Biodegradability of Thermokarst Carbon in a Till‐Associated, Glacial Margin Landscape: The Case of the Peel Plateau, NWT, Canada

Abstract

The Peel Plateau is a characteristic glacial margin landscape, with permafrost comprised of thick, ice-rich glacial tills deposited at the end of the Last Glacial Maximum. Unmodified tills at depth are overlain by a paleo-active layer, created when early Holocene warming deepened regional active layers, enabling organic matter incorporation into now-frozen soils. Ice-rich permafrost encourages retrogressive thaw slumps, which mobilize variable proportions of modern active layer, paleo-active layer, and Pleistocene tills to downstream systems. Here we investigate the biolability of thaw-released dissolved organic carbon (DOC) on the Peel Plateau and compare our results to previous studies from nontill-dominated landscapes. Similar to other Arctic regions, biolability was significantly greater for slump-derived DOC (retrogressive thaw slump runoff) than for DOC from paired, unimpacted locations. However, runoff source was an important control on biolability. Lability was greater for slumps releasing water with a Holocene-like δ 18 O signature than for slumps with a more Pleistocene-like signature, while a small slump, with runoff δ 18 O similar to the modern active layer, showed no biolability increase. Similar to other Arctic regions, biolability was strongly related to DOC aromaticity and molecular weight. However, lability also increased significantly with increasing nutrients, which has not been shown universally. Previous work has shown that DOC concentration dynamics differ sharply on the Peel Plateau when compared to other permafrost thaw landscapes. This work indicates that the lability of permafrost DOC may be relatively uniform across variable Arctic regions, although some factors-such as the importance of nutrient status-may need further exploration.LITTLEFAIR AND TANK 3293

Thermokarst Effects on Carbon Dioxide and Methane Fluxes in Streams on the Peel Plateau (NWT, Canada)

“…Lower p CO 2 downstream than upstream of most RTSs suggests that runoff stream inputs of DIN, less aromatic DOC, and warmer water (Table )—which are considered amenable to aquatic CO 2 production (Campeau & del Giorgio, ; Crawford et al, ; Wickland et al, )—likely did not stimulate substantial CO 2 generation downstream of most RTSs. This could be partly due to the rapid loss of some labile organic matter via mineralization within RTSs (Littlefair & Tank, ), or perhaps the protection of permafrost carbon from microbes via adsorption to RTS sediments (Gentsch et al, ; Littlefair et al, ). Both discharge and pH were significantly greater downstream of RTS compared to upstream (Table ), suggesting that stream turbulence and degassing (MacIntyre et al, ), and/or downstream pH buffering by inputs of HCO 3 – from carbonate weathering within RTSs (Zolkos et al, ) enhanced CO 2 loss to efflux and/or dissociation.…”

“…The relatively stable SO 4 2– concentrations along the transect downstream of SE (mean change = –10 μM, n = 3; Figure S1 in the supporting information) suggests that H 2 SO 4 carbonate weathering and geogenic CO 2 production are relatively greater in RTS runoff (Zolkos et al, ). Less aromatic DOC downstream of RTS SE (mean SUVA 254 = 2.14 versus 3.68 L · mgC ‐1 · m ‐1 at other RTS) possibly enhanced microbial CO 2 production (Littlefair & Tank, ). Yet we did not consistently observe increasing SUVA 254 coupled with decreasing DOC along the transect, as might be expected if microbial oxidation of less aromatic DOC was driving CO 2 production (Drake et al, ).…”

Stream Dissolved Organic Matter in Permafrost Regions Shows Surprising Compositional Similarities but Negative Priming and Nutrient Effects

“…Mean annual precipitation (MAP) ranges from 140 to 600 mm (Table 1). In all permafrost‐affected regions, various types of permafrost degradation have been observed, including thermokarst and thermo‐erosional features such as retrogressive thaw slumps, thermo‐erosional gullies, and thermokarst lakes (Aanderud et al, 2019; Farquharson et al, 2019; Littlefair & Tank, 2018; Liu et al, 2019; Luo et al, 2019; Mu et al, 2019; Olefeldt et al, 2016). Other forms of less visible permafrost warming and degradation are also occurring in the study areas, including active‐layer thickening and talik (pockets of permanently thawed material) formation (Biskaborn et al, 2019; Shiklomanov et al, 2010; Vonk, Tank, Bowden, et al, 2015).…”

“…The northwestern Canada Sites NC1 and NC2, which are underlain by glacial tills, drain watershed areas downstream of a large thaw slump—Site “FM3” in other studies (e.g., Littlefair et al, 2017). Site NC3 is located on the mainstem of the larger Peel River, which receives inputs from numerous slump‐affected tributaries (Kokelj et al, 2017; Littlefair & Tank, 2018; Zolkos et al, 2018). The interior Alaska Sites IA1 and IA2, which occur in thick, ice‐rich Pleistocene silt (Yedoma), are adjacent to a thawing pingo and thermokarst channel, respectively (Ewing et al, 2015; Koch et al, 2013).…”

Downstream Evolution of Particulate Organic Matter Composition From Permafrost Thaw Slumps

“…Unlike from abrupt thaw sites in Siberia and Alaska (e.g., Abbott et al, 2015;Mann et al, 2015;Vonk et al, 2015b;Textor et al, 2019), DOC release from RTS features on the Peel Plateau is not significantly higher, and in places even lower, than that in unimpacted streams (i.e., in streams not influenced by RTS features) (Littlefair et al, 2017). Yet, DOC has been shown to be highly biolabile (Littlefair et al, 2017;Littlefair and Tank, 2018). Release of POC from RTS features is orders of magnitude higher than that of DOC, and it differs in its bulk geochemical composition relative to DOC (Shakil et al, 2020), but molecular composition or degradability of this carbon pool is largely unknown.…”