A relative water-depth model for the Normandy Chalk (Cenomanian–Middle Coniacian,Paris Basin,France) based on facies patterns of metre-scale cycles

A relative water-depth model for the Chalk of the Paris Basin is proposed, based on the lateral variations of the high-frequency metre-scale cycles, which are characteristic features easily identified in the field. The studied outcrops are the Cenomanian–Middle Coniacian cliffs of Normandy. The main result of this study is to highlight the importance of storm activity in the deposition of the Chalk. The relative water-depth model is based on storm-induced shell concentrations observed within the two components of the metre-thick cycles: the depositional interval itself and the top hiatal surface.Six types of shell concentrations are defined, along with seven types of depositional facies making up the depositional units, as well as eight types of hiatal surface. Three cycle associations, differing in their thickness and the amount and type of non-carbonate constituents, can be identified in the Lower to Upper Cenomanian, the Upper Cenomanian to Lower Turonian and the Middle Turonian to Middle Coniacian.A relative water-depth profile model for all these cycles is based on the shell concentrations and a “water-depth equivalence” is proposed between the three cycle associations (lateral “facies” substitution diagram). This model is tested using palaeocological data (irregular echinoids) and by correlating field sections in terms of stacking patterns. Most of the studied deposits accumulated above the storm wave base (upper offshore zone or mid ramp).     

Early maturation processes in coal. Part 2: Reactive dynamics simulations using the ReaxFF reactive force field on Morwell Brown coal structures

This paper reports reactive dynamics (RD) simulations of a macro-model of Morwell Brown coal using the ReaxFF reactive force field. We find that these reactive MD simulations successfully reproduce thermal decomposition processes of defunctionalization, depolymerization and rearrangement of the residual structure observed in various experimental studies. For example, our simulations indicate that the decarboxylation and dehydroxylation of the lignin side chain of the Morwell model involves the formation of double bonds conjugated with the aromatic rings. The process of defunctionalization of the methoxy functions involving the formation of phenolic structures in the residue has been confirmed. We also observe that gaseous hydrocarbons are generated by cleavage of C–C bonds of the lignin side chain.The success in using ReaxFF RD to describe the molecular processes underlying the kinetics in pyrolysis of this model of coal plus the success of a similar previous study on the algaenan of Botryococcus braunii race L biopolymer model of kerogens suggests that such computation can be useful in providing molecular based kinetic models for other pyrolysis processes underlying the organic transformations in sedimentary materials.     

Anhydrite–gypsum transition in the argillites of flooded salt workings in eastern France

This study aims at understanding the physico-chemical interactions between the saturated brine and the rocks enclosing the underground salt workings in Lorraine (eastern France). These anhydrite-rich and argillaceous rocks were characterized in terms of mineralogy, micro-texture and connected porosity. Then, the two main lithofacies, massive anhydrite and anhydrite-rich argillite, were immersed in brine during more than 1 year. During this batch experiment, the argillites were affected by macroscopic splitting, contrarily to the massive anhydrite. Micro-texture and brine chemical analyses clearly show the swelling due to the hydration of anhydrite into gypsum inside the argillites, whereas hydration occurs superficially on the massive anhydrite, due to its very low permeability. Anhydrite–gypsum transformation is promoted by the presence of dissolved strontium and potassium in saturated brine. The low activity of water in saturated brine does not allow the clay fraction to swell significantly during the experiment. Thus, the expansion resulting from the hydration of anhydrite into gypsum might be responsible of the splitting of argillite in a saturated brine environment. The superficial anhydrite hydration on massive anhydrite can be explained by the low amount of connected porosity (less than 1%).     

Impact of the LMDZ atmospheric grid configuration on the climate and sensitivity of the IPSL-CM5A coupled model

The IPSL-CM5A climate model was used to perform a large number of control, historical and climate change simulations in the frame of CMIP5. The refined horizontal and vertical grid of the atmospheric component, LMDZ, constitutes a major difference compared to the previous IPSL-CM4 version used for CMIP3. From imposed-SST (Sea Surface Temperature) and coupled numerical experiments, we systematically analyze the impact of the horizontal and vertical grid resolution on the simulated climate. The refinement of the horizontal grid results in a systematic reduction of major biases in the mean tropospheric structures and SST. The mid-latitude jets, located too close to the equator with the coarsest grids, move poleward. This robust feature, is accompanied by a drying at mid-latitudes and a reduction of cold biases in mid-latitudes relative to the equator. The model was also extended to the stratosphere by increasing the number of layers on the vertical from 19 to 39 (15 in the stratosphere) and adding relevant parameterizations. The 39-layer version captures the dominant modes of the stratospheric variability and exhibits stratospheric sudden warmings. Changing either the vertical or horizontal resolution modifies the global energy balance in imposed-SST simulations by typically several W/m² which translates in the coupled atmosphere-ocean simulations into a different global-mean SST. The sensitivity is of about 1.2 K per 1 W/m² when varying the horizontal grid. A re-tuning of model parameters was thus required to restore this energy balance in the imposed-SST simulations and reduce the biases in the simulated mean surface temperature and, to some extent, latitudinal SST variations in the coupled experiments for the modern climate. The tuning hardly compensates, however, for robust biases of the coupled model. Despite the wide range of grid configurations explored and their significant impact on the present-day climate, the climate sensitivity remains essentially unchanged.     

