Constraints on crustal hydration below the Colorado plateau from Vp measurements on crustal xenoliths

Seismic velocities have been measured as a function of confining pressure to 8 kbar for crustal xenoliths from the Moses Rock Dike and Mule Ear Diatreme, two kimberlite pipes on the Colorado Plateau. Rock types measured include rhyolite, granite, diorite, metasedimentary schists and gneisses, mafic amphibolites and granulites. Many of our samples have been hydrothermally altered to greenschist facies mineral assemblages during transport to the earth's surface. The velocity of compressional waves measured on altered amphibolites and granulites are too low by 0.1–0.3 km/s for such rock types to be characteristic of deep crustal levels. A direct correlation exists between progressive alteration and the presence of microcracks extending into the xenoliths from the kimberlitic host rock. Velocities of pristine samples are compatible with existing velocity profiles for the Colorado Plateau and we conclude that the crust at depths greater than 15 km has probably not undergone a greenschist facies metamorphic event. The xenolith suite reflects a crustal profile similar to that exposed in the Ivrea-Verbano and Strona-Ceneri zones in northern Italy.     

Sorted stripes orientated by wind action: Some observations from sub-antarctic marion island

Sorted stripes found on the volcanic scoria and glacial deposits of sub-Antarctic Marion Island indicate a distinct preferred orientation. Despite uniformity of slope and material, the stripes are predominantly aligned parallel to the wind. It is suggested that melting of needle ice by the early morning sun is of only limited importance in the sub-Antarctic owing to the almost continual overcast conditions. The effect of the wind is so great that in exposed situations stripes are formed on horizontal surfaces.     

Analysis of fluoride, chloride, nitrate and sulphate in natural waters using ion chromatography

The analysis of anion components of thirty natural water samples using an ion-chromatographic method were compared to analyses for F, Cl, NO₃ and SO₄ obtained by presently used techniques. Of these thirty samples, five were replicates. Precision estimates were also calculated from synthetic solution replicates. In addition, a comparison of results obtained through separator columns of two different lengths (250 mm and 500 mm) was completed. An inter-anion interference test indicated that, at pH 6, analysis of low concentrations of any one of the four anions studied was not affected by large concentrations of the other three anions. Reported detection limits for the ion-chromatographic technique are one to two orders of magnitude below routine methods. Replicate sample analysis indicated that a relative standard deviation below 1 percent was possible for F, Cl, NO₃ and SO₄.     

The geochemistry of El Misti volcano,Peru fumaroles

El Misti is a 5822 m strato volcano in southern Peru. The mineralogy of the fumaroles is essentially sulfur, anhydrite, gypsum and ralstonite which are precipitated by acid vapors at 100°C–125°C. There are 2 to 6 parts per thousand chloride concentrations and several parts per million sodium, potassium, magnesium, calcium, iron and zinc concentrations in the condensed vapors. Elemental ratios indicate near complete leaching of the wall rock by the vapors. Arsenic and selenium are contaminants of the sulfur but are absent from the other minerals. Both the gypsum and anhydrite contain strontium and lead and show no tendency to fractionate these metals. The anhydrite alone contains significant thallium and bismuth. Bromine, strontium, thallium and lead are present in the ralstonite. The average δ D and δ O¹⁸ ratios from the fumarolic condensates are ?31.4 ‰ and +4.8 ‰ respectively. A marine contaminated heavy water source at depth is indicated as the parent water for the vapors.     

Dollard canal thwarted

The inner bay of Lourenço Marques receives the untreated sewage of the city, the run-off from five rivers and metal and hydrocarbon pollution from the activities of the port. Iron ore handling results in an increase in iron and copper concentrations in the water, particularly since loading by slurry was introduced. Large quantities of metal pollutants are probably accumulating in muds and are likely to contaminate shellfish which are an important food source in the area. Untreated sewage discharges are at such a level that eutrophication of the bay is threatened, besides constituting a public health risk on bathing beaches.     

Modelling tidally induced sediment-transport paths over the northwest European shelf: the influence of sea-level reduction

A three-dimensional model covering the northwest European Shelf and part of the adjacent Atlantic Ocean is used to examine the influence of water depth change upon the distribution of maximum tidal bed stress. The direction of bed stress is an indicator of sediment movement as bed load and various regions of convergence and divergence in good agreement with observations are identified. Calculations are performed with water depths reduced by 35 m, corresponding to 10 000 years before present (B.P.). Initially, the model is forced by only the M₂ tide, although subsequently five constituents, namely M₂, S₂, N₂, K₁ and O_1, are used for tidal forcing. Although the distribution of extreme bed stresses computed with only M₂ tidal forcing is comparable to that computed with five tides, the additional tidal constituents modify the magnitude of the bed stress. In particular the diurnal tides show regions of local enhanced current amplitude in the shelf-edge region with corresponding changes in bed stress. When water depths are reduced such that the North Sea and English Channel are separated, then there is a significant change in the tidal distribution in the shallow Southern Bight which influences bed-stress distributions and hence bed-load sediment transport in the area. Besides changes in shallow regions, the distribution of tides at the shelf edge is affected. A discussion of the limitations of the present coarse-grid model in shelf-edge regions and how it can be used to provide boundary conditions for limited-area three-dimensional models that can include stratification is presented. Also the importance of stratification for sediment movement at the shelf edge is briefly discussed.Responsible Editor: Phil Dyke     

Coefficients for Penman-based estimation of daily potential evapo-transpiration in Botswana

Reference potential evapo-transpiration (PET) for Botswana was estimated using a Penman-based equation in order to determine the appropriateness of five different sets of values for the 10 input coefficients. Calculated estimates were compared to the only existing set of observed PET values from two free drainage lysimeters at the headquarters of the Department of Meteorological Services (DMS) in Gaborone, Botswana. These values consisted of 36 five-day mean values of PET for grass cover during the period March 21, 1989–20 March, 1990 distributed as follows: fall 9, winter 8, spring 10, summer 9. Estimates were most sensitive to the wind coefficient in the aerodynamic term of the Penman-based equations. Analysis of the residual error between observed and estimated PET showed that the coefficients currently used by DMS best matched the lysimeter observations over the 4 seasons. However, only further comparisons between estimates and long-term, accurate, and multi-locational lysimeter measurements of reference crop PET would establish the set of coefficients appropriate for Penman-based PET estimation across Botswana.     

Calculating the effective permeability of sandstone with multiscale lattice Boltzmann/finite element simulations

The lattice Boltzmann (LB) method is an efficient technique for simulating fluid flow through individual pores of complex porous media. The ease with which the LB method handles complex boundary conditions, combined with the algorithm’s inherent parallelism, makes it an elegant approach to solving flow problems at the sub-continuum scale. However, the realities of current computational resources can limit the size and resolution of these simulations. A major research focus is developing methodologies for upscaling microscale techniques for use in macroscale problems of engineering interest. In this paper, we propose a hybrid, multiscale framework for simulating diffusion through porous media. We use the finite element (FE) method to solve the continuum boundary-value problem at the macroscale. Each finite element is treated as a sub-cell and assigned permeabilities calculated from subcontinuum simulations using the LB method. This framework allows us to efficiently find a macroscale solution while still maintaining information about microscale heterogeneities. As input to these simulations, we use synchrotron-computed 3D microtomographic images of a sandstone, with sample resolution of 3.34 μm. We discuss the predictive ability of these simulations, as well as implementation issues. We also quantify the lower limit of the continuum (Darcy) scale, as well as identify the optimal representative elementary volume for the hybrid LB–FE simulations.