Downstream Evolution of Particulate Organic Matter Composition From Permafrost Thaw Slumps

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

doi:10.3389/feart.2021.642675

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.

Highlights

Air temperatures in the Arctic have been rising twice as fast as the global average during the last decades (IPCC, 2013)
Sediments within thaw slump component for multiple retrogressive thaw slumps (RTS) features on the Peel Plateau are described in Bröder et al (2021), while the focus of this study is on suspended particulate matter
We addressed bulk geochemical and sedimentological properties as well as themolecular composition of suspended and streambank sediments released from two large RTS features and their downstream transport on the Peel Plateau

Summary

Introduction

Air temperatures in the Arctic have been rising twice as fast as the global average during the last decades (IPCC, 2013). Aside from gradual permafrost thaw that establishes itself through an increase in active layer thaw depth, abruptly thawing permafrost sites at locations with high ground-ice content are becoming more common (Segal et al, 2016; Farquharson et al, 2019). These thermokarst features (e.g., retrogressive thaw slumps, eroding coastlines) form through rapid degradation of ice-rich permafrost (Vonk et al, 2012; Kokelj and Jorgenson, 2013; Schuur et al, 2015). Estimates suggest that thermokarstsusceptible landscapes hold about half of the total OC pool of the Arctic, while only covering 20% of the Arctic permafrost region (Olefeldt et al, 2016; Turetsky et al, 2020)

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