Methane, ethane, and propane production in Greenland ice core samples and a first isotopic characterization of excess methane

• Published in: Climate of the Past
• Main Authors: M. Mühl, J. Schmitt, B. Seth, J. E. Lee, J. S. Edwards, E. J. Brook, T. Blunier, H. Fischer
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: Environmental pollution; TD172-193.5; Environmental protection; TD169-171.8; Environmental sciences; GE1-350; Greenland; Greenland ice core; ice core
• Online Access: https://doi.org/10.5194/cp-19-999-2023; https://doaj.org/article/92dccacfa9524de9bf6bad049c223f44

Air trapped in polar ice provides unique records of the past atmospheric composition ranging from key greenhouse gases such as methane (CH 4 ) to short-lived trace gases like ethane (C 2 H 6 ) and propane (C 3 H 8 ). Recently, the comparison of CH 4 records obtained using different extraction methods revealed disagreements in the CH 4 concentration for the last glacial in Greenland ice. Elevated methane levels were detected in dust-rich ice core sections measured discretely, pointing to a process sensitive to the melt extraction technique. To shed light on the underlying mechanism, we performed targeted experiments and analyzed samples for methane and the short-chain alkanes ethane and propane covering the time interval from 12 to 42 kyr. Here, we report our findings of these elevated alkane concentrations, which scale linearly with the amount of mineral dust within the ice samples. The alkane production happens during the melt extraction step of the classic wet-extraction technique and reaches 14 to 91 ppb of CH 4 excess in dusty ice samples. We document for the first time a co-production of excess methane, ethane, and propane, with the observed concentrations for ethane and propane exceeding their past atmospheric background at least by a factor of 10. Independent of the produced amounts, excess alkanes were produced in a fixed molar ratio of approximately 14 : 2 : 1 <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="42pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="21b5996f25aa1e5d07b05d6b9de8bc7e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-19-999-2023-ie00001.svg" width="42pt" height="10pt" src="cp-19-999-2023-ie00001.png"/></svg:svg> , indicating a shared origin. The measured carbon isotopic signature of excess methane is ( - 47.0 ± 2.9 <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="c6173d5bf7057daaac0302665a05af1f"><svg:image ...