THE ROLE Of CONDENSATION-CORROSION IN THERMAL SPELEOGENESIS: STUDy Of A Hy POGENIC SULf IDIC CAVE IN AIx- LES-BAINS, fRANCE VLOGA KONDENZNE KOROZIJE V TERMALNI SPELEOGENEZI: ŠTUDIJA HIPOGENE SULf IDNE JAME V AIx- LES-BAINS, fRANCIJA Philippe AUDRA 1 , f abien HOBLEA 2 , Jean-yves BIGOT 3 & Jean-Claude NOBECOURT 4 1 Équipe Gestion et valorisation de l’ environnement (GVE), UMR 6012 “ESPACE” du CNRS, University of Nice Sophia-Antipolis, 98 boulevard Édouard Herriot, BP 3209, 06204 NICE Cédex 3, f rance; e-mail: audra@unice.fr 2 University of Savoy, EDy TEM, Campus, 73376 LE BOURGET Cédex, f rance; e-mail: f abien.Hoblea@univ-savoie.fr 3 f rench Association of Karstology; e-mail: catherine.arnoux@club-internet.fr 4 f rench Association of Karstology; e-mail: jcnobecourt@free.fr Received/Prejeto: 24.01.2007 COBISS: 1.01 ACTA CARSOLOGICA 36/2, 185–194, POSTOJNA 2007 Abstract UDC 551.435.8:551.3.053(44) Philippe Audra, Fabien Hoblea, Jean-Yves Bigot & Jean- Claude Nobecourt: The role of condensation corrosion in ther- mal speleogenesis. Study of a hypogenic sulfidic cave in Aix- les-Bains, France Condensation-corrosion is an active speleogenetical process in thermal caves where high thermal gradient drives air convection. Wall retreat rates are greater than in meteoric caves. Conversely, evaporation produces depositional processes by replacement of limestone by gypsum and by aerosol decantation leading to the formation of popcorns. The Chevalley Aven belongs to Aix-les- Bains thermal-sulfidic cave system. Condensation occurs at the contact of cool walls of large spheres; conversely, evaporation occurs at the output of the narrow passages where the air sinks down from the upper sphere. A weathered layer and biofilms are present where slow condensation occurs. Corrosion distribution varies according to thermal rock conductivity and causes the sphere to develop upwards, laterally, and divergent. This mor- phodynamic pattern favors the development of stacked spheres, isolated by narrow necks, and arranged in a bush-like pattern. This development is clearly active in the vadose zone above the thermal water table. We propose that some avens above wa- ter table hypogenic caves, like Villa Luz (Mexico), may be of condensation-corrosion origin instead of phreatic. f uture de- velopment will collect physical and chemical data to calculate the condensation-corrosion budget and assess its role in cave development. Key words: condensation-corrosion, thermal caves, sphere genesis, air convection, Aix-les-Bains thermal-sulfidic cave sys- tem, Chevalley Aven, f rance. Izvleček UDK 551.435.8:551.3.053(44) Philippe Audra, Fabien Hoblea, Jean-Yves Bigot & Jean-Claude Nobecourt: Vloga kondenzne korozije v termalni speleogenezi: študija hipogene sulfidne jame v Aix-les-Bains, Francija Kondenzacijska korozija predstavlja aktivni speleogenetski pro- ces v termalnih jamah, kjer kroženje zraka povzroča visok ter- malni gradient. Raztapljanje jamskih sten in stropa je v takšnih jamah večje, kot v vodnih jamah. Izhlapevanje povzroča tudi odlaganje, ki se vrši v obliki nadomeščanja apnenca s sadro in z usedanjem aerosola, kar vodi v tvorbo značilnih morfoloških oblik (popcorn). Brezno Chevalley sodi k termalno-sulfidnemu jamskemu sistemu Aix-les-Bains. Kondenzacija deluje na stiku toplega zraka s hladnimi jamskimi stenami, do izhlapevanja pa prihaja na izhodu ozkih jamskih prehodov. Prepereli sloji kam- nine in prevleke biološkega izvora se pojavljajo na mestih, kjer je počasna kondenzacija. Porazdelitev korozijskega delovanja je odvisna od termalne prevodnosti kamnine in povzroča nas- tanek stropnih, stenskih in divergentnih krožnih oblik. Takšni morfodinamični vzorci omogočajo oblikovanje raznih kupol, ločenih z ožinami in vejasto razvrščenimi. Takšne morfološke oblike so značilne za vadozno cono. Nekatere zožitve nad vod- no gladino v hipogenih jamah (npr. Villa Luz v Mehiki) so kon- denzacijsko-korozijskega in ne freatičnega nastanka. Prihodnje raziskave fizikalno-kemičnih lastnosti bodo pokazale pomen vpliva kondenzacijsko-korozijskih procesov za razvoj jam. Ključne besede: kondenzacijska korozija, termalne jame, sphere genesis, zračna cirkulacija, Aix-les-Bains thermalno- sulfidni jamski sistem, brezno Chevalley, f rancija. INTRODUCTION Convection and condensation-corrosion are generally considered as minor processes for cave development, producing only etching or smoothing of flow-induced features and conduits. However, some authors have put forward the importance of condensation-corrosion in specific conditions. Müller (1974) was among the first to relate spherical cupolas (called spheres in this paper) to air convection above thermal lakes, whereas Rudnicki (1978) had attributed them mainly to phreatic convec- tion. Sphere development by condensation has been simulated numerically, focusing on the role of pressure changes connected to flooding in the epiphreatic zone (Mucke & al., 1983; Lismonde, 2000) or cooling of rising air above thermal lakes (Cigna & f orti, 1986; Szunyogh, 1990; Lismonde, 2003). Dublyansky & Dublyansky (2000) updated a review of worldwide contributions about con- densation. f inally, Dreybrodt & al. (2005) recently per- formed a comprehensive modeling of this process within various boundary conditions. As a rule, most of these authors stated the efficiency of convection-condensation on speleogenesis only in specific conditions that are not widespread in meteoric caves, but more frequent in hy- pogenic caves such as thermal caves or caves with high carbonic or sulfidic atmospheres. The study of Chevalley and Serpents Caves in Aix-les- Bains, f rance, clearly shows the importance of this proc- ess which is enhanced by sulfidic corrosion. Aix-les-Bains Caves, located in the Northern f rench Prealpes in Savoy, are still active and provide an outstanding picture of con- vection processes and related corrosion and deposition phenomena, particularly with gypsum replacement. This paper presents the first results of a study of the morphology of the cave, the distribution of the depos- its, and the distribution of condensation and evaporation zones, in order to make an assessment of the role of con- densation-corrosion role in cave development. A second study is being undertaken to calculate the condensation- corrosion budget through the collection of physical and chemical data. The first part of this paper summarizes current knowledge about condensation-corrosion and the development of spheres. The second part examines condensation-corrosion and the resulting morphologies and deposits in Chevalley and Serpents Caves. f inally, we will discuss the respective role of the different speleoge- netical processes. ACTA CARSOLOGICA 36/2 – 2007 186 CONDENSATION-CORROSION AND THE DEVELOPMENT Of SPHERES SPELEOGENESIS By SULf UR RELEASE AND SULf IDIC CAVES Hypogenic caves formed from sulfur release producing sulfuric acid and limestone corrosion with replacement by gypsum were only recently identified (Morehouse, 1968; Egemeier, 1981). They have been particularly stud- ied in the Guadalupe Mountains, USA (Hill, 1987), in the f rasassi Caves, Italy (Galdenzi & Menichetti, 1990; f orti, 1996), and in f rance (Audra & al., 2002; Audra & Hof- mann, 2004; Audra & Häuselmann, 2004; Audra, 2005). The active participation of microbial processes was iden- tified in Movile Cave, Romania (Sarbu & al., 1996), and in Villa Luz Cave, Mexico (Hose & Pisarowicz, 1999). CONVECTION ABOVE A THERMAL LAKE Air convection occurs when a thermal gradient exists be- tween a lake heated by thermal flow and the upper part of the cave which is cooler. Evaporation from the thermal lake produces warm-moist air that rises, while cooler air sinks. Since the thermal gradient is maintained by heat transfer through the cooler rock mass toward the surface (Lismonde, 2003), permanent convection cells exchange heat and water between the different parts of the cave. The thermal flux is independent of the distance to the surface (Dreybrodt & al., 2005), but high gradients may be present close to topographic surfaces where meteoric seepage can cool the cave roof. CONDENSATION ON COOL WALLS Warm-moist rising air cools at the contact of the colder walls and ceilings producing condensation. Condensa- tion, in turn, releases heat at the wall surface that low- ers the thermal gradient itself (Dreybrodt & al., 2005). f or vapor-saturated air, the condensation rate depends on the thermal gradient. Thus, maximal condensation rates appear when hot thermal water is present within a cave close to the surface, at the ceiling of the upper passages, and more generally in chambers which are relatively isolated by narrow passages from lowermost warmer places. PHILIPPE AUDRA, fABIEN HOBLEA, JEAN-yVES BIGOT & JEAN-CLAUDE NOBECOURT ACTA CARSOLOGICA 36/2 – 2007 187 CORROSION Of THE ROCK WALLS When condensation water quickly reaches equilibrium with carbon dioxide-rich cave atmosphere and residence time of contact between aggressive condensation wa- ter and limestone wall is sufficient, corrosion can occur (Dreybrodt & al., 2005). Such weathering of walls has been demonstrated in Movile Cave, Romania, where soft residue displays C-light isotopic ratios. This depleted car- bon results from the solution of biogenic carbon dioxide (Sarbu & Lascu, 1997). Modeling of wall retreat within thermal conditions shows rates about one order of mag- nitude