The effects of vehicular and pedestrian traffic on dune vegetation in South Africa

The effects of varying intensities of off-road vehicle and pedestrian traffic on vegetation height and percent-cover were investigated at two sites representing pioneer and climax dune shrubland communities in South African dune systems. In all cases, a decreasing trend in vegetation height and percentage cover subsequent to application of the treatments was evident, although this was more clear in the shrub than in the pioneer community. The impact on vegetation variables increased with increasing intensity of vehicle and pedestrian treatments. Vehicles driven along a curved path resulted in greater vegetation destruction than those driven along a straight path. High intensity (single application) treatments resulted in an initial decrease in vegetation height and cover immediately after the impact, which was followed by a further decline over the next 3 months. This two-step response was attributed to the subsequent gradual death of the vegetation which was not evident immediately following the impact. Due to their faster growth rate, pioneer dune communities appear to have the capacity to recover following impacts, while dune shrub communities tend to be more vulnerable, exhibiting signs of damage for longer periods. The results of this study have value for managers of coastal dune systems, particularly those impacted by human recreational activities.     

Temperature dependence of calcite dissolution kinetics between 1 and 62°C at pH 2.7 to 8.4 in aqueous solutions

The kinetics of calcite dissolution in aqueous KCl-solutions far from equilibrium, between 1 and 62°C in the pH-range 2.7 to 8.4 have been investigated using a rotating disc apparatus. At neutral and alkaline pH in the mixed kinetic regime the empirical apparent activation energy (EAAE) for the surface chemical reaction rate constant is 54 ± 4 kJ mole^?1 for Carrara marble and 46 ± 4 kJ mole^?1 for Iceland spar. Under similar conditions the EAAE of the transport rate constant increases with decreasing temperature, but has a mean value of 27 ± 2 kJ mole^?1. The corresponding diffusion coefficient has a mean EAAE of 37 ± 3 kJ mole^?1 and this high EAAE is consistent with transport dependence on product diffusion in this H⁺-independent regime.In contrast, in acid solutions, where the rate approaches end-member transport control, the EAAE of the diffusion coefficient is 16 kJ mole^?1, also decreasing with increasing temperature. This is compatible with H⁺-diffusion to the surface being rate-controlling.In inhibitor-free natural systems, calcite dissolution kinetics far from equilibrium can be described in terms of three regimes: an H⁺-dependent regime (pH < 4 at 25°C), a transition regime (4 < pH < 5.5 at 25°C) and an H⁺-independent regime (pH > 5.5 at 25°C). At lower temperatures these boundaries move to higher pH values. The presence of inhibitors in natural systems may enhance surface controlled kinetics.     

Copper sulphide formation chemistry at low temperatures

Summary Covellite, blaubleibender covellite, djurleite and chalcocite have been synthesized by the reaction between Cu₂O with aqueous Na₂S solutions at low temperatures. Digenite was produced by heating the dried products of the aqueous experimentation at 105°C for 50 days, but not directly from aqueous solution. Anilite was formed through air oxidation of chalcocite at room temperature. The pH and Eh conditions of formation of these phases have been studied and it has been shown that the synthetic results broadly reflect the theoretical stability fields for these compounds, inasmuch as more reduced copper sulphides are formed under higher pH and lower Eh conditions. Studies of the changes in these parameters during the course of the experiments have been used to outline the mechanisms involved in the reactions. It has been demonstrated that the form of the copper sulphide produced around neutral pH values is almost independent of the oxidation state of the copper reactant, but highly sensitive to the presence and concentration of elemental sulphur. A detailed discussion of the formation chemistry of copper sulphides in low temperature aqueous solutions is presented.

