, microfacies successions and fourth‐order trends on different carbonate platform types (platform, ramp, carbonate mound or atoll) during different Devonian stages (Eifelian, Givetian and Frasnian). Average magnetic susceptibility values over a distal–proximal‐trending facies succession vary markedly with depositional setting. On carbonate platforms, average magnetic susceptibility generally increases towards the top of shallowing‐upward sequences. On a distal–proximal transect, average magnetic susceptibility is intermediate for the deepest facies, decreases for the reef belts and increases to a maximum in the back‐reef zone. In ramps and atolls, magnetic susceptibility trends clearly differ; average magnetic susceptibility generally decreases towards the top of shallowing‐upward sequences and is highest in the deepest facies. The strong relationship between magnetic susceptibility, facies and sequences implies a strong environmental influence. However, the different responses in the different platform types suggest that sea‐level changes leading to variation in detrital input is not the only parameter controlling average magnetic susceptibility values. Other primary or secondary processes also probably influenced magnetic mineral distribution. Primary processes such as carbonate production and water agitation during deposition are probably key factors. When carbonate production is high, the proportion of magnetic minerals is diluted and the magnetic susceptibility signal decreases. High water agitation during deposition will also selectively remove magnetic minerals and will lead to low average magnetic susceptibility values. These parameters explain the lowest values observed on the reef platform, inner ramp and atoll crown, which are all in areas characterized by higher carbonate production and greater water agitation during deposition. The lowest values observed in the lagoon inside the atoll crown can be related to detrital isolation by the atoll crown. However, other parameters such as biogenic magnetite production or diagenesis can also influence the magnetic signal. Diagenesis can change magnetism by creating or destroying magnetic minerals. However, the influence of diagenesis probably is linked strongly to the primary facies (permeability, amount of clay or organic matter) and probably enhanced the primary signal. The complexity of the signal gives rise to correlation problems between different depositional settings. Thus, while magnetic susceptibility has the potential to be an important correlation tool, the results of this investigation indicate that it cannot be used without consideration of sedimentary processes and depositional environments and without strong biostratigraphical control.

Lateralized spatial and object memory encoding in entorhinal and perirhinal cortices

The perirhinal and entorhinal cortices are critical components of the medial temporal lobe (MTL) declarative memory system. Study of their specific functions using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI), however, has suffered from severe magnetic susceptibility signal dropout resulting in poor temporal signal-to-noise (tSNR) and thus weak BOLD signal detectability. We have demonstrated that higher spatial resolution in the z-plane leads to improved BOLD fMRI signal quality in the anterior medial temporal lobes when using a 16-element surface coil array at 3 T (Tesla). Using this technique, the present study investigated the roles of the anterior medial temporal lobe, particularly the entorhinal and perirhinal cortices, in both object and spatial memory. Participants viewed a series of fractal images and were instructed to encode either the object's identity or location. Object and spatial recognition memory were tested after 18-sec delays. Both the perirhinal and entorhinal cortices were active during the object and spatial encoding tasks. In both regions, object encoding was biased to the left hemisphere, whereas spatial encoding was biased to the right. A similar hemispheric bias was evident for recognition memory. Recent animal studies suggest functional dissociations among regions of the entorhinal cortex for spatial vs. object processing. Our findings suggest that this process-specific distinction may be expressed in the human brain as a hemispheric division of labor.

