Benthic foraminiferal assemblages of surface sediments were studied in the methane seepage area in the eastern margin of the Japan Sea to identify the effects of methane seep on foraminiferal distribution. Samples were collected from three areas, each including a methane-seep site. Their water depths range from 882 m to 1006 m in the Japan Sea Proper Water. At the methane-seep sites, there were significantly fewer living (stained) foraminifera than at other sites, and ratio of agglutinated foraminifera is small, showing the dominance of calcareous forms. Planktonic foraminifera are very abundant at seep sites. These suggest higher alkalinity at the sediment-water interface of the seep sites.
Cassidulina norvangi, Pseudoparrella takayanagii and Bolivina decussata increase while Trochammina pygmaea, Thalmannammina parkerae, Lagenammina tublata, Reophax cf. dentaliniformis, Saccammina huanghaiensis, etc., decrease at the seep sites. These species are usually common in the Japan Sea Proper Water. Their changes in abundance seem, in part, to relate to alkalinity because the former taxa are calcareous foraminifera and the latter are all agglutinated. There are fewer Bolivina pacifica, Stainforthia fusiformis, Nonionella globosa, and Globobulimina auriculata in the living population at the seep sites, although they are abundant in the total (living + dead) population. These taxa are well-known infauna and can survive in oxygen-depleted environments. This discrepancy may be explained by the mode of methane seepage; the present seeping is not severer than in the near past and is transient at each seep site. The effects of the present methane seeps extend only to less than 40-50 m from each methane-seep site.