Radio observations of Comet 9P/Tempel 1 before and after Deep Impact

Comet 9P/Tempel 1 was the target of a multi-wavelength worldwide investigation in 2005. The NASA Deep Impact mission reached the comet on 4.24 July 2005, delivering a 370-kg impactor which hit the comet at 10.3 km s⁻¹. Following this impact, a cloud of gas and dust was excavated from the comet nucleus. The comet was observed in 2005 prior to and after the impact, at 18-cm wavelength with the Nançay radio telescope, in the millimeter range with the IRAM and CSO radio telescopes, and at 557 GHz with the Odin satellite. OH observations at Nançay provided a 4-month monitoring of the outgassing of the comet from March to June, followed by the observation of H₂O with Odin from June to August 2005. The peak of outgassing was found to be around between May and July. Observations conducted with the IRAM 30-m radio telescope in May and July 2005 resulted in detections of HCN, CH₃OH and H₂S with classical abundances relative to water (0.12, 2.7 and 0.5%, respectively). In addition, a variation of the HCN production rate with a period of 1.73±0.10 days was observed in May 2005, consistent with the 1.7-day rotation period of the nucleus. The phase of these variations, as well as those of CN seen in July by Jehin et al. [Jehin, E., Manfroid, J., Hutsemékers, D., Cochran, A.L., Arpigny, C., Jackson, W.M., Rauer, H., Schulz, R., Zucconi, J.-M., 2006. Astrophys. J. 641, L145-L148], is consistent with a rotation period of the nucleus of 1.715 days and a strong variation of the outgassing activity by a factor 3 from minimum to maximum. This also implies that the impact took place on the rising phase of the “natural” outgassing which reached its maximum ≈4 h after the impact. Post-impact observations at IRAM and CSO did not reveal a significant change of the outgassing rates and relative abundances, with the exception of CH₃OH which may have been more abundant by up to one order of magnitude in the ejecta. Most other variations are linked to the intrinsic variability of the comet. The Odin satellite monitored nearly continuously the H₂O line at 557 GHz during the 38 h following the impact on the 4th of July, in addition to weekly monitoring. Once the periodic variations related to the nucleus rotation are removed, a small increase of outgassing related to the impact is present, which corresponds to the release of ≈5000±2000 tons of water. Two other bursts of activity, also observed at other wavelengths, were seen on 23 June and 7 July; they correspond to even larger releases of gas.     

In situ measurements of oxygen isotopic composition in deep-sea coral, Lophelia pertusa: Re-examination of the current geochemical models of biomineralization

We present ion microprobe measurements of oxygen isotopic compositions in the deep-sea coral Lophelia pertusa. Compared to bulk skeletal aragonite fibres, the EMZ (early mineralization zone), near the inside of the calyx, was systematically depleted in ¹⁸O. Rayleigh fractionation from a semi-closed fluid reservoir does not explain this and other geochemical differences. Furthermore, pH values estimated from skeletal δ¹¹B data are inconsistent with the idea that EMZ (or centres of calcification) ¹⁸O depletion reflects a more alkaline calcification environment. Our data, combined with microstructural and geochemical observations, indicate that the aragonitic fibres and EMZ are formed by a compartmentalized mineralization calicoblastic ectoderm, which exerts strong biological control on the composition of the skeleton. Hence, we propose a new model whereby amorphous calcium carbonates (ACC) are precursors to the EMZ, whereas the fibre precipitation is probably governed by kinetic processes.     

Geochemical characteristics of the Río Verde Complex,Central Hispaniola: Implications for the paleotectonic reconstruction of the Lower Cretaceous Caribbean island-arc

