New strategies to detect life on Mars     

Jeffrey L Bada  Mark A Sephton  Pascale Ehrenfreund  Richard A Mathies  Allison M Skelley  Frank J Grunthaner  Aaron P Zent  Richard C Quinn  Jean-Luc Josset  François Robert  Oliver Botta  Daniel P Glavin 《Astronomy& Geophysics》2005,46(6):6.26-6.27

If we are to find unequivocal evidence for life on Mars, we will need new ways to search for it. Jeff L Bada and the MOD team describe the innovative strategy developed for the ExoMars mission.  相似文献   

Q-band esr studies of fossil teeth: Consequences for esr dating     

R. Bouchez  R. Cox  E. Herve  E. Lopez-Carranza  J.L. Ma  M. Piboule  G. Poupeau  P. Rey 《Quaternary Science Reviews》1988,7(3-4)

At least 14 axial-symmetry ESR lines in Plio-Quaternary enamel fibres were observed by Q-band technique. Some lines follow a law e.g. the main anisotropic A-line whose g = 2.0018 and g = 1.9977 are close to g_eff = 2.0020 and g_eff = 1.9975 respectively of the isotropic doublets B- and E-lines. The other lines are blocked below 220K e.g. one isotropic hyperfine (g = 2.0033, a = 21.6 Gauss) M-septet and one isotropic singlet (g = 2.0007) D-line. By heating, the M-septet intensity increases up to 250°C and is lost at 280°C; B- and E-lines decrease while a new isotropic (g = 2.0056) C-line appears. At 300°C, only the two C- and A-lines usually remain and hold up to 400°C.The natural dose rate of fossil enamel is calculated from U-, Th-, K-content and disequilibrium measured both in enamel and dentine, bones and sediments in contact, by methods of low-background γ-spectrometry, isotopic dilution and fission-tracks mapping.ESR dating of quaternary Proboscidae, Bovidae, Equidae and Cervidae has been attempted by a new comparative method: the A-line intensity, normalized for the same dose rate and weight, of various fossil dental enamels varies quasi-linearly with the age of the geological sites from which they are issued.  相似文献   

Determining past atmospheric HCl mixing ratios from ice core analyses     

Florent Dominé  Emmanuel Thibert  Eric Silvente  Michel Legrand  Jean-Luc Jaffrezo 《Journal of Atmospheric Chemistry》1995,21(2):165-186

Laboratory and field measurements have been performed in order to improve our understanding of the HCl air-snow transfer function. The solubility and diffusion of HCl in laboratory grown single crystals of ice have been measured as a function of HCl partial pressure,P _HCl, between –8 and –25 °C. Measurements ofP _HCl and of the mole fraction of HCl in snow,X'_HCl, have been measured at Summit, Greenland. Comparison of the field and laboratory measurements show that theX'_HCl values are well below equilibrium values. The major processes involved in the formation of snow crystals and in their evolution after deposition are discussed in order to attempt to understand theX'_HCl values and their variations. The discussion is focussed on a few well identified snow layers. It is concluded that sublimation and recrystallization of snow probably play a major role in the evolution ofX'_HCl, but that our understanding of the HCl transfer function is very incomplete. Laboratory and field measurements are suggested to improve this situation.  相似文献   

Numerical modeling of the Amazon River plume     

Oumarou Nikiema  Jean-Luc Devenon  Malika Baklouti 《Continental Shelf Research》2007

Marine circulation above the northern Brazilian continental shelf is subject to energetic forcing factors of various origins: high water buoyancy fluxes induced by the Amazon River freshwater discharge, a strong coastal current associated with a mesoscale current (North Brazil Current (NBC)), a forcing by semidiurnal tide and by Northeast or Southeast trade winds according to the season. Using a three-dimensional (3-D) hydrodynamic numerical model (MOBEEHDYCS), and realistic bathymetry and coastline of the northern Brazilian shelf, this paper aims at studying the influence of some specific physical processes on the morphology of the Amazon plume. The very large volume discharge (180 000 m³/s on average) and the weak effect of Coriolis force are additional characteristics of the studied system, which induce a particular dynamics. The various forcing factors are successively introduced into the model in order to simulate and to determine their respective influences upon the plume extent and the hydrodynamics at the shelf scale. Simulation reveal that the coastal current is at the origin of the permanent northwestward Amazon plume extension while wind effect can either reinforce or moderate this situation. The tide intervenes also to modify the position of the salinity front: a horizontal migration of salinity front is observed under its action.  相似文献   

Foreword     

Jeffrey Johnson  Pablo SamaniegoMinard L. Hall  Jean-Luc Le Pennec 《Journal of Volcanology and Geothermal Research》2008

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Salt crust development in paddy fields owing to soil evaporation and drainage: Contribution of chloride and deuterium profile analysis     

Olivier Grünberger  Peggy Macaigne  Jean-Luc Michelot  Christian Hartmann  Somsak Sukchan 《Journal of Hydrology》2008,348(1-2):110-123

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Potential and limits of double-difference tomographic methods     

Jean-Luc Got  Vadim Monteiller  Jean Virieux  Stéphane Operto 《Geophysical Prospecting》2008,56(4):477-491

