Thermokarst lake inception and development in syngenetic ice-wedge polygon terrain during a cooling climatic trend, Bylot Island (Nunavut), eastern Canadian Arctic

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 of Siberia, Alaska, and the western Canadian Arctic. Here we present the gradual transition from syngenetic ice-wedge polygon terrain to a thermokarst lake in Holocene sediments of the eastern Canadian Arctic. We combine geomorphological surveys with paleolimnological reconstructions from sediment cores in an effort to characterize local landscape evolution from a terrestrial to freshwater environment. Located on an ice- 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 and 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 colder-than-present but 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.

Seasonal patterns in greenhouse gas emissions from lakes and ponds in a High Arctic polygonal landscape

“…However, these yedoma lakes are located within thicker unconsolidated sediments that actively erode once the ice‐rich permafrost thaws. In the case of the thermokarst lake on Bylot Island, younger CH 4 emitted from the littoral zone may be explained by two factors: (1) the active layer underneath the littoral zone is limited in depth as the water column in this section of the lake refreezes to the bottom each winter, restricting the methanogens to carbon from shallower (younger) peat and freshly deposited autochthonous OM, and (2) lake expansion to this area is relatively recent, hence the submerged peat‐rich terrace was formed more recently than the one underlying the deep basin (Bouchard et al 2020). The deep basin of BYL66 (∼ 4 m depth) maintains a layer of water throughout the winter, creating a talik underneath, thus allowing methanogens to access deeper horizons.…”

“…The deep basin of BYL66 (∼ 4 m depth) maintains a layer of water throughout the winter, creating a talik underneath, thus allowing methanogens to access deeper horizons. The oldest age obtained on BYL66 gyttja was 2073 YBP (Bouchard et al 2020), while the age of ebullition CH 4 was older (Table 5), indicating that the underlying peat layer was being used as a carbon source.…”

“…1) and two deeper basins (2–2.5 m deep). Lake BYL66 is the largest and deepest thermokarst lake in the valley, presenting a large central zone with a bumpy bottom (∼ 4 m deep), and a wide shallow littoral zone (< 2 m deep) that continues to expand by thermokarst and thermo‐erosional wave action (see Bouchard et al 2020 for details on the bathymetry and temporal evolution of this lake).…”

Seasonal patterns in greenhouse gas emissions from thermokarst lakes in Central Yakutia (Eastern Siberia)

“…In areas that are dominated by low relief terrain, such as Central Yakutia, thermokarst subsidence often causes ponding and shallow depressions (French 2017). The coalescence and expansion (both laterally and vertically) of these ponds eventually results in the development of larger and deeper lakes, with a portion of unfrozen water remaining under the ice cover in winter (Bouchard et al 2020). Once formed, these lakes profoundly change the local ground thermal regime, sometimes increasing surrounding sediment temperatures by as much as 10 C above the mean annual air temperature (Brouchkov et al 2004).…”

Low vulnerability of Arctic fox dens to climate change-related geohazards on Bylot Island, Nunavut, Canada

“…Thaw settlement of ice-rich permafrost is slower but due to ground ice thawing, the accumulation of water over the permafrost table can inundate the floor of dens and make them unusable. These hazards were also considered relevant because they have been repeatedly found on Bylot Island over the last decade (Fortier et al 2007;Godin and Fortier 2012;Godin et al 2014Godin et al , 2016Beardsell et al 2017;Bouchard et al 2020). Among the different approaches to climate change vulnerability assessment (Tonmoy et al 2014), we used the widely recognized indicator-based vulnerability assessment (IBVA), a method in which indicators are used as proxy measures of processes generating vulnerability (Tonmoy and El-Zein 2013).…”