Comment on ‘Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes’, by G.J. Borradaile and D. Gauthier

Anisotropy of magnetic susceptibility is used as a proxy for the determination of magmatic flow direction in mafic dykes. Here we take advantage of the dataset from G.J. Borradaile and D. Gauthier to comment three points: (1) the sampling strategy; (2) the geometric relationship between magnetic axis dyke, and, (3) an alternative interpretation to obtain a flow direction. The magnetic lineations published by Borradaile and Gauthier correspond to the zone axis of the dyke and magnetic foliation poles, questioning the reliability of the magnetic lineation as a flow estimate. An alternative interpretation is based on the use of the tiling of the magnetic foliation plane against the dyke wall.     

Hydrogeological mapping in Switzerland

The 1:500,000 geological, tectonic and hydrogeological maps of Switzerland have been updated using a new approach for a geographical information system (GIS). Geological, tectonic and hydrogeological data are combined in a single polygon data set. Specific data (point and line elements) are additionally represented on a separate GIS layer for each map respectively. The new 1:500,000 hydrogeological map of Switzerland consists of two different sheets. The first sheet provides information on the near-surface groundwater resources and qualitative estimates of their yield. The second sheet displays the groundwater resource vulnerability and the capacity of lateral contaminant transport. The 1:100,000 hydrogeological map of Switzerland shows the distribution and use of groundwater in a selected area. The near-surface consolidated and unconsolidated rocks are classified in this map according to their permeability and lithological and petrographical properties. The map is available as a pixel map.

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Résumé  Les cartes aux 1:500,000 de la Suisse, portant sur la géologie, la tectonique et l’hydrogéologie, ont été mises à jour en utilisant une nouvelle approche pour un système d’information géographique (GIS, en anglais). Les données géologiques, tectoniques et hydrogéologiques ont été combinées sur un seul set de polygones. Les données plus spécifiques (points et lignes) sont représentées dans une couverture séparée, pour chaque carte respectivement. La nouvelle carte hydrogéologique de Suisse au 1:500,000 est constituée de deux différentes feuilles. La première feuille apporte des informations sur les ressources proches de la surface et sur leur rendement. La seconde feuille montre la vulnérabilité des ressources souterraines et le risque de pollution par transport latéral. La carte hydrogéologique de Suisse au 1:100,000 montre la distribution et l’utilisation de l’eau souterraine dans la région sélectionnée. Les roches consolidées ou meubles de sub-surface, sont classées au regard de leur perméabilité et de leurs propriétés lithologique et pétrographique. La carte est disponible pixellisée.

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Resumen  Los mapas geológico, tectónico e hidrogeológico de Suiza a escala 1:500,000 han sido actualizados utilizando un nuevo enfoque para un sistema de información geográfico (SIG). Los datos geológicos, tectónicos e hidrogeológicos se combinan en un conjunto de datos en un solo polígono. Por otra parte, para cada uno de los mapas, los datos específicos (elementos puntuales y lineales) se representaron en una capa separa del SIG. El nuevo mapa hidrogeológico de Suiza a escala 1:500,000 contiene dos capas diferentes. La primera de ellas suministra información de los recursos hídricos subterráneos poco profundos y estimaciones cualitativas de sus rendimientos. La segunda capa muestra la vulnerabilidad de este recurso y el riesgo debido al transporte lateral de contaminantes. El mapa hidrogeológico de Suiza a escala 1:100,000 muestra la distribución y uso del agua subterránea en un área seleccionada. Los sedimentos consolidados y no consolidados cercanos a la superficie se clasifican en este mapa en función de su permeabilidad y propiedades litológicas y petrográficas. El mapa se encuentra disponible como un mapa de pixeles.

