Large fractionations of C and H isotopes related to methane oxidation in Arctic lakes. (15th August 2016)

• Record Type: Journal Article
• Authors: Cadieux, Sarah B.; White, Jeffrey R.; Sauer, Peter E.; Peng, Yongbo; Goldman, Amy E.; Pratt, Lisa M.
• Is Part Of: Geochimica et cosmochimica acta. Volume 187(2016:Aug. 15)
• Page Start: 141
• Page End: 155
• Publication Date: 2016-08-15
• Subjects: Methane -- Hydrogen isotopes -- Carbon isotopes -- Arctic -- Lakes; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2016.05.004
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Microbial oxidation of methane (CH4 ) plays a central role in carbon cycling in Arctic lakes, reducing potential CH4 emissions associated with warming. Isotopic signatures of CH4 (δ 13 C and δ 2 H) are indicators of microbial oxidation, wherein the process strongly enriches 13 C and 2 H in residual CH4 . We present δ 13 C and δ 2 H measurements obtained from sampling the water column and sediment for dissolved CH4 from three, small Arctic lakes in western Greenland under both open-water and ice-covered conditions from 2013 to 2014. Despite substantial variations in aquatic chemistry among the lakes, δ 13 C and δ 2 H of CH4 suggested that CH4 was produced predominantly by acetoclastic methanogenesis in the littoral sediments and hydrogenotrophic methanogenesis in the profundal sediments in all of the lakes. Surprisingly large variations for both δ 13 C and δ 2 H of CH4 were observed, with δ 13 C extending from −72‰ to +7.4‰ and δ 2 H from −390‰ to +250‰. The CH4 isotopic values reported here were significantly more enriched ( p < 0.0001) in both 13 C and 2 H than values reported from other Arctic freshwater environments. As is characteristic of methanotrophy, the enrichment in 13 C and 2 H was associated with low CH4 concentrations. We suggest that the CH4 most enriched in 13 C and 2 H may reflect unusually efficient methanotrophic communities in Arctic ice-margin lakes. This study provides the first measurement of δ 2 H for CH4 in an Arctic freshwater environment at concentrations <10 μM. The extreme enrichment of 13 C and 2 H of CH4 from Arctic methanotrophy has significant implications for interpreting sources and sinks of CH4 . Without knowledge of local geology, stable isotope values of CH4 higher than −30‰ for δ 13 C and −150‰ for δ 2 H could be misinterpreted as thermogenic, geothermal, or abiogenic origins. Given crystalline bedrock and the strong positive correlation between δ 13 C and δ 2 H throughout the water columns in three Arctic lakes confirms that CH4 heavily enriched in 13 C and 2 H is the result of methanotrophy.