The Lower Wonderfontein Spruit: an exposé

During the nineteenth century the Wonderfontein Valley and Spruit were described as a 'natural jewel' by adventurers. The valley underlain by dolomitic aquifers contained copious volumes of water and partly overlies gold-mining excavations, which were under a threat of flooding. For safety and economic reasons, large-scale dewatering of some of the dolomitic aquifers was decided upon as a matter of policy. This led to the lowering of the water table, which accelerated the formation of sinkholes and dolines in geologically sensitive areas. The streambed along approximately the first 30 km of the Lower Wonderfontein Spruit is particularly vulnerable. The mining sector attempted to rehabilitate the affected parts of the streambed by filling the sinkholes with various materials. Unfortunately, many of the treated sinkholes were reactivated by storm flow.     

A fluid-pressure feedback model of dehydration reactions: experiments, modelling, and application to subduction zones

Dehydration and melting reactions generate large volumes of fluid in the crust and upper mantle, and play an important role in subduction zone seismicity. The fluid pathway must evolve from isolated pockets in low porosity, low permeability rock, coalescing to interconnected permeable pathways to the surface. When fluid pressures generated from a dehydration or melting reaction are sufficient to induce hydrofracture, then hydrofracture significantly influences the porosity–permeability structure within the dehydrating/melting horizon. If a low fluid-pressure boundary is introduced to the dehydrating rock, then fluid will be driven from the rock along the evolved permeable network toward that boundary. The resulting pressure reduction can then accelerate the dehydration reaction and further drive the flow. The sudden introduction of a low fluid-pressure boundary may occur by the co-seismic (dilatant) rupturing of a pressure seal that connects different fluid pressure states. This mechanism is invoked to explain the observed post-seismic evolution of wave velocities (V_p/V_s) following the 1995 Antofagasta, Chile earthquake. We show experimental results and introduce a conceptual and numerical model that reflects this scenario. The model couples the mechanical and thermodynamic effects of fluid pressure with devolitization kinetics, and is quantitatively consistent with experimental studies of the dehydration of gypsum and serpentine. The experimental results show that dehydration is controlled by access to a free (drained) boundary. The model provides a mechanistic explanation for the experimental observations and has applications in understanding the role of transient transport networks on the large-scale behavior of dehydrating and melting systems.     

Modelling of hydrocyclone performance based on spray profile analysis

Spray profile measurements can be used to calculate the underflow rate, and consequently, be related to hydrocyclone performance. The flow geometry of the spray discharge is found to arise from velocity patterns at the outlet orifice. Through a videographic example of an industrial hydrocyclone, it is shown that underflow profiles are typically parabolic, a feature which is indicative of the velocity at which the fluid exits. The inclusion of gravity in this model clearly highlights deficiencies in currently used models. Moreover, an intimate knowledge of factors affecting the profiles of the underflow of a cyclone is essential for the correct interpretation of videographic images. Subsequently, image data are used to estimate outlet velocities, which give an excellent insight into various fluid mechanical phenomena that are not appreciated by analysing basic operational variables. The exit velocities are used to calculate underflow rates, which are related to mass recovery in the underflow.     

Modeling the binding of fulvic acid by goethite: the speciation of adsorbed FA molecules

Under natural conditions, the adsorption of ions at the solid-water interface may be strongly influenced by the adsorption of organic matter. In this paper, we describe the adsorption of fulvic acid (FA) by metal(hydr)oxide surfaces with a heterogeneous surface complexation model, the ligand and charge distribution (LCD) model. The model is a self-consistent combination of the nonideal competitive adsorption (NICA) equation and the CD-MUSIC model. The LCD model can describe simultaneously the concentration, pH, and salt dependency of the adsorption with a minimum of only three adjustable parameters. Furthermore, the model predicts the coadsorption of protons accurately for an extended range of conditions. Surface speciation calculations show that almost all hydroxyl groups of the adsorbed FA molecules are involved in outer sphere complexation reactions. The carboxylic groups of the adsorbed FA molecule form inner and outer sphere complexes. Furthermore, part of the carboxylate groups remain noncoordinated and deprotonated.     

