Hydrothermal vents in turbidite sediments on a Northeast Pacific spreading centre: organisms and substratum at an ocean drilling site

The interaction of substratum, organisms, and hydrothermal venting was studied at Middle Valley on the northern Juan de Fuca Ridge, where venting occurs through turbidite sediments. Presently, hydrothermal activity in Middle Valley is concentrated within the 300 × 800 m High Heat Flow Area vent field, where it is associated with 10–15 m high sulphide mounds topped by active chimneys. Information on the biological nature of the area was derived from video and photographic records of four 1990 dives by the submersible Alvin, samples collected by Alvin, and bottom photographs by a towed camera. Fieldwork was carried out prior to commencement of 1991 Ocean Drilling Program activities at this site. Despite the limited nature of the collections, the Middle Valley fauna is remarkably specious. We documented 18 vent species new to the Juan de Fuca Ridge. Most interesting were the bivalves, which are rare elsewhere on the ridge. Two new species of vesicomyid clams and a new mussel species were present in the collections, as were a previously known mussel (Idasola washingtonia) and the gutless bivalve Solemya johnsoni. Other notable features are the close proximity of vent and non-vent organisms and the evident influence of surficial and subsurface geological features on venting and organism distribution. The number of new species and the small area occupied by vent sites led us to recommend several precautionary measures to the 1991 drilling program to prevent adverse effects on the vent fauna.