Variability of the gas transfer velocity of CO<Subscript>2</Subscript> in a macrotidal estuary (the Scheldt)

We report a large set of 295 interfacial carbon dioxide (CO₂) flux measurements obtained in the Scheldt estuary in November 2002 and April 2003, using the floating chamber method. From concomitant measurements of the air-water CO₂ gradient, we computed the gas transfer velocity of CO₂. The gas transfer velocity is well correlated to wind speed and a simple linear regression function gives the most consistent fit to the data. Based on water current measurements, we estimated the contribution of water current induced turbulence to the gas transfer velocity, using the conceptual relationship of O'Connor and Dobbins (1958). This allowed us to construct an empirical relationship to compute the gas transfer velocity of CO₂ that accounts for the contribution of wind and water current. Based on this relationship, the spatial and temporal variability of the gas transfer velocity in the Scheldt estuary was investigated. Water currents contribute significantly to the gas transfer velocity, but the spatial and temporal variability (from daily to seasonal scales) is mainly related to wind speed variability.     

大别山的构造变形期次和超高压岩石折返的动力学

从构造关系上看,大别山可分为南部,中部和北部3个构造系统,大别山南部构造系统为一套（原地）构造堆叠系统（相对的）,从上到下依次为：（1）未变质的震旦纪一早三叠世沉积盖层,它被造山后的侏罗－白垩纪砂－砾岩不整合覆盖;（2）弱变质的新元古代板岩;（3）高压变质岩系（宿松群）,这些变质岩系大多退变质为绿片岩相;（4）含柯石英榴辉岩超高压变质单元的异地系统;（5）未经历超高压变质作用的原地（相对的）片麻岩系。北西－南东向的拉伸线理在各个地质单元均有表现。从动力学上看,上盘指向NW的剪切运动被后期的褶皱所改变,大别山中部为（混合岩）热穿窿改造系统,系经历了超高压变质作用的地体叠加了混合岩化作用的产物,它与超高压变质地体的界限为一拆离断层,而正是这个拆离断层使超高压变质地体向地表折返并经历了角闪岩相的退变质作用。大别山北部为构造堆叠系统,早期的面理和南北向的挤压线理被轴面北倾的褶皱所改造,这种褶皱对应于大别山中部同折返期的韧性变形的隆升构造。同时建立了地球运力学模型并讨论了逆冲作用和正断层的作用。     

Calcification rate influence on trace element concentrations in aragonitic bivalve shells: Evidences and mechanisms

Trace elements in calcareous organisms have been widely used for paleoclimatic studies. However, the factors controlling their incorporation into mollusc shells are still unclear. We studied here the Sr, Mg, Ba and Mn serial records in the shells of two aragonitic marine bivalve species: Mesodesma donacium and Chione subrugosa from the Peruvian Coast. The elemental concentrations were compared to local temperature and salinity records. The relationships with crystal growth rate G were investigated thanks to well defined periodic growth structures providing a precise shell chronology. Our results show that for both species, environmental parameters only have minor influence, whereas crystal growth rate strongly influences trace elements concentrations, especially for Sr (explaining up to 74% of the variance). The relationship between G and Sr/Ca exhibits variability among the shells as well as inside the shells. For a same growth rate value, Sr/Ca values are higher in more curved shell sections, and the growth rate influence is stronger as well. We show that intercellular and Ca²⁺-pump pathways cannot support the calcification Ca²⁺ flux, leading us to propose an alternative mechanism for ionic transport through the calcifying mantle, implying a major role for calcium channels on mantle epithelial cell membranes. In this new calcification model, Sr/Ca shell ratios is determined by Ca²⁺-channel selectivity against Sr²⁺, which depends (i) on the electrochemical potential imposed by the crystallisation process and (ii) on the Ca²⁺-channel density per surface unit on mantle epithelia.     

