Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

The bulk magnetic mineral record from Lake Ohrid, spanning the past 637 kyr, reflects large-scale shifts in hydrological conditions, and, superimposed, a strong signal of environmental conditions on glacial–interglacial and millennial timescales. A shift in the formation of early diagenetic ferrimagnetic iron sulfides to siderites is observed around 320 ka. This change is probably associated with variable availability of sulfide in the pore water. We propose that sulfate concentrations were significantly higher before  ∼  320 ka, due to either a higher sulfate flux or lower dilution of lake sulfate due to a smaller water volume. Diagenetic iron minerals appear more abundant during glacials, which are generally characterized by higher Fe / Ca ratios in the sediments. While in the lower part of the core the ferrimagnetic sulfide signal overprints the primary detrital magnetic signal, the upper part of the core is dominated by variable proportions of high- to low-coercivity iron oxides. Glacial sediments are characterized by high concentration of high-coercivity magnetic minerals (hematite, goethite), which relate to enhanced erosion of soils that had formed during preceding interglacials. Superimposed on the glacial–interglacial behavior are millennial-scale oscillations in the magnetic mineral composition that parallel variations in summer insolation. Like the processes on glacial–interglacial timescales, low summer insolation and a retreat in vegetation resulted in enhanced erosion of soil material. Our study highlights that rock-magnetic studies, in concert with geochemical and sedimentological investigations, provide a multi-level contribution to environmental reconstructions, since the magnetic properties can mirror both environmental conditions on land and intra-lake processes.

Sural nerve conduction studies using ultrasound-guided needle positioning: Influence of age and recording location.

INTRODUCTION The aim of this study was to compare orthodromic sural nerve conduction study (NCS) results using ultrasound-guided needle positioning (USNP) to surface electrode recordings. METHODS 51 healthy subjects aged 24 - 80 years, divided into 5 age groups, were examined. Electrical stimuli were applied behind the lateral malleolus. Sensory nerve action potentials (SNAPs) were recorded 8 and 15 cm proximally with surface and needle electrodes. RESULTS Mean SNAP amplitudes in µV (surface/needle electrodes) averaged 12.7 (SD 7.6)/40.6 (SD 20.8), P<0.001, for subjects aged 20-29 years, and 5.0 (SD 2.4)/19.8 (SD 9.8), P<0.01, for subjects aged > 60 years. SNAP amplitudes were smaller at the proximal recording location. DISCUSSION NCS using USNP yield higher amplitude responses than surface electrodes in all age groups at all recording sites. SNAP amplitudes are smaller at proximal recording locations due to sural nerve branching. This article is protected by copyright. All rights reserved.

Magnetic properties and chemical composition at DSDP Hole 77-538A

Magnetic properties of doleritic and some metamorphic basement rocks underlying Catoche Knoll are studied. Doleritic rocks show a high saturation magnetic moment (2-5 emu/g) compared to metamorphic rocks (0.1-1 emu/g). Magnetic minerals of rocks from this hole show a high stability when heated in vacuo up to 600°C at a fixed rate of heating. Curie temperatures are distributed close to 550°C. These properties differ markedly from those of common submarine basalts observed before. X-ray microprobe analysis techniques were used to determine internal structures of ferromagnetic minerals; in most of ferromagnetic minerals there exist two different types of magnetic phases (i.e., products of high-temperature and low-temperature oxidations). Interpretations on the coexisting, seemingly contradictory, phases can be made based upon present analyses.

Magnetic properties and oxide petrography of ODP Hole 111-504B

Magnetic properties measurements were performed on 47 samples drilled during Leg 111 of the Ocean Drilling Program and oxide petrography was studied in 32 samples taken at depths throughout the sheeted dike complex in Hole 504B. Integration of these data with results from previous DSDP legs shows that while natural remanent magnetization is constant with depth, magnetic susceptibility increases and median demagnetizing field and the Q ratio decrease with depth in the section. These trends appear to be a result of an increase in deuteric oxidation and a decrease in hydrothermal alteration of primary titanomagnetite with depth. A distinct change in stable magnetic inclination occurs between the extrusive basalts and the sheeted dikes and may be a result of tectonic rotation of the upper extrusive basalts.

