Spatial Distribution of Soda Straws Growth Rates of the Coufin Cave (Vercors, France)
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International Journal of Speleology 39 (2) 61-70 Bologna (Italy) July 2010 Available online at www.ijs.speleo.it International Journal of Speleology Official Journal of Union Internationale de Spéléologie Spatial distribution of soda straws growth rates of the Coufin Cave (Vercors, France). Yves Perrette1 and Stéphane Jaillet1 Abstract: Perrette Y. and Jaillet S. 2010. Spatial distribution of soda straws growth rates of the Coufin Cave (Vercors, France). International Journal of Speleology, 39(2), 61-70. Bologna (Italy). ISSN 0392-6672. The Choranche Cave system (Vercors, France) is an excellent locality for measuring the growth rates of large numbers soda straws. This is especially the case for the Coufin Cave, as enlargement of the cave entrance in 1875 led to a change in stalactite color from brown to white, thus providing a reliable chronomarker. The date of this brown-to-white calcite transition has been confirmed by lamina counting. We measured and georeferenced the growth-lengths of 306 soda straws in a 1m2 area of the roof of the Coufin Cave entrance chamber. Because of the very slow and sometimes inexistent water feeding of those stalactites, hydrochemistry analysis were not achieved and drop rate effect on growth were neglected; this study is based on a geomorphological and geostatistical work. By measuring a large number of soda straws in a very small area for which most of the parameters affecting stalactite growth could be considered uniform, and because flow rates are very slow (frequencies are always superior to 1 drop per half hour), we could ascribe differences in growth rates to variations in the global increase of water flow through the unsaturated matrix. Statistical and geostatistical analyses of the measurements showed that this set of similarly shaped stalactites actually consisted of three Gaussian populations with different mean growth rates: fast growth rate (FGR- mean of 0.92 mm.y-1), medium growth rate (MGR- mean of 0.47 mm.y-1) and low growth rate (LGR- 0.09 mm.y-1). Plotting the lengths and spatial distribution of the 20 longest FGR soda straws revealed that there is a rough pattern to the water flow through the cave roof. Even if no direction is statisticaly different from others, the observed directional pattern is consistent with local and regional tectonic observations. Plots of the spatial distribution of the soda straws show that FGR soda straws follow lines of regional geological stress, whereas MGR and LGR soda straws are more dispersed. Keywords: stalactite, karst, hydrogeology, growth rate, speleothem Received 23 Decembre 2009; Revised 15 February 2010; Accepted 18 February 2010 INTRODUCTION Williams, 1989; Hill and Forti, 1997); Klimchouk et Stalagmites are often used for reconstructing al., 2000). Most studies of the spatial distribution of paleoenvironments and paleoclimates. The dynamics speleothems in caves are relatively large scale. For of the speleothem record have been the subject of a instance, some of the main works on karstogenesis number of studies, although most have focused on (Ford and Williams, 1989; Maire, 1990), associate the geochemical record (Fairchild et al., 2000; Genty stalactite location with classic growth parameters, et al., 2001; Genty et al., 2002; Ramseyer et al., 1997; such as the presence of soils with high partial pressure Verheyden et al., 1999, Fairchild et al., 2006, Spotl of CO2, water flow and partial pressure of CO2 in caves et al., 2005). Some researchers have investigated the in relationship with the ventilation and the water hydrologic controls on speleothem growth (Baker et al., flow in the cave (Bourges et al., 2006) . Cabrol (1978) 2000; Baldini et al., 2006). Most of these studies were showed that speleothem mineralogy is influenced based on hydrologic stalactite series with sampling by the final meters of the water path through the generally being limited to stalactites characterized bedrock, and Hill and Forti (1997) correlated the main by high flow rates allowing enough sampling for speleothem shapes with different feed water regimes, analyses. However, despite its implications for the use but neither study discussed the relationship between of stalactite records (sampling choices, interpretation, speleothems and karst drainage structure. Based on karst filtering effect), very little work has been done on these results, some authors have used the mineralogy flow distributions. of speleothems to determine depositional contexts The literature on karst systems shows that and thereby infer the environment outside the cave differences in drainage and percolation govern both during speleothem formation (Borsato et al., 2000; the location and the shape of stalactites (Ford and Frisia et al., 2000). However, only a few studies have described and discussed the spatial distribution of speleothems in a 1 Edytem, Université de Savoie - CNRS; Campus Scientifique, cave, or even in a chamber or passage of a cave. For Savoie Technolac, F73376 