Geologic methane emissions from the Daginsky thermo-mineral springs in the northeast of Sakhalin Island: 2024 expedition and remote sensing data

The paper deals with the urgent problem of gas-geochemical parameters in the seas and shelf transit zones based on a comparison of field studies and remote sensing data. The results of complex gas-geochemical studies of the Daginsky gas-hydrothermal system are presented, as well as an assessment of methane emissions from the studied area. The Daginsky gas-hydrothermal system is located on the northeastern coast of Sakhalin Island, and is a unique object due to a combination of a number of factors: from zonality due to the proximity of the Okhotsk Sea and the geological structure, to the interaction of deep and surface processes manifested in the presence of biogenic and thermogenic methane, as well as mantle helium. Tectonic faults and oil and gas bearing structures of the northeastern shelf of Sakhalin, which determine the direction of natural gas flows and facilitate its migration to the surface, make a significant contribution to the gas appearance of thermal springs. The main gas is methane up to 90%, homologues of methane up to and including pentane have been established, and isolated high concentrations of helium and hydrogen, both dissolved and in the free gas of bubbles, have also been noted. The conducted isotope studies allow to speak about the complex genesis of the gas. δ¹³C isotopic composition, ranging from -49.4‰ to -60.2‰ shows the dominance of biogenic methane with an admixture of thermogenic component. This is also confirmed by the presence of a fraction of mantle helium. The flow of methane into the atmosphere from the Daginsky area is 963757.5 mol/(km²·day), or about 15.4 t/(km²·year), which indicates the importance of this region for the regional and global carbon cycle. The dynamics of methane emissions can vary depending on various factors, such as seasonal fluctuations and geological activity, which further complicates the understanding of processes in the region.