Consistent past half-century trends in the atmosphere, the sea ice and the ocean at high southern latitudes

Simulations performed with the climate model LOVECLIM, aided with a simple data assimilation technique that forces a close matching of simulated and observed surface temperature variations, are able to reasonably reproduce the observed changes in the lower atmosphere, sea ice and ocean during the second half of the twentieth century. Although the simulated ice area slightly increases over the period 1980–2000, in agreement with observations, it decreases by 0.5 × 10⁶ km² between early 1960s and early 1980s. No direct and reliable sea ice observations are available to firmly confirm this simulated decrease, but it is consistent with the data used to constrain model evolution as well as with additional independent data in both the atmosphere and the ocean. The simulated reduction of the ice area between the early 1960s and early 1980s is similar to the one simulated over that period as a response to the increase in greenhouse gas concentrations in the atmosphere while the increase in ice area over the last decades of the twentieth century is likely due to changes in atmospheric circulation. However, the exact contribution of external forcing and internal variability in the recent changes cannot be precisely estimated from our results. Our simulations also reproduce the observed oceanic subsurface warming north of the continental shelf of the Ross Sea and the salinity decrease on the Ross Sea continental shelf. Parts of those changes are likely related to the response of the system to the external forcing. Modifications in the wind pattern, influencing the ice production/melting rates, also play a role in the simulated surface salinity decrease.     

Measurements and modelling of atmospheric pollution over the Paris area: an overview of the ESQUIF Project

The Étude et Simulation de la QUalité de lair en Ile de France (ESQUIF) project is the first integrated project dedicated to the study of the processes leading to air pollution events over the Paris area. The project was carried out over two years (summer 1998 to winter 2000) to document all types of meteorological conditions favourable to air quality degradation, and in particular to photo oxydant formation. The goals of ESQUIF are (1) to improve our understanding of the relevant chemical and dynamical processes and, in turn, improve their parametrizations in numerical models, and (2) to improve and validate existing models dedicated to pollution analysis, scenarios and/or forecasting, by establishing a comprehensive and thorough database. We present the rationale of the ESQUIF project and we describe the experimental set-up. We also report on the first experiments which took place during the summer of 1998 involving surface networks, and remote sensing instruments as well as several aircraft. Focusing on three days of August 1998, the relative contributions of long-range transported and locally-produced ozone to the elevated ozone concentrations observed during this period are discussed and chemistry-transport model preliminary results on this period are compared to measurements.     

Comparison of methods for the estimation of pyrite oxidation rate in a waste rock pile at Mine Doyon site, Quebec, Canada

Several methods were evaluated and compared for the estimation of pyrite oxidation rates (POR) in waste rock at Mine Doyon, Quebec, Canada. Methods based on data collected in situ, such as the interpretation of temperature and oxygen concentration profiles (TOP) measured in the waste rock pile and pyrite mass balance (PMB) on solid phase samples were compared with the oxygen consumption measurements (OCM) in closed chamber in the laboratory. A 1-D analytical solution to a gas and heat transport equation used temperature and oxygen profiles (TOP) measured in the pile for the preliminary POR estimates at a site close to the slope of the pile (Site 6) and in the core of the pile (Site 7). Resulting POR values were 1.1 × 10^− 9 mol(O₂) kg^− 1 s^− 1 and 1.0 × 10^− 10 mol(O₂) kg^− 1 s^− 1 for the slope site and the core site, respectively. Oxidation rates based on pyrite mass balance (PMB) calculations for solid samples were 2.21 × 10^− 9 mol(O₂) kg^− 1 s^− 1 and 2.03 × 10^− 9 mol(O₂) kg^− 1 s^− 1, respectively, for the same slope and core sites, but the difference between sites was within the error margin. The OCM measurements in the laboratory on fresh waste rock samples yielded higher POR values than field methods, with average oxidation rate of 6.7 × 10^− 8 mol(O₂) kg^− 1 s^− 1. However, the OCM results on weathered and decomposed material from the rock stockpile (average oxidation rate 3.4 × 10^− 9 mol(O₂) kg^− 1 s^− 1) were consistent with results from the field-based estimates. When POR values based on fresh material are excluded, the remaining POR values for all methods range from 1.0 × 10^− 10 to 3.4 × 10^− 9 mol(O₂) kg^− 1 s^− 1. The lowest estimated value (1.0 × 10^− 10 mol(O₂) kg^− 1 s^− 1) was based on TOP estimates in the interior of the pile where oxygen transport was limited by diffusion from the surface. These results suggest that small-scale OCM laboratory experiments may provide relatively representative values of POR in the zones of waste rock piles in which oxygen transport is not dominated by diffusion.     

