173 research outputs found
[Letter] [18]98 March 3, Brooklyn [to] Mr. Gilder / Paul Leicester Ford.
No full textThe letter is written on Clark Street / Brooklyn letterhead.Ford confesses to being "suprised and not a little indignant" to find that "my Franklin has been dropped out of the March Century" without explanation or apology. Since Gilder never expressed dissatisfaction with Ford\u27s work, Ford assumes the omission must be due to "the pressure of your war articles." He closes by stating that it does not seem like a good business practice to brush aside an "author who is likely to write many books, to favor a man who will probably never write another." A historian and novelist, Ford suffered an early injury to his spine which confined his education to his personal reading and private tutors in his parents\u27 Clark Street home in Brooklyn. He engaged in bibliographical research and the republishing of rare materials, and prior to the writing of this letter he had already published a political novel _The Honorable Peter Stirling, and What People Thought of Him_ (1894) as well as edited collections of the works of Thomas Jefferson and Christopher Columbus, biographies of Washington and Franklin, and the historical romance _Janice Meredith: A Story of the American Revolution_ (1899). Ford died after being shot by his brother Malcolm, former amateur athlete, who had been disinherited
Competition of magnetic and hydrodynamic forces on ellipsoidal particles under shear: Influence of the Earth's magnetic field on particle alignment in viscous media
No full textInternational audience[1] We present a model that describes the rotation of ellipsoidal magnetic particles in a viscous fluid under the influence of hydrodynamic and magnetic forces, with an aim to better understand how sediments acquire their remanent magnetizations. Analyses of the governing equations elucidate how magnetic particles will rotate for different values of leading parameters including particle shape, remanent and induced magnetic intensity, magnetic field intensity and direction, strain rate, shear direction, and viscosity. Numerical solution of the governing equations makes it possible to visualize the rotation path and the magnetic direction of a particle through time. Thus the model can discern the timescales and trajectories of magnetic particles rotating due to torque of the magnetic field couple while simultaneously entrained in a velocity gradient. For example, in a layer of viscosity 10 4 Pa s, prolate magnetite starting at any initial orientation and subjected to simple shear with a strain rate of 3.17 Â 10 À8 s À1 needs 4 months to rotate within 3° of the Earth's field direction. Under the same conditions, hydrodynamic forces will govern the orientation of oblate hematite whose moment will be perpetually randomly oriented with respect to the magnetic field direction. When applied to laboratory experiments, the viscous model successfully matches the observed data, particularly after accounting for mechanical interaction and flocculation effects. Magnetic anisotropies calculated from multiparticle systems of hematite yield typical sedimentary fabrics with relatively low percentages of anisotropy (<5%) and maximum principal axes that lie in the sedimentation plane. Citation: Jezek, J., and S. A. Gilder (2006), Competition of magnetic and hydrodynamic forces on ellipsoidal particles under shear: Influence of the Earth's magnetic field on particle alignment in viscous media
Phelps, Elizabeth Stuart, 1844-1911 (SC 2423)
No full textFinding aid, scans and typescripts (Click on Additional Files below) for Manuscripts Small Collection 2423. Letters of Elizabeth Stuart Phelps to Richard Watson Gilder, editor of The Century Magazine. Two letters relate to her submission of the manuscript for the story “Jack” and explain her eagerness to read and correct the proof. She also writes of her home in East Gloucester, Massachusetts and of the challenge of composing short stories. A third letter from Newton Center, Massachusetts discusses her submission of chapters of her memoir, offers to make changes, and advises of her availability by telephone
Impact Structures: How post-impact thermal conditions influence magnetic behavior
No full textLarge scale meteorite impact events are some of the most catastrophic and instantaneous geological processes in nature. These events release high amounts of energy, and generate pressures which will vaporize, melt and metamorphose the target rock. Large impact events create impact structures with very characteristic features, one of which being intense magnetic anomalies. These anomalies are caused by a contrast of the total magnetization in the shocked target, newly formed impactites (that is, rocks formed by, or related to the impact event) and the regional background magnetization. The total magnetization (Mtot) is the sum of natural remanent magnetization (NRM) and induced magnetization (κ*B, κ is magnetic susceptibility and B is Earth’s magnetic field). Shock deformation causes a sharp loss of up to 90% of NRM and κ in the magnetite-bearing target. The demagnetization of magnetite is caused by the shock-induced physical deformation, such as brittle grain fracturing and fragmentation, and ductile crystal lattice defects. Physical defects also enhance magnetic domain wall-pinning, which decreases the apparent domain state of magnetite, from originally multi-domain (MD) towards pseudo-single domain (PSD), or even single domain (SD) states. The change in apparent domain state is concomitant with an increase in magnetic coercivity, and further decreases κ. However, a recent study found that post-shock thermal annealing of shocked magnetite causes a “healing” of some of the reversible ductile lattice defects, which reduces domain wall-pinning and allows for the restoration and recovery of some magnetic properties, including an increase of κ.
