The Geochemical Evolution of Two High Arctic Lakes and the Recurrence of Elevated Bottom Water Conductivity

Abstract

This study investigates the long-term physical and chemical changes in High Arctic lakes to understand the source of seasonally recurring elevated bottom water conductivity (EBWC) (in the bottom 0.5-2.0 m of the lake). To achieve this, two separate investigations were carried out on two cold-monomictic lakes at the Cape Bounty Arctic Watershed Observatory (CBAWO), Nunavut, Canada. More specifically, these studies aim to: (1) investigate and compare the water column chemistry of these lakes to identify the source of emerging lake bottom groundwater signatures; and (2) delineate seasonal changes in physical processes in one of these lakes to determine mechanisms driving the occurrence of EBWC. The first study focused on the chemical analysis of major dissolved ions collected at fixed water column depths. Dissolved ions reveal that both lakes have experienced an unprecedented decadal (2006-2019) shift in water column geochemistry, with ionic ratios transitioning to increasing SO42- ion enrichments observed in catchment rivers. In contrast, the ionic composition of EBWC (<30 m depth) is distinctly different from the rapidly changing water column chemistry, suggesting a separate source of water with a geochemical signature more similar to relic late-glacial marine water. These results suggest that lake bottom sediment porewater is the primary source of EBWC. The second study continued the investigation of EBWC through seasonal analysis of a 2.5-year continuous time series of specific conductivity (SpC), temperature, and density anomaly collected in the deepest region of West Lake (unofficial name) at the CBAWO (32 m depth). The SpC record indicates that under-ice accumulation of EBWC not only occurs in late winter but in early winter, immediately following the complete formation of surficial lake ice. The mechanisms generating EBWC are controlled by patterns in lake ice phenology and occur when the lake is ice covered. These mechanisms include late winter (before the onset of ice melt), radiatively driven convection, and subtle early winter and water column circulation driven by bottom sediment heat flux (following ice-on). Therefore, EBWC occurs through the diffusion of sediment pore-water accumulating in the lake bottom through the downward movement of dense currents driven by under-ice processes.