above those of meteoric karst, ranging from 2 to 20 cm / ka (Mucke & al., 1983; Cigna & f orti, 1986; Drey- brodt & al., 2005), thus allowing the possibility of cave development in a “reasonable” time-span. Modeling has also shown that in “normal” meteoric caves condensa- tion-corrosion does not reach sufficient rates to be con- sidered as a main speleogenetic process. Consequently, condensation-corrosion represents a main process only in the following cases: 1/ when warm- moist air condenses in the entrance of a cold cave in sum- mer; 2/ above thermal lakes where the thermal gradient is high; 3/ where CO 2 or H 2 S-rich atmosphere gives a high aggressivity to the condensed water, even if this pa- rameter is considerably less important than the conden- sation itself (Szunyogh, 1990). Condition 2 may combine with 3. DEPOSITIONAL PROCESSES Several types of depositional processes may occur sepa- rately or in combination: 1/ by evaporation of the con- densed water at the base of walls where air is warmer; 2/ by replacement of limestone by gypsum when H 2 S is released from thermal lakes; 3/ by decantation of aerosols building popcorns in the lowest part of passages (Dub- lyansky & Pashenko, 1997). f ig. 1 resumes the steps in- volved in condensation-corrosion. SPHERE GENESIS By CONVECTION AND CONDENSATION The term sphere roughly corresponds to hemispheric or semi-spherical holes located in the ceiling. Spheres may develop by different processes, involving convection and sometimes additionally condensation: 1/ by pressure rise and condensation after flooding in the epiphreatic zone (Mucke & al., 1983; Lismonde, 1999); 2/ by mixing-cor- rosion in the phreatic zone (Bögli, 1964, 1978); 3/ by con- vection of water in the phreatic zone (Rudnicki, 1978); 4/ by air convection above thermal lakes (Müller, 1974; Szunyogh, 1990). Except for (1), these processes corre- spond to slow moving fluid (water or gas) and convection more or less directly connected to uprising water, con- ditions encountered more frequently in hypogenic cave systems (Osborne, 2004). We will initially consider step (4) alone, that is air convection above thermal lakes mak- ing almost perfect spheres. The thermal conductivity of rock produces higher thermal gradients at the top of ceiling holes (Cigna & f orti, 1986; Szunyogh, 1990), thereby creating the great- Fig. 1: Schematic profile view of movile Cave (Sarbu & Lascu, 1997). A. Lower passage; B. Upper dry passage 1. Evaporation of water at the surface of the lake 2. Thermal flux through the bedrock 3. Condensation of water vapor onto the colder cave walls and ceiling in the upper cave passage 4. Dissolution of CO 2 into the condensate and formation of carbonic acid. 5. Reaction between carbonic acid and the carbonate bedrock with formation of bicarbonate 6. Transport of bicarbonate along the walls 7. Precipitation of aragonite crusts and dissociation of bicarbonate THE ROLE Of CONDENSATION-CORROSION IN THERMAL SPELEOGENESIS ... Fig. 2: Development of a sphere in the ceiling, where thermal gradient is the highest, by condensation-corrosion (Szunyogh, 1982). ACTA CARSOLOGICA 36/2 – 2007 188 est condensation and thus stron- gest corrosion: the sphere con- tinues developing upward, where condensation is at the maximum, and laterally, where seepage occurs, since it is connected to the lower conduit by a narrow neck Fig. 4: Theoretical development of simple and complex spheres. Simple sphere develops upwards and laterally (left); an irregularity in the ceiling produces a new sphere development that fits within the previous one (right); two neighboring spheres diverge toward the greatest zone of heat transfer (right) (after Szunyogh, 1990, modified). Fig. 5: 3D view of Sátorkö-puszta Cave, the bush-like pattern cave type, made of stacked up spheres (müller & Sarvary, 1977). PHILIPPE AUDRA, fABIEN HOBLEA, JEAN-yVES BIGOT & JEAN-CLAUDE NOBECOURT Fig. 3: Condensation occurs at the ceiling. Wall benefits from both condensation plus seepage. Evaporation plus saturated runoff occur at the bottom, thus preventing the expansion of the neck (from Szunyogh, 1990, redrawn). (f ig. 2, 3). Modeling shows that irregularity of the ceiling produces a new sphere development that fits within the previous one (Szunyogh, 1990). The development of two neighboring spheres will be divergent, toward the great- est potential heat transfer, because the rock in between the two spheres has less transfer potential and remains warm (f ig. 4). Such a process explains the development of stacked up spheres arranged in a bush-like structure, as found in the Sátorkö-puszta Cave, Hungary (f ig. 5). The smooth ceiling results from the regular corrosion by a thin film