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 the 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 the results of a follow-up study that aimed at (1) determining the occurrence and origin of sediment-entrapped CH4 in other calcareous glacier forefields across Switzerland and (2) providing an inventory of this sediment-entrapped CH4, i.e., determining the 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 in 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 the determination of CH4 contents, stable carbon-isotope analyses (δ13CCH4), and the 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 the measured forefield area and either literature values of mean sediment thickness 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 δ13CCH4 of sediment of −28.23 (± 3.42) ‰; average gas-wetness ratio of 75.2 (± 48.4)). Whereas sediment-entrapped CH4 contents varied moderately within individual forefields, we noted a large, significant difference in the CH4 content and total CH4 mass (range of 200–3881 t CH4) between glacier forefields at the regional scale. The 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.