Mineral Magnetic Properties Research Articles

Paleomagnetic records and mineral-magnetic properties of deep-sea sediments in the NW pacific: Paleoenvironmental implication

The paleomagnetic records and mineral-magnetic properties of unconsolidated core sediment from the east Mariana Basin of the western Pacific have been analyzed to trace the time-dependent variations in sedimentary environments. Progressive alternating field demagnetization effectively extracts a stable remanent magnetization showing both normal and reverse polarities. Comparison of successive polarity changes, recorded in the sediment core, with reference magnetic polarity time-scale, reveals that the recovered sediment column was deposited since the late Pliocene. From the sediment age model, calculated sedimentation rate during the late Pliocene was 9.8 times higher than that during the Pleistocene. Considering the oceanic environments and geologic setting in the study area, the anomalous high sediment flux during the late Pliocene was probably caused by enhanced current flows, such as North Equatorial Current, associated with atmospheric circulation as well as by debris flows from adjacent sea mounts. In addition, the systematic variation of mineral-magnetic properties indicates periodical fluxes of coarse and magnetically stable particles, on the fine-grained dominant sedimentary environments. Such influxes, however, would not be related to syn-volcanic activities, because the summits of seamounts were totally blanketed by biogenic Pliocene-Pleistocene sediments. It is, hence, reasonable to interpret that paleomagnetic and mineral-magnetic data probably reflect drastic paleoenvironmental changes at the boundary between the Pliocene and Pleistocene, where strong current and atmospheric circulations decreased.

Discriminating fine sediment sources and the application of sediment tracers in burned catchments: a review

AbstractWildfire can cause substantial changes to runoff, erosion and downstream sediment delivery processes. In response to these disturbance effects, the main sources of sediment transported within burned catchments may also change. Sediment tracing offers an approach to determine the proportional contributions of fine sediment (typically <63 µm) from burned catchment sources. In this paper, we review the application of various sediment tracers to discriminate fine sediment sources following wildfire. Fallout radionuclides provide the most effective tracers for discriminating hillslope surface and sub‐surface (usually channel bank) sources. Previous tracer studies quantifying contributions from these sources after wildfire are based exclusively on radionuclides. The potential for fallout radionuclides to discriminate spatial sources defined according to burned and unburned areas depends on burn‐related changes in surface soil concentrations. Tracing of burned and unburned source areas will be problematic where most radionuclides in surface material are bound to ash rather than soil. Geochemical properties of surface soils are probably too susceptible to natural and burn‐related variability to consistently discriminate burn‐defined spatial sources. Mineral magnetic properties have shown potential for discriminating soil burned at different severities as well as unburned areas. More research is needed to assess the use of soil organic compounds as tracers of burned source areas. Linking fallout radionuclide‐based hillslope and channel source discrimination with process measurements and monitoring after fire can provide enhanced insight into fine sediment transfer and related water quality impacts. Adopting such integrated and multi‐scale approaches would contribute to improved understanding of hydrological and geomorphological responses to wildfire. Copyright © 2012 John Wiley & Sons, Ltd.

Mineral magnetic properties of Chinese paddy soils and its pedogenic implications

Abstract Magnetic susceptibility (MS) of paddy soils (Hydragric Anthrosols) and their corresponding dryland soils formed on the same parent materials in Zhejiang Province of Eastern China were measured in order to understand better magnetic profile discrimination and pedogenic magnetic minerals in paddy soils. Magnetic measurements showed that the MS values of paddy soils were much lower than those of dryland soils formed on the same parent materials. The higher the MS value of the original dryland soil is, the more remarkable the decrease in the MS value is when the soil is put under rice cultivation. Gleyed paddy soils exhibited very low MS value (

Magnetic discrimination between Al‐substituted hematites synthesized by hydrothermal and thermal dehydration methods and its geological significance

