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Karst: from palaeogeographic archives to environmental indicators

Le karst : des archives paléogéographiques aux indicateurs de l’environnement
Jean-Jacques Delannoy, Christophe Gauchon, Fabien Hobléa, Stéphane Jaillet, Richard Maire, Yves Perrette, Anne-Sophie Perroux, Estelle Ployon et Nathalie Vanara
p. 83-94

Résumés

Le karst, par sa dimension souterraine, constitue un support privilégié pour les reconstitutions paléogéographiques et paléoenvironnementales et apporte une contribution significative à la géomorphologie actuelle. Deux grandes familles d’archives complémentaires sont abordées dans cet article. Les formes endokarstiques renseignent sur le contexte géomorphologique contemporain de leur genèse, contexte qui dans bien des cas n’est plus visible dans le relief actuel. Les formations endokarstiques sont porteuses d’informations sur l’évolution des environnements et permettent de faire la part entre les facteurs naturels et anthropiques. Plusieurs exemples pris en France et à l’étranger montrent la pertinence des enregistrements karstiques. Les méthodes d’investigation mises en œuvre permettent à la fois de reconstituer l’évolution à long terme, de reconstituer d’anciennes paléogéographies et de décrire les modifications environnementales plus récentes (Holocène). Il convient d’affiner la résolution des enregistrements, de faire la part des mécanismes généraux et des effets de site.

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Notes de la rédaction

Article soumis le 11 juillet 2008, accepté le 2 mars 2009

Texte intégral

We would like to dedicate this article to Michel Chardon, who passed away on 14 May 2008. We owe a lot to Michel who was always supportive of our research and observation-based approach to analyse the underground world. We gratefully thank P. Henderson for the correction of the English style. The authors thank J.-C. Thouret and the reviewers particularly Y. Battiau-Queney for having  improved an earlier version of this paper.

Introduction

1Two French research teams (EDYTEM in Chambéry and ADES in Bordeaux) have developed new methods to analyse and describe surface and underground karst features: high-resolution topography/cartography, monitoring and modelling of liquid and solid (organic and mineral matter) flows, study of sedimentary records (speleothems, underground-lake deposits), geochemical and laser imagery. By applying these methods on different scales, i.e. from mountain range to stalagmite growth laminae, researchers have been able to extract new palaeogeographic information from karst to better understand past and present dynamics in various environments. To get advantage in combining different methods, both research ADES and EDYTEM teams have joined means and savoir-faire in several shared projects. Accurate interpretation of karst records is also quite important to evaluate heritage sites and propose measures for their protection and/or valorisation. Particular attention is given to the limitations of current analytical and methodological approaches. These studies show that karst, due to its specific characteristics, is currently an important field of research in geomorphology.

Karst landforms: valuable palaeogeographic records

2Research in karst hydrogeology and geomorphology, which was carried out in the 1970s, 1980s and 1990s (Delannoy, 1997) has shown that karst gives excellent examples of thermodynamic systems whose structure, function and evolution are determined by surrounding factors, particularly the geomorphological (gravitational energy) and bio-climatic (chemical weathering) ones. This research has also highlighted the fact that surface and subterranean karst landforms provide direct records of palaeogeographic data, which can survive (several million years). It has given rise to the notion of karst immunity (Martin, 1991). Karst landforms, particularly within major cave systems, provide information about the evolution of surrounding area and sometimes paleogeography and palaeo-drainage network. Three French examples are presented below, which show the diversity of information that can be obtained by combining data from surface and underground karst landforms, that is: (i) hydrographic base level drop in Vercors, (ii) marine base level rise in Ardèche, and (iii) erosion of the non-carbonate sedimentary cover in Barrois. The goal of this paper is to show the main interest of karst records either for paleogeographic or morphogenic studies, without any detailed analysis of karstogenesis evolutions.

The Coulmes-Choranche system (Vercors): regional evolution

3The Vercors mountain range in the French pre-Alps is notable for both the diversity of karst landscapes and the number and size of underground cavities, which include the Gouffre Berger, the first sub-1000 m network to be found in the world. The Coulmes region lies on the western side of Vercors. It is a remarkable area, whose surface and underground morphological features have been used to reconstruct the morphogenesis of the Vercors Mountains since the Miocene. The Coulmes region includes cockpit karst, nested palaeo-poljés and cavities truncated by the topographic surface. It has also an active underground system with several generations of passages that extend for more than 50 km (Gournier-Coufin-Chevaline). These surface and subterranean karst features develop from 600 m to 1400 m a.s.l.

4Karst studies have shown that the hydrographic network has undergone several phases of incision, which were driven by geodynamic changes, eustatic movements and Quaternary glaciations (Delannoy, 1997). This reconstruction was based on analyses of two types of karst record: (i) the surface forms that are produced in different gravitational and hydraulic settings, and (ii) the staging of the underground karst drainage, which depends on the regional base level (fig. 1). It has also been possible to reconstruct the regional climatic history thanks to the karstic morphology. By combining different data, a coherent morphogenic framework was proposed for the area. In this part of Vercors, characteristic dolines and conical hills formed in a low gravitational energy and humid tropical climate where chemical weathering was prevailing. Karst landforms, which were covered by sediments deposited during the last stages of the Molasse basin formed when islands were surrounded by the Miocene sea. The evolution of the Coulmes region also involved the development of shallow underground networks that trapped sediment derived from Upper Cretaceous surface formations that have now disappeared. Following the retreat of the Miocene Sea, Vercors went through an inception stage, during which poljés developed in structural depressions. The nesting of the Coulmes poljés reflects the first stages of the Miocene-Pliocene uplift of Vercors, which led to the incision of the hydrographic network (Bourne) and vertical development of the karst. Numerous parts of the drainage system that formed within the limestone bedrock show evidence of staging, with four phases related to eustatic (Pliocene transgression) and tectonic adjustments.

Fig. 1 – Diagrammatic cross section showing the morphogenetic evolution of the Coulmes-Choranche area (Vercors), from surface and subterranean karst features. Inset map showing the Vercors area.
Fig. 1 Coupe synthétique de l’évolution morphogénique du massif des Coulmes-Choranche (Vercors) à partir des formes exo- et endokarstiques. Encart montrant la localisation du Vercors.

