High-resolution isotopic monitoring of cave air CO₂

Rationale: CO₂ is the main driver of many chemical processes in cave environments. Understanding CO₂ fluxes in a given cave system through monitoring campaigns has become a standard procedure in a wide variety of fields such as paleoclimatology or show cave management. However, conventional methods lack the resolution of isotopic data to capture many transient processes occurring in caves.

Methods: A novel approach using isotope ratio infrared spectrometry (IRIS) to monitor cave air pCO₂ , δ¹³ C and δ¹⁸ O values in situ was tested and compared with conventional monitoring methods (handheld pCO₂ meter and discrete cave air samples for conventional isotope ratio mass spectrometry). This also involved the development of a field-deployable experimental setup to operate the equipment in rough cave environments.

Results: Comparison between data obtained by means of a Thermo Fisher Scientific Delta Ray IRIS instrument shows overall good agreement with conventional monitoring methods in terms of pCO₂ and δ¹³ C values. In addition, IRIS allows the δ¹⁸ O values of cave air CO₂ to be measured.

Conclusions: IRIS allows identification and tracking of processes at various timescales ranging from transient visitor impact on the cave atmosphere to seasonal trends in cave ventilation. However, the need for an uninterrupted power supply (110/220 V AC) and the relatively large dimensions of the equipment (698 × 1092 × 704 mm, 80 kg) limit the number of caves where deployment of the instrument is feasible. Copyright © 2017 John Wiley & Sons, Ltd.