A rural “eye” in the Capital: Tuti Island,Khartoum, Sudan

Islands at strategic points have often become the core of a modern city, or have been developed as historic heritage sites. This has been especially true when the settlers have been strangers in the land. This paper examines why such a situation has not occurred with Tuti Island at the confluence of the White and Blue Niles in the centre of Sudan's Three Towns capital, even though the first permanent inhabitants at the confluence were strangers and chose Tuti Island for their first settlement. The paper questions whether Tuti can survive as a green eye in its present form with the increasing pressures from the urban areas surrounding it.     

Towards a political geography of United Nations peacekeeping: Some considerations

Peacekeeping missions should be viewed from various political and spatial perspectives. Firstly, there is the macro-level perspective of the UN Organization itself, particularly the respective interests of the most powerful members of the UN and permanent seat holders on the Security Council. Secondly, there are the national perspectives of the various resources and troop-contributing states to specific UN operations. Thirdly, there are the interests of the conflicting parties themselves, including governments, national armies, local militia groups, factional political leaders and local communities. An analysis of UN peacekeeping involves a complex multi-dimensional geopolitical framework that incorporates the linkages and contradictions between all these differing perspectives, from macro to micro-spatial scales. Peacekeepers themselves are not passive figures in the political and human landscapes within which they operate. Many of the activities of UN blue helmets alter landscapes and conflict scenarios. Whilst some of these changes are deliberate, such as the creation of demilitarized zones, many are unforeseen and less visible, such as the ways in which local civilian populations are affected by the presence of large numbers of foreign peacekeepers. This paper examines some of the political, social and economic consequences of peacekeeping missions in places as diverse as Cambodia, Somalia, the Lebanon and Cyprus.     

Multiple Isotopic Components in Quaternary Volcanic Rocks of the Cascade Arc near Crater Lake, Oregon

