Carbon and hydrogen isotopic systematics of dissolved methane in small seasonally ice-covered lakes near the margin of the Greenland ice sheet

Northern lakes contribute from 6-16% of annual methane inputs to Earth's atmosphere, yet little is known about the seasonal biogeochemistry of CH₄ cycling, particularly for lakes in the Arctic. Studies during ice-free conditions have been conducted in Alaskan, Swedish and Siberian lakes. However, there is little information on CH₄ cycling under ice-covered conditions, and few stable isotopic measurements, which can help elucidate production and consumption pathways. In order to better understand methane dynamics of ice-covered Arctic lakes, 4 small lakes (surface area <1 km²) within a narrow valley extending from the Russells Glacier to Søndre Strømfjord in Southwestern Greenland were examined during summer stratification and winter ice-cover. Lakes in the study area are ice-covered from mid-September to mid-June. In both seasons, variations in the concentrations and isotopic composition of methane with depth were related to redox fluctuations. During late winter under~2 m of ice, the entire water column was anoxic with wide variation in methane concentrationsand isotopic composition from lake to lake. In three of the lakes, CH₄ concentrations and δ¹³C were relatively stable over the depth of the water column, averaging from 120 to 480μM, with δ¹³CH₄ values from -56‰ to -66‰, respectively. Methane concentrations in the other lake increased with depth from <1 μM below the ice to 800 μM at the sediment/water interface, while δ¹³C decreased by 30‰ from -30‰ to -70‰ over this depth. In all the lakes, δ¹³C of sediment porewater was lighter than the overlying water by at least 10‰. The δD-CH₄ in the water column ranged from -370‰ to -50‰, exhibiting covariance with δ¹³C consistent with significant methanotrophic activity. In the sediment, δD-CH₄ values ranged from -330‰ to -275‰, and were inversely correlated with δ¹³C. We will present detailed information on redox dynamics as a controlling factor in methane cycling, and explore the effects of differing microbial communities and carbon supply. Our study suggests that shallow lakes in continuous permafrost landscapes of the Arctic develop distinct methane cycling dynamics despite their close proximity.