Hydrological control on cave ventilation and its effect on the heat balance of Longeaigue cave

The network of fractures and conduits crossing a karst massif drains water and air from the atmosphere deep into the massif, exchanging heat at the boundaries between rock, air and water. The thermal characteristic of the rock together with thermal processes including convection, evapo-condensation, radiation and conduction, concur to fix the cave’s temperature. The thermal length, the distance at which the external temperature fluctuations are damped, and the energy balance of the cave system depend on its geometry and the fluxes therein. Comprehensive knowledge of what modifies these thermal characteristics is of interest for e.g. low-enthalpy geothermal exploitation, mineralisation in water supplies and also for paleoclimatic studies on speleothems.

In Longeaigue cave (Val-de-Travers, Jura mountains, CH), we deployed several sensors measuring airflow and temperature along the main conduit network. The cave is mainly dry and has a lower and upper entrance leading to an intense airflow controlled by the chimney effect. The temperature oscillations observed throughout the cave are chiefly related to external temperature and airflow variations. Results from 8 monitoring stations reveal that 90% of the energy brought in by the air during ventilated periods is exchanged within the first tens of meters from the cave entrances. However, temporary interruptions of the airflow occur during periods of flooding related to rainfall and snowmelt. This situation can take place several times per year. Our observations demonstrate that the transient nature of this airflow modifies the temperature signals in the cave, affecting the cave energy balance in a differentiated way according to seasonal hydrological conditions. With the increasing winter temperatures, we anticipate a progressive shift towards a summer ventilation regime enhanced by limited summer rainfall. A positive feedback is observed on the energy balance of the cave. It is therefore of crucial importance to consider the presence of subsurface ventilation for the thermal characterisation of karstic environments, which can modify the biochemical, physical and thermal characteristics of seeping water and, in turn, impact on the interaction with the encasing rock.