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- Title
Occurrence and Origin of Methane Entrapped in Sediments and Rocks of a Calcareous, Alpine Glacial Catchment.
- Authors
Zhu, Biqing; Henneberger, Ruth; Weissert, Helmut; Zeyer, Josef; Schroth, Martin H.
- Abstract
Glacial ecosystems are an important indicator of climate change, and dynamics of the greenhouse gas methane (CH4) in these systems has become a major research topic of late. We investigated occurrence and origin of recently discovered, sediment‐entrapped CH4 within the Wildstrubel glacial catchment (Switzerland) using geochemical analyses on gas extracted from sediments and rocks, including gas content and CH4 stable isotope measurements, and computation of gas wetness (ratio of CH4 to ethane and propane). We also examined the potential occurrence of microbial CH4 production in subglacial, supraglacial, and glacier forefield sediments based on molecular analyses targeting mcrA (a marker gene for microbial CH4 production) and based on observed CH4 production during laboratory incubation experiments. Substantial amounts of entrapped CH4 were detected in all sediment (65.7 ± 28.6 μg CH4/g) and most rock samples (up to 145.5 μg CH4/g). Similar gas wetness (0.7–126.7) and CH4 stable isotope values (δ13CCH4: −26.9‰ to −31.2‰, δDCH4: −118.5‰ to −158.6‰) in sediment and rock samples provided strong evidence that entrapped CH4 was of common, thermogenic origin. Molecular analyses (up to ~105 mcrA gene copies per gram) and laboratory incubation experiments (production rates up to 1.55 CH4 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 CH4 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 CH4 in glacial systems. Plain Language Summary: The potent greenhouse gas methane was recently discovered to be trapped in sediments of glacier forefields in the Swiss Alps, in particular in those sediments that are derived from limestone bedrock. Here we assess the occurrence and origin of this trapped methane in one specific glacial system, which exhibited highest methane concentrations in a previous survey. We find substantial amounts of methane trapped in sediments and rocks throughout the glacial system. Our geochemical data provide robust evidence that the methane trapped in sediments and rocks is largely derived from ancient organic matter, which was deposited together with the limestone‐forming sediments, and subsequently converted to methane by heat and pressure during formation of the limestone bedrock. Conversely, biological methane production by microorganisms appears to be of minor importance at this site. Our findings show that a combination of methods was essential for identifying the ancient origin of trapped methane in sediments and rocks of this glacial system. Key Points: We detected substantial amounts of methane entrapped in sediments and rocks from diverse areas within a calcareous, glacial catchmentGeochemical data provided important, robust evidence that methane entrapped in sediments and rocks was largely of thermogenic originMicrobial methane production in the field was likely of minor importance despite a noted methanogenic potential in waterlogged sediments
- Subjects
METHANE; SEDIMENT analysis; ORGANIC compounds; CRUST of the earth; EARTH'S mantle
- Publication
Journal of Geophysical Research. Biogeosciences, 2018, Vol 123, Issue 12, p3633
- ISSN
2169-8953
- Publication type
Article
- DOI
10.1029/2018JG004651