The effects of source size on concentration fluctuations in plumes

The effects of source size on plume behaviour have been examined in a 1.2 m wind tunnel boundary layer for isokinetic sources with diameters from 3 to 35 mm at source heights of 230 mm and at ground level. Experimental measurements of mean concentration and the variance, intermittency and probability density functions of the concentration fluctuations were obtained. In addition, a fluctuating Gaussian plume model is presented which reproduces many of the observed features of the elevated emission. The mean plume width becomes independent of source size much more rapidly than the instantaneous plume width. Since it is the meandering of the instantaneous plume which generates most of the concentration fluctuations near the source, these are also dependent on source size. The flux of variance in the plume reaches a maximum, whose value is greatest for the smallest source size, close to the source and thereafter is monotonically decreasing. The intermittency factor reaches a minimum, whose value is lowest for the smallest source, and increases back towards one. Concentration fluctuations for the ground-level source are much less dependent on source size due to the effects of the surface.     

Long-term comparison of macrobenthos within the soft bottoms of the Bay of Banyuls-sur-mer (northwestern Mediterranean Sea)

The macrofauna present at seven of the stations within the Bay of Banyuls-sur-mer (northwestern Mediterranean) initially sampled by Guille during the late 1960s was reassessed in 1994, using the same gear and techniques as those used during the reference study. Results showed an increase both in the number of species and of individuals (all species pooled) per unit of surface area at nearly all stations. This trend was not significant for total biomass. The most important changes were linked to the increase of the polychaete Ditrupa arietina (both within the Spisula subtruncata and the Nephtys hombergii community) and the decrease of the polychaetes Scoloplos armiger and Notomastus latericeus within the Scoloplos armiger community. The possible causes underlying these changes are discussed. The most probable cause is considered to be a slight modification of sediment composition (decrease of fine material in the sediment) within the Spisula subtruncata, the Nephtys hombergii and the Scoloplos armiger communities. Temporal changes in (1) Rhone river input, (2) small coastal river inputs, and (3) frequency of easterly storms may all have contributed to such a decrease.     

Thermal, pressure and wind fields at ground level in the area of the Italian base at Terra Nova Bay, Victoria Land, Antarctica, as observed by a network of automatic weather stations

Ground temperature, pressure and wind speed monthly averages in the area of the Italian Station at Terra Nova Bay, Antarctica, were analyzed for the period 1987–1991 by means of a network of nine AWS (automatic weather stations). Spatial configurations of temperature show a well-defined, relatively warm island in the area of Terra Nova Bay, between Drygalsky and Campbell ice tongues, throughout the year. A second warm island is present to the north along the coast, between Aviator and Mariner ice tongues, for most of the year. From February to March a rapid drop in temperature is observed at all stations. A strong thermal gradient develops during February, March, April and October, November, December, between the coastal region and inner highlands. The baric configuration follows the elevation of the area. Annual average pressure and temperature as functions of stations altitude show linear trends. Severe katabatic wind episodes are recorded at all stations, with wind speed exceeding 25 ms^–1 and direction following the orographic features of the inner areas. Co-occurrences of these episodes were observed for stations located along stream lines of cold air drainage. The autocorrelation function of maximum wind speed time series shows wind persistence of 2–3 days and wind periodicity of about one week.     

Platinum-group element distribution in base-metal sulfides of the Merensky Reef from the eastern and western Bushveld Complex, South Africa

Base-metal sulfides in magmatic Ni-Cu-PGE deposits are important carriers of platinum-group elements (PGE). The distribution and concentrations of PGE in pentlandite, pyrrhotite, chalcopyrite, and pyrite were determined in samples from the mineralized portion of four Merensky Reef intersections from the eastern and western Bushveld Complex. Electron microprobe analysis was used for major elements, and in situ laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) for trace elements (PGE, Ag, and Au). Whole rock trace element analyses were performed on representative samples to obtain mineralogical balances. In Merensky Reef samples from the western Bushveld, both Pt and Pd are mainly concentrated in the upper chromitite stringer and its immediate vicinity. Samples from the eastern Bushveld reveal more complex distribution patterns. In situ LA-ICP-MS analyses of PGE in sulfides reveal that pentlandite carries distinctly elevated PGE contents, whereas pyrrhotite and chalcopyrite only contain very low PGE concentrations. Pentlandite is the principal host of Pd and Rh in the ores. Palladium and Rh concentrations in pentlandite reach up to 700 and 130 ppm, respectively, in the samples from the eastern Bushveld, and up to 1,750 ppm Pd and up to 1,000 ppm Rh in samples from the western Bushveld. Only traces of Pt are present in the base-metal sulfides (BMS). Pyrrhotite contains significant though generally low amounts of Ru, Os, and Ir, but hardly any Pd or Rh. Chalcopyrite contains most of the Ag but carries only extremely low PGE concentrations. Mass balance calculations performed on the Merensky Reef samples reveal that in general, pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts up to 100 % of the Pd and Rh and smaller amounts (10–40 %) of the Os, Ir, and Ru. Chalcopyrite and pyrrhotite usually contain less than 10 % of the whole rock PGE. The remaining PGE concentrations, and especially most of the Pt (up to 100 %), are present in the form of discrete platinum-group minerals such as cooperite/braggite, sperrylite, moncheite, and isoferroplatinum. Distribution patterns of whole rock Cu, Ni, and S versus whole rock Pd and Pt show commonly distinct offsets. The general sequence of “offset patterns” of PGE and BMS maxima, in the order from bottom to top, is Pd in pentlandite?→?Pd in whole rock?→?(Cu, Ni, and S). The relationship is not that straightforward in general; some of the reef sequences studied only partially show similar trends or are more complex. In general, however, the highest Pd concentrations in pentlandite appear to be related to the earliest, volumetrically rather small sulfide liquids at the base of the Merensky Reef sequence. A possible explanation for the offset patterns may be Rayleigh fractionation.     

