François Bourges

Comment on “Carbon uptake by karsts in the Houzhai Basin, southwest China” by Junhua Yan et al.

[1] In a recent paper Yan et al. [2011] estimate the carbon uptake in the Houzhai Basin due to dissolution of carbonate by rainwater using dissolved carbon measurements on this basin. Since both the surface and the underground river networks are known to converge toward a single outlet, this could be achieved with one hydrological station, monitoring water discharge and providing water for chemical analyses. From a nearly monthly record over the 1986–2007 period, they were able to monitor the seasonal variations of dissolved carbon uptake and to compute a mean yearly uptake of 20.7 g C m 2 yr 1 for the Houzhai Basin, which is further extended to estimate the total carbon uptake by karst in south China. Since several karstic basins are located in China, the study by Yan et al. is of great interest for the scientific community. Our concern is that it neglects the gaseous component of the carbon budget. However, it is known that karstic voids contain a few percent volume of CO2, and we will present below evidence that this CO2 is drained downward with rainwater from organic soils, collected by karstic voids and finally advected toward the Earth’s atmosphere, which can significantly alter the carbon budget computed by Yan et al. As the conclusions presented by Yan et al. are designed to constrain global atmospheric models, we suggest that the gaseous CO2 should also be considered in the estimates of the budget of carbon flowing through the Houzhai Basin. [2] Our team has accumulated more than 20 years’ research experience on this topic. Our studies were mainly motivated by the conservation of caves, including prehistoric painted caves, which requires the stability of the inner atmosphere. It is well known that the CO2 concentration can reach a few percent volume in the cave atmosphere [James, 1977], and we monitored its concentration in a large set of French caves [Bourges et al., 2001, 2006]. We have strong indications that this CO2 is mainly biogenic and produced in soils. This inference is supported by numerous dC measurements in the Villard Cave (SW France) with a mean dC of CO2 = 22.7‰ (50 samples from 2005 to 2010), where its variation has been correlated with the CO2 air concentration [Genty, 2008], and in the Chauvet Cave (SE France) with a mean dC of CO2 = 22.7‰ (20 samples from 2000 to 2010). There is also growing evidence that CO2 is transported downward both as dissolved species and in biphasic flow with water seeping toward the phreatic zone [Atkinson, 1977; Bourges et al., 2006;Mudry et al., 2008;Milanolo and Gabrovšek, 2009]. One of our most original results is that the cave atmosphere is not at rest but is continuously flowing toward the Earth’s atmosphere through large openings [Bourges et al., 2001]. Our interpretation is that soil air enriched with CO2 is dragged downward both in biphasic flow and as dissolved species during infiltration of rainwater and collected as gaseous CO2 by the network of macroscopic voids including fissures and caves. Our results indicate that a significant part of this CO2 is directly drained to the outside atmosphere through open fissures and large cave openings. This interpretation is now widely accepted by scientists working on caves [Baldini et al., 2006; Serrano-Ortiz et al., 2010]. After reviewing CO2 data gathered by the flux tower community and showing an anomalously high CO2 rate escaping from karst terrains toward the atmosphere, SerranoOrtiz et al. [2010] proposed an explanation similar to ours, involving a downward infiltration of CO2 with rainwater followed by an outflow of gaseous CO2 toward the terrestrial atmosphere. In some instances, winds flowing on permeable soils can further enhance the exchanges of CO2 between the cave atmosphere and the Earth’s atmosphere [SanchezCañete et al., 2011; Cuezva et al., 2011]. [3] We were able to observe the outward flow of CO2 on several occasions and to estimate the annual flow of gaseous CO2 at one French cave: the Aven d’Orgnac network (Figure 1). In winter, the outside air is colder and denser than the cave atmosphere, and the cave is therefore ventilated by downward flow of the outside air; then, in most parts of the cave the CO2 content is similar to that of the terrestrial atmosphere [Bourges et al., 2006; Kowalski and SanchezCañete, 2010]. This was shown with temperature, CO2 concentration and Rn concentration profiling along the cave system. Due to this thermo-compositional venting, the CO2 derived from soils and conveyed to the cave would be efficiently transferred to the Earth’s atmosphere. However, direct measurements of CO2 flux were impossible in winter, GEConseil, St.-Girons, Ariège, France. IRD/HSM, University of Montpellier II, Montpellier, France. LSCE, UMR CEA/CNRS/UVSQ 1572, L’Orme des Merisiers CEA Saclay, Gif-sur-Yvette, France. EcoEx-Moulis, Moulis, Ariège, France.

La conservation des grottes ornées: un problème de stabilité d'un système naturel (l'exemple de la grotte préhistorique de Gargas, Pyrénées françaises)

Painted caves are karstic cavities here considered as stable physical systems in a state of dynamic equilibrium. From the example of the Gargas cave, we show that introduction of excess energy (visitors, lighting) causes a loss of stability and introduces a risk of degradation in the cavity. From different approaches, we identify the periods and the causes of instability and we determine the maximum level of introduced energy which would preserve the conservative properties of the cavity. These results allow cave equipment and visitor capacity compatible with satisfactory conservation conditions to be defined.