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

• Main Authors: B. Zhu, M. Kübler, M. Ridoli, D. Breitenstein, M. H. Schroth
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-07-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/17/3613/2020/bg-17-3613-2020.pdf
• ISSN: 1726-4170; 1726-4189

<p>Aside from many well-known sources, the greenhouse gas methane (<span class="inline-formula">CH₄</span>) 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 <span class="inline-formula">CH₄</span> 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 <span class="inline-formula">CH₄</span> in other calcareous glacier forefields across Switzerland and (2) providing an inventory of this sediment-entrapped <span class="inline-formula">CH₄</span>, i.e., determining the contents and total mass of <span class="inline-formula">CH₄</span> 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.</p> <p>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 <span class="inline-formula">CH₄</span> contents, stable carbon-isotope analyses (<span class="inline-formula"><i>δ</i></span><span class="inline-formula">¹³C_CH4</span>), and the determination of gas-wetness ratios (ratio of <span class="inline-formula">CH₄</span> to ethane and propane contents). To estimate the total mass of <span class="inline-formula">CH₄</span> 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.</p> <p>Methane was found in all sediments (0.08–73.81&thinsp;<span class="inline-formula">µg CH₄ g⁻¹</span> dry weight) and most rocks (0.06–108.58&thinsp;<span class="inline-formula">µg CH₄ g⁻¹</span>) collected from the five glacier forefields, confirming that entrapped <span class="inline-formula">CH₄</span> is ubiquitous in these calcareous formations. Geochemical analyses further confirmed a thermogenic origin of the entrapped <span class="inline-formula">CH₄</span> (average <span class="inline-formula"><i>δ</i></span><span class="inline-formula">¹³C_CH4</span> of sediment of <span class="inline-formula">−</span>28.23 (<span class="inline-formula">±</span>&thinsp;3.42)&thinsp;‰; average gas-wetness ratio of 75.2 (<span class="inline-formula">±</span>&thinsp;48.4)). Whereas sediment-entrapped <span class="inline-formula">CH₄</span> contents varied moderately within individual forefields, we noted a large, significant difference in the <span class="inline-formula">CH₄</span> content and total <span class="inline-formula">CH₄</span> mass (range of 200–3881&thinsp;<span class="inline-formula">t</span> <span class="inline-formula">CH₄</span>) 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 <span class="inline-formula">CH₄</span> contents. Overall, a substantial quantity of <span class="inline-formula">CH₄</span> 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.</p>