MODELING THE EVOLUTION OF CAVE PASSAGE CROSS-SECTIONAL SHAPE AND THE IMPACTS OF DISCHARGE, SEDIMENT, CHEMISTRY, AND VENTILATION (Invited Presentation)

Cave sediments provide important records of past climate conditions and landscape evolution. However, cave passage morphology itself also contains substantial information about the conditions under which a passage formed. For example, speleologists have long recognized that cave passage cross-sectional shapes indicate whether passages developed in phreatic or vadose conditions. While much is already understood about relationships between cave passage cross-sectional shapes and conditions of passage development, most of this understanding is qualitative rather than quantitative. Here we develop a model for the coupled evolution of cave cross-sections and channel profiles, where model cave passages can adjust in both width and slope as hydrological, chemical, or base level conditions change. In an initial set of experiments, we examine the dynamics of paragenetic cross-section evolution. Within the model, paragenetic channels reach an equilibrium width that scales with both discharge and sediment supply in a relationship that can be predicted from simple theoretical arguments. Next we examine relationships among ventilation, CO₂ exchange, and equilibrium channel forms. Prior field studies have demonstrated that cave ventilation and resulting contrasts in dissolved CO₂ concentrations can produce strong temporal and spatial differences in dissolution rates. Here we model these processes and examine how feedbacks among ventilation, gas exchange, and channel form impact cave passage equilibrium morphology. We show that spatial contrasts in saturation state that are created by cave ventilation drive adjustments in both channel width and slope, whereby channels steepen and narrow to accommodate zones where water is closer to saturation. The model also demonstrates that onset of ventilation within a cave passage can produce systematic decreases in channel width, indicating that keyhole shaped passages may sometimes be created by shifts in chemical conditions rather than simply decreases in discharge or lowering of base level.