On the universal outcome of star formation: is there a link between stars and brown dwarfs?     

Pavel Kroupa †  Jerome Bouvier  Gaspard Duchêne  Estelle Moraux 《Monthly notices of the Royal Astronomical Society》2003,346(2):354-368

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Anoxic nitrification: Evidence from Humber Estuary sediments (UK)   总被引：3，自引：0，他引：3  

Rebecca Bartlett  Robert J.G. Mortimer  Katherine Morris 《Chemical Geology》2008,250(1-4):29-39

Conventional understanding of the nitrogen cycle in marine sediments has changed in recent years with the discovery of an alternative pathway for ammonia oxidation via the reduction of manganese oxides (during anoxic nitrification). In anoxic sediments, the potential for manganese oxides to serve as oxidant for nitrification may be considerable yet previous work on manganese-rich sediments has suggested anoxic nitrification may not be significant. In this study, the potential for anoxic nitrification in a range of sediment types was investigated. Laboratory incubation of sediment from three sites on the Humber Estuary, a microbially diverse environment, showed anoxic accumulation of nitrate, nitrite and dinitrogen gas, with and without the addition of synthetic manganese oxides. Incubation experiments confirmed anoxic nitrification as microbially mediated, with heat-killed controls yielding negative results. The anoxic nitrification reaction significantly depleted ammonia concentrations, and occurred simultaneously with manganese-, iron- and sulphate reduction, and methanogenesis. Taken in conjunction with other studies, results suggest anoxic nitrification may not only be dependent on total manganese concentrations but on manganese dynamics. Anoxic nitrification may be explained as a non-steady state reaction, dependent on the recent stability of a sediment system. Physical perturbation of sediments may cause the redistribution and/or introduction of manganese oxides and promote anoxic nitrification. The significance and persistence of anoxic nitrification is likely to depend on the frequency and magnitude of sediment perturbation, which explains why the reaction varies so widely across studied sites, and why it may not occur in some manganese-rich sediment.  相似文献   

Preliminary Results from a New 157 nm Laser Ablation ICP-MS Instrument: New Opportunities in the Analysis of Solid Samples     

Philippe Telouk  Estelle F. Rose-Koga  Francis Albarede 《Geostandards and Geoanalytical Research》2003,27(1):5-11

Preliminary results are given from an excimer 157 nm laser ablation multiple-collector inductively coupled plasma-mass spectrometer (LA-MC-ICP-MS), used for the isotopic measurements of solid materials. Elements of geological interest with different volatilities such as Pb and U (e.g. zircon geochronology) and Cu and Zn (as examples of geochemical/biochemical tracers) were analysed. The range of ablation rates of 20-150 nm s^-1 enabled us to ablate the sample down to a depth of 45 μm for a 50 μm diameter pit. The Cu and Zn isotopic measurements gave values that were very stable with, on average, a 0.01 % standard error, comparable with that achieved in liquid mode measurements.  相似文献   

Clinoform nucleation and growth in coarse-grained deltas, Loreto basin, Baja California Sur, Mexico: a response to episodic accelerations in fault displacement   总被引：1，自引：0，他引：1  

Estelle Mortimer  Sanjeev Gupta†  Patience Cowie 《Basin Research》2005,17(3):337-359

We investigate the controls on the architecture of coarse‐grained delta progradational units (PUs) in the Pliocene Loreto basin (Baja California Sur, Mexico), a half‐graben located on the western margin of the Gulf of California. Dorsey et al. (1997b) argued that delta progradation and transgression cycles in the basin were driven by episodic fault‐controlled subsidence along the basin‐bounding Loreto fault. Here we test this hypothesis by a detailed analysis of the sedimentary architecture of 11 exceptionally well‐exposed, vertically arranged fluvio‐deltaic PUs, each of which shows lateral facies transition from proximal alluvial facies palaeo‐seaward into distal pro‐delta facies. Of these 11 PUs, seven exhibit a lateral transition from a shoal water to Gilbert‐delta facies associations as they are traced palaeo‐seaward. This transition is characterised by down‐transport development of foresets, which grow in height up to 35 m. Foreset units thicken in a basinward direction, with initially an oblique topset–foreset geometry that becomes increasingly sigmoidal. Each delta is capped by a shell bed that records drowning of the delta top. This systematic transition in delta architecture records increasing water depth through time during individual episodes of progradation. A mechanism that explains this transition is an accelerating rate of fault‐controlled subsidence during each PU. During episodes of low slip rate, shoal‐water deltas prograde across the submerged topography of the underlying delta unit. As displacement rate accelerates, increasing bathymetry at the delta front leads to steepening of foresets and initiation of Gilbert deltas. Subsequent delta drowning results from sediment starvation at the shoreline at high slip rates because of sediment trapping upstream. The observed delta architecture suggests that the long‐term (>100 kyr) history of slip on the Loreto fault was characterised by repetitive episodes of accelerating displacement accumulation. Such episodic fault behaviour is most likely to be because of variations in temporal and spatial strain partitioning between the Loreto fault and other faults in the Gulf of California. A physical explanation for the acceleration phenomenon involves evolving frictional properties on the episodically active Loreto fault.  相似文献   

