DYNAMICS OF THE CHEMICAL COMPOSITION OF THE WATERS EMITTED FROM YUZHNO-SAKHALINSK MUD VOLCANO (SAKHALIN ISLAND) DURING GRYPHON-SALSA ACTIVITY

The article presents the results of hydrogeochemical monitoring conducted on Yuzhno-Sakhalinsk mud volcano (Sakhalin Island) during the field season (May–September) in 2015. Samples of mud erupted by Yuzhno-Sakhalinsk mud volcano were taken once every 14 days from five gryphons/salsas with different levels of activity. Single samples were taken additionally from eight other gryphons/salsas on the volcano. It is shown that mud volcanic waters are characterized by variability of the chemical composition in time and within the mud field area (in different gryphons/salsas). Variations of the chemical composition of mud volcanic waters in time are due to their dilution by fresh atmospheric precipitation on the volcano field. In this case, there is a coordinated decrease in the total mineralization (sometimes by more than 50 %), concentrations of all components of the chemical composition and δ18O and δD values of mud volcanic waters. For correct interpretation of hydrogeochemical data, a method of separating such diluted mud volcanic waters into a separate sample is proposed. The method is based on tracking changes in the correlations between the conservative anion Cl– and other components of the chemical composition of mud volcanic waters. Variations of the chemical composition of mud volcanic waters within the mud field area are mainly due to different levels of activity of the sampled gryphons/salsas. The flow of large amounts of CO2, which dominates in the gas emissions from Yuzhno-Sakhalinsk mud volcano, into some gryphons/salsas leads to an increase in the concentrations of some anions and cations in mud volcanic waters. In the waters of more active gryphons/salsas, the increase in Na+, HCO3– and Mg2+ concentrations can be as high as 12.26 and 65 %, respectively, compared to the waters of less active gryphons/salsas. At the same time, the Cl– concentration is approximately the same for all gryphons/salsas, i.e. does not depend on their activity. The obtained regularities testify to the possibility of using hydrogeochemical indicators for regime observations of mud volcanic activity and monitoring of underground CO2 storages, since Yuzhno-Sakhalinsk mud volcano studied in this work is their natural equivalent.