Hydrogeochemistry and groundwater origin of the Basses-Laurentides sedimentary rock aquifer system, St. Lawrence Lowlands, Québec, Canada

A comprehensive hydrogeochemical study was carried out in the Paleozoic Basses-Laurentides sedimentary rock aquifer system in Québec over a 1500 km² study area. Groundwater samples were collected at 153 sites, characterizing all geological and hydrogeological units to a maximum depth of 140 m. Groundwater was analyzed for major, minor and trace inorganic constituents, stable isotopes δ ²H, δ ¹⁸O, and δ ¹³C of dissolved inorganic carbon (DIC), and some samples were analyzed for ³H, and ¹⁴C of DIC. The regional distribution of groundwater types shows that the hydrogeological conditions exert a dominant control on the major ions chemistry of groundwater. Preferential recharge areas are characterized by tritiated Ca-Mg-HCO₃ groundwater, and confined conditions by submodern Na-HCO₃ and Na-Cl groundwater types. Two groundwater end-members are identified in the aquifer system, modern meteoric water and Pleistocene Champlain Sea water. The region displays significant variations of groundwater geochemistry and quality controlled by glaciation, Champlain Sea invasion, lithological rock diversity, and flow system scales. This situation leads to varied groundwater types and origins within a restricted area.     

3D geologic framework models for regional hydrogeology and land-use management: a case study from a Quaternary basin of southwestern Quebec, Canada

During a regional hydrogeologic survey in the St. Lawrence Lowlands, Canada, a computer-based 3D Geologic Framework Model (GFM) was constructed to obtain a consistent representation of this typical Quaternary glaciated basin over a 1,400 km² area. Such a detailed stratigraphic reconstruction was needed because the Quaternary sediments control the recharge to the underlying regional fractured rock aquifer and also because buried granular aquifers are partly connected to the regional system. The objectives of this geomodeling effort are 1) to improve understanding of subsurface conditions above the regional aquifer and; 2) to provide a common stratigraphic framework for hydrogeologic applications. The method draws on knowledge-driven discrete modeling using gOcad, as well as standardization and quality control procedures to maximize the use of a multisource database. The resulting model represents the bedrock topography and the complex stratigraphic architecture of overlying sediments. The regional till aquitard, the marine clay aquiclude and the buried granular aquifers have been modeled with unprecedented details thus providing a well-constrained 3D hydrostratigraphic framework. The recharge zones of the rock aquifer represent about 35% of the study area. Buried granular aquifers are directly connected to the regional aquifer system over about 10% of the area. The model allows several applications such as assessing aquifer vulnerability and areal groundwater recharge rates; improving the GFM inter-operability with groundwater modeling systems would be the next logical step.

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Resumen Se construyó un Modelo del Marco Geológico (GFM) 3D basado en computadora durante un levantamiento hidrogeológico regional en las Tierras Bajas de St. Lawrence, Canada para obtener una representación consistente de esta cuenca glaciar Cuaternaria típica en un área de 1,400 km². Se necesitó tal grado de reconstrucción estratigráfica debido a que los sedimentos Cuaternarios controlan la recarga del acuífero rocoso fracturado regional subyacente y también porque los acuíferos granulares enterrados están parcialmente conectados con el sistema regional. Los objetivos de este esfuerzo de modelizado geológico son: 1) mejorar el entendimiento de las condiciones subsuperficiales por encima del acuífero regional y; 2) aportar un marco estratigráfico común para aplicaciones hidrogeológicas. El método se basa en el conocimiento de modelizado discreto utilizando gOcad, así como también en estandarización y procedimientos de control de calidad para maximizar el uso de bancos de datos de fuentes múltiples. El modelo obtenido representa la topografía del macizo rocoso y la arquitectura de la compleja estratigrafía de los sedimentos sobreyacentes. El acuitardo regional de tillita, el acuicludo arcilloso marino y los acuíferos granulares enterrados se han modelizado con detalles sin precedentes aportando de este modo un marco hidroestratigráfico 3D bien definido. Las zonas de recarga del acuífero rocoso representan casi el 35% del área de estudio. Los acuíferos granulares enterrados están conectados directamente al sistema acuífero regional sobre aproximadamente el 10% del área. El modelo permite varias aplicaciones tal como evaluar la vulnerabilidad de acuíferos y ritmos de recarga areales de agua subterránea; el paso lógico siguiente sería mejorar la inter-operabilidad del GFM con los sistemas de modelizado de agua subterránea.

