Using carbon-14 and carbon-13 measurements for source attribution of atmospheric methane in the Athabasca oil sands region

Abstract

Abstract. The rapidly expanding and energy-intensive production from the Canadian oil sands, one of the largest oil reserves globally, accounts for almost 12 % of Canada's greenhouse gas emissions according to inventories. Developing approaches for evaluating reported methane (CH4) emission is crucial for developing effective mitigation policies, but only one study has characterized CH4 sources in the Athabasca oil sands region (AOSR). We tested the use of 14C and 13C carbon isotope measurements in ambient CH4 from the AOSR to estimate source contributions from key regional CH4 sources: (1) tailings ponds, (2) surface mines and processing facilities, and (3) wetlands. The isotopic signatures of ambient CH4 indicate that the CH4 enrichments measured at the site were mainly influenced by fossil CH4 emissions from surface mining and processing facilities (56 ± 18 %), followed by fossil CH4 emissions from tailings ponds (34 ± 18 %) and to a lesser extent modern CH4 emissions from wetlands (10 ± <1 %). Our results confirm the importance of tailings ponds in regional CH4 emissions and show that this method can successfully distinguish wetland CH4 emissions. In the future, the isotopic characterization of CH4 sources and measurements from different seasons and wind directions are needed to provide a better source attribution in the AOSR.

Preparation and Handling of Methane for Radiocarbon Analysis at Cologneams

“…These CH 4 sources have characteristic 13 C and 14 C isotopic signatures. CH 4 produced by methanogenesis of fresh organic matter is depleted in its 13 C content and enriched in 14 C, whereas thermogenic degradation of organic matter in sedimentary rocks generates CH 4 that contains more 13 C compared to biogenic sources and no 14 C. other studies applied 14 C analysis to distinguish CH 4 sources in the atmosphere (Graven et al 2019;Zazzeri et al 2021), in peatland (Garnett et al 2016;Cooper et al 2017) or in aquatic systems (Pohlman et al 2009;Joung et al 2019) or the combination of both isotopes has been applied (Gonzalez Moguel et al 2022).…”

“…Unfortunately, in contrast to 13 C, the direct 14 C analysis of CH 4 is not possible and requires an elaborate pre-treatment routine during which the CH 4 is purified and subsequently oxidized to CO 2 for further analysis. Gas samples taken from the environment or from laboratory incubations contain other C carrying gases that need to be separated from CH 4 by utilizing carrier gases (noble gas or synthetic air, Pohlman et al 2009) or pressure differences (Garnett et al 2019) by which the sample is moved through chemical adsorbents (Garnett et al 2019;Zazzeri et al 2021;Gonzalez Moguel et al 2022) and cryogenic traps (Petrenko et al 2008;Pack et al 2015) or a combination of those (Garnett et al 2019;Gonzalez Moguel et al 2022). The approach presented in this study utilizes a combination of methods taken from recent studies including a set of cryogenic traps, an oxidation furnace and synthetic air as carrier gas operating below ambient pressure.…”

“…The choice of the catalyst varies from laboratory to laboratory. Most prominently, cupric oxide (CuO) filled columns (Kessler and Reeburgh 2005;Pack et al 2015), platinized alumina beads contained in quartz glass tubes (Garnett et al 2019;Gonzalez Moguel et al 2022) or platinized quartz wool are used as catalysts (Petrenko et al 2008;Sparrow and Kessler 2017;Zazzeri et al 2021). After the reaction, the CH 4 -derived CO 2 can be quantified and prepared for 14 C analysis via gas injection or graphitization, i.e., conversion to elemental C.…”