New geochronological, trace element and Sr–Nd isotope data for metabasalts, dolerites and amphibolites from the Río Verde Complex, Central Hispaniola, are integrated with existing geochemical data for mafic volcanic rocks and metamorphic derivatives from the Los Ranchos, Amina and Maimón Formations, giving new insights into magma petrogenesis and paleotectonic reconstruction of the Lower Cretaceous Caribbean island-arc–back arc system. U–Pb and ⁴⁰Ar/³⁹Ar age data show that the Río Verde Complex protoliths were in part coeval with volcanic rocks of the Los Ranchos Formation (Upper Aptian to Lower Albian). The geochemical data establish the existence of gradients in trace element parameters (Nb/Yb, Th/Yb, Zr/Yb, Zr/Ba, and normalized Ti, Sm, Y and Yb abundances) and Nd isotope compositions from throughout Hispaniola, which reflect differences in the degree of mantle wedge depletion and contributions from the subducting slab. The Río Verde Complex mafic rocks and some mafic sills and dykes intruding in the Loma Caribe Peridotite, have a transitional IAT to N-MORB geochemistry and a weak subduction-related signature, and are interpreted to form in a rifted arc or evolving back-arc basin setting. The Los Ranchos, Amina and Maimón Formations volcanic rocks have arc-like characteristics and represent magmatism in the volcanic front. Trace element and Nd isotope modeling reproduce observed data trends from arc to back-arc and suggest that the variations in several geochemical parameters observed in a SW direction across the Caribbean subduction system can be explained from the progressively lower subduction flux into a progressively less depleted mantle source. The low Nb contents and high (ε_Nd)_i values in both arc and back-arc mafic rocks imply, however, the absence of a significant Lower Cretaceous plume enriched component. In order to explain these observations, a model of proto-Caribbean oceanic lithosphere subducting to the SW at least in the 120–110 Ma interval, is proposed to cause the observed magmatic variations in the Lower Cretaceous Caribbean island-arc–back-arc system. In this context, arc rifting and initial sea-floor spreading to form the Río Verde Complex protoliths occurred in the back-arc setting of this primitive island-arc, built on the NE edge of the Caribbean plate.     

Relationships between field-aligned currents and convection observed by EISCAT and implications concerning the mechanism that produces region-2 currents: Statistical study

Fluid theories explain the origin of region-2 field-aligned currents as the closure of the ring current, driven itself by the azimuthal pressure gradients generated in the magnetospheric ring plasma by the sunward convection. Although the structure of pressure gradients appears experimentally complex, observations confirm that a close connection exists between the region-2 field-aligned currents and the ring current. The fluid linear theory of the adiabatic transport by convection of the ring plasma gives a first estimate of this process, and leads ultimately to phase quadrature (in terms of magnetic local time) between the region-2 field-aligned currents and the convection potential. When significant non-adiabatic processes are taken into account, such as precipitations at auroral latitudes, the theoretical phase difference rotates toward opposition. We determine experimentally the phase relationship between the region-2 field-aligned currents and the convection potential from recent statistics, depending on the magnetic activity index K_p, and performed from the EISCAT data base. For geometrical reasons of sufficient probing of region 2, it is only computed in the case of a moderate magnetic activity corresponding to 2\leqK_p<4. Region-2 field-aligned currents are found to be in phase opposition with the convection electrostatic potential at auroral latitudes. This confirms the importance of non adiabatic processes, especially ion losses, in the generation of region-2 field-aligned currents, as theoretically suggested.     

Characterization of water in synthetic rhyolitic glasses and natural melt inclusions by Raman spectroscopy

Raman spectroscopy was used to analyze quantitatively water in silicate glasses and melt inclusions and to monitor H₂O–OH speciation. Calibration is based on synthetic glasses with various water contents (0.02–7.67% H₂O); water determination and OH–H₂O differentiation on the area of the Si–O broad band at 468 cm^–1 and the asymmetric O–H band at 3,550 cm^–1. Each Raman spectrum has been decomposed into four Gaussian + Lorentzian components centered at 3,330, 3,458, 3,560, and 3,626 cm^–1 using the Levenberg–Marquardt algorithm. These components are interpreted to be two different types of H₂O molecule sites. The influence of the temperature on the loss of water is more important for molecular water than for the hydroxyl groups. The H₂O–OH partition confirms the typical evolution of water speciation in rhyolitic glasses as a function of the bulk water content. Method limitations have been studied for the application to natural melt inclusions.Editorial responsibility: T.L Grove     

