Carbon dioxide density-driven dissolution in a water-filled laboratory flume of the dimensions 60 cm length, 40 cm height, 1 cm thickness, was visualized using a pH-sensitive color indicator. We focus on atmospheric pressure conditions, like in caves where CO 2 concentrations are typically higher. Varying concentrations of carbon dioxide were applied as boundary conditions at the top of the experimental setup, leading to the onset of convective fingering at differing times. The data were used to validate a numerical model implemented in the numerical simulator DuMu x . The model solves the Navier-Stokes equations for density-induced water flow with concentration-dependent fluid density and a transport equation, including advective and diffusive processes for the carbon dioxide dissolved in water. The model was run in 2D, 3D, and pseudo-3D on two different grids. Without any calibration or fitting of parameters, the results of the comparison between experiment and simulation show satisfactory agreement with respect to the onset time of convective fingering, and the number and the dynamics of the fingers. Grid refinement matters, in particular, in the uppermost part where fingers develop. The 2D simulations consistently overestimated the fingering dynamics. This successful validation of the model is the prerequisite for employing it in situations with background flow and for a future study of karstification mechanisms related to CO 2 -induced fingering in caves.

Karst systems are found in many regions around the world. In the order of 10% of the continental surface is karst (Ford & Williams, 2007;Mangin, 1975). Karst is incredibly complex and manifold, and the processes that dominate karstification strongly depend on the hydrological and geomorphological properties of the karstic systems, which are subject to constant change while karstification is ongoing. Essentially, karstification happens in soluble rocks in contact with water, typically at the earth's surface or close to it. Karst research has evident relations to the disciplines and sub-disciplines of hydrology, geology, speleology, geomorphology, hydrogeology, etc. Karstic rocks are typically carbonate rocks made of Calcium and Magnesium minerals, where limestone (CaCO 3 ) and dolomite ( 𝐴𝐴 CaMg[CO3]2 ) are the most important subtypes. During karstification, these rocks are eroded mechanically, and, more importantly, corroded chemically. The corrosion of calcite and dolomite is driven by the availability of dissolved CO 2 in the water.

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