Sulfate reduction using methane in sediments beneath a bathyal “cold seep” giant clam community off Hatsushima Island, Sagami Bay, Japan

Two sandy sediment cores (Cores D227-120 and D380) were collected from inside a deep-sea giant clam (Calyptogena soyoae) community off Hatsushima Island, western Sagami Bay, central Japan (35°59.9′N, 139°13.6′E; 1160 m deep) and a muddy sediment core (Core D227-202) was obtained from outside the community by the submersibleShinkai 2000. The chloride concentration of the pore waters is constant vertically and sulfate reduction using sedimentary organic matter occurs in Core D227-202 (21 cm long). The chloride concentrations are lower by 7% at the 7.5–9 cm depth in Core D227-120 (9 cm long) and by 3% at the 11–12 cm depth in Core D380 (16 cm long) than those of the overlying bottom waters in the cores from inside of the community. Sulfate concentration decreases remarkably and dissolved inorganic carbon, alkalinity, ammonium-N, and hydrogen sulfide concentrations increase significantly with increasing depth in Core D380.δ³⁴S values of sulfate ions increase from +20.5 to +35.3‰ andδ¹³C values of dissolved inorganic carbon decrease drastically from −7.0 to −45‰ with increasing depth from the top to the bottom of the core, although theδ¹³C values of the organic carbon of the sediments are−23.7 ± 0.9‰ in Core D380. These results indicate that sulfate reduction using methane is active within the sediments just beneath the living clams and that the hydrogen sulfide produced can be used by endosymbiotic sulfur oxidizing bacteria living in the gills ofC. soyoae in the community.