Characterization of Cave Geochemistry

All solid samples (n = 36, contains 3 replicates) were freeze-dried (Alpha 1-2 LD freeze-dryer; Martin Christ, Osterode am Harz, Germany) and passed through a sterile 2-mm sieve. The sieved samples were mixed with ultrapure water (1:5, wt/vol) to get a suspension. The supernatant pH of the suspension was determined using a multiparameter water quality detector (Hach, Loveland, CO, USA) (25 (link)). Dissolved anions and cations were measured with anionic chromatography (ICS-600; Thermo Scientific, USA) and inductively coupled plasma-optical emission spectrometry (ICP-OES) (iCAP 7600+; Thermo, USA), respectively, after filtration with 0.22-μm filters (47 (link)). The concentrations of CH₄ and CO₂ gases and the carbon isotope of CO₂ ([δ¹³C]CO₂) of cave air samples were measured by a high-precision carbon isotope analyzer (G2201-I; Picarro, USA) using cavity decay spectroscopy (cavity ring-down spectroscopy [CRDS]) (5 (link)) at the Institute of Karst Geology, Chinese Academy of Geological Sciences.