Peter Solheid

On the quantitative analysis and evaluation of magnetic hysteresis data

Magnetic hysteresis data are centrally important in pure and applied rock magnetism, but to date, no objective quantitative methods have been developed for assessment of data quality and of the uncertainty in parameters calculated from imperfect data. We propose several initial steps toward such assessment, using loop symmetry as an important key. With a few notable exceptions (e.g., related to field cooling and exchange bias), magnetic hysteresis loops possess a high degree of inversion symmetry (M(H) = −M(−H)). This property enables us to treat the upper and lower half-loops as replicate measurements for quantification of random noise, drift, and offsets. This, in turn, makes it possible to evaluate the statistical significance of nonlinearity, either in the high-field region (due to nonsaturation of the ferromagnetic moment) or over the complete range of applied fields (due to nonnegligible contribution of ferromagnetic phases to the total magnetic signal). It also allows us to quantify the significance of fitting errors for model loops constructed from analytical basis functions. When a statistically significant high-field nonlinearity is found, magnetic parameters must be calculated by approach-to-saturation fitting, e.g., by a model of the form M(H) = Ms + χHFH + αHβ. This nonlinear high-field inverse modeling problem is strongly ill conditioned, resulting in large and strongly covariant uncertainties in the fitted parameters, which we characterize through bootstrap analyses. For a variety of materials, including ferrihydrite and mid-ocean ridge basalts, measured in applied fields up to about 1.5 T, we find that the calculated value of the exponent β is extremely sensitive to small differences in the data or in the method of processing and that the overall uncertainty exceeds the range of physically reasonable values. The “unknowability” of β is accompanied by relatively large uncertainties in the other parameters, which can be characterized, if not rigorously quantified, through the bootstrapped distribution of best fit models. Nevertheless, approach-to-saturation fitting yields much more accurate estimates of important parameters like Ms than those obtained by linear M(H) fitting and should be used when maximum available fields are insufficient to reach saturation.

Magnetic signature of environmental changes in the last 1.2 Myr at ODP Site 1146, South China Sea

Abstract A detailed magnetic analysis has been done on the upper 145 m of the splice of ODP Site 1146 drilled during Leg 184 in the South China Sea. First, a magnetostratigraphic record was obtained. The Bruhnes–Matuyama reversal, preceded by a precursor, and the two Jaramillo reversals were identified at 115, 133 and 137.5 mcd (meter composite depth), respectively. Several known excursions during the Brunhes period, as well as the Kamikatsura event during the Matuyama chron, were also identified. Coupled with the oxygen isotope record obtained from benthic foraminifera, this allows us to establish a precise age model, showing that the studied interval extends to 1.18 Myr. Long-term and short-term variations have been documented in the magnetic parameters, which appear at least in part related to changes in monsoon activity. On the long-term trend, three main intervals with different magnetic trends were identified: 1.2–0.7, 0.7–0.2 and the most recent 0.2 Myr. During the oldest period, the magnetic content decreases, while the grain size of the magnetic mineral increases with large fluctuations. The coercivity of the magnetic minerals also increases consistently with data obtained by other authors from Pacific cores. During the following 500 kyr all the magnetic properties are rather uniform, with rather low abundance and relatively large grain sizes and low S-ratio. This probably illustrates a period of enhanced winter monsoon. Then, in the most recent period, the magnetic grain size decreases again with pronounced fluctuations. On the short-term scale, a 23 kyr component is identified by power spectrum analysis in all magnetic parameters in the bottom part of the studied interval. This component is correlated with the oxygen isotope record, as shown by coherence analysis. Except for the most recent 200 kyr, where the pattern changes, cold/warm periods coincide with low/high magnetic content, and magnetic grains are coarser during cold stages and finer during warm periods. This is consistent with the clay mineral analysis and illustrates changes in the balance between arid/humid periods corresponding to dominant winter/summer monsoon and giving rise to physical erosion and wind blowing/chemical weathering in the main fluvial plains and transport by oceanic current.

Inferred time- and temperature-dependent cation ordering in natural titanomagnetites

Despite years of efforts to quantify cation distribution as a function of composition in the magnetite-ulvöspinel solid solution, important uncertainties remain about the dependence of cation ordering on temperature and cooling rate. Here we demonstrate that Curie temperature in a set of natural titanomagnetites (with some Mg and Al substitution) is strongly influenced by prior thermal history at temperatures just above or below Curie temperature. Annealing for 10(-1) to 10(3) h at 350-400 °C produces large and reversible changes in Curie temperature (up to 150 °C). By ruling out oxidation/reduction and compositional unmixing, we infer that the variation in Curie temperature arises from cation reordering, and Mössbauer spectroscopy supports this interpretation. Curie temperature is therefore an inaccurate proxy for composition in many natural titanomagnetites, but the cation reordering process may provide a means of constraining thermal histories of titanomagnetite-bearing rocks. Further, our theoretical understanding of thermoremanence requires fundamental revision when Curie temperature is itself a function of thermal history.

