Isotope and chemical composition of gases from mud volcanoes in the Taman Peninsula and problem of their genesis

Abstract

Variations in the carbon isotope composition in gases and waters of mud volcanoes in the Taman Peninsula are studied. The δ13C values in CH4 and CO2 vary from −59.5 to −44.0‰ (δ13Cav = −52.4 ± 5.4‰) and from −17.8 to +22.8‰ (δ13Cav = +6.9 ± 9.3‰), respectively. In waters from most mud volcanoes of the peninsula, this parameter ranges from +3.3 to +33.1‰, although locally lower values are also recorded (up to −12‰. Fractionation of carbon isotopes in the CO2-HCO3 system corresponds to the isotope equilibrium under Earth’s surface temperatures. The growth of carbon dioxide concentration in the gaseous phase and increase in the HCO3 ion concentration in their water phase is accompanied by the enrichment of the latter with the heavy 13C isotope. The δ13CTDIC value in the water-soluble carbon depends on the occurrence time of water on the Earth’s surface (exchange with atmospheric CO2, methane oxidation, precipitation of carbonates, and other processes), in addition to its primary composition. In this connection, fluctuations in δ13CTDIC values in mud volcanoes with stagnant waters may amount to 10–20‰. In the clayey pulp, concentrations of carbonate matter recalculated to CaCO3 varies from 1–4 to 36–50 wt %. The δ13C value in the latter ranges from −3.6 to +8.4‰. Carbonate matter of the clayey pulp represents a mixture of sedimentogenic and authigenic carbonates. Therefore, it is usually unbalanced in terms of the carbon isotope composition with the water-soluble CO2 forms.