The influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers: Comment on a paper by marwan A. Hassan and ian reid

Data from Turkey Brook are used to demonstrate that the interaction between gravel bedforms, flow resistance and bedload transport is a dynamic one, both between and within hydrographs. and that creation of a significant component of form drag through construction of microforms (pebble clusters) may precede the eventual break-up of the bed in a transport event. This process of drag augmentation', which can be seen as a feedback mechanism delaying transport and can be likened to dilation of a soil tested in a direct shear apparatus, itself appears to be dependent on the characteristics of turbulence, and therefore involves feedback at a finer resolution than envisaged by Hassan and Reid (1990).     

Soil pore-water distributions and the temperature feedback of weathering in soils

A review of the literature suggests that large variations in pore-water chemistry exist within soils. The heterogeneity indicates that in soil microchemical environments, the chemistry of pore water evolves independently from one pore to another due to differences in surface area/volume ratios and water residence time. A plug-flow reactor model was developed to examine which size classes of pores contribute the most solute to water draining out of the soil profile, and to explore how temperature might affect a soil’s ability to generate solute. The model is based on the simplification that soil pores can be approximated as a suite of capillaries of varying diameter. The model simulates each size class of pores as a plug-flow reactor with an unique water residence time and surface area.In the model, the pores which drain at the highest water contents have low surface area to water volume ratios and contribute relatively little to the overall solute flux from a soil. The smallest pores that drain at the lowest water contents were found to have the highest surface area to volume ratios and contribute the most solute. The calculations also suggest that activation energy and water viscosity have competing effects on the temperature dependence of weathering. As the temperature increases, the dissolution rate constant increases and smaller pores drain; however, water residence time decreases. This decrease in the water residence time is due to decreasing water viscosity, which can be incorporated into the dissolution rate law for quartz with an activation energy of approximately −15 kJ/mole. Studies that parameterize the temperature dependence of weathering using the Arrhenius approach can account for this effect by reducing the predicted activation energy by an appropriate value.     

Integration of Structural,Gravity, and Magnetic Data Using the Weights of Evidence Method as a Tool for Kimberlite Exploration in the Buffalo Head Hills,Northern Central Alberta,Canada

In the Buffalo Head Hills area a weights of evidence statistical approach was used to determine the spatial relationship of NNE-, NE-, NW-, and ENE-trending lineaments to known kimberlite locations. This method outlined different degrees of spatial correlation between and lineaments, with higher correlations defined for the NNE, NE, and ENE lineament data sets. A weights of evidence model then was constructed using the structural lineament maps, the Buffalo High and Buffalo Utikuma terrane boundary, Bouguer gravity data, and magnetic characteristics of the Buffalo High and Buffalo Utikuma terranes. The model reveals maximum favorability for kimberlite exploration along the Buffalo High and Utikuma terrane boundary in correspondence with NNE-trending lineaments and their intersections with NE and ENE lineaments. The relationship of the kimberlite occurrences along the Buffalo High-Buffalo Utikuma terrane boundary and structural lineaments seems to favor an hypothesis of kimberlite emplacement through a major zone of weakness in the basement, here characterized by the boundary between the Buffalo High and Buffalo Utikumaterranes.     

The geology and geochemistry of the Lumwana Cu (± Co ± U) deposits, NW Zambia

The Lumwana Cu (± Co ± U) deposits of NW Zambia are large, tabular, disseminated ore bodies, hosted within the Mwombezhi Dome of the Lufilian Arc. The host rocks to the Lumwana deposits are two mineralogically similar but texturally distinct gneisses, a granitic to pegmatitic gneiss and a banded to augen gneiss which both comprise quartz–feldspar ± biotite ± muscovite ± haematite ± amphibole and intervening quartz–feldspar ± biotite schist. The sulphide ore horizons are typically developed within a biotite–muscovite–quartz–kyanite schist, although mineralization locally occurs within internal gneiss units. Contacts between the ore and host rocks are transitional and characterized by a loss of feldspar. Kinematic indicators, such as S-C fabrics and pressure shadows on porphyroblasts, suggest a top to the north shear sense. The sulphides are deformed by a strong shear fabric, enclosed within kyanite or concentrated into low strain zones and pressure shadows around kyanite porphyroblasts. This suggests that the copper mineralization was introduced either syn- or pre-peak metamorphism. In addition to Cu and Co, the ores are also characterized by enrichments in U, V, Ni, Ba and S and small, discrete zones of uranium mineralization, occur adjacent to the hanging wall and footwall of the copper ore bodies or in the immediate footwall to the copper mineralization. Unlike typical Copperbelt mineralization, unmineralized units show very low background copper values. Whole rock geochemical analyses of the interlayered schist and ore schist, compared to the gneiss, show depletions in Ca, Na and Sr and enrichments in Mg and K, consistent with replacement of feldspar by biotite. The mineral chemistry of muscovite, biotite and chlorite reflect changes in the bulk rock chemistry and show consistent increases in X _Mg as the schists develop. δ³⁴S for copper sulphides range from +2.3?‰ to +18.5?‰, with pyrite typically restricted to values between +3.9?‰ and +6.2?‰. These values are atypical of sulphides precipitated by bacteriogenic sulphate reduction. δ³⁴S data for Chimiwungo (Cu + Co) show a broader range and increased δ³⁴S values compared to the Malundwe (Cu) mineralization. The Lumwana deposits show many characteristics which distinguish them from classical Copperbelt mineralization and which suggests that they are formed by metasomatic alteration, mineralization and shearing of pre-Katangan basement. Although this style of mineralization is reported elsewhere in the Copperbelt, sometimes associated with the more widely reported stratiform ores of the Lower Roan, none of the previously reported occurrences have so far developed the tonnages of ore reported at Lumwana.     

Eddy activity in the lee of the Hawaiian Islands

Persistent northeasterly trade winds have a substantial impact on the oceanic circulation around the Hawaiian Islands. A regional ocean model is applied to understand the effect of different temporal and spatial resolutions of surface momentum forcing on the formation of strong mesoscale vortices and on the simulation of realistic levels of eddy kinetic energy. The higher spatial and temporal resolutions of wind forcing is shown to substantially affect the vorticity and deformation field in the immediate lee of the Hawaiian Islands and produce patterns of eddy kinetic energy similar to observations. This suggests that the surface eddy field in the region is mostly dominated by the local surface momentum forcing. Mesoscale cyclones and anticyclones formed in the lee of the Island of Hawaii are shown to have different propagation patterns. Mesoscale cyclones are more confined to the lee and are hence subject to interactions with the strong wind forcing and deformation field as well as smaller vortices formed in the wake of the other islands. Mesoscale anticyclones show not only a tendency to propagate further westward, but also to persist as coherent features as they propagate, even at relatively lower values of relative vorticity. The large strain rates that affect the propagation of the cyclones cause them to break down into filaments of positive vorticity. Rossby numbers of O(1) within vortices and filaments indicate that nonlinear interactions between the wind stress and the vertical component of the relative vorticity field is potentially important in producing large vertical velocities. Modeled cyclonic eddies show a good resemblance to observations both in terms of vertical structure and propagation patterns.