Décomposition chimique et isotopique d'un hydrogramme de crue d'un torrent méditerranéen - Réflexions méthodologiques

La décomposition chimique et isotopique d'un hydrogramme de crue d'un petit torrent méditerranéen (Cannone, Corse, France), a été tentée dans un cas simple, en confrontant les résultats des deux méthodes et en discutant leurs limites. L'outil isotopique permet de bien différencier l'eau préexistante et l'eau nouvelle. L'analyse de la variation des éléments chimiques met en évidence la participation d'eau du soi, sans qu'il soit possible de quantifier sa contribution; elle démontre que la composition de la pluie n'est pas conservative; le rôle du pluviolessivage est mis en évidence, conjointement à celui du transfert dans la zone non-saturée. Ainsi les décompositions chimiques des hydrogrammes (en particulier celles réalisées avec la conductivité) reposent le plus souvent sur des approximations théoriquement contestables et conduisent à des erreurs notables, à l'exception toutefois de celles réalisées avec la silice.Problems of environmental water quality such as transfer of pollutants and ecosystem acidification call for a new insight to the path and the contact time of water in the different subsurface reservoirs. An isotopic and chemical hydrograph separation is carried out for a flood in a small upland watershed in the Mediterranean region (Cannone, Corsica Island, France) with special attention to a comparison between the two methods and discussion of their limits. Here, old and new water are clearly distinguished by isotopic composition. The pattern of the dissolved constituents of stream water shows the contribution of ground water, often assumed to be negligible. The non-conservative behaviour of rain-water chemistry during its path to the stream channel due to enrichment by throughfall and leaching of soils, is the main obstacle to chemical separation. Chemical separation, especially through specific conductance seems generally to be hazardous, except perhaps for dissolved silica. Checking the variation of all dissolved constituents would prevent questionable approximations

Eutrophisation récente d'un lac de montagne sans occupation humaine (lac de Bastani, Corset : Conséquence d'agents atmosphériques?

