Thermokarst lake development in syngenetic ice-wedge polygon terrain in the Eastern Canadian Arctic (Bylot Island, Nunavut)

Abstract

Abstract. Thermokarst lakes are widespread and diverse across permafrost regions and they are considered significant contributors to global greenhouse gas emissions. Paleoenvironmental reconstructions documenting the inception and development of these ecologically important water bodies are generally limited to Pleistocene-age permafrost deposits (Yedoma) of Siberia, Alaska, and the western Canadian Arctic. Here we present the gradual transition from syngenetic ice-wedge polygon terrains to a thermokarst lake in the Eastern Canadian Arctic. We combine geomorphological surveys with paleolimnological reconstructions from sediment cores in an effort to characterize local landscape evolution from terrestrial to freshwater environment. Located on an ice-rich and organic-rich polygonal terrace, the studied lake is now evolving through active thermokarst, as revealed by subsiding and eroding shores, and was likely created by water pooling within a pre-existing topographic depression. Organic sedimentation in the valley started during the mid-Holocene, as documented by the oldest organic debris found at the base of one sediment core and dated at 4.8 kyr BP. Local sedimentation dynamics were initially controlled by fluctuations in wind activity, local moisture and vegetation growth/accumulation, as shown by alternating loess (silt) and peat layers. Fossil diatom assemblages were likewise influenced by local hydro-climatic conditions and reflect a broad range of substrates available in the past (both terrestrial and aquatic). Such conditions likely prevailed until ~ 2000 BP, when peat accumulation stopped as water ponded the surface of degrading ice-wedge polygons, and the basin progressively developed into a thermokarst lake. Interestingly, this happened in the middle of the Neoglacial cooling period, likely under wetter-than-average conditions. Thereafter, the lake continued to develop as evidenced by the dominance of aquatic (both benthic and planktonic) diatom taxa in organic-rich lacustrine muds. Based on these interpretations, we present a four-stage conceptual model of thermokarst lake development during the late Holocene, including some potential future trajectories. Such a model could be applied to other formerly glaciated syngenetic permafrost landscapes.

Thermokarst lakes formed in buried glacier ice: Observations from Bylot Island, eastern Canadian Arctic

“…depth = 12.2 m and 11.7 respectively), but lake K is smaller than lake L. We also sampled lake G (max. depth = 4.1 m) as lake bottom imagery revealed submerged ice-wedge polygons (~1 m depth) and degraded ice-wedge troughs in shallower lake G, which confirmed that this lake (G) is evolving through the melting of permafrost intrasedimental ice and ice wedges as revealed by subsiding and eroding shores (see video supplement in Bouchard et al, 2020). Two sediment cores of 109 cm and 114 cm were collected in spring 2015 from lakes G and K, respectively.…”

“…2). At the lake scale, these shallow lakes have relatively flat and homogeneous lake beds with a deeper central basin surrounded by shallower nearshore zones (< 2 m deep), where submerged polygons are clearly visible (see the video supplement in Bouchard et al, 2020). These lakes have maximum depths ranging between 1 and 4 m, with mean depth reaching 1.4 ± 0.7 m across their platform (Fig.…”

“…Today, Bylot Island remains 40% glacierized as numerous valleys and piedmont glaciers still flow from the local ice cap and terminate in lowlands underlain by sedimentary rock of Cretaceous-Tertiary ages (Dowdeswell et al, 2007). The Qarlikturvik Valley is one of the many U-shaped glacial valleys with ice-rich sediments dating back to the Late Pleistocene and Holocene, which are highly susceptible to thermokarst (Fortier and Allard, 2004;Bouchard et al, 2020). The valley contains abundant and diverse water bodies, including a proglacial river, lakes, through and polygon ponds, small streams, and gullies (Godin et al, 2014;Muster et al, 2017;Prėskienis et al, 2021).…”

“…Syngenetic ice wedge growth has created an extensive polygonal patterned ground. Thermokarst is an active landscape change mechanism operating in the valley, as demonstrated by the abundance of lakes, thermo-erosional gullies and thaw slumps within the study area (Bouchard et al, 2020;Fortier et al, 2007;Godin et al, 2014). Previous work in the area have examined various aspects of thermokarst lake dynamics such as GHG exchanges (Bouchard et al, 2015a;Prėskienis et al, 2021), photochemical and microbial decomposition of organic matter (Laurion et al, 2021), microbial diversity (Negandhi et al, 2014), nutrient inputs from the goose colony (Côté et al, 2010) and methylmercury , as well as lake development in syngenetic ice-wedge polygon terrain (Bouchard et al, 2020).…”

“…Calibrated ages (cal yr BP) were calculated using "CALIB 8.2" (Stuiver et al, 2021;IntCal20 dataset, Reimer et al, 2020). In the case of lake G, facies are described in more details based on other proxies, such as organic content and fossil diatoms (Bouchard et al, 2020;Figs. 4 and 5).…”