Biological and granulometric controls on sedimentary organic matter of an intertidal mudflat

A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter.     

The structure of Jebel El Mourra,Tunisia: a diapiric structure causing a positive gravity anomaly

Detailed gravity data integrated with geological data and mining well data are analysed to constrain the shape of the Triassic evaporitic body at Jebel El Mourra (northern Tunisian Atlas) and the mechanism of its emplacement at the surface. The gravity data analysis included the construction of a gravity anomaly maps, and synthetic and forward 2.5D gravity models. The complete Bouguer and residual gravity anomaly maps indicate a positive amplitude gravity anomaly over the Triassic evaporitic outcrops and prominent NE–SW‐trending features associated with the boundary of the Triassic rocks and surrounded layers. A NW–SE‐trending gravity model that crosses the Triassic evaporitic outcrop at Jebel El Mourra shows that the positive gravity anomaly can be explained by a deep‐rooted salt diapir. Conventional models of salt dome formation suggest that they produce negative gravity anomalies; however, this study shows that this model is not universal. The studied area is an example of a diapir expressed by positive gravity anomaly and this result is supported by synthetic gravity models at different stages of salt piercing.     

A multi‐level parallelized substructuring‐frontal solution for coupled thermo/hydro/mechanical problems in unsaturated soil

This paper proposes a multi‐level parallelized substructuring–frontal combined algorithm for the analysis of the problem of thermo/hydraulic/mechanical behaviour of unsaturated soil. Temperature, displacement, pore water pressure and pore air pressure are treated as the primary variables in a non‐linear analysis. Details are given firstly of the substructuring–frontal combined approach. The incorporation of the algorithm in a multi‐level parallel strategy is then discussed. The parallel processing can thus be carried out at different substructural levels. The method thus developed impacts, in a positive way, on both computer storage requirement and execution time. Copyright © 2003 John Wiley & Sons, Ltd.     

Flexure of Europa's lithosphere due to ridge-loading

Using photoclinometry, topographic profiles across europan ridges have been produced. These profiles allow the identification of bulges in the terrain adjacent to the ridges. The bulges are assumed to have been produced by flexure of the elastic lithosphere due to the load of the ridges, which lie along cracks in the crust. The distance from the crack to these “fore-bulges” depends on the thickness of the elastic plate being flexed. Based on a survey of ridges in Galileo images with resolution <300 m/pixel, the thickness of the elastic lithosphere has been determined by this method at a wide variety of sites along the leading and trailing hemispheres of Europa. The average thickness is about 200 m. The elastic lithosphere underneath smooth dilational bands tends to be thicker than plains morphology, an effect that is pronounced at Thynia Linea and Astypalaea Linea. Among the ridges investigated here, more recent loading correlates with a thicker elastic lithosphere, which may either reflect an intrinsically thicker layer, or less viscous relaxation over the shorter time period.     

Flows in clumpy planetary nebulae

The dynamics of clumps observed in planetary nebulae are considered. The possibility that SiO maser spots in evolved stars and the planetary nebula clumps are formed by the Parker instability behind shocks in pulsating stars' atmospheres is raised. Molecular observations of the clumps are suggested. The effects of the ablation of clumps on the global flow structure of a more tenuous plasma in which they are embedded are reviewed.