Proton Irradiation of Centaur,Kuiper Belt,and Oort Cloud Objects at Plasma to Cosmic Ray Energy

Times for accumulation of chemically significant dosages on icy surfaces of Centaur, Kuiper Belt, and Oort Cloud objects from plasma and energetic ions depend on irradiation position within or outside the heliosphere. Principal irradiation components include solar wind plasma ions, pickup ions from solar UV ionization of interstellar neutral gas, energetic ions accelerated by solar and interplanetary shocks, including the putative solar wind termination shock, and galactic cosmic ray ions from the Local Interstellar Medium (LISM). We present model flux spectra derived from spacecraft data and models for eV to GeV protons at 40 AU, a termination shock position at 85 AU, and in the LISM. Times in years to accumulate dosages ～100 eV per molecule are computed from the spectra as functions of sensible surface depth less than one centimeter at unit density.The collisional resurfacing model of Luu and Jewitt is reconsidered in thecontext of depth-dependent dosage rates from plasma, suprathermal,and higher energy protons, and global exposure, by micrometeoroiddust grain impacts, of moderately irradiated red material below athin crust of heavily irradiated neutral material. This material should be more visible on dynamically `cold’ objects in the ～40 AU region.     

Cosmological Parameters from Cosmic Microwave Background Anisotropies: Status and Prospects

A survey is made of the present constraints on cosmological parameters from Cosmic Microwave Background Anisotropies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Monitoring selective availability dither frequencies and their effect on GPS data

Summary The signals transmitted by Block II satellites of the Global Positioning System (GPS) can be degraded to limit the highest accuracy of the system (10 m or better point positioning) to authorized users. This mode of degraded operation is called Selective Availability (S/A). S/A involves the degradation in the quality of broadcast orbits and satellite clock dithering. We monitored the dithered satellite oscillator and investigated the effect of this clock dithering on high accuracy relative positioning. The effect was studied over short 3-meter and zero-baselines with two GPS receivers. The equivalent S/A effects for baselines ranging from 0 to >10,000 km can be examined with short test baselines if the receiver clocks are deliberately mis-synchronized by a known and varying amount. Our results show that the maximum effect of satellite clock dithering on GPS double difference phase residuals grows as a function of the clock synchronization error according to: S/A _effect =0.04 cm/msec, and it increases as a function of baseline length like: S/A _effect =0.014 cm/100 km. These are equations for maximum observed values of post-fit residuals due to S/A. The effect on GPS baselines is likely to be smaller than the 0.14 mm for a baseline separation of 100 km. We therefore conclude, for our limited data set, and for the level of S/A during our tests, that S/A clock dithering has negligible effect on all terrestrial GPS baselines if double difference processing techniques are employed and if the GPS receivers remain synchronized to better than 10 msec. S/A may constitute a problem, however, if accurate point processing is required, or if GPS receivers are not synchronized. We suggest and test two different methods to monitor satellite frequency offsets due to S/A. S/A modulates GPS carrier frequencies in the range of-2 Hz to +2 Hz over time periods of several minutes. The methods used in this paper to measure the satellite clock dither could be applied by the civilian GPS community to continuously monitor S/A clock dithering. The monitored frequencies may aid high accuracy point positioning applications in a postprocessing mode (Malys and Ortiz 1989), and differential GPS with poorly synchronized receivers (Feigl et al. 1991).     

Main sequence for rotating stars of intermediate mass

We have investigated the evolutionary behaviour of intermediate mass (2, 3, 4, 5, and 7M ) Population I stars, assuming two different rates of rotation at the threshold of stability.In the first part of the study, stars are assumed to start with a critical rotation (fast rotation model) and to progress to the point of rotational instability. The stars evolve by losing mass and become rotationally unstable before they reach the zero-age Main Sequence. It is argued that multiple star systems might be formed through the evolution of rapidly rotating stars. An expression for the rotational mass loss rate is derived as a function of the physical parameters of stars.In the second part of the study, stars are assumed to rotate at a rate below the critical value (slow rotation model). The evolution of slowly rotating stars is followed as far as zero-age Main Sequence on the theoretical Hertzsprung-Russell diagram and compared with that of normal stars. The evolutionary paths are found to be more or less similar to those of normal stars; but their positions on the Main Sequence are characterized by effective temperatures and luminosities lower than those of normal stars. The zero-age Main-Sequence times of these stars are longer than those of normal stars. The rotational rates obtained for the zero-age Main Sequence are in good agreement with observed values.     