Magnetic mineralogy and geochemistry of ODP Holes 137-504B and 140-504B samples

Leg 140 of the Ocean Drilling Program deepened Hole 504B to a total depth of 2000.4 m below seafloor (mbsf), making it the deepest hole drilled into ocean crust. Site 504, south of the Costa Rica Rift, is considered the most important in-situ reference section for the structure of shallow ocean crust. We present the results of studies of magnetic mineralogy and magnetic properties of Hole 504B upper crustal rocks recovered during Legs 137 and 140. Results from this sample set are consistent with those discussed in Pariso et al. (this volume) from Legs 111, 137, and 140. Coercivity (Hc) ranges from 5.3 to 27.7 mT (mean 12 mT), coercivity of remanence (HCR) ranges from 13.3 to 50.6 mT (mean 26 mT), and the ratio HCR/HC ranges from 1.6 to 3.19 (mean 2.13). Saturation magnetization (JS) ranges from 0.03 to 5.94 * 10**-6 Am**2, (mean 2.52 * 10**-6 Am**2), saturation remanence (JR) ranges from 0.01 to 0.58 * 10**-6 Am2 (mean 0.37 * 10**-6 Am**2), and the ratio JR/JS ranges from 0.08 to 0.29 (mean 0.16), consistent with pseudo-single-domain behavior. Natural remanent magnetization (NRM) intensity ranges from 0.029 to 7.18 A/m (mean 2.95 A/m), whereas RM10 intensity varies only from 0.006 to 4.8 A/m and has a mean of only 1.02 A/m. Anhysteretic remanent magnetization (ARM) intensity ranges from 0.04 to 6.0 A/m, with a mean of 2.46 A/m, and isothermal remanent magnetization (IRM) intensity ranges from 0.5 to 1683 A/m, with a mean of 430.7 A/m. Volume susceptibility ranges from 0.0003 to 0.043 SI (mean 0.011 SI). In all samples examined, high-temperature oxidation of primary titanomagnetite has produced lamellae or pods of magnetite and ilmenite. Hydrothermal alteration has further altered the minerals in some samples to a mixture of magnetite, ilmenite, titanite, and a high-titanium mineral (either rutile or anatase). Electron microprobe analyses show that magnetite lamellae are enriched in the trivalent oxides Cr2O3, Al2O3, and V2O5, whereas divalent oxides (MnO and MgO) are concentrated in ilmenite lamellae.

Granulometry, biogenic silica content, magnetic susceptibility and bulk densities of ODP Site 188-1165

The Prydz Bay area is a key region for studying and understanding the history of the eastern Antarctic Continental Ice Sheet (O'Brien, Cooper, Richter, et al., 2001, doi:10.2973/odp.proc.ir.188.2001). Ocean Drilling Program (ODP) Site 1165 is situated in a water depth of 3357 m on the continental rise offshore from Prydz Bay and lies in front of the outlet for the Lambert Glacier-Amery Ice Shelf system that today drains 22% of East Antarctica. The site was drilled into mixed pelagic and hemipelagic sediments from the southwestern side of the Wild Drift. The drift is an elongate sediment body formed by the interaction of sediment supplied from continental shelf and slope with westward-flowing bottom currents. The sedimentary sequence is characterized by alternations between a generally gray to dark gray facies and a green to greenish gray facies. The greenish facies are structureless diatom-bearing clays with common bioturbation and larger amounts (>15%-20%) of biogenic silica, dispersed clasts, and lonestones than the dark gray facies, which are mostly less bioturbated clay with some silt laminations (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.188.103.2001). High-quality advanced piston corer and extended core barrel cores containing nearly complete sections of middle Miocene to early Pliocene age allow a detailed characterization of sedimentary cycles and can provide indications for ice advances of the Lambert Glacier system into Prydz Bay, for the extent of sea ice, and for changes in oceanic circulation. The purpose of this work is to provide a data set of coarse-fraction mass percentage (>63, >125, and >250 µm) and biogenic silica content measured on sediments of late Miocene to early Pliocene age drilled at Site 1165. Additionally, high-resolution records of magnetic susceptibility (MS) and gamma ray attenuation (GRA) bulk density are presented. These shipboard data sets were edited postcruise. Furthermore, I provide a high-resolution dry bulk density record that is derived from GRA bulk density and can be used for the calculation of mass accumulation rates. These sedimentological and physical parameters will be used in future work to understand the depositional pattern of alternating biogenic and terrigenous sediments that was observed at Site 1165 (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.188.103.2001).

