A literature review of Algerian salt lakes: values,threats and implications

Chotts are salt lakes located in the arid regions of Northern Africa. These unique wetlands are recognized for their role in the migration and breeding of waterbirds and economic values generated from mineral extraction, agriculture and tourist activities. Despite their importance for flora and fauna biodiversity, they continue to face important threats due to human activities. This article consolidates a literature review of Algerian chotts from global to local importance. An in-depth view demonstrates the changes in land use of two large chotts in the southeastern region of Algeria: Chotts Merouane and Melghir. Land use images highlight a net expansion of agricultural lands, a decrease in grazing area around the lakes and a decrease of salt lake surface area. The local focus highlights the threats and changes of Algerian and Northern African chotts and proposes some perspectives for the future.     

Premiers résultats d'une étude géophysique sur le flanc nord des glissements de Storegga (Norvège)

The Storegga slides, off Norway, are among the largest submarine slides ever known on a continental slope. The HYDRATECH cruise on N/O Le Suroît aimed at a high-resolution survey of an area at the northern boundary of the slides. This survey images in great detail the bottom simulating reflector (BSR) extent and properties, the various fluid escape structures and the sediment deformations. The combination and the quality of the data help to understand the still poorly constrained relationships between fluid escapes, gas hydrates and slope stability in the survey area. To cite this article: H. Nouzé et al., C. R. Geoscience 336 (2004).     

Comparative analysis between analytical approximations and numerical solutions describing recession flow in unconfined hillslope aquifers

Recession flow of aquifers from a hillslope can be described by the non-linear Boussinesq equation. Under strong assumptions and for specific conceptual formulations, different authors derived analytical approximations or linearized versions to this partial differential equation. A comparative analysis between some analytical approximations of the Boussinesq equation and the numerical solution of the recession flow of an unconfined homogeneous aquifer (horizontal, inclined and concave aquifer floor) was carried out. The objective was to define the range where the analytical solutions approximate the numerical solution. The latter was considered in this study as the reference method, because it requires fewer assumptions. From the considered analytical approximations, exponential decay relationships were found to be mainly valid for fine domain materials when horizontal, mild slopes (less than 2%) and concave aquifer floors were considered, but failed to reproduce coarse aquifer numerical model outflows, in contrast to the quadratic decay relationship, which better reproduce outflows in such domains. On the basis of the comparative analysis, it has been found that recession flows obtained with the considered analytical approximations yield similar values only for certain ranges of aquifer properties and geometries.

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Résumé L’écoulement de décrue dans les aquifères de versant peut être décrit par l’équation non-linéaire de Boussinesq. En considérant d’importantes hypothèses et des formulations conceptuelles spécifiques, différents auteurs ont dérivé des approximations analytiques ou des versions linéarisées de cette équation partielle différentielle. Une analyse comparative entre des approximations analytiques de l’équation de Boussinesq et une solution numérique de la décrue d’un aquifère homogène libre (limite inférieure de l’aquifère horizontale, inclinée et concave) a été effectuée. L’objectif était de définir la plage pour laquelle les solutions analytiques approchent la solution numérique. Cette dernière a été considérée dans cette étude comme la méthode de référence car elle demande moins d’hypothèses. Parmi les approximations analytiques considérées, les relations de décroissance exponentielle se sont avérées être principalement valides pour des matériaux fins lorsque des couches aquifères horizontales, concaves et de pentes douces (moins de 2%) étaient considérées; en revanche, celles-ci n’ont pas réussi à reproduire les écoulements sortants d’un modèle numérique pour un aquifère à matériaux grossiers, contrairement à la relation de décroissance quadratique qui reproduit mieux les écoulements sortants dans de tels milieux. Sur la base de l’analyse comparative, il s’est avéré que les écoulements en décrue obtenus avec les approximations analytiques considérées présentent des valeurs similaires seulement pour certains critères géométriques et propriétés de l’aquifère.

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Resumen Puede describirse el flujo de recesión de acuíferos de una ladera mediante la ecuación no-lineal de Boussinesq. Bajo fuertes supuestos y para formulaciones conceptuales específicas, distintos autores han derivado aproximaciones analíticas o versiones lineales para esta ecuación parcial diferencial. Se realizó un análisis comparativo entre algunas aproximaciones analíticas de la ecuación de Boussinesq y la solución numérica del flujo de recesión de un acuífero homogéneo no confinado (piso de acuífero cóncavo, inclinado, y horizontal). El objetivo consistió en definir el rango donde las soluciones analíticas aproximan la solución numérica. En este estudio se consideró la solución numérica como el método de referencia debido a que requiere menores supuestos. De las aproximaciones analíticas consideradas se encontró que las relaciones de desintegración exponencial eran principalmente válidas para materiales de ámbitos finos donde se consideraron pisos de acuíferos horizontal, pendientes medianas (menores a 2%) y cóncavos pero fallaron en reproducir flujos de salida de modelos numéricos de ámbito grueso, en contraste con la relación de desintegración cuadrática la cual reproduce mejor los flujos de salida en tales ámbitos. En base al análisis comparativo se encontró que los flujos de recesión obtenidos con las aproximaciones analíticas consideradas aportaron valores similares solo para ciertos rangos de propiedades y geometrías de acuíferos.

