Temperature as a marker for karstic waters hydrodynamics. Inferences from 1 year recording at La Peyrére cave (Ariège, France)

We present temperature measurements recorded at a 15 min time step between October 2002 and April 2003 inside the La Peyrére cave (Baget karstic system, Central Pyrenees) on six 0.01 °C sensitivity sensors distributed on the whole recognized cave, down to a depth of 57 m. Rainy events are associated to a steep signal of a few hundredth to a few tenth of degrees in amplitude starting 1 h to one day after rain, and with a sign depending mostly of the outside temperature. The temperature signal does not correlate well with the rainfall, while the water level does with a maximum water rising rate 2.5 h after rain. The spectral density of the temperature signal displays one peak at a period of 3 days, which is also present in the rain signal and which is likely to be related to climatic parameters, and a broad smooth zone of spectral energy close to a 1 day period. By comparing a ‘superficial’ and a ‘deepest’ group of sensors, we note that the flood signal is more pronounced on the deepest sensor group whatever the outside temperature, and that the annual temperature variation is also larger on this sensor group. This could indicate that rain water flows in the cave through its base and explain the development in depth of the cave. Due to the arrival of water with different temperatures and mineralizations at different levels, double diffusive convection is likely to be triggered in the cave. We suggest that the onset of convection could be associated to the oscillations observed on the temperature signal at the onset of the flood.