Enregistrements sismologiques de l'explosion sur le site de l'usine AZF (Toulouse,France)Seismological records of an explosion at the AZF chemical complex in Toulouse,France

This study presents a detailed analysis of the seismic records of a strong explosion that occurred on 21 September 2001 at a chemical complex located south of Toulouse, France, and provoked important damages. The explosion, which is equivalent to a 3.4 magnitude earthquake, has been recorded at most of the stations of the National Seismological Network, as well as at a station under test at the ‘Observatoire Midi-Pyrénées’, 4.2 km away from the epicentre. The main seismic phases are interpreted using the known crustal structures, and a modelling with synthetic seismograms is performed. To cite this article: A. Souriau et al., C. R. Geoscience 334 (2002) 155–161.     

Modeling the ionospheric electron content for the correction of altimetric measurements

The TOPEX-POSEIDON oceanographic satellite (due to be launched in 1992) will proceed to high accuracy altimetric measurements of the sea surface. Since the altimeter signals will propagate through the ionosphere, they will be retarded with respect to their free-space propagation delay. As a result, the measured altitude will exhibit an apparent lengthening which must be considered. In order to correct this effect, the ionosphere total electron content (TEC) beneath the satellite has to be known. This paper addresses the problem of determining the TEC form Doppler measurements performed on telemetric signals propagating between the satellite and the ground stations of the DORIS positioning system. This is an inverse problem which, in general, does not admit a single-valued solution. Physical observations of the ionophere lead us to assume that the TEC along each half-revolution is regular such that we can select an appropriate solution. This solution is approximated by cubic splines. The computed results are compared to simulation results, based on the Bent ionospheric model and seem to be particularly promising.     

A numerical simulation of the surface water budget during HAPEX-MOBILHY

A simple land surface parameterization scheme previously validated at the daily time scale is studied for monthly periods using the HAPEX-MOBILHY data set. For a set of six locations, the scheme reproduces with good accuracy two components of the surface water budget (evaporation and soil moisture depletion) when the observed rainfall rate at the ground surface is specified. For two sites where the superficial soil moisture content has been measured, the model provides a reasonable evolution for this quantity. The validation has been completed by examining the screen-level temperature and relative humidity. The agreement with observations is fair, except when the local properties of the site strongly differ from those of the surroundings.     

First direct geodetic link between Europe,Africa and South America with a multi station VLBI array

The first geodetic experiment tying Europe, Africa and South-America was realized in July 1985 by Very Long Baseline Interferometry with a network of 5 radiotelescopes. TheVLBI technique and data analysis are presented, with special emphasis on the ionosphere modeling because of its importance in this particular experiment. Comparisons of the results with other geodetic information confirm the achievement of decimetric accuracy.     

87Rb87Sr chronology of H chondrites: Constraint and speculations on the early evolution of their parent body

A precise⁸⁷Rb-⁸⁷Sr whole-rock isochron for H chondrites and an internal isochron for Tieschitz (H3) have been determined. The age and⁸⁷Sr/⁸⁶Sr initial ratio of the whole rocks are4.52 ± 0.05 b.y. and0.69876 ± 0.00040(λ(⁸⁷Rb) = 1.42 × 10^?11yr^?1). For Tieschitz, whereas handpicked separates plot on a well-defined line, heavy liquid separates scatter in the⁸⁷Rb/⁸⁶Sr vs.⁸⁷Sr/⁸⁶Sr diagram. Leaching experiments by heavy liquids indicate that they might have a sizeable effect on Tieschitz minerals. The age and⁸⁷Sr/⁸⁶Sr initial ratio as determined by handpicked separates are4.53 ± 0.06 b.y. and0.69880 ± 0.00020, indistinguishable from the whole-rock isochron.These results are interpreted as “primitive isochrons” dating the condensation of chondrites from the solar nebula. The best value of this event is given by joining both isochrons together at4.518 ± 0.026 b.y. and⁸⁷Sr/⁸⁶Sr= 0.69881 ± 0.00016. The near identity of this initial ratio with the one of Allende white inclusions argues in favor of a sharp isochronism of condensation from a⁸⁷Sr/⁸⁶Sr homogeneous nebula. Data from Guaren?a [11] and Richardton [48] are interpreted as secondary internal isochrons, 100 m.y. after the condensation of the whole rocks.The data are then used to constrain a thermal evolution model of the H chondrite parent body. This body might have a 150–175 km radius, and might have been heated by²⁶Al. An²⁶Al/²⁷Al ratio of 4–6 × 10^?6 is enough for heating such a body. Further tests for this model are proposed.     