Hematite, a ubiquitous mineral in aerobic sediments and soils of temperate and warm areas, is weakly magnetic. However, it carries a stable natural remanent magnetization, and thus can reflect paleoenvironment changes. To quantify the influence of Al content in hematite on its magnetic properties, two series of hematite particles were prepared by hydrothermal transformation of ferrihydrite in aqueous suspension (HFh* series) and by thermal dehydration of goethite (HG* series). Crystal morphological and mineral magnetic properties of these two types of hematites differ distinctively. More specifically, the HFh* series samples display oblate (plate‐like) morphologies, while the HG* series samples are prolate (highly acicular). HFh* series samples display higher saturation magnetization but lower magnetic coercivity than that of the HG* series. It is tenable that a better lattice ordering of Al substitution occurs during the process of dehydration of goethite than after transformation from ferrihydrite, resulting in weaker saturation magnetization for HG* series samples. The origin of single domain (SD) hematite in nature can be diagnosed by the correlation of unblocking temperature and magnetic coercivity: a positive correlation indicates the presence of pure (Al‐free) SD hematite, while a negative correlation indicates a chemical origin of SD Al‐substituted hematite. These results bear new information on decoding the complex magnetic properties of SD Al‐hematite in nature environments, and thus deepen our understanding of the mechanism of variations in both paleomagnetic and paleoenvironmental signals carried by Al‐hematite.

The possible role of magnetic measurements in the discrimination of Sahara/Sahel dust sources

ABSTRACTThe mineral magnetic properties of deposited dusts collected along a broadly north‐to‐south transect across Niger have been investigated on both a bulk and particle size‐specific basis. Dusts display a general north‐to‐south gradient in fine‐grained ferrimagnetic mineral (magnetite/maghemite) concentrations, with samples south of the Sahara/Sahel transition (south of ~15·5o N) generally containing greater concentrations than dusts from further north where the climate is much drier. This distinction is seen especially clearly in the clay (< 2 µm) fraction, which harbours the products of weathering and pedogenesis. This gradient in ferrimagnetic mineral concentrations broadly parallels that previously reported for surface soils/sands across a similar latitudinal range. We suggest that the regional distinction in both dust and surface soil/sand magnetic properties can be related to differences in weathering regime between the arid Saharan parts of the transect and the Sahel sites where higher rainfall has permitted stronger weathering and pedogenesis. Given that the weathering‐related magnetic signatures in the clays dominate the magnetic properties of the < 16 µm fractions in these samples, and that this particle size component is most likely to be involved in long‐range transport, magnetic measurements hold out the potential of discriminating Sahara and Sahel dusts deposited in remote areas. Copyright © 2011 John Wiley & Sons, Ltd.

Coastal paleo-environment and sea-level change at Macassa Bay, southern Mozambique, since c 6600 cal BP

Changes in Holocene coastal paleo-environment and sea-level variations have been recorded in estuary sediments from Macassa Bay, southern Mozambique. Methods include analysis of fossil diatoms, sediment stratigraphy, mineral magnetic properties, stable carbon and nitrogen isotope composition and carbon and nitrogen content. The chronology, based on six AMS 14C dates, suggests a basal age of the sediment core of c 6600 cal BP (calibrated years Before Present). The multi-proxy dataset implies two phases when the site was experiencing marine conditions and tentative sea-level high-stands: 1) between c 6600–6300 cal BP which is contemporary with the last phase of the global transgression and the Holocene climatic optimum, and 2) between c 4700–1000 cal BP, a period when sea level curves from the region suggest occasional minor sea level high-stands. Between these phases (c 6300–4700 cal BP), the environment within the Macassa Bay estuary was less influenced by marine processes, and studied proxies indicate a freshwater phase associated with relatively low sea levels. After c 1000 cal BP, a terrestrial environment prevailed at the site, probably as a result of a combined effect of sea level lowering and high accumulation of organic peat within the estuary.