Le Lac de Bastani (Corse - France), petit lac d'altitude (2089 m) à bassin versant rocheux, est dépourvu de toute implantation ou activité humaine. Ce lac est mésotrophe à eutrophe par certains de ses caractères alors que 4 autres lacs étudiés dans le même contexte sont oligotrophes. De plus, il était décrit au début du siècle comme oligotrophe.Dans ce type de lacs, les apports atmosphériques constituent la majeure partie de la source externe de nutriments. L'apport d'azote inorganique dissous atmosphérique au bassin versant est important, de l'ordre de 0,8 g.N.m-2 par an, soit l'équivalent du rejet d'une trentaine d'habitants permanents. Les trois-quarts sont relargués à la fonte, pendant environ un mois; cela représente l'équivalent des rejets d'environ 300 habitants sur le bassin qui, du fait de la faible activité biologique dans les sols à cette température, arrive pratiquement en entier au lac. Ces apports de nutriments à la fonte sont responsables du démarrage très précoce de la production primaire.En l'absence d'un bilan complet des nutriments la comparaison du lac de Bastani avec les autres lacs oligotrophes et en particulier avec celui de Capitello, aux caractéristiques voisines, permet de cerner les facteurs responsables de la différence de leur niveau de trophie. Bastani et Capitello recevant une charge externe semblable, la source interne est probablement responsable de la situation de Bastani. Les concentrations estivales de N-NO3 notables (0,2-0,3 mg.l-1) des lacs oligotrophes sont un indice de la limitation de leur production primaire par le phosphore. Par contre à Bastani, l'azote étant pratiquement entièrement consommé en été le lac ne semble pas limité par le phosphore. Le vent violent et fréquent qui règne à Bastani semble le facteur déterminant de son niveau trophique supérieur : il assure le brassage de l'eau du lac et la remontée du phosphore (et aussi de l'azote) libéré des sédiments jusque dans la zone photique. Le phytoplancton consomme presque entièrement l'azote, alors que dans les lacs oligotrophes abrités du vent où le phosphore est limitant, les nitrates restent à des teneurs notables.L'eutrophisation récente du lac de Bastani peut s'expliquer par l'accroissement des apports atmosphériques, les seuls qui aient varié. Le phosphore étant disponible la production primaire devait être limitée auparavant par l'azote. Les apports d'azote inorganique dissous ont été dans cette région multipliés par 2 au moins par la pollution à longue distance. Cette dernière pourrait entraîner à elle seule l'évolution trophique d'un lac de haute montagne, dans un site vierge de toute occupation humaine, lorsque le brassage par le vent assure la mobilisation du phosphore des sédiments.Lake Bastani (Corsica, France), a small alpine lake (2089 m), is located in a pristine rocky watershed. It presents some characteristics of eutrophic lakes, whereas four other similar lakes are oligotrophic or ultraoligotrophic. Moreover, at the beginning of the century it was described as being oligotrophic.The lake is ice-covered from November to June. Intense phytoplankton activity begins before the ice melts completely and goes on until fall freezing. Consequently Secchi disk transparency is between 3.2 m in June and 1.8 m in August. The algal biomass, as expressed by chlorophyll-a content, is maximum at the bottom. Chlorophyll-a is about 4-5 mg-m-3 at ice melting and reaches a maximum of 46 mg.m-3 in August. Primary production is highest in the upper levels of water with 41 mg.C.m-3.d-1 at - 5 m. These values are 20 to 80 times that of the other Corsican lakes. Regarding the different trophic classifications this lake presents some features of eutrophic lakes.In 1923 a survey of phytoplankton by Pestalozzi showed that the lake was oligotrophic. This trophic evolution is therefore recent; it is shown by the complete disappearance of Desmidiaceae, the near disappearance of Diatoms, the appearance of green algae with Oedogonium and Spirogyra dominant in summer and autumn, and the development of blue-green algae with Oscillatoria.The seasonal pattern of phytoplankton composition displays diatom development at ice- and snow-melting with the green algae Oedogonium. During the summer warmth, green algae are dominant with the blue-green Oscillatoria. Green algae Oedogonium and Spirogyra increase until the fall turnover.The lake is 2nd-order dimictic with two turnovers in spring and autumn, and two periods of thermal stratification. A strong and frequent wind stirs the lake water inducing homogeneisation of the epilimnion and sometimes the disappearance of the hypolimnion. Dissolved oxygen is always present even at the bottom where O2 saturation is about 50% in winter. Nitrogen is very low - 0.01-0.04 mg.l-1 N-NO3 - and phosphorus (PO4) is below the detection level in the epilimnion during summer, due to intense phytoplankton activity, The hypolimnion is richer in NH4, NO2 and PO4. In spring when the snow begins to melt the surficial waters are enriched with N (NO3 + NH4) and