ANSTO Environmental Research, New Illawarra Rd., Lucas Heights, NSW 2234, Australia.
University of Wollongong, Centre for Atmospheric Chemistry, Wollongong, NSW 2522, Australia.
Sci Rep. 2017 Aug 16;7(1):8314. doi: 10.1038/s41598-017-07769-6.
Methane concentration in caves is commonly much lower than the external atmosphere, yet the cave CH 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 CH 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 CH 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 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.
洞穴中的甲烷浓度通常比外部大气中的甲烷浓度低得多，但洞穴 CH 消耗的因果机制仍存在争议，与外部昼夜和季节性温度循环的动态联系也未知。在这里，我们报告了澳大利亚杰诺兰洞穴（Jenolan Caves）连续 3 年的洞穴甲烷和其他痕量气体记录，该记录显示了 CH 极度消耗的季节性循环，从环境中的约 1775 ppb 到夏季接近零，到冬季约 800 ppb。在洞穴表面和外部喀斯特土壤中，一些新发现的甲烷氧化细菌会迅速消耗甲烷，其在洞穴中的寿命只有几个小时。由于洞穴环境中没有替代的生长碳源，导致细菌的极端选择，使得存在于洞穴中的甲烷氧化菌的比例极高，占总细菌的 2-12%。我们对洞穴 CH 消耗记录的意外季节性偏差的解释，是由涉及洞穴上方有氧喀斯特土壤中的甲烷氧化作用、夏季通过表生带将土壤气体输送到洞穴，以及随后进一步消耗洞穴 CH 的三个步骤的过程。通过追踪来源和汇来区分洞穴气体变化的因果关系，确定了洞穴石笋生长的季节性偏差，这暗示了古气候记录的偏差。
