Benthic foraminifers on the continental shelf and upper slope, Russian River area, Northern California

Abstract

We analyzed benthic foraminifers from 71 surface samples collected from the sea floor of the northern California continental margin adjacent to the Russian River. One hundred and six different taxa were identified, and Q-Mode factor analysis was used to identify assemblages. Six formainiferal assemblage factors explain 94% of the variation in the data matrix. The spatial distribution of the assemblages correlates with water depth, distance from shore, and dissolved oxygen at the sediment-water interface. The Inner Shelf Assemblage is characterized by Trichohyalus ornatissima, Rotalia columbiensis, Cassidulina limbata, Cibicides fletcheri, Elphidiella hannai and Elphidium sp. 1 and occupies water depths less than 50 m. The Middle Shelf Assemblage is characterized by Nonionella basispinata, Elphidium excavatum and Florilus labradoricus and occupies water depths between 50 and 90 m. A Middle Shelf to Upper Bathyal Assemblage is characterized by Uvigerina juncea, Globobulimina spp. and Nonionella basispinata and occupies depths between about 90 and 450 m. Two overlapping assemblages make up the Upper Middle Bathyal Assemblage. One of these assemblages is dominated by Uvigerina peregrina, and the other is principally composed of Epistominella pacifica, E. smithi, Bolivina spissa, Bulimina spicata, Buliminella tenuata and Globobulimina spp. These species are most abundant between water depths of 500 and 1,300 m and are associated with low-oxygen conditions. The Mid-Bathyal Assemblage is dominated by Uvigerina proboscidea and occurs on the slope at water depths ranging from 1,200 to 2,500 m.