P-PO4, showing the contribution of concentrated snow water.In fact atmospheric input is the dominant external loading in such a watershed. Nitrogen atmospheric input is extrapolated from data obtained at Lake Bavella 30 km southwards for 3 years - 1984 to 1986. The dissolved inorganic nitrogen input is about 0.8 g.m-2 .y-1 , equivalent to wastes from some 30 permanent inhabitants for the whole watershed, About 80 % of annual rainfall is stored in the snowpack, as are ¾ of the dissolved inorganic nitrogen. This nitrogen returns to the lake during a period of about one month which represents 100 kg and a loading of 80 mg.m-2.d-1 for 30 days; it is equivalent to wastes from some three hundred capita during one month. Moreover it is known that the first melwaters are considerably enriched with several ions, especially those of ecological concern, H+, SO4--, NO3¯, NH4+. Using findings from literature we can estimate that in the first two weeks of melting, up to half the annual atmospheric input of dissolved inorganic nitrogen reaches the lake with meltwater concentrations of more than 1 mg.N.l-1. The total dissolved nitrogen input is estimated at 1.2 g.m-2.y-1 and the total nitrogen input at 1.3 g.m-2 .y-1.No data are available for the phosphorus atmospheric input but estimations from literature are about 10 mg.m-2 .y-1 for orthophosphate and 60 mg.m-2 .y-1 for total phosphorus. The release of P-PO4 from the melting snowcover and from ice, added to the tremendous release of inorganic nitrogen, explains the start of vigorous phytoplankton production before the ice melts completely. By this early start primary production may avoid the flush of nutriments resulting from the flow of melting water.Trophic conditions in Lake Bastani are a matter of question as the other four lakes studied with the same edaphic conditions are oligo- or ultraoligotrophic. A comparison between Lake Bastani and the ultraoligotrophic Capitello Lake, which has the same geomorphological and ecological environment, reveals the determining factors of this paradoxical situation, though a nutrient budget is still not available.A low flushing rate (17 months) cannot be the explanation, as that of Lake Capitello is similar (12 months).The external loading (from watershed and atmosphere)being the same, the difference between the two lakes is to be found in the internal loading.The fact that Lake Capitelto and the other oligotrophic lakes present notable N concentrations in summer (0.2-0.3 mg.l-1 ), when Bastani displays N- concentrations 10 times lower, is a sign that they are limited in phosphorus. Conversely Lake Bastani is not phosphorus-limited.The determining factor in this situation seems to be windstirring. Lake Bastani is downwind of a pass in the axial range of Corsica and is submitted to strong frequent winds. Windstirring induces an epilimnion homogeneisation, the downward migration of the thermo- and chemoclines. Thus phosphorus from the sediments can be transferred to the photic zone and is not limiting for phytoplankton. Phytoplankton exhausts nitrogen which is very low at the end of summer. In Lake Capitello as in the other lakes, shielded from the wind by rockwall screens, no mixing occurs, phosphorus cannot be supplied from the sediments and is limiting : nitrogen is not used up and remains at relatively high levels of 0.2-0.3 mg.l -1.Recent eutrophication cannot be explained by an increase of nutrient availability from the sediments due to windstirring, as no climatic change has occurred in the fast century. The probability that internal loading had reached a threshold by progressive accumulation of nutrients in the sediment is very slight if there is no change in the external loading (the unique source of "new" nutrients) and/or the trophic status of the lake. Thus an increase in the external loading seems to be responsible for the eutrophication of Lake Bastani. The atmospheric inupt is the only external reason for a change in the nutrient rate. An increase in the phosphorus atmospheric input by local (forest fires) and regional (long range pollution) human activities is suspected, but does not seem sufficient to change the trophic status of the lake as it has no effect on other lakes, which remain oligotrophic. Moreover, atmospheric P loading is well below the level of phosphorus release from sediments, as estimated from literature. Conversely, an atmospheric inorganic nitrogen input has increased at least twice due to long range pollution (agricultural, industrial and domestic activities). This was estimated from a typologic classification of rain events in Corsica. This increase in atmospheric nitrogen loading might explain the change in the trophic status of the lake, N having been limiting. In the other P-limited lakes the increased atmospheric N input has only increased the N-NO3 concentration in lake water. This explanation being the most realistic, it is of great interest to note that long range atmospheric pollution could induce the eutrophication of a pristine oligotrophic lake

