Depths of Partial Crystallization of H2O-bearing MORB: Phase Equilibria Simulations of Basalts at the MAR near Ascension Island (7 11{degrees}S)

Phase equilibria simulations were performed on naturally quenchedbasaltic glasses to determine crystallization conditions priorto eruption of magmas at the Mid-Atlantic Ridge (MAR) east ofAscension Island (7–11°S). The results indicate thatmid-ocean ridge basalt (MORB) magmas beneath different segmentsof the MAR have crystallized over a wide range of pressures(100–900 MPa). However, each segment seems to have a specificcrystallization history. Nearly isobaric crystallization conditions(100–300 MPa) were obtained for the geochemically enrichedMORB magmas of the central segments, whereas normal (N)-MORBmagmas of the bounding segments are characterized by polybariccrystallization conditions (200–900 MPa). In addition,our results demonstrate close to anhydrous crystallization conditionsof N-MORBs, whereas geochemically enriched MORBs were successfullymodeled in the presence of 0·4–1 wt% H₂O in theparental melts. These estimates are in agreement with direct(Fourier transform IR) measurements of H₂O abundances in basalticglasses and melt inclusions for selected samples. Water contentsdetermined in the parental melts are in the range 0·04–0·09and 0·30–0·55 wt% H₂O for depleted and enrichedMORBs, respectively. Our results are in general agreement (within±200 MPa) with previous approaches used to evaluate pressureestimates in MORB. However, the determination of pre-eruptiveconditions of MORBs, including temperature and water contentin addition to pressure, requires the improvement of magma crystallizationmodels to simulate liquid lines of descent in the presence ofsmall amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation     

Particulate barium fluxes and export production in the northwestern Mediterranean

Biogenic barium, mostly in the barite (BaSO₄) form, has been proposed as a tracer for export production in the ocean. Here we report on biogenic barium (Ba_xs) and particulate organic carbon (POC) fluxes from sediment traps deployed at the DYFAMED site in the Northwestern Mediterranean Sea. Ba_xs fluxes display average values of 37 ± 45 and 50 ± 58 μg/m²/d at 200 and 1000 m respectively, and are linearly correlated to POC fluxes (mean values of 7.9 ± 9.3 and 6.8 ± 6.8 mg C/m²/d at 200 and 1000 m). Export production estimates, calculated using published Ba_xs- or POC-based algorithms, all fall below or close to the lower limit of potential export values proposed in the literature. This work clearly demonstrates the usefulness of Ba_xs as a tracer of oceanic export production in the Northwestern Mediterranean Sea. However, development of a quantitative export production proxy requires a clear understanding of the underlying cause(s) for the observed spatial variations in the relationship between Ba_xs and POC fluxes. The present study confirms that the processes leading to barite formation differ between margin and open-ocean sites and probably account for much of the regional variability in the POC/Ba_xs ratio.     

The Loyalty—New Hebrides Arc collision: Effects on the Loyalty Ridge and basin system,Southwest Pacific (first results of the ZoNéCo programme)

The ZoNéCo 1 and 2 cruises of Ifremer's Research Vessel L'Atalante, collected new swath bathymetry and geophysical data over the southern and northern segments of the basins and ridges forming the Loyalty system. Between the two surveyed areas, previous studies found evidence for the resistance of the Loyalty Ridge to subduction beneath the New Hebrides trench near 22°S–169°E. On the subducted plate, except for seismicity related to the downbending of the Australian plate, recorded shallow seismicity is sparse within the Loyalty system (Ridge and Basin) where reliable focal mechanism solutions are almost absent.Swath bathymetry, seismic reflection and magnetic data acquired during the ZoNéCo 1 and 2 cruises revealed a transverse asymmetric morphology in the Loyalty system, and an along-strike horst and graben structure on the discontinuous Loyalty Ridge. South of 23°50S and at 20°S, the two WSW-ENE-trending fault systems, respectively, sinistral and dextral, that crosscut the southern and northern segments of the Loyalty system, are interpreted as due to the early effects of collision with the New Hebrides Arc. A NNW-SSE trend, evident along the whole Loyalty system and on the island of New Caledonia, is interpreted as an inherited structural trend that may have been reactivated through flexure of the Australian lithospheric plate at the subduction zone.Overall then, the morphology, structure and evolution of the southern and northern segments of the Loyalty system probably result from the combined effects of the Australian plate lithospheric bulge, the active Loyalty-New Hebrides collision and the overthrust of the New Caledonian ophiolite.     

Comparison of high spatial resolution trace metal distributions with model simulations for surface waters of the Gulf of Cadiz

A multidisciplinary study in the Gulf of Cadiz is revisited, using additional diagnostic modelling tools. The dissolved trace metal (Cu, Ni, Zn, Co) distributions in the Gulf of Cadiz are analysed using modelled tracer evolutions, field observations and the concept of tracer ages. This study shows that a significant part of the observed metal distributions can be explained by the metal inputs of three river systems (Guadiana, Rio Tinto and Odiel, Guadalquivir) discharging into the Gulf of Cadiz, while the remainder of the signal is most likely associated with the benthic metal remobilisation along the shelf of this coastal region.     

Coupling a two-way nested primitive equation model and a statistical SST predictor of the Ligurian Sea via data assimilation

A primitive equation model and a statistical predictor are coupled by data assimilation in order to combine the strength of both approaches. In this work, the system of two-way nested models centred in the Ligurian Sea and the satellite-based ocean forecasting (SOFT) system predicting the sea surface temperature (SST) are used. The data assimilation scheme is a simplified reduced order Kalman filter based on a constant error space. The assimilation of predicted SST improves the forecast of the hydrodynamic model compared to the forecast obtained by assimilating past SST observations used by the statistical predictor. This study shows that the SST of the SOFT predictor can be used to correct atmospheric heat fluxes. Traditionally this is done by relaxing the model SST towards the climatological SST. Therefore, the assimilation of SOFT SST and climatological SST are also compared.     

