LARGE FRACTIONATIONS OF C AND H ISOTOPES RELATED TO METHANE OXIDATION IN ARCTIC LAKES

Microbial oxidation of methane (CH₄) plays a central role in Arctic carbon cycling, reducing potential CH₄ emissions from lakes associated with climate warming. Isotopic signatures of CH₄ (δ¹³C and δ²H) are indicators of microbial oxidation, strongly enriching ¹³C and ²H in residual CH₄. We present measurements of δ¹³C and δ²H for dissolved CH₄ in three, small Greenlandic lakes under both ice-free and ice-covered conditions from 2013-2014. The region was deglaciated ~10,000 years ago and the bedrock is dominated by garnet-grade, Archean gneisses, therefore CH₄ recovered from these lakes is minimally influenced by ancient organic matter, allowing recent cycling of biogenic CH₄ to be studied in isolation. Surprisingly large ranges for both δ¹³C and δ²H of CH₄ were observed, with δ¹³C extending from -72 ‰ to +7.4 ‰ and δ²H from -390 ‰ to +250 ‰. The CH₄ isotopic values reported here were significantly more enriched (p < 0.0001) in both ¹³C and ²H than values reported from other Arctic freshwater environments. As is characteristic of methanotrophy, the strongest enrichment in ¹³C and ²H was associated with low CH₄ concentrations. There are no other reported values for δ¹³C and δ²H of CH₄ at concentrations < 10 mM that we can make comparisons with. The extreme enrichment of δ¹³C and δ²H 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 heavier than -30 ‰ for δ¹³C and -150 ‰ for δ²H could be misinterpreted as thermogenic, geothermal, or abiogenic signals. Given crystalline bedrock and the statistically significant relationship between δ¹³C and δ²H throughout the water columns in three Arctic lakes confirms that CH₄ heavily enriched in ¹³C and ²H is the result of methanotrophy.