Karst landscapes can serve as carbon sinks when carbon dioxide (CO2) reacts with water to form carbonic acid, which then weathers carbonate rocks. However, CO2 can also move through the subsurface via gas diffusion, a process that is not well-understood in karst systems. This study focused on quantifying CO2 diffusion within a karst sinkhole. The objectives of this study were to: 1) identify the depth of the zero-flux plane (i.e., depths of local maximum CO2 concentrations), analyze the distributions of concentration gradients, and investigate the validity of a uniform concentration gradient throughout the profile; and 2) assess the influences of vertical position and seasonality on CO2 fluxes within this sinkhole. The study site contained three locations within the sinkhole, including shoulder, backslope, and toeslope locations. Each location had three soil CO2 and three soil water content/temperature sensors placed at 20, 40, and 60 cm depths. Zero-flux planes were seldom detectable during the warm season (April-September) but were frequently found near the surface (20 or 40 cm) during the cool season (October-March). The common assumption of a uniform concentration gradient was often invalid based on relative concentrations between sensor pairs. As for the second objective, CO2 fluxes generally followed a trend of upward fluxes in warmer months that was partially offset by downward fluxes during the cooler months. These study results provide new insight into CO2 dynamics in a karst system, and suggest that subsurface processes such as chemical weathering and cave ventilation affect the direction and magnitude of CO2 fluxes.