Mexico City topsoils: Heavy metals vs. magnetic susceptibility

The degree of anthropogenic impact in the urban environments can be evaluated in terms of metal concentration of soils, often established in terms of integrated pollution index (IPI). Detailed geochemical investigations on urban soils require a great amount of chemical analyses, which are time consuming and expensive. The magnetic susceptibility signal detected in some urban topsoils is mainly attributed to the ferromagnetic components of anthropogenic origins, which include in their molecules a portion of heavy metals. Therefore, the application of susceptibility measurements of soils allows the assessment of polluted areas reducing the amount of chemical analyses. The magnetic survey reported here for first time on Mexico City's topsoils tries to establish the link between magnetic proprieties and the content of heavy metals (Fe, Pb, Cu, Zn, Ni, Cr and V). The best correlation between susceptibility values and the heavy metal content was found for highly contaminated samples yielding susceptibility values higher than 400 χ × 10 − 8 SI. In contrast, low contamination levels (metals contents near to background values) don't have significant correlation with magnetic susceptibility. The results show that magnetic susceptibility has a significant correlation with IPI, Fe, Cu and Zn. It is concluded that magnetic susceptibility can be used as a first approach for heavy metal contamination in terms of IPI, allowing to recognize areas that would require a detailed chemical study.

Magnetic susceptibility evolution and sedimentary environments on carbonate platform sediments and atolls, comparison of the Frasnian from Belgium and Alberta, Canada

Magnetic susceptibility (MS) measurements on carbonate rocks are considered as a proxy for impurities delivered to the carbonate environments. In the absence of strong climatic or tectonic variations, bulk MS values have been linked to sea level variations, because sea-level fall increases clastic supply and therefore increases in magnetic mineral deposition. In this paper we explore the relationship between the average magnitude of bulk MS, with shallowing-up sequences and facies evolution in different Devonian carbonate complexes. Similarities and differences between these parameters have been scrutinized in carbonate attached platform and detached platforms (mounds and/or atolls) from Belgium and Canada. In the carbonate attached platforms from Belgium and Canada, the MS patterns are directly related to depositional environment. Mean MS values increase from the most distal towards the most proximal facies and towards the top of the majority of fourth-order shallowing-up sequences. These trends are in agreement with theoretical background (MS increases with regression). In the Belgian detached platform, the average MS pattern generally shows an opposite behaviour to that observed in the attached carbonate platforms. Average MS decreases towards the most proximal facies and towards the top of a majority of the fourth-order shallowing-up sequences. This behaviour can be explained by the influence of sedimentary rate and water agitation during deposition. A high sedimentary rate will dilute the magnetic minerals in the atoll facies and the high water agitation during deposition may be expected to have prevented the deposition of the magnetic grains. So, the combination of these two effects will result in the observed low values in the atoll crown and lagoonal facies. In the Canadian detached platform, MS is mainly negative. This means that the limestones are very pure. The technique does not appear to be appropriate in these rocks. The variations of average MS behaviour by platform type can imply difficulties in correlating carbonates from different settings. A comparison of time equivalent mound and platform deposits shows that after an important regressive surface, the MS values are increasing for the platform deposits and decreasing for the mound. So MS evolution can be in complete opposition (caused by highly different sedimentary rates) in different depositional settings. The MS signal preserved in carbonate rocks is probably mainly related to 1) varying clastic supplies; 2) varying carbonate accumulation rates (dilution of the magnetic minerals by high carbonate production) and 3) potentially diagenesis.

Astronomical calibration of the Early Oxfordian (Vocontian and Paris basins, France): Consequences of revising the Late Jurassic time scale

Magnetic susceptibility (MS) variations record Earth's orbital parameters in the clays and marls of the Early Oxfordian in the Paris and Vocontian basins of France. Climatically driven weathering of surrounding emergent areas and the resulting detrital input to the basins is a significant source of the MS signal. MS proves to be an effective tool for deciphering the orbital forcing signal in these sediments, and for assessing the chronology of these formations. The 405-kyr orbital eccentricity cycle is clearly visible in the MS signal and, consequently, is a valuable geochronometer for this portion of the Jurassic time scale. Astronomical calibration of the Mariae ammonite zone (basal Oxfordian stage) indicates a duration of ~ 2.2 myrs, whereas current time scales assign only 0.6 myrs to this biozone. However, the Late Jurassic time scale has large uncertainties in stage boundary ages (± 4 myrs), hence of interval durations. These results could lead to significant revisions in the M-sequence magnetic anomaly block model, and greatly improve the resolution of the Jurassic time scale.