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Résumé Dans le cadre dune étude dhydrogéologie régionale dans les basses terres du Saint-Laurent (Canada), un Modèle surfacique géologique en 3D (MSG) a été développé pour obtenir une représentation cohérente par ordinateur de ce bassin quaternaire sur une superficie de plus de 1,400 km². Une telle reconstitution stratigraphique était nécessaire en raison du contrôle quexercent les sédiments quaternaires sur la recharge de laquifère régional fracturé sous-jacent et aussi parce que les aquifères granulaires enfouis sont partiellement connectés au système régional. Les objectifs de cet effort de géomodélisation sont: 1) daméliorer la compréhension des conditions de sous-surface au-dessus de laquifère régional et; 2) de fournir un cadre stratigraphique commun à des fins de caractérisation hydrogéologique. La méthode repose sur lintégration de linterprétation géologique dans la modélisation discrète effectuée à laide du logiciel gOcad, ainsi que sur des procédures duniformisation et de contrôle de la qualité des données afin doptimiser lutilisation dune base de données multisources. Le MSG représente la topographie du roc et larchitecture stratigraphique des sédiments sus-jacents. Laquitard régional (till), laquiclude dargile marine et les aquifères granulaires ont été modélisés à un niveau de détail sans précédent, fournissant ainsi un cadre hydrostratigraphique solidement établi. Les zones de recharge de laquifère rocheux représentent environ 35% de la zone détude. Les aquifères granulaires enfouis sont connectés directement au système aquifère régional sur au moins 10% du territoire. Le MSG permet de multiples usages tels lévaluation de la vulnérabilité des aquifères et lestimation de la distribution de la recharge; la prochaine étape logique serait lamélioration de linteropérabilité avec les systèmes de modélisation numérique de lécoulement.

     

A unified hydrogeological conceptual model of the Milk River transboundary aquifer,traversing Alberta (Canada) and Montana (USA)

A conceptual model of the transboundary Milk River Aquifer (MRA), extending across the Canada–USA border, was developed based on literature, focused fieldwork and a three-dimensional geological model. The MRA corresponds to the Virgelle Member of the Milk River Formation (Eagle Formation in Montana, USA) and it is an important groundwater resource over a large area (25,000 km²). The Virgelle outcrops near the international border and along the Sweet Grass Arch in Montana. The down-gradient limit of the MRA is the unconformity separating the Virgelle from the gas-bearing sandy shale of the Alderson Member. The MRA is confined above by the Pakowki/Claggett Formations aquitards and below by the Colorado Group aquitard. The MRA contains higher transmissivity areas resulting in preferential flowpaths, confirmed by natural geochemical tracers. Tritium and ¹⁴C delineate restricted recharge areas along the outcrops on both sides of the international border. Drastic decreases in horizontal hydraulic gradients indicate that the Milk River intercepts a large proportion of groundwater flowing to the north from the recharge area. Downgradient of the Milk River, groundwater movement is slow, as shown by ³⁶Cl residence times exceeding 1 Ma. These slow velocities imply that groundwater discharge downgradient of the Milk River is via vertical leakage through the Colorado Group and upward along buried valleys, which act as drains and correspond to artesian areas. When confined, the MRA contains a fossil groundwater resource, not significantly renewed by modern recharge. Groundwater exploitation thus far exceeds recharge, a situation requiring properly managed MRA groundwater depletion.     

Laboratory Study of Polymer Solutions Used for Mobility Control During In Situ NAPL Recovery

The use of surfactant solutions for the in situ recovery of residual NAPL in aquifers is increasingly considered as a viable remediation technique. The injection of a few pore volumes of high concentration surfactant solutions can mobilize most of the residual NAPL contacted by the solutions. However, the washing solutions'physico-chemical properties (low density and high viscosity), combined with the natural porous media heterogeneity, can prevent a good sweep of the entire contaminated volume. From the petroleum industry, it is well-known that polymer solutions can be injected following a surfactant solution slug to act as a mobility buffer and increase the overall sweep efficiency. The objective of our laboratory study is first to select and characterize polymers that would be suitable for aquifer restoration. Our experiments showed that among several polymers, xanthan gum solution rheology was made in order to predict shear rates, xanthan gum concentrations, salinity, and temperature effects on solution viscosity. The second set of experiments were made with a sand box which was designed to reproduce a simple heterogeneous media consisting of layers of sand with different permeability. These tests illustrate the xanthan gum solution's ability to increase surfactant solution's sweep efficiency and limit viscous fingering. The tests established that: (1) the injection of xanthan solution behind a surfactant solution slug decreases fluid velocity in high permeability layers and increases it in low-permeability ones, thus increasing the sweep efficiency (2) xanthan solutions eliminate viscous fingering at the polymer/surfactant solution front; (3) a xanthan solution preflush is desirable to limit surfactant solution mobility and prevent surfactant adsorption on solids; and (4) depending on site heterogeneity injection strategies should be applied to limit overriding by low-density surfactant solution.     

