2) causing a rapid (~15 min) change in air-flow direction along open passages measured at Flitch of Bacon (Fig. In spring and autumn diurnal external temperature may dip below cave temperature and also rise above cave temperature (Fig. On any one day throughout the year air-flow may change direction abruptly in response to changing external weather (temperature). Summer air-flow is dominantly from the ridge overlying Chifley Cave including soil gas seepage with winter air-flow in the opposite direction (Fig. Air flows more slowly through minor passages & narrow fissures and ultimately through a multitude of cracks and through the soil. The open passages provide the path for the bulk of cave air-flow. The differential air density at different air temperatures inside the cave compared to external temperature drives bi-directional convective air-flow through the major passages, measured at the Flitch of Bacon (Fig. The karst soil overlying Chifley Cave was continuously monitored for soil temperature, soil water fraction and CO 2, supplemented by hourly CH 4 and δ 13C-in-CO 2 measured during a 10 day experiment to define soil function as a response to weather and gas composition as a potential source for cave air (Supplementary Fig. High precision gas analysers for CO 2, CH 4, CO, N 2O and δ 13C-in-CO 2 (CRDS and FTIR, see methods) were installed approximately 20 m from the lower (Grand Arch) entrance and sequentially analysed air sampled once per hour drawn along tubing from four locations in the cave (Fig. We also measured cave air-temperature, air-flow, air-pressure, drip-rate, and drip pH continuously for 3 years from September 2012 to July 2015. 1), represents a well-ventilated tourist cave hosting several tours per day. Our continuous data for CH 4, CO 2, δ 13C-in-CO 2, Rn, CO, and N 2O trace gases from Chifley Cave (Fig. CO 2 sources that would be expected to show antithetical behaviour relative to CH 4 are ground air 7 and the overlying soil gas, where CO 2 is increased by plant root and microbial respiration and CH 4 is depleted by microbial oxidation. CO 2 has several potential sources, including breath from cave visitors, ground air 7, speleothem growth 8, 9 and karst soil gas 10. CO 2 is enhanced relative to the external ambient atmosphere, to as high as 10,000 ppm in summer but remains near external ambient in winter. However, an in-situ steady-state model is inconsistent with our observations of strong diel and seasonal bias in CO 2, CH 4 and other trace gases.Ĭave CO 2 variations are strongly anti-correlated with those of CH 4. A methanotrophic bacterial oxidation mechanism was first proposed to account for rapid methane loss over a few hours 1, 2, 5 in opposition to a recently proposed radiolytic CH 4 destruction mechanism 4.Ī steady-state model of in-situ depletion of methane by cave dwelling methanotrophs or radiolytic destruction by ions derived from radon decay might be expected from the year round constant cave air temperature ± 2 ☌. Disentangling cause and effect of cave gas variations by tracing sources and sinks has identified seasonal speleothem growth bias, with implied palaeo-climate record bias.Ĭave methane depletion is near universal for 44 caves reported 1, 2, 3, 4, 5, 6. Unexpected seasonal bias in our cave CH 4 depletion record is explained by a three-step process involving methanotrophy in aerobic karst soil above the cave, summer transport of soil-gas into the cave through epikarst, followed by further cave CH 4 depletion. Extreme bacterial selection due to the absence of alternate carbon sources for growth in the cave environment has resulted in an extremely high proportion 2–12% of methanotrophs in the total bacteria present. Methanotrophic bacteria, some newly-discovered, rapidly consume methane on cave surfaces and in external karst soils with lifetimes in the cave of a few hours. Here, we report a continuous 3-year record of cave methane and other trace gases in Jenolan Caves, Australia which shows a seasonal cycle of extreme CH 4 depletion, from ambient ~1,775 ppb to near zero during summer and to ~800 ppb in winter. Methane concentration in caves is commonly much lower than the external atmosphere, yet the cave CH 4 depletion causal mechanism is contested and dynamic links to external diurnal and seasonal temperature cycles unknown.
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