Apport de la cathodoluminescence à haute résolution à l'étude de la diagenèse météorique dans les formations sédimentaires carbonatées

Different diagenetic transformations and their relative chronological sequence are studied in the meteoric diagenetic zone from the Upper Oligocene limestone at the North of the Aquitaine Basin (France), by combining high-resolution cathodoluminescence spectroscopy and electron-microprobe analyses. More than 128 spot analyses by electron microprobe and 60 analyses by cathodoluminescence spectroscopy are done on different meteoric cements firstly identified by classic optical microscopy. Three cement types are identified according to the relative intensity of the bands of their respective cathodoluminescence spectra (350, 380, 430, 500, and 620 nm). From these investigations, we could identify for each meteoric cement different phases of crystalline growth and crystalline dissolution. As a result, a better and more realistic meteoric diagenetic model is proposed. It illustrates the cyclic transformations from vadose zone (unsaturated) to meteoric zone (saturated). To cite this article: R. Chapoulie et al., C. R. Geoscience 337 (2005).     

A simple model accounting for the uptake,transport, and deposition of wind‐eroded mineral particles in the hyperarid coastal Atacama Desert of northern Chile

As previously observed in marine sediments collected downwind of African or South American continental sources, recent studies of sediment cores collected at the bottom of Mejillones Bay in north Chile (23°S) show a laminated structure in which the amount of particles of aeolian origin and their size create significant differences between the layers. This suggests inter‐annual to inter‐decadal variations in the strength of the local southerly winds responsible for (1) the erosion of the adjacent hyperarid surface of the Mejillones Pampa, and (2) the subsequent transport of the eroded particles towards the bay. A simple model accounting for the vertical uptake, transport, and deposition of the particles initially set into motion by wind at the surface of the pampa is proposed. This model, which could be adapted to other locations, assumes that the initial rate of (vertical) uptake is proportional to the (horizontal) saltation flux quantified by means of White's equation, that particles are lifted to a height (H), increasing with the magnitude of turbulence, and that sedimentation progressively removes the coarsest particles from the air column as it moves towards the bay. In this model, the proportionality constant (A) linking the vertical flux of particles with the horizontal flux, and the injection height (H) control the magnitude and size distribution of the deposition flux in the bay. Their values are determined using the wind speed measured over the pampa and the size distribution of particles collected in sediment traps deployed in the bay as constraints. After calibration, the model is used to assess the sensitivity of the deposition flux to the wind intensity variations. The possibility of performing such quantitative studies is necessary for interpreting precisely the variability of the aeolian material in the sediment cores collected at the bottom of Mejillones Bay. Copyright © 2010 John Wiley & Sons, Ltd.     

The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection

The LMDZ4 general circulation model is the atmospheric component of the IPSL–CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley–Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtke’s convection scheme, used in previous versions, the Emanuel’s scheme improves the representation of the Hadley–Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtke’s parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.     

Observed dependence of the water vapor and clear-sky greenhouse effect on sea surface temperature: comparison with climate warming experiments

This study presents a comparison of the water vapor and clear-sky greenhouse effect dependence on sea surface temperature for climate variations of different types. Firstly, coincident satellite observations and meteorological analyses are used to examine seasonal and interannual variations and to evaluate the performance of a general circulation model. Then, this model is used to compare the results inferred from the analysis of observed climate variability with those derived from global climate warming experiments. One part of the coupling between the surface temperature, the water vapor and the clear-sky greenhouse effect is explained by the dependence of the saturation water vapor pressure on the atmospheric temperature. However, the analysis of observed and simulated fields shows that the coupling is very different according to the type of region under consideration and the type of climate forcing that is applied to the Earth-atmosphere system. This difference, due to the variability of the vertical structure of the atmosphere, is analyzed in detail by considering the temperature lapse rate and the vertical profile of relative humidity. Our results suggest that extrapolating the feedbacks inferred from seasonal and short-term interannual climate variability to longer-term climate changes requires great caution. It is argued that our confidence in climate models' predictions would be increased significantly if the basic physical processes that govern the variability of the vertical structure of the atmosphere, and its relation to the large-scale circulation, were better understood and simulated. For this purpose, combined observational and numerical studies focusing on physical processes are needed.     