Quaternary lavas and pyroclastic rocks of Mount Mazama, CraterLake caldera, and the surrounding area have variable Sr, Nd,and Pb isotopic compositions. High-alumina olivine tholeiites(HAOT) have ⁸⁷Sr/⁸⁶Sr ratios of 0.70346–0.70364; basalticandesite, 0–70349–0.70372; shoshonitic basalticandesite, 0.70374–0.70388; and andesite, 0.70324–0.70383.Dacites of Mount Mazama have ⁸⁷Sr/⁸⁶Sr ratios of 0.70348–0.70373.Most rhyodacites converge on 0.7037. However, rhyodacite ofthe caldera-forming, climactic eruption has ⁸⁷Sr/⁸⁶Sr=0.70354because of an admixed low-⁸⁷Sr/⁸⁶Sr component. Andesitic tomafic-cumulate scoriae of the climactic eruption, and enclavesin preclimactic rhyodacites, cluster in two groups but shownearly the entire ⁸⁷Sr/⁸⁶Sr range of the data set, confirmingpreviously suggested introduction of diverse parental magmasinto the growing climactic chamber. Pb and Nd isotope ratiosdisplay less variation (²⁰⁶Pb/²⁰⁴Pb= 18.838–18.967, ²⁰⁷Pb/²⁰⁴Pb=15.556–15.616,²⁰⁸Pb/²⁰⁴Pb=38.405–38.619; _Nd= +3.9 to +6.1) and generallycovary with ⁸⁷Sr/⁸⁶ Sr ratios. Radiogenic isotope data fromCrater Lake plot with published data for other Cascade volcanoeson isotope ratio correlation diagrams. The isotopic data for the Crater Lake area require sources ofprimitive magmas to consist of depleted mantle and a subductioncomponent, introduced in variable quantity to the depleted mantlewedge. Variable degrees of melting of this heterogeneous mantle,possibly at different depths, produced the diversity of isotopiccompositions and large-ion lithophile element (LILE) abundancesin primitive magmas. Trace element ratios do not indicate presenceof an ocean island basalt (OIB) source component that has beenreported in lavas of some other Cascade volcanoes. Crustal contamination may have affected isotope ratios and LILEconcentrations in evolved HAOT, where initial LILE concentrationswere low. Contamination is more difficult to detect in the calcalkalinelavas because of their higher LILE concentrations and the smallisotopic contrast with likely contaminants, such as mid- tolower-crustal rocks thought to be equivalents of igneous rocksof the Klamath Mountains and associated lower crust. Crustalassimilation appears to be required for calcalkaline rocks onlyby ¹⁸O values, which vary from lows of +5.6 to + 6.0% in HAOTand primitive basaltic andesites to a high of +7.0% in dacite,a range that is too high to be explained by plagioclase-dominatedclosed-system fractional crystallization. Elevated ¹⁸O valuesof differentiated lavas may be attributed to interaction withrelatively ¹⁸O-rich, ⁸⁷Sr-poor crustal rocks. Variably fused granitoid blocks ejected in the climactic eruption,and rarely in late Pleistocene eruptive units, have ¹⁸Opl of–3.4 to +6.5% and ¹⁸O_qz of –2.2 to +8.0% but haveSr, Nd, and Pb isotope ratios similar to volcanic rocks (e.g.⁸⁷Sr/⁸⁶Sr0.7037). Rb and Sr data for glass separates from granodioritessuggest that the source pluton is Miocene. Glass from granodioritehas ⁸⁷Sr/⁸⁶Sr ratios as high as 0.70617. Oxygen isotope fractionationbetween quartz, plagioclase, and glass indicates requilibrationof O isotopes at magmatic temperatures, after ¹⁸O/¹⁶O had beenlowered by exchange with meteoric hydrothermal fluids. Unmeltedgranodiorite xenoliths from pre-climactic eruptive units have¹⁸O values that are consistent with onset of hydrothermal exchangeearly during growth of the climactic magma chamber. Assimilationof such upper-crustal granodiorite apparently lowered ¹⁸O valuesof rhyodacites without significantly affecting their magmaticcompositions in other ways.     

Aqueous alteration of the Bali CV3 chondrite: evidence from mineralogy, mineral chemistry, and oxygen isotopic compositions

A petrographic, geochemical, and oxygen isotopic study of the Bali CV3 carbonaceous chondrite revealed that the meteorite has undergone extensive deformation and aqueous alteration on its parent body. Deformation textures are common and include flattened chondrules, a well-developed foliation, and the presence of distinctive (100) planar defects in olivine. The occurrence of alteration products associated with the planar defects indicates that the deformation features formed prior to the episode of aqueous alteration. The secondary minerals produced during the alteration event include well-crystallized Mg-rich saponite, framboidal magnetite, and Ca-phosphates. The alteration products are not homogeneously distributed throughout the meteorite, but occur in regions adjacent to relatively unaltered material, such as veins of altered material following the foliation. The alteration assemblage formed under oxidizing conditions at relatively low temperatures (<100 degrees C). Altered regions in Bali have higher Na, Ca, and P contents than unaltered regions which suggests that the fluid phase carried significant dissolved solids. Oxygen isotopic compositions for unaltered regions in Bali fall within the field for other CV3 whole-rocks, however, the oxygen isotopic compositions of the heavily altered material lie in the region for the CM and CR chondrites. The heavy-isotope enrichment of the altered regions in Bali suggest alteration conditions similar to those for the petrographic type-2 carbonaceous chondrites.     

