Temporal variations in pore water and gas geochemistry in shallow marine sediments over four years following offshore CO2 injection in Yeongil Bay, South Korea

Carbon capture and storage (CCS) is a promising strategy to mitigate anthropogenic carbon dioxide (CO₂) emissions. In January 2017, 100 tons of anthropogenic CO₂ were injected into a deep saline aquifer beneath Yeongil Bay, South Korea, as part of an offshore CCS demonstration project. This study investigates the potential impact of CO₂ injection on pore water and gas geochemistry in shallow marine sediments, based on repeated sampling and analyses conducted before and up to four years after injection. No significant post-injection changes in total alkalinity (TA), δ¹³C of dissolved inorganic carbon, and gas-phase CO₂ concentrations are observed at the same sites, indicating negligible CO₂ leakage during the monitoring period. Instead, spatial variations in fluid and gas properties are attributed to natural geochemical processes, including microbial degradation of organic matter, carbonate precipitation, and marine silicate weathering (MSiW). Pore water is primarily derived from seawater with minor freshwater contributions, and methane (CH₄) is predominantly generated by microbial CO₂ reduction, as supported by isotopic evidence. Sites 16P01A, 16P17, 18P17, and 20P17, characterized by acoustic blanking seismic feature and shallow sulfate-methane transition (SMT; ∼1.0-1.2 m below the seafloor), exhibit high TA and CH₄ concentrations, along with downcore decreases in Ca²⁺ and Sr²⁺ concentrations, indicating active methanogenesis and carbonate precipitation. Additionally, high TA (>60 mM) and Mg²⁺ concentrations (>52 mM) at Sites 16P17 and 20P17 suggest enhanced MSiW. In contrast, other sites with non-acoustic blanking features display high SO₄^2- (>14 mM), Mn, and Fe concentrations, reflecting particular organic matter sulfate reduction coupled with Mn and Fe reduction. Overall, pore water and gas chemistry in Yeongil Bay have remained stable over the four years, with geochemical variations governed predominantly by natural sources and diagenetic processes rather than by anthropogenic CO₂ migration.