Magnetic properties in an ash flow tuff with continuous grain size variation: A natural reference for magnetic particle granulometry

The Tiva Canyon Tuff contains dispersed nanoscale Fe-Ti-oxide grains with a narrow magnetic grain size distribution, making it an ideal material in which to identify and study grain-size-sensitive magnetic behavior in rocks. A detailed magnetic characterization was performed on samples from the basal 5 m of the tuff. The magnetic materials in this basal section consist primarily of (low-impurity) magnetite in the form of elongated submicron grains exsolved from volcanic glass. Magnetic properties studied include bulk magnetic susceptibility, frequency-dependent and temperature-dependent magnetic susceptibility, anhysteretic remanence acquisition, and hysteresis properties. The combined data constitute a distinct magnetic signature at each stratigraphic level in the section corresponding to different grain size distributions. The inferred magnetic domain state changes progressively upward from superparamagnetic grains near the base to particles with pseudo-single-domain or metastable single-domain characteristics near the top of the sampled section. Direct observations of magnetic grain size confirm that distinct transitions in room temperature magnetic susceptibility and remanence probably denote the limits of stable single-domain behavior in the section. These results provide a unique example of grain-size-dependent magnetic properties in noninteracting particle assemblages over three decades of grain size, including close approximations of ideal Stoner-Wohlfarth assemblages, and may be considered a useful reference for future rock magnetic studies involving grain-size-sensitive properties.

The paleoenvironmental-magnetic record of the Gold Hill Steps loess section in central Alaska

Abstract In contrast to the loess-paleosol sequences of China and Central Europe, paleosols of the Halfway House and Gold Hill Steps loess sections in central Alaska show no enhancement in magnetic susceptibility. Evidence for maghemitization and ultrafine superparamagnetic minerals is found in the topsoil and in the lowest weathered loess of the sequence. These minerals are not detected in the three paleosols within the profiles. Rock-magnetic characteristics also indicate a smaller magnetic grain size in the loess with the lowest magnetic susceptibility values. This finding supports the interpretations of Beget et al. (1990) that magnetic concentration and corresponding susceptibility variations may be related to changes in wind intensity.

Deconvolution of u channel magnetometer data: Experimental study of accuracy, resolution, and stability of different inversion methods

We explore the effects of sampling density, signal/noise ratios, and position-dependent measurement errors on deconvolution calculations for u channel magnetometer data, using a combination of experimental and numerical approaches. Experiments involve a synthetic sample set made by setting hydraulic cement in a 30-cm u channel and slicing the hardened material into ∼2-cm lengths, and a natural lake sediment u channel sample. The cement segments can be magnetized and measured individually, and reassembled for continuous u channel measurement and deconvolution; the lake sediment channel was first measured continuously and then sliced into discrete samples for individual measurement. Each continuous data set was deconvolved using the ABIC minimization code of Oda and Shibuya (1996) and two new approaches that we have developed, using singular-value decomposition and regularized least squares. These involve somewhat different methods to stabilize the inverse calculations and different criteria for identifying the optimum solution, but we find in all of our experiments that the three methods converge to essentially identical solutions. Repeat scans in several experiments show that measurement errors are not distributed with position-independent variance; errors in setting/determining the u channel position (standard deviation ∼0.2 mm) translate in regions of strong gradients into measurement uncertainties much larger than those due to instrument noise and drift. When we incorporate these depth-dependent measurement uncertainties into the deconvolution calculations, the resulting models show decreased stability and accuracy compared to inversions assuming depth-independent measurement errors. The cement experiments involved varying directions and uniform intensities downcore, and very good accuracy was obtained using all of the methods when the signal/noise ratio was greater than a few hundred and the sampling interval no larger than half the length scale of magnetization changes. Addition of synthetic noise or reduction of sampling density decreased the resolution and accuracy of all the methods equally. The sediment-core experiment involved uniform (axial) magnetization direction and strongly varying intensities downcore. Intensity variations are well resolved and directions are accurate to within about 5 degrees, with errors attributable to omission and/or inaccurate calibration of cross terms in the instrument response function.

