The effects of a constant volume deformation on the magnetization of an artificial sediment

Anhysteretic remanent magnetization (DC field = 1 Oe, peak AC field = 1000 Oe) was given to an artificial sediment consisting of a kaolinite matrix and a 0.03% magnetic fraction of needle-shaped magnetite grains. The mean grain size of the clay flakes and the magnetite needles was approximately 0.5 μm. This sediment was subjected to plane strain while maintaining constant volume. The axis of maximum compression was shortened by as much as 331/3%, which produced no significant change in the direction of magnetization with a decrease in intensity of 28%. A continuous deformation model, in which the magnetite needles are embedded in a plastically deforming linearly homogeneous medium, failed to explain these results. A discontinuous deformation model was more successful. In this model, deformation occurs mainly in shear zones bounding blocks which translate along the shears. Only those magnetite grains in the shear zones rotate, and from symmetry considerations no net change in the direction of magnetization is predicted. Because magnetite grains rotate in opposite directions in complementary sets of shears, the model predicts a decrease in intensity comparable to that which was observed.     

The Lake Lahontan highstand: age, surficial characteristics, soil development, and regional shoreline correlation

The Lake Lahontan basin has been the site of numerous pluvial lakes during the Pleistocene. We address the question of whether or not the highest remnant shoreline features around the perimeter of the lake were produced during the most recent Sehoo highstand (13 ka), the penultimate Eetza highstand (140–280 ka), or both. To do so, we document surficial characteristics, morphologic preservation, and soil development on multiple Sehoo beach barriers in the Jessup embayment to define the range in characteristics displayed by latest Pleistocene beach features. Sehoo barriers generally exhibit original constructional morphology that has been little modified by erosion. Soils developed on Sehoo barriers are generally thin and weakly developed and are strongly influenced by the introduction of eolian fines into the predominately clast-supported coarse beach gravels. Similar observations from 13 other highstand barriers and from seven older-than-latest Pleistocene paleosols located around the basin form the basis for a regional comparison. Based on similar characteristics, including the degree of morphologic preservation and weak soil development, we conclude that the widespread and nearly continuous high shoreline around the perimeter of Lake Lahontan dates from the most recent major lake cycle in all areas except in the Walker Lake subbasin. In the Walker Lake subbasin, isolated early to middle Pleistocene lacustrine outcrops and landforms are elevated as much as 70 m above the late Pleistocene limit, but are differentiated by their degraded form and lack of continuity around the subbasin. Similar unambiguous landforms were not observed elsewhere and at similar elevations in the northern subbasins of Lake Lahontan.     

Trace metal concentrations in offshore,deep-water seaweeds in the western North Atlantic ocean

Ambient, natural levels of selected heavy metals were assayed from deep, offshore populations of attached seaweeds from 1980 through 1983 from Pigeon Hill, 37 km from the nearest shoreline in the Gulf of Maine, and from Block Island, 23 km from the nearest mainland in Rhode Island Sound. Concentration levels of barium, cadmium, chromium, copper, mercury, lead and zinc were low when compared with concentration values published for a variety of seaweeds from the eastern and western North Atlantic. Values from Pigeon Hill and Block Island provide baseline data for long term monitoring of a relatively pristine, offshore, benthic community in the western North Atlantic.     

Late Holocene vegetation changes in Greenwater Valley,Mojave Desert,California

Small-scale late Holocene vegetation changes were determined from a series of 13 modern and fossil packrat middens collected from a site in the Greenwater Valley, northern Mojave Desert, California. Although the site is above the modern lower limit of Coleogyne ramosissima (black-brush), macrofossils of this shrub are only present in samples younger than 270 yr B.P. In order to measure changes more subtle than presence vs absence, macrofossil concentrations were quantified, and principal components and factor analyses were used to distinguish midden plant assemblages. Both the presence/absence data and the statistical analyses suggest a downward shift of 50 to 100 m for Coleogyne (blackbrush) communities between 1435 and 1795 A.D.     