Relationship between footwall composition,crustal contamination,and fluid–rock interaction in the Platreef,Bushveld Complex,South Africa

In the northern limb of the 2.06-Ga Bushveld Complex, the Platreef is a platinum group elements (PGE)-, Cu-, and Ni-mineralized zone of pyroxenite that developed at the intrusion margin. From north to south, the footwall rocks of the Platreef change from Archaean granite to dolomite, hornfels, and quartzite. Where the footwall is granite, the Sr-isotope system is more strongly perturbed than where the footwall is Sr-poor dolomite, in which samples show an approximate isochron relationship. The Nd-isotope system for samples of pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of 2.17 ± 0.2 Ga and initial Nd-isotope ratio of 0.5095. Assuming an age of 2.06 Ga, the ɛNd values range from −6.2 to −9.6 (ave. −7.8, n = 17) and on average are slightly more negative than the Main Zone of the Bushveld. These data are consistent with local contamination of an already contaminated magma of Main Zone composition. The similarity in isotope composition between the Platreef pyroxenites and the hanging wall norites suggests a common origin. Where the country rock is dolomite, the Platreef has generally higher plagioclase and pyroxene δ ¹⁸O values, and this indicates assimilation of the immediate footwall. Throughout the Platreef, there is considerable petrographic evidence for sub-solidus interaction with fluids, and the Δ _{plagioclase–pyroxene} values range from −2 to +6, which indicates interaction at both high and low temperatures. Whole-rock and mineral δD values suggest that the Platreef interacted with both magmatic and meteoric water, and the lack of disturbance to the Sr-isotope system suggests that fluid–rock interaction took place soon after emplacement. Where the footwall is granite, less negative δD values suggest a greater involvement of meteoric water. Consistently higher values of Δ _{plagioclase–pyroxene} in the Platreef pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with CO₂-rich fluid derived from decarbonation of the footwall rocks. The overprint of post crystallization fluid–rock interaction is the probable cause of the previously documented lack of correlation between PGE and sulfide content on the small scale. The Platreef in contact with dolomite is the focus of the highest PGE grades, and this suggests that dolomite contamination played a role in PGE concentration and deposition, but the exact link remains obscure. It is a possibility that the CO₂ produced by decarbonation of assimilated dolomite enhanced the process of PGE scavenging by sulfide precipitation.     

A geological and geotechnical investigation of the settlement area of Zümrüt Building (Konya, Turkey) which caused 92 fatalities due to its collapse

This study examines the local geological conditions and soil structure as possible causes of the collapse of the Zümrüt Building 2 February 2004. This catastrophe resulted in 92 fatalities and 35 injuries. This study also examines other views which claim weak soil structure, elastic and consolidation settlement of soil and excessive groundwater extraction as well as subsidence resulting from the underground silt erosion as possible factors. Zümrüt Building was constructed on normally consolidated, low plasticity clay. The underground water table was 30 m in depth. The internal friction angle of soil was 8°–30°, its cohesion was between 34 and 127 kN/m² and standard penetration test numbers varied between 11 and 50. The underground water level beneath Zümrüt Building had risen 4.5 m since its construction. Therefore the claim that subsidence resulting from the decrease of underground water level contributed to the collapse is incorrect. Secondly the settlement, resulting from the filling up of the pores created by the silt receding with the underground water, was 4.4 mm in total, and attributing this as the primary cause of the collapse is also incorrect. Soil properties, in situ and laboratory test results showed that the existing and/or expected settlement and the differential ground settlement in the Zümrüt building vicinity had the potential to cause structural damage. The tensile stresses caused by differential settlements recorded here are thought to be an indicator, but not the main cause contributing to the collapse of the building. The Zümrüt Building collapse was due to several compounding mistakes during the construction phase. These were geotechnical and other project faults and the use of low quality construction materials. The resulting catastrophe caused 92 fatalities, 35 injuries and a material loss of approximately US$7 million.     

