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91.
Continental shelf systems are highly dynamic sedimentary environments, where sediments from biogenic production as well as
from terrigenous sources are redistributed in the shelf depositional system, and partly exported off the shelf to the slope
and the deep sea. The Golfe d’Arguin (Mauritania, NW Africa) is dominated by such redistribution processes, involving clastic
silt imported as dust from the Sahara desert and biogenic carbonates of marine origin. Indeed, surface-sediment grain size
and mineralogy show a clear north–south partitioning of sediment type. Fine material is winnowed from the northern part of
the gulf, and transported toward the southern part off the Banc d’Arguin, where coarse silt settles on the outer shelf and
upper slope, at least down to 600 m water depth. Particles of the fine silt fraction, estimated in terms of eolian material
collected aboard the research vessel, are thought to be exported further offshore as they correspond to grain sizes previously
reported from adjacent deep-sea sediments. These findings suggest that the interpretation of dust records from the continental
slope and rise off NW Africa must consider reworking and partitioning processes active on the Mauritanian shelf. 相似文献
92.
Julien Malzac 《Astrophysics and Space Science》2007,311(1-3):149-159
In the last decade multi-wavelength observations have demonstrated the importance of jets in the energy output of accreting
black hole binaries. The observed correlations between the presence of a jet and the state of the accretion flow provide important
information on the coupling between accretion and ejection processes. After a brief review of the properties of black hole
binaries, I illustrate the connection between accretion and ejection through two particularly interesting examples. First,
an INTEGRAL observation of Cygnus X-1 during a ‘mini-’ state transition reveals disc jet coupling on time scales of orders
of hours. Second, the black hole XTEJ1118+480 shows complex correlations between the X-ray and optical emission. Those correlations
are interpreted in terms of coupling between disc and jet on time scales of seconds or less. Those observations are discussed
in the framework of current models. 相似文献
93.
Jaime E. Forero-Romero Jérémy Blaizot Julien Devriendt Ludovic Van Waerbeke Bruno Guiderdoni 《Monthly notices of the Royal Astronomical Society》2007,379(4):1507-1518
We present the Lensed Mock Map Facility ( lemomaf ), a tool designed to perform mock weak-lensing measurements on numerically simulated chunks of the Universe. Coupling N -body simulations to a semi-analytical model of galaxy formation, lemomaf can create realistic lensed images and mock catalogues of galaxies, at wavelengths ranging from the ultraviolet to the submillimetre. To demonstrate the power of such a tool, we compute predictions of the source–lens clustering (SLC) effect on the convergence statistics, and quantify the impact of weak lensing on galaxy counts in two different filters. We find that the SLC effect skews the probability density function of the convergence towards low values, with an intensity which strongly depends on the redshift distribution of galaxies. On the other hand, the degree of enhancement or depletion in galaxy counts due to weak lensing is independent of the SLC effect. We discuss the impact on the two-point shear statistics to be measured by future missions like SNAP and LSST . The SLC effect would bias the estimation of σ8 from two-point statistics up to 5 per cent for a narrow redshift distribution of mean z ∼ 0.5 , and up to 2 per cent in small angular scales for a redshift distribution of mean z ∼ 1.5 . We conclude that accurate photometric redshifts for individual galaxies are necessary in order to quantify and isolate the SLC effect. 相似文献
94.
Julien M. J. Racca Robert Racca Reinhard Pienitz Yves T. Prairie 《Journal of Paleolimnology》2007,38(3):467-472
Transfer functions that implement organism–environment relationships are now commonly used for inferring past environmental
conditions in paleoecology. Specific software for developing and evaluating commonly used modelling techniques such as Weighted
averaging (WA), Weighted averaging partial least square (WA-PLS), Maximum likelihood (ML), and Modern analog technique (MAT)
are available. A new software programme, PaleoNet, is now available for modelling organism–environment relationships which is specifically designed for the development and
the evaluation of artificial neural network (ANN) based transfer functions in paleoecology. Here we present the main characteristics
of this new software PaleoNet (User guide version 1.01) and discuss in more detail one of its specific features: the pruning. 相似文献
95.
