Application of a mixing-ratios based formulation to model mixing-driven dissolution experiments

We address the question of how one can combine theoretical and numerical modeling approaches with limited measurements from laboratory flow cell experiments to realistically quantify salient features of complex mixing-driven multicomponent reactive transport problems in porous media. Flow cells are commonly used to examine processes affecting reactive transport through porous media, under controlled conditions. An advantage of flow cells is their suitability for relatively fast and reliable experiments, although measuring spatial distributions of a state variable within the cell is often difficult. In general, fluid is sampled only at the flow cell outlet, and concentration measurements are usually interpreted in terms of integrated reaction rates. In reactive transport problems, however, the spatial distribution of the reaction rates within the cell might be more important than the bulk integrated value. Recent advances in theoretical and numerical modeling of complex reactive transport problems [De Simoni M, Carrera J, Sanchez-Vila X, Guadagnini A. A procedure for the solution of multicomponent reactive transport problems. Water Resour Res 2005;41:W11410. doi: 10.1029/2005WR004056, De Simoni M, Sanchez-Vila X, Carrera J, Saaltink MW. A mixing ratios-based formulation for multicomponent reactive transport. Water Resour Res 2007;43:W07419. doi: 10.1029/2006WR005256] result in a methodology conducive to a simple exact expression for the space–time distribution of reaction rates in the presence of homogeneous or heterogeneous reactions in chemical equilibrium. The key points of the methodology are that a general reactive transport problem, involving a relatively high number of chemical species, can be formulated in terms of a set of decoupled partial differential equations, and the amount of reactants evolving into products depends on the rate at which solutions mix. The main objective of the current study is to show how this methodology can be used in conjunction with laboratory experiments to properly describe the key processes that occur in a complex, geochemically-active system under chemical equilibrium conditions. We model three CaCO₃ dissolution experiments reported in Singurindy et al. [Singurindy O, Berkowitz B, Lowell RP. Carbonate dissolution and precipitation in coastal environments: Laboratory analysis and theoretical consideration. Water Resour Res 2004;40:W04401. doi: 10.1029/2003WR002651, Singurindy O, Berkowitz B, Lowell RP. Correction to Carbonate dissolution and precipitation in coastal environments: laboratory analysis and theoretical consideration. Water Resour Res 2005;41:W11701. doi: 10.1029/2005WR004433], in which saltwater and freshwater were mixed in different proportions. The integrated reaction rate within the cell estimated from the experiments are modeled independently by means of (a) a state-of-the-art reactive transport code, and (b) the uncoupled methodology of [12, 13], both of which use dispersivity as a single, adjustable parameter. The good agreement between the results from both methodologies demonstrates the feasibility of using simple solutions to design and analyze laboratory experiments involving complex geochemical problems.     

Architecture of the deep critical zone in the Río Icacos watershed (Luquillo Critical Zone Observatory,Puerto Rico) inferred from drilling and ground penetrating radar (GPR)

In the critical zone, surficial bedrock interactions result in the formation of a mantle of chemically‐ and physically‐altered material defined here as regolith. In the watershed of the Río Icacos, an upland river draining the Luquillo Mountains in tropical Puerto Rico, we explored the influence of lithology (quartz diorite versus hornfels‐facies volcaniclastic rock) on weathering. Regolith profiles were studied by drilling boreholes and imaging the subsurface using ground penetrating radar (GPR). Overall, the regolith structure is not laterally continuous but rather is punctuated by zones of deep fractures that host in situ weathering, corestones, and colluvial material. GPR images of these vertical zones show reflectors at 15–20 m depth. Thus, the architecture of the critical zone in the upper Luquillo Mountains is highly dependent on lithology and its influence on fracture development. At the highest elevations where hornfels overlies quartz diorite, positive feedbacks occur when the water table drops so that oxidative weathering of biotite in the more felsic rock creates microfractures and allows deeper infiltration of meteoric waters. Such exposure results in some of the fastest weathering rocks in the world and may contribute to formation of the knickpoint in the Río Icacos watershed. This work represents the first study combining GPR and drilling to look at the structure of the deep critical zone and demonstrates: (1) the importance of combining direct methods (such as drilling) with indirect methods (such as GPR) to understand the architecture of the critical zone in tropical systems; (2) the interplay of the surficial stress regime, lithology and climate in dictating the architecture of weathering. Copyright © 2016 John Wiley & Sons, Ltd.     

Interaction between a surface quasi-geostrophic buoyancy filament and an internal vortex

This paper focuses on the nonlinear interaction between a surface quasi-geostrophic buoyancy filament and an internal vortex. We first revisit the stability of an isolated buoyancy filament. The buoyancy profile considered is continuous and leads to a continuous velocity field, albeit one with infinite shear just outside its edge. The stability properties of an isolated filament help to interpret the unsteady interaction with a sub-surface (internal) vortex studied next. We find that, in all cases, the filament breaks into billows, analogous in form to those occurring in Kelvin–Helmholtz shear instability. For intense buoyancy filaments, the vortex itself may undergo strong deformations, including being split into several pieces. Generally, the nonlinear interaction causes both the filament and the vortex to lose their respective “self”-energies to the energy of interaction. The flow evolution depends sensitively on whether the vertical vorticity of the filament and the vortex have the same or opposite signs – termed “cooperative” and “adverse” shear respectively. In cooperative shear, the filament rolls up into a coherent surface eddy above a vortex initially placed below it, whereas in adverse shear, buoyancy is expelled above the vortex. Although sufficiently great shear induced by the buoyancy filament may split the vortex in both cases, adverse shear is significantly more destructive.     