Natural post-impact magnetic recovery through temperature has not been studied to date, and is particularly relevant for large impact craters such as the Chicxulub (Mexico). In this study we compared the large Chicxulub (diameter ca. 200 km) with the smaller Nördlinger Ries structure (Germany, diameter ca. 25 km). In both impact structures, high temperature impactites (>900°C) and post-impact hydrothermalism may lead to optimal conditions for natural annealing to take place. The effect that this phenomenon has on the characteristic magnetic signature of the craters remains unclear until now.
To address this knowledge gap, magnetite samples from the Chicxulub and Nördlinger Ries craters were investigated through rock-magnetic, petrographic and mineral chemical methods. Two generations of magnetite common to both craters were observed: (1) a pure, stoichiometric shocked magnetite with large (~100 μm) grains in the basement; and (2) a newly formed, generally low-cation substituted (Ti-) magnetite, with smaller (~10-50 μm) grains and no visible shock deformation, in the impactites.
Naturally shocked MD magnetite was found to show an apparent domain state decrease towards PSD, similar to magnetite shocked experimentally in laboratory conditions. Temperatures above 540°C create an irreversible increase in κ, and restore some apparent MD state contributions. The threshold for annealing and magnetic recovery is estimated to be ~540°C. If the magnetite is oxidized and hematite is present in the sample, it transforms back to magnetite if experimentally heated above 560°C in an argon atmosphere. The transformation creates very small magnetite grains with mottled textures and SD to superparamagnetic (SP) domain states, in a process that masks annealing if both occur concomitantly. This is an underappreciated phenomenon, that may lead to NRM remagnetization and overprint in nature, under certain conditions.
In Chicxulub, the shocked magnetite of the uplifted basement shows shock-reduced NRM and κ values, leading to a low total magnetization in the peak-ring. We attribute the negative anomaly in the peak-ring to this lack of magnetization. The hydrothermal system did not significantly overprint the magnetic signal, and its temperatures (~450°C) were not high enough to anneal the magnetite naturally, and thus it did not affect significantly the magnetic anomaly in the peak-ring. Natural annealing takes place in contact with the impact melt, where basement magnetite shows increased κ, and a transformation of pre-impact oxidation-derived hematite into newly formed SD-SP magnetite. The impact melt shows strong NRM and κ values, but in the peak-ring it constitutes only a thin layer, which also does not significantly contribute to the anomaly.
In Nördlinger Ries, the basement magnetite is similarly demagnetized, and natural annealing occurs locally due to the prevalence of melt bearing suevite dykes in the basement, with high emplacement temperatures (>900°C). The hydrothermal system in Nördlinger Ries had shorter duration and lower temperatures than in Chicxulub (max. 300°C), and also does not significantly affect the magnetic anomalies of the crater. The impact melt shows weak magnetization, with κ values comparable to the shocked and demagnetized basement. On the other hand, impact breccia (suevite) in Nördlinger Ries show high κ and strong reverse polarity magnetization, which contributes to the negative magnetic anomalies
Evidence for coeval Late Triassic terrestrial impacts from the Rochechouart (France) meteorite crater
No full textHigh temperature impact melt breccias from the Rochechouart (France) meteorite crater record magnetization component with antipodal, normal and reverse polarities. The corresponding paleomagnetic pole for this component lies between the 220 Ma and 210 Ma reference poles on the Eurasian apparent polar wander path, consistent with the 214 ± 8 Ma 40Ar/39Ar age of the crater. Late Triassic tectonic reconstructions of the Eurasian and North American plates place this pole within 95% confidence limits of the paleomagnetic pole from the Manicouagan (Canada) meteorite impact crater, which is dated at 214 ± 1 Ma. Together, these observations reinforce the hypothesis of a Late Triassic, multiple meteorite impact event on Earth
Biogenic and hydrogenous magnetic mineral formation in upward oxygenated sediments of the East Pacific Rise
