Thermal conduction in karst terrains dominating cave atmosphere temperatures: Quantification of thermal diffusivity

Abstract

[EN]The relatively stable temperature of most cave interiors is caused by the transfer of the surface atmosphere temperature signal to the karst underground by thermal conduction. Transferring underground thermal signals by conduction implies the amplitude attenuation of the external thermal anomaly as well as a signal delay. The magnitude of these attenuation and delay effects is proportional to the thickness of bedrock cover above the cave and is controlled by the bedrock thermal diffusivity. Here we present 5-year temperature record of Los Pilones Cave, in central Spain, where advection has a limited control on thermal anomalies all year round. Thus, thermal conduction dominates the thermal variability in the cave, with simulations of a 1-D thermal conduction model explaining up to 94% of the variability observed. Thermal attenuation and signal delays are highly correlated with the thickness of bedrock cover (r2 = 0.95 in both cases), enabling the calculation of a thermal diffusivity of 5.07*10ð€€€ 7 ±1.27*10ð€€€ 7 m2/s. The calculated thermal diffusivity is in the range of available values measured in laboratory for carbonate lithologies, although differs beyond uncertainty from the limited number of studies that calculated this value from field observations in karst. Thus, local rock properties significantly impact thermal diffusivities, and more experimental studies are required to show a complete distribution of this value in karst settings. The reported uncertainty, that was calculated using different time windows within the studied period, represents a variability of 23% on the thermal diffusivity. Karst terrains are mostly composed of three phases, rock, air and water, that have different thermal properties. Thus, variability of the water content in the rock porosity through time is likely a significant control on the measured uncertainty. Since temperature in the studied cave has a minor component affected by advection, this cave represents a paradigmatic location to observe the important role of thermal conduction in caves. Therefore, conclusions from this study can be useful to better understand thermal regimes and interannual trends of most cave interiors despite occurrence of variable impacts of seasonal ventilation dynamics in cave temperature records. In addition, the mechanism of thermal conduction results in a thermal decoupling between the surface and cave atmospheres affecting cave microclimate, which has important implications for multiple disciplines interested in karst underground environments