Fig. 1 – Diagrammatic cross section showing the morphogenetic evolution of the Coulmes-Choranche area (Vercors), from surface and subterranean karst features. Inset map showing the Vercors area.Fig. 1 – Coupe synthétique de l’évolution morphogénique du massif des Coulmes-Choranche (Vercors) à partir des formes exo- et endokarstiques. Encart montrant la localisation du Vercors.

5This example highlights the unique value of karst records to reconstruct the regional landscape development. By comparing surface and underground forms and mapping their spatial distribution, it has been possible to reconstruct in detail the morphological story of the Coulmes-Choranche area since the Miocene. In fact it can be considered as a paradigm of alpine karst systems (Maire, 1990; Audra, 1994; Hobléa, 1999). Compared to the Coulmes, similarities have been observed in the karstic development of the Arbailles (French western Pyrenees; Vanara, 2000).

The Aven d’Orgnac: recording a rising hydrological base level

6The Aven d’Orgnac karst, between the Ardèche and Cèze Gorges (fig. 2), provides a unique record of rising hydrological base level during the Pliocene. Two karst planation surfaces have been recognised (400 m and 260 m a.s.l.). The first formed before the Messinian salinity crisis (Clauzon, 1982); the second is connected to the end of the Pliocene continental aggradation (Mocochain et al., 2006; Mocochain, 2007). These sub-horizontal features, which indicate a low gravitational energy, are in sharp contrast with the deep incision of Ardèche and Cèze Gorges. However, studies of the Orgnac underground system have shown that both planation and gorge formation do not tell the entire story.

Fig. 2 – Diagrammatic cross section showing the karst evolution of the Aven d’Orgnac (Ardèche): subterranean karst records of Mio-Pliocene eustatic variations. Variation in the level of the Rhône (Clauzon, 1982; Mocochain, 2007). Inset map showing the Orgnac area.
Fig. 2 – Coupe synthétique de l’évolution karstogénique de l’aven d’Orgnac (Ardèche) : les enregistrements endokarstiques des variations eustatiques mio-pliocènes. Variation du niveau du Rhône d’après Clauzon (1982) et Mocochain (2007). Encart montrant la localisation d’Orgnac.

Fig. 2 – Diagrammatic cross section showing the karst evolution of the Aven d’Orgnac (Ardèche): subterranean karst records of Mio-Pliocene eustatic variations. Variation in the level of the Rhône (Clauzon, 1982; Mocochain, 2007). Inset map showing the Orgnac area.Fig. 2 – Coupe synthétique de l’évolution karstogénique de l’aven d’Orgnac (Ardèche) : les enregistrements endokarstiques des variations eustatiques mio-pliocènes. Variation du niveau du Rhône d’après Clauzon (1982) et Mocochain (2007). Encart montrant la localisation d’Orgnac.

7The Aven d’Orgnac consists of a series of vast chambers stretching over a distance of more than five kilometres. Recent work, based on the morphology of the passages and analyses of clay infillings (Jaillet et al., 2007), has revealed a phase of major cave formation, with upward karst dissolution enlargement in phreatic condition. This type of erosion, which is referred to as paragenesis (Renault, 1967), is characterised by detrital argillaceous sedimentation and synchronous scouring of the upper part of the passage, which led to the formation of lateral benches at the contact with the argillaceous filling (“banquettes limites”: Renault, 1967). Laser-scanning and 3D reconstructions of the spatial distribution of these lateral features have revealed different phases in the ascendant scouring within the Orgnac network. Because they are found almost continuously between 180 m and 270 m a.s.l., they provide a unique record of a major episode in the regional story, maybe in relationship with the rising base level of River Cèze during the Pliocene (Delannoy et al., 2007).

8The Pliocene marine transgression that followed the Messinian salinity crisis, combined with two million years continental aggradation in the Rhône ria and its tributaries, resulted in a substantial reduction of the hydraulic gradient. This led to more than 100 m rise of the Cèze Valley level and drowning of pre-existing networks, including the Orgnac network. The staging of the lateral benches in the Aven d’Orgnac, over a height of almost 100 m, is a textbook example of a diachronous underground karst feature recording the evolution of an area. The detailed analysis of this record clarifies the picture first suggested by studies of external features and deposits in the same area (Clauzon, 1996). Other studies in Ardèche, for example in and around the Chauvet cave (Delannoy et al., 2004) or in the Saint Marcel Aven (Mocochain et al., 2006) confirm the role of the Pliocene transgression in karstogenesis. Thus the Ardèche karst evolution improves the general karstic model developed on the Mediterranean outer boundary during the Messinian salinity crisis (Delannoy, 1997; Bini, 1994; Bruxelles, 2001; Camus, 2003).

The Barrois karst: recording the erosion of the non-carbonate cover

9Karst studies can also provide information about the erosion of the geological cover. The determining parameter in such studies is the maturity of the karst system. A study of the Barrois area in Lorraine/Champagne has enabled S. Jaillet (2005) to decipher three stages in the erosion of the Cretaceous argillaceous-arenaceous cover by combining an analysis of the maturity of the karst systems with their spatial distribution (fig. 3). Three karsts were analysed in this study: Poissons, Cousance and Rupt du Puits. They belong to an active system that is evolving both under the cover and at the contact between the bare and covered karst. It contains landforms that are typical of covered-karst terranes (Gamez, 1995), for example, dome-pits or alignment of sinkholes due to infiltrations along the karst and Cretaceous cover contact. Similar landforms have been found in older karsts in the same region, suggesting that they, too, were once covered by Cretaceous sediments: this assumption is supported by the identification of Cretaceous sediments based on U/Th ages of overlaying speleothems.

Fig. 3 – Diagrammatic cross section of the Poissons, Cousance and Rupt du Puits karsts (Lorraine): synchronous records of the ablation of the Cretaceous sedimentary cover and the incision of the regional hydrographic network.
Fig. 3 – Coupe synthétique des karsts de Poissons, Cousance et du Rupt du Puits (Lorraine) : enregistrements synchrones de l’ablation de la couverture géologique crétacée et de l’incision du réseau hydrographique.

Fig. 3 – Diagrammatic cross section of the Poissons, Cousance and Rupt du Puits karsts (Lorraine): synchronous records of the ablation of the Cretaceous sedimentary cover and the incision of the regional hydrographic network. Fig. 3 – Coupe synthétique des karsts de Poissons, Cousance et du Rupt du Puits (Lorraine) : enregistrements synchrones de l’ablation de la couverture géologique crétacée et de l’incision du réseau hydrographique.

