Preliminary data on natural CO2 emissions from Nurra karst terrain (NW Sardinia, Italy)

The Mesozoic carbonate platform of NW Sardinia (Italy) pertains to the historical region of Nurra and since 1999 to the Regional Natural Park of Porto Conte. The area is also part of the Fluxnet project, a global network that coordinates the analysis of the net ecosystem carbon balance by direct measurement of gas exchanges across canopy-atmosphere interface using micrometeorological flux tower. The carbonate terrains of this sector host well-developed cave systems. The karst aquifer has a high dissolved carbon and the underground atmosphere is often characterised by anomalous level of carbon dioxide that has never been investigated before. As geological diffusive background emission over the carbonate outcrop was previously neglected, this study is aimed to assess the magnitude of the CO2 outgassing from caves to outside atmosphere. An environmental monitoring programme has been carried out in the Monte Doglia cave, a 100 m-deep vertical shaft located at 320 m asl and 2 km far from the coast. It opens in an area where a significant part of the surface is bare rock. The soil cover, where it exists, rarely exceeds 20 cm in thickness and consists of fine red, sandy material. The natural vegetation consists mainly of shrubs. The cave is monitored with multiple meteorological sensors that record continuously air CO2 and microclimatic parameters. Carbon dioxide concentration is measured at -10 m depth from the entrance by an infrared spectrometer (NDIR technology, range 0-10,000 ppm). Preliminary data indicate periodic oscillations of cave air CO2 levels ranging from 500 ppm to 1600 ppm, punctuated by events that provide clues to ventilation and degassing mechanisms. These gas plumes reach concentration >10,000 ppm. This huge CO2 content is hidden when a prevailing advective-renewal of cave air is established. Ventilation occurs via density driven flow and by wind across the entrance. Subsurface airflow in unsatured zones can be also induced by atmospheric pressure fluctuations, topographic effects, water table oscillations and meteoric water infiltration. About the origin of this CO2-rich gas, examples of volatiles escapes along faults have been documented in Sardinia. They have been related to mantle-derived fluxes associated to Quaternary volcanism. Other non-volcanic degassing involved on natural CO2 emissions from Nurra karst terrain might derive from carbonate rock weathering, oxidation of sulphur deposits and surface processes. Future stable isotopes analyses will be devoted to clarify these points. Finally, by considering the density of cave entrances and the contribution of numerous fractures widespread in the karst terrain, the temporal CO2 pattern of Nurra region provides evidence that the amounts of carbon that might be released from subsurface atmosphere in the vadose zone could be noticeable at both local and regional scale.