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Methane (CH4) emissions from wetlands are likely increasing and important in global climate change assessments. However, contemporary terrestrial biogeochemical model predictions of CH4 emissions are very uncertain, at least in part due to prescribed temperature sensitivity of CH4 production and emission. While statistically consistent apparent CH4 emission temperature dependencies have been inferred from meta-analyses across microbial to ecosystem scales, year-round ecosystem-scale observations have contradicted that finding. Using flux observations and mechanistic modeling in two heavily studied high-latitude research sites (Stordalen, Sweden, and Utqiaġvik, Alaska, USA), we show here that substrate-mediated hysteretic microbial and abiotic interactions lead to intra-seasonally varying temperature sensitivity of CH4 production and emission. We find that seasonally varying substrate availability drives lower and higher modeled methanogen biomass and activity, and thereby CH4 production, during the earlier and later periods of the thawed season, respectively. Our findings demonstrate the uncertainty of inferring CH4 emission or production from temperature alone, and highlight the need to represent microbial and abiotic interactions in wetland biogeochemical models. [ABSTRACT FROM AUTHOR]

The release of long‐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 (CH4) clumped isotope (Δ18) and 14C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δ18 values are negatively correlated with CH4 production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CH4 Δ18 and the fraction modern carbon. These correlations imply that CH4 derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δ18 values, are not positively correlated with CH4 flux estimates, highlighting the likely importance of environmental variables other than CH4 production rates in controlling ebullition fluxes. Plain Language Summary: There is concern that carbon from thawed permafrost will be emitted to the atmosphere as methane (CH4). It is currently uncertain whether old organic carbon from thawed permafrost can be converted to CH4 as rapidly as organic carbon recently fixed by primary producers. We address this question by combining radiocarbon and clumped isotope measurements of CH4 from lakes in permafrost landscapes. Radiocarbon (14C) measurements indicate the age of CH4 carbon sources. We present data from culture experiments that support the hypothesis that clumped isotope values are dependent on microbial CH4 production rate. In lake bubble samples, we observe a strong correlation between these two measurements, which implies that CH4 formed from older carbon is produced relatively slowly. We also find that higher rates of CH4 production, as inferred from clumped isotopes, are not linked to higher rates of CH4 emissions, implying that variables other than CH4 production rate strongly influence emission rates. Key Points: Experimental results indicate CH4 clumped isotopes are controlled by rates of microbial CH4 productionPaired clumped isotopes and 14C measurements imply CH4 formed from older substrates in northern lakes is produced relatively slowlyComparison of clumped isotope data and flux estimates suggests that CH4 production rate is not a strong determinant of CH4 ebullition flux [ABSTRACT FROM AUTHOR]

Characterizing methane sources in the Arctic remains challenging due to the remoteness, heterogeneity and variety of such emissions. In situ campaigns provide valuable datasets to reduce these uncertainties. Here we analyse data from the summer 2014 SWERUS-C3 campaign in the eastern Arctic Ocean, off the shore of Siberia and Alaska. Total concentrations of methane, as well as relative concentrations of 12CH4 and 13CH4 , were measured continuously during this campaign for 35 d in July and August. Using a chemistry-transport model, we link observed concentrations and isotopic ratios to regional emissions and hemispheric transport structures. A simple inversion system helped constrain source signatures from wetlands in Siberia and Alaska, and oceanic sources, as well as the isotopic composition of lower-stratosphere air masses. The variation in the signature of lower-stratosphere air masses, due to strongly fractionating chemical reactions in the stratosphere, was suggested to explain a large share of the observed variability in isotopic ratios. These results point towards necessary efforts to better simulate large-scale transport and chemistry patterns to make relevant use of isotopic data in remote areas. It is also found that constant and homogeneous source signatures for each type of emission in a given region (mostly wetlands and oil and gas industry in our case at high latitudes) are not compatible with the strong synoptic isotopic signal observed in the Arctic. A regional gradient in source signatures is highlighted between Siberian and Alaskan wetlands, the latter having lighter signatures (more depleted in 13C). Finally, our results suggest that marine emissions of methane from Arctic continental-shelf sources are dominated by thermogenic-origin methane, with a secondary biogenic source as well. [ABSTRACT FROM AUTHOR]