1: Palaeo base level (horizontal karst passages); 2: Altitude relative to the palaeo base levels; 3: Inactive karst sinkholes; 4: Active karst sinkholes; 5: Infra-Cretaceous erosion surface (under a layer of cover or exposed); 6: Infra-Cretaceous erosion surface, eroded today; 7: U/Th dates; 8: Karst spring. Inset map showing the Barrois area.
1 : Paléo-niveau de base (drains karstiques subhorizontaux) ; 2 : Altitude relative des paléo-niveaux de base karstique ; 3 : Pertes karstiques abandonnées ; 4 : Pertes karstiques fonctionnelles ; 5 : surface d’érosion infra-crétacée ; 6 : surface d’érosion infra-crétacée actuelle ; 7 : âges U/Th ; 8 : Sources karstiques. Encart montrant la localisation du Barrois.

10Similar studies have also been carried out on the Poissons and Cousance karsts. The Poissons palaeokarst system is located in the southeast of the region, at about 390 m a.s.l. Studies of its internal structure have been facilitated by excavations carried out in the 19th century to mine iron-rich sediments. Some Cretaceous ferruginous fillings were identified in karstic pockets showing that karst process was active under a Cretaceous sedimentary cover, which is now found only 10 km further west. This palaeokarst forms a marker perched on the major planation surface of southern Lorraine (Le Roux and Harmand, 2003). The Cousance karst lies to the northwest of the Poissons karst at 240 m a.s.l. It shows drowned palaeomorphologies that are related to an intermediate stage in the erosion of the Cretaceous cover connected with the incision of the Marne hydrographic network (Jaillet et al., 2004). The Rupt du Puits karst lies further northwest. It provides an archetypal example of the ongoing karstic evolution where the effects of the sediment-cover ablation are combined with those of the karst migration due to the river incision. Although the relative ages of the three systems have been confirmed by U/Th dating on speleothems (> 400 ky for Poissons, 20 to 150 ky for Cousance, and < 25 ky for Rupt du Puits; Jaillet et al., 2006), it has not been possible to precisely date the stages in their evolution. In each of the three examples, coherent and often unique pictures of the region’s evolution have been drawn up by combining analyses of underground and surface features with studies of the mechanisms governing their spatial distribution.

Karst formations: invaluable environmental and climatic indicators

11Karst features are valuable sources of information for studying the morphogenesis of landscapes and reconstructing paleogeographies; however, karsts can also be used on another temporal scale (Pleistocene, Holocene) to study environmental and climatic variations, which are contemporary with human activities. Karst sedimentary processes are controlled by climate and biogeography. As these deposits consist of transported or remobilised material from the surrounding environment, they provide indirect records of that environment.

12A distinction is usually made between speleothems and clastic deposits. Since the 1970s, speleothem studies have been extensively used in palaeogeographic reconstructions, first because they provide excellent geochronometers, especially through U/Th dating, and, second, because they are excellent climate and environmental archives, thanks to their sensitivity to climatic and bio-pedological conditions (CO2, organic matter) (Ford and Williams, 2007, and references therein). Recent speleothem studies have combined U/Th isotope dating with analyses of stable isotopes of oxygen and carbon, in order to obtain information about the evolution of the continental climate and environment (Genty et al., 2003; Frisia et al. 2005; Spötl and Mangini, 2007; Zanchetta et al., 2007; Mattey et al., 2008). Due to their position in a sequence, speleothems can also provide relative dates for subterranean sedimentary sequences, e.g. alternation of detrital and carbonate formations (Quinif and Maire, 1998; Lignier and Desmet, 2002; Jaillet et al., 2006; Lans et al., 2007) and for archaeological horizons (Couchoud, 2006).

13The use of infillings as environmental archives is more recent. High resolution studies are based on methodological and analytical protocols developed for lake, river and glacial sediments. By combining data from analyses of large detrital sediment series and from speleothems, it has been possible to decipher the contemporaneous morphogenic dynamics of cold periods and/or climate deterioration (Audra, 1994; Quinif and Maire, 1998; Hobléa, 1999; Lignier and Desmet, 2002). Studies of underground lake deposits (Perroux, 2005, 2006) and sedimentary sequences in tropical environments (Maire et al., 2004) have highlighted their value as records of rapid and/or catastrophic events: floods, sediment discharges related to manmade changes in land-use, etc. Finally, petrographical analyses of detrital sediments trapped in karsts can be used to trace former hydrological flow patterns and palaeogeographies (Bruxelles, 2001; Camus, 2003).

Speleothems: recording environmental dynamics and human activities

14Speleothem studies generally use analyses of the stable isotopes (18O or 13C). The examples presented below, in the Coufin Cave (Choranche, Vercors) and the Aven de la Portalerie (Causses) highlight the diversity of stalagmite archives and concern research at the interface between geomorphological effects of the site and palaeoenvironmental studies.

15Speleothems are formed by the precipitation of minerals from saturated solutions percolating through karst cavities but they are not only controlled by chemical parameters; their formation is also affected by ventilation and flows. For example a sudden change in crystallisation may either reflect a change in climate or result from geomorphological factors (e.g., opening/closing of a cave entrance or a sump). Figure 4 illustrates this environmental sensitivity via a cross-section through a stalactite from the Coufin Cave (Choranche, France). The changes in stalactite growth and crystallisation rates are related to the opening (clearance of the entrance) of the Choranche Cave at the end of the 19th century, and then the draining of a sump in the cave entrance during the 20th century. In this case, the detailed knowledge of local cave exploration and manmade events was required to correctly interpret the record.

Fig. 4 – Stalactite growth sensitivity to changing dynamics of the karst environment.
Fig. 4 – Sensibilité de la croissance des stalactites aux changements de dynamique du milieu karstique.

Fig. 4 – Stalactite growth sensitivity to changing dynamics of the karst environment. Fig. 4 – Sensibilité de la croissance des stalactites aux changements de dynamique du milieu karstique.