Magnetic minerals in Calcic Luvisols (Chromic) developed in a warm Mediterranean region of Spain: Origin and paleoenvironmental significance

Soil formation under aerobic conditions usually enhances magnetic signals, mainly through neoformation of sub-micron magnetite and/or maghemite. In order to better understand the nature and magnitude of the enhancement, in this work we characterized the iron (Fe) oxides and magnetic properties of two Calcic Luvisols (Chromic) developed on calcareous orthoquartzites in a Spanish region with a warm Mediterranean climate. Weathering of the Fe-bearing minerals in the parent material (mostly Fe-rich smectite) had resulted in the neoformation of hematite and maghemite rather than goethite, thus causing a substantial increase in magnetic susceptibility, χ, saturation magnetization, M s, and other magnetic signals. The absolute frequency-dependent magnetic susceptibility, χ FD, which is a typical proxy for the concentration of sub-micron superparamagnetic pedogenic maghemite, was strongly correlated with the concentration of hematite (Hm). The estimated percent frequency-dependent magnetic susceptibility, χ FD%, was 15.7%. This and various other magnetic indicators indicate that the grain size distribution (GSD) of the pedogenic maghemite is narrower around the superparamagnetic/single domain threshold than is that of maghemite in soils and paleosols developed under other temperate climates (e.g., the Chinese Loesss Plateau, where χ FD% < 13%). Our results are consistent with predictions based on the ferrihydrite → maghemite → hematite transformation model. Also, they suggest that a wise combination of the Hm/ χ FD ratio, χ FD%, the hematite/goethite ratio, and several indicators of the degree of weathering provides a better tool for paleoclimatic reconstructions than does the current use of χ alone.

Climate variability of southern Chile since the Last Glacial Maximum: a continuous sedimentological record from Lago Puyehue (40°S)

This paper presents a multi-proxy climate record of an 11 m long core collected in Lago Puyehue (southern Chile, 40°S) and extending back to 18,000 cal yr BP. The multi-proxy analyses include sedimentology, mineralogy, grain size, geochemistry, loss-on-ignition, magnetic susceptibility and radiocarbon dating. Results demonstrate that sediment grain size is positively correlated with the biogenic sediment content and can be used as a proxy for lake paleoproductivity. On the other hand, the magnetic susceptibility signal is correlated with the aluminium and titanium concentrations and can be used as a proxy for the terrigenous supply. Temporal variations of sediment composition evidence that, since the Last Glacial Maximum, the Chilean Lake District was characterized by three abrupt climate changes superimposed on a long-term climate evolution. These rapid climate changes are: (1) an abrupt warming at the end of the Last Glacial Maximum at 17,300 cal yr BP; (2) a 13,100–12,300 cal yr BP cold event, ending rapidly and interpreted as the local counterpart of the Younger Dryas cold period, and (3) a 3,400–2,900 cal yr BP climatic instability synchronous with a period of low solar activity. The timing of the 13,100–12,300 cold event is compared with similar records in both hemispheres and demonstrates that this southern hemisphere climate change precedes the northern hemisphere Younger Dryas cold period by 500 to 1,000 years.

Lithological and pedological influences on the magnetic susceptibility of soil: Their consideration in magnetic pollution mapping

Magnetic susceptibility measurements are widely used to map and monitor the heavy metal pollution of soils. However, the magnetic properties of soils are influenced significantly by the bedrock lithology and soil-forming processes. Therefore, a main challenge in the data interpretation is to filter out the anthropogenic pollution signal. In this study we address this problem by analysing susceptibility values, heavy metal concentrations, as well as pedological parameters in a large soil data set from the eastern segment of Austria, covering a wide range of different lithologies and soil types. The statistic assessment demonstrates an influence of lithology and soil type on the magnetic susceptibility signal. Therefore anomalies are defined in sub sets of different soil types separately. Three different methods were applied to detect susceptibility anomalies: the median absolute deviation method, the boxplot method, and the population modelling method. These methods evaluate topsoil data only and can therefore also be applied to field measurements of magnetic susceptibility. The results were compared to the conventional method of calculating the difference of topsoil and subsoil susceptibility. All three approaches identify the main anomalies in the study area and are successful in circumventing the problem of erroneous anomaly definition due to pedological processes. However, knowledge of the lithological background is still necessary for a meaningful interpretation and can only be substituted by a large amount of data. The tested methods lead to thresholds of different height and therefore act as filters of different strength for the definition of anomalies.

