Gas Emission from the Yuzhno-Sakhalinsk Mud Volcano: A First-Order Assessment of CH4 and CO2 Fluxes in a Tectonically Active Setting

Mud volcanoes are geological structures that transport deep-sourced fluids and gases, primarily methane and carbon dioxide, to the surface through fracture networks and fault systems. However, in certain settings, particularly within continent–ocean transition zones, the existing flux estimates are still uncertain and require regional clarification. This study presents a first quantitative, first-order assessment of methane (CH4) and carbon dioxide (CO2) emissions from the terrestrial Yuzhno-Sakhalinsk mud volcano (YSMV) on Sakhalin Island, Russia—a feature confined to the Central Sakhalin fault zone. Based on field research in 2023–2025, we measured free gas composition (CH4, CO2, He, and minor hydrocarbon components), helium isotopes, and gas fluxes from active gryphons (mud- and gas-emitting vents), combined with remote sensing data for the period of 2020–2025. The new contributions of this study are the first quantitative, first-order flux estimates. During the dormant state, the total CH4 flux is estimated at 18.5 t yr−1 and CO2 flux at 184.1 t yr−1. Unlike CH4-dominated mud volcanoes, YSMV is strongly CO2-dominated (60–80 vol.%), with a mantle helium contribution averaging 23.7%, confirming deep fluid sources rather than shallow biogenic or crustal radiogenic origins. Remote sensing observations under calm wind conditions (≤1.5 m s−1) reveal a persistent local CO2 anomaly of 418 ppm (8–12 ppm above background) spatially coinciding with the volcano’s location, which is qualitatively consistent with the presence of a persistent local CO2 source. While globally the YSMV’s CH4 emission is modest, its CO2 output represents a sustained natural source that should be accounted for in local carbon budgets, particularly given the volcano’s proximity to populated areas.