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Chemie der Kupfersulfidbildung bei niedrigen Temperaturen

Zusammenfassung Covellin, blaubleibender Covellin, Djurleit und Chalkosin wurden durch Reaktion zwischen Cu₂O und wäßrigen Na₂S-Lösungen bei niedrigen Temperaturen synthetisiert. Digenit entstand, wenn die getrockneten Produkte aus wäßrigen Lösungen 50 Tage bei 105°C getempert wurden, aber nicht direkt aus wäßriger Lösung. Anilit bildete sich aus Chalkosin bei Zimmertemperatur durch Oxidation an der Luft. Die pH- und Eh-Bedingungen für die Bildung dieser Phasen wurden untersucht und es wurde gezeigt, daß die Syntheseergebnisse im wesentlichen den theoretischen Stabilitätsfeldern dieser Verbindungen entsprechen, indem sich die stärker reduzierten Kupfersulfide unter höherem pH und kleinerem Eh bilden. Untersuchungen über die Änderung dieser Parameter im Verlauf der Experimente wurden zur Umreißung der Reaktionsmechanismen benützt. Es wurde gezeigt, daß die Art des Kupfersulfides, welches sich um neutrale pH-Werte bildet, fast nicht vom Oxidationszustand des reagierenden Kupfers, aber stark von der Anwesenheit und der Konzentration von elementarem Schwefel abhängt. Die Chemie der Bildung von Kupfersulfiden in wäßrigen Lösungen niedriger Temperatur wird ausführlich diskutiert.

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With 2 Figures     

Timing of megakinks and related structures: Constraints from the Devonian Bunga‐Wapengo Basin,Mimosa Rocks National Park,New South Wales

The Bunga beds are bounded by faults adjacent to which are fanglomerates that form part of an early Late Devonian volcanic rift basin. Some cross‐faults acted as precursors for later regional deformation that kinked the Ordovician basement and gently folded the basin sediments in a northwest‐southeast direction. Details of the faulted junction at Picnic Point and orientation of cleavage microstructures in the fanglomerates support the dating of the north‐south regional F₂ deformation as Middle Devonian or older and the northwest‐southeast F₃ kinking as post‐early Late Devonian.     

Free oscillations of a rotating fluid contained between two spheroidal surfaces

Abstract

A theoretical study is made of the free periods of oscillation of an incompressible inviscid fluid, bounded by two rigid concentric spheroids. It is shown that the topography of the bounding surfaces can have a significant effect on the distribution of the eigenvalues. The significance of this result is discussed with reference to the observed westward drift of the geomagnetic secular variation.     

Evolution of the Martian atmosphere and hydrosphere: Solar wind erosion studied by ASPERA-3 on Mars Express

The evolution of the Martian atmosphere and the potential existence of a past hydrosphere is a scientific issue of great interest in planetary research. Although the first missions to Mars had a focus on surface features and atmospheric properties, some of the missions (e.g., The Soviet Mars 2, 3 and 5) also carried instruments addressing the solar wind interaction with the Martian atmosphere and ionosphere and the potential existence of an intrinsic magnetic field on Mars. However, it took until 1989 before a spacecraft, Phobos-2, was able to carry out a more detailed investigation of the solar wind interaction with Mars. Phobos-2 gave valuable data on the Solar wind interaction with Mars during about 2 months of operations, leading to a better understanding of the solar wind impact on a weakly magnetized planet. However, Phobos-2 also raised a number of critical issues that has left science without adequate data since 1989.Investigations planned for Mars Express will cast new light on important aspects of the solar wind interaction with Mars. ASPERA-3 (Analyzer of Space Plasma and Energetic Atoms) on Mars Express will focus on the overall plasma outflow and monitor remotely the outflow and inflow of energetic neutral atoms produced by charge exchange processes. This report will discuss some of the unsolved issues about the solar wind interaction with Mars and how we plan to address these issues with Mars Express.     

Rocket observations of electron spectral and angular characteristics in an “inverted V” event

Energy spectra and pitch angle distributions of auroral electrons in the energy range 2.5–11 keV observed on a rocket flight launched from Andøya on 13 November 1970 are presented. Strong rapidly fluctuating fluxes during the first part of the flight were succeeded by fluxes below or close to the level of detectability. Before the rocket passed through the northern precipitation boundary two spectral events of “inverted V” character occurred. Both events were associated with field aligned pitch angle distributions. While anisotropies with the flux peaked near 0° were in general associated with the spectral peak energy, isotropy over the upper hemisphere was the dominant distribution for other energies. The observations made during these events provide strong support for the theory of a parallel potential drop close to the ionosphere as an important accelerating mechanism for auroral electrons in connection with “inverted V” events.