Hydrogeological control on carbon dioxide input into the atmosphere of the Chauvet-Pont d’Arc cave François Bourges, Dominique Genty, Frédéric Perrier, Bruno Lartiges, Édouard Régnier, Alexandre François, Johann Leplat, Stéphanie Touron, Faisl Bousta, Marc Massault, et al. To cite this version: François Bourges, Dominique Genty, Frédéric Perrier, Bruno Lartiges, Édouard Régnier, et al.. Hy- drogeological control on carbon dioxide input into the atmosphere of the Chauvet-Pont d’Arc cave. Science of the Total Environment, Elsevier, 2020, 716, pp.136844. 10.1016/j.scitotenv.2020.136844. hal-03089428 HAL Id: hal-03089428 https://hal-cnrs.archives-ouvertes.fr/hal-03089428 Submitted on 28 Dec 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Science of the Total Environment xxx (xxxx) 136844 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: http://ees.elsevier.com Hydrogeological control on carbon dioxide input into the atmosphere of the Chauvet-Pont d'Arc cave François Bourges a, Dominique Genty b, Frédéric Perrier c,⁎, Bruno Lartiges d, Édouard Régnier b, Alexandre François e, Johann Leplat e, Stéphanie Touron e, Faisl Bousta e, Marc Massault f, Marc Delmotte b, Jean-Pascal Dumoulin g, Frédéric Girault c, Michel Ramonet b, Charles Chauveau h, Paulo Rodrigues h a Géologie Environnement Conseil, 30 rue de la République, F-09200 Saint-Girons, France b Laboratoire des Sciences du Climat et de l'Environnement, CNRS, F-91691 Gif-sur-Yvette, France c Institut de Physique du Globe de Paris, Université de Paris, 1 rue Jussieu, F-75005 Paris, France PROOF d Université de Toulouse III Paul Sabatier, Géosciences Environnement-Toulouse, 14 av. Edouard Belin, F-31400 Toulouse, France e Laboratoire de Recherches des Monuments Historiques (CRC, USR3224), Museum national d'Histoire naturelle, Sorbonne Universités, Ministère de la Culture, CRNS, 29 rue de Paris, F-77420 Champs-sur-Marne, France f Laboratoire Interactions et Dynamique des Environnements de Surface, Université Paris Sud, F-91405 Orsay, France g Laboratoire de Mesure du Carbone 14 (LMC14), LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France h Service de la Conservation de la Grotte Chauvet, Ministère de la Culture, F-07150 Vallon-Pont-d'Arc, France ARTICLE INFO ABSTRACT Article history: Carbon dioxide (CO2) concentration (CDC) is an essential parameter of underground atmospheres for safety and Received 27 March 2019 cave heritage preservation. In the Chauvet cave (South France), a world heritage site hosting unique paintings Received in revised form 16 January 2020 dated 36,000 years BP, a high-sensitivity monitoring, ongoing since 1997, revealed: 1) two compartments with Accepted 20 January 2020 a spatially uniform CDC, a large volume (A) (40,000 to 80,000 m3) with a mean value of 2.20 ± 0.01% vol. in Available online xxx 2016, and a smaller remote room (B) (2000 m3), with a higher mean value of 3.42 ± 0.01%; 2) large CDC annual Editor: Jay Gan variations with peak-to-peak amplitude of 2% and 1.6% in A and B, respectively; 3) long-term changes, with an increase of CDC and of its annual amplitude since 1997, then faster since 2013, reaching a maximum of 4.4% in Keywords B in 2017, decreasing afterwards. While a large effect of seasonal ventilation is ruled out, monitoring of seepage Underground at two dripping points indicated that the main control of CDC seasonal reduction was transient infiltration. Dur- Carbon dioxide ing periods of water deficit, calculated from surface temperature and rainfall, CDC systematically increased. The Vadose zone carbon isotopic composition of CO2, correlated with water excess, is consistent with a time-varying component Natural ventilation of CO2 seeping from above. The CO2 flux, which is the primary driver of CDC in A and B, inferred using box Painted cave modelling, was found to confirm the relationship between water excess and reduced CO2 flux into A, compatible Preservation with a more constant flux into B. A buoyancy-driven horizontal CO2 flow model in the vadose zone, hindered by water infiltration, is proposed. Similarly, pluri-annual and long-term CDC changes can likely be attributed to vari- ations of water excess, but also to increasing vegetation density above the cave. As CDC controls the carbonate geochemistry, an increased variability of CDC raises concern for the preservation of the Chauvet cave paintings. © 2020 1. Introduction et al., 1999; Quindos et al., 1987), caves are affected by global and local changes (Pla, 2016; Baker and Genty, 1998), by visitors The preservation of the exceptionally precious heritage of painted (Saiz-Jimenez et al., 2011; Fernandez-Cortes et al., 2011; Hoyos caves in the context of a rapidly evolving environment (Pla, 