Environmental changes at the Frasnian–Famennian boundary in Central Morocco (Northern Gondwana): integrated rock-magnetic and geochemical studies

Abstract Rock magnetic (magnetic susceptibility and hysteresis parameters) and geochemical analyses (major and trace elements) were carried out on whole rock samples of two Frasnian–Famennian boundary sections, Anajdam and Bou-Ounebdou in the Central Morocco (Western Meseta). During the Frasnian, the decreasing trend of the magnetic susceptibility signal, mainly carried by low-coercivity magnetite grains, indicates a gradual reduction of detrital influx. This decrease in detrital input parallels a Frasnian long-term sea-level rise. In the Late Frasnian Kellwasser Horizons, that are classically considered to represent highstand deposits, the magnetic signal exhibits the lowest intensities in connection with maximum diamagnetic contribution of the carbonate fraction. With respect to geochemical data, the two black carbonate-rich Kellwasser Horizons are characterized by noticeable positive anomalies of bottom-water dysoxic proxies and of marine primary productivity markers. Our data thus suggest that in Central Morocco, the Late Frasnian marine environments were marked by a relatively important biogenic productivity favouring the onset of oxygen-depleted conditions during periods of maximum transgression on the continental platforms.

Gulf of Mexico sediment sources and sediment transport trends from magnetic susceptibility measurements of surface samples

The magnetic properties from 200 trigger core-top and Van Veen grab sediment samples recovered from throughout the Gulf of Mexico have been analyzed and used to characterize sediment source and flow pattern distributions. Magnetic parameters included are anhysteretic remanent magnetism (ARM) and magnetic susceptibility (MS) measurements. Results from these measurements are compared to previously determined calcium carbonate percentages, and clay and hematite influx trajectories into the Gulf of Mexico for the same samples reported by Balsam and Beeson [Balsam, W.L. and Beeson, J.P., 2003. Sea-floor sediment distribution in the Gulf of Mexico, Deep-Sea Res. I, 50, 1421–1444.]. The ARM results give an estimate of magnetic grain size distributions, and by analogy, grain size distributions in general, whereas MS patterns show high detrital sediment accumulation zones within the Gulf. The dominant influx of modern high susceptibility sediment into the Gulf of Mexico appears to originate from the Red River, flow into Atchafalaya River Basin and out into the Gulf from Atchafalaya Bay, with significant additional contributions from the Mississippi River through the Southwest Pass of the Mississippi River Delta. This material then moves across the continental shelf and down through the Mississippi Canyon into the deep Gulf where it is redistributed at depths > 3600 m. The eastern shelf margins in the Gulf, offshore from Alabama and Florida, are accumulating calcite- or quartz-rich medium to fine-grained sediment that has a very low or diamagnetic MS signature. From the Louisiana to Texas Gulf coast margins, MS is moderate to high, suggesting a river influx of magnetic constituents from the volcanic fields in New Mexico, and from igneous and metamorphic sources in the Mississippi Basin. Offshore from western Mexico, the MS is high to moderate, but the Yucatan Shelf margin is characterized by low to diamagnetic MS values due to sediment dominated by calcite sands and oozes, a trend that continues to the east onto the West Florida Shelf. Additional measurements of samples collected in association with sites characterized by hydrocarbon seepage exhibit anomalously low MS values. The samples from the lower shelf and slope areas are typified by iron reduction by bacterial organisms in these samples. These results produce anomalous localized lows in the MS trends observed.