Permafrost thaw leads to thermokarst lake formation and talik growth tens of meters deep, enabling microbial decomposition of formerly frozen organic matter (OM). We analyzed two 17-m-long thermokarst lake sediment cores taken in Central Yakutia, Russia. One core was from an Alas lake in a Holocene thermokarst basin that underwent multiple lake generations, and the second core from a young Yedoma upland lake (formed similar to 70 years ago) whose sediments have thawed for the first time since deposition. This comparison provides a glance into OM fate in thawing Yedoma deposits. We analyzed total organic carbon (TOC) and dissolved organic carbon (DOC) content, n-alkane concentrations, and bacterial and archaeal membrane markers. Furthermore, we conducted 1-year-long incubations (4 degrees C, dark) and measured anaerobic carbon dioxide (CO2) and methane (CH4) production. The sediments from both cores contained little TOC (0.7 +/- 0.4 wt%), but DOC values were relatively high, with the highest values in the frozen Yedoma lake sediments (1620 mg L-1). Cumulative greenhouse gas (GHG) production after 1 year was highest in the Yedoma lake sediments (226 +/- 212 mu g CO2-C g(-1) dw, 28 +/- 36 mu g CH4-C g(-1) dw) and 3 and 1.5 times lower in the Alas lake sediments, respectively (75 +/- 76 mu g CO2-C g(-1) dw, 19 +/- 29 mu g CH4-C g(-1) dw). The highest CO2 production in the frozen Yedoma lake sediments likely results from decomposition of readily bioavailable OM, while highest CH4 production in the non-frozen top sediments of this core suggests that methanogenic communities established upon thaw. The lower GHG production in the non-frozen Alas lake sediments resulted from advanced OM decomposition during Holocene talik development. Furthermore, we found that drivers of CO2 and CH4 production differ following thaw. Our results suggest that GHG production from TOC-poor mineral deposits, which are widespread throughout the Arctic, can be substantial. Therefore, our novel data are relevant for vast ice-rich permafrost deposits vulnerable to thermokarst formation.

The feedback between the atmosphere and permafrost soils containing large carbon stocks is currently considered the most important carbon-cycle feedback, but it is missing from climate models due to many uncertainties. Knowledge of how differences in post-thaw hydrological conditions affect carbon (C) release is critical for predicting permafrost feedback, but this knowledge remains limited. In this study, permafrost and active layer soils from the southern margin of the Eurasian boreal permafrost region in Northeast China were collected and incubated under experimentally modified moisture to monitor their CO2 and CH4 productions under simulated natural, drier and flooded conditions. We also characterized soil properties related to soil organic carbon (SOC) quality and microbial activities to determine their relations with measured C productions. We found that permafrost had higher C release per gram of SOC basis (C vulnerability) than the active layer, which suggested that there would be a high risk for C emissions in the permafrost region when permafrost thaws in the warming future. However, hydrological conditions following the permafrost thaw control these emissions. Permafrost C decomposed in relatively aerobic upland systems had higher C emissions than that decomposed in anaerobic wetland environments. Our results suggest a greater climate forcing of C release in aerobic than anaerobic conditions, but there are some uncertainties resulting from the unknown long-term CH4 production rates. Moreover, we found that the aerobic C production in permafrost after thawing could be constrained by water stress, suggesting that previous predictions based on soil incubations at natural soil moisture might overestimate the aerobic permafrost C release.

Cryosols contain roughly 1700 Gt of Soil organic carbon (SOC) roughly double the carbon content of the atmosphere. As global temperature rises and permafrost thaws, this carbon reservoir becomes vulnerable to microbial decomposition, resulting in greenhouse gas emissions that will amplify anthropogenic warming. Improving our understanding of carbon dynamics in thawing permafrost requires more data on carbon and nitrogen content, soil physical and chemical properties and substrate quality in cryosols. We analyzed five permafrost cores obtained from the North Slope of Alaska during the summer of 2009. The relationship between SOC and soil bulk density can be adequately represented by a logarithmic function. Gas fluxes at -5 degrees C and -5 degrees C were measured to calculate the temperature response quotient (Q(10)). Q(10) and the respiration per unit soil C were higher in permafrost-affected soils than that in the active layer, suggesting that decomposition and heterotrophic respiration in ciyosols may contribute more to global warming. (C) 2014 Published by Elsevier B.V.