Experiments on the settling, overturning and entrainment of bivalve shells and related models

An experimental and theoretical examination has been made of the settling, entrainment and overturning of 176 valves representing 16 common Northwest European marine bivalve species, together with a comparative study of 15 plastic models in the form of segments from cylindrical tubes. Settling behaviour in both stagnant and moving water depends on particle mass, symmetry and concavo-convexity. Separated empty bivalve shells spin and spiral while settling and, if sufficiently elongated, also pitch. At the observed Reynolds numbers, the shells and models fall concave-up, the terminal fall velocity increasing as the square root of the unit immersed mass or weight. The drag coefficient is independent of Reynolds number but increases with surface roughness and, particularly, particle elongation. Turbulence slightly lowers the critical elongation for pitching. A separation vortex lies captive on the upper side of each descending particle. Consequently, an empty bivalve shell traversing a suspension of sand traps grains on its upper side at a rate proportional to their volume concentration and terminal fall velocity. This process, increasing the effective shell mass, is limited only by the capacity of the shell and grain spillage due to the possible onset of pitching. The ratio (non-dimensional) of a quantity proportional to the applied fluid force and the particle unit immersed weight consistently describes the entrainment of concave-up and convex-up particles, and also the immediate overturning of a valve on settling concave-up to the bed. These thresholds vary in relative magnitude with bed-particle friction and particle concavo-convexity. In general, convex-up particles are the most stable; the concave-up entrainment and overturning thresholds are of a substantially lower but similar magnitude. The high frequency of concave-up bivalve attitudes in turbidites is understandable largely in terms of the ability of a settling valve to increase in effective mass by grain entrapment. Convex-up attitudes in the lower parts of turbidites may record currents stronger than the overturning threshold.     

Correlations in aftershock and seismicity patterns

Correlations in space and time play a fundamental role in earthquake processes. One direct manifestation of the effects of correlations is the occurrence of aftershocks due to the stress transfer in the vicinity of a main shock. Less obvious and more speculative changes in correlations may occur in the background seismicity before large earthquakes. Using statistical physics it is possible to introduce a measure of spatial correlations through a correlation length. This quantity characterizes how local fluctuations can influence the occurrence of earthquakes over distances comparable with the correlation length. In this work, the physical basis of spatial correlations of earthquakes is discussed in the context of critical phenomena and the percolation problem. The method of two-point correlation function is applied to the seismicity of California. Well defined variations in time of the correlation length are found for aftershock sequences and background seismicity. The scaling properties of our obtained distributions are analyzed with respect to changes in several scaling parameters such as lower magnitude cutoff of earthquakes, the maximum time interval between earthquakes, and the spatial size of the area considered. This scaling behavior can be described in a unified manner by utilizing the multifractal fit. Utilizing the percolation approach the time evolution of clusters of earthquakes is studied with the correlation length defined in terms of the radius of gyration of clusters. This method is applied to the seismicity of California.     

Microbial reduction of chromium from the hexavalent to divalent state

We demonstrate that Shewanella oneidensis, a metal-reducing bacteria species with cytoplasmic-membrane-bound reductases and remarkably diverse respiratory capabilities, reduced Cr(VI) to Cr(II) in anaerobic cultures where chromate was the sole terminal electron acceptor. Individual cell microanalysis by transmission electron microscopy (TEM) using electron energy-loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS) demonstrates Cr(II) concentrated near the cytoplasmic membrane, suggesting the terminal reduction pathway is intracellularly localized. Further, estimated cellular Cr(II) concentrations are relatively high at upwards of 0.03-0.09 g Cr/g bacterium. Accumulation of Cr(II) is observed in S. oneidensis cells prior to the formation of submicron-sized precipitates of insoluble Cr(III) on their surfaces. Furthermore, under anaerobic conditions, Cr(III) precipitates that encrust cells are shown to contain Cr(II) that is likely bound in the net negatively charged extracellular biopolymers which can permeate the surfaces of the precipitates. In otherwise nearly identical incubations, Cr(III) precipitate formation was observed in cultures maintained anaerobic with bubbled nitrogen but not in three replicate cultures in an anaerobic chamber.     

The subsystem of open clusters in the post-hipparcos era: Cluster structural parameters and proper motions

The wide neighborhoods of 401 open clusters are analyzed using the modern, high-precision, homogeneous ASCC-2.5 all-sky catalog. More than 28000 possible cluster members (including about 12500 most probable members) are identified using kinematic and photometric criteria. Star counts with the ASCC-2.5 and USNO-A2.0 catalogs are used to determine the angular and linear radii of the cluster cores and coronas, which exceed the previously published values by factors of two to three. The segregation (differing central concentration) of member stars by magnitude is observed. The mean proper motions are determined directly in the Hipparcos system for 401 clusters, for 183 of them for the first time. The heliocentric distances of 118 clusters are determined for the first time based on color-magnitude diagrams for their identified members.     

Exploration for scheelite in East Greenland — A case study

An intensive scheelite exploration program was carried out in Precambrian crystalline and sedimentary rocks intruded by granites of Precambrian and Caledonian ages in East Greenland (70–74°30′N). Previous heavy-mineral panning (2100 samples within an area of 100,000 km²) formed the basis for selection of scheelite-anomalous subregions (1550 km²)In the subregions, pan-concentrate samples were taken from first- and second-order rivers and from mid and side moraines of active glaciers. All samples were studied in the field under UV light, and scheelite grains were counted. A consideration of the distribution of scheelite in the samples together with the river and glacial drainage systems, led to definition of the potential source areas of the scheelite within localities of 2–5 km².Within the localities, panning of scree fines (samples every 100–200 m along talus slopes) and UV-light traverses at night led to the finding of outcropping or sub-outcropping scheelite mineralisation. Scheelite was observed associated with granite-carbonate contact zones, quartz vein stockworks, and fault zones in limestones, at nine localities within the 300-km-long zone of investigation.The heavy-mineral panning method with the counting of scheelite grains in the field and the subsequent definition of potential scheelite-bearing areas has the advantage that it is possible to execute a program from the subregional to the outcropping mineralisation stage in one field season. The investigation in this case was performed by five geologists during the 1979 2.5-month field season.