Gas Emission From Permafrost - New Possible Mechanism of the Lake Formation

Limnicity of Yamal is 10% on average (Trofimov eds. 1975). The origin of these lakes is described in numerous publications (Beletskaya, 1987, Romanenko, 1999). However, no publications describe the possible gas-emission mechanisms for a lake basin formation. After the appearance of gas-emission crater (GEC) in Yamal peninsula (Leibman et al., 2014) we started to monitor this permafrost feature and observed that it can form a new lake in three consecutive summer seasons. Here we suggest that other lakes in the North of Wester Siberia might have similar origin. For proving this hypothesis, we analyze hydrochemical and morphometric features of GEC-lakes (on Yamal and Gydan) and compare them with background lakes in this area. GEC1 lake was characterized by elevated dissolved methane (dCH₄) concentration at the bottom layer throughout 2015 and 2017 (winter and summer). Values of dCH₄ were almost 50 times higher than in Yamal lakes on average (45 ppm). Given this distribution it can be assumed that source of methane existed after the GEC appearance. Most likely the gas is continuously delivered from the gas saturated sediments and further oxidized at the upper part of the lake. Our dataset on methane isotopes suggests that the source of methane is primarily biogenic as the values of 𝛿¹³C are less than -60‰ (Bernard et al., 1976). Values of 𝛿¹³C of methane extracted from deep boreholes of Bovanenkovo gas field (depths 28 - 120) varies from -74.6 to -70.4‰ also corresponding to biogenic origin (Skorobogatov et al., 1998). Geochemistry of lakes in central Yamal with a domination of Na and Cl ions reflect the marine origin of sediments. The domination of HCO₃ anion in GEC lakes can be a signal of close connection to tabular ground ice (TGI). This implies that thaw of TGI is a considerable source of water in GEC lakes. Isotopic composition of water also supports this hypothesis. In western Siberian lakes crater-like depressions have been observed and named as gas-explosion craters (Kuzin, 1975, 1980, Sizov 2015, Bogoyavlenskiy et al., 2017). Detailed bathymetric survey in 21 lakes revealed several crater-like depressions with relative depth of 10-20 m below the average surface of lake bottom and steep slopes (>36 degrees, 8.4 on average). Such depressions can be a remnant of former gas flows from permafrost.