Gunther Kletetschka

The Global Magnetic Field of Mars and Implications for Crustal Evolution

The Mars Global Surveyor spacecraft obtained globally-distributed vector magnetic field measurements approximately 400 km above the surface of Mars. These have been compiled to produce the first complete global magnetic field maps of Mars. Crustal magnetization appears dichotomized, with intense magnetization mainly confined to the ancient, heavily cratered highlands in the south. The global distribution of sources is consistent with a reversing dynamo that halted early in Mars evolution. Intense crustal magnetization requires an increased oxidation state relative to mantle-derived rock, consistent with assimilation of an aqueous component at crustal depths.

Magnetic stratigraphy of Chinese Loess as a record of natural fires

Magnetic susceptibility records of paleosols and loess show high correlation with oxygen-isotope stratigraphy of ocean sediments [Kukla, 1987], providing a global paleoclimatic record. Different models have been put forth to explain the nature and cause of susceptibility variations, but consensus has not yet been achieved. Our low-temperature studies reveal a secondary magnetite component in paleosols that is characterized by a higher Verwey transition (115K) than that for the magnetite (100 K) in unaltered loess. The same shift in the Verwey transition can be achieved by heating and cooling loess samples. This is consistent with a new hypothesis that the magnetic signal from paleosols may be produced by natural fires in the past. Natural fire intensity is sensitive to the amount of annual precipitation, so that increased fire-induced susceptibilities should reflect an increase in the humidity of regional climate.

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

Mineral magnetic properties of soils and parent materials have been interpreted in terms of paleoclimate and rates of soil formation but it is important to understand which minerals contribute to the mineral magnetic signal. Citrate-bicarbonate-dithionite (CBD) treatment has been used to determine the amounts of fine-grained, often pedogenic, ferrimagnetic minerals relative to coarse-grained, often inherited, magnetic minerals. However, questions have been raised about the effect of particle size on the efficacy of CBD in dissolving magnetite and maghemite grains. In this paper we use magnetic susceptibility and its frequency dependence, and the low-temperature behavior of a saturation isothermal remanent magnetization, to track the dissolution of carefully sized magnetite grains. We found that the standard CBD procedure dissolves fine magnetite particles (ca. 1 etm) essentially intact. Thin oxidized coatings, presumably maghemite, are also dissolved by the CBD procedure. These results support previous interpretations that the CBD procedure can be used to distinguish between pedogenic and lithogenic magnetic grains, assuming that most pedogenic magnetic grains are sufficiently small (ca. 1 /im). These results also show that the standard procedure is too harsh to differentiate between 1 grm grains of magnetite and maghemite. A modified CBD extraction that uses half as much dithionite reduces the magnetic susceptibility of 1 Jam magnetite grains by only 10%. This method may be useful in distinguishing between magnetite and maghemite grains in this size range.

Determining the ages of comets from the fraction of crystalline dust.

The timescale for the accretion of bodies in the disk surrounding a young star depends upon a number of assumptions, but there are few observational constraints. In our own Solar System, measurements of meteoritic components can provide information about the inner regions of the nebula, but not the outer parts. Observations of the evolution of more massive protostellar systems (Herbig Ae/Be stars) imply that significant changes occur in the physical properties of their dust with time. The simplest explanation is that thermal annealing of the original, amorphous grains in the hot inner nebula slowly increases the fractional abundance of crystalline material over time. Crystalline dust is then transported outward, where it is incorporated into comets that serve as a long-term reservoir for dust disks, such as that surrounding Beta Pictoris. Here we show that when applied to our own Solar System, this process can explain observed variations in both the volatile and dusty components of comets, while also providing a natural indicator of a comets mean formation age. Studies of comets with different dust contents can therefore be used to investigate the timescales of the early Solar System.

Multidomain hematite: A source of planetary magnetic anomalies?

Thermoremanent magnetization (TRM) in hematite is larger than TRM in magnetite for grain sizes ≥ 10 μm. We show that hematites weak spontaneous magnetization M s causes its strong TRM, since the self-demagnetizing field H d opposing large domain wall displacements is proportional to M s . In hematite, H d is comparable to the Earths magnetic field but in magnetite, H d is 1000 times larger. As a result, Earths field TRM of MD hematite (0.3 Am 2 /kg) outweighs TRM and induced magnetization of MD magnetite ( 0.01-0.02 Am 2 /kg) and rivals TRM of single-domain and PSD magnetite as a source of magnetic anomalies on Earth and perhaps on Mars.

Unique thermoremanent magnetization of multidomain sized hematite: Implications for magnetic anomalies

Intense magnetic remanence (100^1000 A/m) associated with MD hematite and/or titanohematite and associated with high Koenigsberger ratios (40^1000) indicate that magnetic remanence may dominate the total magnetization if these minerals are volumetrically significant. Titanohematite behaves similarly to hematite and, thus, the grain size dependence of TRM acquisition in hematite is considered as a generalization. The transition between truly MD behavior and tendency towards SD behavior in hematite has been established to be between grain sizes of 0.1 and 0.05 mm. In contrast to magnetite and titanomagnetite, hematite exhibits inverse grain size dependence, with MD hematite acquiring a relatively intense TRM in the geomagnetic field, comparable to sub-micrometer sized magnetite and only an order of magnitude less than SD magnetite. Consequently MD hematite (and by analogy titanohematite) remanence may be of significance as a source of magnetic anomalies at all scales. MD hematite exhibits TRM weak field acquisition behavior that is different from all other magnetic minerals, being the only magnetic mineral having an REM (TRM/SIRM) value E0.1 for TRM acquisition in the geomagnetic field. The very different TRM behavior of MD hematite in contrast to magnetite is due to two factors. The first is the lesser influence of demagnetizing energy with respect to wall pinning energy, at temperatures almost up to the Curie temperature for hematite. The second is the greater importance of the magnetostatic energy in the applied field, which for hematite dominates the total energy at high temperatures. fl 2000 Published by Elsevier Science B.V. All rights reserved.

Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

Significance We present detailed geochemical and morphological analyses of nearly 700 spherules from 18 sites in support of a major cosmic impact at the onset of the Younger Dryas episode (12.8 ka). The impact distributed ∼10 million tonnes of melted spherules over 50 million square kilometers on four continents. Origins of the spherules by volcanism, anthropogenesis, authigenesis, lightning, and meteoritic ablation are rejected on geochemical and morphological grounds. The spherules closely resemble known impact materials derived from surficial sediments melted at temperatures >2,200 °C. The spherules correlate with abundances of associated melt-glass, nanodiamonds, carbon spherules, aciniform carbon, charcoal, and iridium. Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world’s premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

It has been proposed that fragments of an asteroid or comet impacted Earth, deposited silica-and iron-rich microspherules and other proxies across several continents, and triggered the Younger Dryas cooling episode 12,900 years ago. Although many independent groups have confirmed the impact evidence, the hypothesis remains controversial because some groups have failed to do so. We examined sediment sequences from 18 dated Younger Dryas boundary (YDB) sites across three continents (North America, Europe, and Asia), spanning 12,000 km around nearly one-third of the planet. All sites display abundant microspherules in the YDB with none or few above and below. In addition, three sites (Abu Hureyra, Syria; Melrose, Pennsylvania; and Blackville, South Carolina) display vesicular, high-temperature, siliceous scoria-like objects, or SLOs, that match the spherules geochemically. We compared YDB objects with melt products from a known cosmic impact (Meteor Crater, Arizona) and from the 1945 Trinity nuclear airburst in Socorro, New Mexico, and found that all of these high-energy events produced material that is geochemically and morphologically comparable, including: (i) high-temperature, rapidly quenched microspherules and SLOs; (ii) corundum, mullite, and suessite (Fe3Si), a rare meteoritic mineral that forms under high temperatures; (iii) melted SiO2 glass, or lechatelierite, with flow textures (or schlieren) that form at > 2,200 °C; and (iv) particles with features indicative of high-energy interparticle collisions. These results are inconsistent with anthropogenic, volcanic, authigenic, and cosmic materials, yet consistent with cosmic ejecta, supporting the hypothesis of extraterrestrial airbursts/impacts 12,900 years ago. The wide geographic distribution of SLOs is consistent with multiple impactors.

Grain size limit for SD hematite

Grain sizes in the range (10 −4 to 10 −1 mm) are common in some rocks. Because thermal and/or chemical remanent magnetization of hematite in this range approaches intensities of single domain (SD) magnetite, careful exploration of this transition, may serve to develop new applications in rock magnetism that relate to magnetic anomaly source identification, and various paleomagnetic and grain size-dependent investigations. Grain size-dependent magnetic behavior of hematite reveals a SD–multidomain (MD) transition at 0.1 mm. This transition is recognized by variation in magnetic coercivity and susceptibility and is related to an anomaly in remanence recovery when cycling through the Morin transition. The coercivity decrease with increasing grain size occurs much more gradually above 0.1 mm than below this value. Magnetic susceptibility of the grains smaller than 0.1 mm has negligible dependence on the amplitude of the applied alternating magnetic field. For the larger grains a new amplitude-dependent susceptibility component is observed. The grain size of 0.1 mm is also associated with loss of most of the remanence when cycling through the Morin transition. This behavior is ascribed to a transition from the metastable SD to the MD magnetic state. The increase in magnetized volume causes the demagnetizing energy to destabilize the SD state, resulting in a transition where the demagnetizing energy is reduced by nucleation of the domain wall for grains larger than 0.1 mm. The 0.1 mm transition has no significant effect on shape of the temperature-dependent coercivity and saturation magnetization.

Lodestone: Natures only permanent magnet‐What it is and how it gets charged

Magnetite and Titanomagnetite exhibit magnetic properties which are attributable to the micro-structures developed during oxidation and exsolution: All magnetite iron ores which are lodestones contain maghemite. These lodestones have Hc between 10 and 30 mT, SIRM between 8 and 18 Am²kg¹ and RI between 0.10 and 0.26. Magnetite, titanomagnetite and metals have REM values (ratio of NRM to SIRM) < 0.05. Samples (called fulgarites) obtained from the Smithsonian Institution have REM values ranging from 0.45 to 0.92. The REM value serves as a witness parameter to the magnetic fields associated with the lightning bolt. If a high REM value (say ≫ 0.1) can be verified as not to be due to contamination by man and does not contain MD hematite then the rock has LRM (lightning remanent magnetization). The magnetic field associated with lightning can be revealed from an isothermal remanent acquisition (RA) curve.