Deposits related to degradation processes on Piton des Neiges Volcano (Reunion Island): overview and geological hazard

Piton des Neiges (PN) Volcano on Reunion Island offers a rare opportunity to study deposits related to degradation processes in a deeply eroded oceanic shield volcano. Both the inner parts and flanks reveal a large amount of resedimented volcaniclastic material, including extensive debris avalanche deposits. PN litho–structural units, first studied by Upton and Wadsworth [1965, Philos. Trans. R. Soc. Lond., A 271, pp. 105–130], are re-examined. This review highlights the importance of long volcanic repose periods and erosion processes during PN history. volcaniclastic deposits have been studied in the field in order to evaluate the spatial and temporal distribution of the three main types of PN degradation processes. The deposits of these processes have been classified into: (1) talus, (2) mudflow and debris flow, and (3) debris avalanche. Lithology, frequency and estimated volumes of each deposit type imply that the structural evolution of PN can be considered in terms of the competition between the volcanic productivity and the degradation and erosion processes. The occurrence of huge catastrophic avalanches produced by flank failure is convincingly linked to the basaltic activity of PN, which implies a very low risk at present. On the contrary, mudflows and debris flows pose an important risk due to the high population density focussed around the basin outlets. Moreover, if smaller debris avalanches can occur in the cirques of PN, another major risk must be evaluated.     

Longitudinal variability of soils and structural response of sewer networks

A simple model which describes the soil–pipe interaction and accounts for the longitudinal soil variation has been developed. It is used for the analysis of the static response of a section of a buried sewer. A probabilistic analysis (Monte-Carlo method) enabling to quantify the influence of spatial variability of the geomechanical characteristics of the soil makes possible to study the parameters which can influence and drive the longitudinal response of a section of sewer. The system response is complex, soil–structure interaction depending on three different stiffnesses: soil stiffness, pipe components stiffness and joints stiffness. Various analyses have been performed to identify the parameters whose influence is the larger. A specific attention has been devoted to the fluctuation scale of the soil properties and to the stiffness of joints.     

Magnetic properties of sediments in the Bay of Bengal and the Andaman Sea: impact of rapid North Atlantic Ocean climatic events on the strength of the Indian monsoon

The results of a high-resolution mineral magnetic study combined with major element geochemistry analysis, oxygen isotopes and ¹⁴C AMS stratigraphy are reported for deep-sea gravity cores MD77-169 and MD77-180 located in the Andaman Sea and the Bay of Bengal, respectively. Core MD77-169 covers the last 280 kyr and core MD77-180 covers the last 160 kyr. In both cores, rock magnetic parameters indicate that the magnetic assemblage is dominated by pseudo-single domain titanomagnetite grains, with grain-size variations following a strong 23 kyr periodicity. Smaller magnetic grain sizes are observed during periods characterized by a strong summer monsoon. In addition, in core MD77-180, we observe a correlation between magnetic grain size and a chemical index of alteration. This suggests that these magnetic grain-size changes are related to chemical weathering driven by summer monsoon rainfall. A comparison of the GISP2 ice core isotopic record and the magnetic grain-size record of the Bay of Bengal shows that rapid temperature variations documented in the ice core (Dansgaard–Oeschger cycles and Heinrich events), during the last glacial period are also present in the magnetic grain-size record. Heinrich events and cold stadial events are characterized by relatively large magnetic grain sizes. Furthermore, Heinrich events are characterized by lower values of the chemical index of alteration implying a lower degree of chemical weathering related to significantly drier conditions on the continent. We suggest that rapid cold events of the North Atlantic (Heinrich events) during the last glacial stages are characterized by a weaker summer monsoon rainfall over the Himalaya via an atmospheric teleconnection.     

Natural attenuation of TCE, As, Hg linked to the heterogeneous oxidation of Fe(II): an AFM study

Hydrous ferric oxide (HFO) colloids formed, in strictly anoxic conditions upon oxidation of Fe²⁺ ions adsorbed on mineral surface, were investigated under in situ conditions by contact mode atomic force microscopy (AFM). Freshly cleaved and acid-etched large single crystals of near endmember phlogopite were pre-equilibrated with dissolved Fe(II) and then reacted with Hg(II), As(V) and trichlorethene (TCE)-bearing solutions at 25 °C and 1 atm. HFO structures are found to be of nanometer scale. The As(V)–Fe(II) and Hg(II)–Fe(II) reaction products are round (25 nm) microcrystallites located predominantly on the layer edges and are indicative of an accelerated Fe(II) oxidation rate upon formation of Fe(II) inner sphere surface complexes with the phyllosilicate edge surface sites. On the other hand, TCE–Fe(II)–phlogopite reaction products are needle-shaped (45 nm long) particles located on the basal plane along the Periodic Bond Chains (PCBs) directions. Experiments with additions of sodium chloride confirm the importance of the Fe(II) adsorption step in the control of the overall heterogeneous Fe(II)–TCE electron transfer reaction.

