A non-mass-dependent isotopic fractionation effect

Isotopic fractionation is discussed in terms of collision cross sections. Under thermal equilibrium conditions, isotopically non-reactive collisions and collisions involving indistinguishable isotopes have no effect on the concentration of isotopic species: isotopic exchange reactions give rise to the well known mass-dependent isotopic fractionation (MDF). Under non-thermal equilibrium conditions, the non-reactive and indistinguishable cross sections must be taken into account since they participate in the loss of energy (thermalisation) of the hot species. If an isotope exchange between hot and cold species occurs during the thermalisation, the relative isotopic abundances contribute to the reaction rates and the MDF rule is not valid anymore. A general formulation of the isotopic fractionation, accounting for thermal and non-thermal situations, is proposed. The experimental results obtained during the synthesis of ozone [1] are well explained by the present treatment, which could be extended to other isotopic anomalies observed in meteorites.     

Le principe d'extremum et la stabilité de certains états de mouvement de l'air atmosphérique

Résumé Une particule qui ne subit que des transformations adiabatiques, occupe sa position d'équilibre au sein d'une masse d'air en équilibre hydrostatique, lorsque la sommeS de son enthalpie spécifique et de son énergie potentielle par unité de masse est extremum. Suivant que cet extremum est un minimum ou un maximum, l'état d'équilibre hydrostatique est stable ou instable. On peut généraliser cette proposition au cas de l'état de mouvement géostrophique, à condition d'ajouter, à la sommeS, l'énergie cinétique longitudinale, par unité de masse, de la particule. On peut étendre immédiatement ce dernier résultat au cas du tourbillon circulaire permanent.

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Summary A particle of air which undergoes but adiabatic changes of state occupies its position of equilibrium inside a mass of air in hydrostatic equilibrium if the sumS of its specific enthalpy and of its potential energy per unit of mass assumes an extreme value. According as this extreme is a minimum or a maximum the state of hydrostatic equilibrium is stable or unstable. This postulate can be generalized in case of geostrophic state of motion on condition that the kinetic longitudinal energy of the particle per unit of mass is added to the sumS. This last result can be immediately extended on the case of a permanent circular vortex.

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Zusammenfassung Ein Luftpartikelchen, das ausschließlich adiabatischen Zustandsänderungen unterliegt, nimmt seine Gleichgewichtslage im Innern einer im hydrostatischen Gleichgewicht befindlichen Luftmasse ein, wenn die SummeS seiner spezifischen Enthalpie und seiner potentiellen Energie in der Masseneinheit ein Extrem wird; je nachdem dieses Extrem ein Minimum oder ein Maximum ist, ist das hydrostatische Gleichgewicht stabil oder instabil. Diese Forderung läßt sich für den Fall der geostrophischen Bewegung verallgemeinern, indem man zur SummeS die kinetische Longitudinalenergie der Partikel (je Maßeinheit) addiert. Dieses letztere Resultat läßt sich unmittelbar auf den Fall des permanenten Kreiswirbels ausdehnen.

     

Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank

The Algiers region, northern Algeria, is known to be seismically active, with recurrent large (M>6) earthquakes. Because of the lack of high-resolution bathymetry, the offshore structures remained for a long time poorly known. Thanks to a new marine data base (MARADJA 2003 cruise), the offshore part of the margin is accurately mapped, and new active and recent structures are described. West of the bay of Algiers, the margin enlarges, forming the Khayr al Din bank, interpreted as a tilted block of the passive margin born during the opening of the Algero-Provençal basin. At the slope break, a 80 km-long fault-tip Quaternary fold, namely the Khayr al Din fault, extends at the foot of the margin off NW Algiers and represents the largest active structure of the coastal area, together with the Sahel anticline. We also map for the first time a set of overlapping, en echelon active folds in the upper part of the Khayr al Din bank, located off previously known active structures on land. Most of these faults represent actually a threat for the Algiers region in terms of seismic hazard but also geological hazards, such as tsunamis, as most of them depicts significant dimensions and slip rates. The highest long-term horizontal shortening rate is found on the Khayr al Din fault and is estimated at 0.5 ± 0.1 mm/yr, with a maximal magnitude of 7.3, which provides one of the highest seismogenic potential in the region.A new tectonic framework for the Algiers region is proposed, in which the main south-dipping offshore structure, of opposite vergence relative to most thrusts on land, appears to be nowadays the main driving fault system, as also found further east in the Boumerdès (M 6.8) 2003 rupture zone. The overall apparent pop-up structure of the recent and active faults may result from a progressive migration of the plate limit from the Late Miocene, north-dipping suture zone on land, to the Quaternary, south-dipping main Khayr al Din fault at sea, suggesting a process of subduction inception.     

Estimation of model errors generated by atmospheric forcings for ocean data assimilation: experiments in a regional model of the Bay of Biscay

The characterization of model errors is an essential step for effective data assimilation into open-ocean and shelf-seas models. In this paper, we propose an experimental protocol to properly estimate the error statistics generated by imperfect atmospheric forcings in a regional model of the Bay of Biscay, nested in a basin-scale North Atlantic configuration. The model used is the Hybrid Coordinate Ocean Model (HYCOM), and the experimental protocol involves Monte Carlo (or ensemble) simulations. The spatial structure of the model error is analyzed using the representer technique, which allows us to anticipate the subsequent impact in data assimilation systems. The results show that the error is essentially anisotropic and inhomogeneous, affecting mainly the model layers close to the surface. Even when the forcings errors are centered around zero, a divergence is observed between the central forecast and the mean forecast of the Monte Carlo simulations as a result of nonlinearities. The 3D structure of the representers characterizes the capacity of different types of measurement (sea level, sea surface temperature, surface velocities, subsurface temperature, and salinity) to control the circulation. Finally, data assimilation experiments demonstrate the superiority of the proposed methodology for the implementation of reduced-order Kalman filters.     

Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines)

Central Mindanao was the locus of a Pliocene (4–5 Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3 Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts are among the magmatic products. All the lavas are Na-rich (up to 4.88%), with Na₂O/K₂O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere.     

A Note on a General Algorithm for Two-Body Expansions

This note describes briefly a technique, easily implemented with most computer algebra systems, for the purpose of computing the two-body expansions (in powers of the eccentricity and in Fourier series of the mean-anomaly) of a large class of functions of the distance, the true anomaly and/or the eccentric anomaly. This revised version was published online in July 2006 with corrections to the Cover Date.     

Twenty years of Belgian North Sea aerial surveillance: a quantitative analysis of results confirms effectiveness of international oil pollution legislation

Over the years many policy measures have been taken to prevent illegal oil discharges from ships, like the MARPOL 73/78 Convention (1983) and the Bonn Agreement (1969/1983). However, the number of discharges remained high, leading to chronic oiling of seabirds and sensitive coastlines, therefore further measures were taken. The aim of this study is to quantify the effectiveness of two key legislative regulations: the IMO-designation of the North Sea as MARPOL Special Area which took effect in 1999, and the adoption of the EU Directive on Port Reception Facilities in 2000. Under study is the heavily navigated Belgian Surveillance Area, monitored since 1991, characterised by shallow waters with ecologically important sandbanks. The aerial surveillance data from 1991 to 2010 show a stepwise decrease in ship-source oil pollution. Three time periods can be distinguished with two turning points coinciding with the actual implementation of these key legislative measures, confirming their effectiveness.