Early- and mid-Holocene palaeoenvironments as revealed by mineral magnetic, geochemical and palynological data of sediments from Bai Nuur and Ulan Nuur, southeastern inner Mongolia Plateau, China

Bai Nuur (41°38.590′N, 114°30.922′E, ∼1346 m a.s.l.) and Ulan Nuur (41°44.214′N, 115°05.630′E, ∼1246 m a.s.l.) are two small lakes situated in the transition zone between semi-humid and semi-arid climate regimes that runs parallel to the present limit of the southeast monsoon along the southeastern Inner Mongolia Plateau in north China. A 215 cm-deep sediment sequence was recovered from Bai Nuur and a 244 cm-deep one from Ulan Nuur. Soils were sampled from the catchments of the two lakes and dune-materials from a sandy land lying further north and northwest. Mineral magnetic measurements were performed on the sediments, soils and dune-materials. AMS 14C dating and analyses of particle-size, TOC and C/N and pollen composition were also made for the sediments. The mineral magnetic properties of the two sediment sequences, in combination with the other proxy-climate data, reveal the palaeoenvironmental development during early- and mid-Holocene around the two lakes. Climate began to ameliorate at 10,600 or 10,500 cal. BP and maximum humidity and warmth were reached during 10,600 or 10,500–9700 or 9600 cal. BP around Bai Nuur. Environments remained generally wet and/or warm until 7700 or 7600 cal. BP. Climate deterioration commenced at 7700 or 7600 cal. BP around Bai Nuur and at 7200 or 7100 cal. BP around Ulan Nuur. This deterioration intensified from 6900 or 6800 cal. BP onwards around Bai Nuur and from 6200 or 6100 cal. BP onwards around Ulan Nuur. The differences in timing of the reconstructed environmental changes at the two short-distanced sites may be due to the differences across the narrow climatic and vegetative transition zone, errors and uncertainties of dating and/or different chronostratigraphic frames of the two sequences. The palaeoenvironments reconstructed around the two small lakes (2.3 km 2 and 8.9 km 2) coincide with a palaeoenvironmental record from Anguli-nuur Lake, a much larger lake (47.6 km 2) in the same region. These data also agree with reconstructions at several other sites in different parts of the Inner Mongolia Plateau, although the Holocene amelioration took place earlier than in northern Mongolia.

Comparing mineral magnetic properties of sediments in two reservoirs in “strongly” and “mildly” eroded regions on the Guizhou Plateau, southwest China: A tool for inferring differences in sediment sources and soil erosion

Shibanqiao Reservoir (0.24km2) and Xiaohe Reservoir (0.28km2) have catchments of 6.00km2 and 22.69km2, respectively. They essentially represent “strongly eroded” (Shibanqiao) and “mildly eroded” (Xiaohe) regions in Guizhou Plateau, southwestern China. Sediment cores were retrieved from the two reservoirs and soils were also sampled from their catchments. Mineral magnetic measurements were performed on the sediments and soils, and the particle-size analysis of selected sediment cores and soil profiles were also made. SIRM versus χlf was used to compare sediment and soil samples. In addition, correlations between percentages of different particle-size fractions and magnetic parameters/ratios of sediment samples were also checked. These results revealed differences in sediment sources and soil erosion in these two representative catchments. The sediments in Shibanqiao Reservoir are derived from both the topsoil and subsoil. When erosion is relatively weak, it is limited to the topsoil and preferentially transports the relatively finer magnetic grains into the reservoir, which may imply rain splash and/or sheet erosion. In this relatively steep catchment, stronger erosional forces are capable of not only moving relatively coarser ferrimagnetic grains from the topsoil, but also incising the subsoil and moving anti-ferromagnetic minerals into the reservoir, which seems to hint at not only sheet-wash but also rill and gully erosion. As there are no stable streams nor channel networks developed in this mountainous catchment, contributions from river-bank erosion to sediment yield are almost negligible. The sediments in Xiaohe Reservoir, which has a lower relief, are predominately derived from topsoil erosion. Here, erosion is concentrated to the topsoil and preferentially transports the relatively fine magnetic grains into the reservoir when erosional forces are weak. However, even when erosional forces increase in this relatively flat catchment, erosion is still restricted to the topsoil. Thus, relatively coarser ferrimagnetic grains from the topsoil are transported to the reservoir, but the subsoil remains undisturbed and there are no significant changes in the proportion of anti-ferromagnetic minerals in the sediments. Consequently, only sheet-washing occurs even when erosion is intensifying in this hilly plain catchment. In general, the more effective erosion in the Shibanqiao Catchment is probably due to its steeper topography than Xiaohe Catchment, although higher and more seasonal rainfalls and lower percentages of woodlands may also contribute to the differences in susceptibility to erosional forces.

