Preferential export of permafrost-derived organic matter as retrogressive thaw slumping intensifies

Lisa Bröder,Tommaso Tesi,Steve V Kokelj,Timothy I Eglinton,Scott Zolkos,Suzanne E Tank,Bart E Van Dongen,Kirsi Keskitalo,Negar Haghipour,Sarah Shakil,Jorien E Vonk

doi:10.1088/1748-9326/abee4b

Abstract

Enhanced warming of the Northern high latitudes has intensified thermokarst processes throughout the permafrost zone. Retrogressive thaw slumps (RTS), where thaw-driven erosion caused by ground ice melt creates terrain disturbances extending over tens of hectares, represent particularly dynamic thermokarst features. Biogeochemical transformation of the mobilized substrate may release CO2 to the atmosphere and impact downstream ecosystems, yet its fate remains unclear. The Peel Plateau in northwestern Canada hosts some of the largest RTS features in the Arctic. Here, thick deposits of Pleistocene-aged glacial tills are overlain by a thinner layer of relatively organic-rich Holocene-aged permafrost that aggraded upward following deeper thaw and soil development during the early Holocene warm period. In this study, we characterize exposed soil layers and the mobilized material by analysing sediment properties and organic matter composition in active layer, Holocene and Pleistocene permafrost, recently thawed debris deposits and fresh deposits of slump outflow from four separate RTS features. We found that organic matter content, radiocarbon age and biomarker concentrations in debris and outflow deposits from all four sites were most similar to permafrost soils, with a lesser influence of the organic-rich active layer. Lipid biomarkers suggested a significant contribution of petrogenic carbon especially in Pleistocene permafrost. Active layer samples contained abundant intrinsically labile macromolecular components (polysaccharides, lignin markers, phenolic and N-containing compounds). All other samples were dominated by degraded organic constituents. Active layer soils, although heterogeneous, also had the highest median grain sizes, whereas debris and runoff deposits consisted of finer mineral grains and were generally more homogeneous, similar to permafrost. We thus infer that both organic matter degradation and hydrodynamic sorting during transport affect the mobilized material. Determining the relative magnitude of these two processes will be crucial to better assess the role of intensifying RTS activity in CO2 release and ecosystem carbon fluxes.

Highlights

Abrupt thaw caused by enhanced warming has become a widespread phenomenon throughout the northern permafrost zone, causing increased geomorphic disturbances such as thermo-erosional gullies, thermokarst lakes, active layer detachment slides and retrogressive thaw slumps (RTS; e.g. Kokelj and Jorgenson 2013, Nitze et al 2018)
Sedimentological and biogeochemical characteristics of Retrogressive thaw slumps (RTS) features Large differences in sediment properties, as well as bulk andmolecular organic matter composition were observed between the active layer, Holocene and Pleistocene permafrost
These results corroborate findings that RTS-mobilized particulate organic material is more recalcitrant than particulate organic material present in non-impacted streams (Shakil et al 2020), and are consistent with the observation of more biodegradable organic carbon (OC) (e.g. Oalkyl-C) in the active layer compared to more aromatic compounds in permafrost based on 13C nuclear magnetic resonance (NMR) analysis (Lacelle et al 2019)

Summary

Introduction

Abrupt thaw caused by enhanced warming has become a widespread phenomenon throughout the northern permafrost zone, causing increased geomorphic disturbances such as thermo-erosional gullies, thermokarst lakes, active layer detachment slides and retrogressive thaw slumps (RTS; e.g. Kokelj and Jorgenson 2013, Nitze et al 2018). Recent studies estimate that up to 20% of the northern permafrost zone may become affected by rapid thaw processes by 2300, leading to disturbance of up to half of the total organic matter stored in permafrost (Olefeldt et al 2016, Turetsky et al 2020) Upon thaw, this large carbon pool is subjected to photochemical and microbial degradation, potentially releasing CO2 to the atmosphere and amplifying global warming (e.g. Schuur et al 2015), as well as affecting downstream ecosystems (e.g. Vonk et al 2015). The loads of dissolved OC can decrease in RTS-impacted streams (Littlefair et al 2017, Shakil et al 2020) This pattern of a strong dominance of particulate over dissolved organic matter mobilization can be widespread (Kokelj et al 2020), and stands in contrast to other permafrost environments characterized by a gradual increase in active layer thickness and talik formation (Vonk et al 2015). In the Arctic coastal shelf seas, permafrost-derived material can be further degraded (e.g. Vonk et al 2012, Bröder et al 2018) or buried and sequestered in marine sediments (e.g. Hilton et al 2015)

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Conclusion