The Use of Invertebrates in Ground Water Monitoring: A Rising Research Field

The use of invertebrates as biomonitors of ground water quality is a relatively new approach that has come of age with the development of ground water ecology. The benefits of such an approach are illustrated by four examples of field biomonitoring from several sites in various hydrogeological settings. Contamination of the interstitial zone by heavy metals in some sectors of the Rhóne River (France) was shown by the scarcity of insect species; sewage pollution in the saturated zone of a karstic aquifer was indicated by the low relative abundances of stygobites as compared with those of stygophiles and stygoxenes; and enrichment with organic matter of an underflow was clearly demonstrated by the extremely high density of ground water invertebrates such as oligochaetes, ostracods, and isopods. Examination of the spatial changes in the composition and abundance of invertebrate assemblages was also useful in determining the direction and intensity of water fluxes between a river and its underflow, as well as in delineating the reduced or oxidized zones in a manganese-polluted aquifer. Finally, the selected case studies emphasized the variety of methodological approaches that could be developed in ground water contamination biomonitoring, as well as the complementary and sometimes new information provided by this innovative method in comparison with that obtained by conventional pollution monitoring techniques.     

Does the Port 2000 harbour construction have an effect on the Seine estuary suprabenthic community?

Using a Before/During/After protocol, the effects of Port 2000 (the Le Havre harbour extension) on the suprabenthic communities in the Northern Channel of the Seine estuary (English Channel) were examined from March 2001 to March 2009. Since the beginning of Port 2000 construction in 2002, there have been several changes in the hydrological and sedimentary characteristics in the North Channel, including changes in current velocity, bottoms salinity and sand accumulation, as well as in the benthic communities themselves, moving from mud to medium and coarse sand communities. For the suprabenthos, changes were moderate, corresponding to an increase in species richness in the upper-part of the North Channel and a decrease in species density in the dominant mysids species (i.e., mainly Mesopodospsis slabberi) over time. Due to hydrological changes since the beginning of the Port 2000 project (several years of low freshwater input) it has been difficult to attribute the biological changes to Port 2000 construction only. In the future, the results obtained for the suprabenthos will have to be compared to those obtained for the macrobenthos. Similarly, since the suprabenthos is known to play an important role in the estuarine trophic chain, it will also be essential to estimate the fish distribution in the North Channel and to examine their stomach contents to evaluate the probable changes since the Port 2000 project began.     

Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission II: Aerosol extinction profiles in the 600–1420 cm spectral range

We have analyzed the continuum emission of limb spectra acquired by the Cassini/CIRS infrared spectrometer in order to derive information on haze extinction in the 3–0.02 mbar range (∼150–350 km). We focused on the 600–1420 cm⁻¹ spectral range and studied nine different limb observations acquired during the Cassini nominal mission at 55°S, 20°S, 5°N, 30°N, 40°N, 45°N, 55°N, 70°N and 80°N. By means of an inversion algorithm solving the radiative transfer equation, we derived the vertical profiles of haze extinction coefficients from 17 spectral ranges of 20-cm⁻¹ wide at each of the nine latitudes. At a given latitude, all extinction vertical profiles retrieved from various spectral intervals between 600 and 1120 cm⁻¹ display similar vertical slopes implying similar spectral characteristics of the material at all altitudes. We calculated a mean vertical extinction profile for each latitude and derived the ratio of the haze scale height (H_haze) to the pressure scale height (H_gas) as a function of altitude. We inferred H_haze/H_gas values varying from 0.8 to 2.4. The aerosol scale height varies with altitude and also with latitude. Overall, the haze extinction does not show strong latitudinal variations but, at 1 mbar, an increase by a factor of 1.5 is observed at the north pole compared to high southern latitudes. The vertical optical depths at 0.5 and 1.7 mbar increase from 55°S to 5°N, remain constant between 5°N and 30°N and display little variation at higher latitudes, except the presence of a slight local maximum at 45°N. The spectral dependence of the haze vertical optical depth is uniform with latitude and displays three main spectral features centered at 630 cm⁻¹, 745 cm⁻¹ and 1390 cm⁻¹, the latter showing a wide tail extending down to ∼1000 cm⁻¹. From 600 to 750 cm⁻¹, the optical depth increases by a factor of 3 in contrast with the absorbance of laboratory tholins, which is generally constant. We derived the mass mixing ratio profiles of haze at the nine latitudes. Below the 0.4-mbar level all mass mixing ratio profiles increase with height. Above this pressure level, the profiles at 40°N, 45°N, 55°N, at the edge of the polar vortex, display a decrease-with-height whereas the other profiles increase. The global increase with height of the haze mass mixing ratio suggest a source at high altitudes and a sink at low altitudes. An enrichment of haze is observed at 0.1 mbar around the equator, which could be due to a more efficient photochemistry because of the strongest insolation there or an accumulation of haze due to a balance between sedimentation and upward vertical drag.