Environmental Magnetism of Late Pleistocene / Holocene Sequences from Lake Victoria, East Africa

The East African region has undergone significant landscape modification since the last glacial maximum, and this history is archived in sediments from Lake Victoria. Comparison of systematic variations in sediment magnetic properties with changes in lithic grain size, water content and other proxies provides a sensitive proxy record for evaluation of transport energy and weathering in the drainage basin. Seven cores were recovered from Lake Victoria in 1995 as part of an International Decade for East African Lakes (IDEAL) multidisciplinary study of the lake. Whole core magnetic susceptibility of these seven cores which extend to more than 14,000 years B.P. show a general intra-basin correlation of stratigraphy overlying a firm soil. One core was selected as a representative type core. Volume magnetic susceptibility identified soil horizons below the Holocene muds, and mass normalized susceptibility determined that this soil zone derives from the precursor lake muds with some evidence of diagenesis of the magnetic carrier. Prior to 12,700 yr B.P. the proxies reflect very low water levels with some periods of complete desiccation adequate to form thick crumbly soils, oxidize organic matter and form hematite. During the filling stage to ca. 7,500 yr B.P., magnetic mineral sources remain stable, although lake level fluctuated. There is a general increase in magnetic susceptibility of sediments through the Holocene once the lake overflows. This suggests that although the pre-Holocene magnetic mineral components are soils derived from precursor lake sediments, the Holocene magnetic mineral components are derived from the drainage basin. An anomalous magnetic spike at 180 to 160 cm depth has been identified as increased amounts of the same source material, possibly a major wash-in event. Increase in magnetic properties and lithic grain size in the uppermost 30 cm is probably due to increased soil erosion, a reflection of human forest clearance activity. Although the Holocene magnetic record has diamagnetic and possibly diagenetic overprint, a 2,000-year oscillation of concentration and composition shows that the record is sensitive to major changes in water balance history.

Full vector low-temperature magnetic measurements of geologic materials

GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS, VOL. ???, XXXX, DOI:10.1029/, Full vector low-temperature magnetic measurements of geologic materials Joshua M. Feinberg 1 , Peter A. Solheid 1 , Nicholas L. Swanson-Hysell 1,2 , Mike J. Jackson 1 , and Julie A. Bowles 1,3 The magnetic properties of geologic materials offer insights into an enormous range of important geophysical phenomena ranging from inner core dynamics to pale- oclimate. Often it is the low-temperature behavior (<300 K) of magnetic minerals that provides the most useful and highest sensitivity information for a given problem. Con- ventional measurements of low-temperature remanence are typically conducted on instru- ments that are limited to measuring one single axis component of the magnetization vec- tor and are optimized for measurements in strong fields. These instrumental limitations have prevented fully optimized applications and have motivated the development of a low-temperature probe that can be used for low-temperature remanence measurements between 17 and 300K along three orthogonal axes using a standard 2G Enterprises SQuID rock magnetometer. In this contribution, we describe the design and implementation of this instrument and present data from five case studies that demonstrate the probe’s con- siderable potential for future research: a polycrystalline hematite sample, a polycrystalline hematite and magnetite mixture, a single crystal of magnetite, a single crystal of pyrrhotite and samples of Umkondo Large Igneous Province diabase sills. Abstract. Measurement System (MPMS), most often with the inten- tion of revealing information about the dominant magnetic mineral phases and grain size distribution. While these in- struments are adept at a range of low-temperature exper- iments, understanding the full behavior of a rock’s natu- ral remanence at low-temperature is hampered by the mea- surement capabilities being limited to a single axis and the instrument not providing an ultra-low field environment. If low-temperature measurements of a natural remanence (NRM) are desired using such instrumentation, great care must be taken to align the NRM with the measurement axis, and any directional change during thermal cycling will not be captured. In such an instrument, deviation from the single-axis will result in a measured magnetization that is less than the specimen’s actual total magnetization. In this contribution, we describe a low-temperature probe developed for use with superconducting rock magnetometers (SRM) at the Institute for Rock Magnetism (IRM ), Uni- versity of Minnesota. This instrument allows for three-axis full-vector measurements of magnetic remanence at temper- atures between 300 and 17 K in low-field environments (<10 nT). It was developed with different engineering, but the same intent, as a low-temperature insert that has previously been implemented at the University of Rochester Paleomag- netic Laboratory [Smirnov and Tarduno, 2011]. 1. Introduction Magnetic behavior at low temperatures (<300 K) is one of the most sensitive indicators of the iron mineral phases and their concentrations and grain size distributions in nat- ural samples. Changes in magnetocrystalline anisotropy and crystallographic structure give rise to low-temperature tran- sitions that are diagnostic of specific mineral phases. The Morin transition of hematite (at ∼262 K; Morin [1950]), the Verwey transition of magnetite (at ∼122 K; Verwey [1939]) and the Besnus transition of pyrrhotite (at ∼32 K, Besnus and Meyer [1964]) are all diagnostic of common magnetic minerals that carry remanence at Earth surface tempera- tures. Other phases that acquire remanence at low tem- perature, such as siderite (with a Ne´el temperature of 38 K; Frederichs et al. [2003]) and superparamagnetic grains [Worm and Jackson, 1999], can also be readily identified through their low-temperature behavior. In addition to the utility of low-temperature data as a diagnostic tool for mag- netic mineral identification and characterization, irreversible changes in remanence that are associated with cycling to low temperatures are often used as a tool in paleomagnetic stud- ies. Low-temperature steps in paleodirectional and paleoin- tensity study are applied in some protocols with the goal of preferentially removing magnetic remanence held by mul- tidomain grains and thereby isolating magnetizations held by single-domain grains [e.g., Schmidt [1993]; Dunlop [2003]; Yamamoto et al. [2003]]. Low-temperature remanence experiments are routinely conducted on the Quantum Designs Magnetic Properties 2. Instrument design In order to develop the capacity to make three-axis mea- surements of remanence in an ultra-low-field environment, a cryostat insert was developed at the IRM in coopera- tion with ColdEdge Technologies (Allentown, PA) (Figure 1). This low-temperature instrument (IRM-LTI) allows for three-axis measurements to be made between room temper- ature and ∼17 K using horizontal-loading SRMs. There are many advantages to outfitting a superconducting rock mag- netometer for measurements at low-temperatures. First, these instruments are specifically designed for three-axis re- manence measurements, and ambient fields are minimized using a superconducting lead shield. Nulling fields are ap- plied by external coils while the shield cools to superconduct- ing temperatures, ultimately trapping a ∼2-3 nT field along 1 Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota, USA 2 Department of Earth and Planetary Science, University of California, Berkeley, California, USA 3 Department of Geosciences, University of Wisconsin, Milwaukee, WI, USA Copyright 2015 by the authors.

