The South Sakhalin mud volcano (Sakhalin Island, Russia) emits HCO₃-Cl/Na-Mg water, emanates CO₃ prevailing over CH₃ in the gas phase, and extrudes mud bearing five carbonate mineral species. The study focuses on the distribution, diversity, and origin of the carbonate minerals from the mud volcano (MV) ejecta, in terms of carbon cycle processes. The data presented include a synthesis of field observations, compositions of MV gases and waters, chemistry of carbonate minerals, as well as stable isotope geochemistry of MV waters (δ¹³C, δ D, and δ¹⁸O) and carbonates (δ¹³C and δ¹⁸O). The sampled MV waters are isotopically heavy, with δ ¹⁸O = +5.7 parts per thousand to +7.5 parts per thousand VSMOW, δ D = -18.0 parts per thousand to -11.0 parts per thousand VSMOW, and ¹³C (δ ¹³C(DIC) = +6.9 parts per thousand to +8.1 parts per thousand VPDB). This composition may be due to the dilution of basinal water with dehydration water released during the diagenetic illitization of smectite. Carbonates in the sampled mud masses belong to three genetically different groups. Mg-rich siderite, (Fe_0.54-0.81Mg_0.04-0.30Ca_0.05-0.23Mn_0.00-0.08)CO₃, disseminated in abundance throughout the mud masses, coexists with common calcite and sporadic ankerite. The trace-element chemistry of Mg-siderite, as well as the oxygen (δ¹⁸O = +34.4‰ to +36.8‰ parts per thousand VSMOW) and carbon (δ¹³C = -1.3‰ +0.6‰ parts per thousand VPDB) isotopic signatures, confirms its authigenic origin. Siderite formed during early diagenesis of the Upper Cretaceous sandy and clayey marine sediments mobilized by mud volcanism in the area. Another assemblage, composed of dawsonite, siderite, and vein calcite (+/- kaolinite), represents altered arkose sandstones found as few fragments in the mud. This assemblage may be a marker of later CO2 flooding into the sandstone aquifer in the geological past. The trace-element chemistry, particular morphology, and heavy C (δ¹³C = +5.5‰ to +7.0‰ VPDB) and O (δ ¹⁸O = +39.1‰ to +39.5‰ VSMOW) isotope compositions indicate that aragonite is the only carbonate species that is related to the current MV activity. It crystallized in a shallow reservoir and was maintained by CO₂ released from rapidly ascending liquefied mud and HCO₃-Cl/Na-Mg-type of MV waters.