Abstract
Methane concentration in caves is commonly much lower than the external atmosphere, yet the cave CH4 depletion causal mechanism is contested and dynamic links to external diurnal and seasonal temperature cycles unknown. 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 CH4 depletion, from ambient ~1,775 ppb to near zero during summer and to ~800 ppb in winter. 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. 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. Unexpected seasonal bias in our cave CH4 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 CH4 depletion. 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.
Highlights
Methane concentration in caves is commonly much lower than the external atmosphere, yet the cave CH4 depletion causal mechanism is contested and dynamic links to external diurnal and seasonal temperature cycles unknown
Methanotrophic bacteria living on limestone and sediment cave surfaces, or in the overlying karst soil, may cause the observed depletion of methane in cave air
We have identified similar levels of methanotrophs in the Orient, Lucas, and Temple of Baal caves at Jenolan, and in the Wolondilly and Junction caves at Wombeyan Caves
Summary
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 CH4 depletion, from ambient ~1,775 ppb to near zero during summer and to ~800 ppb in winter. 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. A 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 °C. CO2 has several potential sources, including breath from cave visitors, ground air[7], speleothem growth[8,9] and karst soil gas[10]. CO2 sources that would be expected to show antithetical behaviour relative to CH4 are ground air[7] and the overlying soil gas, where CO2 is increased by plant root and microbial respiration and CH4 is depleted by microbial oxidation
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