Caractérisation de l'effluent de vidange d'un étang de pisciculture extensive

Résumé Afin d'évaluer l'impact potentiel de la vidange, les flux de nutriments et de matières sont quantifiés lors du remplissage et de la vidange d'un étang construit en dérivation, à l'aval d'une chaîne d'étangs. La cinétique de vidange a été établie à l'aide d'un modèle numérique de terrain restituant la surface topographique de l'étang. Les matières en suspension et les principaux nutriments ont été régulièrement quantifiés. La qualité hydrobiologique du cours d'eau récepteur a été contrôlée pendant et après la vidange. L'étang a exporté 8.5 tonnes de matières en suspension et des quantités de nutriments relativement élevées. Nos résultats confirment l'existence de deux phases critiques, ne concernant cependant qu'un très faible volume d'effluent. Compte tenu des capacités élevées de sédimentation des matières en suspension, la courte durée d'exposition associée à un linéaire de fossés suffisant pourrait limiter l'impact de ces matières rejetées.

Citation Vallod, D. & Sarrazin, B. (2010) Caractérisation de l'effluent de vidange d'un étang de pisciculture extensive. Hydrol. Sci. J. 55(3), 394–402.     

The Merzbacher & Eggler (1984) Geohygrometer: a Cautionary Note on its Suitability for High-K Suites

The use of the popular Merzbacher & Eggler (1984, Geology12, 587–590) experimental geohygrometer for calcalkalineandesites and dacites is critically evaluated and two pitfallsare found. First, calculation of the correct projection parametersis problematic because two endmember calculation schemes arefound in the literature; Baker & Eggler (1983, Journal ofVolcanology and Geothermal Research 18, 387–404) and Walkeret al. (1979, Contributions to Mineralogy and Petrology 70,111–125). Although related, these two schemes have crucialdifferences that can result in very different projection parametersfor the plagioclase component. This is most crucial for high-Kcompositions; H₂O contents estimated using the Walker et al.(1979) scheme can be as much as 100% higher than those estimatedusing the Baker & Eggler (1983) projection. Incorrect projectionparameter calculation has led to overestimation of water contentsin high-K andesites and dacites from the Central Andes. Second,for medium-K and high-K andesitic–dacitic compositionswater contents derived using the Merzbacher & Eggler (1984)geohygrometer deviate considerably from water contents estimatedusing other methods. Experimental data from the literature,and our studies of water contents inferred from melt inclusionsand plagioclase–melt equilibrium for dacites from theAltiplano–Puna Volcanic Complex of the Central Andes indicatethat the Merzbacher & Eggler (1984) geohygrometer shouldnot be applied to compositions with K₂O >1·9 wt %,as originally calibrated. KEY WORDS: magmatic volatiles; pre-eruptive water; experimental geohygrometer; high-K suites; melt inclusion     

Hydrodynamic behaviour of coarse bioclastic sand from shelly cheniers

Cheniers from Mont‐Saint‐Michel bay (France) are coarse shelly sand ridges migrating on the mudflat up to the salt marshes where they accumulate and merge in a littoral barrier. In this macrotidal setting and low wave forcing, the cheniers are rarely submerged. However, they are found to move up to several metres during coincidence of spring tide and wave activity. Their processes of migration, morphology and internal structure (composition of the beddings, grain size, sorting and grain arrangement) are thought to be closely related to the hydrodynamic behaviour of the coarse and shelly sediment. This paper focuses on the hydrodynamic behaviour of bioclastic sand sampled from the cheniers: settling velocities of the shell fragments were measured using a 2 m long sedimentation tube. Thresholds of motion under unidirectional current, velocity and turbulence vertical profiles were characterized in a small recirculating flume using Laser Doppler Anemometry (LDA). The flat‐shaped bioclastic particles feature low settling velocities and reveal a good resistance to the re‐suspension effect of the flow when imbricated in a sediment bed. The shear stress in the bottom boundary layer has been measured in the viscous and log sub‐layers. Nikuradse roughness heights (k_s) for shell debris beds of different sizes have been quantified. It is found that k_s ≈ 2·56d₅₀. This value is close to the ones used for classic rounded sand grains despite their major differences of shape. The dual behaviour of the shell fragments (low settling velocity, good resistance to unidirectional flow) should be considered as a key to understanding how this coarse material is transported across the tidal flat, and finally accumulated as cheniers. Further flume experiments including wave activity and tidal fluctuations are necessary to better quantify these complex processes. Copyright © 2010 John Wiley & Sons, Ltd.