Downstream Evolution of Particulate Organic Matter Composition From Permafrost Thaw Slumps

Abstract

Permafrost soils, which store almost half of the global belowground organic carbon (OC), are susceptible to thaw upon climate warming. On the Peel Plateau of northwestern Canada, the number and size of retrogressive thaw slumps (RTS) has increased in recent decades due to rising temperatures and higher precipitation. These RTS features caused by the rapid thaw of ice-rich permafrost release organic matter dominantly as particulate organic carbon (POC) to the stream network. In this study, we sampled POC and streambank sediments along a fluvial transect (∼12 km) downstream from two RTS features and assessed the composition and degradation status of the mobilized permafrost OC. We found that RTS features add old, Pleistocene-aged permafrost POC to the stream system that is traceable kilometers downstream. The POC released consists mainly of recalcitrant compounds that persists within stream networks, whereas labile compounds originate from the active layer and appear to largely degrade within the scar zone of the RTS feature. Thermokarst on the Peel Plateau is likely to intensify in the future, but our data suggest that most of the permafrost OC released is not readily degradable within the stream system and thus may have little potential for atmospheric evasion. Possibilities for the recalcitrant OC to degrade over decadal to millennial time scales while being transported via larger river networks, and within the marine environment, do however, still exist. These findings add to our understanding of the vulnerable Arctic landscapes and how they may interact with the global climate.

Low biodegradability of particulate organic carbon mobilized from thaw slumps on the Peel Plateau, NT, and possible chemosynthesis and sorption effects

“…These findings are consistent with measures of little change in CO2 concentrations downstream relative to upstream of slumps (Zolkos et al, 2019), despite orders of magnitude increases in POC and TOC (Shakil et al, 2020). We note that rapid within-slump processing of labile components prior to entrainment within streams may still occur, as supported by high NH4 + concentrations (Shakil et al, 2020) indicative of decomposition (Tanski et al, 2017), low representation of labile compounds in the slump scar zone and stream sediments relative to headwall sources (active layer) (Keskitalo et al, 2021), and excess CO2 in slump runoff resulting from both heterotrophic respiration and geogenic production (Zolkos et al, 2019)(Fig. 3F).…”

“…Our incubations, coupled with multiple studies examining slump-POC composition (Shakil et al, 2020;Bröder et al, 2021;Keskitalo et al, 2021) indicate that slump-mobilized POC in glacial landscapes of western Canada has low biodegradability on the Peel Plateau. We found no significant losses of POC or TOC or evidence that %ΔTOC increased in streams due to slump-mobilized POC across slump sites and for varying size fractions and distances downstream of slump SE.…”

“…1) typically mobilizes terrestrial material from three distinct sources: (1) Pleistoceneage tills that have remained preserved within permafrost since deposition by the Laurentide Ice Sheet and subsequent permafrost aggradation; (2) Holocene-age permafrost developed from tills following active layer deepening and/or slumping in previous warm periods; and (3) a contemporary active layer. Thus, relative contribution of sources to streams can depend on thaw depth (Shakil et al, 2020;Bröder et al, 2021). Variation in source composition can also vary west-east as vegetation (elevation) and geology changes along this gradient (Shakil et al, 2020).…”

“…For further details, see Appendix A. Data for this publication is available through Shakil et al (2021).…”

“…Slump-DOC in the region has an elevated lability compared to reference streams without slumps, as shown by chemical composition and incubations (Littlefair et al, 2017;Littlefair and Tank, 2018). Slump-POC chemical composition suggests lower bioavailability (Shakil et al, 2020;Bröder et al, 2021), though POC bioavailability has not been experimentally assessed. Given that slump-released carbon occurs almost entirely as POC in this region (Shakil et al, 2020), understanding the fate of this carbon remains a critical knowledge gap.…”