Sedimentology and geochemistry of thermokarst ponds in discontinuous permafrost, subarctic Quebec, Canada

Abstract

[1] Thermokarst (thaw) ponds and lakes are distributed throughout arctic and subarctic regions; however their natural variability and temporal evolution recorded in the bottom sediments are poorly understood. This paper presents a multiproxy study conducted in a subarctic site with many thermokarst ponds near Kuujjuarapik-Whapmagoostui, on the eastern shore of Hudson Bay. Sedimentological, geochemical and chronological analyses have been performed on short sediment cores (10-20 cm) retrieved from limnologically contrasted ponds. 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). Despite displaying strikingly different water colors, the study ponds showed similar long-term developmental patterns regarding their physico-chemical 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 bottom water oxygen concentrations (from well-oxygenated to anoxic/hypoxic conditions).

Landscape-gradient assessment of thermokarst lake hydrology using water isotope tracers

“…The southern KWK and SAS are situated in the boreal-subarctic zone within spruce-lichen woodland overlying sporadic permafrost. Lakes at KWK are located on postglacial marine clay soils of low permeability covered with dense forest and shrub vegetation where local permafrost is at an advanced stage of degradation (Bouchard et al, 2011). The elevation of the lakes at KWK are well above the nearby creek shown in Figure 2.…”

“…Permafrost landscapes cover more than 50 % of Canada including 30 % of subarctic Québec (Nunavik; Bouchard et al, 2011). Rapid degradation of permafrost since the mid-20 th century along the eastern coast of Hudson Bay has contributed to an increase in the number of shallow thermokarst lakes (Payette et al, 2004).…”

Distribution and identity of Bacteria in subarctic permafrost thaw ponds

“…The ponds belong to the first type, featuring a more defined depression with reduced watershed influence. These ponds are filled with water from precipitation, lack ground water inputs, and are typically highly turbid due to fine silt and clay depositions (Bouchard et al 2011). They are surrounded by dense shrubs (Betula glandulosa, Salix sp., Alnus sp., Myrica gale) and few trees (Picea mariana, P. glauca, Larix laricina; details in Breton et al 2009).…”

“…The ponds are surrounded by thick shrubs, sparse trees, and variable underlying peat (Bhiry et al 2011, Bouchard et al 2011. Bacteria in the ponds could originate from the surrounding soils, or be transported by wind or rain (Harding et al 2011).…”

“…A larger number of ponds would be needed to discern any regional patterns driven by pond morphology, age, or catchment (Bouchard et al 2011), which may result in different bacterial assemblages within a specific region (Logue & Lindström 2008). As suggested by the origins of the best matches, community composition was mostly determined by the environmental conditions within the ponds and not by the surrounding environment.…”