Living benthic foraminifera of the Hess Rise and Suiko Seamount, central North Pacific

Rose-Bengal-stained benthic foraminifera in six pilot-core samples and one multicore sample collected from the Hess Rise and Suiko Seamount in August 1994 were studied in order to understand foraminiferal distributions betwee n two areas divided by an oceanic front in the central North Pacific. Sampl es from the Hess Rise were collected in depths of 2167-3354 m under the war m, saline Kuroshio Extension, while samples from Suiko Seamount came from d epths of 1811-1955 m under the cold, less-saline subarctic current. Sedimen t-trap results for the year prior to our sediment sampling show that organi c matter fluxes were about 2.5 times greater at Suiko Seamount than at the Hess Rise. However, the hydrographic structure between 1800 and 3400 m. bas ed on CTD observations, is almost the same at both sites. Temperature decre ases from 2.2 to 1.7 degreesC over the depth range of 1800-3400 m, salinity increases from 34.5 to 34.7, and the dissolved oxygen content gradually in creases from 1.5 to 3.0 mi l(-1). The faunal populations at the Hess Rise a re quite different from those at Suiko Seamount. The abundant species at th e Hess Rise are Epistominella exigua, Brizalina pacifica, Fursenkoina cedro sensis and Alabaminella weddellensis. These species characteristically inha bit phytodetrital aggregates deposited on an oligotrophic seafloor. The pop ulations at Suiko Seamount are dominated by Triloculina frigida, Lagenammin a cf. arenulata, Reophax subfusiformis, and Reophax scorpiurus. The reason for differences between these populations is unclear. However, the typical phytodetritus-dwelling species E. exigua is dominant at the Hess Rise, whic h is located in a subtropical area that has a pulsed supply of settling org anic matter in the spring. On the other hand, E. exigua is rare at Suiko Se amount, a subarctic site where there are more stable and greater fluxes of organic matter in summer and autumn. Occurrences of this species may be rel ated to the seasonally short supply of organic matter that reaches the seaf loor in the oceanic North Pacific. (C) 2001 Elsevier Science Ltd. All right s reserved.