Benthic foraminiferal distribution at Middle Valley, Juan de Fuca Ridge, a northeast Pacific hydrothermal venting site

Abstract

Fourteen benthic foraminiferal bearing surface samples collected from the Area of Active Venting (AAV) in Middle Valley at a depth of 2430 m at the northern end of Juan de Fuca Ridge, northeast Pacific Ocean, yielded 156 identifiable taxa. Sample sites were selected near active or recently active vents, where temperatures up to 274 °C have been measured. The sea floor beyond hydrothermal mounds is characterized by unaltered soft hemipelagic mud and is dominated by a calcareous foraminiferal assemblage which includes numerous allochthonous species derived from the shelf via turbidite flows into the valley. Within the AAV, agglutinated foraminifera dominate with proximity to hydrothermal vent activity. Species of the family Hormosinidae, subfamily Reophacinae, dominate in the hydrothermal habitat because of their success at colonization. Attached agglutinated foraminifera densely colonize the indurated sediment in close proximity to vent sites. However, like the macrofauna observed near hydrothermal vents, foraminifera do not venture into the extreme habitats where temperatures are greater than 20 °C. Where clam beds are found in association with active hydrogen sulphide venting, benthic foraminifera occur rarely. These beds occur in loose pyritic mud that is toxic to all but species (e.g. the clam Calyptogena) that co-exist symbiotically with sulphur-oxidizing bacteria. The sparse distribution of calcareous benthic foraminifera in the vent areas is caused by enhanced carbonate dissolution at pH values of 5.2. Low foraminiferal abundance and species diversity at the immediate site of active venting is attributed to rapid changes in physical and chemical conditions caused by circulating hydrothermal vent waters.