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

Abstract

Glacial ecosystems are an important indicator of climate change, and dynamics of the greenhouse gas methane (CH 4 ) in these systems has become a major research topic of late. We investigated occurrence and origin of recently discovered, sediment-entrapped CH 4 within the Wildstrubel glacial catchment (Switzerland) using geochemical analyses on gas extracted from sediments and rocks, including gas content and CH 4 stable isotope measurements, and computation of gas wetness (ratio of CH 4 to ethane and propane). We also examined the potential occurrence of microbial CH 4 production in subglacial, supraglacial, and glacier forefield sediments based on molecular analyses targeting mcrA (a marker gene for microbial CH 4 production) and based on observed CH 4 production during laboratory incubation experiments. Substantial amounts of entrapped CH 4 were detected in all sediment (65.7 ± 28.6 μg CH 4 /g) and most rock samples (up to 145.5 μg CH 4 /g). Similar gas wetness (0.7–126.7) and CH 4 stable isotope values (δ 13 C CH4 : −26.9‰ to −31.2‰, δD CH4 : −118.5‰ to −158.6‰) in sediment and rock samples provided strong evidence that entrapped CH 4 was of common, thermogenic origin. Molecular analyses (up to ~10 5  mcrA gene copies per gram) and laboratory incubation experiments (production rates up to 1.55 CH 4 g −1 day −1 ) provided evidence for local hot spots of viable methanogens in waterlogged, glacier forefield sediments but not in subglacial sediments. Nonetheless, microbial CH 4 production appeared to be only of minor importance at our field site, as indicated by results of our geochemical analyses. Our findings illustrate the crucial importance of appropriate geochemical analyses to provide solid evidence on the origin of CH 4 in glacial systems.