[A-CG53] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG53-06] Effects of CO₂ Gas Venting from the Seafloor along the Coast of Himeshima on Seafloor Pore Water

*Makoto Yamada¹, Masahiko Fujii², Yu Horiuchi³, Tsuneo Ono⁴, Shigeki Wada⁵ (1.Faculty of Economics, Ryukoku University, 2.International Coastal Research Center, Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Oita Himeshima Geopark Promotion Office, 4.Japan Fisheries Research and Education Agency, 5.Seto Inland Sea Carbon-neutral Research Center, Hiroshima University)

Keywords:Seafloor Pore Water, pH, CO2 seep, Shallow coastal waters

In several marine areas off the coast of Himeshima, Oita Prefecture, CO₂ venting from the seafloor has been observed. These CO₂ venting sites are regarded as natural analogs of the future marine environment predicted if CO₂ emissions from human activities are not significantly reduced. Consequently, they have attracted considerable attention in the marine sciences—particularly in ocean acidification research—both in Japan and abroad. As part of our investigation in Himeshima, we observed a marked decrease in seawater pH during low tide. Moreover, spatial pH distribution maps revealed low-pH water masses in locations slightly offset from the actual CO₂ venting points. If CO₂ gas directly lowered pH where it is emitted, we would expect a significant drop precisely at or immediately above the venting sites. However, our field observations did not consistently support this expectation, indicating that additional processes linked to CO₂ venting may be contributing to the observed pH reduction. In this study, we hypothesized that CO₂ venting lowers the pH of pore water in shallow seafloor sediment layers, which may then be released into overlying seawater under the influence of tides, giving rise to low-pH water masses. To test this hypothesis, we first attempted to characterize pH distributions in seafloor pore water. Because direct sampling in deeper water—where pH declines were observed during low tide—proved challenging, we selected a coastal site within the broader Himeshima CO₂ venting region where pore water collection was more feasible. We then investigated whether seafloor CO₂ venting affects the pH of sediment pore water. Within the surveyed venting zone, CO₂ emissions were confined to a distinct area. We extracted pore water samples from approximately 20 cm below the seafloor both inside and outside the venting zone and measured pH. Inside the venting zone, pH ranged from 6.8 to 7.4, notably lower than typical seawater values. Outside the zone, pH exceeded 8, closer to normal seawater levels (albeit slightly lower than open-ocean averages), and clearly higher than the pore water from within the venting zone. These findings suggest that a distinct low-pH water mass exists in the shallow subsurface of the CO₂ venting area. Although we have not yet confirmed whether this low-pH water emerges into the water column under tidal influences, our observations confirm the presence of water with significantly lower pH than seawater in the shallow sediments of the venting zone.