Relationship between definitive datum levels and magnetic polarity and revised geomagnetic polarity time-scale at DSDP Hole 72-516F

This paper contains magnetobiostratigraphic correlation charts for each of the four sites occupied during DSDP Leg 72. Microfossil zonal boundaries and magnetic polarity determinations for Sites 515 through 518 are summarized in Figures 1 through 4, respectively. Our discussion focuses on the correlations derived for the Paleogene and late Cretaceous (Coniacian-Maestrichtian) of Site 516, because of the value of this site as a stratigraphic reference section for the South Atlantic.

Magnetic susceptibility and electric conductivity of marine surficial sediments by benthic electromagnetic profiling

Distribution, accumulation and diagenesis of surficial sediments in coastal and continental shelf systems follow complex chains of localized processes and form deposits of great spatial variability. Given the environmental and economic relevance of ocean margins, there is growing need for innovative geophysical exploration methods to characterize seafloor sediments by more than acoustic properties. A newly conceptualized benthic profiling and data processing approach based on controlled source electromagnetic (CSEM) imaging permits to coevally quantify the magnetic susceptibility and the electric conductivity of shallow marine deposits. The two physical properties differ fundamentally insofar as magnetic susceptibility mostly assesses solid particle characteristics such as terrigenous or iron mineral content, redox state and contamination level, while electric conductivity primarily relates to the fluid-filled pore space and detects salinity, porosity and grain-size variations. We develop and validate a layered half-space inversion algorithm for submarine multifrequency CSEM with concentric sensor configuration. Guided by results of modeling, we modified a commercial land CSEM sensor for submarine application, which was mounted into a nonconductive and nonmagnetic bottom-towed sled. This benthic EM profiler Neridis II achieves 25 soundings/second at 3-4 knots over continuous profiles of up to hundred kilometers. Magnetic susceptibility is determined from the 75 Hz in-phase response (90% signal originates from the top 50 cm), while electric conductivity is derived from the 5 kHz out-of-phase (quadrature) component (90% signal from the top 92 cm). Exemplary survey data from the north-west Iberian margin underline the excellent sensitivity, functionality and robustness of the system in littoral (~0-50 m) and neritic (~50-300 m) environments. Susceptibility vs. porosity cross-plots successfully identify known lithofacies units and their transitions. All presently available data indicate an eminent potential of CSEM profiling for assessing the complex distribution of shallow marine surficial sediments and for revealing climatic, hydrodynamic, diagenetic and anthropogenic factors governing their formation.