of condensed water rather than discrete drips (Mucke & al., 1983); slow runoff of condensed water at the base of the sphere makes corrosion furrows, and cal- cite deposits as popcorn at the outlet of the sphere after evaporation (Cigna & f orti, 1986). AIx -LES-BAINS THERMAL-SULf IDIC CAVES (CHEVALLEy AVEN AND SERPENTS CAVE) BUSH-LIKE SPHERES AND WATER TABLE PATTERN Aix-les-Bains is a thermal resort in Savoy, located between the Bourget Lake shore and the foot of the Bauges massif in Northern f rench Prealps. Chevalley Aven and Serpents Cave belong to the same system (f ig. 6) (Hobléa, 1999; Gallino 2006). Chevalley Aven is made of stacked spheres arranged in a bush-like pattern (Sátorkö-puszta Cave type). Chevalley Aven rises from the thermal water table and does not break through the surface, and the present entrance is arti- ficial. Serpents Cave is a water table cave, with spheres and avens at the ceiling. The active Alum thermal spring flows into the cave at the upstream end. The gallery is gently sloping downstream and is plugged at the end with till. A SULf IDIC AND THERMAL UPWELLING f LOW The discharge of the Alum spring ranges from 8 to 42 Ls -1 ; the temperature oscillates seasonally between 33.5 and ACTA CARSOLOGICA 36/2 – 2007 189 46.6 °C on account of some mixing with meteoric com- ponent (Muralt, 2003). Water has a high concentration of calcium, sulfate, and secondary sodium, magnesium, and silica (Tab. 1). It degasses some H 2 S and CO 2 and brings up microbial soft flakes. The temperature, high silica and salt content, and the presence of trace elements suggest a deep artesian flowpath (about 2000 m), confined under the Bourget Lake syncline, where Triassic evaporites are leached (Carfantan & al., 1998). H 2 S degassing seems less strong in Chevalley Aven, probably due to the standing water. On the contrary, the water flowing turbulently out from the Alum spring pro- duces a stronger degassing, as is evidenced by the char- acteristic rotten egg smell, together with a thick coating of replacement gypsum and native sulfur covering walls around the spring pool (Audra et. al., 2007). Martel (1935) was the first to identify the sulfidic origin of Ser- pents Cave. THE CONVECTION CELLS IN CHEVALLEy AVEN (f ig. 7) The aerial thermal gradient, between the pool at about 32 to 34 °C and the top of Chevalley Aven at 26 °C ranges from 0.2 to 0.3 °C/m, so, air convection cells occur. The distribution of air loops is controlled by the presence in the main passage of a cool dripping originating from both condensation and seepage. The cool airflow sinks into the main passage whereas the warm airflow rises from the thermal pool through lateral passages. The highest air- flow velocity is about 15 cm/s in the narrow passages. Before 1996, water seepage originated from natural rain- water percolation through the soil. Since the building of the thermal Spa, seepage originates from leakage from the thermal pool. We present here the observations made in the small vent connecting the Lower Gallery and the Aixinoise gallery (f ig. 7). Preliminary measures were done the 10/26/2006 using a heated wire anemometer (Testo V1) which includes a digital thermometer. Device accuracy is 1 cm/s for air velocity and 0.1 °C for air tem- perature. STUDy Of C ONDENSATION-CORROSION AND EVAPORATION-DEPOSITION PROCESSES OCCURRING INTO THE VENT (f ig. 8) This vent is a narrow inaccessible passage about 3 m long. Temperature is 32.7 °C at the base, and 28.4 °C at the upper mouth. The air flow velocity is very low, about 1 cm/s or less. Warm air rises up through the vent and some sinking secondary loops appear at the contact of the cooler lateral walls of the central sphere. The ac- cumulation of condensation produces runoff as a film along the sphere walls and dripping at the tip of down- facing pendants. Airflow is considered close to satura- tion regarding moisture. Consequently, cooling from ris- ing air produces condensation in the narrow fissures, at Fig. 6: Outline of Chevalley Aven and Serpents Cave, cross-section view (survey SC Savoy, EDy TEm). Serpents Cave is a water table cave, with spheres and avens at the ceiling. The active Alum thermal spring flows into the cave at the upstream end. Chevalley Aven is made of stacked up spheres arranged in a bush-like pattern and reaches the thermal water table. The “paleowater table” corresponds to the water level before the digging of the adit in 1859. Tab. 1: main physical and chemical data from the Alum spring, Serpents Cave, Aix-les-Bains (main data after muralt, 2003). Physico- chemistry Values Reference, if other than Muralt (2003) Temperature 33,5 – 46,6 °C Discharge 8 - 42 L.s -1 Conductivity 576 – 691 