Inverse modeling of the hydrological and the hydrochemical behavior of hydrosystems: Characterization of Karst System Functioning

International audienceInverse modeling of mass transfer characterizes the dynamic processes affecting the function of karst systems and can be used to identify karst properties. An inverse model is proposed to calculate unit hydrographs as well as impulse response of fluxes from rainfall-runoff or rainfall-flux data, the purpose of which is hydrograph separation. Contrary to what hydrologists have been doing for years, hydrograph separation is carried out by using transfer functions in their entirety, which enables accurate separation of fluxes, as was explained in the companion paper [Pinault et al., this issue]. The unit hydrograph as well as impulse response of fluxes is decomposed into a quick and a slow component, and, consequently, the effective rainfall is decomposed into two parts, one contributing to the quick flow (or flux) and the other contributing to the slow flow generation. This approach is applied to seven French karstic aquifers located on the Larzac plateau in the Grands Causses area (in the south of France). Both hydrodynamical and hydrogeochemical data have been recorded from these springs over several hydrological cycles. For modeling purposes, karst properties can be represented by the impulse responses of flow and flux of dissolved species. The heterogeneity of aquifers is translated to time-modulated flow and transport at the outlet. Monitoring these fluxes enables the evaluation of slow and quick components in the hydrograph. The quick component refers to the "flush flow" effect and results from fast infiltration in the karst conduit network when connection is established between the infiltration and phreatic zones, inducing an increase in water head. This component reflects flood events where flow behavior is nonlinear and is described by a very short transfer function, which increases and decreases according to water head. The slow component consists of slow and fast infiltration, underground runoff, storage in annex-to-drain systems, and discharge from the saturated zone. These components can be further subdivided by measuring chemical responses at the karst outlet. Using such natural tracers enables the slow component of the unit hydrograph to be separated into preevent water, i.e., water of the reservoir and event water, i.e., water whose origin can be related to a particular rainfall event. These measurements can be used to determine the rate of water renewal. Since the preevent water hydrograph is produced by stored water when pushed by a rainfall event and the event water hydrograph reflects rainwater transfer, separating the two components can yield insights into the characteristics of karst aquifers, the modes of infiltration, and the mechanisms involved in karstification, as well as the degree of organization of the aquifer

Discovery of a composite reefal terrace of middle and late Pleistocene age in Great Inagua Island, Bahamas. Implications for regional tectonics and sea-level history

International audienceWe provide here new 234U/230Th ages measured on coral samples collected from a reefal terrace exposed on Great Inagua Island (Bahamas) that was, up to now, wholly attributed to the last interglacial period (Marine Isotope Stage 5e). Our results from the upper part of the terrace confirm the previously reported MIS 5e age, whereas ages obtained from the lower part range between 139,000 and 193,000 years BP, spanning most of MIS 6. Petrographic examination showed that secondary aragonite cement and internal sediment occur in the coral chambers of these samples, indicating they were rejuvenated and likely date from the penultimate interglaciation (MIS 7). The studied terrace is thus a composite build-up and its lower part represents the first coral reef of MIS 7 age ever described in the Bahamas archipelago. Our results further suggest (1) that Great Inagua Island recently underwent a phase of tilting, and (2) that sea level was close present datum during MIS 7. © 2006 Elsevier B.V. All rights reserved

Deformation features within an active fault zone in carbonate rocks: The Gubbio fault (Central Apennines, Italy)

The Gubbio fault is an active normal fault defined by an important morphological scarp and normal fault focal mechanism solutions. This fault truncates the inherited Miocene Gubbio anticline and juxtaposes Mesozoic limestones in the footwall against Quaternary lacustrine deposits in the hanging wall. The offset is more than 2000 m of geological throw accumulated during a poly-phased history, as suggested by previous works, and has generated a complex zone of carbonate-rich fault-related structures. We report the results of a multidisciplinary study that integrates detailed outcrop and petrographic analysis of two well-exposed areas along the Gubbio fault zone, geochemical analysis (fluid inclusions, stable isotopes, and trace elements) of calcite-sealed fault-related structures and fault rocks, and biostratigraphic controls. Our aims are: (i) the characterization of the deformation features and their spatialetemporal relationships, and (ii) the determination of the P/T conditions and the fluid behaviour during deformation to achieve a better understanding of fluiderock interaction in fault zones. We show that few of the observed structures can be attributed to an inherited shortening phase while the most abundant structures and fault rocks are related to extensional tectonics. The outcropping extensional patterns formed at depths less than 2.5e3 km, in a confined fluid system isolated from meteoric water, and the fault structures are the response to a small amount of cumulated displacement, 12e19% of the total geological throw

Constraints for the evolution of the Gubbio fault zone: preliminary results from micro-structural analysis, fluid inclusions, cathodoluminescence and stable isotopes analyses

Serie Special

Assessment of doline geometry using geophysics on the Quercy plateau karst (South France)

In the protection of karstic aquifers, the role played by dolines is of interest as these shallow structures can be preferential infiltration pathways for groundwater recharge. In this study, three depressions in South France (Orniac, Lot) were identified and characterized using combined geophysical methods. Apparent conductivity mapping allows precise definition of dolines corresponding to topographic depressions. It also reveals other dolines that are hidden by soil cover. Electrical resistivity and seismic refraction tomography provide detailed cross-sections. Even though the dolines studied are close one to another, they exhibit great variability in shape and content. Moreover, time-lapse data show that the most saturated regions are on the edges of the first doline. Such information helps to evaluate human impacts and forecast potential groundwater contamination

Datations U/Th (TIMS) et 14C (AMS) des stalagmites de la grotte Chauvet (Ardèche, France) : intérêt pour la chronologie des événements naturels et anthropiques de la grotte

International audienceHuit stalagmites, deux planchers stalagmitiques ainsi que des charbons de bois trouvés piégés sous les concrétionnements prélevés dans la grotte Chauvet (Vallon-Pont-d'Arc, Ardèche) ont été datés par des méthodes radiométriques par spectrométrie de masse (U/Th et 14C). Les résultats donnent des jalons chronologiques précis de l'histoire de la grotte et confirment aussi, pour certains, l'âge avancé des charbons de bois. Ils montrent également quelles ont été les périodes climatiques favorables à la croissance des stalagmites. Pour citer cet article : D. Genty et al., C. R. Palevol 3 (2004).Abstract : TIMS U/Th and 14C AMS ages of the Chauvet cave stalagmites: interest for the chronology of natural and human events of the cave. Eight stalagmites, two flowstones and several pieces of charcoal found trapped under the speleothems from the Chauvet cave (Vallon-Pont-d'Arc, Ardèche, France) have been dated by radiometric mass spectrometric methods (U/Th et 14C). Results give accurate ages for some geological features of the cave, and, for some of them, confirm the old ages of the prehistoric charcoal. They also indicate the timing of the climatic periods that were favourable to the stalagmites growth. To cite this article: D. Genty et al., C. R. Palevol 3 (2004)