The first example concerns the analysis of a soda straw (hollow stalactite) from the entrance to the Coufin Cave (Choranche, France). The straw was seen to consist of two units. Unit 1 (U1) corresponded to the part of the stalactite that grew before the cave was discovered. Its brown colour is due to sediment deposited in the entrance chamber during floods of the underground river. Unit 2 (U2) is the part of the stalactite that grew after the cave entrance was enlarged, thereby reducing the frequency of floods. Parts U2.1 and U2.2 (close up 1 and close up 2, respectively) show substantially different growth rates (3 to 5 times higher for U2.2). Lamina-count dating of this transition has shown that it corresponds to the increased ventilation of the chamber (reductions in pCO2 and hygrometry).
Cette fistuleuse (stalactite creuse) de la zone d’entrée de la grotte de Coufin (Choranche, France) est divisée en deux unités principales. L’U1, partie marron, correspond à la croissance de la stalactite avant la découverte de la cavité. La couleur est liée à des mises en charge de la salle d’entrée liées aux crues de la rivière souterraine. L’U2 est la partie ayant poussé depuis l’ouverture de la cavité. L’élargissement de l’entrée a réduit la fréquence des mises en charge du réseau. Les parties U2.1 et U2.2 (respectivement agrandissement 1 et 2) sont distinguées par des vitesses de croissance différentes (3 à 5 fois supérieures pour U2.2). Par comptage de lamines, on peut dater cette transition et montrer qu’elle correspond à l’augmentation de la ventilation de la cavité (diminution des pCO2 et de l’hygrométrie).

16Macroscopic and microscopic colours of speleothem can also provide information about changing environment, as they owe their colour to two factors: the crystalline structure (opacity; Genty et al., 1997; Perrette et al, 2005) and chemical components (either organic or metallic) trapped in the crystal matrix (White, 1981; Ramseyer et al, 1997; Van Beynen et al., 2001; Hill and Forti, 1997, and references therein). Detrital or organic fragments can also colour the speleothems. The Portalerie speleothem, from the Causse du Larzac (fig. 5), provides a good example of colour changes, which are caused by carbon/soot inclusions within the calcite, in relation with human activity in and around the chamber around 4 500 years BP (uncalibrated 14C).

Fig. 5 – Record of human activities in a stalagmite from the Aven de la Portalerie (Causse du Larzac, France).
Fig. 5 – Enregistrement des activités humaines dans une stalagmite de l’aven de la Portalerie (Causse du Larzac, France).

Fig. 5 – Record of human activities in a stalagmite from the Aven de la Portalerie (Causse du Larzac, France).Fig. 5 – Enregistrement des activités humaines dans une stalagmite de l’aven de la Portalerie (Causse du Larzac, France).

This speleothem provides a good illustration of the morphological and petrographical indicators contained within stalagmites. It shows five growth stages (drawn below, 1 to 5) associated with the presence or absence of human activity: 1: initial growth phase; 2: first breakage of the speleothem. The broken part has not been found (possibly broken by a human); 3: new growth phase with soot and carbon inclusions (fires in or above the modern chamber, contemporaneous with the growth); 4: phase of “clean” growth (the archaeological site was no longer occupied); 5: new breakage of the speleothem, this time the broken section was found on the ground, trapped in a stalagmitic floor containing carbon fragments dated at 4 500 yr BP (uncalibrated 14C date).
Cette concrétion illustre bien les indicateurs morphologiques et pétrographiques utilisables dans les stalagmites. Cette concrétion présente cinq stades (représentés dessous de 1 à 5) associés ou non à des activités humaines : 1 : phase de croissance initiale ; 2 : première rupture de la concrétion, la partie cassée n’est pas retrouvée (hypothèse d’une cassure d’origine anthropique) ; 3 : nouvelle phase de croissance avec des inclusions de suies et charbons (feux dans ou au dessus de la cavité contemporains de la croissance) ; 4 : phase de croissance "propre" (le site archéologique n’est plus occupé) ; 5 : nouvelle cassure de la concrétion qui cette fois-ci est retrouvée au sol et piégée dans un plancher stalagmitique englobant des charbons datés à 4 500 ans BP (non calibrés).

Detrital deposits: record of a monsoon phenomenon

17The karsts of southern China contain a large number of extensive underground networks that have recorded the area’s environmental history over a long period of time. The 11-km long Dadong Cave (Hubei) is one of several natural tunnels in China’s sub-tropical monsoon region (fig. 6). The entrance to the sinkhole, which opens at the Cambrian-Ordovician contact, is formed by a 100-m high horizontal cave entrance at the end of a blind canyon containing an ephemeral stream, whose flood-flow discharge can reach several tens of m3.s-1. Three groups of trapped detrital sediments have been recognised in several parts of the network: 1) an upper group consisting of a 20-m thick sequence of red horizontal rhythmites, at 100 m above the level of the cave entrance (not yet dated); 2) a lower group consisting of a 25-m thick sequence of brown rhythmites that have been scoured by the stream. Dates on wood fragments suggest that the bottom of the sequence was formed around 18 590 years BP (fig. 6) nested and entrenched fluvial terraces.

Fig. 6 – Diagrammatic cross section through the rhythmites of the Dadong Cave (Hubei, China).
Fig. 6 – Coupe de synthèse des rythmites de la grotte de Dadong (Hubei, Chine).

Fig. 6 – Diagrammatic cross section through the rhythmites of the Dadong Cave (Hubei, China). Fig. 6 – Coupe de synthèse des rythmites de la grotte de Dadong (Hubei, Chine).

These rhythmites provide a record of large monsoon floods over a period of 20,000 years. Only the lower group and the fluviatile terraces are shown in the figure. Inset map showing the location of the Dadong cave.
Ces rythmites présentent un enregistrement des grandes crues de mousson depuis 20 000 ans. Seul l’ensemble inférieur et les terrasses fluviatiles sont représentés sur la figure. Encart montant la localisation de la grotte du Dadong.

18In the lower sequence of brown rhythmites, which are located in a large chamber that acts as a settling pond, it is possible to follow the record of large monsoon floods over a period of 20 000 years. A scoured discordance was found, one metre below the top of the sequence. It is characterised by several sandy layers, followed by a charcoal-rich horizon. Several data provide information on the climate and human occupation of the site (fig. 6): (i) natural or manmade fires at the base of the sequence with micro-charcoal and ash in several laminae of isotope stage 2; (ii) soil erosion, fires and driftwood linked to massive deforestation during the 20thcentury; this phase corresponds to the scoured discordance; (iii) the large monsoon flood of 1997, which severely affected this area, as shown by numerous tree trunks. Analyses and dates on driftwood and charcoal samples from this detrital sequence, combined with information contained in the stalagmites above the chamber, have given a detailed history of the environmental evolution of the area (Pomel and Maire, 1997). This data can be used to distinguish the current effects of climate change from the results of human activities in an area of the world where important new data has been obtained on the role of palaeo-monsoons during the last 224 000 years, thanks to 18O analyses of a stalagmite from the Sanbao Cave, Hubei (Wang et al., 2008).

