The timing and radiative forcing of the permafrost carbon feedback (PCF) is not well understood. This is manifested in PCF projections of 0.05 to 0.39 °C temperature increases by the year 2300. Modelled CH4 contributes only ~16% of the PCF despite its high radiative forcing compared to CO2. Short-term batch experiments also resulted in a minor CH4 contribution to the PCF compared to CO2. This contribution had long lag phases but on longer time scales equal amounts of CO2 and CH4 may be produced anaerobically. CH4 concentrations and isotopic signatures in submarine permafrost furthermore showed oxidation of CH4 after it was released by permafrost thaw. Large uncertainties in projected CH4release thus depend on the microbial response to permafrost thaw. Important questions include: i) what constrains methanogenic activity in recently thawed sediment and in permafrost and ii) are there unrecognized yet relevant microbial methane filters associated with sub-aquatic permafrost thaw?
Using molecular techniques such as Illumina sequencing, quantitative PCR, total cell counts and biomarker analysis combined with batch experiments, geochemistry and statistics, we addressed these questions on ten permafrost cores from Canada, Alaska and Siberia of Holocene and Pleistocene origin that range from frozen to completely thawed permafrost sediments.
Results/Conclusions
The low number of methanogenic cells is a primary initial constraint on CH4 production in thawing permafrost. The methanogenic population size is thereby a function of carbon density and serves as a good predictor for the production of CH4 given permafrost thaw. We suggest that thousands of years of exposure to permafrost conditions bred high-affinity methanogenic communities, since in most of our incubations they were unable to exhaust large substrate concentrations or to substantially build up biomass, even after one year of incubation. Comparative sequencing analysis on incubations with weak versus strong responses to permafrost thaw will clarify community level constraints on microbial CH4production.
We further identified microbial communities responsible for anaerobic methane oxidation (AOM) in sediments undergoing sub-aquatic permafrost thaw. Specifically, ANME-2a/b and ANME-2d assemblages probably oxidize CH4 in submarine permafrost and ANME-2d in thermokarst lake sediments. Coarse estimates of potential AOM rates show that up to 120 Tg C per year are consumed in circum-arctic submarine permafrost, which is comparable to other global AOM habitats such as seeps and wetlands. We thus propose that AOM is active where subaquatic permafrost thaws but that this CH4 oxidation is currently ignored in global methane budgets.