Technical Note: Disturbance of soil structure can lead to release of entrapped methane in glacier forefield soils

摘要

pstrongAbstract./strong Investigations of sources and sinks of atmospheric CHsub4/sub are needed to understand the global CHsub4/sub cycle and climate-change mitigation options. Glaciated environments might play a critical role due to potential feedbacks with global glacial meltdown. In an emerging glacier forefield, an ecological shift occurs from an anoxic, potentially methanogenic subglacial sediment to an oxic proglacial soil, in which soil-microbial consumption of atmospheric CHsub4/sub is initiated. The development of this change in CHsub4/sub turnover can be quantified by soil-gas profile analysis. br/br We found evidence for CHsub4/sub entrapped in glacier forefield soils when comparing two methods for the collection of soil-gas samples a modified steel rod (SR) designed for one-time sampling and rapid screening (samples collected ∼1 min after hammering the SR into the soil), and a novel multilevel sampler (MLS) for repetitive sampling through a previously installed access tube (samples collected weeks after access-tube installation). In glacier forefields on siliceous bedrock, sub-atmospheric CHsub4/sub concentrations were observed with both methods. Conversely, elevated soil-CHsub4/sub concentrations were observed in calcareous glacier forefields, but only in samples collected with the SR, while MLS samples all showed sub-atmospheric CHsub4/sub concentrations. Time-series of SR soil-gas sampling (additional samples collected 2, 3, 5, and 7 min after hammering) confirmed the transient nature of the elevated soil-CHsub4/sub concentrations, which were decreasing from ∼100 ??L Lsupa??1/sup towards background levels within minutes. This hints towards the existence of entrapped CHsub4/sub in calcareous glacier forefield soil that can be released when sampling soil-gas with the SR. br/br Laboratory experiments with miniature soil cores collected from two glacier forefields confirmed CHsub4/sub entrapment in these soils. Treatment by sonication and acidification resulted in a massive release of CHsub4/sub from calcareous cores (on average 0.3a??1.8 ??g CHsub4/sub (g d.w.)sup−1/sup) (d.w. a?? dry weight); release from siliceous cores was 1a??2 orders of magnitude lower (0.02a??0.03 ??g CHsub4/sub (g d.w.)sup−1/sup). Clearly, some form of CHsub4/sub entrapment exists in calcareous glacier forefield soils, and to a much lesser extent in siliceous glacier forefield soils. Its nature and origin remain unclear and will be subject of future investigations./p.