Mud Volcanic Fluids of the Kerch–Taman Region: Geochemical Reconstructions and Regional Trends: Communication 2. Genesis of Mud Volcanic Gases and Regional Geochemical Trends

Abstract

The chemical and isotopic signatures of mud volcanic fluids were determined for 42 mud volcanoes of the Kerch–Taman region (Crimea–Caucasus district). The analysis of spatial geochemical trends of mud-volcanic waters and gases and estimates of formation temperatures using the Mg–Li geothermometer allowed us to attribute the mud volcanic fluids to sediments of the Maikop Formation, the thickness of which in the studied region varies from 1 to 5–6 km. It was shown that an increase of CO₂ and ${\text{HCO}}_{3}^{ - }$ concentrations and values of δ¹⁸O in H₂O and δ¹³C in CH₄, as well as a decrease of Cl^– concentrations and δ¹³C values in ${\text{HCO}}_{3}^{ - }$ and CO₂ correlate with increasing thickness of the Maikop sediments in the central part of the Kerch–Taman region. At the periphery of this region, in the foothills of the Caucasus and Crimea orogens, this correlation is absent. Active tectonic processes in these areas provided contribution of additional “higher temperature” sources of water and gas to fluids in the mud volcanic systems. Using the Kerch–Taman region gases as an example, it was shown for the first time that the enrichment of mud volcanic fluids in isotopically heavy CO₂ (δ¹³C CO₂ to +22.8‰; δ¹³C HCO₃ to +38.9‰) occurs within a narrow temperature range (40–80°C). At higher temperatures, the accumulation of isotopically heavy CO₂ is ceased. These trends suggest that the high-δ¹³C CO₂ is related to the hydrocarbon biodegradation. Using δ¹³C (in CH₄ and CO₂) and δ₂H (in CH₄) classification, it was shown that at least three methane sources could participate in the CH₄ balance: microbial (primary), connected with “biodegradation” (secondary), and thermogenic. The content of “secondary” methane in some cases can reach almost 100%. In the studied mud volcanic gases, the isotopic composition of molecular nitrogen was determined for the first time. The δ¹⁵N values vary from –5.2 to –0.1‰ (δ¹⁵Nav = –2.3 ± 0.9‰, n = 35) and indicate a significant admixture of the non-atmospheric nitrogen. A negative correlation between values of δ¹⁵N in N₂, δ¹³C in ${\text{HCO}}_{3}^{ - }$, and the calculated fluid generation temperatures (t(Mg-Li)) make it possible to attribute the origin of this nitrogen to the transformation of organic matter during the maturation of sediments.