The limitations of current karst analysis methods

19The data provided by karst studies are increasingly being used to answer a wide range of questions in palaeogeographic reconstructions, climate reconstructions, and human sciences (e.g., prehistory). Modern data acquisition and processing methods, high-resolution temporal and spatial analysis tools, have greatly increased the acuity of our interpretations of karst landforms, flows and sedimentary archives. New research is pushing back the limits set by our current knowledge of karsts and by existing analysis methods. Here, we look at three difficulties that modern karst research is striving to overcome: (i) the need to determine reliable chronologies for karst events over both long and very short time scales; (ii) the need to take into account “site effects”, which can mask the transfer of environmental data and/or the recording of this information in karst archives; (iii) the need to compare past and contemporary dynamics using a diachronic approach.

Cosmogenic isotopes help to determining long-term chronologies

20Until the beginning of the 2000s, the chronological calibration of karstogenic information was mostly based on speleothems dating with the 234U/230Th method (Quinif, 1991), so that it was possible to give an age to karst networks and place them in the overall evolution of limestone areas. The obtained dates disproved a number of paradigms, most notably the belief, held until the beginning of the 1980s, that the formation of the world’s large cave networks was related with the Quaternary glaciations. However, in the 1980s and 1990s, it was realised that many underground systems were formed during the Tertiary period. In that case, the 234U/230Th method proves to be limited as it cannot go further back than 400 000 years. Other dating methods were developed including palaeomagnatism on infillings and speleothems (Audra, 1994), and micropalaeontological studies of karst sediments (Delannoy et al., 1997). More recently, measurements of the 26Al/10Be cosmogenic isotope contents of endokarst sediments have opened new perspectives for karst reconstructions (Granger et al., 2001; Häuselmann and Granger, 2004). The 26Al/10Be method has been applied to the perched karst systems of the eastern part of the Chartreuse Mountains. It shows palaeodrainage structures that are very different from the current karst drainage system (Hobléa, 1999). The Granier karst system, which occupies the north-eastern corner of the Chartreuse, consists of more than 70 km of galleries over a vertical interval of 635 m (fig. 7). Data obtained by applying newly developed 3D geomatic analysis tools suggest that the two highest stages of the system were formed before the Quaternary. The earliest 26Al/10Be dates on quartz grains from detrital sediments trapped in the Granier network support this hypothesis, as they give a Pliocene age for the formation of the Granier’s uppermost palaeodrainage system (tab. 1).

Fig. 7 – The perched karst of Mount Granier (Chartreuse massif, France).
Fig. 7 – Le karst perché du Mont Granier (Massif de la Chartreuse, France).

Fig. 7 – The perched karst of Mount Granier (Chartreuse massif, France). Fig. 7 – Le karst perché du Mont Granier (Massif de la Chartreuse, France).

A : Sketch map showing the location of Mount Granier and surroundings ; B : 3D view of the underground networks in their geographical setting ; C : The stages underground networks of the Granier karst system.
A : Schéma de localisation du Mont Granier et des environs ; B : Vue en 3D des réseaux souterrains dans leur contexte géographique ; C : Les étages des réseaux souterrains du système karstique du Mont Granier.

Tab. 1 – Burial ages for two samples of quartzitic endokarst infillings from Mont Granier (Chartreuse, France) based on the 26Al/10Be method (26Al measurements: National Laboratory California, USA; 10Be measurements: PSI/ETH, Zurich, Switzerland; geochronological calculations: IAG, Vienna, Austria).
Tab. 1 – Âge d’enfouissement de deux échantillons de remplissages endokarstiques quartzeux du Mont Granier (Chartreuse, France) obtenu par la méthode 26Al / 10Be (mesures 26Al : National Laboratory Californie, USA ; mesures 10Be : PSI/ETH, Zurich, Suisse ; calculs géochronologiques : IAG, Vienne, Autriche).

Sample

[26Al](103 at/g)

[10Be](103 at/g)

26Al /10Be

Age (Ma)of burial

Erosion rate(m/Ma)

1 LIL 3

148 ± 19

161.4 ± 8.2

0.92 ± 0.13

4.28 ± 0.27

7 ± 1

PULS 4

180 ± 21

129 ± 8.5

1.39 ± 0.19

3.42 ± 0.27

14 ± 3

Note : The values are standardised for a mean life of 1.34 Ma (uncertainties = 1σ).

High-resolution time studies

21A major purpose of any study of natural archives is to get a reliable chronology of events. Accurate chronologies are essential to determine relative influences of human activities and climate on changing environment. For example, in Vercors, the maximum impact of the Little Ice Age (LIA) occurred almost simultaneously with the demographic peak of the 19th century (Astrade et al., 2008); however, “almost” signifies a period of several tens of years! Because of this uncertainty, it is impossible to use endokarst sediment records to categorically determine whether periods of erosion, of increased organic-matter flow, or of increased hydrological flow during the LIA are primarily related to human causes or to natural climate change.

22Hence, accurate age models are vital for studies of recent environments. However, the U/Th ages obtained for many samples deposited in the French pre-Alps during the last 2000 years are unreliable. In the 1990s, a large research effort was made to use stalagmite laminae counts as geochronometers and get chronologies with a one-year resolution. The method was based on the assumption that laminae are produced annually. Unfortunately, stalagmite laminae are very different from tree growth rings and the annual-growth hypothesis is very difficult to verify without absolute chronological markers (tephras, U/Th, 14C ages, stratigraphic reference points). In fact, recent work has shown that, under identical biopedological conditions, laminae may be annual in some speleothems and supra-annual or infra-annual in others (Perrette, 1999; Perrette, 2000). This difference is linked to site effects, which affect both the transfer and recording of environmental signals. Research currently being carried out by EDYTEM as part of the ANR Climanthrope project (Climate-Anthropisation, 2007-2010) is using geochemical analyses and spectroscopic methods (spectrofluorescence, XRF, µXRF, µFTI) to try and find a reliable annual signal within speleothems. Defining such a signal is essential to work on short time periods of centuries and millennia and to correctly differentiate the effects of climate and man on karst records (Carcaillet et al., 2007).