The impact of artisanal fishing on coral reef fish health in Hat Thai Mueang,Phang-nga Province,Southern Thailand     

Estelle Jones  Tim Gray  Chanin Umponstira 《Marine Policy》2009

This study investigates whether there has been a decline in fish stocks of a coral reef in Thailand, and if so, whether that decline is due to small-scale fishing. The research methods used included fish sampling by UVC, interviews and questionnaires with key informants and artisanal fishers. The results confirmed that there has been a decline in piscivores and carnivores (though not herbivores), and the main perceived threat came not from artisanal but from commercial fishing. Recommendations are to replace the largely unrestricted fishery with a more regulated regime backed by effective enforcement.  相似文献   

The Monitor project: rotation of low-mass stars in the open cluster NGC 2516     

Jonathan Irwin  Simon Hodgkin  Suzanne Aigrain  Leslie Hebb  Jerome Bouvier  Cathie Clarke  Estelle Moraux  D. M. Bramich 《Monthly notices of the Royal Astronomical Society》2007,377(2):741-758

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The Monitor project: JW 380 – a 0.26-, 0.15-M_⊙, pre-main-sequence eclipsing binary in the Orion nebula cluster     

Jonathan Irwin  Suzanne Aigrain  Simon Hodgkin  Keivan G. Stassun  Leslie Hebb  Mike Irwin  Estelle Moraux  Jerome Bouvier  Aude Alapini  Richard Alexander  D. M. Bramich  Jon Holtzman  Eduardo L. Martín  Mark J. McCaughrean  Frédéric Pont  P. E. Verrier  María Rosa Zapatero Osorio 《Monthly notices of the Royal Astronomical Society》2007,380(2):541-550

We present a general recipe for constructing N -body realizations of galaxies comprising near spherical and disc components. First, an exact spherical distribution function for the spheroids (halo and bulge) is determined, such that it is in equilibrium with the gravitational monopole of the disc components. Second, an N -body realization of this model is adapted to the full disc potential by growing the latter adiabatically from its monopole. Finally, the disc is sampled with particles drawn from an appropriate distribution function, avoiding local-Maxwellian approximations. We performed test simulations and find that the halo and bulge radial density profile very closely match their target model, while they become slightly oblate due to the added disc gravity. Our findings suggest that vertical thickening of the initially thin disc is caused predominantly by spiral and bar instabilities, which also result in a radial re-distribution of matter, rather than scattering off interloping massive halo particles.  相似文献   

Temporal and geochemical evolution of the Cenozoic intraplate volcanism of Zealandia     

Christian Timm  Kaj Hoernle  Reinhard Werner  Folkmar Hauff  Paul van den Bogaard  James White  Nick Mortimer  Dieter Garbe-Schönberg 《Earth》2010,98(1-2):38-64

In order to constrain better the distribution, age, geochemistry and origin of widespread Cenozoic intraplate volcanism on Zealandia, the New Zealand micro-continent, we report new ⁴⁰Ar/³⁹Ar and geochemical (major and trace element and Sr–Nd–Hf–Pb isotope) data from offshore (Chatham Rise, Campbell and Challenger Plateaus) and onland (North, South, Auckland, Campbell, Chatham and Antipodes Islands of New Zealand) volcanism on Zealandia. The samples include nephelinite, basanite through phonolite, alkali basalt through trachyte/rhyolite, and minor tholeiite and basaltic andesite, all of which have ocean island basalt (OIB)-type trace element signatures and which range in age from 64.8 to 0.17 Ma. Isotope ratios show a wide range in composition (⁸⁷Sr/⁸⁶Sr = 0.7027–0.7050, ¹⁴³Nd/¹⁴⁴Nd = 0.5128–0.5131, ¹⁷⁷Hf/¹⁷⁶Hf = 0.2829–0.2831, ²⁰⁶Pb/²⁰⁴Pb = 18.62–20.67, ²⁰⁷Pb/²⁰⁴Pb = 15.54–15.72 and ²⁰⁸Pb/²⁰⁴Pb = 38.27–40.34) with samples plotting between mid-ocean-ridge basalts (MORB) and Cretaceous New Zealand intraplate volcanic rocks.Major characteristics of Zealandia's Cenozoic volcanism include longevity, irregular distribution and lack of age progressions in the direction of plate motion, or indeed any systematic temporal or spatial geochemical variations. We believe that these characteristics can be best explained in the context of lithospheric detachment, which causes upwelling and melting of the upper asthenospheric mantle and portions of the removed lithosphere. We propose that a large-scale seismic low-velocity anomaly, that stretches from beneath West Antarctica to Zealandia at a depth of > 600 km may represent a geochemical reservoir that has been in existence since the Cretaceous, and has been supplying the upper mantle beneath Zealandia with HIMU-type plume material throughout the Cenozoic. In addition, the sources of the Cenozoic intraplate volcanism may be at least partially derived through melting of locally detached Zealandia lower lithosphere.  相似文献   

