At Kujukuri Beach in Chiba, Japan, the Holocene series unconformably overlies the Pleistocene Kazusa Group, which was deposited in a deep-sea environment; notably, the strata contain natural gas dissolved in water. The region is known particularly for gas seeps (from underground plains and coasts) observed in paddy fields, rivers, and beach zones in the area. Off Kujukuri Beach, gas bubbles emerging from the seafloor are observed on the sea surface; gas plumes can be confirmed using a fish-finder echo sounder. The distribution of gas seeps across the coastal and offshore Kujukuri is investigated. In the southern region of Kujukuri Beach, numerous gas seeps are observed; these seeps are not distributed regularly, but appear sporadically. Their distribution pattern extends from north-northeast to south-southwest, and north-to-south. Offshore Kujukuri, there are two gas-plume concentration zones that extend northeastward from the shore to offshore areas. The southern gas-plume concentration zone is influenced by the strike and faulting of the Otadai Formation of the Kazusa Group. The central gas-plume concentration zone is influenced by the strike of the Umegase Formation of the Kazusa Group and the morphology of a buried valley formed during the postglacial age. A few notable effects of gas seepage in the region are observed. High concentrations of ammonia nitrogen (derived from natural gas brine discharged into the Kujukuri coast) are observed in the study area. Furthermore, high concentrations of phosphate ions are detected in groundwater at gas-seep locations on the beach. The release of PO43−-P from sediments under strongly reducing conditions induced by gas seepage may contribute to biological productivity. Satellite images reveal high chlorophyll-a concentrations on Offshore Kujukuri.