Satellite-Derived ERS scatterometer sea-surface wind-stress curl in the southwestern Indian Ocean

Surface marine wind data base from ERS-1 scatterometer has been processed to define the characteristic seasonal distribution of wind-stress curl throughout the southwestern Indian Ocean between January and December 1994. A compact model to obtain satellite-derived wind-stress curl fields on smaller scales than previously available is proposed and evaluated. The results indicate the strong capability of the ERS-1 scatterometer to monitor wind-driven variations in the mesoscale ocean patterns. To cite this article: M. Petit et al., C. R. Geoscience 338 (2006).     

Rhine flood deposits recorded in the Gallo-Roman site of Oedenburg (Haut-Rhin, France)

From the first to the fourth century AD, the Gallo-Roman town of Oedenburg developed in the alluvial landscape of the southern Upper Rhine Graben. Throughout this period, the landscape mosaic, composed of palaeochannels, stable palaeoislands and river terraces, continued to evolve. A district of this town, situated on a lateral Rhine channel system, was archaeologically excavated. Large-scale excavation and cross-section analysis provide evidence of changing fluvial conditions during the period under study. At about AD 20 or earlier, this lateral part of the floodplain, affected by very fine sedimentation, was occupied by moribund marshy palaeochannels. When the first Gallo-Roman settlers occupied the site, they filled parts of the channels with woven brushwood in order to create an efficient circulation surface. The sedimentary infill of this palaeochannel records four different flood deposits interlayered with dated anthropogenic units (pavements, road embankments, and other structures). Archaeological analysis and dendrochronological dating indicate that these four flooding events occurred during a short time period between AD 20 and AD 145/146. These geoarchaeological observations focus on floods that do not seem to have significantly affected human occupation in this part of the Rhine floodplain. These results are set in the broader context of the Rhine catchment and the Alps.     

Response of hurricane-type vortices to global warming as simulated by ARPEGE-Climat at high resolution

Atlantic hurricanes and their sensitivity to anthropogenic warming are investigated using very high (0.5°×0.5° over the Atlantic domain) resolution global simulations. The ARPEGE-Climat variable resolution grid demonstrates its usefulness in regional climate studies since resolution can be multiplied by a factor of 2.5 over the domain of interest compared to a uniform grid, for a similar computer cost. The question of hurricane characteristics dependence on anthropogenic warming is tackled trough the implementation of a tracking method. Changes in the total number, as well as locations, of hurricanes appear to depend more on sea surface temperature (SST) spatial patterns anomaly than Atlantic mean intensity, essentially through the change in large scale vertical wind shear. A uniform SST anomaly forcing produces increased and eastward shifted systems while a spatially contrasted anomaly leads to a decrease. Comparison between cyclogenesis density calculated from tracking or large scale combined variables (as a modified Gray parameter) brings some confidence in the use of the latter to investigate low resolution simulations. Mean hurricane dynamical characteristics are weakly changed by the warming but precipitation core and latent heat flux are enhanced in all scenarios.     

Statistical analysis of extreme events in a non-stationary context via a Bayesian framework: case study with peak-over-threshold data

Statistical analysis of extremes currently assumes that data arise from a stationary process, although such an hypothesis is not easily assessable and should therefore be considered as an uncertainty. The aim of this paper is to describe a Bayesian framework for this purpose, considering several probabilistic models (stationary, step-change and linear trend models) and four extreme values distributions (exponential, generalized Pareto, Gumbel and GEV). Prior distributions are specified by using regional prior knowledge about quantiles. Posterior distributions are used to estimate parameters, quantify the probability of models and derive a realistic frequency analysis, which takes into account estimation, distribution and stationarity uncertainties. MCMC methods are needed for this purpose, and are described in the article. Finally, an application to a POT discharge series is presented, with an analysis of both occurrence process and peak distribution.     

A mixed-dimensional finite volume method for two-phase flow in fractured porous media

We present a vertex-centered finite volume method for the fully coupled, fully implicit discretization of two-phase flow in fractured porous media. Fractures are discretely modeled as lower dimensional elements. The method works on unstructured, locally refined grids and on parallel computers with distributed memory. An implicit time discretization is employed and the nonlinear systems of equations are solved with a parallel Newton-multigrid method. Results from two-dimensional and three-dimensional simulations are presented.