Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides,elemental sulfur,and pyrite

Rates of sulfate reduction, oxygen uptake and carbon dioxide production in sediments from a short Spartina alterniflora zone of Great Sippewissett Marsh were measured simultaneously during late summer. Surface sediments (0–2 cm) were dominated by aerobic metabolism which accounted for about 45% of the total carbon dioxide production over 0–15 cm. Rates of sulfate reduction agreed well with rates of total carbon dioxide production below 2 cm depth indicating that sulfate reduction was the primary pathway for sub-surface carbon metabolism. Sulfate reduction rates were determined using a radiotracer technique coupled with a chromous chloride digestion and carbon disulfide extraction of the sediment to determine the extent of formation of radiolabelled elemental sulfur and pyrite during shortterm (48 hr) incubations. In the surface 10 cm of the marsh sediments investigated, about 50% of the reduced radiosulfur was recovered as dissolved or acid volatile sulfides, 37% as carbon disulfide extractable sulfur, and only about 13% was recovered in a fraction operationally defined as pyrite. Correlations between the extent of sulfate depletion in the marsh sediments and the concentrations of dissolved and acid volatile sulfides supported the results of the radiotracer work. Our data suggest that sulfides and elemental sulfur may be major short-term end-products of sulfate reduction in salt marshes.     

The simulation of ore-dressing plants

An efficient and automated ore-dressing plant simulator has been developed.In this simulator, stream variables can be used to describe a large number of unique solid particles that have a wide range of properties. Consequently, a large number of simultaneous equations need to be solved.This paper demonstrates how particles can be categorized and the mass balance for complex recycle systems rapidly achieved. The simulator that has been developed uses unit modules that are linked together by an executive computer programme. The executive, by treating each module in turn, obtains a set of simultaneous equations that are solved iteratively. The sequence in which the modules are treated is determined by partitioning and tearing algorithms that select tear streams, thus rendering the system acyclic. The algorithms are designed to choose the tear set (as there are usually several) that will allow convergence in the fewest possible iterations. This choice is based on a criterion that can be extended to apply to all the convergence methods considered. A simultaneous convergence technique has been developed that is applicable specifically to linear ore-dressing plants, but can also be applied to non-linear systems. This technique, which is called the reduced Newton method, is shown to exhibit local convergence under reasonable conditions and is generally superior to direct substitution and to the bounded Wegstein methods.     

Recent immigration to Italy: Character,causes and consequences

During the 1970s Italy changed from being a country of mass emigration to one of mass immigration, taking over from Germany the role of Europe's main recipient of immigrants from less developed countries. By 1991 the officially registered foreign population in Italy stood at 860,000; however, clandestine migrants push the real figure above 1 million. Italy was generally unprepared for this immigration and policy has been slow to evolve. Analysis of residence permit data show that the immigrants come increasingly from Third World, especially African, countries, and that there is a relative concentration in the north of Italy. One third of the immigrants are Moslem. Employment data are scarce but indicate that around two–thirds are involved in low-grade service sector activities (street-trading, domestic service, hotel work etc.). There is a high degree of occupational specialisation amongst certain national groups (Senegalese street-hawkers, Tunisian fishermen, Filipino domestics etc.). The immaturity of the immigration is also revealed by marked gender and age assymetry. Five main causes are suggested as being behind the immigration: ease of entry; Italy's increasing prosperity; segmentation of the Italian labour market, opening up specific niches for immigrant employment; dominance of push factors from the countries of origin; and the demographic collapse in Italy. Within Italy, the reaction to immigration has not been very favourable. Opinion polls indicate that Italians have mainly negative and stereotyped views of immigrants and there is disturbing evidence of growing racism. Further inflows of immigrants are likely, whatever policies Italy attempts to put in place.     

A comparison of bowen ratio and eddy correlation sensible and latent heat flux measurements above deciduous forest

Sensible and latent heat flux densities (H and E) were measured above a mature, 18 m deciduous forest during July and August, 1988, using the Bowen ratio-energy balance (BREB) and eddy correlation (EC) methods. EC estimates ofH and E underestimated day-time surface available energy by 11%. EC also partitioned available energy differently than BREB. for/L<0.0, EC favouredH and BREB favoured E. Practical and theoretical limitations of the BREB and EC methods above forests are discussed. The most plausible causes for the failure of EC to close the surface energy balance are a low frequency loss of flux and the failure of a single point measurement to account for the spatial dispersive flux. The most plausible causes of the EC-BREB energy partitioning anomaly are the invalidity of the BREB similarity assumption and the violation of flux-gradient diffusion assumptions in the near-field diffusion region.     

