Marked variation in aqueous chemistry and methane concentrations in small thermokarst lakes in western Greenland

The western ice-free margin of Greenland contains thousands of small (<1 km²) thermokarst lakes perched on permafrost and ice-covered for 9 months of the year. A semi-arid climate and lack of established drainage networks result in limited surface inflow and compositions ranging from saline to dilute. Depths are generally > 2 m allowing bottom waters to remain unfrozen year around. Despite their abundance, the majority of published literature on Greenlandic lakes focuses on physicochemical limnology and paleobotany of lakes with areas >1 km². Minimal data are available on methane concentrations through water columns for large or small lakes in western Greenland. Here, we concentrate on a chain of 7 small lakes (<1 km²), spanning a distance < 6 km along a narrow valley overlying a structural shear zone and extending from the Russell Glacier to the Søndre Strømfjord. Due to close proximity and location in a valley < 50 m in width, we anticipated similar physical parameters and methane concentrations. Preliminary results from four weeks of field work in summer 2012 indicate that our assumptions were incorrect. The 7 lakes have similar stratified thermal properties but there are substantial variations in the aqueous chemistry of these lakes. Hypolimnions range from pH 6.5 to 9.5. One lake is buffered by bicarbonate and varies vertically only from pH 9.5 to 9.1 while another lake varies vertically from pH 9.4 to 7.2. Conductivity varies markedly from lake to lake, with 4 lakes categorized as saline (>800 μS cm^-1) and 3 lakes classified as dilute (<500 μS cm^-1). Variability is not dependent on distance; nearly an order of magnitude difference in conductivity (2,900 to 250 μS cm^-1) is observed between two lakes separated by only 0.25 km. In addition, distinct variation is observed in methane concentrations for these two adjacent lakes. A methane concentration of 15,000 ppm was measured in the hypolimnion of the lake with a conductivity of 3,500 μS cm^-1 and a methane concentration of 15 ppm was measured in the hypolimnion of the lake with a conductivity of 1,200 μS cm^-1. Covariation of methane and conductivity is only weakly observed in the other lakes, with a coefficient of determination of 0.4. No other unifying trends are observed between depth, surface area, aquatic chemistry, and methane concentrations. We conclude that despite the close proximity, there is a substantial spectrum of physicochemical diversity observed in these small thermokarst lakes, likely due to localized ecological and hydrogeochemical factors. Further work is needed to identify the dominant factors for each lake, but differences in bedrock and vegetation are plausible hypotheses.