Seasonal temperature variations controlling cave ventilation processes in Cueva Larga, Puerto Rico

Abstract

Two years of cave monitoring investigate ventilation processes in Cueva Larga, a tropical cave in Puerto Rico. The cave is 1,440 m long with a large main passage (about 120,000 m3). Cave air pCO2 in the main passage varies seasonally, between 600 ppm in winter and 1,800 ppm in summer. The seasonal variability in cave pCO2 permits the estimation of a cave air exchange time of 36 ± 5 days and a winter ventilation rate of 3,300 ± 1,000 m3/day for the main cave passage. Calculations of virtual temperature and differences between cave and surface temperature indicate that the seasonal temperature cycle is the main driver of the alternation between a well-ventilated winter mode and a near-stagnant summer mode. The winter mode is characterized by a positive buoyancy contrast at night leading to maximal cave ventilation, while cave ventilation is at a minimum during summer. Between winter and summer, a transitional mode of partial cave ventilation is observed. On shorter time scales (diurnal to weekly), cave pCO2 is also influenced by atmospheric pressure but this variation is one order of magnitude lower than the seasonal pCO2 change. The cave morphology of Cueva Larga including its large volume, tubular shape and the obstructed cave entrance geometry are important boundary conditions for the observed ventilation patterns. Our findings emphasize that cave systems with varying morphology have to be studied individually in order to correctly describe ventilation processes.

Keywords

• environmental monitoring
• cave ventilation
• carbon dioxide
• virtual temperature
• air exchange time