Temperature-driven air circulation in caves with blind passages

Authors

Chris Hendy Dr, University of Waikato, New ZealandFollow
David J. Merritt, The University of Queensland, AustraliaFollow
Shannon Corkill, Tourism Holdings Limited, New ZealandFollow
Cross Cross, University of Auckland, New ZealandFollow

Highlights

• Two blind-passage single-entrance caves in New Zealand were monitored for temperature and CO₂
• When the outside was warmer than inside, CO₂ partial pressures increased
• When the outside was cooler, CO₂ partial pressures decreased
• Both caves end in upper breakdown zones through which air percolates, accumulating CO₂ from the soil

Abstract

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.

DOI

https://doi.org/10.5038/1827-806X.ijs2532

Creative Commons License

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License.

Recommended Citation

Hendy, C.H., Merritt, D.J., Corkill, S., Cross, T., 2025. Temperature-driven air circulation in caves with blind passages. International Journal of Speleology, 54(1), ijs2532. https://doi.org/10.5038/1827-806X.ijs2552