Sudden stratospheric warmings and tropospheric blockings in a multi-century simulation of the IPSL-CM5A coupled climate model

The relation between sudden stratospheric warmings (SSWs) and blocking events is analyzed in a multi-centennial pre-industrial simulation of the Institut Pierre Simon Laplace coupled model (IPSL-CM5A), prepared for the fifth phase of the coupled model intercomparison project. The IPSL model captures a fairly realistic distribution of both SSWs and tropospheric blocking events, albeit with a tendency to overestimate the frequency of blocking in the western Pacific and underestimate it in the Euro-Atlantic sector. The 1000-year long simulation reveals statistically significant differences in blocking frequency and duration over the 40-day periods preceding and following the onset of SSWs. More specifically, there is an enhanced blocking frequency over Eurasia before SSWs, followed by an westward displacement of blocking anomalies over the Atlantic region as SSWs evolve and then decline. The frequency of blocking is reduced over the western Pacific sector during the life-cycle of SSWs, while the model simulates no significant relationship with eastern Pacific blocks. Finally, these changes in blocking frequency tend to be associated with a shift in the distribution of blocking lifetime toward longer-lasting blocking events before the onset of SSWs and shorter-lived blocks after the warmings. This study systematically verifies that the results are consistent with the two pictures that (1) blockings produce planetary scale anomalies that can force vertically propagating Rossby waves and then SSWs when the waves break and (2) SSWs affect blockings in return, for instance via the effect they have on the North Atlantic Oscillation.     

Toward an experimental synthesis of the chondritic insoluble organic matter

Based on the statistical model proposed for the molecular structure of the insoluble organic matter (IOM) isolated from the Murchison meteorite, it was recently proposed that, in the solar T‐Tauri disk regions where (photo)dissociation of gaseous molecules takes place, aromatics result from the cyclization/aromatization of short aliphatics. This hypothesis is tested in this study, with n‐alkanes being submitted to high‐frequency discharge at low pressure. The contamination issue was eliminated using deuterated precursor. IOM was formed and studied using solid‐state nuclear magnetic resonance, pyrolysis coupled to gas chromatography and mass spectrometry, RuO₄ oxidation, and high‐resolution transmission electron microscopy. It exhibits numerous similarities at the molecular level with the hydrocarbon backbone of the natural IOM, reinforcing the idea that the initial precursors of the IOM were originally chains in the gas. Moreover, a fine comparison between the chemical structure of several meteorite IOM suggests either that (i) the meteorite IOMs share a common precursor standing for the synthetic IOM or that (ii) the slight differences between the meteorite IOMs reflect differences in their environment at the time of their formation i.e., related to plasma temperature that, in turn, dictates the dissociation–recombination rates of organic fragments.     

Effect of Surface Morphology on the Dissipation During Shear and Slip Along a Rock–Rock Interface that Contains a Visco-elastic Core

High resolution topography measurements of the Vuache–Sillingy fault (Alps, France) reveal a characteristic roughness of the fault zone. We investigate the effect of roughness on the rheology of a planar shear configuration by using a model system consisting of a visco-elastic layer embedded into a rigid solid. The model is discussed in the context of several geological cases: a damage fault zone, a fault smeared with a clay layer, and a shear zone with strain weakening. Using both analytical approaches and finite element simulations, we calculate to linear order the relation between wall roughness and the viscous dissipation in the fault zone as well as the average shear rate.     

Active Deformation along the Southern End of the Tosco-Abreojos Fault System: New Insights from Multibeam Swath Bathymetry

The relative motion of the Pacific plate with respect to the North America plate is partitioned between transcurrent faults located along the western margin of Baja California and transform faults and spreading ridges in the Gulf of California. However, the amount of right lateral offset along the Baja California western margin is still debated. We revisited multibeam swath bathymetry data along the southern end of the Tosco-Abreojos fault system. In this area the depths are less than 1,000?m and allow a finer gridding at 60?m cell spacing. This improved resolution unveils several transcurrent right lateral faults offsetting the seafloor and canyons, which can be used as markers to quantify local offsets. The seafloor of the southern end of the Tosco-Abreojos fault system (south of 24°N) displays NW?CSE elongated bathymetric highs and lows, suggesting a transtensional tectonic regime associated with the formation of pull-apart basins. In such an active tectonic context, submarine canyon networks are unstable. Using the deformation rate inferred from kinematic predictions and pull-apart geometry, we suggest a minimum age for the reorganization of the canyon network.     

The M<Subscript>W</Subscript> 4.5 Vallorcine (French Alps) earthquake of 8 September 2005 and its complex aftershock sequence

On 8 September 2005 a moderate M_W 4.5 earthquake occurred in the north-western Alps midway between Chamonix (France) and Martigny (Switzerland). The focal mechanism corresponds to a right-lateral strike-slip on a N60°E fault plane. The foreshock–mainshock–aftershock sequence is investigated on the basis of data recorded by a temporary network of 28 stations deployed for 1 month just after the mainshock, and data from permanent, regional seismic networks. Absolute and relative locations of more than 400 events are obtained with a mean uncertainty of approximately 0.2 km. Small foreshocks, the mainshock, and early and late aftershocks are located relative to the main aftershock set. The seismic sequence exhibits a surprisingly complex structure, with at least five clusters on distinct fault planes. The main elongated cluster agrees with the location of the mainshock, its hypocenter being 4.3 km below sea level. We discuss the relationship between the right-lateral fault beneath the Loriaz peak (the source of the Vallorcine event), the nearby normal Remuaz fault, and the regional seismotectonic stress field.