Paleomagnetic and environmental magnetic studies are commonly conducted on samples containing mixtures of magnetic minerals and/or grain sizes. Major hysteresis loops are routinely used to provide information about variations in magnetic mineralogy and grain size. Standard hysteresis parameters, however, provide a measure of the bulk magnetic properties, rather than enabling discrimination between the magnetic components that contribute to the magnetization of a sample. By contrast, first-order reversal curve (FORC) diagrams, which we describe here, can be used to identify and discriminate between the different components in a mixed magnetic mineral assemblage. We use magnetization data from a class of partial hysteresis curves known as first-order reversal curves (FORCs) and transform the data into contour plots (FORC diagrams) of a two-dimensional distribution function. The FORC distribution provides information about particle switching fields and local interaction fields for the assemblage of magnetic particles within a sample. Superparamagnetic, single-domain, and multidomain grains, as well as magnetostatic interactions, all produce characteristic and distinct manifestations on a FORC diagram. Our results indicate that FORC diagrams can be used to characterize a wide range of natural samples and that they provide more detailed information about the magnetic particles in a sample than standard interpretational schemes which employ hysteresis data. It will be necessary to further develop the technique to enable a more quantitative interpretation of magnetic assemblages; however, even qualitative interpretation of FORC diagrams removes many of the ambiguities that are inherent to hysteresis data.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900326

First-order reversal curve diagrams: A new tool for characterizing the magnetic properties of natural samples

Dust storms are recognized as having a very wide range of environmental impacts. Their geomorphological interest lies in the amount of deflation and wind erosion they indicate and their role in loess formation. Atmospheric mineral-dust loading is one of the largest uncertainties in global climate-change modeling and is known to have an important impact on the radiation budget and atmospheric instability. Major gaps remain in our understanding of the geomorphological context of terrestrial sources and the transport mechanisms responsible for the production and distribution of atmospheric dust, all of which are important in reducing uncertainties in the modeling of past and future climate. Using meteorological data from ground stations, from the space-borne Total Ozone Monitoring Spectrometer (TOMS), and from the National Center for Environmental Prediction–National Center for Atmospheric Research reanalysis project, we illustrate the key source regions of dust and demonstrate the primacy of the Sa...

Dust-Storm Source Areas Determined by the Total Ozone Monitoring Spectrometer and Surface Observations

https://researchcommons.waikato.ac.nz/bitstream/handle/10289/4616/Tephrochronology%20and%20its%20application.pdf;sequence=2

Tephrochronology and its application: A review

The Earth's climate has undergone a global transition over the past four million years, from warm conditions with global surface temperatures about 3 °C warmer than today, smaller ice sheets and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation ∼2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. Here we compare climate records from high latitudes, subtropical regions and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. We also find that high-latitude climate sensitivity to variations in solar heating increased gradually, culminating after cool tropical and subtropical upwelling conditions were established two million years ago. Our results suggest that mean low-latitude climate conditions can significantly influence global climate feedbacks.

Regional climate shifts caused by gradual global cooling in the Pliocene epoch.

[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

Des mesures de 14C par spectrometrie de masse sont effectuees sur des varves provenant du Lac Suigetsu durant le Pleistocene ancien/Holocene. Les dates obtenues sont en accord avec d'autres dates calibrees dans un intervalle de 10,5 a 11,5 ka cal BP

/pdf/ams-14c-dating-of-varved-sediments-from-lake-suigetsu-1eqdzyfs2e.pdf

AMS 14C Dating of Varved Sediments from Lake Suigetsu, Central Japan and Atmospheric 14C Change During the Late Pleistocene

The 3.2-m.y. whole core magnetic susceptibility record obtained during Ocean Drilling Program leg 117 from the Owen Ridge in the western Arabian Sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information. The salient features of this record are that (1) it correlates strongly with variations in the concentration and flux of eolian dust; (2) it is driven strongly at Earth orbital periodicities; and (3) there is a significant change in the spectral character at 2.4 m.y., which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas. In view of its potential paleoclimatic significance, we have examined the origin of the rock magnetic signal from the Owen Ridge in more detail. We find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content, there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis. We ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process. The first occurs at about 1.5 m depth and is characterized by a loss of fine-grained ferrimagnetic material. The second stepshift in magnetic properties occurs at a depth of 6–8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity (magnetite) and high coercivity (hematite and/or goethite) magnetic mineral components. Scanning electron microscope and energy dispersive Xray analyses show that the bulk of the coarse-grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the Fe:Ti ratio of these grains across the lower diagenetic front. This suggests that titanomagnetite grains with higher Ti contents may be more resistant to the process of reductive diagenesis, and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front.

Origin of the sedimentary magnetic record at Ocean Drilling Program Sites on the Owen Ridge, western Arabian Sea

Paleomagnetic directions of a volcanic and sedimentary sequence of the Kani Basin, ranging from 22 to 15 Ma, show a clockwise declination shift of about 50{degree}. This result implies that detectable rotation of Southwest Japan did not occur in this period, but after 15 Ma. The initial opening of the Japan Sea before 15 Ma might have caused a southeastward translation of Southwest Japan, which was then followed by clockwise rotation between 16 and 14 Ma.

Paleomagnetism of the Early Miocene Kani Group in southwest Japan and its implication for the opening of the Japan Sea

Paleoposition of Southwest Japan at 16 Ma: Implication from paleomagnetism of the Miocene Ichishi Group

Monoclinic Y4Al2O9 and hexagonal YAlO3 crystallize at low temperatures from amorphous materials prepared by the hydrolysis of yttrium and aluminium double alkoxides. Hexagonal YAlO3 transforms to the cubic phase with a garnet structure as an intermediate product at elevated temperatures. The formation process of YAlO3 is described. Solid solutions of hexagonal YAlO3 crystallize between 50 and 62.5 mol % Al2O3. Yttrium aluminium garnet Y3Al5O12(YAG) is formed by transformation of the solid solution.

Akira Hayashida

Papers

AMS 14C Dating of Varved Sediments from Lake Suigetsu, Central Japan and Atmospheric 14C Change During the Late Pleistocene

Origin of the sedimentary magnetic record at Ocean Drilling Program Sites on the Owen Ridge, western Arabian Sea

Paleomagnetism of the Early Miocene Kani Group in southwest Japan and its implication for the opening of the Japan Sea

Paleoposition of Southwest Japan at 16 Ma: Implication from paleomagnetism of the Miocene Ichishi Group

Formation of alkoxy-derived yttrium aluminium oxides