23The example of stalagmite laminae demonstrates the impact of site effects on the environmental signals within different karst systems. Thus, it is essential to take tem into account to get meaningful information about past climates, environments and human activities. It is the focus of the ANR Climanthrope research programme, which was set up to define the factors that influence the formation of karst records. Different underground archives are being analysed in terms of their (i) karstogenic environment, (ii) past, recent and current climatic and biopedological changes, (iii) current flow and sedimentation conditions (ventilation, hydrodynamics), and (iv) morphological, chemical and geochemical analyses of sediments.

24This approach has shown that the environmental information contained in stalagmites depends on the hydrological system: a transmissive system favours the transfer of ashes, soil, and micro-charcoal particles formed by human activity; whereas a filtering system favours the recording of less-disturbed, climate cycle signals (Perrette, 1999). Similarly, in the same geographical setting, information contained in underground lake sediments varies according to the position of the lake in the karst system, the existence and nature of upstream sediment traps (basins, sumps), and the geometry and dimensions of karst channels (Perroux, 2005).  Site effects may at first appear to be a limiting factor in the use of endokarst deposits as environmental archives. In fact, the opposite is true. When the influence of site effects is taken into account in the sampling and analysis of subterranean karst deposits, the records within a single cave can provide a variety of important data on past changes in climate or on the impact of human activities on the environment.

Current dynamics and the calibration of natural archives

25A prior calibration is required to correctly interpret the natural karst archives (either landforms or formations), especially in case of site effects. For example, it is important to understand hydrology and hydrochemistry of the flows feeding stalagmites (Verheyden et al., 1999). In case of cave morphology and detrital formations, such a “calibration” is gradually being refined as karst research and cave exploration advance. For example, investigation of underground lakes, exploration of sumps and underwater passages and studies of young karsts found in pacific atolls, provide modern analogues of older systems.

26Advances in high-resolution analysis methods considerably help to characterise modern sedimentary chemical and detrital flows, and thus to decipher natural archives. As a result, it is now possible to study the geochemistry of deposits on an annual-scale or even sub-annual-scale, thanks to X-ray fluorescence analysis. This is important to facilitate comparisons with modern analogues, most notably in combining descriptive methods with the monitoring of flows on an annual, seasonal, or even single meteorological event scale. These approaches are promising, especially to understand the sedimentary dynamics involved in the deposition of detrital subterranean sequences. For example, the Choranche Caves (Vercors) have already been the focus of numerous studies combining flow hydrological, hydrochemical measurements and geomorphological analyses of carbonate formations (stalagmites, straws, tufas, etc.) (Delannoy et al., 1999; Perrette et al., 2001). Due to the presence of underground lakes and sumps, this subterranean natural laboratory is particularly suited to study detrital sediment traps and karst lake formations compared to “modern analogues” (Perroux, 2005). The resulting typology, which is based on sediment size and sorting, has given satisfactory results when applied to non-functional karsts. Calibration and the active analogue approach are essential parts of the process of extracting environmental information from recent and modern subterranean karst records. This approach is compulsory for working on subterranean karst sediments formed during former karstogenic phases and in other climate settings. The comparison between modern karsts and ancient karsts has a double benefit, in that it leads to better interpretations of fossil archives and a better understanding of functional karsts.

Conclusion

27The objective of the present paper was to summarise current research interests in the karstic areas and processes studied by French geomorphologists. The given examples highlight the fact that karst forms and features provide records that can be analysed on different spatial and temporal scales. Recent research shows two paradoxes. First, although karsts are sensitive to external changes, they are also able to conserve geomorphological features over long periods of time. Second, although the underground parts of karsts are disconnected from the external environment, karst sediments in cave networks are capable to record environmental changes.

28This double paradox, combined with new tools for acquiring and processing morphological, geochemical and isotopic data, has allowed scientists to reveal stages in a karst’s evolution that had been previously masked by the paucity of surface evidence, and determine the responsible driving forces responsible. Three of the examples described above (Vercors, Ardèche, Lorraine) provide a good illustration of the wide range of morphological information given by karst features developed in very different geodynamic settings. Other examples, based on the sediments found in cave networks, show the value of karst analyses to draw up environmental reconstructions. The development of data acquisition and processing tools, and new geochemical analysis methods, together with studies of the mechanisms involved in the formation of sedimentary deposits within karst systems, have enabled researchers to carry out high-resolution analyses of environmental changes and determine their causes (climatic, anthropogenic, etc). The examples also highlight the complementary information provided by chemical and detrital deposits. Speleothem data now plays an important role in environmental and anthropological research, and it seems certain that, within a few years, first-class data will also be provided by analyses of underground detrital sediments, especially to understand environmental impacts either from changing climate or human activities. These long-ignored deposits are currently the focus of intensive research and methodology development.

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Annexe

Version française abrégée

Objet géomorphologique particulier, le karst est de plus en plus sollicité dans les reconstitutions paléogéographiques et l’analyse des impacts anthropiques et climatiques dans les dynamiques environnementales. Cet intérêt est lié à sa capacité à enregistrer et à conserver des informations morphogéniques dans les conduits souterrains. Deux grandes familles d’archives sont abordées : les formes et les formations endokarstiques. Cette distinction permet d’aborder les temps de la géologie et des environnements récents.

Les formes exo- et endokarstiques constituent des enregistrements directs des données paléogéographiques et ont la faculté de se conserver sur plusieurs millions d’années. L’étude des formes du karst apporte un éclairage complémentaire, souvent unique, aux reconstitutions paléogéographiques issues d’autres indicateurs généralement géologiques. Trois exemples régionaux permettent de saisir la diversité des enregistrements karstiques. Les deux premiers cas permettent de souligner l’intérêt du karst dans la morphogenèse d’un massif en relation avec les événements isostatiques et eustatiques majeurs (fig. 1 et fig. 2). L’étude karstogénique du massif du Vercors (Alpes – France) a permis d’identifier plusieurs phases d’incision du réseau hydrographique régional et d’en préciser les moteurs. Cette reconstitution s’est appuyée sur les formes issues de contexte à faible énergie hydraulique (aplanissements et poljés) et sur l’étagement de paléodrains endokarstiques (fig. 1). Les paléogéographies miocènes, antérieures et postérieures au soulèvement alpin ainsi que les réajustements engendrés par les glaciations quaternaires ont pu être précisées. L’exemple de l’aven d’Orgnac situé entre les vallées de la Cèze et de l’Ardèche (Sud de la France) souligne un autre intérêt du karst : enregistrer les remontées du niveau de base régional (fig. 2). Si les incidences morphogéniques de la crise messinienne sont de mieux en mieux cernées, les effets de la transgression pliocène dans la morphogénèse des karsts méditerranéens restaient encore à préciser. L’étude des réseaux d’Orgnac a permis de définir l’impact de cette transgression bien au-delà des seuls indices géologiques et géomorphologiques de surface. L’étude croisée de la maturité des systèmes endokarstiques et de leur distribution spatiale a permis dans le cas du Barrois (Lorraine – France) d’identifier différentes phases d’ablation de la couverture géologique crétacée et leurs incidences sur l’organisation du réseau hydrographique régional (fig. 3).