Analysis of Radionuclide Releases from the Fukushima Dai-ichi Nuclear Power Plant Accident Part II

The present part of the publication (Part II) deals with long range dispersion of radionuclides emitted into the atmosphere during the Fukushima Dai-ichi accident that occurred after the March 11, 2011 tsunami. The first part (Part I) is dedicated to the accident features relying on radionuclide detections performed by monitoring stations of the Comprehensive Nuclear Test Ban Treaty Organization network. In this study, the emissions of the three fission products Cs-137, I-131 and Xe-133 are investigated. Regarding Xe-133, the total release is estimated to be of the order of 6 × 10¹⁸ Bq emitted during the explosions of units 1, 2 and 3. The total source term estimated gives a fraction of core inventory of about 8 × 10¹⁸ Bq at the time of reactors shutdown. This result suggests that at least 80 % of the core inventory has been released into the atmosphere and indicates a broad meltdown of reactor cores. Total atmospheric releases of Cs-137 and I-131 aerosols are estimated to be 10¹⁶ and 10¹⁷ Bq, respectively. By neglecting gas/particulate conversion phenomena, the total release of I-131 (gas + aerosol) could be estimated to be 4 × 10¹⁷ Bq. Atmospheric transport simulations suggest that the main air emissions have occurred during the events of March 14, 2011 (UTC) and that no major release occurred after March 23. The radioactivity emitted into the atmosphere could represent 10 % of the Chernobyl accident releases for I-131 and Cs-137.     

Air masses and aerosols chemical components in the free troposphere at the subtropical northeast atlantic region

The dynamics and the aerosol chemistry of the air masses reaching the free troposphere of the subtropical Northeast Atlantic region during the period 1995–98 have been studied. Seven days backward trajectories were calculated daily with HYSPLIT-4 model for Izaña Global Atmospheric Watch (GAW) Observatory (28.3^°N 16.5^°W, 2367 m a.s.l.). These back-trajectories were classified by means of a k-means clustering strategy. The daily air masses have been coded using 16 variables to detect the aerosol load of each one of them. Four clusters were found: Cluster 1, representative of Atlantic oceanic middle troposphere air masses, (OMT), has an average frequency of occurrence of 50.6%. Cluster 2, which includes air masses originated in the African continent (AfD), has been recorded in a 19.8% of time. Cluster 3 represents a mixture at least of two of the next sources: Europe, Africa and Ocean, (EAM), with a frequency of 12.7%. Finally, Cluster 4 includes air masses with a high load of maritime aerosols, (MaA), and it has been detected in a 16.9%. An analysis of four aerosol components: NO₃ ^?, NH₄ ⁺, non-sea-salt-SO₄ ^2?, and mineral dust and its relation with the origin and transport of the air masses have been done. The highest quantities of mineral dust and nss-SO₄ ^2? are linked with African air masses with a mean value of 86.5 and 1.9 μg/m³ respectively. Whereas the highest levels of NO₃ ^?, 1.0 μg/m³, and NH₄ ⁺, 0.4 μg/m³, were obtained for AfD and EAM. The lowest levels were associated with OMT and MaA air masses types: 12.7, 0.6, 0.2, and 0.5 μg/m³ for dust, NO₃ ^?, NH₄ ⁺, and nss-SO₄ ^2? in average for the four studied years. However, it is remarkable that the values of the median for dust are 2.2 and 3.5 μg/m³ in clusters MaA and OMT respectively. Using non-parametric statistical tests the distributions of concentrations in each cluster by year have been compared in order to detect similarities. The results show that the aerosol loads of OMT and MaA air masses are quite similar and the same occurs for AfD and EAM air masses. However, the correlation analysis between the levels of anions and ammonium evidenced important differences among the air mass types. In AfD air masses is clear a low correlation between levels of nss-SO₄ ^2? and NH₄ ⁺ (r ² = 0.08) suggesting that the sulfate speciation was dominated by sulfate species others than ammonium sulfate, such as calcium sulfate. CaSO₄ ?2H₂O (gypsum) is mainly present in the coarse mode, where the radiative effects of sulfate are less important that in the accumulative mode. For OMT air masses is noticeable an important increasing on the correlation between the levels of anions and those of NH₄ ⁺ for the two last years of the study period (1997–1998, r ² = 0.61 –0.85%) with respect to the first ones (1995–1996, r ² = 0.25–0.49%), coinciding with the second strongest ENSO (El Niño Southern Oscillation) event recorded. This behavior indicates a change in the speciation of the aerosol component.     

