Hydrogen and oxygen isotopic anomalies in pore waters suggesting clay mineral dehydration at gas hydrate-bearing Kedr mud volcano, southern Lake Baikal, Russia

A multibeam echosounder survey was conducted (deeper than ca. 300 m water depth, total area: ca. 1.8 × 10 3 km 2 ) in the southern basin of Lake Baikal, Russia, in June 2015, 2016, and 2017. Characteristic morphology of the lake floor was mapped on the ancient Tankhoy stratum, covered with present sediment, by high-resolution bathymetry. Sediment core sampling operations were conducted in August 2015 and August 2016 at a characteristic mound-like landform named Kedr (after the Kedrovaya River). Sub-surface gas hydrates (GH), containing not only microbial but also thermogenic gases (Hachikubo et al. 2016 ), were retrieved. Core lithology and sub-bottom profiler survey suggest that Kedr is a mud volcano (MV). Hydrogen and oxygen isotopic anomalies were observed in the sediment pore waters. This suggests the water results from clay mineral dehydration, which is the first observation of this process in Lake Baikal sediment pore water. The thermogenic gases, mud breccia, and water from clay mineral dehydration suggest potential ascending gas as well as water from greater depths, presumably from the ancient Tankhoy stratum under the Kedr MV.