Environmental magnetic and petroleum hydrocarbons records in sediment cores from the north east coast of Tamilnadu, Bay of Bengal, India

In this study, mineral magnetic properties and petroleum hydrocarbons were statistically analysed in four sediment cores (C1, A1, T1 and K1) from the north east coast of Tamilnadu, India to examine the feasibility of PHC concentrations assessment using magnetic susceptibility. The C1 and A1 cores reveal a clear horizon of increase in PHC above 35 and 50cm respectively suggesting the excess anthropogenic loading occurred in the recent past. Magnetic properties which were enhanced in the upper part of the sediment cores were the result of ferrimagnetic minerals from anthropogenic sources. Factor analysis confirmed that the input of magnetic minerals and petroleum hydrocarbons in Chennai coastal sediments are derived from the same sources. The present study shows that instead of expensive and destructive PHC chemical methods, magnetic susceptibility is found to be a suitable, cheap and rapid method for detailed study of petroleum hydrocarbon contamination in marine sediments.

Assessment of heavy metal contamination and mineral magnetic characterization of the Asopos River sediments (Central Greece)

The content and distribution of heavy metals in sediments of the intermittent and contaminated Asopos River, located in Central Greece, was assessed by means of total dissolution, dilute acid and sequential extraction procedures. Mineral magnetic properties were used as proxy parameters of transport mechanisms of land-derived material to the sea. The combination of enrichment factors estimated against local background levels and the levels of labile metals revealed that surface sediments are enriched in Cu, Cd, Ni and Cr. The low flow of the system allows particles and organic matter to accumulate in the estuarine shallow “pools” where they undertake a series of redox reactions, authigenic formation, etc. The most fine of these particles, which are metal rich, are transported to relatively long distances off the Asopos River mouth, even under low energy conditions, converting the system to a secondary source of pollution for the adjacent marine environment.

Characterization of diamictons on the basis of their mineral magnetic properties in Murchisonfjorden, Nordaustlandet, Svalbard

Different grain-size fractions of 12 lithologically defined and 2 non-defined Weichselian glacial stage diamicton units from Nordaustlandet, Svalbard were studied for their mineral magnetic properties. The applied measurements included magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM), saturation isothermal remanent magnetization (SIRM), ‘backfield’ isothermal remanent magnetization (IRM) and several of their parametric ratios. The results indicated a poor correlation between the magnetic fingerprint of dry-sieved and wet-sieved diamicton samples. The dry-sieved samples had higher χ, ARM and SIRM values, suggesting a higher concentration of ferrimagnetic grains. This was probably because washing of the samples resulted in the loss of the most fine-grained ferrimagnetic material. Both sieving methods seem to be capable of discriminating different diamictons, but they cannot be used contemporaneously. The results only partially support previous findings of the chronostratigraphic correlation of diamictons. They indicate that in suitable conditions and within a limited area it is possible to discriminate (and group) glacigenic sequences based on the magnetic signature of their fine-grained fractions in the sample. For example and based on the mineral magnetic fingerprint, a diamicton unit that was described from the marine core (MUR 09/sor1/125–150) obtained from Isvika region most probably represents diamicton material that were derived from provenance areas related to the Middle Weichselian glacial stage. It was also noted that certain mineral magnetic parameters, such as SIRM/χ and SIRM/ARM ratio, had greater potential in discriminating diamicton units than others.

