Response of littoral chironomid communities and organic matter to late glacial lake—level,vegetation and climate changes at Lago dell’Accesa (Tuscany,Italy)

This study focuses on the response of lacustrine littoral chironomid communities to late glacial changes in limnological, environmental and climate conditions in the Mediterranean context. Late glacial chironomid (Diptera: Chironomidae) assemblages, organic petrography and geochemistry were analysed in a sediment core from the littoral zone of Lago dell’Accesa (Tuscany, Italy), where the lake-level fluctuations and the vegetation history have been previously reconstructed. Comparison of the chironomid stratigraphy to other proxies (pollen assemblages, organic petrography and geochemistry, lake-level) and regional climate reconstruction suggested the predominant influence of lake-level changes on the littoral chironomid fauna. The main lowering events that occurred during the Oldest and the Younger Dryas were followed by higher proportions of taxa typical of littoral habitats. A complementary study of organic matter suggested the indirect impact of lake-level on the chironomids through changes in humic status and habitat characteristics, such as the type of substrate and aquatic macrophyte development. Several chironomid taxa, such as Glyptotendipes, Microtendipes and Cricotopus type patens, were identified as possible indicators of low lake-level in the late glacial records. Nevertheless, this study suggested that parallel analyses of organic matter and chironomid assemblages may be needed to circumvent misinterpretation of littoral chironomid assemblage stratigraphy. There was a weak response of the chironomid assemblages to small lake-level lowerings that corresponded to the Older Dryas and Preboreal oscillations. A higher level of determination, e.g. to the species group level, may be necessary to increase the sensibility of the indicators to lake-level changes.     

Stochastic delineation of wellhead protection area in fractured aquifers and parametric sensitivity study

 In many countries, the setup of protection areas around every drinking water well was instituted at a national level in order to preserve the quality of water as well as the perennially of the resource. Wellhead protection surfaces have been defined using capture zones showing the area influenced by a well within a certain time. A stochastic method is developed for delineating time-related capture zones in fractured aquifers characterised by a low porosity and a high degree of fracturing. The flow velocity within the fractures is determined statistically depending on the distribution of the fracture features and the mass transfer solution is obtained through a particle tracking algorithm. Probabilistic capture zone curves are obtained as a function of the travel time of particles to the well and the percentage of particles apt to be extracted up to this time. A sensitivity study of fracture network parameters leads to the conclusion that orientations and aperture distribution of the fracture sets are of primary importance to the wellhead protection delineation.     

Petrogenesis of the amphibole-rich veins from the Lherz orogenic lherzolite massif (Eastern Pyrenees, France): a case study for the origin of orthopyroxene-bearing amphibole pyroxenites in the lithospheric mantle

The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic K₂O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase). The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few centimetres by vein–wallrock exchange reactions which continued down to subsolidus temperatures for Mg–Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P ≥ 13 kbar and recrystallized at P < 12 kbar. The high Al^VI/Al^IV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with about 1–3 wt% H₂O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4–6 wt% H₂O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process. Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P ≤ 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical boundary layer/thermal boundary layer transition (about 45–50 km deep). Continuous fluid ingress allowed remelting of the deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths. Received: 15 September 1999 / Accepted: 31 January 2000     

Study of the air-sea interactions at the mesoscale: the SEMAPHORE experiment

The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m⁻². This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.     

Nature et origine des „andésites“ et „trachyandésites“ cénozoiques du Massif Central Français

The intermediate lavas (andesites, trachyandesites, doreites, sancyites) of the Cenozoïc volcanic districts of the Massif Central (France) are compared with lavas of the alkaline and calc-alkaline suites. On ground of mineralogical and chemical data, two groups may be recognized. The rocks of the first group bear a close resemblance with the intermediate rocks that result of the differentiation of an alkaline basaltic magma. The rocks of the second group, less numerous, possess one or several characters identical to those of calc-alkaline andesites.The contamination suffered by some lavas of both groups has not been sufficient to blot out their comagmatic characteristics.     

Le Bassin Cambrien de Tisrouima (Haut-Atlas Occidental,Maroc)

Résumé Dans le Haut-Atlas marocain, entre l'oued Sous au Sud, Asif AÏt Moussa à l'Ouest, le bled Seksaoua au Nord et Tizi n'Test à l'Est, plus exactement dans la partie sud-ouest du Massif du Tichka et au Nord-Ouest de ce massif affleurent en de nombreux endroits des roches charbonneuses: grès, arkoses, pélites, schistes, cipolins et coméennes dont la principale est la tisrouimite (7). Cette dernière s'est déposée un peu avant les calcaires àArchaeocyatha du Cambrien inférieur, puis en mÊme temps que leurs couches de base. Les autres roches charbonneuses sont sensiblement contemporaines de la tisrouimite. On peut en conclure qu'il a existé un bassin sédimentaire riche en matière organique à une époque correspondant, en première approximation, a l'Assadasien.

