Methane release and the carbon cycle on Mars

It has been suggested that the present release rate of methane to the Martian atmosphere could be the result of serpentinization in the deep subsurface, followed by the conversion of H₂ to CH₄ in a CO₂-rich fluid. Making this assumption, we show that the cryosphere could act as a buffer storing, under the form of micron-size methane clathrate particles, the methane delivered from below by hydrothermal fluids and progressively releasing it to the atmosphere at the top. From an extrapolation of the present CH₄ release rate back to the past, we calculate that up to several hundred millibars (～200–2000 mbar) of CO₂, resulting from the oxidation of the released CH₄, in addition to the volcanic supply (～400 mbar), should have accumulated in the atmosphere in the absence of a CO₂ sink. We reassess the capability of escape to have removed CO₂ from the atmosphere by C non-thermal escape and show that it is not significant. We suggest that atmospheric carbon is recycled to the crust through an active subsurface hydrological system, and precipitates as carbonates within the crust. During episodic periods of magmatic activity, these carbonates are decomposed to CO₂ dissolved in running water, and CO₂ can react with H₂ formed by serpentinization to build CH₄. CH₄ is then buffered in the subsurface cryosphere, above the water table, and finally released to the atmosphere, before being recycled to the subsurface hydrological system, and converted back to carbonates. We propose a typical evolution curve of the CO₂ pressure since the late Noachian based on our hypothesis. Contrary to the steady state carbon cycle at work on Earth, a progressive damping of the carbon cycle occurs on Mars due to the absence of plate tectonics and the progressive cooling of the planet.     

A data‐mining approach for assessing consistency between multiple representations in spatial databases

When different spatial databases are combined, an important issue is the identification of inconsistencies between data. Quite often, representations of the same geographical entities in databases are different and reflect different points of view. In order to fully take advantage of these differences when object instances are associated, a key issue is to determine whether the differences are normal, i.e. explained by the database specifications, or if they are due to erroneous or outdated data in one database. In this paper, we propose a knowledge‐based approach to partially automate the consistency assessment between multiple representations of data. The inconsistency detection is viewed as a knowledge‐acquisition problem, the source of knowledge being the data. The consistency assessment is carried out by applying a proposed method called MECO. This method is itself parameterized by some domain knowledge obtained from a second method called MACO. MACO supports two approaches (direct or indirect) to perform the knowledge acquisition using data‐mining techniques. In particular, a supervised learning approach is defined to automate the knowledge acquisition so as to drastically reduce the human‐domain expert's work. Thanks to this approach, the knowledge‐acquisition process is sped up and less expert‐dependent. Training examples are obtained automatically upon completion of the spatial data matching. Knowledge extraction from data following this bottom‐up approach is particularly useful, since the database specifications are generally complex, difficult to analyse, and manually encoded. Such a data‐driven process also sheds some light on the gap between textual specifications and those actually used to produce the data. The methodology is illustrated and experimentally validated by comparing geometrical representations and attribute values of different vector spatial databases. The advantages and limits of such partially automatic approaches are discussed, and some future works are suggested.     

Mytilus californianus as an indicator of heavy metals on the northwest coast of Baja California, Mexico.

Samples of Mytilus californianus were analyzed to determine the spatial and temporal distribution of Ag, Al, Cd, Cr, Cu, Mn, Pb and Zn concentrations on the northwest coast of Baja California and to compare the results with levels reported for California, USA. The samples were collected at eight sites from the US-Mexican border to San Quintín, BC (300 km south of the border) in February and July 1982 and 1994. During both years, north-south gradients in the concentration of Ag and Pb were observed, with the highest values close to the US-Mexican border (Ag = 0.60-1.54 and Pb = 0.21-2.89 micrograms g-1) and the lowest in the southern part of the study area (Ag = 0.05-0.10 and Pb = not detectable-0.03 microgram g-1). The Ag and Pb gradients suggest that their distributions are affected by anthropogenic activities close to the US-Mexican border. South-north gradients in the concentration of Cd (minimum = 0.59-2.16 and maximum = 2.61-12.93 micrograms g-1), for both 1982 and 1994, were observed. The geographic annual means of Pb for California (average of six stations along the entire state) were significantly higher than those of this study (average of the eight sampling sites) in 1982 and 1994. Cu in California was significantly greater than that of this study only in 1994. Cluster analyses on all metals indicate three similar geographic zones in 1982 and 1994, suggesting a relative consistency in the processes that determine the spatial variability of the concentration of some metals in M. californianus on the northwest coast of Baja California.     

Tectonic and seismic conditions for changes in spring discharge along the Garni right lateral strike slip fault (Armenian Upland)

Abstract

This study presents the relationship between local seismicity and springs discharge, in which the microseismicity modifies the state of stress of the aquifer matrix and these modifications of the local stress control the aquifer behaviour from an example of a basaltic reservoir crossed by a seismic fault.

Along the right-lateral Garni seismic wrench fault, the distribution in space and time of more than 500 micro-earthquakes has been studied providing a pattern of stress distribution. Together with this tectonic survey, the hydrographs of springs issuing from an aquifer located in basaltic lava flows and crossed by the fault have been gauged for 4 years. According to its tectonic, geologic and hydrodynamic properties, the reservoir has been divided in to several parts. The hydrological behaviour of the reservoir and of each of its parts has been modelled, based on the rainfall and hydrodynamic properties of the basaltic reservoir and of nearby reservoirs.

