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

• Main Authors: Mühl, Michaela, Schmitt, Jochen, Seth, Barbara, Lee, James Edward, Edwards, Jon Shelley, Brook, Edward J., Blunier, Thomas, Fischer, Hubertus
• Format: Text
• Language: English
• Published: 2022
• Subjects: Greenland; Greenland ice core; Greenland ice cores; ice core
• Online Access: https://doi.org/10.5194/egusphere-2022-1133; https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1133/

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 ). Provided that the analyzed species concentrations and their isotopic fingerprints accurately reflect the past atmospheric composition, biogeochemical cycles can be reconstructed. 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 other 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 occurring in dust-rich sections of Greenland ice cores. The alkane production happens during the melt extraction step ( in extractu ) of the classic wet extraction technique and reaches 14 to 91 ppb for CH 4 excess in dusty ice samples. We document for the first time a co-production of excess methane, ethane, and propane (excess alkanes) 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, indicating a shared origin. The amount of excess alkanes scales linearly with the amount of mineral dust within the ice samples. The isotopic characterization of excess CH 4 reveals a relatively heavy carbon isotopic signature of -46.4 ‰ (± 2.4 ‰) and a light deuterium isotopic signature of -318 (± 53 ‰) in the samples analyzed. With the co-production ratios of excess alkanes and the isotopic composition of excess methane we established a fingerprint that allows ...