Diurnal to seasonal ventilation in Brazilian caves

Abstract

Limited studies, particularly in the low-latitudes, exist that monitor cave ventilation, an important process dictating cave geochemical properties. To investigate cave ventilation, we present multi-year monitoring results, including observations of cave-air CO2, cave-air temperature, and cave-air relative humidity, from three Brazilian caves: Paraíso (4.07°S, 55.45°W), Tamboril (16.19°S, 46.59°W), and Jaraguá (21.08°S, 56.58°W). Cave ventilation regimes on numerous temporal scales are established. Highest concentrations of cave-air CO2 are recorded during austral summers from a multi-year monitoring effort at Jaraguá Cave. Conversely, lowest cave-air CO2 concentration are recorded during austral winters. The variability in cave-air CO2 is attributed to seasonally varying temperature contrasts between surface- and cave-air, which results in buoyancy-driven cave-air CO2 exchange with surface-air CO2. Speleothem growth as calcite precipitation is expected to be biased towards the austral winter season when cave-air CO2 is lowest. This understanding of cave ventilation affects the geochemical interpretation of speleothems as recorders of past climate change from caves in the low-latitudes. Next, continuous CO2 measurements near the entrance of Tamboril Cave suggest diurnal ventilation attributed to diurnal cave- and surface-air temperature differences. CO2 monitored from the deeper part of Paraíso and Jaraguá Cave do not exhibit this diurnal cycle. Further, virtual temperature calculations suggest that buoyancy contrasts between cave- and surface-air drive daily to weekly cave ventilation in all three caves. The established cave ventilation regimes provide an additional framework to interpret speleothem based terrestrial environmental change spanning the low- and mid-latitudes. By investigating spatially disparate caves across Brazil, we find buoyancy driven cave ventilation to be a unifying mechanism. Our results suggest that exchange between cave- and surface-air occurs seasonally in caves across the low- and mid-latitudes. These results suggest buoyancy driven ventilation is important not only at mid-latitudes but also in the tropics where speleothems are typically suited to investigate paleomonsoon variability. Therefore, prior knowledge of cave ventilation in caves that contain speleothems suitable for paleoclimate reconstructions is critical to robustly infer subsequent geochemical trends across different timescales (seasonal - centennial).