Disentangling natural signal from anthropogenic contribution in the temperature dynamics within show caves

The aim of this study was to understand the impacts of both visitation and external atmosphere in Su Mannau, a show cave receiving around 15,000 visitors per year, in the Geomineral, Historical and Environmental Park of Sardinia (Italy). This cave system develops in Cambrian carbonates with large rooms and long tunnels for more than 8 km in length, with mostly horizontal profile and only few vertical drops. The first 600 m of the cave is equipped with tourist path for visits, opens seasonally from April to October. The underground air temperature was measured at 10-minutes intervals with HOBO U23 Temp/RH datalogger (accuracy: ±0.25°C; resolution: 0.04°C) at three different locations, two along the tour route and one in the wild part of the karst system, 800 m distant from the cave entrance. Data from cave atmosphere were compared to hourly air temperature from surface, measured within 100 m outside of the cave opening. Year-around monitoring at all cave sites shows both daily and annual temperature fluctuations that decrease increasing the distance from entrance. Daily temperature variations are less than 1 °C, a stable regime compared to nearby surface locations that in summer vary daily by more than 15 °C. Results show a direct correlation of cave and external temperatures during the warm season when the modification of the ventilation regime due to cave morphology causes the entrance of external air into the cave. This micrometeorological condition leads to a rhythmic daily oscillations in cave temperature, even in remote passages, strongly associated to the outside atmosphere but with a hourly delay. No anthropogenic source of temperature rise attributed to visitation was detected, except for a single occurrence in spring when tourist presence reached more than 500 units in only a day. Fortunately, the cave recovered during night closure.