Constraining Soil Hydrothermal CO2 Degassing Across the Changbaishan Volcanic Area: Insights From 13C-14C Perspective

Abstract

Soil hydrothermal CO₂ represents important forms of diffuse degassing in volcanic areas, but the corresponding CO₂ output remains unclear. Herein, we investigated geochemical features of soil gas, that is, soil gas components, carbon isotopic compositions of soil CO₂ (δ¹³C-CO₂ and Δ¹⁴C-CO₂), and soil CO₂ fluxes in the Changbaishan volcanic area (CHV). The geochemical characteristics of soil gas in diffuse degassing structures differed significantly from those in volcano-affected structures. Specifically, soil CO₂ concentrations were up to 2.5 × 10⁵ ppm, with δ¹³C-CO₂ values ranging from −6.2‰ to −1.6‰ and Δ¹⁴C-CO₂ values spanning from −996‰ to −845‰, showing significant hydrothermal CO₂ signatures. The isotopic mixing model was constructed to estimate the contribution of hydrothermal, biogenic, and atmospheric CO₂ to soil CO₂. The results showed that soil CO₂ in diffuse degassing structures was predominantly derived from hydrothermal CO₂, while soil CO₂ in the volcano-affected structures was mainly derived from biogenic CO₂. Combined with the soil CO₂ fluxes, it was concluded that the hydrothermal CO₂ output through soil degassing was 5.83 × 10⁴ t yr⁻¹ in CHV, significantly lower than previous estimates. We found that hydrothermal CO₂ release in CHV exceeded the CO₂ consumption through silicate weathering, thereby acting as the carbon source. We also estimated the hydrothermal soil CO₂ output from global volcanoes based on literature-published values from 80-degassing volcanoes, which was ∼150 Mt yr⁻¹. Our results highlighted the importance of conducting ¹³C-¹⁴C investigations in global volcanic areas to replenish the database of hydrothermal CO₂ output in different forms and subsequently evaluate their climatic implications.