Loading and fate of particulate organic carbon from the Himalaya to the Ganga-Brahmaputra delta

We use the evolution of river sediment characteristics and sedimentary C_org from the Himalayan range to the delta to study the transport of C_org in the Ganga-Brahmaputra system and especially its fate during floodplain transit.A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and C_org content in order to evaluate the controls on C_org loading. Contributions of C3 and C4 plants are estimated from C_org stable isotopic composition (δ¹³C_org). We use the evolution of δ¹³C_org values from the Himalayan range to the delta in order to study the fate of C_org during floodplain transit.Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4-5 times higher specific area, Ganga sediments have similar C_org content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on C_org loading. The general correlation between the total C_org content and Al/Si ratio indicates that C_org loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates.Bed and suspended sediments have distinct δ¹³C_org values. In bed sediments, δ¹³C_org values are compatible with a dominant proportion of fossil C_org derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ¹³C_org values (−24.8‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ¹³C_org values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ¹³C_org values (−22.4‰ to −20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic C_org coming from the Himalaya is oxidised and replaced by floodplain C_org. This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain.     

Magnesium isotope systematics of the lithologically varied Moselle river basin, France

Magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the Moselle basin, northeastern France. This small basin is remarkable for its well-constrained and varied lithology on a small distance scale, and this is reflected in river water Sr isotope compositions. Upstream, where the Moselle River drains silicate rocks of the Vosges mountains, waters are characterized by relatively high ⁸⁷Sr/⁸⁶Sr ratios (0.7128-0.7174). In contrast, downstream of the city of Epinal where the Moselle River flows through carbonates and evaporites of the Lorraine plateau, ⁸⁷Sr/⁸⁶Sr ratios are lower, down to 0.70824.Magnesium in river waters draining silicates is systematically depleted in heavy isotopes (δ²⁶Mg values range from −1.2 to −0.7‰) relative to the value presently estimated for the continental crust and a local diorite (−0.5‰). In comparison, δ²⁶Mg values measured in soil samples are higher (∼0.0‰). This suggests that Mg isotope fractionation occurs during mineral leaching and/or formation of secondary clay minerals. On the Lorraine plateau, tributaries draining marls, carbonates and evaporites are characterized by low Ca/Mg (1.5-3.2) and low Ca/Sr (80-400) when compared to local carbonate rocks (Ca/Mg = 29-59; Ca/Sr = 370-2200), similar to other rivers draining carbonates. The most likely cause of the Mg and Sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer. δ²⁶Mg of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year (from −1.4‰ to −1.6‰ and from −1.2‰ to −1.4‰, respectively), within the range defined for the underlying rocks. Downstream of Epinal, the compositions of the Moselle River samples in a δ²⁶Mg vs. ⁸⁷Sr/⁸⁶Sr diagram can be explained by mixing curves between silicate, carbonate and evaporite waters, with a significant contribution from the Vosgian silicate lithologies (>70%). Temporal co-variation between δ²⁶Mg and ⁸⁷Sr/⁸⁶Sr for the Moselle River throughout year is also observed, and is consistent with a higher contribution from the Vosges mountains in winter, in terms of runoff and dissolved element flux. Overall, this study shows that Mg isotopes measured in waters, rocks and soils, coupled with other tracers such as Sr isotopes, could be used to better constrain riverine Mg sources, particularly if analytical uncertainties in Mg isotope measurements can be improved in order to perform more precise quantifications.