µS.cm -1 Hobléa, 1999 pH 6,5 Hobléa, 1999 TDS 496 mg.L -1 HCO3 - 262 mg.L -1 SO4 -- 60 - 230 mg.L -1 Cl - 15 - 30 mg.L -1 Na + 20 - 40 mg.L -1 Ca ++ 100 - 150 mg.L -1 K + 3 - 6 mg.L -1 Mg ++ 10 - 25 mg.L -1 SiO 2 22 - 26 mg.L -1 H 2 S 5 mg.L -1 Iundt & al., 1987 Trace elements Al, Fe, Mn, Pb, B, Sr, Sn, Sb, Ba, Li Iundt & al., 1987 THE ROLE Of CONDENSATION-CORROSION IN THERMAL SPELEOGENESIS ... ACTA CARSOLOGICA 36/2 – 2007 190 the ceiling, and on the walls of the central sphere which is covered with condensation drips; conversely, cool air that sinks from an uppermost chimney warms up and produces evaporation at the upper mouth of the vent, which is dry. At the upper part of the lowermost neck evaporation also occurs from secondary convection cells in the central sphere. The corrosion rate depends on condensation; thus corrosion occurs mainly on the ceiling and walls of the sphere that is gradually enlarged. The sphere ceiling and the upper part of the walls display a regular and smooth cupola shape made of a weathered lime- stone layer, 2-3 cm in thickness, where boxwork veins protrude. The condensation film dissolves the limestone micrite cement, dissociates the sparite grains, and pro- gresses deepwards into the host rock as an incomplete weathering front (Zupan-Hajna, 2003). Along the walls of the sphere, the runoff from the accumulation of con- densation dissolves the walls; consequently, no weath- ered material subsists over the lower rockwalls. At the base of the sphere, the condensation flow becomes satu- rated, and calcite deposits as small transparent crystals in a soft and wet paste. Sulfate produced by replacement corrosion on the limestone wall is also washed down by condensation runoff. This process has also been noticed in f rasassi (Cigna & f orti, 1986). Runoff arriving at the base of the sphere becomes saturated with gypsum. Evaporation onto the “calcite grains wet paste” causes microcrystal- line gypsum to deposit in a thin crust. Dripping from the top of the sphere may re-dissolve the gypsum that re-deposits downward into the narrow passage as gyp- sum flowstones, stalactites and columns made of massive gypsum crystals. Biofilms coat most of walls, except: where intense condensation or runoff washes the walls; or where evap- oration prevents the microbial development that needs permanent moisture. Evaporation dries up the upper lip of the vent. Water is attracted by capillarity toward this dry zone that maintains a continuous recharge of dissolved species of carbonate. The evaporation allows calcite precipitation as a rim, similar to a small drapery, shaped by both airflow and evaporation (f ig. 9). Such cave features are characteristic of caves having a high thermal gradient and develop at the upper mouth of chimneys connecting cave levels where convection airflow cells occur. They may develop in the following situations: 1- in avens above a thermal water table located at depth, such as Jószef-Hegy barlang, Budapest (auth. obs.); 2- where impervious layers covering karst prevent meteoric seepage that may homogenize temperatures throughout the cave profile (Wind Cave, Black Hills); Fig. 9: Twin upper vents at the bottom of the Aixinoise gallery (photo. Ph. Audra). Fig. 7: Chevalley Aven cross-section. Air convection is shown, together with present water seepage originating from leakage of the thermal swimming pool located above. Before the building of the new thermal center in 1996, this water seepage was provided by natural rainwater infiltration through the soil. PHILIPPE AUDRA, fABIEN HOBLEA, JEAN-yVES BIGOT & JEAN-CLAUDE NOBECOURT ACTA CARSOLOGICA 36/2 – 2007 191 Fig. 8: Overview of the condensation-corrosion and evaporation-deposition processes. See text for details. 3- in arid areas for the same reason (Endless Cave, Guadalupe Mountains; see photography in Hill & f orti, 1997, p. 91); 4- in tropical caves where strong humidity and tem- perature gradients may exist between active galleries fed by surface seepage and galleries where strong airflow between entrances dries walls (NT2 Cave System, Kam- mouan, Laos; auth. obs.) THE VENT, A CAVE f EATURE COMBINING CONDENSATION / EVAPORATION PLUS CORROSION / DEPOSITION PROCESSES f igure 8 shows that the morphological setting strongly influences the distribution of processes. - The upper mouth of the vent opens above the bot- tom of the uppermost sphere: the condensation water of the uppermost sphere flows away in another direction. Consequently, the vent collects its own condensation only that becomes visible in the lowest part of the narrow where clear surfaces covered with a weathered layer are visible. THE ROLE Of CONDENSATION-CORROSION IN THERMAL SPELEOGENESIS ... Fig. 10: The vent viewed from below, in the Lower Gallery; width of the vent is about 20 cm (photo. Ph. Audra). ACTA CARSOLOGICA 36/2 – 2007 192 - The lowermost vent located below the central sphere acts as a funnel that collects condensation from the upper narrow, from the central sphere that is the main runoff provider, and from its own condensation occur- ring in the lowermost part. Consequently, condensation runoff is important. Thus, by an overtopping effect, con - densation runoff transfers the processes normally occur- ring at the base of the central sphere downwards. Instead of evaporating slowly, condensation water with high sul- fate content flows downward into the narrow where it fi- DISCUSSION A POSITIVE f EEDBACK DEVELOPS SPHERES IN BUSH PATTERN The distribution of corrosion and deposition processes produces a morphodynamic evolution according to the size of the conduits: - spheres, where condensation-corrosion is high, evolve faster and tend to enlarge; - narrow passages evolve slowly due to the small amount of condensed water collected, by neutralization of the condensation runoff after some distance, or evapo- ration. Thus, a positive feedback tends to enlarge the wid- er places that evolve to form a spherical shape, whereas narrows remain small. Such a morphodynamic tends towards the development of stacked up spheres ar- ranged in a bush-like pattern that are isolated by nar- row necks. THE ORIGIN Of SPHERES IN BUSH PATTERN REMAINS ENIGMATIC The accurate observation of condensation-corrosion and evaporation-deposition processes clearly demonstrates that aerologic convections tend to exaggerate the differ- ence in size of the voids located in the vadose zone just above thermal water table, and finally may develop as spheres in bush pattern and avens. However, the origin of the Chevalley Aven is still obscure: - did it develop exclusively from this aerologic con- vection process by hollowing into massive rock? - did an older phreatic lift exist, that was drained by base level lowering and which evolved afterwards under aerologic conditions? And in this case, could a phreatic origin have initiated the development of the spheres? flooding from meteoric invasions is also evoked as a contribution to the aven genesis, which would have en- hanced mixing-corrosion processes typical of “transi- tional karst” (Hobléa, 1999; Dublyansky, 1997). If the origin of the proto-cave is still under debate, we propose that a major part of the cave volume, if not the entire cave, could develop under aerologic conditions involving condensation-corrosion by a progressive up- ward propagation of voids into the rock mass which fi- nally produces spheres in bush pattern. Similarly, in Mo- vile Cave, condensation-corrosion is considered to be the main process of current wall retreat for air-filled passages above the water table (Sarbu & Lascu, 1997). IMPLICATION f OR THE UNDERSTANDING Of Hy POGENIC WATER TABLE CAVES (VILLA LUZ, SERPENTS CAVE) The Villa Luz Cave (Tabasco, Mexico) is similar to Ser- pents Cave, since it has an active water table cave with sulfidic springs upstream. Numerous avens are present, some being blind, some having broken through the sur- face as skylights. These avens are considered as phreatic lifts (due to their spectacular scallops showing rising flow) older than the drainage of the cave and the development of the main gallery (Hose & Pisarowicz, 1999). However, Villa Luz is a typical water table cave, clearly connected to the bottom of a small local valley and displaying a low gradient free surface flow. The avens starting from this water table gallery and developing upward cannot be old- er than the gallery itself. Consequently, they must have developed simultaneously to the main water table gallery (or later), thus not in phreatic conditions. The “phreatic scallops” that have been taken as indicators of upflow- ing, in fact would result from corrosive convection in the warm sulfidic atmosphere. Such airflow would produce features very similar to the scallops. Some of the avens still contain huge masses of replacement gypsum made of the accumulation of gypsum crust detached from walls. Other avens have breached through the surface, allow- ing the meteoric infiltration to wash away the gypsum deposits (Palmer, 2003). PHILIPPE AUDRA, fABIEN HOBLEA, JEAN-yVES BIGOT & JEAN-CLAUDE NOBECOURT nally evaporates and where it deposits secondary gypsum as speleothems made of massive crystals. These gypsum deposits develop downward till they arrive at the ceiling of the lowermost sphere, where condensation and corro- sion are predominant (f ig. 10). Thus, a calcite rim may line the upper mouth of a vent if it is located above