The model allows us to define the ‘normal behaviour’ of the springs, when their discharge is not affected by an earthquake. Anomalies to the normal hydrometric curves are defined, and correlated to small (M < 3.5) earthquakes along the Garni fault. We propose that the circulation of underground water in the area around the fault depends largely on the fracture pattern of the basalts and the aquifer basement, as pointed out from field observations. Changes in spring discharges are explained by variations in the state of stress around the Garni fault, induced by the alternation of elastic strain and stick-slip movement on the fault, and by creep far from the fault. A model of stress distribution is proposed which explains changes in fracture charateristics before and after earthquakes. © Elsevier, Paris     

Life in the sublittoral zone of long-lived Lake Pannon: paleontological analysis of the Upper Miocene Szák Formation,Hungary

Life and depositional environments in the sublittoral zone of Lake Pannon, a large, brackish Paratethyan lake from the Late Miocene, were reconstructed from fossils and facies of the Szák Formation. This formation is exposed in several, roughly coeval (9.4–8.9 Ma) outcrops, located along strike of the paleo-shelf-break in northwestern Hungary. The silty argillaceous marl of the formation was deposited below storm wave base, at 20–30 to 80–90 m water depth. The abundance of benthic organisms indicates that the bottom water was usually well oxygenated. Interstitial dysoxia, however, may have occurred immediately below the sediment–water interface, as evidenced by occasional preservation of trace fossils such as Diplocraterion. The fauna comprised endemic mollusks, including brackish cockles of the subfamily Lymnocardiinae, dreissenid mussels (Congeria), and highly adapted, uniquely large-sized deep-water pulmonate snails (planorbids and lymnaeids). Ostracods were dominated by endemic species and, in some cases, endemic genera of candonids, leptocytherids, cypridids, and loxoconchids. Fish remnants include a sciaenid otolith and the oldest skeletal occurrence of Perca in Europe. The phytoplankton comprised exclusively endemic coccolithophorids, mostly endemic dinoflagellates (prevailingly Spiniferites), and cosmopolitan green algae. The Late Miocene fauna and flora of Lake Pannon were in many ways similar to the modern Caspian biota, and in particular cases can be regarded as its precursor.     

Polybrominated diphenyl ethers and their methoxylated analogs in mullet (Mugil cephalus) and sea bass (Dicentrarchus labrax) from Bizerte Lagoon, Tunisia

Concentrations of ten polybrominated diphenyl ethers (PBDEs) and eight methoxylated polybrominated diphenyl ethers (MeO-PBDEs) in mullet (Mugil cephalus) and sea bass (Dicentrarchus labrax) collected from the Bizerte Lagoon and the Mediterranean Sea were investigated. To the best of our knowledge, this is the first report of these compounds in marine fishes from Tunisia. The PBDE mean concentrations in fish from Bizerte Lagoon were 45.3 and 96.2 ng g(-1) lw respectively in mullet and sea bass, while the concentrations of these compounds in mullet and sea bass from Mediterranean Sea were 7.80 and 27.9 ng g(-1) lw respectively. MeO-PBDE concentrations in mullet and sea bass from Bizerte Lagoon ranged from 6.46 to 286 ng g(-1) lw and from 49.4 to 798 ng g(-1) lw respectively, while the concentrations of these compounds in mullet and sea bass from Mediterranean Sea ranged from 190 to 401 ng g(-1) lw and from 353 to 578 ng g(-1) lw respectively. The total PBDEs and total MeO-PBDEs concentration in fish from Bizerte Lagoon were similar or slightly lower than those reported for other species from other locations around the world.     

Subfilter-scale Fluxes over a Surface Roughness Transition. Part I: Measured Fluxes and Energy Transfer Rates

A wind-tunnel experiment was designed and carried out to study the effect of a surface roughness transition on subfilter-scale (SFS) physics in a turbulent boundary layer. Specifically, subfilter-scale stresses are evaluated that require parameterizations and are key to improving the accuracy of large-eddy simulations of the atmospheric boundary layer. The surface transition considered in this study consists of a sharp change from a rough, wire-mesh covered surface to a smooth surface. The resulting magnitude jump in aerodynamic roughnesses, M = ln(z ₀₁/z ₀₂), where z ₀₁ and z ₀₂ are the upwind and downwind aerodynamic surface roughnesses respectively, is similar to that of past experimental studies in the atmospheric boundary layer. The two-dimensional velocity fields used in this study are measured using particle image velocimetry and are acquired at several positions downwind of the roughness transition as well as over a homogeneous smooth surface. Results show that the SFS stress, resolved strain rate and SFS transfer rate of resolved kinetic energy are dependent on the position within the boundary layer relative to the surface roughness transition. A mismatch is found in the downwind trend of the SFS stress and resolved strain rate with distance from the transition. This difference of behaviour may not be captured by some eddy-viscosity type models that parameterize the SFS stress tensor as proportional to the resolved strain rate tensor. These results can be used as a benchmark to test the ability of existing and new SFS models to capture the spatial variability SFS physics associated with surface roughness heterogeneities.