ABSTRACT: Recent dynamics of thermokarst ponds in discontinuous permafrost: a paleolimnological study from subarctic Quebec, Canada.

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Abstract

Accelerated thawing and erosion of permafrost is leading to the release of organic carbon through greenhouse gas emissions, especially in thermokarst (thaw) ponds and lakes. These aquatic ecosystems are widespread throughout arctic and subarctic regions; however their natural variability and temporal evolution recorded in the bottom sediments are poorly understood.Here we present a multi-proxy study conducted in a subarctic site with many thermokarst ponds near Kuujjuarapik-Whapmagoostui, on the eastern shore of Hudson Bay, at the southern edge of the discontinuous and scattered permafrost zone. Sedimentological, geochemical and biological (diatoms) analyses have been performed on short sediment cores (10-20 cm) retrieved from limnologically contrasted ponds. Long-term (14C) and short-term (210Pb, 137Cs) chronologies were also established. Analyses revealed two distinct sedimentary facies, from bottom to top: 1) massive marine silts and clays deposited during postglacial Tyrrell Sea transgression (ca. 8000 to 6000 cal yr BP), subsequently emerged by glacio-isostatic rebound and more recently (ca. 1500 to 400 cal yr BP) affected by permafrost inception and growth; 2) laminated organic-rich lacustrine muds deposited since permafrost thawing and subsidence, i.e. since thermokarst pond inception (the last centuries). Almost absent from the bottom sediments (lower facies), benthic and planktonic diatoms appeared highly abundant in surface lacustrine sediments (upper facies) and reflected past changes in bottom water properties (e.g., pH, dissolved organic carbon, water color). Despite displaying strikingly different water colors, the study ponds showed similar long-term developmental patterns regarding their biogeochemical properties (as recorded in the sediments), such as: decreasing mineral grain size (from silts to clays); decreasing major chemical element concentrations; increasing organic matter content and decreasing pH (establishment of peatland vegetation/soils); decreasing bottom water oxygen concentrations (from well-oxygenated to anoxic/hypoxic conditions). Contributing to a growing literature on thermokarst pond evolution at northern latitudes, this study provides the litho/bio-stratigraphic background on which present-day conditions (e.g., geomorphology, biology, optical properties) can be interpreted.

Bibliography of Canadian Geomorphology