UNDERSTANDING EXTREME ISOTOPE ENRICHMENT CAUSED BY METHANE OXIDATION

Microbial oxidation of methane (CH₄) plays a central role in Arctic carbon cycling, potentially reducing CH₄ emissions from lakes associated with atmospheric warming. Isotopic signatures of methane (δ¹³C and δ²H) are indicators of microbial oxidation, strongly enriching ¹³C and ²H in residual methane. Previous measurements of δ¹³C and δ²H for dissolved methane in three, small Greenlandic lakes under both ice-free and ice-covered conditions from 2013-2014 revealed surprisingly large ranges for both δ¹³C and δ²H. Under open-water conditions, δ¹³C ranged from from -68.7 ‰ to +7.4 ‰ and δ²H from -370 ‰ to +250 ‰, while under ice-covered conditions, δ¹³C ranged from extending from -72.2 ‰ to -33.2 ‰ and δ²H from -388 ‰ to -29 ‰ The most enriched C and H isotopic values were observed in a single lake, Teardrop Lake, under open-water conditions, coinciding with a metalimnetic oxygen minimum at 4 m depth in summer 2013. The enrichment in ¹³C and ²H was associated with low methane concentrations, as is characteristic with consumption of methane by microbial oxidation of methane. Such extreme enriched isotopes suggest unusually efficient methanotrophic communities in Arctic ice-margin lakes. In summer of 2015, we returned to Teardrop Lake and measured enriched isotopic values of -13.2‰ and -27.1‰ for δ¹³C and δ²H, respectively, at 4.0 m depth again. This enrichment was observed at two locations in the lake, both at 4.0 m depth. Coinciding with the enrichment, concentrations of methane significantly decreased from 35 uM to 3.5 uM, suggesting consumption by methane oxidation. Microbial community composition responsible for the enrichment in methane was characterized with amplicon sequencing of 16S rRNA genes on the MiSeq Illumina platform. The occurrence of extreme isotope enrichment will be explored though detailed analysis of water column chemistry.