Quantity and distribution of methane entrapped in sediments of calcareous, Alpine glacier forefields

Abstract. Aside from many well-known sources, the greenhouse gas methane (CH4) was recently discovered entrapped in sediments of Swiss Alpine glacier forefields derived from calcareous bedrock. A first study performed in one glacial catchment indicated that CH4 was ubiquitous in sediments and rocks, and was largely of thermogenic origin. Here we present results of a follow-up study, which aimed at (1) determining occurrence and origin of sediment-entrapped CH4 in other calcareous glacier forefields across Switzerland, and (2) providing an inventory for this sediment-entrapped CH4, i.e., determining contents and total mass of CH4 present, and its spatial distribution within and between five different Swiss glacier forefields situated on calcareous formations of the Helvetic Nappes of the Central Alps. Sediment and bedrock samples were collected at high spatial resolution from the forefields of Im Griess, Griessfirn, Griessen, Wildstrubel, and Tsanfleuron glaciers, representing different geographic and geologic regions of the Helvetic Nappes. We performed geochemical analyses on gas extracted from sediments and rocks, including determination of CH4 contents, stable carbon-isotope analyses (δ13CCH4), and determination of gas-wetness ratios (ratio of CH4 to ethane and propane contents). To estimate the total mass of CH4 entrapped in glacier-forefield sediments, the total volume of sediment was determined based on measured forefield area and either literature values of mean sediment depth or direct depth measurements using electrical-resistivity tomography. Methane was found in all sediments (0.08–73.81 μg CH4 g−1 dry weight) and most rocks (0.06–108.58 µg CH4 g−1) collected from the five glacier forefields, confirming that entrapped CH4 is ubiquitous in these calcareous formations. Geochemical analyses further confirmed a thermogenic origin of the entrapped CH4 (average δ13C-CH4 of sediment: −28.23 (± 3.42) ‰; average gas-wetness ratio: 75.2 (± 48.4)). Whereas sediment-entrapped CH4 contents varied moderately within individual forefields, we noted a large, significant difference in CH4 content and total CH4 mass (range: 200–3881 t CH4) between glacier forefields at the regional scale. Lithology and tectonic setting within the Helvetic Nappes appeared to be dominant factors determining rock and sediment CH4 contents. Overall, a substantial quantity of CH4 was found to be entrapped in Swiss calcareous glacier forefields. Its potential release and subsequent fate in this environment is the subject of ongoing studies.