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

Abstract

Abstract. Investigations of sources and sinks of atmospheric CH4 are needed to understand the global CH4 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 CH4 is initiated. The development of this change in CH4 turnover can be quantified by soil-gas profile analysis. We found evidence for CH4 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 CH4 concentrations were observed with both methods. Conversely, elevated soil-CH4 concentrations were observed in calcareous glacier forefields, but only in samples collected with the SR, while MLS samples all showed sub-atmospheric CH4 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-CH4 concentrations, which were decreasing from ∼100 μL L−1 towards background levels within minutes. This hints towards the existence of entrapped CH4 in calcareous glacier forefield soil that can be released when sampling soil-gas with the SR. Laboratory experiments with miniature soil cores collected from two glacier forefields confirmed CH4 entrapment in these soils. Treatment by sonication and acidification resulted in a massive release of CH4 from calcareous cores (on average 0.3–1.8 μg CH4 (g d.w.)−1) (d.w. – dry weight); release from siliceous cores was 1–2 orders of magnitude lower (0.02–0.03 μg CH4 (g d.w.)−1). Clearly, some form of CH4 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.

Occurrence and Origin of Methane Entrapped in Sediments and Rocks of a Calcareous, Alpine Glacial Catchment

“…Moreover, a seemingly widespread CH 4 source was discovered during a field survey on CH 4 turnover in sediments of five Swiss glacier forefields situated on calcareous bedrock (Nauer et al, ). Subsequent analyses confirmed that this CH 4 was entrapped in the sediment matrix and could be released upon acidification or mechanical impact (Nauer et al, ). As measurements had indicated oxic conditions and low organic carbon (OC) content in these sediments, ongoing microbial CH 4 production was considered unlikely (Nauer et al, ).…”

High Temporal and Spatial Variability of Atmospheric-Methane Oxidation in Alpine Glacier Forefield Soils

“…As our measurements represent net fluxes, i.e., the sum of simultaneous CH 4 transport, production, and consumption (53), differences in any of these processes may explain differences in CH 4 flux between the landforms. Although we were unable to distinguish between individual processes, occasionally observed CH 4 emissions at locations F and T support the presence of a CH 4 source of as yet unknown origin in these calcareous glacier forefield soils (54). We found no evidence of microbial CH 4 production on screening of the top 15 cm of soils at locations S, F, and T for presence of the mcrA gene, a biomarker for methanogenic Archaea (data not shown).…”

“…It may indicate that Methylocystis-like MOB possess an ecological advantage in calcareous glacier forefield soils, which may be linked to the existence of two pMMO isoenzymes, with different affinities for low (2 to 600 l liter Ϫ1 ) and high (Ͼ600 l liter Ϫ1 ) CH 4 concentrations, previously detected in cultures of a Methylocystis strain (11). Thus, Methylocystis-like MOB may profit from transient release of CH 4 (at elevated concentrations) entrapped in calcareous soil aggregates (54).…”

“…We found no evidence of microbial CH 4 production on screening of the top 15 cm of soils at locations S, F, and T for presence of the mcrA gene, a biomarker for methanogenic Archaea (data not shown). However, we cannot fully exclude the possibility of microbial CH 4 production in deeper, water-logged soils at locations F and T. Alternatively, the CH 4 source may be related to slow dissolution of carbonate rock or particle aggregates in water-logged soils, thereby releasing entrapped CH 4 (54).…”

“…We performed sampling and measurements in the forefield of Griessfirn Glacier (Canton Uri, Switzerland), which has been described extensively elsewhere (39,54). Sampling locations were positioned along two transects (Fig.…”