ALBERT

Description: Methane production in thawing permafrost can be substantial, yet often evolves after long lag phases or is even lacking. A central question is to which extent the production of methane after permafrost thaw is determined by the initial methanogenic community. We quantified the production of methane relative to carbon dioxide (CO2) and enumerated methanogenic (mcrA) gene copies in long-term (2–7 years) anoxic incubations at 4 °C using interglacial and glacial permafrost samples of Holocene and Pleistocene, including Eemian, origin. Changes in archaeal community composition were determined by sequencing of the archaeal 16S rRNA gene. Long-term thaw stimulated methanogenesis where methanogens initially dominated the archaeal community. Deposits of interstadial and interglacial (Eemian) origin, formed under higher temperatures and precipitation, displayed the greatest response to thaw. At the end of the incubations, a substantial shift in methanogenic community composition and a relative increase in hydrogenotrophic methanogens had occurred except for Eemian deposits in which a high abundance of potential acetoclastic methanogens were present. This study shows that only anaerobic CO2 production but not methane production correlates significantly with carbon and nitrogen content and that the methanogenic response to permafrost thaw is mainly constrained by the paleoenvironmental conditions during soil formation.

Description: Permafrost deposits have been a sink for atmospheric carbon for millennia. Thaw-erosional processes, however, can lead to rapid degradation of ice-rich permafrost and the release of substantial amounts of organic carbon (OC). The amount of the OC stored in these deposits and their potential to be microbially decomposed to the greenhouse gases carbon dioxide (CO2) and methane (CH4) depends on climatic and environmental conditions during deposition and the decomposition history before incorporation into the permafrost. Here, we examine potential greenhouse gas production as a result of degrading ice-rich permafrost deposits from three locations in the northeastern Siberian Laptev Sea region. The deposits span a period of about 55 kyr from the last glacial period and Holocene interglacial. Samples from all three locations were incubated under aerobic and anaerobic conditions for 134 days at 4 °C. Greenhouse gas production was generally higher in deposits from glacial periods, where 0.2 %–6.1% of the initially available OC was decomposed to CO2. In contrast, only 0.1 %–4.0% of initial OC was decomposed in permafrost deposits from the Holocene and the late glacial transition. Within the deposits from the Kargin interstadial period (Marine Isotope Stage 3), local depositional environments, especially soil moisture, also affected the preservation of OC. Sediments deposited under wet conditions contained more labile OC and thus produced more greenhouse gases than sediments deposited under drier conditions. To assess the greenhouse gas production potentials over longer periods, deposits from two locations were incubated for a total of 785 days. However, more than 50% of total CO2 production over 785 days occurred within the first 134 days under aerobic conditions, while 80% were produced over the same period under anaerobic conditions, which emphasizes the nonlinearity of the OC decomposition processes. Methanogenesis was generally observed in active layer samples but only sporadically in permafrost samples and was several orders of magnitude smaller than CO2 production