On the H i content, dust-to-gas ratio and nature of Mg ii absorbers   总被引：1，自引：0，他引：1  

Brice Ménard  Doron Chelouche 《Monthly notices of the Royal Astronomical Society》2009,393(3):808-815

We estimate the mean dust-to-gas ratio of Mg  ii absorbers as a function of rest equivalent width W ₀ and redshift over the range  0.5 < z < 1.4  . Using the expanded Sloan Digital Sky Survey/ Hubble Space Telescope sample of low-redshift Lyman-α absorbers, we first show the existence of a 8σ correlation between the mean hydrogen column density  〈 N _{H  i} 〉  and W ₀, an indicator of gas velocity dispersion. By combining these results with recent dust-reddening measurements, we show that the mean dust-to-gas ratio of Mg  ii absorbers does not appreciably depend on rest equivalent width. Assuming that, on average, dust-to-gas ratio is proportional to metallicity, we find its redshift evolution to be consistent with that of   L ^★  galaxies from   z = 0.5  to 1.4, and we show that our constraints disfavour dwarf galaxies as the origin of such absorbers. We discuss other scenarios and favour galactic outflows from  ∼ L ^★  galaxies as the origin of the majority of strong Mg  ii absorbers. Finally, we show that, once evolutionary effects are taken into account, the Bohlin et al. relation between A _V and N _H is also satisfied by strong Mg  ii systems down to lower column densities than those probed in our Galaxy.  相似文献   

High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)     

Thierry Mulder  Yannick Callec  Philippe Joseph  Cécile Robin  Tristan Salles  Jonathan Allard  Emmanuelle Ducassou  Brice Ferger  Vincent Hanquiez  Elodie Marchès  Sébastien Zaragosi 《Sedimentary Geology》2010,229(3):160-191

The Lauzanier area represents the northernmost extension of the Annot Sandstone series and contains deposits between 650 and 900 m-thick. This basin was active from upper Bartonian or lower Priabonian to early Rupelian. It is composed of two superposed units separated by a major unconformity. The sediment supply is due to channelled flows coming from the south. Flow processes include mass flow to turbidity currents. The size of the particles and the absence of fine-grained sediment suggest a transport over a short distance. The Lower Unit is made of coarse-grained tabular beds interpreted as non-channelled lobe deposits. The Upper Unit is made of massive conglomerates interpreted as the channelled part of lobes. These lobe deposits settle in a tectonically confined basin according to topographic compensation that occurs from bed scale to unit scale. The abrupt progradation between the lower and the upper unit seems related to a major tectonic uplift in the area. This uplift is also suggested by a change in the petrographic nature of the source and an abrupt coarsening of the transported clasts.This field example allows providing high resolution analysis for depositional sedimentary sequences of terminal lobe deposits in a coarse-grained turbidite system. The outcrop analysis shows the lateral evolution of deposits and the system progradation allows a longitudinal analysis of facies evolution by superposing on the same outcrops the channelled lobe system and the non-channelled lobe system. These results of high-resolution outcrop analysis can be extrapolated to results obtained on sedimentary lobes in recent deep-sea turbidite system that are either restricted to cores, or with a lesser resolution (seismic).  相似文献   

Application of evidential reasoning to improve the mapping of regenerating forest stands     

Brice Mora  Richard A. Fournier  Samuel Foucher 《International Journal of Applied Earth Observation and Geoinformation》2011

This study confirmed the ability of the Dempster–Shafer theory (DST) and the Dezert–Smarandache (Free DSm model) theory to significantly improve the quality of maps of regenerating forest stands in southern Quebec, Canada compared to a classical Maximum Likelihood Algorithm (MLA). The proposed approach uses data fusion methods that allow the integration of remotely sensed imagery with conventional maps of ecophysiographic features. While the MLA provided an overall accuracy of 82.75%, the DST and Free DSm models had overall accuracies of 90.14% and 91.13% respectively. In addition, this study showed that the data fusion methods can model the influence of biophysical parameters (e.g., surface deposits and drainage) on the growth potential of regenerating forest stands. This study illustrates the importance of the mass function allocation for each ancillary data source. We found that a Bayesian belief configuration provided results equivalent to those obtained when representing data uncertainty. This demonstrates the difficulty in modelling uncertainty associated with each ancillary source.  相似文献   