Use of unit response functions for management of regional multilayered aquifers: application to the North Aquitaine Tertiary system (France)

Although there are many numerical applications used in aquifer management, few methodologies seem to be sufficiently adapted to provide solutions for regional-scale problems. Their applications for multilayered aquifers are also too limited. A new groundwater management tool adapted to such large physical systems has been developed, using the unit response function (URF) approach. This tool is based on the link between a numerical groundwater flow model and a mathematical optimisation tool. Pre- and post processing are based on a geographical information system (GIS). The developed tool has been applied to a real management case—the regional Aquitaine multilayered aquifer (France). A representative hydrodynamic model was built using MODFLOW 2000 code. First tests show that the linearity condition needed for the application of URF methodology is respected despite the high complexity of the system. Two management scenarios were tested. Optimal solutions thus obtained allow for viable exploitation alternatives to be proposed, which integrate complex environmental constraints.

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Résumé Malgré un nombre important d’applications existant dans le domaine de la gestion des aquifères, peu de méthodologies semblent adaptées pour proposer des solutions à des problèmes réels de dimension régionale. Leurs application pour des systèmes multicouches sont souvent trop limitées. Un outil de gestion adapté à ces systèmes de large taille a été développé, basé sur la théorie des Fonctions de Réponses Unitaires (FRU). Cet outil est basé sur le couplage d’un modèle numérique d’écoulement et d’un code d’optimisation mathématique. Les entrées et sorties du modèle sont réalisées à l’aide d’un Système d’Information Géographique (SIG). L’outil développé est appliqué à un cas de gestion réel—le système aquifère régional multicouche aquitain (France). Un modèle hydrodynamique représentatif a été construit, basé sur le code MODFLOW 2000. Les premiers tests montrent que les conditions de linéarité nécessaires à l’application de la méthode FRU sont respectées, malgré l’importante complexité du système. Deux scénarii de gestion sont testés. Les solutions optimales ainsi obtenues permettent de proposer des solutions d’exploitation alternatives viables, intégrant des contraintes environnementales complexes.

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Resumen Aunque hay muchas aplicaciones numéricas usadas en la gestión de acuíferos, unas pocas metodologías parecen estar adaptadas suficientemente para dar las soluciones a los problemas de escala regional. También son limitadas sus aplicaciones para los acuíferos multicapa. Se ha desarrollado una nueva herramienta para el manejo del agua subterránea, adaptada a tales sistemas físicos grandes, usando el avance de la Función de Respuesta Unitaria (URF). Esta herramienta se basa en la combinación entre un modelo numérico de flujo de agua subterránea y una herramienta de optimización matemática. El pre-proceso y el post-proceso se basan en un Sistema de Información Geográfico (GIS). La herramienta desarrollada se ha aplicado a un caso real de manejo en el acuífero regional multicapa de Aquitania (Francia). Se construyó un modelo hidrodinámico representativo, que usó el código MODFLOW 2000. Las primeras pruebas muestran que la condición de linearidad, necesaria para la aplicación de la metodología URF, se respeta a pesar de la complejidad alta del sistema. Se probaron dos escenarios de manejo. Las soluciones óptimas así obtenidas permiten la proyección de alternativas viables de explotación, las cuales integran restricciones medioambientales complejas.

     

Some comments on the limb shift of solar lines

We searched for a variation with heliographic latitude of the solar limb effect by comparing the relative wavelengths of weak and strong Fraunhofer lines. The blue shifts associated with the limb effect appear 9%±5% larger in the polar radius vector than in an equatorial radius vector at cos = 0.5. This should perhaps be interpreted as an increase with latitude of either solar convection or of convective overshoot. Recent observations of poleward meridional flows of 30m s^–1 should be corrected for this limb effect variation. This correction increases this flow velocity to 70 m s^–1. A search for a similar variation in plages and in network boundaries had negative results, the variation being +1%±5% and -1%±6% respectively.Now at the Multiple Mirror Telescope Observatory, University of Arizona, Tucson, Ariz. 85721, U.S.A. The MMTO is jointly operated by the University of Arizona and the Smithsonian Institution.The Sacramento Peak Observatory is operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.     

A time evolution study of limb spicule spectra

Time sequences of simultaneous spectra of limb spicules, obtained using the Sacramento Peak Observatory's tower telescope and echelle spectrograph are analyzed. Intensity determinations of H and K, H, 8498 and 8542 of calcium are tabulated for three observing heights. Electron densities averaged over the entire visible lifetimes of spicules are -6 × 10¹⁰ cm^–3 at observing heights of 6000km, while maximum and minimum values were -1.1 × 10¹¹ cm^–3 at 6000km and - 2 × 10¹⁰ cm^–3 at 10000km. Electron temperatures range between 12 000 K and 16 000 K. Profile halfwidths indicate turbulent velocities of 12 to 22 km s^–1, and spectral tilts are interpreted as caused by differential velocity fields of -3 km s^–1 per 1000 km. No large scale spicule expansions or contractions are observed, although possible expulsions or accretions of material are observed. Spicules may be wider in the calcium K and H lines than in H.Now at School of Science and Engineering, The University of Alabama in Huntsville, Huntsville, Alabama.