     

Cementation of kerogen-rich marls by alkaline fluids released during weathering of thermally metamorphosed marly sediments. Part I: Isotopic (C,O) study of the Khushaym Matruk natural analogue (central Jordan)

The Khushaym Matruk site in central Jordan may represent a natural analogue depicting the interaction of alkaline solutions with a clayey sedimentary formation or with clay-rich confining barriers at the interface with concrete structures in waste disposal sites. In this locality, past spontaneous combustion of organic matter in a clayey biomicritic formation produced a ca. 60 m-thick layer of cement-marble containing some of the high-temperature phases usually found in industrial cements (e.g., spurrite, brucite, and Ca-aluminate). A vertical cross-section of the underlying sediments was used in order to study the interaction between cement-marbles and neighbouring clayey limestones under weathering conditions. A thermodynamic approach of the alteration parageneses (calcite–jennite–afwillite–brucite and CSH phases) in the cement-marbles constrains the interacting solutions to have had pH-values between 10.5 and 12. Over 3 m, the sediments located beneath the metamorphic unit were compacted and underwent carbonation. They display large C and O isotopic variations with respect to “pristine” sediments from the bottom of the section. Low δ¹³C-values down to −31.4‰/PDB show the contribution of CO₂ derived from the oxidization of organic matter and from the atmosphere to the intense carbonation process affecting that particular sedimentary level. The size of the C isotopic anomalies, their geometrical extent and their coincidence with the variations of other markers like the Zn content, the structure of organic matter, the mineralogical composition, all argue that the carbonation process was induced by the percolation of high pH solutions which derived from the alteration of cement-marbles. The temperature of the carbonation process remains conjectural and some post-formation O isotopic reequilibration likely affected the newly-formed carbonate. Carbonation induced a considerable porosity reduction, both in fractures and matrixes. The Khushaym Matruk site may have some bearing to the early life of a repository site, when water saturation of the geological formations hosting the concrete structures is incomplete, enabling simultaneous diffusion of alkaline waters and gaseous CO₂ in the near field.     

Cementation of kerogen-rich marls by alkaline fluids released during weathering of thermally metamorphosed marly sediments. Part II: Organic matter evolution,magnetic susceptibility and metals (Ti,Cr, Fe) at the Khushaym Matruk natural analogue (Central Jordan)

Spontaneous combustion, less than 1 Ma ago, affected a 60-m thick sediment pile of biomicrite at the Khushaym Matruck site (Jordan). The present study shows that three retrograde alteration stages occurred: weathering, thermal stress and oxidative alkaline perturbation. μ-FT-i.r. spectra of isolated kerogens and oxygen index of whole rocks indicate that oxidation of organic matter occurred down to ∼10 m beneath the metamorphosed zone at Khushaym Matruck. The occurrence of the oxidative weathering bacterially mediated, as suggested by the mass chromatograms of saturated hydrocarbons, can explain high Rock-Eval T_max values and low petroliferous potential measured along the sedimentary pile. On the other hand, the thermal extent of combustion events was limited to the first 2 m from the contact. The mean reflectance of 0.20–0.24% and porosity of ca. 50% of the grey clayey biomicrites indicate that organic matter was very immature and sediments were unconsolidated at the time of the combustion event. Using mineralogy, microscopic analyses of vegetable debris and magnetic susceptibility, a suite of characteristic points corresponding to the thermal imprint can be assessed: (i) x = 0m, T ∼ 1000 °C, (ii) x = 1 m, T ∼ 350 °C, (iii) x = 2 m, T ∼ 150 °C and (iv) x > ∼ 8 m, T ∼ 30 °C. Paleocirculation of meteoric groundwater in the ‘cement-marbles’ generated high-pH fluids that have circulated via fractures and through the matrix porosity of the underlying biomicrites but have also induced alkaline hydrolysis and oxidative attack of the organic matter. The polysaccharide/lignin ratio derived from μ-FT-i.r. analyses shows that the delignification of vegetable debris and degradation of polysaccharides progressively decline in the indurated zone, which indicates a decrease in the pH of migrating solutions. The latter also severely oxidized organic matter at 2.10 and 3.05 m as revealed by the oxygen index and induced the generation of bitumen. The spatial correlation between the oxidation levels of organic matter and the metal contents (Fe, Ti and Cr) suggests that redox reactions were responsible for the immobilization of metals in the indurated biomicrites. The intensity of these reactions is attributed to changes in the fluid flow regime within the sedimentary column.     

ESR chronology of the Somme River Terrace system and first human settlements in Northern France

The Somme Valley, Northern France, is famous for its archaeological sequence, where numerous rich Palaeolithic sites, such as Saint-Acheul, the type site of the Acheulian, have been discovered. The archaeological levels are often directly associated with fossil alluvial sediments of the River Somme or with slope deposits, including loess and palaeosols. In the middle reaches of the valley, near Amiens, the system of fossil-stepped fluvial terraces is particularly well developed and preserved, and occurs on 10 alluvial formations. These terraces, from +5 to +55 m above the present-day valley bedrock, allow the study of the environmental changes and the human settlement of this area through the Pleistocene.