Rock magnetism of sediments in the Angola-Namibia upwelling system with special reference to loss of magnetization after core recovery

A rock magnetic study was performed on sediment cores from four sites in the South Atlantic off the western coast of Africa, which were taken during the Ocean Drilling Program Leg 175 (Sites 1078, 1082, 1084, and 1085). The sites are within the Angola-Namibia upwelling system, and the sediments have a high total-organic-carbon content. Concentration of ferrimagnetic minerals at these sites is very low, and the magnetic susceptibility is dominated by paramagnetic and diamagnetic minerals. Severe and rapid loss of remanent magnetization occurred during storage of the cores, with less than 10% of the initial intensity remaining a few months after core recovery. The loss of magnetization may prevail in organic-rich sediments. Changes of magnetic properties with time were examined using samples that were kept frozen before the experiment. Hysteresis parameters and the ratio of ARM (anhysteretic remanent magnetization) to SIRM (saturation isothermal remanent magnetization) indicate increases in the average magnetic grain size with the decay of magnetization, which suggests preferential dissolution of finer magnetic minerals. Loss of low-coercivity magnetic minerals with time was estimated from the decrease of S ratios. Low-temperature magnetometry revealed the presence of magnetite in the sediments even after the completion of sulfate reduction. Magnetization attributable to magnetite decreased with the loss of magnetization. This suggests the transformation of magnetite into non-magnetic phases, which is consistent with the decrease of S ratios.

Research‐oriented data base for rock and paleomagnetism to be developed

Plans for constructing a second-generation paleomagnetic data base incorporating paleosecular variation, paleo-intensity, paleopole positions, and reversal records are expanding to include fundamental and applied rock magnetic data. The Institute for Rock Magnetism (IRM) at the University of Minnesota held an initial planning meeting in early September to discuss design considerations for a new rock magnetic data base, and whether it should be subsumed under the new paleomagnetic umbrella. Technical aspects of modern data base design and how to deal with the particular difficulties of storing, searching, and retrieving multi-variable rock magnetic data were the focus of the meeting.