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

Carbon dioxide (CO2) 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 m3) with a mean value of 2.20 ± 0.01% vol. in 2016, and a smaller remote room (B) (2000 m3), 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 CO2, correlated with water excess, is consistent with a time-varying component of CO2 seeping from above. The CO2 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 CO2 flux into A, compatible 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 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.

Mbaka lakes isotopic ((18)O and (2)H) and water balances: discussion on the used atmospheric moisture compositions.

The water balances of small crater lakes Masoko, Katubwi, Kyambangunguru, Ilamba, Kingiri, located north of lake Malawi (Tanzania), are established with the Craig-Gordon model of evaporation. In this approach, it is difficult to evaluate the isotopic composition of evaporated vapour (deltaE), which is indirectly estimated from the atmospheric moisture signature (deltaA). Given that 'precipitation equilibrium' approach does not lead to realistic solutions in this tropical region, the lake balances are established and discussed from both the sampled deltaA and those derived from the terminal lake Rukwa. The mean local sampled signature (deltaAm : delta(18)O =-13.7per thousand and delta(2)H =-76per thousand) is inferior to that derived from lake Rukwa (deltaAtl : delta(18)O =-10.4per thousand and delta(2)H =-64per thousand), which may be due to altitudinal and latitudinal effects but both are influenced by recycling continental vapour. Water balances enable us to confirm and quantify high inflows and groundwater outflows that account for typologies of Mbaka lakes. The proposed sampling and measurement method allows us then to access reasonable moisture compositions in far-away regions.

Estimate of recharge of a rising water table in semiarid Niger from 3H and 14C modeling.

A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O, 2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.