Kinetic measurements at the nanomolar level of Hg° formed upon reduction of Hg(II) by Fe(II) in presence of phlogopite particles provide further convincing evidence for reduction of Hg(II)_aq coupled to the oxidation of Fe(II) adsorbed at the phlogopite–fluid interface, and indicate that sorption of Fe(II) to mineral surfaces enhances the reduction rate of Hg(II) species. The Hg(II) reduction reaction follows a first-order kinetic law. Under our experimental conditions, which were representative of many natural systems, 80% of the mercury is transferred to the atmosphere as Hg° in less than 2 h.

The reduction of a heavy metal (Hg), a toxic oxyanion (arsenate ion) and a chlorinated solvent (TCE) thus appear to be driven by the high reactivity of adsorbed Fe(II). This is of environmental relevance since these three priority pollutants are that way reductively transformed to a volatile, an immobilizable and a biodegradable species, respectively. Such kinetic data and reaction pathways are important in the evaluation of natural evaluation scenarios, in the optimization of Fe(II)/mineral mixtures as reductants in technical systems, and in general, in predicting the fate and transport of pollutants in natural systems.     

Integration of local participatory and regional planning: A GIS data aggregation procedure

This paper presents an approach for GIS integrating local participatory land management information used for regional planning, and contributing to a bottom-up approach to land use planning. In participatory planning, the integration between local and regional levels should facilitate the communication and co-operation among the parties at both levels, for an efficient use of available resources. For coherently linking these two levels it is necessary to transform the data produced at one level, in order to be usable by the other. This transformation consists of a spatial procedure, which allows scaling-up the local participatory rural appraisal (PRA) information for regional purposes and scaling down the regional information for local use, using a GIS. Such an integration procedure is presented and discussed using data from a case study in south-western Burkina Faso.     

Strain analysis of a shear zone in a granodiorite

A ductile shear zone in a late Precambrian granodiorite, from the Rouergue (southwest part of the French Massif Central) has been studied.A single episode of deformation is responsible for the formation of a foliation and a well-defined lineation which are localized into an elongated zone, a few decimeters wide.The strain features can be attributed to a simple-shear mechanism (Ramsay and Graham, 1970), so that the main parameters of the deformation are defined.At stages of increasing deformation, the quartz isotropic sub-fabric of the undeformed host rock is progressively transformed into an anisotropic fabric composed of a single oblique girdle while the subgrain size progressively decrease and the dislocation density remains constant. It is suggested that the gliding planes of quartz are the basal plane (0001) and a predominant prismatic plane 101̄0 the slip directions may be a for both glide-planes.The results obtained in this investigation provide a basis for a high voltage electron microscope (H.V.E.M.) study which shows that the fabrics development may be related to dislocation processes. The difference of strain rates in the host rock and in the shear zone is calculated from the dislocation microstructures.     

Japan Sea: a pull-apart basin?

Recent field work in the Hokkaido Central Belt and marine geology studies along the eastern margin of Japan Sea in addition to previously published data lead us to propose a new model of opening of the Japan Sea. The synthesis of both on-land and offshore structural data gives new constraints about the structural evolution of the system. The rhombohedral shape of the Japan Basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early Eo-Oligocene rifting of a pull-apart basin accommodated along two large right-lateral shear zones, east of Korea and west of northeast Japan and Sakhalin. It is followed, during Upper Oligocene/Lower Miocene, by the main opening of the Japan Basin as a mega pull-apart. Then a back-arc spreading probably related to the subduction process, induced the creation of the Yamato and Tsushima Basins at the end of Lower Miocene and in Middle Miocene. Clockwise rotation of southwest Japan larger than 20° or major bending of Honshu mainland deduced from paleomagnetic studies is unlikely at this time. Since 1 or 2 My B.P. to Present, compression prevails along the eastern margin of the Japan Sea. The generation of marginal basins as pull-apart basins along intracontinental strike-slip faults is a mechanism which has been proposed by other authors concerning the South China Sea, the question then is whether the fragmentation of the Asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one.     

Evolution qualitative à long terme des rotifères du Lac Léman

Seventy-five species and lower taxonomic units of rotifers have been identified in Lake Geneva (= lac Léman); seven species and one form are new for the lake. When using indicators of trophic conditions, the qualitative composition of the biocenosis is still dominated by oligo-mesotrophic lake indicators, mixed with eutrophic and oligotrophic ones. However the rotifer biocenosis has not yet reached a steady state.