Regulation of deep carbon degassing by gas-rock-water interactions in a seismic region of Southern Italy

Abstract

This study is focused on fluids characterization and circulations through the crust of the Irpinia region, an active seismic zone in Southern Italy, that has experienced several high-magnitude earthquakes, including a catastrophic one in 1980 (M = 6.9 Ms). Using isotopic geochemistry and the carbon‑helium system in free and dissolved volatiles in water, this study aims to explore the processes at depth that can alter pristine chemistry of these natural fluids. Gas-rock-water interactions and their impact on CO2 emissions and isotopic composition are evaluated using a multidisciplinary model that integrates geochemistry and regional geological data. By analyzing the He isotopic signature in the natural fluids, the release of mantle-derived He on a regional scale in Southern Italy is verified, along with significant emissions of deep-sourced CO2. The proposed model, supported by geological and geophysical constraints, is based on the interactions between gas, rock, and water within the crust and the degassing of deep-sourced CO2. Furthermore, this study reveals that the Total Dissolved Inorganic Carbon (TDIC) in cold waters results from mixing between a shallow and a deeper carbon endmember that is equilibrated with carbonate lithology. In addition, the geochemical signature of TDIC in thermal carbon-rich water is explained by supplementary secondary processes, including equilibrium fractionation between solid, gas, and aqueous phases, as well as sinks such as mineral precipitation and CO2 degassing. These findings have important implications for developing effective monitoring strategies for crustal fluids in different geological contexts and highlight the critical need to understand gas-water-rock interaction processes that control fluid chemistry at depths that can affect the assessment of the CO2 flux in atmosphere. Finally, this study highlights that the emissions of natural CO2 from the seismically active Irpinia area are up to 4.08·10+9 mol·y-1, which amounts is in the range of worldwide volcanic systems.