Future productivity and phenology changes in European grasslands for different warming levels: implications for grassland management and carbon balance

Background

Europe has warmed more than the global average (land and ocean) since pre-industrial times, and is also projected to continue to warm faster than the global average in the twenty-first century. According to the climate models ensemble projections for various climate scenarios, annual mean temperature of Europe for 2071–2100 is predicted to be 1–5.5 °C higher than that for 1971–2000. Climate change and elevated CO₂ concentration are anticipated to affect grassland management and livestock production in Europe. However, there has been little work done to quantify the European-wide response of grassland to future climate change. Here we applied ORCHIDEE-GM v2.2, a grid-based model for managed grassland, over European grassland to estimate the impacts of future global change.

Results

Increases in grassland productivity are simulated in response to future global change, which are mainly attributed to the simulated fertilization effect of rising CO₂. The results show significant phenology shifts, in particular an earlier winter-spring onset of grass growth over Europe. A longer growing season is projected over southern and southeastern Europe. In other regions, summer drought causes an earlier end to the growing season, overall reducing growing season length. Future global change allows an increase of management intensity with higher than current potential annual grass forage yield, grazing capacity and livestock density, and a shift in seasonal grazing capacity. We found a continual grassland soil carbon sink in Mediterranean, Alpine, North eastern, South eastern and Eastern regions under specific warming level (SWL) of 1.5 and 2 °C relative to pre-industrial climate. However, this carbon sink is found to saturate, and gradually turn to a carbon source at warming level reaching 3.5 °C.

Conclusions

This study provides a European-wide assessment of the future changes in productivity and phenology of grassland, and their consequences for the management intensity and the carbon balance. The simulated productivity increase in response to future global change enables an intensification of grassland management over Europe. However, the simulated increase in the interannual variability of grassland productivity over some regions may reduce the farmers’ ability to take advantage of the increased long-term mean productivity in the face of more frequent, and more severe drops of productivity in the future.

     

The impact of the last European deglaciation on the deep-sea turbidite systems of the Celtic-Armorican margin (Bay of Biscay)

The compilation of results obtained on three giant piston cores from the Whittard, Shamrock and Guilcher turbidite levees reveals a high-resolution stratigraphic record for the Bay of Biscay. Due to the abundance of reworked sediments in these sedimentary environments, a specific methodological approach, based on an X-ray-assisted subsampling phase associated with sedimentological, geochemical and micropalaeontological analyses, was implemented. With an accurate chronological framework, this multi-proxy investigation provides observations on the ‘Fleuve Manche’ palaeoriver and the British-Irish Ice Sheet (BIS) histories over the last 20,000 years. The results obtained highlight the direct influence of the decay of the BIS on the Bay of Biscay deep-sea clastic sedimentation during the last European deglacial phase. During this period, the annual BIS cycle of meltwater seems enough to generate seasonal turbidity currents associated with exceptional sedimentation rates in all the Celtic and Armorican turbidite systems. With very high sedimentation rates, the turbidite levees represent the main deep-sea clastic depositional area. Long coring combined with a very careful subsampling method can provide continuous high-resolution palaeoenvironmental signals.     

The contribution of marine magnetics in the Gulf of Saint-Malo (Brittany,France) to the understanding of the geology of the North Armorican Cadomian belt

Marine magnetic data extracted from the geophysical database of the SHOM (the French Hydrographic and Oceanographic Service) offer a first overview of the magnetic offshore environment in the Gulf of Saint-Malo (Brittany, France). Their cross-interpretation with available geological and geophysical knowledge provides a new land and sea model of the western part of the Late Proterozoic North Armorican Cadomian belt. In particular, marine data exhibit relatively intense and heterogeneous magnetic signatures, mainly interpreted as the offshore prolongation of plutonic, volcanic, and metamorphic geologic formations recognized onshore. Imprints of major faults well known on land can be inferred at sea from discontinuities and shifting of magnetic anomalies or followed by high-resolution bathymetry. An impressive and dense dolerite dyke swarm propagated on more than 50 km from the coast seaward. Dykes are characterized by both 1-km-deep and less than 50-m-deep superficial magnetic responses, suggesting the existence of deeply rooted super dykes to which individual small-scale structures are connected.