| Summary: | The release of longâ€stored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CHâ‚„) clumped isotope (Δâ‚₈) and ¹â´C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δâ‚₈ values are negatively correlated with CHâ‚„ production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CHâ‚„ Δâ‚₈ and the fraction modern carbon. These correlations imply that CHâ‚„ derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δâ‚₈ values, are not positively correlated with CHâ‚„ flux estimates, highlighting the likely importance of environmental variables other than CHâ‚„ production rates in controlling ebullition fluxes. © 2020 American Geophysical Union. Received 19 DEC 2019; Accepted 5 MAR 2020; Accepted article online 09 MAR 2020. We thank Nami Kitchen for help with Δâ‚₈ analyses, Sarah Murseli for help with ¹â´C analyses, and Nathan Dalleska for help with CHâ‚„ concentration measurements. Victoria Orphan and Jared Leadbetter provided laboratory facilities for culture experiments. This research was partially supported by funds from the Trottier Institute for Science and Public Policy to PMJD, from Royal Dutch Shell to JME and ALS, from the Agouron Institute to MKL, from the Heisingâ€Simons Foundation to DA Stolper, and from VetenskapsrÃ¥det 2013â€5562 to PC. Data supporting this article are available at https://doi.org/10.6084/m9.figshare.11831340.v1. Published - 2019GL086756.pdf Supplemental Material - grl60364-sup-0001-2019gl086756-si.pdf Supplemental Material - grl60364-sup-0002-2019gl086756-ts01.pdf
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