Windblown sand on Venus: Preliminary results of laboratory simulations

Small particles and winds of sufficient strength to move them have been detected from Venera and Pioneer-Venus data and suggest the existence of aeolian processes on Venus. The Venus wind tunnel (VWT) was fabricated in order to investigate the behavior of windblown particles in a simulated Venusian environment. Preliminary results show that sand-size material is readily entrained at the wind speeds detected on Venus and that saltating grains achieve velocities closely matching those of the wind. Measurements of saltation threshold and particle flux for various particle sizes have been compared with theoretical models which were developed by extrapolation of findings from Martian and terrestial simulations. Results are in general agreement with theory, although certain discrepancies are apparent which may be attributed to experimental and/or theoretical-modeling procedures. Present findings enable a better understanding of Venusian surface processes and suggest that aeolian processes are important in the geological evolution of Venus.     

Hydrochemical characterization of acute acid mine drainage at Iron Duke mine, Mazowe, Zimbabwe

 Acid mine drainage (AMD) with a minimum pH of 0.52 was recorded at Iron Duke mine near Mazowe, Zimbabwe during an investigation of the environmental geochemistry of mine waters in the Greenstone Belts of Zimbabwe. Hydrochemical data for waters emanating from the Iron Duke waste-rock pile indicate their super-saturation with respect to Fe and SO₄ ^2–. Extremely high dissolved concentrations of Al, Zn, Cu, Co, Ni, V, Cr, Cd and As also prevail. Substantial losses of metals from solution occur within 400 m of the AMD source through the precipitation of crystalline sulphates, principally melanterite. Further downstream, hydrous oxide precipitation forms the dominant mechanism of metal attenuation in waters characteristically under-saturated with respect to Fe sulphates. Speciation and saturation index data generated using the equilibrium model WATEQ4F, suggest that such codes have broad utility for generic prediction of the mineralogical contraints on metal mobility in acute AMD systems. Major discrepancies between modelled and empirical hydrochemistries are, however, evident for super-saturated waters in which the kinetics of Fe precipitation are slow, and in which total ionic strengths markedly exceed their theoretical maximum. Received: 28 August 1998 · Accepted: 7 December 1998     

Cone-penetrometer exploration of Sinkholes: Stratigraphy and soil properties

Four sinkholes with varying surficial expressions were subjected to detailed stratigraphic and soil analysis by means of Standard Penetration Tests (SPT) and Electric Friction Cone Penetration Tests (CPT) in order to evaluate applications of CPT to sinkhole investigations. Although widely used, SPT data are of limited value and difficult to apply to sinkhole mapping. CPT is sensitive to minor lithologic variability and is superior to SPT as a cost-effective technique for determining geotechnical properties of sinkholes. The effectiveness of CPT data results from the force measurements made along the sleeve of the cone. The friction ratio (ratio of sleeve to tip resistance) is a good indicator of soil stratigraphy and properties. By smoothing the friction-ratio data, general stratigraphy and changes in soil properties are easily identified. Stratigraphy of the sinks has been complicated by intense weathering, karstification and marine, transgressions. The resulting deposits include five stratigraphic units. I and II represent Plio-Pleistocene marine sediments with Unit II being the zone of soil clay accumulation. III and IV are horizons residual from Miocene strata and indicate an episode of karstification prior to deposition of Units I and II. Conduit fill is a mixture of materials with low cohesion. The fill materials indicate centripetal and downward movement of insoluble sediments derived from the surrounding strata. Loss of cohesion results in near-zero friction ratios. Very low friction ratios, coupled with materials with little cohesion, indicate potentially-liquefiable soils in the immediate vicinity of zones where piping failure may be imminent. SPT does not provide sufficient data to predict these zones of potential, failure. CPT provides sufficient information for recognition of sinkhole stratigraphy and geotechnical properties. When coupled with laboratory soil analysis, CPT provides unique information about sinkhole geometry and dynamics. In contrast, SPT data fail to produce consistent indicators of sinkhole stratigraphy or properties. With laboratory soil data, SPT indicates general, inconclusive trends.     

Cyclicity in the late Precambrian Elatina Formation,South Australia: Solar or tidal signature?

New observations from two late Precambrian (~800-650 Ma) sedimentary rhythmites in South Australia (Reynella Siltstone and Chambers Bluff Tillite) reveal cycles structurally similar to the ~12-laminae cycles of the ~650-Ma Elatina Formation but which comprise from 15 to 26 or more laminae. The new data are difficult to accomodate in depositional models for the Elatina Formation whereby its laminae are regarded as annual and the cyclicity ascribed to solar variability (Williams and Sonett, 1985) or to the combined influences of the sunspot cycle and the lunar nodal tide (Zahnle and Walker, 1987). If, however, the long-term Elatina Cycle and not the individual lamina is taken as a yearly climatic signal, the Elatina and other rhythmites studied may be interpreted as the deposits of marine ebb-tidal deltas that record variability in the velocity and range of paleo-ebb tides. The basic laminae-cycles would represent lunar fortnightly cycles of diurnal and/or semidiurnal laminae, commonly truncated through non-deposition at neap tides. Despite numerous empirical similarities between the Elatina and sunspot series, the ebb-tidal model for deposition is preferred because it accounts plausibly for observations from all three rhythmites studied.     

Interplanetary scintillation observations of interaction regions in the solar wind

Co-rotating interaction regions (CIRs) between fast and slow streams of plasma are a prominent feature of the solar wind. Measurements of interplanetary scintillation (IPS) using the three widely separated antennas of the EISCAT facility have been used to detect the compression regions at the leading edges of interaction regions and to determine the location and velocity of the structure. Observations show that interaction regions have developed as close to the Sun as 25–30 solar radii, a result supported by theoretical modelling which shows that the conditions needed for CIRs to develop exist inside 30 solar radii.