Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science最新文献

Here are presented the renewable energies composed of all energy sources deriving directly or indirectly from the sun, including solar, wind and hydraulic energies as well as the energies available from bioconversion. These energies are characterised by the fact that they are inexhaustible but available only in limited quantities at a given place and time. They have only a very slight impact on the environment. The development of renewable energy sources in Europe will receive a strong stimulus in the coming decade by the adoption of a European directive on electricity from renewable sources, designed to foster compliance with the European commitments made in Kyoto. In France, the goal is to increase the portion of electrical energy from renewable sources from 15 to 21% by 2010.

The 21st century will have to deal with three big categories of new problems, each of them having its own schedule: (i) the effects of climate change will be felt progressively with more intensity; this will lead to a change in the public opinion, which will accept the necessity to fight against greenhouse warming and which will consequently ask the political world for more adapted and efficient decisions; (ii) various scenarios are being constructed to answer by different means the high energy demand, depending upon the availability of fossil fuels, the more or less rapid coming of new and renewable energies... – all these scenarios are subject to their own schedule and decision calendars; (iii) keeping on with nuclear energy production implies that decisions be taken soon, either for extending the time during which the actual plants can be exploited, or for starting to prototype and build new-technology plants, or finally for deciding how to deal with the fission products. Looking simultaneously at the above calendars shows that the main options will have to be agreed as soon as 2005–2006. It suggests also strongly that the corresponding decisions will depend partly on the availability of new technological solutions, but also, and as importantly, on the society attitude toward the alternative between nuclear energy and climate change.

The world demand for primary energy keeps increasing and has little chance to decrease over the next decades, when considering the desirable and necessary development of the less wealthy countries. The two weightiest questions are to provide electricity to megalopolis, and fuels for transportation. In the present situation, fossil fuels (crude oil, natural gas and coal) provide 90 % of primary energy. Furthermore, hydrocarbons (crude oil and gas) are considered as closing energies to balance the level of demand. However, the link between fossil fuel consumption for one and a half century and climate change is now considered as highly probable. Crude oil and natural gas reserves are substantial, but limited. One half of the proven reserves of crude oil, and one third of gas reserves will be already used in year 2020. Gas reserves will be sufficient to reach 2050, but proven reserves of crude oil will not last until that year. Coal only, which is the largest source of CO₂ and pollutants, provides enough reserves for several centuries. A considerable effort of scientific and technological research is the only way to reach the end of the century using oil and gas, plus a drastic effort for saving energy. This effort in research is discussed in the paper, but it does not provide until now an answer to the climate problem.

By the year 2020 or so, oil and gas, which contribute currently to 62 % of the whole of carbon dioxide anthropogenic emissions, will go on to cover roughly half of the world demand of primary energy. Thence, the fight against the CO₂ emissions requires the implementation of preventive actions as regards processing (refining of crude oil and natural gas) and uses (combustion in boilers for energy production and engines for automotive transportation) of fossil hydrocarbons and curative actions aiming at the capture and sequestration of the given off CO₂. The appropriate technical solutions to do that are reviewed in this paper.

We report the results of a comparative thermomagnetic investigation on natural volcanic samples. Susceptibility vs temperature (k–T curves), induced magnetization vs temperature and remanent (saturated) magnetization vs temperature continuous curves were recorded on the same virgin samples. Continuous susceptibility curves appear more sensitive to the magnetic mineralogy, in agreement with the theoretical suggestions. In addition, some new continuous thermomagnetic curves were obtained using VSTM (vibrating sample thermomagnetometer) apparatus, which allows estimating the domain state of magnetic minerals through the study of partial thermoremanent (pTRM) magnetizations. Alternatively, the domain state estimation on the sister samples was derived using hysteresis parameters at room temperature. The interpretation differs depending on the method used. This disagreement is probably due to the fact that the domain state estimation using room temperature hysteresis parameters [Phys. Earth Planet. In. 13 (1977) 260–267] in terms of the plot of magnetization ratio vs coercivity ratio has no resolution for mixture of grain sizes of a single mineral or an assemblage of different minerals. Complex magnetic mineral assemblage with more than one characteristic domain state likely occurs in most natural rocks.

We have compared the sedimentological record of the Pleistocene coastal deposits in southeastern Tunisia with the climatic data obtained on two deep-sea cores recovered in the Mediterranean Sea. Coastal marine deposits are composed of two distinct lithostratigraphic units separated by an erosion surface. The radiochemical dating shows that these two units were developed during the unique marine isotopic substage 5e (the Last Interglacial). The lower unit mainly consists of quartz-rich sands whereas the upper one consists of carbonate deposits. This petrographic contrast indicates that the palaeoclimatic evolution in the southeastern coast of Tunisia was identical to that observed in the western Mediterranean Sea and the surrounding continents. In particular, the establishment of humid conditions at the beginning of the Last Interglacial was responsible for a supply of terrigenous material and siliciclastic sedimentation along the coastal area of southeastern Tunisia, whereas the regression of these wet conditions during the second half of the Last Interglacial favoured carbonate sedimentation.

Coesite and graphite were discovered as inclusions in zircon separates from pelitic gneiss associated with a large eclogite body in the North Qaidam UHP terrane. This finding suggests UHP metamorphism at pressures below the diamond stability field. This supports previous indirect UHP evidences, such as polycrystalline quartz inclusions in eclogitic garnet, quartz lamellae in omphacite and P–T estimates for both eclogite and garnet peridotite. The U/Pb and Sm/Nd isotopic ages from the North Qaidam eclogite indicated that continental subduction occurred in Early Palaeozoic, most probably in relation with the collision between the Sino-Korean and Yangtze plates.

A survey of CO₂ concentrations in the atmosphere of the Aven d'Orgnac shows that aerodynamic transfer can be a major process in karst system dynamics. The local meteorological conditions and the geometry of the cavity govern exchanges between the cave atmosphere and the exterior. Air enriched with biogenic CO₂ is transferred through the microfissural network by diphasic infiltration from soil to caves where it is continuously produced from rock walls. Analysis of the aerodynamic emptying of confined zones and direct flow measurement give a mean CO₂ production per surface unit of the cave rock wall.

Eclogites have been discovered within units constituting the prolongation of the Hercynian nappes of the Western Sahara (Northern Mauritanides). The eclogitic assemblage comprises Grt + Omp (Jd_30–40) + barroisite + Qtz + Rt ± phengite + Ep. Estimated pressure–temperature conditions are 550–600 °C, 13–15 kbar. Geochronological data obtained by the single evaporation method on zircons characterise a Panafrican event (595 Ma), which could be attributed to the magmatic crystallisation of the protolith. Sm–Nd dating by using garnet and omphacite argue for an Hercynian metamorphic event at 330 Ma.