No full textThe Clarion-Clipperton Zone is a deep-sea area in the eastern Pacific Ocean. Due to low sedimentation rates, the sediment cover in this region is thin and discontinuous. This allows low-temperature hydrothermal activity in the underlying crustal rocks, bringing oxygen and other solutes into the sediments from below. Therefore, the geochemical zonation in the deeper sediments is reversed. The sediment composition, mineral diagenesis and (bio)geochemical processes in this environment have not yet been studied in detail. This work, based on a combination of sediment-magnetic methods and geochemical analyses, investigates the mineralogy and the processes in these hydrothermally influenced sediments. The influx of oxygen from below supports the preservation of magnetic minerals in the sediments and enables reliable magnetic dating over several million years. Hydrothermal oxygen and chemoremanent overprinting have no significant effects. However, it was discovered that microaerophilic magnetotactic bacteria can live deep inside the hydrothermally oxygenated sediments, where no such bacteria have been expected in the past. The secondary biogenic magnetite produced by these bacteria accumulates locally and results in magnetic anomalies and overprinting. This discovery has far-reaching consequences for hydrothermally active deep-sea regions and, in particular, for ridge flank systems
I Have A Dream Speech Marked Civil War Centennial
No full textAuthor and historian David Blight connected the Civil War and the Civil Rights Movement during the recent Gilder-Jordan Lecture in Southern History event. A professor at Yale and director of the Gilder Lehrman Center, Blight\u27s latest book, American Oracle: The Civil War in the Civil Rights Era and Our Own Time focuses on Civil War memory. The following is a short piece from that lecture. Produced by Mary Stanton.https://egrove.olemiss.edu/umvideo/1839/thumbnail.jp
Oligo-Miocene magnetostratigraphy and rock magnetism of the Xishuigou section, Subei (Gansu Province, western China) and implications for shallow inclinations in central Asia.
No full textMagnetostratigraphy of 222 remanent directions together with late Oligocene to early Miocene mammal and charophyte paleontology suggest that 2179 m of the Xishuigou section (Subei, Gansu Province, China) were deposited from ~26 to ~19 Ma. Stratigraphic patterns of bulk susceptibility, anisotropy of magnetic susceptibility parameters, and natural and anhysteretic remanent magnetization intensities demonstrate that (1) faulting does not significantly affect the record, (2) sediment deposition was relatively continuous, (3) sediment source changed around 23 Ma, and (4) rapid uplift near Subei occurred at 21 Ma. Subei rotated 27° ± 5° counterclockwise with respect to the 20 Ma pole from the Eurasian synthetic apparent polar wander path. Folding and rotation of the section took place after 19 Ma. The paleolatitude of Subei is 14° less than at present and 19° ± 3° less than predicted from the reference pole. Both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing, a phenomenon which has likely affected other paleomagnetic data from central Asia
Contrasting Recording Efficiency of Chemical Versus Depositional Remanent Magnetization in Sediments
No full textHow and when sedimentary rocks record Earth's magnetic field is complex. Most studies assume a time-progressive lock-in mechanism during sediment deposition called depositional remanent magnetization (DRM). However, magnetic minerals can also form in situ, recording a chemical remanent magnetization (CRM) that is discontinuous in time. Disentangling the two mechanisms represents a major hurdle, and differences in their recording efficiencies remain unexplored. Here, our theoretical solutions demonstrate that CRM intensities exceed DRM by a factor of six when acquired in the same magnetic field. Novel experiments growing greigite (Fe3S4) in sediments and subsequent redeposition under identical magnetic field conditions confirm the predicted difference in recording efficiency. Thus, if left unrecognized, CRM leads to overestimated paleointensity and deserves more attention when interpreting Earth's magnetic history from sedimentary records. Recognition of fundamental differences between CRM and DRM characteristics provide a way forward to distinguish the recording mechanisms through routine laboratory protocols
Magnetic Properties of Hexagonal Closed-Packed Iron Deduced from Direct Observations in a Diamond Anvil Cell
No full textThe attraction of hexagonal closed packed (hcp) iron to a magnet at 16.9 gigapascals and 261 degrees centigrade suggests that hcp iron is either paramagnetic or ferromagnetic with susceptibilities from 0.15 to 0.001 and magnetizations from 1800 to 15 amperes per meter. If dominant in Earth's inner core, paramagnetic hcp iron could stabilize the geodynamo.</jats:p
- …
