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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 CO213C 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.

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Notes

  1. In this work, we distinguish the microbial (primary) methane, which is formed by the bacterial decomposition of organic matter from sedimentary rocks; thermogenic methane released by the thermal decomposition of kerogen in sedimentary rocks; and secondary or “biodegradationbiodegraded” methane formed by the bacterial decomposition of heavy fractions of oil hydrocarbons in upper parts of the sedimentary sequence.

  2. Since mud volcanic waters have neutral pH values, the predominant dissolved CO2 species is \({\text{HCO}}_{3}^{ - }\) ion. Therefore, our determinations characterize δ13C in HCO3.

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ACKNOWLEDGMENTS

We are grateful to B.G. Pokrovsky for a great deal of O and H isotope determinations in collected water samples. I. Vigano and A. Zuiderweg (IMAU) are thanked for δ2H measurements in methane from mud volcanoes of the Taman Peninsula.

Funding

The study was supported by the Russian Science Foundation, project no. 17-17-01056P.

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Correspondence to V. Yu. Lavrushin or E. V. Sokol.

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Translated by M. Bogina

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Lavrushin, V.Y., Aydarkozhina, A.S., Sokol, E.V. et al. Mud Volcanic Fluids of the Kerch–Taman Region: Geochemical Reconstructions and Regional Trends: Communication 2. Genesis of Mud Volcanic Gases and Regional Geochemical Trends. Lithol Miner Resour 57, 1–24 (2022). https://doi.org/10.1134/S0024490222010059

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Keywords:

  • mud volcanoes
  • stable H, C and N isotopes in gases
  • Kerch and Taman peninsulas