Coupling of sedimentological and limnological dynamics in subarctic thermokarst ponds in Northern Québec (Canada) on an interannual basis

Abstract

Landscapes are changing at fast rates in subarctic regions due to recent climate warming and related permafrost thaw. As a consequence, thermokarst lakes and ponds are forming and their properties are changing rapidly. Here, we report on the interannual (2012-2014) variability of sedimentological and limnological conditions in a recently formed thermokarst pond in discontinuous permafrost terrain in Northern Quebec, and we discuss various aspects of pond sedimentation processes. Sediment samples from collecting traps and a short core were analyzed for particle size, organic matter content and geochemical composition, as well as 14C dating of a peat sample from the core. Results reveal the preponderance of silts containing 2 to 13% organic matter and an age of 1825–1950cal. yr. BP for the peat sample. A hypoxic hypolimnion formed in the pond during the short summers. Apparent sedimentation rates (up to 5.5mm/d) varied in relation to local meteorological conditions and snow cover. The results also reveal major parameters associated with sediment composition, most notably dissolved oxygen in the water column, sampling depth and the year of sampling. Microplankton (20–200μm) is likely the main source of organic matter, which represents up to 10 to 13% of sediment trap samples, considering its size matching a major grain size mode (44.9–59.0μm). Using sedimentation rates and an estimation of long-term sediment compaction, the pond's “life span” was calculated at 370 to 600years. This represents a baseline for the general understanding of the development of young (15–20years) subarctic thermokarst ponds overlying impermeable soils, and provides an approximate time frame for the potential response of such systems to climate change impacts on northern landscapes.