Double-difference tomographic methods use directly accurate time delays computed between similar signals. Such methods are designed to image very heterogeneous media, such as volcanoes or fault zones. In seismological applications, similar signals are recorded at a given station from earthquakes sharing similar and close-by sources. In seismic exploration experiments, similar signals are often recorded at neighbouring receivers. After a brief presentation of the tomographic algorithm used, a seismological application is summarized. The potential and limits of double-difference tomographic methods are explored using various numerical experiments. They show that two effects are competing in double-difference tomography: (i) the degradation of the stability of the inversion due to the geometrical proximity of the rays used in the differentiation and (ii) the decrease in modelling error, which allows improving the stability of the inversion and using smaller quantities of a priori information when data are sufficiently accurate. The best resolution is obtained for an optimal value of the inter-source or inter-receiver distance. For optimal values of these distances and a priori information, tomography using traveltime differences provides significantly better resolved results than using traveltimes.  相似文献   

Changes in vegetation cover on the Younghusband Peninsula transgressive dunefields (Australia) 1949–2017     

Martim A.B. Moulton  Patrick A. Hesp  Graziela Miot da Silva  Camille Bouchez  Muriel Lavy  Guilherme B. Fernandez 《地球表面变化过程与地形》2019,44(2):459-470

Studies have shown that the impact of climate change, human and animal actions on coastal vegetation can turn stabilized dunes into active mobile dunes and vice versa. Yet, the driving factors that trigger vegetation changes in coastal dunes are still not fully understood. In the transgressive dunefields of the Younghusband Peninsula (south-east coast of South Australia) historical aerial photographs show an increase in vegetation cover over the last ~70 years. This study attempts to identify the causes of the changes in vegetation cover (1949 to 2017) observed in a typical section of the coastal dune systems of the Peninsula. Vegetation cover was first estimated for various years using the available historical aerial photography (long-term changes – 1949 to 2017) and recent satellite imagery (short-term annual changes – 2010 to 2017) for the area, and then results were discussed against the observed changes in climatic variables and rabbit density, factors that could have played a role in this transformation. Results of long-term changes show that the vegetation cover has increased significantly from 1949 to 2017, from less than 7% vegetation cover to almost 40%, increasing dune stabilization and forming parabolic dune systems. Periods with the largest growth in vegetation cover (1952-1956 and 2009-2013) coincide with a significant decline in rabbit numbers. Rabbit density was found to be the primary factor linked to the rapid vegetation growth and stabilization of the dunefield, for both decadal long-term (last 68 years) and annual short-term changes (last 8 years). Other factors such as changes in rainfall, aeolian sediment transport, land use practices, and the introduction of invasive plants have apparently played a limited to negligible role in this stabilization process. © 2018 John Wiley & Sons, Ltd.  相似文献   

Evolution of the Upper Rhone River discharge and suspended sediment load during the last 80 years and some implications for Lake Geneva   总被引：1，自引：0，他引：1  

Jean-Luc Loizeau  Janusz Dominik 《Aquatic Sciences - Research Across Boundaries》2000,62(1):54-67

: During this century, the characteristics of the Upper Rhone River discharge, flowing into Lake Geneva, have been altered in response to the changes which have occurred in its watershed. Principally, numerous hydroelectric dams have been constructed on the course of the Rhone River tributaries. At present the major reservoirs can hold about 122010⁶m³ of water, which represents 1/5 of the total annual Rhone River flow. Flow regime characteristics of the river have been modified by the dam operations. Water is released from reservoirs during winter and stored in summer. Large floods have also been reduced in amplitude and frequency. From the available literature data, sediment rating curves have been calculated and used to estimate the evolution of the sediment load from the Rhone River to Lake Geneva. They show that sediment input has decreased by at least a factor 2. This reduction has impacted the occurrence of underflows along the lake bottom, due to the reduction of sediment-laden floods, which in turn may have considerable negative effects on the reoxygenation of Lake Geneva deep waters.  相似文献   

Terrace reconstruction and long profile projection: a case study from the Solent river system near Southampton,England     

Rebecca M. Briant  Martin R. Bates  Gilbert D. Marshall  Jean-Luc Schwenninger  Francis F. Wenban-Smith 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(3):438-449

River terrace sequences are important frameworks for archaeological evidence and as such it is important to produce robust correlations between what are often fragmentary remnants of ancient terraces. This paper examines both conceptual and practical issues related to such correlations, using a case study from the eastern part of the former Solent River system near Southampton, England. In this region two recent terrace schemes have been constructed using different data to describe the terrace deposits: one based mainly on terrace surfaces; the other on gravel thicknesses, often not recording the terrace surface itself. The utility of each of these types of data in terrace correlation is discussed in relation to the complexity of the record, the probability of post-depositional alteration of surface sediments and comparison of straight-line projections with modern river long profiles. Correlation using age estimates is also discussed, in relation to optically stimulated luminescence dating of sand lenses within terrace gravels in this region during the PASHCC project. It is concluded that the need for replication at single sites means that this approach has limited use for correlative purposes, although dating of sediments is important for understanding wider landscape evolution and patterns of human occupation.  相似文献