Comparative gas geochemistry and degassing behavior of mud volcanoes: insights from Azerbaijan and Northern Italy

Mud volcanoes represent key natural pathways for the transfer of deep-seated fluids to the surface, yet their gas composition and degassing behavior can vary significantly depending on geological setting and post-genetic processes. Here we present a comparative geochemical and monitoring-based study of mud volcano systems from Azerbaijan and Northern Italy, integrating molecular composition, stable isotopes (δ¹³C-CH₄, δ²H-CH₄, δ¹³C-CO₂) and soil gas flux measurements, to investigate the dynamics of crustal fluid circulation and the release of climate-relevant gases to the atmosphere.

Azerbaijani mud volcanoes are characterized by CH₄-dominated gases with variable contributions of CO₂ and higher hydrocarbons, wide ranges in C₁/C₂⁺ ratios, and isotopic signatures indicating predominantly thermogenic methane, locally affected by secondary microbial processes and mixing during migration. These systems commonly display significant and spatially focused CH₄ and CO₂ fluxes, reflecting active and deep-rooted fluid pathways, and highlighting an efficient transfer of deep fluids to the atmosphere and a potentially significant role in natural greenhouse gas emissions.

Northern Italian mud volcanoes are also characterized by CH₄-dominated gases with low content of CO₂ and wide ranges of C₁/C₂⁺ ratios, but isotopic signatures indicate a dominant secondary microbial methane origin, associated with biodegradation of hydrocarbons and subsequent methanogenesis, producing isotopically heavy CO₂. Soil gas flux measurements are generally lower than those reported for Azerbaijan mud volcanoes, suggesting that deep-sourced gases are largely attenuated by shallow processes and limited near-surface permeability.

The comparison highlights how mud volcanoes with similar surface expressions can reflect markedly different subsurface processes, fluid sources and degassing dynamics. These results emphasize the importance of integrated geochemical characterization and monitoring to 1) properly assess mud volcano activity, 2) their contributions to greenhouse gas emissions and 3) their environmental and societal implications including associated geohazards.