The theory, measurement and interpretation of frequency-dependent susceptibility (Xfd) are examined. A new model is proposed which explains Xfd in terms of the behaviour of all superparamagnetic grains (SP) with diameters between 0 and ~0.03 um. The model predicts maximum Xfd percentage values of 14-17 per cent for spherical SP ferrimagnetic grains in the grain size range 0.01-0.025 um, and a maximum value of 10-12 per cent for grain assemblages spanning a wider range of grain sizes (0-0.03 um). Synthetic and experimental data support the model predictions in terms of both maximum Xfd percentage values and the relationship between Xfd percentage and mass specific Xfd, which exhibits an envelope of data points partly related to grain size distributions within the SP range. When the Xfd percentage is at a maximum, the mass specific Xfd term can be used to estimate the concentration of SP grains in a sample. Lower values of Xfd percentage in soils are caused by the presence of narrow distributions of ultrafine SP grains, frequency-independent stable single and multi-domain ferrimagnetic grains. Some soils with low susceptibilities may have low Xfd percentages because of an appreciable content of paramagnetic and canted antiferromagnetic minerals. A simple mixing model predicts proportions of SP grains in mixed grain assemblages, but model validation requiring further characterization of grain interaction and grain size distributions is needed before it can be applied to environmental data.

Frequency-dependent susceptibility measurements of environmental materials

[1] In environmental magnetism, rock and mineral magnetic techniques are used to investigate the formation, transportation, deposition, and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes. All materials respond in some way to an applied magnetic field, and iron-bearing minerals are sensitive to a range of environmental processes, which makes magnetic measurements extremely useful for detecting signals associated with environmental processes. Environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics, chemistry, and biology and environmental science, including research on climate change, pollution, iron biomineralization, and depositional and diagenetic processes in sediments to name a few applications. Magnetic parameters are used to routinely scan sediments, but interpretation is often difficult and requires understanding of the underlying physics and chemistry. Thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities, complexities, and limitations to interpretations. In this review, we evaluate environmental magnetic parameters based on theory and empirical results. We describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations. We also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology, chemistry, and physics. Finally, we discuss directions in which environmental magnetism is likely to develop in the future.

/pdf/environmental-magnetism-principles-and-applications-42hbntvvit.pdf

Environmental magnetism: Principles and applications

Environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport, deposition, or transformation of magnetic grains is influenced by environmental processes in the atmosphere, hydrosphere, and lithosphere. The first explicit description of environmental magnetism as a distinct field was in 1980. Since that time environmental magnetism has become a broad field that is finding application in an ever-increasing array of scientific disciplines. In this review of the present state of environmental magnetic studies, we divide the field into three broad, but arbitrary, categories. The first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments. This category includes the correlation of sediment cores using magnetic susceptibility measurements, studies of geomagnetic field behavior, the analysis of depositional and postdepositional mechanical processes that affect sediments, and the examination of magnetic parameters that might represent proxies for paleoclimatic variation. The second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment. These provenance investigations include studies of changes in catchment-derived sediment in lakes, fluctuations in contributions from terrigenous, aeolian and glaciogenic components in deep-sea sediments, and the origin of atmospheric particulates. The final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments, including pedogenesis, authigenetic/diagenetic formation of ferrimagnetic phases, dissolution of magnetic minerals due to reductive diagenesis, and contributions of biomagnetism to sedimentary magnetism. Because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques, it is likely that the scope of environmental magnetism will continue to expand rapidly. Environmental magnetism is capable of providing important data for studies of global environmental change, climatic processes, and the impact of humans on the environment, all of which are major research initiatives in the international scientific community. These factors suggest that environmental magnetism has a bright and diverse future.

Environmental magnetism: Past, present, and future

Geochemical characterization of the Luochuan loess-paleosol sequence, China, and paleoclimatic implications

At the Verwey transition (Tv≈110–120 K), magnetite transforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change markedly at Tv in the case of coarse grains but only slightly or inappreciably in the case of fine (<1 µm) grains. We find on the contrary that remanence changes at Tv by 50–80% in both large and small crystals, if they are stoichiometric. However, minor surface oxidation suppresses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 µm which were initially non-stoichiometric (oxidation parameter z from 0.2–0.7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero-field warming to 300 K with little or no indication of the Verwey transition. After the oxidized surface of each crystal was reduced to stoichiometric magnetite, the SIRM decreased sharply during warming by 50–80% around 110 K. The change in SIRM for the 0.22 µm grains was almost identical to that measured for a 1.5 mm natural magnetite crystal. Thus a 1012 change in particle volume does not materially affect the remanence transition at Tv but oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on Tv provides a sensitive test for maghemitization in soils, sediments and rocks.

