LONGITUDINAL VARIATION IN DISSOLUTION RATES IN A CAVE STREAM: LESSONS FROM LENGTH SCALES

Characteristic process length scales emerge in karst conduits as a result of the combination of process time scales and flow velocities in the conduits. These length scales can be employed to improve the understanding of a given process, potentially suggesting, or ruling out, a variety of mechanisms or controls. Frequently, longitudinal variations in processes will be associated with entry effects in conduits as a result of the strong gradient between surface and sub-surface conditions. We present a study of the longitudinal variation of dissolution rates in Lekinka Cave, Slovenia. The integrated weight loss of limestone tablets placed in the stream is an exponentially decreasing function of stream distance, with an e-folding scale of around 300 m. One possible mechanism for the decrease of dissolution rates is that the water is quickly dissolving limestone and approaching saturation as it moves downstream. However, an analysis of dissolution length scales, which are found to be orders of magnitude longer than 300 m, demonstrates that dissolution itself not likely to be responsible for the decrease in rates. Alternatively, carbon dioxide (CO₂) degassing is expected to occur over length scales similar to that observed, suggesting a more plausible mechanism. To explore this hypothesis, we took spot measurements of pH and specific conductivity and analyzed the water chemistry of samples from the surface, cave stream, and infeeding cave streams under a variety of flow conditions. These observations provide evidence that changing concentrations of dissolved CO₂ are responsible for the longitudinal decrease in dissolution rates. We discuss the possible mechanisms for the reduction in CO₂, and the resulting potential feedbacks between the hydrological, biological, and geomorphological systems involved. This study is part of a longer-term exploration of the time variation in dissolution rates and the relative importance of mechanical erosion and dissolution in cave streams.