A model for the internal structures of asteroids

Interpretation of reflectance spectra indicates that most belt asteroids are composed of materials similar to carbonaceous chondrites. Also, there is considerable evidence to support the origin of many, if not most, lunar and meteoritic chondrules by impact processes. The accretional histories of the carbonaceous asteroids must have influenced greatly their internal structures and textures. A model for this accretional history can be divided conveniently into three temporal stages that produce distinctly different lithologies: (1) low-velocity accretion of fine silicate and carbonaceous grains producing chondrule-free petrologic type 1 lithology; (2) continued accretion of low-velocity fine silicate and carbonaceous grains, but with a few larger, higher-velocity bodies also impacting the surface thereby producing both fluid drop and lithic chondrules (the resultant lithology would be that of petrologic type 2 and 3 carbonaceous chondrites); and (3) dominance of high-velocity low-mass meteoroid impacts, producing a sparse, thin, erosive lunar-like regolith. The lithologic product of stage 3 is not ideally represented among the presently described carbonaceous chondrites, but texturally analogous samples are known from the achondrites. The greater proportion of chondrules in the CV group meteorites, in contrast to the CM2 and CO3 groups, may be due to the origin of the CV chondrites on larger asteroid parent bodies that could withstand more numerous and higher-energy chondrule-producing impacts prior to fragmentation.     

Surface particle sizes on armoured gravel streambeds: effects of supply and hydraulics

Most gravel‐bed streams exhibit a surface armour in which the median grain size of the surface particles is coarser than that of the subsurface particles. This armour has been interpreted to result when the supply of sediment is less than the ability of the stream to move sediment. While there may be certain sizes in the bed for which the supply is less than the ability of the stream to transport these sizes, for other sizes of particles the supply may match or even exceed the ability of the channel to transport these particles. These sizes of particles are called ‘supply‐limited’ and ‘hydraulically limited’ in their transport, respectively, and can be differentiated in dimensionless sediment transport rating curves by size fractions. The supply‐ and hydraulically limited sizes can be distinguished also by comparing the size of particles of the surface and subsurface. Those sizes that are supply‐limited are winnowed from the bed and are under‐represented in the surface layer. Progressive truncation of the surface and subsurface size distributions from the ?ne end and recalculation until the size distributions are similar (collapse), establishes the break between supply‐ and hydraulically limited sizes. At sites along 12 streams in Idaho ranging in drainage area from about 100 to 4900 km², sediment transport rating curves by size class and surface and subsurface size distributions were examined. The break between sizes that were supply‐ and hydraulically limited as determined by examination of the transport rate and surface and subsurface size distributions was similar. The collapse size as described by its percentile in the cumulative size distribution averaged D₃₆ of the surface and D₇₃ of the subsurface. The discharge at which the collapse size began to move averaged 88 per cent of bankfull discharge. The collapse size decreased as bed load yield increased and increased with the degree of selective transport. Copyright © 2003 John Wiley & Sons, Ltd.     

Distribution d'un élément trace (Cr) dans un sol développé sur roches métamorphiques : variabilité à l'échelle d'un versantDistribution of chromium in soils developed into metamorphic rocks: variability at the hillslope scale

The influence of pedogenetic factors onto spatial variability of chromium in soil surface horizon was studied in a metamorphic area (Massif Central, France) at the hillside scale. A grid survey was performed. Surface soil horizons were sampled at each point and analysed in Cr. Results show the determinant influence of the rock type on Cr content distribution along the slope. However, surface horizons of these soils exhibit a lower Cr content heterogeneity than the deeper horizons. This was explained by soils' material erosion and deposition along the slope. Tillage and hedges also influence the Cr distribution in surface horizon along the slope. To cite this article: S. Salvador-Blanes et al., C. R. Geoscience 334 (2002) 51–58     

Spatial distribution and geomorphology of lunette dunes on the High Plains of Western Kansas: implications for geoarchaeological and paleoenvironmental research

The High Plains are a vast, low relief landscape that contains thousands of upland-embedded depressional playa wetlands. Lunettes are isolated dunes that form from prolonged periods of aeolian deposition along the downwind margin of some larger playas. Playa-lunette systems are important landscape elements that have the potential to preserve high-resolution records of past environments, and several archeological sites have been associated with playa-lunette systems throughout the High Plains. While a comprehensive playa wetland geospatial database has been developed for the High Plains, lunettes have only been mapped for the Southern High Plains of Texas and New Mexico. The purpose of this research was to map and characterize the geomorphology of all lunettes associated with playas in the High Plains and bordering regions of western Kansas. Results indicate 129 lunettes are associated with 104 playas in Kansas. All lunettes are associated with playas larger than 5 ha, and about 10% of these larger playas have a lunette. Lunette height ranges from approximately 1.5–12 m, with a median height of 3 m. Most lunettes (60%) are in Finney, Gray, Lane, and Scott counties, which have broad, flat uplands with few major drainages upwind of playas and lunettes.