Sr\u3csub\u3e3\u3c/sub\u3eMgPtO\u3csub\u3e6\u3c/sub\u3e

Single crystals of the mixed alkaline earth platinate, tristrontium magnesium platinum hexaoxide, Sr3MgPtO6, were grown from a KOH flux at 1273 K. The compound adopts the rhombohedral K4CdCl6 structure type, featuring chains of face-shared, distorted MgO6 trigonal prisms (Mg site symmetry 32) and PtO6 octahedra (Pt site symmetry ) surrounded by columns of Sr2+ ions (Sr site symmetry 2)

Sr\u3csub\u3e3\u3c/sub\u3eZnPtO\u3csub\u3e6\u3c/sub\u3e and Sr\u3csub\u3e3\u3c/sub\u3eCdPtO\u3csub\u3e6\u3c/sub\u3e

The flux synthesis of single crystals of the isostructural compounds tristrontium zinc platinum hexaoxide, Sr3ZnPtO6, and tristrontium cadmium platinum hexaoxide, Sr3CdPtO6, is reported. The compounds adopt the pseudo-one-dimensional rhombohedral K4CdCl6 structure type, and feature chains of face-shared distorted ZnO6 or CdO6 trigonal prisms and PtO6 octahedra, surrounded by columns of Sr2+ ions. All transition metals are located on the threefold axis of symmetry, while the Sr2+ cations lie on twofold axes. Formula: Sr3ZnPtO6 and Sr3CdPtO

Exotic magnetism on the quasi-FCC lattices of the d3d^3 double perovskites La2_2NaB′'O6_6 (B′' == Ru, Os)

We find evidence for long-range and short-range ($\zeta$ $=$ 70 \AA~at 4 K) incommensurate magnetic order on the quasi-face-centered-cubic (FCC) lattices of the monoclinic double perovskites La$_2$NaRuO$_6$ and La$_2$NaOsO$_6$ respectively. Incommensurate magnetic order on the FCC lattice has not been predicted by mean field theory, but may arise via a delicate balance of inequivalent nearest neighbour and next nearest neighbour exchange interactions. In the Ru system with long-range order, inelastic neutron scattering also reveals a spin gap $\Delta$ $\sim$ 2.75 meV. Magnetic anisotropy is generally minimized in the more familiar octahedrally-coordinated $3d^3$ systems, so the large gap observed for La$_2$NaRuO$_6$ may result from the significantly enhanced value of spin-orbit coupling in this $4d^3$ material.Comment: 5 pages, 4 figure

Two different one-dimensional structural motifs in the same coordination polymer: a novel interpenetration of infinite ladders by bundles of infinite chains

A coordination polymer with a novel structural motifconsisting of stacks of infinite ladders interpenetrated bybundles of infinite chains is described; geometrical argumentsare made for the requirements that can lead to suchinterpenetration as a function of ligand dimensions

“[Cu(pyrazine-2-carboxylate)\u3csub\u3e2\u3c/sub\u3e]\u3csub\u3e2\u3c/sub\u3eCd\u3csub\u3e4\u3c/sub\u3eI\u3csub\u3e8\u3c/sub\u3e: Unprecedented 1-D serpentine chains and regular 2-D regular metal-organic square grids in a 3-D framework.

A novel three-dimensional coordination polymer containingan unprecedented one-dimensional serpentine motif conjoinedwith a two-dimensional square grid motif is presented

Ca\u3csub\u3e4\u3c/sub\u3eIrO\u3csub\u3e6\u3c/sub\u3e, Ca\u3csub\u3e3\u3c/sub\u3eMgIrO\u3csub\u3e6\u3c/sub\u3e and Ca\u3csub\u3e3\u3c/sub\u3eZnIrO\u3csub\u3e6\u3c/sub\u3e

Single crystals of tetracalcium iridium hexaoxide, Ca4IrO6, tricalcium magnesium iridium hexaoxide, Ca3MgIrO6, and tricalcium zinc iridium hexaoxide, Ca3ZnIrO6, were prepared via high-temperature flux growth and structurally characterized by single-crystal X-ray diffraction. The three compounds are isostructural and adopt the K4CdCl6 structure type, comprised of chains of alternating face-shared [CaO6], [MgO6] or [ZnO6] trigonal prisms and [IrO6] octahedra, surrounded by columns of Ca2+ ions. Formula: Ca4IrO6, Ca3.34Mg0.66IrO6 and Ca3.50Zn0.50IrO

Crystal growth, Structure Determination and Magnetism of a New Hexagonal Rhodate: Ba9Rh8O24

Single crystals of Ba9Rh8O24, grown from a molten potassium carbonate flux, crystallize in the spacegroup R¯3c with lattice parameters of a = 10.0899(4) and c = 41.462(2) Å. Magnetic measurements on oriented single crystals reveal the existence of magnetic anisotrop