Origin of carbon released from ecosystems affected by permafrost degradation in Northern Siberia

Permafrost soils and peatlands store half of the soil organic carbon stock worldwide, and are rapidly evolving as a result of permafrost thaw. Determining the origin (permafrost or recent photosynthesis) of carbon which is released to surface waters and the atmosphere is crucial to assess Arctic ecosystems' potential feedback to climate change. In order to evaluate it, we investigated the stable and radioactive content of carbon in solid organic matter, dissolved organic matter (DOM) and dissolved CO2 and CH4 in a discontinuous permafrost area of Siberia affected by permafrost degradation (Igarka, Graviyka catchment (67°27'11''N, 86°32'07''E)). We collected samples from the active layer, permafrost, surface water and bubbles from thermokarst lakes. We further investigated DOM and dissolved CO2 and CH4 in porewater profiles, streams and the catchment outlet. In thermokarst lakes, DOM of surface water as well as CO2 and CH4 from bubbles from lake sediments predominantly originate from modern carbon. In two locations, CO2 and CH4 from bubbles have relatively low 14C contents, with ages greater than 700 yr BP, but still younger that what was previously reported in Eastern Siberia. In all samples the Δ14C of CH4 and CO2 were strongly correlated, with CH4 being consistently older than CO2, indicating strong interrelation between CO2 and CH4 cycles. In our study, permafrost influenced CO2 and CH4 is found in small ponds where palsa collapse and the resulting bank erosion has mobilized sequestered carbon. In peatland porewater, the Δ14C of DOM, CO2 and CH4 increases with depth (DOM: 1385 ±45 yr BP at 2m), indicating a contribution from Holocene peatlands affected by permafrost. In deep layers, CO2 reduction is the dominant pathway of CH4 production, whereas acetate fermentation dominates in thermokarst lakes. In summary, the majority of dissolved CO2 and CH4 analyzed from thermokarst lakes and degraded peatlands is modern and originates from recently fixed carbon. Additionally, the DOM exported in small streams draining peatlands is also modern. However, at the catchment scale, an additional contribution from deep groundwater or thawing permafrost results in an intermediate Δ14C of DOM (300-400 yr BP) at the outlet of the Graviyka River.