Magnetic viscosity and paleomagnetism of the basement complex at DSDP Hole 77-538A

The basement at Catoche Knoll consists of Paleozoic gneiss and amphibolite intruded by several generations of early Jurassic diabase dikes. Upon exposure to a 1-oersted field for 9 days, the diabase and amphibolite acquire a viscous remanent magnetization (VRM) which ranges from 42 to 2047% of their natural remanent magnetization (NRM). A magnetic field of similar intensity is observed in the paleomagnetic facility of the Glomar Challenger, and it is therefore doubtful if accurate measurements of magnetic moments in such rocks can be made on board unless the facility is magnetically shielded. The significant VRM also indicates the futility of attempting to discern magnetic lineations from an ocean floor composed of such rocks. No strong correlation exists between the Königsberger ratio, which is usually less than 1, and the tendency to acquire a VRM. The VRM decay is typical of a Richter aftereffect, but the relaxation times vary widely among the samples studied. A stable remanence is observed after alternating field demagnetization to 200 Oe. The range of magnetic inclinations in the diabase dikes is consistent with 40Ar/39Ar dates of 190 and 160 Ma. The inclinations suggest that the Catoche Knoll block tilted more than 20° to the north after the final dike intrusion.

Magnetic properties and heavy metals at DSDP Hole 83-504B

We report measurements of magnetic intensity, inclination, initial susceptibility, Koenigsberger's ratio, saturation magnetization, and Curie temperatures of 68 basalt samples from the Leg 83 section of Hole 504B. As in the upper part of the hole, reversely magnetized units predominate. Intensities of natural remanent magnetization vary widely, but the range of variation is an order of magnitude less than in the upper part of the hole. This and the other properties measured indicate that the magnetic characteristics of basalts from Hole 504B have been strongly affected by hydrothermal alteration, particularly in the deeper, Leg 83 section. The alteration states of the magnetic samples were studies using Xray diffraction, electron microprobe, X-ray fluorescence, and ion coupled plasma. Our results suggest three alteration zones in Hole 504B: a low-temperature zone (274.5-890 m) and two high-temperature zones (890-1050 m and 1050- 1350 m), differing in the number of veins observed in the samples and presumably differing in the volumes of hydrothermal fluids which reacted with the basalts.

Rock magnetic properties and titanomagnetite oxydation state of ODP Hole 109-648B (Table 1)

The titanomagnetite oxidation state of "zero age" ocean floor basalts was investigated. For this purpose the oxidation parameter, z, of Hole 648B basalts was determined by SEM observation of "shrinkage cracks" in individual titanomagnetite grains and by Curie temperature measurements. A mean z-value of 0.1 has been deduced for the Hole 648B basalts. Assuming a linear relationship between titanomagnetite low-temperature oxidation state and age of the oceanic basalt, an age of 0.7 m.y. is deduced for Hole 648B.

Magnetic properties and paleomagnetism of basalts from ODP Leg 107 basalts

The basalts recovered at Holes 651A and 655B appear to carry a single component remanent magnetization, which is generally of reversed polarity. These magnetizations are consistent with eruption during the Matuyama (651A) and Gilbert (655B) polarity epochs. The blocking temperature spectra and the Js/T curves indicate that titanomaghemite is the principal remanence carrier. The lower mean destructive field (MDF) and higher susceptibility at 651A probably indicates a lower mean oxidation state at this hole relative to 655B, which may simply reflect the age difference between the two basalt sequences. At both holes, a decreasing downcore trend both in natural remanent magnetization (NRM) and susceptibility probably indicates that maghemitization (from primary titanomagnetite) increases downcore. An interval of high coercivity at hole 655B (119.80-151.45 mbsf) appears to define a magnetically distinct unit within the basalt sequence.

Magnetic properties and geochemistry in DSDP Holes 69-504B an d 69-505B

More than 60 basalt samples from two Deep Sea Drilling Project holes on the Costa Rica Rift were studied for magnetic properties and were found to have no properties significantly different from other DSDP basalts. Opaque mineralogical and thermomagnetic properties of these samples, however, to some extent show differences from normal submarine basalts; a new type of thermomagnetic curve and wide range of chemical compositions were recognized. Oxidized samples possibly containing incipient ilmenite exsolution lamellae were reduced and re-equilibrated during heating. The Curie temperatures of the re-equilibrated titanomagnetites are interpreted to be those of the original crystallized phase before oxidation.