The role of near-surface cavities in the carbon dioxide cycle of karst areas: evidence from the Carburangeli Cave Natural Reserve (Italy)

摘要

Hydrological, chemical and meteorological data collected during the years 2006-2007 at Carburangeli Cave (Italy) have provided new insights on the near-surface cycle of carbon dioxide, particularly concerning the role played by fractures and karst conduits. Carbon dioxide is trapped in the underground atmosphere essentially when its temperature is lower than the outer one. By contrast, convective air circulation disperses all the excess CO_2 in the external environment when the thermal differential is inverted. The network of fractures and karst conduits then works, in the vadose zone, as a re-circulator of CO_2 from the soil to the atmosphere. The total amount of CO_2 fixed in the underground is controlled, during the wet season, by the amount of infiltrating waters, which act as the main carrier of CO_2 in the subsoil. By contrast, during the dry season, gravitational drainage is responsible for the accumulation of carbon dioxide in the underground voids. The quantitative balance demonstrated that the degassed CO_2 amounts are one order of magnitude higher than the dissolved CO_2. In light of this, if the near-surface outgassing processes are not taken into account, CO_2 budgets may be affected by significant errors.