Biogeochemical properties of shallow-water CO2 seeps on Himeshima Island and Showa Iwojima Island, Japan

Abstract Volcanic gases erupt from the seafloor in several regions around Japan. Volcanological and geochemical gas seep studies have mainly focused on coastal shallow-water areas that are relatively accessible and important to human society. Shallow-water CO 2 seeps are thought to foreshadow future marine environments that may develop if CO 2 emissions are not drastically reduced. Thus, CO 2 seeps provide important insights for assessing and projecting the impacts of ocean acidification on marine ecosystems. This study is the first to investigate two shallow-water CO 2 seeps near Japan from the perspective of ocean acidification. We observed biotic transitions and reduced biodiversity around these CO 2 seeps, as well as high CO 2 concentrations, low pH, and low calcium carbonate saturation—conditions expected to occur by the end of this century unless anthropogenic CO 2 emissions are significantly reduced. These results suggest that, from a marine life conservation perspective, it is essential to mitigate ocean acidification through substantial reductions in anthropogenic CO 2 . Shallow-water CO 2 seeps serve as natural experimental sites that illustrate ocean acidification and its effects on marine ecosystems. Given that the shallow-water CO 2 seeps examined in this study are both located in geoparks, study tours and ecotourism field trips should utilize these sites to enhance awareness of the consequences of ocean acidification and climate change.