Author Correction: Seasonal total methane depletion in limestone caves.

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Seasonal total methane depletion in limestone caves.

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.

Use of multiply charged atomic ions for isotope ratio mass spectrometry.

We have investigated the use of multiply charged atomic ions for the measurement of isotopic ratios of gaseous and vapour samples. We use a mass spectrometer system incorporating an electron cyclotron resonance (ECR) ion source for this purpose. In the cases of carbon, nitrogen and oxygen, the selection of the 2+ atomic species is found to be the most effective for obtaining reliable isotopic ratios. Using samples of carbon dioxide, nitrogen, air and water vapour, we have demonstrated the determination of the isotopic ratios 13C/12C, 15N/14N, 17 O/16 O and 18 O/16 O. For oxygen, this technique offers an alternative to the equilibration or purification methods normally required to obtain isotopic ratios of water or other oxygen-containing samples. In particular, 17 O/16 O can be measured directly without isobaric interference from OH+. With typical ionization efficiencies of greater than 10%, ECR ion sources have the potential to enable measurements on very small samples. In addition to those evaluated in the present work, there is scope for application of this method to other sample types, to a variety of sampling methods, and to other elements.