On the interest of using field primary production data to calibrate phytoplankton rate processes in ecosystem models

In many ecosystem models based on empirical formulations, parameters generally are calibrated in order to achieve the best fit between measured and simulated chlorophyll a standing stocks. An accurate calibration of rate processes as primary production rarely is taken into account. In this paper, we test the usefulness of calibration of phytoplankton photosynthetic processes in an ecosystem model using field primary production data. We used 18 months of photosynthetic process data from the Baie des Veys ecosystem (Normandy, France). Five empirical formulations of photosynthesis–irradiance curve models amongst the most widely used were tested. In each formulation, the variability of photosynthetic parameters (i.e. the light-saturated rate of photosynthesis (P_max^B) and the initial slope of the photosynthesis–light curve (α^B)) was considered depending on environmental factors (temperature and nutrient availability). The fit of the five equations as well as the calibration of parameters on field measurements (i.e. the light-saturated rate of photosynthesis (P_ref^B), the initial slope of the photosynthesis–light curve (α_ref^B), the half-saturation constant for nitrogen (K_N) and silicates uptake (K_Si), and the coefficient in the exponential thermal effect (K_T)) was performed using the whole available data set of P vs. E curves (n = 143, P vs. E curves). Then, the Smith formulation allowing the best simulation of the Baie des Veys primary production and corresponding parameters were introduced in an ecosystem box model. This formulation led directly to a satisfactory representation of the Baie des Veys phytoplankton dynamics without additional calibration. Results obtained were compared with a more classical approach in which ecosystem models were calibrated using published values of parameters. This comparison showed that for the two years studied, annual primary production estimated through the ecosystem model was 13% and 26% higher with our approach than with the more classical approach. This work emphasizes the importance of accurately representing rate processes in ecosystem models in order to adequately simulate production as well as standing stocks.     

On the theory of the oblateness of the Sun

In this paper we first emphasize why it is important to know the successive zonal harmonics of the Sun's figure with high accuracy: mainly fundamental astrometry, helioseismology, planetary motions and relativistic effects. Then we briefly comment why the Sun appears oblate, going back to primitive definitions in order to underline some discrepancies in theories and to emphasize again the relevant hypotheses. We propose a new theoretical approach entirely based on an expansion in terms of Legendre's functions, including the differential rotation of the Sun at the surface. This permits linking the two first spherical harmonic coefficients (J ₂ and J ₄) with the geometric parameters that can be measured on the Sun (equatorial and polar radii). We emphasize the difficulties in inferring gravitational oblateness from visual measurements of the geometric oblateness, and more generally a dynamical flattening. Results are given for different observed rotational laws. It is shown that the surface oblateness is surely upper bounded by 11 milliarcsecond. As a consequence of the observed surface and sub-surface differential rotation laws, we deduce a measure of the two first gravitational harmonics, the quadrupole and the octopole moment of the Sun: J ₂=−(6.13±2.52)×10⁻⁷ if all observed data are taken into account, and respectively, J ₂=−(6.84±3.75)×10⁻⁷ if only sunspot data are considered, and J ₂=−(3.49±1.86)×10⁻⁷ in the case of helioseismic data alone. The value deduced from all available data for the octopole is: J ₄=(2.8±2.1)×10⁻¹². These values are compared to some others found in the literature. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005238718479     

Use of a high-resolution profiling sonar and a towed video camera to map a Zostera marina bed,Solent, UK

Seagrasses are flowering plants that develop into extensive underwater meadows and play a key role in the coastal ecosystem. In the last few years, several techniques have been developed to map and monitor seagrass beds in order to protect them. Here, we present the results of a survey using a profiling sonar, the Sediment Imager Sonar (SIS) and a towed video sledge to study a Zostera marina bed in the Solent, southern UK. The survey aimed to test the instruments for seagrass detection and to describe the area for the first time.