Seasonal variations in planktonic foraminifera at three sediment traps in the Subarctic, Transition and Subtropical zones of the central North Pacific Ocean

The distribution and flux of planktonic foraminifera in the North Pacific Ocean were studied for a 1 year period beginning in July 1993 using time-series samples collected in three sediment traps deployed in a north–south transect along 175°E longitude. Sediment trap locations allowed monitoring of faunas within the Subarctic, Transition and Subtropical water masses in this region. Seasonally high foraminiferal flux rates were recorded at the Subarctic site during periods of intense stratification of surface water accompanied by high fluxes of organic matter and biogenic opal contents. Faunal composition dominated by sinistral forms of Neogloboquadrina pachyderma and Globigerina umbilicata at the latter site characterized the Subarctic water mass feature. Data collected at the Transitional and Subtropical sites illustrate differences in seasonal flux rates and in relation to those recorded at the Subarctic site. Faunal composition at the Transitional water mass site was dominated by dextral phenotypes of N. pachyderma, whereas this taxon was absent at the Subtropical site. Also, during the period of warmest surface temperature (October), Globigerinoides ruber became dominant at the Transitional water mass site in contrast to the Subtropical water mass site where Globigerinoides sacculifer became dominant while associated with common occurrences of Globigerinoides tenellus and Globorotalia tumida. Much lower fluxes of planktonic foraminifera, organic matter and biogenic opal also marked Subtropical water. The distinctive differences in faunal composition and flux rates at the Transitional and Subtropical sites suggest that little if any exchange of surface water occurred between these two sites and, in particular, in the area representing the distal Kuroshio Extension.