Mineral magnetic properties of surface soils and sands across four North African transects and links to climatic gradients

In light of their earlier success in characterizing and differentiating soils, sediments and atmospheric particulates, magnetic measurements were carried out on samples of North African surface soils and sands. The main aims were to document spatial variations in magnetic properties, explore their possible links to climatic gradients and their likely basis, and thereby help to differentiate potential sources for Sahara/Sahel‐derived dusts. Samples were collected along four broadly north‐to‐south orientated transects located in Egypt, Niger, Mali, and Benin and southern Togo. These transects cover major climatic gradients ranging from hyper‐arid to tropical. Across the rainfall range spanned by the Niger and Mali Transects (<100 to >800 mm/yr) there are significant north‐to‐south gradients in magnetic mineral concentrations. This is seen most clearly in the concentrations of the finest ferrimagnetic minerals (magnetite/maghemite), which are strongly represented in the clay fraction. Concentrations of the imperfect antiferromagnetic minerals (predominantly haematite) also increase with increasing rainfall. By analogy with studies spanning a similar rainfall gradient in areas of more uniform lithology, we suggest that the most likely cause for these trends is increased chemical weathering linked to the progressively wetter climate across the Sahara/Sahel transition. Within the set of samples from Egypt (rainfall >100 to <5 mm/yr), there is a marked gradient in the coercivity of remanence in the antiferromagnetic minerals present, with coercivity increasing southwards, paralleling the trend toward increasing aridity. Given the strong spatial discrimination demonstrated, especially in the Niger and Mali transects, these potentially climate‐linked gradients hold out the promise of discriminating between dust source regions.

Climatic interpretation of a 1.9 Ma environmental magnetic record of loess deposition and soil formation in the central eastern Pampas of Buenos Aires, Argentina

Much of what we know about Quaternary climate has been learned from sedimentary records from the world’s oceans. With the exception of the extensive studies of the Chinese loess/paleosol sequence and more recent studies of long lake records, there are few long terrestrial climate records, particularly from the southern hemisphere. The loess record of Argentina provides an important opportunity to further our understanding of climate change from a terrestrial environment, but its complexity and discontinuity have led to difficulty in formulating a climatological model of depositional and pedogenic processes. In this study, we present one of the longest and most continuous loess/loessoid records from the central eastern Pampas of Argentina. Our age model is based on optically stimulated luminescent dates and a paleomagnetic reversal stratigraphy and indicates a basal age around 1.9 Ma. Within the age model uncertainties, we characterize the environmental magnetic properties associated with loess deposition and soil formation with respect to wind patterns, moisture availability, and temperature. Major changes in magnetic grain size are linked to a differential northward shift of the subtropical high-pressure cell during glacial periods. We suggest that coarser (finer) magnetic grains correspond to weaker (stronger) glacial periods when the high-pressure cell is located in a more southerly (northerly) position and the source region is more proximal (distal) to our study area. An abrupt increase in the ultrafine-grained magnetic material around 0.9 Ma is related to an increase in moisture transport from the South Atlantic driven by an increase in summer sea surface temperatures at the mid-Pleistocene transition (∼1 Ma). In addition to these grain size variations, there is a relative decrease in the amount of goethite compared to hematite beginning around 0.5 Ma, which has been related to the temperature increase observed after the mid-Brunhes Event (∼450 ka) in the EPICA ice core temperature record. A more detailed comparison to insolation indicates that, for portions of the record, ferrimagnetic minerals are depleted during periods of low insolation. This result suggests that Argentine loess deposition and soil formation follows a model more similar to the Alaskan loess sequences than the Chinese loess sequences. Although further work is needed to validate the models and mechanisms proposed in this study, our record indicates that the mineral magnetic properties of the loess and paleosol deposits record major changes in deposition and soil formation and provide insight into possible mechanisms relating to global and/or hemispheric climate change.

Mineral Magnetic Properties of Sediments of Beaches, Redi–Vengurla Coast, Central West Coast of India: A Seasonal Characterization and Provenance Study