Detection of sinkholes using 2D electrical resistivity imaging

Sinkholes in dolomitic areas are notoriously difficult geophysical targets, and selecting an appropriate geophysical solution is not straightforward. Electrical resistivity imaging, or tomography (RESTOM) is well suited to mapping sinkholes because of the ability of the technique for detecting resistive features and discriminating subtle resistivity variations. RESTOM surveys were conducted at two sinkhole sites near Pretoria, South Africa. The survey areas are located in the dolomites of the Lyttelton Formation, which forms part of the Malamani Subgroup and Chuniespoort Group of the Transvaal Supergroup. The survey results suggest that RESTOM is an ideal geophysical tool to aid in the detection and monitoring of sinkholes and other subsurface cavities.     

Quasi-conservative behaviour of Be in deep waters of the Weddell Sea and the Atlantic sector of the Antarctic Circumpolar Current

We present the first transect of dissolved ¹⁰Be depth profiles across the Antarctic Circumpolar Current (ACC) in the Atlantic sector. North of the Polar Front the ¹⁰Be concentrations increase continuously from very low values at the surface to values of up to 1600 atoms/g at depth. Deep water ¹⁰Be concentrations of particular water masses are consistent with earlier results obtained further north. South of the Polar Front and in the Weddell Sea the distribution of ¹⁰Be is also characterised by low surface concentrations but below 1000 m depth the concentrations are relatively constant and significantly higher (up to 2000 atoms/g) than further north, probably as a result of mixing and advection of water masses of Pacific origin. Overall the deep water ¹⁰Be distribution is obviously not significantly affected by scavenging processes or ice melt and comparison with the density distribution suggests that ¹⁰Be can be viewed as a quasi-conservative tracer. This provides a tool for an improved understanding of the behaviour of other more particle reactive trace metals in the Southern Ocean such as ²³⁰Th: in deep waters north of the ACC/Weddell Gyre boundary (AWB) ¹⁰Be/²³⁰Th has a relatively constant value (1.7±0.3×10⁹ atoms/dpm) over a wide density range whereas south of the AWB the ratio is significantly lower (1.1±0.2×10⁹ atoms/dpm). This normalisation to ¹⁰Be corroborates that ²³⁰Th is enriched by 50% due to accumulation south of the AWB as a consequence of minimal particulate fluxes. The quasi-conservative behaviour deduced from our results also implies that ¹⁰Be can only be used as a tracer for Southern Ocean particle fluxes in the past if ocean circulation patterns and water mass residence times did not change significantly.     

Palynological evidence for the astronomical origin of lignite-detritus sequence in the Middle Pleistocene Marathousa Member, Megalopolis, SW Greece

The Marathousa Member, Middle Pleistocene strata in the fluvio-lacustrine Megalopolis basin, southwest Greece, displays distinct but complicated lithological cycles comprising first-order alternation of lignites and detrital muds and second-order alternation expressed by frequent intercalation of organic layers. Palynological evidence indicates that the lithological cycles are driven by the Earth’s orbital forcing. All the lignite seams yield temperate oak forest whereas the detrital beds provide semi-arid steppe mainly of Artemisia. This means that the first-order lithological cycle represents the glacial/interglacial cycle (i.e., the 100-kyr eccentricity cycle), providing a timescale of at least 350 kyr to the Marathousa Member. Pollen also detects smaller-scale climate fluctuations in many of the subordinate organic layers, with the total number of fluctuations being five in a complete lignite-detritus couplet. This means that the second-order lithological cycle reflects the 21-kyr insolation cycle. A tentative phase relation between the lithological cycles and the astronomical cycles is shown based on palynostratigraphy and electron spin resonance dating. Lacustrine environments with increased water tables are implied for the glacial periods sedimentologically, in contrast to local swamp vegetation for the interglacial periods. The subordinate organic layers were formed under intermediate environments (climate, water depth, etc.) between full glacials and interglacials.