Effect of deep-sourced CO2 release on marine silicate weathering as a carbon sink

Abstract

Natural carbon dioxide (CO₂) is produced through various geological and biological processes, including volcanic activity, tectonic movements, and microbial actions in marine sediments. Geochemical analyses of the pore water and gas samples obtained from four sites on the Korea Plateau in the East Sea combined with one-dimensional (1D) modeling was conducted to investigate the origins and behavior of CO₂ in the liquid and gas phases. The sites were classified into two groups based on CO₂ concentration and stable C isotopic composition. The first group, comprising sites 22GH-P02 and 22GH-P09, primarily exhibited biogenic CO₂ from microbial degradation of organic matter. 1D reactive transport (diffusion) modeling indicated that sulfate reduction dominated geochemical processes affecting the vertical distribution of aqueous and gaseous species in the shallow sediments of site 22GH-P02. The modeled biogenic CO₂ concentration in pore water reached up to 38,500 ppmv at a depth of 9.5 m below the seafloor (mbsf). The accumulated biogenic CO₂ contributed to authigenic carbonate precipitation based on the variation of Ca²⁺ and Mg²⁺ profiles and saturation state for carbonates with depth. In contrast, the second group, including sites 22GH-P04 and 22GH-P06, exhibited significantly higher CO₂ concentrations (up to 152,517 and 232,307 ppmv, respectively) with isotopic signatures indicative of a non-biogenic origin. These elevated levels suggest an external CO₂ flux likely originating from thermogenic sources associated with volcanic sills and lava flows, alongside basement faults impacting these sites. This interpretation is further supported by the chemical and isotopic signatures of CO₂ and diverging trends in Ca²⁺ and Mg²⁺ profiles, suggesting that processes other than authigenic carbonate precipitation dominate in Group II. Collectively, this study demonstrates the complex interactions between biogenic and non-biogenic CO₂ sources, highlighting the importance of these dynamics in marine carbon cycling and climate change predictions.