Hydrogeological control on carbon dioxide input into the atmosphere of the Chauvet-Pont d'Arc cave

Abstract

Carbon dioxide (CO 2 ) concentration (CDC) is an essential parameter of underground atmospheres for safety and cave heritage preservation. In the Chauvet cave (South France), a world heritage site hosting unique paintings dated 36,000 years BP, a high-sensitivity monitoring, ongoing since 1997, revealed: 1) two compartments with a spatially uniform CDC, a large volume (A) (40,000 to 80,000 m 3 ) with a mean value of 2.20 ± 0.01% vol. in 2016, and a smaller remote room (B) (2000 m 3 ), with a higher mean value of 3.42 ± 0.01%; 2) large CDC annual 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 B in 2017, decreasing afterwards. While a large effect of seasonal ventilation is ruled out, monitoring of seepage at two dripping points indicated that the main control of CDC seasonal reduction was transient infiltration. During periods of water deficit, calculated from surface temperature and rainfall, CDC systematically increased. The carbon isotopic composition of CO 2 , correlated with water excess, is consistent with a time-varying component of CO 2 seeping from above. The CO 2 flux, which is the primary driver of CDC in A and B, inferred using box modelling, was found to confirm the relationship between water excess and reduced CO 2 flux into A, compatible with a more constant flux into B. A buoyancy-driven horizontal CO 2 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 variations 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.

CO2 dynamics and heterogeneity in a cave atmosphere: role of ventilation patterns and airflow pathways

“…As a result, the CO 2 reservoir expands into a deeper vadose zone, where it is often referred to as a subsoil carbon source or ground air (Atkinson 1977;Serrano-Ortiz et al 2010;Mattey et al 2016;Baldini et al 2018). This CO 2 reservoir is now well recognized through contemporary research of the karst vadose zone pCO 2 in boreholes (Benavente et al 2010(Benavente et al , 2015, direct measurements in the soil zone (Blecha and Faimon 2014;Garcia-Anton et al 2017), carbon isotope records in speleothems (Noronha et al 2015), hydrochemistry of drip water (Bergel et al 2017), or case studies with long-term monitoring (Mattey et al 2016(Mattey et al , 2021Bourges et al 2020).…”

CO2 Dynamics and Heterogeneity in a Cave Atmosphere: Role of Ventilation Patterns and Airflow Pathways

“…Although it appears that the chimney effect and wind effect control the CO 2 dynamics in PP, there may be other factors involved. In some cases, CO 2 dynamics in mature caves has been found to be controlled by variation of drip rate, water in ltration or saturation of vadose zone (Milanolo and Gabrovšek 2015;Bourges et al 2020). Theoretically, air ow and in ltration water pathways are shared within a vadose zone, and in this way saturation with water can limit or block the gas exchange between the cave and the exterior (Cuezva et al 2011;Garcia-Anton et al 2014;Pla et al 2016b).…”

“…As a result, the CO 2 reservoir expands into a deeper vadose zone, where it is often referred to as a subsoil carbon source or ground air (Atkinson 1977;Serrano-Ortiz et al 2010;Mattey et al 2016;Baldini et al 2018). This CO 2 reservoir is now well recognized through contemporary research of the karst vadose zone pCO 2 in boreholes (Benavente et al 2010(Benavente et al , 2015, direct measurements in the soil zone (Blecha and Faimon 2014; Garcia-Anton et al 2017), carbon isotope records in speleothems (Noronha et al 2015), hydrochemistry of drip water (Bergel et al 2017), or case studies with long-term monitoring (Mattey et al 2016(Mattey et al , 2021Bourges et al 2020).…”

“…As an exception, CO 2 seasonality can be induced by winds in a tropical environment that may otherwise have no seasonal variations (Noronha et al 2017), as suggested by James et al (2015). In paleoclimate studies, CO 2 seasonality has been recognized as an important modulating factor introducing a seasonal bias in speleothem geochemistry (Spötl et al 2005 Bourges et al 2020). In a conceptual model of CO 2 transport through the unsaturated karst zone, direct advection from the soil zone has become a preferred mechanism over diffusion to explain the spatiotemporal variation of CO 2 in caves (Covington 2016;Lang et al 2017).…”

“…pCO 2 maxima are often observed in warm periods when CO 2 production in the soil is highest and the cave air is static (e.g. formation of a cold trap in a descending cave) or when the air ow is directed from the soil zone and the surrounding karst massif towards the cave (Milanolo and Gabrovšek 2009 (Houillon et al 2017;Bourges et al 2020). As an exception, CO 2 seasonality can be induced by winds in a tropical environment that may otherwise have no seasonal variations (Noronha et al 2017), as suggested by James et al (2015).…”