Carbon Dioxide and Methane Release Following Abrupt Thaw of Pleistocene Permafrost Deposits in Arctic Siberia

Abstract

The decomposition of thawing permafrost organic matter (OM) to the greenhouse gases (GHG) carbon dioxide (CO 2 ) and methane forms a positive feedback to global climate change. Data on in situ GHG fluxes from thawing permafrost OM are scarce and OM degradability is largely unknown, causing high uncertainties in the permafrost-carbon climate feedback. We combined in situ CO 2 and methane flux measurements at an abrupt permafrost thaw feature with laboratory incubations and dynamic modeling to quantify annual CO 2 release from thawing permafrost OM, estimate its in situ degradability and evaluate the explanatory power of incubation experiments. In July 2016 and 2019, CO 2 fluxes ranged between 0.24 and 2.6 g CO 2 -C m −2 d −1 . Methane fluxes were low, which coincided with the absence of active methanogens in the Pleistocene permafrost. CO 2 fluxes were lower three years after initial thaw after normalizing these fluxes to thawed carbon, indicating the depletion of labile carbon. Higher CO 2 fluxes from thawing Pleistocene permafrost than from Holocene permafrost indicate OM preservation for millennia and give evidence that microbial activity in the permafrost was not substantial. Short-term incubations overestimated in situ CO 2 fluxes but underestimated methane fluxes. Two independent models simulated median annual CO 2 fluxes of 160 and 184 g CO 2 -C m −2 from the thaw slump, which include 25%-31% CO 2 emissions during winter. Annual CO 2 fluxes represent 0.8% of the carbon pool thawed in the surface soil. Our results demonstrate the potential of abrupt thaw processes to transform the tundra from carbon neutral into a substantial GHG source.Plain Language Summary Thawing of permanently frozen soils (permafrost) in the northern hemisphere forms a threat to global climate since these soils contain large amounts of frozen organic carbon, which might be decomposed to the greenhouse gases (GHGs) carbon dioxide (CO 2 ) and methane upon thaw. How fast these GHGs are produced is largely unknown, since field observations of greenhouse gas fluxes from thawing permafrost are too sparse. Consequently, simulations on the effect of thawing permafrost soils on future climate are highly uncertain. We measured CO 2 and methane fluxes from soils affected by abrupt permafrost thaw in Siberia during two summer seasons. We used these field observations and long-term incubation data to calibrate two models that simulate the CO 2 release over a whole year. We found that greenhouse gas fluxes were dominated by CO 2 and that the minor importance of methane was due to the absence of methane producing microorganisms in the Pleistocene permafrost. The CO 2 release in the first year accounted for 0.8% of thawed permafrost carbon but decomposition rates decreased after the depletion of the rapidly decomposable organic matter. Abrupt permafrost thaw turned the tundra into a substantial source of CO 2 , of which 25%-31% was released in the non-growing season.

Enhanced trends in spectral greening and climate anomalies across Europe

“…Anthropogenic impact on regional vegetation and surface change Global warming and coupled rising CO 2 atmospheric concentrations enhance ecosystem productivity in the high latitudes, increase the decomposition of organic matter in the permafrost region, and shift the climatic limitations of arable land towards the north (Liu et al, 2021). These feedbacks not only amplify CO 2 and methane release from thawing permafrost (Knoblauch et al, 2021;Zhu et al, 2019), but also increase the carbon sink rate due to accelerated vegetation productivity during prolonged growing seasons in the forested zones of Northern Europe. Theoretically, this leads to the expansion of the boreal zone as well as the migration of temperate forest species into the boreal zone across the Northern Hemisphere (Boisvert-Marsh and Blois 2021).…”

Equilibrium isotope fractionation factors of H exchange between steam and soil clay fractions

“…Sample RU‐17 was collected from the 0–30 cm layer of a Luvisol, RU‐PS2 from the 0–8 cm layer of a Cryosol, and RU‐S20 from the 0–10 cm layer of a Luvisol. The Russian samples had a clay content of 4%–13% and an organic C concentration of 20–78 g kg −1 62–64 . Sample SK‐Oso originated from the 0–10 cm layer of a Cambisol in Slovakia and had a clay content of 15% and an organic C concentration of 60 g kg −1 65,66 .…”

“…The Russian samples had a clay content of 4%-13% and an organic C concentration of 20-78 g kg À1 . [62][63][64] Sample SK-Oso originated from the 0-10 cm layer of a Cambisol in Slovakia and had a clay content of 15% and an organic C concentration of 60 g kg À1 . 65,66 Finally, UZ-K1 originated from the 0-10 cm layer of a Calcisol in Uzbekistan with a clay content of 30% and an organic C concentration of 14 g kg À1 .…”

Microbiome assembly in thawing permafrost and its feedbacks to climate

“…In a collapse scar where thermokarst was generated on a short time span, drift could be particularly important (Figure 1). Thermokarst also likely increases dispersal and colonization of active layer microbes into the recently thawed permafrost, via slumping and mixing at small spatial and short temporal scales and via wind‐ and water‐transport at the landscape scale (Knoblauch et al, 2021). Layering deterministic processes onto these stochastic ones, newly introduced active layer microbes might then be at a selective advantage in circumstances where abrupt thaw increases the availability of labile C, selecting for rapidly growing organisms and excluding permafrost organisms (Ernakovich & Wallenstein, 2015).…”

“…We expect this to be particularly pronounced for metabolic processes that have narrow phylogenetic breadth—that is, where functional guild members (microorganisms sharing a metabolic trait) are phylogenetically clustered—as there is a greater potential for stochastic processes to remove that entire function. For example, methanogens and nitrifiers are phylogenetically narrow guilds, and even under suitable environmental conditions, the absence of methanogens can limit CH 4 production (Figure 2, Example 1; Ernakovich et al, 2017; Knoblauch et al, 2018, 2021) and the absence of ammonia oxidizers can limit N 2 O production rates (Figure 2, Example 2; Alves et al, 2019; Siljanen et al, 2019; Monteux et al, 2020; Marushchak et al, 2021). Stochastic factors are particularly impactful for phylogenetically and ecologically constrained guilds (Vellend, 2010).…”