Storm extreme levels and coastal flood hazards: A parametric approach on the French coast of Languedoc (district of Leucate)   总被引：1，自引：0，他引：1  

Brice Anselme  Paul Durand  Yves-François Thomas  Alexandre Nicolae-Lerma 《Comptes Rendus Geoscience》2011,343(10):677-690

Coastal flooding is a significant risk on the shores of Languedoc-Roussillon. The storms that periodically hit the coast can generate strong swells and storm surges. Most beach resorts, built on a low elevation dune ridge, are periodically flooded during major storms. Although risks zoning regulations take into consideration coastal flood hazards, the delineation of vulnerable areas is still insufficient and the commonly accepted threshold is regularly exceeded during most severe storms. This paper presents a method to improve the assessment of extreme storm-related water levels. It relies on fieldwork carried out in the Leucate commune (Aude), which is particularly exposed to the risk of sea level rise. It considers both storm surges and wave phenomena that occur within the surf zone (set-up and swash), calculated from the Simulating WAves Nearshore (SWAN^®) numerical wave model and the Stockdon formula. Water levels reached during several recent storm events have been reconstructed and simulations of submerged areas were carried out by numerical modelling.  相似文献   

Assessment of shallow groundwater in Lake Nyos catchment (Cameroon,Central-Africa): implications for hydrogeochemical controls and uses   总被引：1，自引：0，他引：1  

Brice T. Kamtchueng  Wilson Y. Fantong  Akira Ueda  Edwige R. Tiodjio  Katsuro Anazawa  Mengnjo J. Wirmvem  Joseph O. Mvondo  Luc S. Nkamdjou  Minoru Kusakabe  Takeshi Ohba  Gregory Tanyileke  Joseph V. Hell 《Environmental Earth Sciences》2014,72(9):3663-3678

In 1986, carbon dioxide gas exploded from Lake Nyos and killed about 1,800 people. After that disaster, various administrative and research activities have been conducted to mitigate subsequent disasters. However, none of those endeavors have characterized the groundwater chemistry to identify hydrogeochemical processes that control the water chemistry, and the quality of the water for domestic and agricultural uses that support the lives of un-official resettlers around Lake Nyos. Conventional hydrochemical techniques coupled with statistical and graphical analysis were therefore employed to establish the baseline hydrochemical conditions, assess processes controlling solutes distribution in shallow groundwater in the Lake Nyos catchment and explore its usability. Groundwater samples were analyzed for their physical and chemical properties. The wide ranges of electrical conductivity and total dissolved solid values reveal the heterogeneous distribution of groundwater within the watershed. The relative abundance of major dissolved species was Ca > Mg > Na > K for cations and HCO₃ >>> Cl > SO₄ > NO₃ for anions. Piper diagram classified almost all water samples into mixed CaMg–HCO₃ water type. Major ion geochemistry reveals that, in addition to silicates weathering (water–rock interaction), ion exchange processes regulate the groundwater chemistry. Principal component analysis supports the occurrence of water rock interaction. Hierarchical cluster analysis showed that the chemistry of groundwater in the study area is controlled by three main factors, and suggests no hydraulic connectivity between deep lake water and groundwater in the catchment. The quality assessment of the groundwater showed that groundwater parameters are within the acceptable limit of the World Health Organization and Nigeria guidelines for drinking and domestic uses, and water found to be good for irrigation.  相似文献   

Low-Mass Young Stars in Cep OB2: Ages, Distribution and Accretion Disks     

Aurora Sicilia-Aguilar  Lee Hartmann  César Briceño  María Eugenia Contreras  James Muzerolle  Nuria Calvet 《Astrophysics and Space Science》2004,292(1-4):457-463

