GLOBAL CHANGE AND CO2 VARIATIONS IN KARST. INDICATIONS FROM THE MILANDRE UNDERGROUND LABORATORY (JURA MOUTAINS, SWITZERLAND)

Water mineralization in a (limestone) karst is mainly controlled by the CaCO₃ dissolution which is controlled by the dissolved CO₂ in infiltration water, giving its aggressiveness to water. The CO₂is provided by soil activity from the catchment. Variations of soil activity may impact the mineralization of karst waters.

Observations in the Milandre cave Laboratory make it possible to assess this effect both in space and time. It provides some keys for the assessment of global change to water mineralization, as well as some indications considering calcite deposition in stalagmites.

The groundwater catchment area of the cave stream (explored ~5 km) is exclusively of autogenic type. 90% of the water feeding the stream comes from 5 main inlets, and about 10% from low rate seepage. The stream reacts swiftly to precipitation. Therefore, it seems that the epikarst layer efficiently concentrates water towards the main flow routes, only a small proportion of infiltration water is stored.

Previous works showed that land-use can be correlated to the mineralization of the respective tributaries, the highest rate being observed below thick soils with cultivated land. Measured pCO₂in soils are from 5’000 to 15’000 ppm in late winter/spring, and between 20’000 and 50’000 ppm in summer.

The highest rates of cave air pCO₂(2003-2012) are observed from Sep. to Jan. (max 35’000 ppm). They are correlated with high discharge rates in inlets, bringing CO₂ from the soils (lag of ~3 months). Tracer tests and stable isotopes confirmed this transit time. Short time variations were also observed evidencing a strong effect of the cave ventilation on cave air pCO₂. In summer, CO₂daily cycles are observed (between 1.2 and 2%).

A long term trend could also be observed: between 1990 and 2012 water CaCO₃ raised from about 450 to 490 mg/L. This corresponds to equilibrium pCO₂of 43’000 ppm in 1990 to 55’000 ppm in 2012. This trend seems to be mainly due to climate warming (+0.5°C measured) but could also be related to changes in agriculture practices.

Further measurements are being undertaken to precise pCO₂ in cave, as well as in the soils. Also an experimental device has been set up and makes it possible to inject water on top of the cave with controlled rate and pCO₂.

One basic question remains: Observed pCO₂in Milandre cave are very high compared to many other caves, why so?