Seasonal changes in planktonic foraminifera in the northwestern North Pacific Ocean: sediment trap experiments from subarctic and subtropical gyres

Upper ocean environments such as seawater temperature, salinity, thermal structure in the water column, light intensity and food supply affect the assemblage of planktonic foraminifera. Since detailed information on planktonic foraminifera was not available for the northwestern North Pacific, we examined seasonal changes in fluxes and composition of planktonic foraminifera there. Data were collected by sediment traps deployed at three sites (Site 50N (50degrees01'N, 165degrees02'E), Site KNOT (43degrees58'N, 155degrees03'E), and Site 40N (39degrees60'N, 165degrees00'E)) in this area in order to better understand the relationship between the foraminiferal assemblage and surface-ocean environments. Although each planktonic foraminiferal species showed specific seasonal variations in flux, some exhibited similar flux profiles. In order to define these more accurately, correlations among species were calculated and the foraminiferal species classified into four groups: (1) Group A (subpolar species: Neogloboquadrina pachyderma, Globigerina quinqueloba, Globigerina bulloides, and Globigerinita glutinata); (2) Group B (subtropical and tropical species: Globigerinoides ruber and Globigerinoides sacculifer); (3) Group C (post-upwelling species: Neogloboquadrina dutertrei); and (4) Group D (deep-water species: Globorotalia scitula and Globorotalia truncatulinoides). The common environments for each period, based on foraminiferal production and composition, were observed among the three sites. Based upon predominant foraminiferal groups, total foraminiferal fluxes (TFFs), organic matter (OM) fluxes and hydrographic conditions, including sea-surface temperatures (SST) and thermal structure, the surface-ocean environments in the northwestern North Pacific could be generalized into five types. Type I is characterized by a dominance of Group B, with low TFFs under high SST, and Type II is marked by a high flux of Group A. On the other hand, Type III shows low TFFs and low OM fluxes due to low insolation during winter. Type IV represents a high flux of Group A, and Group D at Site 40N, under a well-mixed surface ocean, and Type V is characterized by a prominent peak of Group C under a developed thermocline. A comparison of annual mean foraminifera fluxes at 50degreesN between the northwestern (Site 50N) and the northeastern North Pacific (Station PAPA) during the normal cold mode demonstrated higher fluxes of foraminifera at Site 50N due to higher nutrient concentrations. The results also showed that G. bulloides is not always a proxy for upwelling and that N. pachyderma is not solely controlled by seawater temperature in the northwestern Pacific, where the surface water is enriched in nutrients. The fluxes and composition of foraminifera suggest that similar ocean environments are expected in large areas of the northern North Pacific, from Site KNOT to Station PAPA during the warm mode, which is affected by El Ni (n) over tildeo events. (C) 2002 Elsevier Science Ltd. All rights reserved.