Temperature-driven air circulation in caves with blind passages

• Published in: International journal of speleology Vol. 54; no. 1
• Main Authors: Merritt, David, Hendy, Chris, Corkill, Shannon, Cross, Cross
• Format: Journal Article
• Language: English
• Published: 01.01.2025
• ISSN: 0392-6672; 1827-806X
• DOI: 10.5038/1827-806X.ijs2532

Two blind-passage show-caves in the Waitomo district of New Zealand, Ruakuri Cave and Aranui Cave, have been monitored for the impact of visitors on their environment, especially focusing on partial pressures of carbon dioxide because high levels cause speleothem degradation. In Ruakuri Cave, annual cycles of daily mean pCO2 correspond with annual cycles of visitor numbers, both peaking in summer. The origin of the high pCO2 had been assumed to be anthropogenic due to respiration from visitors. Prolonged intervals with no visitation during the Covid-19 pandemic saw the daily mean pCO2 continuing to show the annual cycle, suggesting a natural source where CO2 entered under certain conditions. Finer scale monitoring in the Drum Passage of Ruakuri Cave and the Fairy Walk of Aranui Cave showed that when the outside air was warmer than the cave air, the pCO2 increased and it rapidly decreased to near outside concentrations when outside air temperatures fell below the cave temperatures. Levels could be high at certain locations within Ruakuri Cave, for example, spot measurements within a breakdown at the Drum Passage section showed pCO2 values as high as 8000 ppm. We hypothesised that when outside air was warmer than cave air the temperature difference caused air to flow through breakdown zones and possibly epikarst, transporting soil gases, including CO2, and heat. When the outside air was cooler, air flowed inward through a low-level entrance bringing pCO2to near-external levels. Although there are no obvious higher entrances in the two caves, they mimic the chimney-effect ventilation that predominates in nearby Waitomo Glowworm Cave where air flows between a lower and upper entrance at a speed and direction determined by the temperature difference. Neither Ruakuri nor Aranui Caves have obvious upper entrances to allow a through-flow of air, but both caves end in breakdown zones through which the air is assumed to percolate. These observations demonstrate that there is a source of carbon dioxide in the air flowing into these two blind-ended passages when downflow conditions are likely to prevail but not during upflow conditions.