Carbon isotopic fractionation in lipids from methanotrophic bacteria: relevance for interpretation of the geochemical record of biomarkers

Experiments with cultured aerobic methane oxidising bacteria confirm that their biomarker lipids will be significantly depleted in 13C compared to the substrate. The methanotrophic bacteria Methylococcus capsulatus and Methylomonas methanica, grown on methane and using the RuMP cycle for carbon assimilation, show maximum 13C fractionation of approximately 30% in the resultant biomass. In M. capsulatus, the maximum fractionation is observed in the earliest part of the exponential growth stage and decreases to approximately 16% as cells approach stationary phase. This change may be associated with a shift from the particulate form to the soluble form of the methane monooxygenase enzyme. Less than maximum fractionation is observed when cells are grown with reduced methane availability. Biomass of M. capsulatus grown on methanol was depleted by 9% compared to the substrate. Additional strong 13C fractionation takes place during polyisoprenoid biosynthesis in methanotrophs. The delta 13C values of individual hopanoid and steroid biomarkers produced by these organisms were as much as l0% more negative than total biomass. In individual cultures, squalene was 13C-enriched by as much as 14% compared to the triterpane skeleton of bacteriohopaneaminopentol. Much of the isotopic dispersion in lipid metabolites could be attributed to shifts in their relative abundances, combined with an overall reduction in fractionation during the growth cycle. In cells grown on methanol, where there was no apparent effect of growth stage on overall fractionation there were still significant isotopic differences between closely related lipids including a 5.3% difference between the hopane and 3 beta-methylhopane skeletons. Hopane and sterane polyisoprenoids were also 13C-depleted compared to fatty acids. These observations have significant implications for the interpretation of specific compound isotopic signatures now being measured for hydrocarbons and other lipids present in sediments and petroleum. In particular, biomarker lipids produced by a single organism do not necessarily have the same carbon isotopic composition.     

Inner part observation of theβ pictoris disk

The inner regions of the circumstellar disk around Pictoris might be dust-free due to a possible planet which may have cleared up dust particles. We present a new observational technique based on the use of an anti-blooming CCD in order to directly image this zone. The structure of the disk is revealed down to 2 arcsec from the star (30 AU). We show that the Pic disk brightness is neutral inV, R, andI _C , but drops down when going inward inB, possibly related to a change in the dust composition. Also, a slight disk asymmetry is present but inverted from the outer to the inner zones.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Wind variability in intermediate luminosity B supergiants

We discuss wind variability in intermediate luminosity B supergiants. Because these are not the hottest, most luminous or just about other kind of -est stars, they have not attracted much attention. However, since their wind lines are well-developed, but unsaturated, they make ideal diagnostics for time dependent wind studies. We show that their wind profiles undergo large scale variability and that this phenomenon is quite common. We also discuss a time series for the B0.5 Ib HD 64760 which suggests the long sought after connection between wind and photospheric activity. Newly developed analysis techniques needed to extract physical information from the wind lines are discussed, and results are applied to the time series.     

A coronal loop model for the production of X-ray and radio flares in the RS CVn binary HR 1099

The RS CVn binary stellar system HR 1099 is a source of both X-ray and radio flares. We present here a model of the system in which the two types of flare are produced by the same population of mildly-relativistic ( 10) electrons, injected into a coronal loop. After reviewing possible radiation mechanisms we conclude that, given the probable conditions in the flaring region, the radio emission is gyrosynchrotron radiation and the X-ray emission is thermal bremsstrahlung. The thermal X-ray source must lie in the stellar chromosphere, but the apparent absence of plasma absorption at radio frequencies indicates that the radio source is located high in the coronal loop. Using the relationships given by Dulk and Marsh (1982) for the radio emission from a power-law electron energy spectrum,N() ( - 1)^–, we conclude that 3 7, with 30% of the electron population trapped in the radio source. Some implications of these results for one particular version of the model are discussed.     

Strategy of DENIS observations

Doing a homogenious and fast survey requires an intelligent model for the observing-strategy. This contribution shows a model simulating the parameters for the survey of the southern hemisphere. Input parameters are weather statistics and requirements of the photometry and of the technical equipment. It does not include priorities for special zones of the sky or deeper observations, even if the program is already able to consider such parameters.     

A photochemical model of the martian atmosphere

The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HOx radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations.