Évolution de l'onde semi-diurne M2 de la marée à Brest de 1846 à 2005

The work of searching, recovering and quality control of ancient sea-level measurements at Brest is presented. This work enables us to complete a study carried out by Cartwright in 1972, which showed a decrease in the tidal M2 semi-diurnal amplitude of 1% per century. After including these ancient data, as well as the last four decades of observations in the analysis, our results show an increase of the amplitude of M2 after 1960 and a decrease before 1880, suggesting a long-period oscillation rather than a steady secular trend. To cite this article: N. Pouvreau et al., C. R. Geoscience 338 (2006).     

Estimating uncertainty associated with water stages from a single SAR image

It is the goal of remote sensing to infer information about objects or a natural process from a remote location. This invokes that uncertainty in measurement should be viewed as central to remote sensing. In this study, the uncertainty associated with water stages derived from a single SAR image for the Alzette (G.D. of Luxembourg) 2003 flood is assessed using a stepped GLUE procedure. Main uncertain input factors to the SAR processing chain for estimating water stages include geolocation accuracy, spatial filter window size, image thresholding value, DEM vertical precision and the number of river cross sections at which water stages are estimated. Initial results show that even with plausible parameter values uncertainty in water stages over the entire river reach is 2.8 m on average. Adding spatially distributed field water stages to the GLUE analysis following a one-at-a-time approach helps to considerably reduce SAR water stage uncertainty (0.6 m on average) thereby identifying appropriate value ranges for each uncertain SAR water stage processing factor. For the GLUE analysis a Nash-like efficiency criterion adapted to spatial data is proposed whereby acceptable SAR model simulations are required to outperform a simpler regression model based on the field-surveyed average river bed gradient. Weighted CDFs for all factors based on the proposed efficiency criterion allow the generation of reliable uncertainty quantile ranges and 2D maps that show the uncertainty associated with SAR-derived water stages. The stepped GLUE procedure demonstrated that not all field data collected are necessary to achieve maximum constraining. A possible efficient way to decide on relevant locations at which to sample in the field is proposed. It is also suggested that the resulting uncertainty ranges and flood extent or depth maps may be used to evaluate 1D or 2D flood inundation models in terms of water stages, depths or extents. For this, the extended GLUE approach, which copes with the presence of uncertainty in the observed data, may be adopted.     

Extension at the coast of the Makran subduction zone (Iran)

In the Makran subduction zone, earthquake focal mechanisms and geodetic data indicate that the deforming prism currently experiences N–S compression. However, palaeostress inversions performed on normal faults observed along the coast reveal local stress components consistent with N‐S extension. Previously proposed mechanisms such as gravitational collapse are not favoured by N–S compression and surface uplift. We propose that the observed kinematics result from transient stress reversals following large earthquakes. During the interseismic period (now), the region experiences N–S compression. However, following a large reverse rupture on the subduction interface, stresses in the inner wedge relax, enabling a brief period of extensional faulting before a compressive stress state is re‐established. This mechanism, also observed in other subduction zones, requires low overall stresses in the upper plate and that the margin ruptures in large megathrust earthquakes that result in nearly complete stress drops.     

The lacustrine microbial carbonate factory of the successive Lake Bonneville and Great Salt Lake,Utah, USA

The Bonneville Basin is a continental lacustrine system accommodating extensive microbial carbonate deposits corresponding to two distinct phases: the deep Lake Bonneville (30 000 to 11 500 ¹⁴C bp ) and the shallow Great Salt Lake (since 11 500 ¹⁴C bp ). A characterization of these microbial deposits and their associated sediments provides insights into their spatio‐temporal distribution patterns. The Bonneville phase preferentially displays vertical distribution of the microbial deposits resulting from high‐amplitude lake level variations. Due to the basin physiography, the microbial deposits were restricted to a narrow shoreline belt following Bonneville lake level variations. Carbonate production was more efficient during intervals of relative lake level stability as recorded by the formation of successive terraces. In contrast, the Great Salt Lake microbial deposits showed a great lateral distribution, linked to the modern flat bottom configuration. A low vertical distribution of the microbial deposits was the result of the shallow water depth combined with a low amplitude of lake level fluctuations. These younger microbial deposits display a higher diversity of fabrics and sizes. They are distributed along an extensive ‘shore to lake’ transect on a flat platform in relation to local and progressive accommodation space changes. Microbial deposits are temporally discontinuous throughout the lake history showing longer hiatuses during the Bonneville phase. The main parameters controlling the rate of carbonate production are related to the interaction between physical (kinetics of the mineral precipitation, lake water temperature and runoff), chemical (Ca²⁺, Mg²⁺ and HCO₃^? concentrations, Mg/Ca ratio, dilution and depletion) and/or biological (trophic) factors. The contrast in evolution of Lake Bonneville and Great Salt Lake microbial deposits during their lacustrine history leads to discussions on major chemical and climatic changes during this interval as well as the role of physiography. Furthermore, it provides novel insights into the composition, structure and formation of microbialite‐rich carbonate deposits under freshwater and hypersaline conditions.     