the drainage point of the upper sphere; on the contrary, condensation runoff washes and transports dissolved minerals downwards, to the lower narrow. ACTA CARSOLOGICA 36/2 – 2007 193 In Serpents Cave, the Hell’s Aven, which is the natu- ral entrance, probably has a similar origin, that is to say a breakthrough surface of an aven made by corrosive “air” convections. However, and contrary to Villa Luz, the Hell’s Aven is located just above the Alum spring (fig. 6); in this case, there remains the possibility that the aven could be an older phreatic lift drained after a base level lowering (Hobléa, 1999). CONCLUSION Observation of condensation and evaporation areas, cor- rosional morphologies, and of the distribution of the deposits have shown the strong relationship between condensation and corrosion, and conversely between evaporation and deposition. Moreover, the distribution of the condensation and evaporation processes strongly depends on the morphology of the cave. Condensation- corrosion tends to enlarge the largest voids, leading final- ly to a pattern of stacked spheres. These first conclusions, based on the study of cave features, indicate the possibil- ity of speleogenesis mainly through condensation-corro- sion. This hypothesis is currently under investigation. We are collecting data (temperature, airflow speed, pCO 2 and pH 2 S, chemistry of condensate water), in order to calcu- late a budget of condensation rates and corrosion vol- umes which will be compared to the cave dimensions. ACKNOWLEDGEMENT We are grateful to the Head of Chevalley thermal resort for the permission to access the caves, particularly Mr. J.f . Michel, S. Bienvenue, f . Canella, J.P . Morin, and Mrs. A. Labrosse. THE ROLE Of CONDENSATION-CORROSION IN THERMAL SPELEOGENESIS ... REf ERENCES Audra, Ph., Bigot, J.-y. & Mocochain, L. 2002: Hypo- genic caves in Provence (f rance). Specific features and sediments.- Acta carsologica, 31, 3, 33-50. Ljub- ljana Audra Ph. & Hofmann B. A. 2004: Les cavités hypogènes associées aux dépôts de sulfures métalliques (MVT).- Le Grotte d’Italia. 5, 35-56. Audra Ph. & Häuselmann Ph. 2004: Hydrothermal origin of two hypogenic karst caves in f rench Provence: Preliminary results from fluid inclusions.- Journées AfK, 2003, Rouen, 32-34. Audra P . & Hobléa, f. 2007: f irst occurennce of jurbanite [AI(O minerals: alunogen [Al 2 (SO 4 ) 3 · 17H 2 O] and tsche- rmigite [NH 4 ]. Journal of Cave and Karst Studies, 69, 2 Audra Ph. 2005: Hydrothermal karst and caves in Southern f rance. Cave features, related sediments and genesis. Genesis and formations of hydrothermal caves. Inter- national Conference, Budapest 2004, Papers, 5-13. Bögli, A. 1964: Corrosion by mixing of water.- Interna- tional Journal of Speleology, 1, 61-70, Bologna. Bögli, A. 1978: Karsthydrographie und physische Speläolo- gie.- Springer, 292 p Berlin, Heidelberg, New york. Carfantan, J.-C., Nicoud, G. & Iundt, f . 1998: L ’ origine et le parcours des eaux thermo-minérales d’ Aix- les-Bains, Savoie (The origin and the path of ther- mo-mineral waters of Aix-les-Bains, Savoy).- Cir- culations hydrothermales en terrains calcaires, 10 e Journée technique du Comité français de l’Association internationales des hydrogéologues, Carcassone, 28 novembre 2003, 7-14. AIH-CfH, Orléans Cigna, A. A. & f orti, P . 1986: The speleogenetic role of air flow caused by convection. 1 st contribution.- Inter- national Journal of Speleology, 15, 41-52, Bologna. Dreybrodt, W ., Gabrovšek, f . & Perne, M. 2005: Conden- sation corrosion: a theoretical approach.- Acta car- sologica, xxxIV/II, 317-348. Ljubljana. Dublyansky, y.V ., 1997: Transition between hydrothermal and cold-water karst. Proceedings of the 12 th Interna- tional Congress of Speleology, La Chaux-de-Fonds, 2, 267-270. Union internationale de spéléologie & So- ciété suisse de spéléologie, Genève. Dublyansky, y. V . & Pashenko, S. E. 1997: Cave popcorn - an aerosol Speleothem?- Proceedings of the 12 th International Congress of Speleology, La Chaux-de- Fonds, 1 (Physical speleology and karst geomorphol- ogy), 271-274. Union internationale de spéléologie & Société suisse de spéléologie, Genève. ACTA CARSOLOGICA 36/2 – 2007 194 Dublyansky, V . N. & Dublyansky, y. V . 2000: The problem of condensation in karst studies.- Journal of Cave and Karst Studies, 60, 1, 3-17, Huntsville. Egemeier, S. J. 1981: Cavern development by thermal wa- ters.- NSS Bulletin, 43, 2, 31-51, Huntsville. f orti, P . 1996: Thermal karst systems.- Acta carsologica, xx V , 99-117, Ljubljana. Galdenzi, S. & Menichetti, M. 1990: Evolution of under- ground karst systems in the Umbria Marche Apen- nines in Central Italy.