Carbonates in the Ejecta of South Sakhalin Mud Volcano, Sakhalin Island, Russia: Diversity, Origin, and Sources

The South Sakhalin mud volcano (Sakhalin Island, Russia) emits HCO₃-Cl/Na-Mg water, emanates CO₂ prevailing over CH₄ 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 (δ¹³С, δD, and δ¹⁸O) and carbonates (δ¹³С and δ¹⁸O). The sampled MV waters are isotopically heavy, with δ¹⁸O = +5.7 to +7.5 ‰ VSMOW, δD = −18.0 to −11.0 ‰ VSMOW, and ¹³С (δ¹³С_DIC = +6.9 to +8.1 ‰ 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 ‰ VSMOW) and carbon (δ¹³C = −1.3 to +0.6 ‰ 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 (δ¹³С = +5.5 to +7.0 ‰ VPDB) and O (δ¹⁸О = +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 СО₂ released from rapidly ascending liquefied mud and HCO₃-Cl/Na-Mg-type of MV waters.