Carbon isotopic composition of CO2 in interstitial soil gases from Southern Puna calderas, Central Andes: Decoding hydrothermal and shallow sources

Abstract

The Southern Puna hosts active geothermal systems associated with various volcano-magmatic settings, among which caldera-hosted systems stand out as promising targets for geothermal exploration. This study focuses on two key calderas that currently exhibit active geothermal manifestations: Cerro Blanco Caldera (CBC), the youngest caldera system in the region (4.2 ka), and Cerro Galán Caldera (CGC), the largest caldera system in this sector of the Andes. The main objective was to decode the contributions of deep (hydrothermal) and shallow (biogenic and atmospheric) sources to diffuse CO₂ emitted from the soil, using a three-component mixing model. The results revealed the presence of these three sources: deep hydrothermal, shallow biogenic, and atmospheric. At CBC and CGC, high CO₂ concentrations (>5000 ppm) and δ¹³C-CO₂ values around −5 ‰ (vs. V-PDB) indicate a dominant hydrothermal contribution. In contrast, more ¹³C-depleted values (down to −21.3 ‰) and lower CO₂ concentrations suggest microbial or soil respiration origin. Analysis of profiles in the soil revealed considerable variability, with deviations from the expected theoretical patterns in some cases. These anomalies are attributed to a combination of factors, including atmospheric contamination during sampling in low-permeability soils, isotopic fractionation under low gas flow conditions and local secondary processes, such as carbonate dissolution/precipitation. Despite these complexities, the combination of isotopic and concentration analysis robustly confirms the presence of hydrothermal CO₂ in the shallow soil gases in both calderas providing valuable insights for geothermal exploration and volcanic monitoring in the Central Andes.