The role of multipolar magnetic fields in pulsar magnetospheres     

Estelle Asseo  David Khechinashvili 《Monthly notices of the Royal Astronomical Society》2002,334(4):743-759

We explore the role of complex multipolar magnetic fields in determining physical processes near the surface of rotation powered pulsars. We model the actual magnetic field as the sum of global dipolar and star-centred multipolar fields. In configurations involving axisymmetric and uniform multipolar fields, 'neutral points' and 'neutral lines' exist close to the stellar surface. Also, the curvature radii of magnetic field lines near the stellar surface can never be smaller than the stellar radius, even for very high-order multipoles. Consequently, such configurations are unable to provide an efficient pair-creation process above pulsar polar caps, necessary for plasma mechanisms of generation of pulsar radiation. In configurations involving axisymmetric and non-uniform multipoles, the periphery of the pulsar polar cap becomes fragmented into symmetrically distributed narrow subregions where curvature radii of complex magnetic field lines are less than the radius of the star. The pair-production process is only possible just above these 'favourable' subregions. As a result, the pair plasma flow is confined within narrow filaments regularly distributed around the margin of the open magnetic flux tube. Such a magnetic topology allows us to model the system of 20 isolated subbeams observed in PSR B0943+10 by Deshpande & Rankin. We suggest a physical mechanism for the generation of pulsar radio emission in the ensemble of finite subbeams, based on specific instabilities. We propose an explanation for the subpulse drift phenomenon observed in some long-period pulsars.  相似文献   

Behaviour of REE and mass balance calculations in a lateritic profile over chlorite schists in South Cameroon     

Veronique Kamgang Kabeyene Beyala  Estelle Ndome Effoudou Priso  Georges Emmanuel Ekodeck 《Chemie der Erde / Geochemistry》2009,69(1):61-13

An in situ weathering profile overlying chlorite schists in the Mbalmayo-Bengbis formations (South Cameroon) was chosen for the study of the behaviour of REE and the evaluation of geochemical mass balance. After physical and mineralogical studies, the chlorite schists and the undisturbed weathered materials were chemically analyzed for major elements (X-ray fluorescence and titrimetry) and REE (ICP-MS). The behaviour of the REE in the Mbalmayo weathering system was established in comparison with the REE of the reference parent rock. Mass balance calculations were applied to both major elements and REE. The mineralogy of the materials was determined with the aid of a Philips 1720, diffractometer. The chlorite schists of the Mbalmayo sector show low REE contents (Σ=153.44 ppm). These rocks are relatively rich in LREE (about 125 times the chondritic value) and relatively poor in HREE (about 20 times the chondritic value). The REE diagram normalized to chondrites shows a slightly split graph ((La/Yb)_N=6.18) with marked enrichment in LREE (LREE/HREE=9.50) in relation to HREE. Moreover, these spectra do not present any Ce anomaly, but a slightly positive Eu anomaly. The imperfectly evolved profile, whose materials are genetically linked, shows an atypical behaviour of REE. In effect, the LREE are more mobile than the HREE during weathering ((La/Yb)NASC<1) with weak Ce anomalies. This has been rarely reported in lateritic profiles characterized by higher HREE mobility than LREE during weathering processes with high Ce anomalies. This is either due to the difference in the stability of REE-bearing minerals, or to the weak acidic to basic pH conditions (6.70<pH<7.80), or even due to the average evolution of the weathering materials. The pathway of the REE along the profile is as follows: (1) leaching in the saprolites and summit of the profile, except for Ce, which precipitates very weakly in the nodular materials and the coarse saprolite materials, (2) at the base of the profile, solutions come in contact with chlorite schist formations, at this level, the pH increases (pH=7.79), HREE and a part of LREE partially void of Ce precipitate and (3) the other part of LREE precipitates further up in the profile. The geochemical mass balance calculations reveal that these elements are leached in the same phases as the relatively high Si, Al, K and Fe²⁺ contents.  相似文献