The geomagnetic field and secular variation exhibit asymmetrical spatial features which are possibly originating from an heterogeneous thermal control of the Earth's lower mantle on the core. The identification of this control in magnetic data is subject to several difficulties, some of which can be alleviated by the use of core surface flow models. Using numerical dynamos driven by heterogeneous boundary heat flux, we confirm that within the parameter space accessible to simulations, time average surface flows obey a simple thermal wind equilibrium between the Coriolis and buoyancy forces, the Lorentz, inertial and viscous forces playing only a secondary role, even for Elsasser numbers significantly larger than 1. Furthermore, we average the models over the duration of three vortex turnovers, and correlate them with a longer time average which fully reveals the signature of boundary heterogeneity. This allows us to quantify the possibility of observing mantle control in core surface flows averaged over a short time period. A scaling analysis is performed in order to apply the results to the Earth's core. We find that three vortex turnovers could represent between 100 and 360 years of Earth time, and that the heat flux heterogeneity at the core-mantle boundary could be large enough to yield an observable signature of thermal mantle control in a time average core surface flow within reach of the available geomagnetic data. 相似文献
96.
Philippe Guéguen Cécile Cornou Stéphane Garambois Julien Banton 《Pure and Applied Geophysics》2007,164(1):115-134
The H/V spectral ratio method based on seismic noise (HVSRN) was used in the Grenoble Basin (France), an Alpine valley characterized
by a small apex ratio. The resonance frequencies obtained in the experiments were compared to the thickness of the sediments
deduced from a microgravimetric survey and to the 1-D theoretical assessment of site responses. Given the abundance of data
on the sediments and depth of the basin, the values of the theoretical resonance frequency fo can be determined quite accurately. However, it has been observed that the effects of basin geometry can disturb fo measurements using the HVSR method, in particular for a case like the Grenoble Basin, which has a small apex ratio (w/H)
and strong suspected 2-D and/or 3-D effects. Interpretation of fo values in terms of bedrock depth gives rise to estimation errors of about 10% in certain cases, with the most significant
errors (>50%) occurring on the edges of the basin, where subsurface layers are characterised by larger heterogeneities and
where the basin topography is accentuated. This study suggests that great care must be taken when using the HVSRN method as
an exploration tool, at least in valleys with a small apex ratio. 相似文献
97.
98.
Climate Dynamics - We investigate heat waves defined as periods of at least 3 consecutive days of extremely high daily maximum temperature affecting at least 30 % of western Europe. This... 相似文献
99.
The ability of state-of-the-art climate models to capture the mean spatial and temporal characteristics of daily intense rainfall events over Africa is evaluated by analyzing regional climate model (RCM) simulations at 90- and 30-km along with output from four atmospheric general circulation models (AGCMs) and coupled atmosphere–ocean general circulation models (AOGCMs) of the Climate Model Intercomparison Project 5. Daily intense rainfall events are extracted at grid point scale using a 95th percentile threshold approach applied to all rainy days (i.e., daily rainfall ≥1 mm day?1) over the 1998–2008 period for which two satellite-derived precipitation products are available. Both RCM simulations provide similar results. They accurately capture the spatial and temporal characteristics of intense events, while they tend to overestimate their number and underestimate their intensity. The skill of AGCMs and AOGCMs is generally similar over the African continent and similar to previous global climate model generations. The majority of the AGCMs and AOGCMs greatly overestimate the frequency of intense events, particularly in the tropics, generally fail at simulating the observed intensity, and systematically overestimate their spatial coverage. The RCM performs at least as well as the most accurate global climate model, demonstrating a clear added value to general circulation model simulations and the usefulness of regional modeling for investigating the physics leading to intense events and their change under global warming. 相似文献
100.
Y. Julien J.A. SobrinoJ.-C. Jiménez-Muñoz 《International Journal of Applied Earth Observation and Geoinformation》2011
Several previous studies have shown that the inclusion of the LST (Land Surface Temperature) parameter to a NDVI (Normalized Difference Vegetation Index) based classification procedure is beneficial to classification accuracy. In this work, the Yearly Land Cover Dynamics (YLCD) approach, which is based on annual behavior of LST and NDVI, has been used to classify an agricultural area into crop types. To this end, a time series of Landsat-5 images for year 2009 of the Barrax (Spain) area has been processed: georeferenciation, destriping and atmospheric correction have been carried out to estimate NDVI and LST time series for year 2009, from which YLCD parameters were estimated. Then, a maximum likelihood classification was carried out on these parameters based on a training dataset obtained from a crop census. This classification has an accuracy of 87% (kappa = 0.85) when crops are subdivided in irrigated and non-irrigated fields, and when cereal crops are aggregated in a single crop, and performs better than a similar classification from Landsat bands only. These results show that a good crop differentiation can be obtained although detailed crop separation may be difficult between similar crops (barley, wheat and oat) due to similar annual NDVI and LST behavior. Therefore, the YLCD approach is suited for vegetation classification at local scale. As regards the assessment of the YLCD approach for classification at regional and global scale, it will be carried out in a further study. 相似文献