The beaches under study are characterized by distinctly different magnetic signatures in terms of their concentration and magnetic grain sizes. Seasonal variation in accumulation and erosion is seen at Vengurla Beach (Stations 1–7), which is moderate to very low premonsoon, high to low during monsoon season, and low to very low postmonsoon. Presence of fine single domain magnetic grains is moderate to high premonsoon, moderate to low during monsoon, and low postmonsoon at stations 1, 4, 7 (Vengurla beach), and 8 (Aravali beach). Aravali Beach has a very low concentration of magnetic minerals, precluding realistic assessment of its seasonal accretion–erosion pattern. At Redi Beach (Stations 15–20), the concentration levels of magnetic minerals are high premonsoon, which further increases during monsoon season, although at certain locations the rise continues postmonsoon. The sediments of these three beaches have variable proportions of magnetite, titanomagnetite, and hematite. The concentration of magnetic minerals is more at the northern (Stations 1 and 2) and southern (Stations 16–19) ends of Vengurla and Redi beaches, respectively. The provenance of magnetite and titanomagnetite can be attributed to Deccan traps and relict sands.This technique can complement the conventional methods and underlines the utility of magnetic parameters in studying sediment movement along the beaches.

Ophiolite Tectonics, Rock Magnetism and Palaeomagnetism, Cyprus

Magnetic properties of minerals may be sensitive indicators of provenance. Remanence-bearing minerals (RBM) such as iron–titanium oxides, and matrix-forming minerals such as paramagnetic phyllosilicate or diamagnetic calcite yield different clues to provenance, strain history and tectonics, and are essential supplements for the full interpretation of palaeomagnetic data. Moreover, mineral magnetic properties provide magnetic-petrofabric indicators of tectonic strain, determine the suitability of sites for palaeomagnetism, and permit the restoration of palaeomagnetic vectors in some strained rocks. In the Cretaceous Troodos ophiolite (~88 Ma) magnetic properties are dictated by the relative importance of mafic silicates and largely primary, ophiolite-derived RBM. In its cover of deformed pelagic sedimentary rock, magnetic properties are dictated by the balance of clastic RBM versus matrix calcite and in some cases clay. The two larger Cretaceous ophiolite outcrops (Troodos & Akamas) share a common orientation of their plutonic flow fabrics, determined by magnetic methods. The dike complex shows fabrics indicating plume-like feeders spaced along and perpendicular to the spreading axis, with longevities >0.5 Ma. South of the ophiolite, its Cretaceous-Miocene limestone cover possesses ubiquitous tectonic petrofabrics inferred from anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent susceptibility (AARM). Its foliation and maximum extension dip and plunge gently northward, sub-parallel to a common but previously unreported North-dipping stylolitic cleavage. In well-known localized areas, there are S-vergent thrusts and overturned folds. The S-vergent deformation fabrics are due to Late Miocene (pre-Messinian ~8 Ma) deformation. The structures are geometrically consistent with overthrusting of the Cretaceous Troodos-Akamas ophiolite, and its sedimentary cover, onto the underlying Triassic Mamonia terrane. The northern limit of pre-Messinian tectonic fabrics, the Troodos-Mamonia terrane boundary and the Arakapas-Transform fault form an approximate E–W composite boundary that we term the Troodos Tectonic Front. Miocene deformation remagnetized the ophiolite and its sedimentary cover in many places and also affects the Mamonia terrane to the SW, with which the Troodos terrane docked in the late Cretaceous. Magnetic mineralogy, particularly of the RBM traces the progressive un-roofing of the ophiolite during the deposition of its sedimentary cover. During the submarine exposure and erosion of the ophiolite, the contribution of RBM clasts to the overlying sedimentary cover changed qualitatively and quantitatively. Thus, magnetic mineralogy of the sedimentary rock cover records the progressive denudation of the ophiolite from lavas, down through dikes, to gabbros and deeper mantle rocks. Palaeomagnetic studies previously revealed the anticlockwise rotation of the Troodos terrane and its northwards migration. Characteristic remanent magnetism (ChRM) is most reliable for lavas and dikes although it is usually carried by recrystallized RBM. These correspond to the age of greenschist facies ocean-floor metamorphism, perhaps 7–15 Ma after igneous crystallization with an extent and depth dependent on depth and degree of hydrothermal circulation. The gabbros and mantle rocks commonly bear young ( 8 Ma), the remagnetizing effects of penetrative strain have been under-estimated. Where strain has occurred, un-tilting procedures produce erroneous restorations for the remanence vectors, and thus for the associated paleopoles. We find that de-straining of limestone sites most appropriately restores ChRM vectors to their original orientation. The best-determined and restored ChRMs define an apparent polar wander path (APWP). Since the APWP terminates at the present N-pole, we inverted it to determine the true plate-motion of the Troodos-terrane. Thus, in present-day coordinates, Troodos rocks moved ~1,000 km South; then ~4,500 km East and finally ~900 km North at an approximate rate of 75 km/Ma [1 km/Ma = 1 mm/a]. This true motion path commenced ~88 Ma ago and rates of motion since 65 Ma may be too high due to the limited precision of strain-corrections of the ChRM orientations in limestone. This true motion path is compatible with the eastward and then northward rotation of Africa relative to Europe although other workers show relative motion paths.

