CO2 dynamics and heterogeneity in a cave atmosphere: role of ventilation patterns and airflow pathways

Abstract Understanding the dynamics and distribution of CO 2 in the subsurface atmosphere of carbonate karst massifs provides important insights into dissolution and precipitation processes, the role of karst systems in the global carbon cycle, and the use of speleothems for paleoclimate reconstructions. We discuss long-term microclimatic observations in a passage of Postojna Cave, Slovenia, focusing on high spatial and temporal variations of p CO 2 . We show (1) that the airflow through the massif is determined by the combined action of the chimney effect and external winds and (2) that the relationship between the direction of the airflow, the geometry of the airflow pathways, and the position of the observation point explains the observed variations of p CO 2 . Namely, in the terminal chamber of the passage, the p CO 2 is low and uniform during updraft, when outside air flows to the site through a system of large open galleries. When the airflow reverses direction to downdraft, the chamber is fed by inlets with diverse flow rates and p CO 2 , which enter via small conduits and fractures embedded in a CO 2 -rich vadose zone. If the spatial distribution of inlets and outlets produces minimal mixing between low and high p CO 2 inflows, high and persistent gradients in p CO 2 are formed. Such is the case in the chamber, where vertical gradients of up to 1000 ppm/m are observed during downdraft. The results presented in this work provide new insights into the dynamics and composition of the subsurface atmosphere and demonstrate the importance of long-term and spatially distributed observations.

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