- Proceedings of the 10 th Interna- tional Congress of Speleology, Budapest 1989, III, 745- 747. Hungarian Speleological Society, Budapest. Gallino, S. 2006: Le karst du dôme anticlinal d’ Aix-les- Bains. Nouvelles données sur le panache hydro- thermal (The karst of the anticline dome of Aix-les- Bains. New data about the hydrothermal plume).- Karstologia, 48, 29-32 Hill, C. A. 1987: Geology of Carlsbad cavern and other caves in the Guadalupe Mountains, New Mexico and Texas.- New mexico Bureau of mines and min- eral Resources, n° 117, 150 p. Hill, C. A. & f orti, P . 1997: Cave microclimate and spe- leothems.- Cave minerals of the world, 258-261. Na- tional Speleological Society, Huntsville. Hobléa, f . 1999: Contribution à la connaissance et à la gestion environnementale des géosystèmes karstiques montagnards : études savoyardes (Contribution to the knowledge and to the environmental management of mountain karstic systems: Savoyan studies).- PhD Thesis, University of Lyon, 995 p. Hose, L. D. & Pisarowicz, J. A. 1999: Cueva de Villa Luz, Tabasco, Mexico: reconnaissance study of an active sulfur spring cave and ecosystem.- Journal of Cave and Karst Studies, 61, 1, 13-21, Huntsville Iundt, f r., Lopoukine, M., Malatrait, A. & Martelat, M. 1987: Étude du système thermal et minéral d’Aix- les-Bains, Savoie.- Rapport n° 87-SGN.434 RMA. Bureau de recherches géologiques et minières, Or- léans, Unpubl. Lismonde, B. 1999: Quelques mécanismes chimiques du creusement des cavernes, plus particulièrement pour l’ étude de la zone épinoyée (Some karst corro- sion mechanisms, particularly regarding epiphreatic zone).- Karstologia , 33, 41-50 Lismonde, B. 2000: Corrosion des coupoles de plafond par les fluctuations de pression de l’air emprisonné (Corrosion of ceiling pockets associated with pres- sure of confined air).- Karstologia, 35, 39-46 Lismonde, B. 2003: Limestone wall retreat in a ceiling cupola controlled by hydrothermal degassing with wall condensation (Szunyogh model) (Comments to Wolfgang Dreybrodt remark “On feasibility of condensation processes in caves” .- Speleogenesis and Evolution of Karst Aquifers (www.speleogen- esis.info), 1, 4, 3p. Martel, E.-A. 1935: Contamination, protection et améliora- tion des sources thermominérales (Contamination, pro- tection and improvement of thermomineral springs).- Congrès international des mines, de la métallurgie et de la géologie appliquée, 7e session, 2, 791-798, Paris Morehouse, D. f . 1968: Cave development via the sulfuric acid reaction.- NSS Bulletin, 30, 1, 1-10, Huntsville. Mucke, B., Vicker, V . & Wadewitz, S. 1983: Cupola for- mation in occasionally inundated cave roofs.- Euro- pean Regional Conference on Speleology, Sofia 1980, 2, 129-132, Bulgarian f ederation of Speleology Müller, P . 1974: A melegforrás-barlangok és gömbfülkék leb- etheziséröl (On the origin of thermal caves and spheri- cal niches).- Karszt és Barlang, 1, 7-10, Budapest Müller, P . & Sarvary, I. 1977: Some aspects of develop- ments in Hungarian speleology theories during the last ten years.- Karszt és Barlang, special issue, 53- 60, Budapest Muralt, R. 2003: Processus hydrogéologiques et hydrochi- miques dans les circulations d’ eaux thermales à Aix-les- Bains (Savoie).- Circulations hydrothermales en terrains calcaires, 10 e Journée technique du Comité français de l’ Association internationales des hydrogéologues, Carcas- sone, 28 novembre 2003, 65-72, AIH-Cf H, Orléans, Osborne, R. A. L. 2004: The troubles with cupolas.- Acta carsologica, 33, 29-36. Ljubljana Palmer, A. N. 2003: Sulfuric caves of North America.- Le Grotte d Rudnicki, J. 1978: Role of convection in shaping subter- ranean karst forms.- Kras i Speleologia, xI, 2, 92- 101, Katowice. Sarbu, S. M., Kane, T. C. & Kinkley, B. K. 1996: A chemo- autotrophically based groundwater ecosystem.- Sci- ence, 272, 1953-1955. Sarbu, S. M. & Lascu, C. 1997: Condensation corrosion in Movile cave, Romania.- Journal of Cave and Karst Studies, 59, 3, 99-102, Huntsville. Szunyogh, G. 1982: A hévizes eredetü gömbfülkék ki- oldódásának elméleti vizsgálata (Theory of the dis- solving of spherical cavities formed by thermal wa- ter).- Karszt és Barlang, 2, 83-88, Budapest Szunyogh, G. 1990: Theoretical investigation of the de- velopment of spheroidal niches of thermal water origin – Second approximation.- Proceedings of the 10th International Congress of Speleology, Budapest 1989, III, 766-768, Hungarian Speleological Society, Budapest. Zupan-Hajna, N. 2003: Incomplete solution: weathering of cave walls and the production, transport and deposi- tion of carbonate fines.- Thesis, Karst Research Insti- tut, Postojna & Založba ZRC, 168 p., Ljubljana. PHILIPPE AUDRA, fABIEN HOBLEA, JEAN-yVES BIGOT & JEAN-CLAUDE NOBECOURT