Les dépôts endokarstiques permettent de travailler à une autre échelle temporelle : celle des variations environnementales contemporaines de l’anthropisation. Trois exemples mettent en avant la complémentarité des concrétions et des dépôts détritiques. Deux études de cas soulignent la qualité des informations contenues dans les concrétions. Dans le premier cas, les modifications climatiques de la grotte de Choranche (Vercors- France), liées aux aménagements spéléologiques et touristiques durant le dernier siècle, ont pu être identifiées grâce à l’étude des lamines de croissance d’une stalactite (fig. 4). Dans le second cas, plusieurs stades d’occupation préhistoriques du Causse du Larzac (sud du Massif Central – France) ont pu être décelés grâce aux indicateurs morphologiques et pétrographiques contenus dans les stalagmites (fig. 5). L’étude des dépôts détritiques de la grotte de Dadong (Chine ; fig. 6) a permis de préciser l’impact des moussons passées et actuelles dans l’évolution environnementale de cette région, notamment l’érosion des sols.

L’article précise les écueils auxquels sont actuellement confrontées les recherches sur le karst. Ceux-ci portent sur le calage chronologique des objets karstiques dans le temps géologique (plusieurs millions d’années), la haute résolution temporelle et la calibration des archives karstiques et la déformation d’informations environnementales due à des effets de filtre du drainage karstique. L’exemple du massif de la Chartreuse (Alpes du nord, France) souligne l’intérêt des isotopes cosmogéniques dans le calage d’anciennes phases karstogéniques rapportées au Pliocène (fig. 7). Dans une toute autre dimension temporelle, les formations endokarstiques, par leur caractère laminé ou rythmé, constituent un support privilégié pour travailler à haute résolution temporelle. L’apport de l’imagerie laser (fluorescence, réflectance, etc.) permet d’identifier des signaux environnementaux annuels voire saisonniers et de faire la part des effets climatiques et anthropiques dans les dynamiques environnementales passées et actuelles.