Since 1988, ESR dating was systematically applied on bleached quartz extracted from the fossil fluvial deposits, in order to better describe the geological evolution of the stepped system. More recently, U-series/ESR dating has also been performed on teeth collected from the different terrace deposits. Here we present a synthetic review of the main ESR results, and propose an ESR chronology for the geological evolution of the Somme fluvial system and for the Middle Pleistocene human settlement of northern France.     

Influence of the oceanic biology on the tropical Pacific climate in a coupled general circulation model

The influence of chlorophyll spatial patterns and variability on the tropical Pacific climate is investigated by using a fully coupled general circulation model (HadOPA) coupled to a state-of-the-art biogeochemical model (PISCES). The simulated chlorophyll concentrations can feedback onto the ocean by modifying the vertical distribution of radiant heating. This fully interactive biological-ocean-atmosphere experiment is compared to a reference experiment that uses a constant chlorophyll concentration (0.06 mg m⁻³). It is shown that introducing an interactive biology acts to warm the surface eastern equatorial Pacific by about 0.5°C. Two competing processes are involved in generating this warming: (a) a direct 1-D biological warming process in the top layers (0–30 m) resulting from strong chlorophyll concentrations in the upwelling region and enhanced by positive dynamical feedbacks (weaker trade winds, surface currents and upwelling) and (b) a 2-D meridional cooling process which brings cold off-equatorial anomalies from the subsurface into the equatorial mixed layer through the meridional cells. Sensitivity experiments show that the climatological horizontal structure of the chlorophyll field in the upper layers is crucial to maintain the eastern Pacific warming. Concerning the variability, introducing an interactive biology slightly reduces the strength of the seasonal cycle, with stronger SST warming and chlorophyll concentrations during the upwelling season. In addition, ENSO amplitude is slightly increased. Similar experiments performed with another coupled general circulation model (IPSL-CM4) exhibit the same behaviour as in HadOPA, hence showing the robustness of the results.     

A maximum entropy method for combining AOGCMs for regional intra-year climate change assessment

This paper deals with different responses from various Atmosphere-Ocean Global Climate Models (AOGCMs) at the regional scale. What can be the best use of AOGCMs for assessing the climate change in a particular region? The question is complicated by the consideration of intra-year month-to-month variability of a particular climate variable such as precipitation or temperature in a specific region. A maximum entropy method (MEM), which combines limited information with empirical perspectives, is applied to assessing the probability-weighted multimodel ensemble average of a climate variable at the region scale. The method is compared to and coupled with other two methods: the root mean square error minimization method and the simple multimodel ensemble average method. A mechanism is developed to handle a comprehensive range of model uncertainties and to identify the best combination of AOGCMs based on a balance of two rules: depending equally on all models versus giving higher priority to models more strongly verified by the historical observation. As a case study, the method is applied to a central US region to compute the probability-based average changes in monthly precipitation and temperature projected for 2055, based on outputs from a set of AOGCMs. Using the AOGCM data prepared by international climate change study groups and local climate observation data, one can apply the MEM to precipitation or temperature for a particular region to generate an annual cycle, which includes the effects from both global climate change and local intra-year climate variability.     

Apport des isotopes stables dans l'estimation des altitudes de recharge de sources thermales du MarocRecharge altitude estimation of thermal springs using stable isotopes in Morocco

The recharge altitude estimation of thermal springs from northern and eastern Morocco using ¹⁸O and ²H contents requires the definition of regional isotopic altitudinal gradients (?0.25‰ for 100 m for the Rif and ?0.30‰ for the East) and the calculation of residence time using ¹⁴C. The altitudes of emergence vary widely between 170 and 1040 m under the altitude of the recharge areas. The ¹⁸O and ²H compositions of palaeowaters ($\text{>10}\text{000}$ yr BP) indicate two effects, altitude and palaeoclimate. To cite this article: A. Winckel et al., C. R. Geoscience 334 (2002) 469–474.