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The area of study in the moroccan High-Atlas area is included between wady Sous to the South, Asif AÏt Moussa to the West, bled Seksaoua to the North and Tizi n'Test to the East; more exactly it is located in the south-western and nordwestern Tichka. In this area we have noticed numerous outcrops of carbonaceous rocks, such as sandstones, arkoses, mudstones, shales, cipolins and hornfels, the principal variety of which is tisrouimite (7). Tisroumite first was deposited shortly before the Lower Cambrian Archaeocyathid limestones, and again later contemporaneously with the basal layers of these limestones. The other carbonaceous rocks are more or less contemporaneous with the tisrouimite. Therefore we conclude approximately at the time of the Assedasian (early Cambrian II) a sedimentary basin existed the sediments of which were rich in organic matter

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Zusammenfassung Im marokkanischen Hoch-Atlas, zwischen Wadi Sous im Süden, Asif AÏt Moussa im Westen, der Gegend um Seksoua im Norden und Tizi n'Test im Osten, sind im SW-Teil und NE-Teil des Tichka-Gebirgsstockes dunkle, kohlenstoffreiche Gesteine aufgeschlossen: Sandstein, Arkose, Pelit, Schiefer, grüner Marmor und Homfels, dessen Hauptvariante tisrouimite genannt wird. Die Hornfelse sind etwas älter als die Archaeocyathiden-Kalke des Unterkambriums und bilden ihre Basis Die übrigen Gesteine sind etwa gleich alt wie die Hornfelse. Wir schließen daraus, daß hier zur Zeit des Assadassien (tieferes Kambrium II) ein Sedimentationsbecken lag, dessen Sedimente reich an organischen Substanzen waren.

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Comportement géochimique des éléments métalliques en milieu réducteur et diagrammes (log S,pH)

Résumé L'existence de relations entre les teneurs en carbonates et en sulfures dans les eaux des bassins réducteurs, ainsi que l'irreversibilité de la réaction sulfate-sulfure, conduisent à proposer le remplacement des diagrammes Eh. pH, par les diagrammes log S, pH (S désignant la concentration totale de S (-II)). Nous donnons ici le principe de construction, des exemples de diagrammes et quelques applications.

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The existence of relations between the amounts of carbonates and the sulfides in the water of anoxic basins and the irreversibility of the reaction sulphate-sulfide lead us to propose the substitution of diagrams Eh, pH by diagrams log S, pH (S being the total concentration of S (-II)). The principle of how diagrams should be drawn is outlined and some examples and applications are given.

     

Les caractères magmatiques du volcanisme plio-quaternaire des Andes dans le Sud du Pérou

Chemical data on late cenozoic lavas (83 new analyses) from southern of Peru indicate a zonal arrangement of lavas types according to their distance from the trench. The nearest belong to a calc-alkaline suite and the farthest are shoshonites rocks. The calc-alkaline rocks show progressive increase in K₂O content and in K₂O/Na₂O ratio in northeastern direction. The shoshonitic rocks are more potassic and titaniferous, and the K₂O/Na₂O ratio is approximatively constant. The potash content of the two series correlate with the depth of the inclined seismic zone beneath the volcanoes, but this correlation is less well established than in island arcs.     

生物质燃烧颗粒物有机示踪化合物的测定和应用

使用气相色谱-质谱法对2002年7月-2003年7月的北京市大气粗、细颗粒物样品中的左旋葡聚糖等糖类化合物进行了测定。结果表明，左旋葡聚糖主要存在于细颗粒中，可以作为示踪化合物来研究生物质燃烧现象。生物质燃烧对北京市大气颗粒物有较重要的贡献。对应于12％-40％的PM2.5有机碳和10％-33％的PM10有机碳。北京市在2002年10月和11月受到明显的生物质燃烧的影响，可能由于农田秸杆焚烧和秋季落叶焚烧。2003年5月7日颗粒物样品受到直线距离约为1000km以外的内蒙古自治区呼伦贝尔市大兴安岭林区森林大火烟雾的影响。生物质燃烧事件具有突发性，可以长距离传输；生物质作为农村生物燃料的使用其燃烧排放具有经常性和持久性的特征。     

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.