Kenneth L. Verosub

Bio: Kenneth L. Verosub is an academic researcher from University of California, Davis. The author has contributed to research in topics: Paleomagnetism & Earth's magnetic field. The author has an hindex of 48, co-authored 147 publications receiving 9075 citations. Previous affiliations of Kenneth L. Verosub include Centre national de la recherche scientifique.

Characterizing interactions in fine magnetic particle systems using first order reversal curves

Abstract: We demonstrate a powerful and practical method of characterizing interactions in fine magnetic particle systems utilizing a class of hysteresis curves known as first order reversal curves. This method is tested on samples of highly dispersed magnetic particles, where it leads to a more detailed understanding of interactions than has previously been possible. In a quantitative comparison between this method and the δM method, which is based on the Wohlfarth relation, our method provides a more precise measure of the strength of the interactions. Our method also has the advantage that it can be used to decouple the effects of the mean interaction field from the effects of local interaction field variance.

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

Abstract: 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.

Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization

Abstract: The asteroid impact near the Russian city of Chelyabinsk on 15 February 2013 was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding one million. Because it occurred in an era with modern consumer electronics, field sensors, and laboratory techniques, unprecedented measurements were made of the impact event and the meteoroid that caused it. Here, we document the account of what happened, as understood now, using comprehensive data obtained from astronomy, planetary science, geophysics, meteorology, meteoritics, and cosmochemistry and from social science surveys. A good understanding of the Chelyabinsk incident provides an opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.

Wasp-waisted hysteresis loops: Mineral magnetic characteristics and discrimination of components in mixed magnetic systems

Abstract: Rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials. Hysteresis loops that are constricted in the middle section, but are wider above and below the middle section, are commonly observed in mixed magnetic assemblages. Such “wasp-waisted” hysteresis loops have been widely documented, particularly with respect to rare earth permanent magnets, basaltic lava flows, remagnetized Paleozoic carbonate rocks, and an increasingly wide range of other rocks. Our modelling, combined with a review of previous work, indicates that there are several conditions that give rise to, as well as magnetic properties that are characteristic of, wasp-waisted hysteresis loops. First, at least two magnetic components with strongly contrasting coercivities must coexist. This condition can arise from either mixtures of grain sizes of a single magnetic mineral, or a combination of magnetic minerals with contrasting cocrcivities, or a combination of these two situations. Second, materials that give rise to wasp-waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force (B cr /B c ) because B0 is controlled by the soft (low coercivity) component, whereas Bcris controlled by the hard (high coercivity) component. Third, values of B cr /B c ? 10 usually only occur for strongly wasp-waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains. Fourth, a given mixture of superparamagnetic and single-domain (SD) grains is more likely to give rise to wasp-waisted hysteresis loops than an equivalent mixture of SD and multidomain grains. Fifth, our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component, in the absence of significant magnetic interaction between particles. Thus to contribute significantly to wasp-waisted behavior, a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock. As a result, minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp-waistedness in materials that also contain ferrimagnetic minerals. We outline a method for determining the magnetic components that can give rise to wasp-waisted hysteresis loops. This method is based on high- and low-temperature magnetic measurements that are used to identify the dominant remanence-bearing mineral/s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states. The method is illustrated with several examples from archaeological, geological, and synthetic materials.

Environmental magnetism: Past, present, and future

Abstract: 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.

Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic

Abstract: Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic. The age of 66 Ma for the Cretaceous/Paleogene (K/P) boundary used for calibration in the geomagnetic polarity timescale of Cande and Kent (1992) (hereinafter referred to as CK92) was supported by high precision laser fusion Ar/Ar sanidine single crystal dates from nonmarine strata in Montana. However, these age determinations are now

Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data

Abstract: [1] Although most paleomagnetic and environmental magnetic papers incorporate a Day plot of the hysteresis parameters Mrs/Ms versus Hcr/Hc, a comprehensive theory covering superparamagnetic (SP), single-domain (SD), pseudo-single-domain (PSD), and multidomain (MD) (titano)magnetites is lacking. There is no consensus on how to quantify grain-size trends within the Day plot, how to distinguish MD from SP trends/mixtures, or whether magnetite, titanomagnetites, and other minerals have distinctive trends by which they might be identified. This paper develops the theory of the Day plot parameters for MD, MD + SD, PSD, and SP + SD grains of titanomagnetite (Fe 3– xTixO4) with compositions x = 0 (TM0 or magnetite) and x = 0.6 (TM60). MD grains have a separate trend that intersects the curve for SD + MD mixtures. SP + SD mixtures generate a variety of trends, depending on the SP grain size. All SP + SD curves lie much above those for MD or SD + MD trends, as has been proposed, but not demonstrated, previously. Data for PSD-size magnetites of many different origins fall along a single trend, but different levels of internal stress shift points for similar grain sizes along the ‘‘master curve.’’ In order to use the Day plot to determine grain size, one must have independent information about the state of internal stress. Theoretical model curves for SD + MD mixtures match the PSD magnetite and TM60 data quite well, although the SD!MD transition region in grain size is much narrower for TM60 than for magnetite. The agreement between PSD data and SD + MD mixing curves implies that PSD behavior is due to superimposed independent SD and MD moments, either in individual or separate grains, and not to exotic micromagnetic structures such as vortices. The theory also matches Mrs and Hc values in mechanical mixtures of very fine and very coarse grains, although nonlinear mixing theory is required to explain some Hcr and Hcr/Hc data. INDEX TERMS: 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques; 1533 Geomagnetism and Paleomagnetism: Remagnetization; 1512 Geomagnetism and Paleomagnetism: Environmental magnetism;

Correlation between climate events in the North Atlantic and China during the last glaciation

Abstract: EPISODES of massive iceberg release (Heinrich events)1-3 into the North Atlantic Ocean during the last glaciation were associated with recurring episodes of unusually cold North Atlantic surface water (Bond cycles)4 and cold air temperatures over Greenland (Dansgaard-Oeschger events)5,6. Four of the youngest of these cold events have also been reported in climate records from sites outside the North Atlantic region7, but until now the entire suite has been identified only in North Atlantic marine sediments, Greenland ice-core records and, tentatively, in French lake sediments8. Here we examine grain-size data from Chinese loess and intercalated accretionary palaeosols of last-glacial age for evidence of similar climate signals remote from the North Atlantic region. We see grain-size maxima with ages that match those of the last six Heinrich events, which we interpret as an indication of the changing strength of the East Asian winter monsoon, which largely controls the transport and deposition of central Asian aeolian dust. Thus it seems that these Heinrich events have left their signature in the Chinese loess record. This is consistent with simulations of the glacial climate9, which imply that the climates of the North Atlantic and China were linked by the effect of westerly winds.