HIRM variations in the Chinese red-clay sequence: Insights into pedogenesis in the dust source area

Although extensive rock magnetic studies have been performed on the Chinese loess–paleosol sediments, few comparable studies have been applied to the underlying red-clay sediments. Because of this paucity of data, our understanding of the mineral magnetic properties of the red-clay sequence, and the paleoenvironmental information encoded in them, is limited. Here we report “hard” isothermal remanent magnetization (HIRM) variations from ∼8.1 to ∼1.2 Ma in the lower part of the loess–paleosol and the underlying red-clay sequences of the Chaona section on the Chinese Loess Plateau. The results indicate that there is a negative correlation between HIRM and frequency dependence of magnetic susceptibility χ fd ( χ fd = χ lf − χ hf, where χ lf and χ hf are magnetic susceptibility measured at 470 Hz and 4700 Hz, respectively) from ∼3.4 to ∼2.6 Ma and a positive correlation between these two proxies from ∼8.1 to ∼3.4 Ma and ∼2.6 to ∼1.2 Ma. We argue that the HIRM variations from ∼8.1 to ∼3.4 Ma and from ∼2.6 to ∼1.2 Ma reflect hematite availability in the dust source region, determined by the size of the area and climatic conditions of the dust source region. The negative correlation between HIRM and χ fd between ∼3.4 and ∼2.6 Ma suggests that magnetic enhancement during this time interval may be related to the alteration of hematite/goethite to magnetite/maghemite. We also find that HIRM data between 8.1 and 3.4 Ma show high coherence with the 100-kyr eccentricity signals, suggesting eccentricity forcing of concentration variations of hematite in the dust source area. Higher resolution samples are needed to determine if concentration variations of hematite in the dust source area are coherent with the orbital obliquity and precession forcing signals.

A late Pleistocene and Holocene mineral magnetic record from sediments of Lake Aibi, Dzungarian Basin, NW China

We studied mineral magnetic properties of a 6-m-long, late Pleistocene through Holocene sediment sequence from Lake Aibi in Dzungaria (Zunggary, Junggar), northern Xinjiang, China. Results were used to infer environmental changes and are compared with previously studied cores from Lake Manas. Both water bodies occupy the deepest parts of the Dzungarian Basin and are remnants of large Holocene lakes. During the Late Pleistocene, the magnetic mineralogy in both lakes was dominated by detrital, iron oxide minerals. Oxic conditions, which dominated during sedimentation and early diagenesis, persisted over the Pleistocene–Holocene transition. Later, during the middle Holocene, lake bottom conditions enabled authigenic formation of iron sulphide minerals such as pyrite (FeS2) in Lake Aibi, and pyrite and greigite (Fe3S4) in Lake Manas. This iron sulphide mineralogy suggests increased biological activity in stagnant, anoxic bottom waters. Anoxic bottom conditions started about 9.8 cal kyr BP in Lake Manas and at about 7.2 cal kyr BP in Lake Aibi. A short dry event recorded in Lake Manas between 6.8 and 5.2 cal kyr BP is not clearly observed in Lake Aibi. In the late Holocene, i.e. the last 2.8 cal kyr, sediments of both lakes are again characterised by iron oxides, suggesting well-mixed, shallow water bodies. For this recent period, it seems that the detrital material in the two lakes had a common origin. Magnetic properties of sediments in Lakes Aibi and Manas show broadly similar environmental evolution during the late Pleistocene and Holocene. Nevertheless, despite the close proximity of the two lakes (~200 km) in the same basin, they display some different magnetic properties and record environmental changes at different times.