We are studying the young clusters Tr37 and NGC7160 in the Cep OB2 region as part of a program to understand the evolution of accretion disks at the ages of disk dissipation and planet formation. Here, we present the first identifications of low mass (spectral types K-M) members of the clusters and study the presence and characteristics of their accretion disks, finding evidences of disk evolution. Using optical photometry and spectroscopy, we have identified ～70 members in Tr37 and ～20 in NGC7160, confirming age estimates of 3 and 10 Myr respectively. Accretion rates are ～10^?8 M _⊙yr^?1 in Tr37. We have not found any accreting members in NGC7160, suggesting that disk accretion generally ends before the age of 10 Myr, which is consistent with the results from other populations.  相似文献   

The environmental dependence of the relations between stellar mass, structure, star formation and nuclear activity in galaxies   总被引：2，自引：0，他引：2  

Guinevere Kauffmann  Simon D. M. White  Timothy M. Heckman  Brice Ménard  Jarle Brinchmann  Stéphane Charlot  Christy Tremonti  Jon Brinkmann 《Monthly notices of the Royal Astronomical Society》2004,353(3):713-731

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Holocene changes in climate and land use drove shifts in the diversity of testate amoebae in a subalpine pond     

France Oris  Mariusz Lamentowicz  Aurélie Genries  Brice Mourier  Olivier Blarquez  Adam A. Ali  Laurent Bremond  Christopher Carcaillet 《Journal of Paleolimnology》2013,49(4):633-646

Testate amoebae that inhabit peat are sensitive indicators of water table position. In this study, we used testate amoebae in sediments from a mire in the western Alps (Lac du Thyl) to: (1) reconstruct the hydrology of the site over the last 7,000 years, (2) determine how hydrological changes affected testate amoebae diversity and (3) infer past trophic state shifts. The study site is located in one of the driest valleys of the Alps and is thus very sensitive to hydrological changes. Our study revealed that the water table depth increased (dry conditions) between 5,800 and 4,000 cal year BP. This triggered establishment of a Sphagnum-type peat and acidic conditions from 5,700 to 4,000 cal year BP. These processes were independent of ongoing transformations of the terrestrial vegetation and soil in the catchment area. After 1,690 cal year BP, the depth to the water table decreased (wetter conditions) and a minerotrophic fen developed. At the same time, the diversity of testate amoebae increased, probably as a result of deforestation that supported the expansion of grassland. Climate and land use were apparently more important factors controlling the lake hydrology than were changes in vegetation and soil in the catchment. Testate amoebae diversity was linked to land cover. Changes in pH were controlled indirectly by external forcing (climate), but more directly by fluctuations in the level of the water table (internal forcing) and autogenous expansion of Sphagnum.  相似文献   

Variation of hydrogeochemical characteristics of water in surface flows,shallow wells,and boreholes in the coastal city of Douala (Cameroon)     

Wilson Y. Fantong  Brice T. Kamtchueng  Beatrice Ketchemen-Tandia  Doris Kuitcha  Josephine Ndjama  Alain T. Fouepe 《水文科学杂志》2013,58(16):2916-2929

ABSTRACT

Groundwater is used by 3?million inhabitants in the coastal urban city of Douala, Cameroon, but comprehensive data are too sparse for it to be managed in a sustainable manner. Hence this study aimed to (1) assess the potability of the groundwater; (2) evaluate the spatial variation of groundwater composition; and (3) assess the interaction and recharge mechanisms of different water bodies. Hydrogeochemical tools and methods revealed the following results in the Wouri and Nkappa formations of the Douala basin, which is beneath Douala city: 30% of water samples from hand-dug wells in the shallow Pleistocene alluvium aquifer were saline and highly mineralized. However, water from boreholes in the deeper (49–92 m depth) Palaeocene aquifer was saline-free, less mineralized and potable. Water in the shallow aquifer (0.5–22 m depth) was of Na⁺-K⁺-Cl^?-NO₃^? type and not potable due to point source pollution, whereas Ca⁺-HCO₃^? unpolluted water dominates in the deeper aquifer. Water in the deep and shallow aquifers indicates the results of preferential flow pass and evaporative recharge, respectively. Possible hydrogeochemical processes include point source pollution, reverse ion exchange, remote recharge areas and mixing of waters with different chemical signatures.