A Zonal Pathway for NADW in the South Atlantic

Several large deployments of neutrally buoyant floats took place within the Antarctic Intermediate (AAIW), North Atlantic Deep Water (NADW), and the Antarctic Bottom Water (AABW) of the South Atlantic in the 1990s and a number of hydrographic sections were occupied as well. Here we use the spatially and temporally averaged velocities measured by these floats, combined with the hydrographic section data and various estimates of regional current transports from moored current meter arrays, to determine the circulation of the three major subthermocline water masses in a zonal strip across the South Atlantic between the latitudes of 19°S and 30°S. We concentrate on this region because the historical literature suggests that it is where the Deep Western Boundary Current containing NADW bifurcates. In support of this notion, we find that a net of about 5 Sv. of the 15–20 Sv that crosses 19°S does continue zonally eastward at least as far as the Mid-Atlantic Ridge. Once across the ridge it takes a circuit to the north along the ridge flanks before returning to the south in the eastern half of the Angola Basin. The data suggest that the NADW then continues on into the Indian Ocean. This scheme is discussed in the context of distributions of dissolved oxygen, silicate and salinity. In spite of the many float-years of data that were collected in the region a surprising result is that their impact on the computed solutions is quite modest. Although the focus is on the NADW we also discuss the circulation for the AAIW and AABW layers.     

The responses of different ethnic groups in the Sudan to rural-urban migration

The ethnic identities of the various population groups in the Sudan are a product of their specific natural and social environments. The migration of these groups to the towns, especially to Greater Khartoum, involves a drastic change of cultural norms and values. However, the capability of change and the subsequent results are strongly conditioned by the specific background of each ethnic group and by the conditions in its area of origin. This is being proved by a comparative study on three ethnic groups in the Sudan: the Nuba, the Zaghawa and the Hadandawa.     

The afterglow of a dense molecular cloud after the passage of a cosmological gamma-ray burst

Bolometric light curves for the afterglow resulting from the passage of a gamma-ray burst through a molecular cloud are computed. The profile and duration of the afterglow light curve depend strongly on the distribution of matter in the cloud, the degree of collimation of the gamma-ray radiation, and the observing conditions. The peak can be reached as soon as seven days (the gamma-ray burst is located at some distance from the center of a molecular cloud with small-scale density enhancements), or as long as one to three years (the gamma-ray burst is located at the center of a uniform molecular cloud) after the burst. The bolometric luminosity of the re-radiated signal can reach 6.5 × 10⁴² erg/s.     

Pyroxenite xenoliths from an alkali trachybasalt in the Glen Innes area,northeastern New South Wales

Pyroxenite xenoliths are relatively common in an alkali trachybasalt in the Glen Innes area in northeastern New South Wales where they coexist with peridotite xenoliths, probably lherzolitic. The pyroxenites vary widely in modal composition. Several pyroxenite xenoliths are characterised by megacrystals of subcalcic clinopyroxene and enstatite, the former comprising a unique group of high pressure pyroxenes in which exsolution of orthopyroxene has proceeded on a megascopic scale. Garnet is absent from all mineral assemblages. Mineralogical and experimental data suggest that the subcalcic clinopyroxene and orthopyroxene megacrystals equilibrated initially at temperatures and pressures of the order of 1350–1450° C and 10–23 kb respectively. The most common xenoliths, namely diopsideorthopyroxene assemblages in which there is evidence of subsolidus annealing, equilibrated at comparable pressures but a significantly lower temperatures (ca. 1000° C). It is suggested that the pyroxenites and associated peridotite xenoliths are samples of essentially unmodified layered upper mantle.