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Table des illustrations

Titre Fig. 1 – Diagrammatic cross section showing the morphogenetic evolution of the Coulmes-Choranche area (Vercors), from surface and subterranean karst features. Inset map showing the Vercors area.Fig. 1 Coupe synthétique de l’évolution morphogénique du massif des Coulmes-Choranche (Vercors) à partir des formes exo- et endokarstiques. Encart montrant la localisation du Vercors.
URL http://geomorphologie.revues.org/docannexe/image/7520/img-1.png
Fichier image/png, 61k
Titre Fig. 2 – Diagrammatic cross section showing the karst evolution of the Aven d’Orgnac (Ardèche): subterranean karst records of Mio-Pliocene eustatic variations. Variation in the level of the Rhône (Clauzon, 1982; Mocochain, 2007). Inset map showing the Orgnac area.Fig. 2 – Coupe synthétique de l’évolution karstogénique de l’aven d’Orgnac (Ardèche) : les enregistrements endokarstiques des variations eustatiques mio-pliocènes. Variation du niveau du Rhône d’après Clauzon (1982) et Mocochain (2007). Encart montrant la localisation d’Orgnac.
URL http://geomorphologie.revues.org/docannexe/image/7520/img-2.png
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Titre Fig. 3 – Diagrammatic cross section of the Poissons, Cousance and Rupt du Puits karsts (Lorraine): synchronous records of the ablation of the Cretaceous sedimentary cover and the incision of the regional hydrographic network. Fig. 3 – Coupe synthétique des karsts de Poissons, Cousance et du Rupt du Puits (Lorraine) : enregistrements synchrones de l’ablation de la couverture géologique crétacée et de l’incision du réseau hydrographique.
Légende 1: Palaeo base level (horizontal karst passages); 2: Altitude relative to the palaeo base levels; 3: Inactive karst sinkholes; 4: Active karst sinkholes; 5: Infra-Cretaceous erosion surface (under a layer of cover or exposed); 6: Infra-Cretaceous erosion surface, eroded today; 7: U/Th dates; 8: Karst spring. Inset map showing the Barrois area.1 : Paléo-niveau de base (drains karstiques subhorizontaux) ; 2 : Altitude relative des paléo-niveaux de base karstique ; 3 : Pertes karstiques abandonnées ; 4 : Pertes karstiques fonctionnelles ; 5 : surface d’érosion infra-crétacée ; 6 : surface d’érosion infra-crétacée actuelle ; 7 : âges U/Th ; 8 : Sources karstiques. Encart montrant la localisation du Barrois.
URL http://geomorphologie.revues.org/docannexe/image/7520/img-3.png
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Titre Fig. 4 – Stalactite growth sensitivity to changing dynamics of the karst environment. Fig. 4 – Sensibilité de la croissance des stalactites aux changements de dynamique du milieu karstique.
Légende The first example concerns the analysis of a soda straw (hollow stalactite) from the entrance to the Coufin Cave (Choranche, France). The straw was seen to consist of two units. Unit 1 (U1) corresponded to the part of the stalactite that grew before the cave was discovered. Its brown colour is due to sediment deposited in the entrance chamber during floods of the underground river. Unit 2 (U2) is the part of the stalactite that grew after the cave entrance was enlarged, thereby reducing the frequency of floods. Parts U2.1 and U2.2 (close up 1 and close up 2, respectively) show substantially different growth rates (3 to 5 times higher for U2.2). Lamina-count dating of this transition has shown that it corresponds to the increased ventilation of the chamber (reductions in pCO2 and hygrometry).Cette fistuleuse (stalactite creuse) de la zone d’entrée de la grotte de Coufin (Choranche, France) est divisée en deux unités principales. L’U1, partie marron, correspond à la croissance de la stalactite avant la découverte de la cavité. La couleur est liée à des mises en charge de la salle d’entrée liées aux crues de la rivière souterraine. L’U2 est la partie ayant poussé depuis l’ouverture de la cavité. L’élargissement de l’entrée a réduit la fréquence des mises en charge du réseau. Les parties U2.1 et U2.2 (respectivement agrandissement 1 et 2) sont distinguées par des vitesses de croissance différentes (3 à 5 fois supérieures pour U2.2). Par comptage de lamines, on peut dater cette transition et montrer qu’elle correspond à l’augmentation de la ventilation de la cavité (diminution des pCO2 et de l’hygrométrie).
URL http://geomorphologie.revues.org/docannexe/image/7520/img-4.png
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Titre Fig. 5 – Record of human activities in a stalagmite from the Aven de la Portalerie (Causse du Larzac, France).Fig. 5 – Enregistrement des activités humaines dans une stalagmite de l’aven de la Portalerie (Causse du Larzac, France).
Légende This speleothem provides a good illustration of the morphological and petrographical indicators contained within stalagmites. It shows five growth stages (drawn below, 1 to 5) associated with the presence or absence of human activity: 1: initial growth phase; 2: first breakage of the speleothem. The broken part has not been found (possibly broken by a human); 3: new growth phase with soot and carbon inclusions (fires in or above the modern chamber, contemporaneous with the growth); 4: phase of “clean” growth (the archaeological site was no longer occupied); 5: new breakage of the speleothem, this time the broken section was found on the ground, trapped in a stalagmitic floor containing carbon fragments dated at 4 500 yr BP (uncalibrated 14C date).Cette concrétion illustre bien les indicateurs morphologiques et pétrographiques utilisables dans les stalagmites. Cette concrétion présente cinq stades (représentés dessous de 1 à 5) associés ou non à des activités humaines : 1 : phase de croissance initiale ; 2 : première rupture de la concrétion, la partie cassée n’est pas retrouvée (hypothèse d’une cassure d’origine anthropique) ; 3 : nouvelle phase de croissance avec des inclusions de suies et charbons (feux dans ou au dessus de la cavité contemporains de la croissance) ; 4 : phase de croissance "propre" (le site archéologique n’est plus occupé) ; 5 : nouvelle cassure de la concrétion qui cette fois-ci est retrouvée au sol et piégée dans un plancher stalagmitique englobant des charbons datés à 4 500 ans BP (non calibrés).
URL http://geomorphologie.revues.org/docannexe/image/7520/img-5.png
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Titre Fig. 6 – Diagrammatic cross section through the rhythmites of the Dadong Cave (Hubei, China). Fig. 6 – Coupe de synthèse des rythmites de la grotte de Dadong (Hubei, Chine).
Légende These rhythmites provide a record of large monsoon floods over a period of 20,000 years. Only the lower group and the fluviatile terraces are shown in the figure. Inset map showing the location of the Dadong cave.Ces rythmites présentent un enregistrement des grandes crues de mousson depuis 20 000 ans. Seul l’ensemble inférieur et les terrasses fluviatiles sont représentés sur la figure. Encart montant la localisation de la grotte du Dadong.
URL http://geomorphologie.revues.org/docannexe/image/7520/img-6.png
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Titre Fig. 7 – The perched karst of Mount Granier (Chartreuse massif, France). Fig. 7 – Le karst perché du Mont Granier (Massif de la Chartreuse, France).
Légende A : Sketch map showing the location of Mount Granier and surroundings ; B : 3D view of the underground networks in their geographical setting ; C : The stages underground networks of the Granier karst system.A : Schéma de localisation du Mont Granier et des environs ; B : Vue en 3D des réseaux souterrains dans leur contexte géographique ; C : Les étages des réseaux souterrains du système karstique du Mont Granier.
URL http://geomorphologie.revues.org/docannexe/image/7520/img-7.png
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Jean-Jacques Delannoy, Christophe Gauchon, Fabien Hobléa, Stéphane Jaillet, Richard Maire, Yves Perrette, Anne-Sophie Perroux, Estelle Ployon et Nathalie Vanara, « Karst: from palaeogeographic archives to environmental indicators », Géomorphologie : relief, processus, environnement, vol. 15 - n° 2 | 2009, 83-94.

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Jean-Jacques Delannoy, Christophe Gauchon, Fabien Hobléa, Stéphane Jaillet, Richard Maire, Yves Perrette, Anne-Sophie Perroux, Estelle Ployon et Nathalie Vanara, « Karst: from palaeogeographic archives to environmental indicators », Géomorphologie : relief, processus, environnement [En ligne], vol. 15 - n° 2 | 2009, mis en ligne le 01 juillet 2011, consulté le 23 août 2017. URL : http://geomorphologie.revues.org/7520 ; DOI : 10.4000/geomorphologie.7520

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Auteurs

Jean-Jacques Delannoy

Laboratoire EDYTEM, université de Savoie-CNRS, 73376 le Bourget du Lac cedex (France), jean-jacques.delannoy@univ-savoie.fr

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Christophe Gauchon

Laboratoire EDYTEM, université de Savoie-CNRS, 73376 le Bourget du Lac cedex (France)

Fabien Hobléa

Laboratoire EDYTEM, université de Savoie-CNRS, 73376 le Bourget du Lac cedex (France)

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Stéphane Jaillet

Laboratoire EDYTEM, université de Savoie-CNRS, 73376 le Bourget du Lac cedex (France)

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Richard Maire

Laboratoire ADES, université de Bordeaux III-CNRS, Maison des Suds, 12 Esplanade des Antilles 33607 PESSAC cedex (France), rmaire@ades.cnrs.fr

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Yves Perrette

Laboratoire EDYTEM, université de Savoie-CNRS, 73376 le Bourget du Lac cedex (France)

Anne-Sophie Perroux

Laboratoire EDYTEM, université de Savoie-CNRS, 73376 le Bourget du Lac cedex (France)

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Estelle Ployon

Laboratoire EDYTEM, université de Savoie-CNRS, 73376 le Bourget du Lac cedex (France)

Articles du même auteur

Nathalie Vanara

Laboratoire ADES, université de Bordeaux III-CNRS, Maison des Suds, 12 Esplanade des Antilles 33607 PESSAC cedex (France)

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