/pdf/the-effect-of-oxidation-on-the-verwey-transition-in-lpis99fhta.pdf

The effect of oxidation on the Verwey transition in magnetite

This is an updated collation of magnetic parameters of rocks and minerals for geologists, geochemists, and geophysicists. Since the publication of the previous edition of Handbook of Physical Constants [74], two other collations have appeared [16, 18]. In addition, selected magnetic parameters have also been assembled [19, 22, 38, 41, 88]. Rather than produce a fully comprehensive collection, we have aimed for high-precision data obtained from wellcharacterized samples. Both tables and figures have been used for presenting the data, and best-fit equations have been provided for some of the displayed ata so that interpolations can be made easily. In an attempt to discourage the use of the outdated cgs system, all values are in the SI system (see Moskowitz, this volume). References have been cited for the sources used here. However, a more comprehensive bibliography has also been provided from which information can be extracted for samples which have not been included. The single-crystal constants and their variation with temperature and composition are for use by rock magnetists. Paleomagnetists and magnetic anomaly modelers have been provided with the magnetic properties of rocks and polycrystalline mineral samples. Lastly, we have made an effort to address the needs of environmental magnetism, a new group of researchers who require the values of sizeand

http://www.alaska-gold.com/RF003p0189.pdf

Magnetic Properties of Rocks and Minerals

SUMMARY Results of the first detailed study of the climate proxy record in the loess-palaeosol sequence at Xining-one of the few palaeoclimate sites in the currently arid western Loess Plateau of China-illustrate the importance of making many types of rockmagnetic measurements other than susceptibility. A multiparameter approach yielded confirmation that here, as elsewhere in the Loess Plateau, the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite. Nevertheless, magnetic enhancement was caused not exclusively by changes in relative grain size, but also by variations in concentration and mineralogy of the magnetic fraction. The effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence, respectively. The resulting signal was ascribed more confidently to variation in magnetic grain size, which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility. Variations in magnetic mineralogy were also determined to constrain interpretations further. The data were then used to discuss climate history at Xining. Finally, results from Xining were compared with other western sites and contrasted with eastern sites. In summary: (1) data is presented from a new Loess Plateau site which also appears to yield a global climate signal; (2) a demonstration is made of a more rock-magnetically robust way to separate concentration, composition and grain-size controls on susceptibility and other magnetic parameters; and (3) models are provided for inter-regional comparisons of palaeoclimate proxy records.

https://conservancy.umn.edu/bitstream/handle/11299/175069/Geophys.%20J.%20Int.-1995-Hunt-232-44.pdf?sequence=1

Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China

We propose a method based on thermal unblocking of low-temperature saturation remanent magnetization for a quantitative estimation of the superparamagnetic [Cullity, 1972] fraction (size, d < 30 nm) of magnetite produced by pedogenesis in the Chinese loess plateau [Liu, 1988] We applied this method to the proxy climatic records of the last 130 ka from two sites 250 km apart, but separated by the mountain range Liupan-shan Xifeng to the east (357°N, 1076°E) and Baicaoyuan to the west (362°N, 1050°E) currently have humid and arid microclimates, respectively As expected, the superparamagnetic fraction increases during known warm temperature intervals at each site Furthermore, the more humid site clearly has higher overall superparamagnetic fractions during most of the last 130 ka However, during the period 5 to 10 ka ago, the relative humidity at both sites was the same within experimental errors Bulk grain size evidence confirms the magnetic data, and we suggest that the present easterly summer monsoon in China came from a more southerly direction during this time to flow parallel to Liupan-shan, resulting in very similar summer humidity at Xifeng and Baicaoyuan

/pdf/separation-of-local-signals-from-the-regional-paleomonsoon-3v5jt8m7d2.pdf

Separation of local signals from the regional paleomonsoon record of the Chinese Loess Plateau: A rock‐magnetic approach

http://web.natur.cuni.cz/uhigug/kletetschka/dithionite.pdf

Effect of citrate-bicarbonate-dithionite treatment on fine-grained magnetite and maghemite

The role of maghemite in the enhancement of magnetic signal in the Chinese loess-paleosol sequence: An extensive rock magnetic study combined with citrate-bicarbonate-dithionite treatment

Christopher P. Hunt

Papers

Magnetic Properties of Rocks and Minerals

Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China

Separation of local signals from the regional paleomonsoon record of the Chinese Loess Plateau: A rock‐magnetic approach

Effect of citrate-bicarbonate-dithionite treatment on fine-grained magnetite and maghemite

The role of maghemite in the enhancement of magnetic signal in the Chinese loess-paleosol sequence: An extensive rock magnetic study combined with citrate-bicarbonate-dithionite treatment