This paper discusses the results of long-term monitoring observations on the chemical and isotopic composition of free gases in gryphons of the South Sakhalin mud volcano on Sakhalin Island. It was shown that during seismic activation in the region, the chemical composition of free gases changes, whereas the isotopic composition of carbon in methane and carbon dioxide remains constant. A possible interpretation of the obtained data was suggested. The depth of the gas reservoir that supplies the South Sakhalin mud volcano was estimated as well.

This paper reports detailed evidence on the dynamics of the gryphon activity of the Yuzhno-Sakhalinsk gas-water lithoclastic (mud) volcano. It was obtained by visual observations during periods between the short-term eruptions of 1959, 1979, and 2001 and, especially, during continuous monitoring between June 18 and September 3, 2005. In addition to the direct observations, the monitoring included measurements (three times per day) of the air temperature and pressure, the temperature of the liquid lithoclastic mass filling the crater hollows of several gryphons of various types, and the amounts of gases released by the two largest and most active gryphons. Liquid lithoclastic mass was collected daily in the crater of the largest (main or central) gryphon for the subsequent ICP AES analysis for five elements (Al, Fe, Ca, Mn, and Ba). The results of the monitoring were compared with the measurements of natural seismicity using the system of Dat and Datamark digital seismic stations. The acquired information was used to unequivocally demonstrate the existence of a direct causal relation between the activity of the Yuzhno-Sakhalinsk mud volcano, the dynamics of the chemical composition of the liquid lithoclastic mass ejected from its main gryphon, and the regional and local natural seismicity, which was previously conjectured. The dynamics of one of the measured elements (Al) is potentially useful for the prediction of eruptions.

This paper is the world’s first to generalize and analyze worldwide data on the chemical composition of waters from 175 onshore mud volcanoes. Empirical probability density functions are obtained for the dominant anions and cations, whose average concentrations (Clarke values) were also estimated. Correlations between the geochemical parameters of the samples (Na and Cl concentrations, Na/Cl ratio and \({\text{HCO}}_{3}^{ - }\) concentration, Cl/Br ratio and Cl concentration, Cl/B ratio and \({\text{HCO}}_{3}^{ - }\) concentration, etc.) were determined and used in the further analysis of conditions under which mud volcanic waters are formed. The waters are mainly slightly alkaline, with mineralization (TDS) in the range of 5 to 25 g/L and mostly belong to the Cl–Na type (about 58%), with waters of the Cl–HCO₃–Na (about 23%) and HCO₃–Cl–Na (about 12%) types being less common. When mud volcanic chambers are formed, mainly connate (sedimentation–buried seawaters) waters are localized in them. The further evolution of mud volcanic waters is controlled largely by interaction in the water–rock–gas system (in the channel of the mud volcanoes). These interactions involve, first of all, the influx of CO₂ in mud volcanoes, hydrolysis of aluminosilicates, sorption–desorption from the exchange complex of clay, smectite illitization, sulfate reduction, and the precipitation of carbonate minerals. Therewith the processes of dilution (with water formed during the dehydration of clay minerals and by meteoric waters) and evaporation are demonstrated to also play important roles. An interplay of all of these processes, whose intensity varies from one mud volcano to another, determines the diversity of the chemical composition of mud volcanic waters.

We characterize the mineralogy and geochemistry of Fe, Zn, Pb, Sb, As, Hg, Ag sulfide assemblages from mud masses currently extruded by the onshore South Sakhalin and Pugachev mud volcanoes (Sakhalin Island, Russia). Abundant Tl-rich pyrite in sulfide concentrate samples from the mud volcanoes coexists with common Hg- and Ge-rich sphalerite, as well as with sporadic boulangerite, robinsonite, bournonite, galena, realgar, metacinnabar, cinnabar, acanthite, and chalcopyrite. Sphalerites are remarkably enriched in Hg (locally reaching 27 wt%) and coupled zwith permanent abnormal enrichment in Ge (3008–3408 ppm). According to single-crystal XRD analyses and Raman spectroscopy, both Hg-poor and Hg-rich sphalerites are single-phase (Zn,Hg)S_cub compounds. Pyrite is of diagenetic origin, judging by its trace-element chemistry, particular morphology, and heavy S isotope composition. Another assemblage, composed of Pb-Sb-(Hg) sulfide minerals and lesser As, Cu, Ag, and Bi compounds, results from hydrothermal alteration and is genetically related to Neogene volcano-sedimentary rocks found among the ejecta of the mud volcanoes. The composition of impurities in sphalerite from mud masses indicates crystallization at temperatures lower than ~100 °C, under the leaching effect of mud volcano waters.

The results of hydrogeochemical observations on the Yuzhno-Sakhalinsk mud volcano in 2010–2014 are considered. The chemical analysis of samples of mud–volcanic waters was carried out at various analytical centers, which is similar to the common situation where hydrochemical data for a volcano are obtained by different researchers. It is shown that the chemical composition of the mud–volcanic waters is relatively stable in time and space (for different gryphons of the volcano). This allows us to determine the characteristic range of hydrogeochemical indicators. For each year of observations, the coefficients of variation for the concentrations of Na, Mg, Ca, K, and HCO₃ mostly range from 10 to 30%. However, the concentrations analyzed in individual samples may differ significantly from each other. These natural variations are a likely source of errors in the interpretation of hydrochemical data. In addition, it is necessary to account for the specifics of mud–volcanic waters as an object of analytical chemical investigations.