     

Nouvelles données structurales et datations 40K–40Ar sur les roches métamorphiques de la région de Neyriz (zone de Sanandaj–Sirjan,Iran méridional). Leur intérêt dans le cadre du domaine néo-téthysien du Moyen-Orient

The metamorphic rocks of the Neyriz area (Sanandaj–Sirjan zone) represent a Palaeozoic sequence, the upper part of which being palaeontologically dated from the Carboniferous and the Permian. Field structural analysis of the whole sequence, detailed in laboratory by microstructural one and ⁴⁰K–⁴⁰Ar dating carried on separated minerals, lead to establish that the whole sequence, from gneisses to Permian rocks, has suffered a unique synmetamorphic deformation, of variable intensity, marked by a foliation. Isotopic ages measured on extracted amphiboles and micas, clustered in four groups between 300 and 60 Ma, show the successive stages of their slow exhumation, which ended by the end of the Cretaceous. To cite this article: R. Sheikholeslami et al., C. R. Geoscience 335 (2003).     

The coupling of optimal economic growth and climate dynamics

In this paper, we study optimal economic growth programs coupled with climate change dynamics. The study is based on models derived from MERGE, a well established integrated assessment model (IAM). We discuss first the introduction in MERGE of a set of “tolerable window” constraints which limit both the temperature change and the rate of temperature change. These constraints, obtained from ensemble simulations performed with the Bern 2.5-D climate model, allow us to identity a domain intended to preserve the Atlantic thermohaline circulation. Next, we report on experiments where a two-way coupling is realized between the economic module of MERGE and an intermediate complexity “3-D-” climate model (C-GOLDSTEIN) which computes the changes in climate and mean temperature. The coupling is achieved through the implementation of an advanced “oracle based optimization technique” which permits the integration of information coming from the climate model during the search for the optimal economic growth path. Both cost-effectiveness and cost-benefit analysis modes are explored with this combined “meta-model” which we refer to as GOLDMERGE. Some perspectives on future implementations of these approaches in the context of “collaborative” or “community” integrated assessment modules are derived from the comparison of the different approaches.     

Geochemical and isotopic evidence for seawater contamination of the hydrothermal system of Taal Volcano, Luzon, the Philippines

 The hydrologic structure of Taal Volcano has favored development of an extensive hydrothermal system whose prominent feature is the acidic Main Crater Lake (pH<3) lying in the center of an active vent complex, which is surrounded by a slightly alkaline caldera lake (Lake Taal). This peculiar situation makes Taal prone to frequent, and sometimes catastrophic, hydrovolcanic eruptions. Fumaroles, hot springs, and lake waters were sampled in 1991, 1992, and 1995 in order to develop a geochemical model for the hydrothermal system. The low-temperature fumarole compositions indicate strong interaction of magmatic vapors with the hydrothermal system under relatively oxidizing conditions. The thermal waters consist of highly, moderately, and weakly mineralized solutions, but none of them corresponds to either water–rock equilibrium or rock dissolution. The concentrated discharges have high Na contents (>3500 mg/kg) and low SO₄/Cl ratios (<0.3). The Br/Cl ratio of most samples suggests incorporation of seawater into the hydrothermal system. Water and dissolved sulfate isotopic compositions reveal that the Main Crater Lake and spring discharges are derived from a deep parent fluid (T≈300  °C), which is a mixture of seawater, volcanic water, and Lake Taal water. The volcanic end member is probably produced in the magmatic-hydrothermal environment during absorption of high-temperature gases into groundwater. Boiling and mixing of the parent water give rise to the range of chemical and isotopic characteristics observed in the thermal discharges. Incursion of seawater from the coastal region to the central part of the volcano is supported by the low water levels of the lakes and by the fact that Lake Taal was directly connected to the China sea until the sixteenth century. The depth to the seawater-meteoric water interface is calculated to be 80 and 160 m for the Main Crater Lake and Lake Taal, respectively. Additional data are required to infer the hydrologic structure of Taal. Geochemical surveillance of the Main Crater Lake using the SO₄/Cl, Na/K, or Mg/Cl ratio cannot be applied straightforwardly due to the presence of seawater in the hydrothermal system. Received: 12 February 1997 / Accepted: 26 January 1998