Le Bourget du Lac – France, yves. example, as part of a study to calibrate speleothem [email protected] paleoclimate isotope records, Verheyden et al. (1999) 62 Yves Perrette and Stéphane Jaillet showed that the chemical behavior of the stalactite from 900 to 1400m a.s.l. The vegetation covering water supply was different at each sampling site. the plateau changes with elevation from an oak and This observation, together with similar findings by boxwood assemblage to a pine and beech assemblage. Cabrol (1978), suggests that geology has a significant Where they exist, soils are typical calco-humic brown influence on water chemistry. Thus, stalactite shape soils (Muxart, 1978). Mineralizations of infiltration and chemistry appear to be governed by the two main measured on fast flows (from 1hz to continuous water flow processes in limestone: 1) porosity water flow) of massive stalagmites in the entrance chamber flow, which tends to homogenize the growth, shape show as supposed a calcite saturation index varying and chemistry of stalactites; 2) fracture network water from 0.4 to 0.9 with a calcium concentration from flow, which usually causes heterogeneities in porosity 80 to 100 mg.l-1 and a manganesium concentration resulting in variations in stalactite growth rate, shape always inferior to 0.5 mg.l-1 (unpublished data). The and chemistry. continuous saturation of water is in accordance with This work investigates the respective roles of these the observation of continuous flowstone deposition. two processes by studying the spatial distribution From an hydrological point of view, drop rate were of growth rates for a large population of stalactites monitored for fast flows (used for mineralization in one small area of a cave. Our working hypothesis study) and slow flow ( faster soda straws) (Destombes is the uniformity of cave ventilation and lithology et al., 1997, Perrette, 2000, Perrette et al., 2003). No at sampling site and site’s position with respect to relationship has been shown between rainfall and drop geological macro-structures (faults and diaclases) rate for sodastraws. The key control of these drop rate allowed us to assume that most of the variables was found to be atmospheric pressure at the event affecting stalactite growth were fixed (ventilation, time scale and high vs low flows at the year time scale. partial pressure of CO2, temperature, bedrock depth Those data are pertinent to give a general hydrological above the cave). Furthermore, due to the slow drip and hydrochemical background for infiltration waters rates (faster drip frequencies are about 2 to 3 drops in Choranche cave ; this information has to be applied per hour but are commonly lower than one drop cautiously to the soda straws whose water feeding are per hour), the continuance of the water feeding of very low. stalactite by percolating water is assumed to be the only significant variable controlling stalactite growth A large number of sodastraws chronomarked in 1875 in such a small area. Because this water flow is In 1875 AD, the Coufin Spring, which emerged from generally around the milli-hertz magnitude order, a scree slope, dried up. When the villagers started increasing growth rate by increasing drop interval digging away the scree to find out what had happened (Baldini, 2001) can be exclude. to their water source, they discovered the previously unknown Coufin Cave (Gauchon, 1997). This SITE AND METHOD excavation work enlarged the outlet from the cave, Study site inducing a major change in the hydrological dynamics Geological, geomorphological and geographical of the entrance chambers of the Choranche system. frame Before enlargement, the large entrance chamber was This work was carried out at Choranche, on the periodically subject to backflooding, which resulted western side of the Vercors Mountains in southeast in the stalactites being “dirtied” with clays. Since France. The Choranche system consists of three caves 1875, the entrance has been large enough to prevent with more than 50km of passages and chambers backflooding; therefore, the stalactites are no longer (see Figure 1). The system rises at the bottom of the “dirtied” by clays. Thus, the change of stalactite Urgonian (Barremo-Bedoulian) escarpment that forms color from “dirty” brown to “clean” white provides an the northern side of the Bourne Gorge (Delannoy, accurate chronomarker corresponding to 1875 (see 1981). Figure 2 shows the geological setting of the Figure 3). caves (Arnaud, 1966; Arnaud, 1981; Werner et al., The research presented here is based on the 1980) measurement of growth rates for white calcite, that is Although relics of pre-Miocene karstification have to say, the parts of stalactites formed since 1875. As been observed on the plateau, the Choranche network well as showing the “dirty” to “clean” color change, a mostly developed during the Pleistocene (Delannoy, feature of all the soda straws in the entrance chamber, 1997). The vertical structure of the karst (pits) is Photo 1 also confirms the chronology of the transition. linked to the Vercors uplift (Alpine orogenesis).