Ancient stable magnetism of the Richardton H5 chondrite

Investigating mineral magnetic properties of meteorites is essential to understanding the formation and evolution of planetary bodies in the solar system. In order to decipher ancient magnetic records, demagnetization experiments were carried out for the ∼4550 Ma Richardton H5 chondrite. Alternating-field demagnetization revealed a soft fraction as well as a hard fraction. Conventional thermal demagnetization in air showed severe alterations. But, a few thermal demagnetizations in vacuum were successful in isolating a stable paleomagnetic record. On the basis of microscopic analysis, we found that fine-grained clinopyroxene-hosted kamacite is abundant, responsible for the stable and permanent magnetic record of Richardton. The experimental data imply a thermal or thermochemical origin for the stable paleomagnetic record of Richardton. However, the possibility of pressure (re)magnetization cannot be evaluated because the effect of pressure on magnetization for the Fe–Ni system is unknown.

Magnetic properties and environmental changes recorded in Lake Lehmilampi (Finland) during the Holocene

Standard mineral magnetic measurements, including magnetic susceptibility, ARM and IRM, were carried out on two Holocene sediment cores from Lake Lehmilampi (Finland), which span the last ~9,700 years. Paleoenvironmental changes were inferred from stratigraphic variations in the sediment mineral magnetic properties. Total organic carbon (TOC) measurements supplement the bulk mineral magnetic data. Development of Lake Lehmilampi is closely connected with Lake Pielinen, from which Lake Lehmilampi was gradually isolated ~5,066 Cal. years BP. Since the isolation of Lake Lehmilampi, its sediment has been annually laminated, with clastic-organic varves in the deep basin. Magnetic mineralogy is dominated by ferrimagnetic minerals, interpreted as magnetite throughout the sediment column, with a variable, but minor contribution of antiferromagnetic minerals. Sediment magnetic properties indicate a series of changes in sediment composition during the Holocene, which correlate well with sediment lithology and TOC. Magnetic properties differ in pre- and post-isolation sediments with respect to concentration of fine-grained magnetite, as expressed by χARM and χARM/SIRM, showing variation in response to organic matter content in post-isolation varved sediments. This may reflect authigenic bacterial production of magnetite in the weakly anoxic environment. Preliminary data suggest that magnetosome production in Lake Lehmilampi is controlled by lake productivity, and ultimately by climate.

Clay minerals in SW Indian continental shelf sediment cores as indicators of provenance and palaeomonsoonal conditions: a statistical approach

Two sediment cores from the inner continental shelf of SW India, SK‐148/13 (4.66 m long; from 50 m water depth) and SK‐148/14 (5.37 m long; from 22 m water depth), were studied for provenance and palaeomonsoonal implications. Sediment layers at different depth intervals in these cores were estimated for clay minerals and organic matter content. Five surface sediment samples from the nearby Kali River estuary of the adjacent hinterland were also analysed for clay mineral contents. In both cores and Kali River sediments, smectite is the dominant clay followed by illite, and kaolinite or chlorite. Based on the similarities of the abundances of clay minerals and their relative wt‐% in the inner shelf sediment cores and in surface sediments of the estuary as demonstrated by significance tests, Al‐rich illite, and negligible contribution of clay from the deep‐sea and aeolian sources, it can be inferred that the sediments of the two inner continental shelf cores were derived from the adjacent hinterland. Application of statistical discordancy and significance tests on the down‐core variations in the crystallinity index (CI) of illite and organic matter content in the sediment cores indicates intense monsoonal (high rainfall) conditions at the adjacent hinterland during about 4300–6200 and 9300–10,400 years BP. These inferences are comparable to those from other well‐established palaeomonsoonal indicators such as sedimentation rates and mineral magnetic properties. The CI of illite, an easily determinable and climatically sensitive parameter, can thus be a reliable palaemonsoonal indicator for inner shelf sediment cores. The statistical methodology used in this work highlights the advantages of a quantitative interpretation of the data instead of the conventional qualitative visual examination.