The major petrological and geochemical characters of the 3,600 m.y. Uivak gneisses from Labrador

The 3,622±72 m.y. Uivak gneisses of northern Labrador consist of two major rock suites: regionally developed layered granodioritic gneisses (Uivak I suite) interpreted as derived from earlier tonalitic igneous parents by the massive introduction of potassium and rubidium, and a less extensive group of iron-rich porphyritic granodiorites and ferro-diorites (Uivak II suite) emplaced after at least one major period of deformation and migmatisation had affected the Uivak I gneisses. Samples from both suites fall on the 3,622 m.y. Rb/Sr whole-rock isochron.The age is provisionally interpreted as that of Rb metasomatism and homogenisation affecting both suites. It is suggested that the low initial ratio of 0.7014±0.0008 obtained from both suites need not be diagnostic of a short crustal residence if the evidence for massive addition of Rb to this level of the crust at 3,622 m.y. is accepted.     

Studies of quaternary saline lakes—II. Isotopic and compositional changes during desiccation of the brines in Owens Lake,California, 1969–1971

Owens Lake is an alkaline salt lake in a closed basin in southeast California. It is normally nearly dry, but in early 1969, an abnormal runoff from the Sierra Nevada flooded it to a maximum depth of 2·4 m. By late summer of 1971, the lake was again nearly dry and the dissolved salts recrystallized. Changes in the chemistry, pH, and deuterium content were monitored during desiccation.During flooding, salts (mostly trona, halite, and burkeite) dissolved slowly from the lake floor. Their concentration in the lake waters increased as evaporation removed water and salts again crystallized, but winter temperatures caused precipitation of some salts and the following summer warming caused their solution, resulting in seasonal variations in the concentration patterns of some ions. The pH values (9·4–10·4) changed with time but showed no detectable diurnal pattern.The deuterium concentration increased during evaporation and appeared to be in equilibrium with vapor leaving the lake according to the Rayleigh equation. The effective α(D/H in liquid/D/H in vapor) decreased as salinity increased; the earliest measured value was 1·069 [as total dissolved solids (TDS) of lake waters changed from 136,200 to 250,400 mg/1]and the last value (calc.) was 1·025 (as TDS changed from 450,000 to 470,300 mg/1). Deuterium exchange with the atmosphere was apparently small except during late desiccation stages when the isotopic contrast became great. Eventually, atmospheric exchange, combined with decreasing α and lake size and increasing salinity, stopped further deuterium concentration in the lake. The maximum contrast between atmospheric vapor and lake deuterium contents was about 110%.     

A deterministic approach to the prediction of free field ground motion and response spectra from stick-slip earthquakes

Theoretical and experimental results from computational physics and non-linear rock mechanics have been merged in order to obtain a deterministic model of a stick-slip earthquake. The model has been exercised to uncover the dependence of peak ground motion and response spectra on fault length, rupture velocity and dynamic stress drop during rupture. A particular method for generating a design spectrum has been tested against results from the model. Utilization of this deterministic technique seems especially appropriate when design information is required at sites located near the epicentre.     

Electromagnetic induction in spherical cap current layers under lunar and terrestrial conditions

We present the theory of electromagnetic induction in spherical cap current sheets of arbitrary angular size, with arbitrary axisymmetric integrated electrical conductivity variations and located at any radial position with respect to the surface of observation. The external time-varying magnetic field may be arbitrarily oriented with respect to the current layer cap and the induced fields are derived for vacuum boundary conditions appropriate to terrestrial induction and plasma confinement boundary conditions relevant to lunar induction in the solar wind or magnetosheath plasmas. Numerical evaluations show the induced magnetic field as a function of position over the current sheet cap, depth to the current layer, size of the cap, integrated electrical conductivity of the current sheet, and frequency of the fluctuating external field. The local vertical magnetic field component and the horizontal field component which is normal to the periphery of the cap exhibit peak inductive responses above the edge of the current sheet for external magnetic fields perpendicular to the axis of the cap. Thus, induced magnetic field fluctuations observed over the edge of a conductivity anomaly may exhibit a highly directional, or polarized behavior. This may provide an explanation for the asymmetric character of induced magnetic field fluctuations observed on the lunar surface.