2016; et al., 1998; Villar et al., 1984, 1986), and by mistaken remedia- Dominguez-Villar et al., 2014; Beltrami et al., 2005; Perrier tion strategies, difficult to reverse, as in the case of the Lascaux cave et al., 2005a; Badino, 2004) represents a challenging issue for the in France (Martin-Sanchez et al., 2012, 2014). One essential para- scientific community (Bourges et al., 2014a, 2014b; Baker and meter controlling the chemical conditions at the wall surface, and in Genty, 1998). While the prehistoric paintings have reached present particular dissolution and precipitation of carbonates and speleothem times owing to particularlyUNCORRECTEDstable underground conditions (Mangin growth, is the CO2 concentration (CDC) in the cave atmosphere (Houil- lon et al., 2017; Spötl et al., 2005; Genty and Deflandre, 1998). Conversely, this parameter provides information about the evolution of the underground microclimate and of its thermodynamical condi- ⁎ Corresponding author. E-mail address: [email protected] (F. Perrier) tions, as well as of its surroundings (Bourges et al., 2012; Fernandez- https://doi.org/10.1016/j.scitotenv.2020.136844 0048-9697/© 2020. 2 F. Bourges et al. / Science of the Total Environment xxx (xxxx) 136844 Cortes et al., 2011). Thus, one key issue for the understanding of the ently displace air masses and produce cave breathing (Perrier and preservation and degradation processes, and for the monitoring of the Le Mouël, 2016; Wigley, 1967). A decrease in atmospheric pressure effect of remediation, is the characterization of CO2 sources and trans- leads to the extraction of porous space air and of the cavity air to the port modes into and out of the underground cavities (Houillon et al., outside, while an increase in atmospheric pressure leads to outside air 2017). being pushed into underground spaces. Close to the entrances or in Various atmospheric and underground CO2 sources can be distin- the case of important porous dry volumes, large effective air volumes guished. In volcanic regions, the major source of CO2 is the degassing downstream can lead to spectacular motions referred to as “cave wind” of mantle rocks, characterized by an isotopic carbon ratio δ13C of (Conn, 1966). This pumping process is frequency-dependent and effi- −2.3 ± 0.9‰ (Chiodini et al., 2008). Crustal CO2 from metamorphic ciently mixes cave air and outside air; it supplements natural ventilation, decarbonation (Groppo et al., 2017), characterized by δ13C values in and sometimes even dominates (Perrier and Richon, 2010). the range − 0.9 to −0.7‰, was shown to accumulate in a tunnel near ac- Additional processes for CO2 transport occur within the underground tive faults in the Nepal Himalayas (Girault et al., 2014, 2018). While cavities. One important process, particularly in the epikarst, is the de- important in principle, given the large fluxes from the ground, often in gassing of dripping water (Houillon et al., 2017; Balakowicz and 5 2 Jusserand, 1986), which can be significant in CO -poor atmospheres, excess of 10 g/m /day, mantle and crustal CO2 sources are not main 2 contributors in the case of European painted caves. Instead, the major especially during episodes of enhanced transient infiltration and sub- relevant production source is the biogenic production from the soil and sequent seepage. Reservoirs of SC and GC contribute to cave air both the epikarst (Mattey et al., 2016; Faimon et al., 2012; Bourges et by diffusion processes, which dominates in the poorly-connected poros- al., 2001; Atkinson, 1977), or in the underground part of the critical ity of the surrounding micro-fractured rock, and by buoyancy-driven zone (CZ), defined as the domain included from the base of the aquifer processes (Mattey et al., 2016; Badino, 2009), dominant through to the top of the canopy (Lin, 2010). well-connected porosity networks. Indeed, CO2-richer, drier or colder air tends to flow downwards while CO -poorer, more humid or warmer Two main zones of CO2 production can be distinguished (Peyraube 2 air raises through connected networks, for example through a discrete et al., 2013, 2018; Mattey et al., 2016): 1) soil CO2 (SC), which in- PROOF number of well-connected fracture networks, while most of the encas- cludes the emission of CO2 by microorganisms and root respiration, and ing porosity remains as a passive static reservoir. In the case of karstic 2) ground CO2 (GC), which includes CO2 released by the decay of or- ganic matter in the vadose zone. The combined analysis of δ13C ratio domains, networks can include, besides fractures, large conduits, some- and 14C activity of carbon dissolved in seepage water and in speleothems times occupied by water (Zhang et al., 2017).