Capture-zone design in an aquifer influenced by cyclic fluctuations in hydraulic gradients

Design of a groundwater pumping and treatment system for a wood-treatment facility adjacent to the tidally influenced Fraser River estuary required the development of methodologies to account for cyclic variations in hydraulic gradients. Design of such systems must consider the effects of these cyclic fluctuations on the capture of dissolved-phase contaminants. When the period of the cyclic fluctuation is much less than the travel time of the dissolved contaminant from the source to the discharge point, the hydraulic-gradient variations resulting from these cycles can be ignored. Capture zones are then designed based on the average hydraulic gradient determined using filter techniques on continuous groundwater-level measurements. When the period of cyclic fluctuation in hydraulic gradient is near to or greater than the contaminant travel time, the resulting hydraulic-gradient variations cannot be ignored. In these instances, procedures are developed to account for these fluctuations in the capture-zone design. These include proper characterization of the groundwater regime, assessment of the average travel time and period of the cyclic fluctuations, and numerical techniques which allow accounting for the cyclic fluctuations in the design of the capture zone. Electronic Publication     

IN-SITU observations of irregular ionospheric structure associated with the plasmapause

Additional studies of the ion composition results obtained from the OGO-6 satellite support earlier observations of irregularities in the distribution of H⁺ and He⁺ within the light ion trough near L = 4, which has been associated with the plasmapause. These irregularities are in the form of sub-troughs superimposed upon the major mid latitude decrease of the light ions. In the sub-troughs, ionization depletions and recoveries of as much as an order of magnitude are observed within a few degrees of latitude, usually exhibited in a pattern which changes significantly with longitude as the Earth rotates beneath the relatively fixed satellite orbit. The location and properties exhibited by these sub-troughs appear to be consistent with the concept of a plasmasphere distortion in the form of “plasmatails” resulting from the combined effects of magnetospheric convection plus corotation. Like the light ion trough, the “plasmatail” irregularity in H⁺ may be obscured on the day side by the dominant topside distribution of O⁺. Consequently, these light ion irregularities are seen as an important factor for studies of plasmapause-trough relationships.     

The retrieval of cloud microphysical properties using satellite measurements and an in situ database

By combining AVHRR data from the NOAA satellites with information from a database of in situ measurements, large-scale maps can be generated of the microphysical parameters most immediately significant for the modelling of global circulation and climate. From the satellite data, the clouds can be classified into cumuliform, stratiform and cirrus classes and then into further sub-classes by cloud top temperature. At the same time a database of in situ measurements made by research aircraft is classified into the same sub-classes and a statistical analysis is used to derive relationships between the sub-classes and the cloud microphysical properties. These two analyses are then linked to give estimates of the microphysical properties of the satellite observed clouds. Examples are given of the application of this technique to derive maps of the probability of occurrence of precipitating clouds and of precipitating water content derived from a case study within the International Cirrus Experiment (ICE) held in 1989 over the North Sea.     

Oscillations in radiation-driven outflows

In this paper we investigate the dynamical behaviour of radiation-driven winds, specifically winds that arise when Compton scattering transfers momentum from the radiation field to the gas flow. Such winds occur during strong X-ray bursts from slowly accreting neutron stars, and also may be driven from the inner regions of a black hole or neutron star accretion disc when the mass transfer rate is very high. By linearizing the radiation hydrodynamic equations around steady spherical outflow, we evaluate the time-dependent response of these winds to perturbations introduced at their inner boundaries. We find that although radiation-driven winds are generally stable, they act as mechanical filters that should produce quasi-periodic oscillations or peaked noise in their radiation output when perturbations force them stochastically. This behaviour may underlie the photospheric oscillations observed in some strong Type I X-ray bursts.