Microclimatic monitoring of coastal caves: the example of Bue Marino

The Gulf of Orosei (Central-East Sardinia) is one of the most interesting coastal karst areas of Italy. Its 37 km long vertical Jurassic limestone cliff is characterised by well developed karst phenomena. It hosts many cave systems fed by allogenic recharge, while the interaction between fresh and sea water, combined with the waves action, have allowed for the development of large cave entrances along the coastline. Bue Marino is certainly the most important cave along this coast. Composed of more than 17 km of passages, it is organized in 3 main branches (Ramo Nord, Ramo Di Mezzo and Ramo Sud) and opens to the sea with several entrances, some of which are more than 20 metres wide. With the exception of the innermost part of the southern branch and the completely dry entrance of Ramo Nord, all cave tributaries have water circulation and are mostly drowned. During flood events, the Ramo Sud branch drains the overflow of Codula Ilune karst system, with over 42 km of passages, representing its palaeo-spring. Since September 2006, a microclimatic monitoring project has been carried out in this cave, with the aim to evaluate the critical environmental parameters (water and air temperature, relative humidity, CO2 concentration) for the conservation of biodiversity and subterranean ecosystem protection and the possible impact derived from the visitors (approximately 80,000 tourists between Easter and October). Cave temperature have been monitored hourly by 5 iButton Maxim High Resolution Thermochron Digital thermometer (Resolution: 0.0625 °C; Accuracy: ±0.5 °C) distributed at different distances from the entrances, three of them along the tourist trails and two more in the internal part of Ramo Sud. One similar sensor has been placed at the surface, in order to compare the underground record with the external one. Two HOBO Pendant® Temperature Data Logger (Resolution: 0.14 °C; Accuracy: ± 0.53 °C) recorded air temperature in two different stations in Ramo Nord. Furthermore, temperature has been measured in water at a depth of 3 metres with HOBO Water Temperature Data Logger (Resolution: 0.14 °C; Accuracy: ± 0.53 °C) and in rock with HOBO U23 Pro v2 External Temperature probe (Resolution: 0.02 °C; Accuracy: ± 0.2 °C). Single temperature and relative humidity measurements have been made by BABUC-M (Resolution: 0.01 °C). CO2 data acquisition has been carried out continuously during tourist peak with Telaire 7001 Carbon Dioxide sensor (Measurement Range: 0 to 4000 ppm; Resolution: ±1 ppm; Accuracy: ±50 ppm) while single point measurements have been performed with a Zenith CO2 meter equipped with a NDIR sensor (Range 0-10000 ppm; Resolution: 1 ppm; Accuracy: ±50 ppm). The results show an air temperature spatial pattern characterized by a seasonal trend from the entrance to the deepest parts of the cavity. The external influence propagates from the surface to cave atmosphere up to 500 metres from the entrance. Even daily cycles have been observed at the innermost monitoring stations. Furthermore, the temperature in the deepest part is unaffected by external climatic variables and remains almost unchanged, with values close to the annual average (around 16 °C), except during floods which can produce drastic decreases. CO2 concentration is typically of karst system with values ranging between about 1000 and 5000 ppm, also in non-touristic pathways.