Dynamical linkage of tropical and subtropical weather systems to the intraseasonal oscillations of the Indian summer monsoon rainfall. Part II: Simulations in the ENSEMBLES project

We assess the ability of individual models (single-model ensembles) and the multi-model ensemble (MME) in the European Union-funded ENSEMBLES project to simulate the intraseasonal oscillations (ISOs; specifically in 10–20-day and 30–50-day frequency bands) of the Indian summer monsoon rainfall (ISMR) over the Western Ghats (WG) and the Bay of Bengal (BoB), respectively. This assessment is made on the basis of the dynamical linkages identified from the analysis of observations in a companion study to this work. In general, all models show reasonable skill in simulating the active and break cycles of the 30–50-day ISOs over the Indian summer monsoon region. This skill is closely associated with the proper reproduction of both the northward propagation of the intertropical convergence zone (ITCZ) and the variations of monsoon circulation in this band. However, the models do not manage to correctly simulate the eastward propagation of the 30–50-day ISOs in the western/central tropical Pacific and the eastward extension of the ITCZ in a northwest to southeast tilt. This limitation is closely associated with a limited capacity of models to accurately reproduce the magnitudes of intraseasonal anomalies of both the ITCZ in the Asian tropical summer monsoon regions and trade winds in the tropical Pacific. Poor reproduction of the activity of the western Pacific subtropical high on intraseasonal time scales also amplify this limitation. Conversely, the models make good reproduction of the WG 10–20-day ISOs. This success is closely related to good performance of the models in the representation of the northward propagation of the ITCZ, which is partially promoted by local air–sea interactions in the Indian Ocean in this higher-frequency band. Although the feature of westward propagation is generally represented in the simulated BoB 10–20-day ISOs, the air–sea interactions in the Indian Ocean are spuriously active in the models. This leads to active WG rainfall, which is not present in the observed BoB 10–20-day ISOs. Further analysis indicates that the intraseasonal variability of the ISMR is generally underrepresented in the simulations. Skill of the MME in seasonal ISMR forecasting is strongly dependent on individual model performance. Therefore, in order to improve the model skill with respect to seasonal ISMR forecasting, we suggest it is necessary to better represent the robust dynamical links between the ISOs and the relevant circulation variations, as well as the proportion of intraseasonal variability in the individual models.     

Baossi–Warack monogenetic volcanoes,Adamawa Plateau,Cameroon: petrography,mineralogy and geochemistry

Acta Geochimica - Three monogenetic cones in the Baossi–Warack area, Ngaoundéré, Adamawa Plateau forming part of the Cameroon Volcanic Line (CVL) are documented in this study....     

Gaussian Process Emulators for Computer Experiments with Inequality Constraints

Physical phenomena are observed in many fields (science and engineering) and are often studied by time-consuming computer codes. These codes are analyzed with statistical models, often called emulators. In many situations, the physical system (computer model output) may be known to satisfy inequality constraints with respect to some or all input variables. The aim is to build a model capable of incorporating both data interpolation and inequality constraints into a Gaussian process emulator. By using a functional decomposition, a finite-dimensional approximation of Gaussian processes such that all conditional simulations satisfy the inequality constraints in the entire domain is proposed. To show the performance of the proposed model, some conditional simulations with inequality constraints such as boundedness, monotonicity or convexity conditions in one and two dimensions are given. A simulation study to investigate the efficiency of the method in terms of prediction is included.     

Influence of hydro-sedimentary factors on mollusc death assemblages in a temperate mixed tide-and-wave dominated coastal environment: Implications for the fossil record

Mollusc death assemblages were recovered in 98 subtidal sampling stations on the seafloor of the shallow Pertuis Charentais Sea (Atlantic coast of France). Taxonomic composition and spatial distribution of death assemblages were investigated, as well as their response to sediment grain size (field data), bottom shear stress (coupled tide and wave hydrodynamic modelling), and sediment budget (bathymetric difference map). Results showed that molluscs are likely to be reliable paleoenvironmental indicators since death assemblages were able to acquire ecological changes within years (decadal-scale taphonomic inertia), and live–dead agreement inferred from existing data on living benthic communities was high, except close to river mouths and intertidal mudflats that provide terrestrial and intertidal species to subtidal death assemblages, respectively. Taxonomic composition of these within-habitat death assemblages strongly depended on sediment grain size and bottom shear stress, similarly to living subtidal communities. Post-mortem dispersal of shells, owing to relatively low bottom shear stress in the area, was only of a few 10s to 100s of meters, which shows that death assemblages preserved environmental gradients even at a fine spatial scale. Sediment budget had also a significant influence on death assemblages. Thick-shelled epifaunal species were correlated with erosion areas on one side, and thin-shelled infaunal species with deposition on the other, showing that mollusc fossil assemblages could be used as indicators of paleo-sedimentation rate. This new proxy was successfully tested on a previously published Holocene mollusc fossil record from the same area. It was possible to refine the paleoenvironmental interpretation already proposed, in accordance with existing stratigraphic and sedimentological data.