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. Thermokarst lakes are widespread and diverse across permafrost regions,and they are considered significant contributors to global greenhousegas emissions. Paleoenvironmental reconstructions documenting theinception and development of these ecologically important water bodiesare generally limited to Pleistocene-age permafrost deposits ofSiberia, Alaska, and the western Canadian Arctic. Here we present thegradual transition from syngenetic ice-wedge polygon terrain to athermokarst lake in Holocene sediments of the eastern CanadianArctic. We combine geomorphological surveys with paleolimnologicalreconstructions from sediment cores in an effort to characterize locallandscape evolution from a terrestrial to freshwaterenvironment. Located on an ice- and organic-rich polygonalterrace, the studied lake is now evolving through active thermokarst,as revealed by subsiding and eroding shores, and was likely created bywater pooling within a pre-existing topographic depression. Organicsedimentation in the valley started during the mid-Holocene, asdocumented by the oldest organic debris found at the base of onesediment core and dated at 4.8  kyr BP . Local sedimentationdynamics were initially controlled by fluctuations in wind activity,local moisture, and vegetation growth and accumulation, as shown byalternating loess (silt) and peat layers. Fossil diatom assemblageswere likewise influenced by local hydro-climatic conditions andreflect a broad range of substrates available in the past (bothterrestrial and aquatic). Such conditions likely prevailed until ∼2000 BP , when peat accumulation stopped as water ponded thesurface of degrading ice-wedge polygons, and the basin progressivelydeveloped into a thermokarst lake. Interestingly, this happened in themiddle of the Neoglacial cooling period, likely undercolder-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 lacustrinemuds. Based on these interpretations, we present a four-stageconceptual model of thermokarst lake development during the lateHolocene, including some potential future trajectories. Such a modelcould be applied to other formerly glaciated syngenetic permafrostlandscapes.

the cryosphere

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