Azumi Kuroyanagi

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

Abstract 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 (50°01′N, 165°02′E), Site KNOT (43°58′N, 155°03′E), and Site 40N (39°60′N, 165°00′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 50°N 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 Nino events.

Distribution of Recent Benthic Foraminifera off Western Costa Rica in the Eastern Equatorial Pacific Ocean

Abstract. Benthic foraminifera provide essential information for paleobathymetric reconstructions. However, the modern distribution of benthic foraminifera, especially at depths below 1000 meters below sea level (mbsl), is still obscure in the offshore regions near Central and South America. To characterize the bathymetric scale in the eastern equatorial Pacific Ocean, we examined the depth distribution of benthic foraminifera using piston core samples taken off the coast of Costa Rica. Foraminiferal assemblages vary according to water depth: 1) U1 (mainly composed of Ammonia beccarii, Cancris sagra, Elphidium tumidum, Hanzawaia concentrica, Pseudononion basispinata, and Planulina exorna) represent inner shelf faunas (shallower than 50 mbsl). 2) U2 (mainly composed of Ammobaculites foliaceus, Bolivina striatula, Cassidulina minuta, Hanzawaia concentrica, Uvigerina incilis, Bulimina denudata, and Cancris sagra) is correlated with mid-shelf depth assemblages, from 50 to 100 mbsl. 3) U3 (mainly composed of Uvigerina incilis, Hanzawaia concentrica, Angulogerina semitrigona, Bolivina acuminata, Bolivina bicostata, and Cibicorbis inflatus) is assigned to outer shelf assemblages from 100 to 200 mbsl. 4) U4 (mainly composed of Bolivina humilis, Bolivina seminuda, Bolivina subadvena, Cassidulina tumida, Epistominella obesa, Angulogerina carinata, and Cibicorbis inflatus) is the upper bathyal faunas (200–600 mbsl). 5) U5 (mainly composed of Brizalina argentea, Uvigerina peregrina, Uvigerina auberiana, Brizalina seminuda, Bulimina striata, Epistominella smithi and Globocassidulina subglobosa) is the mid-bathyal faunas (600–1000 mbsl). 6) U6 (mainly composed of Uvigerina auberiana, Uvigerina peregrina, Brizalina argentea, Bulimina mexicana, Cassidulina carinata, Epistominella smithi, and Lenticulina cushmani) represents the lower bathyal assemblage (1000–2000 mbsl). 7) U7 (mainly composed of Uvigerina auberiana, Brizalina argentea, and Eubuliminella tenuata) represent upper abyssal faunas (2000–3000 mbsl). 8) U8 (mainly composed of Glomospira sp. A, Lagenammina arenulata, Chilostomella oolina, Hoeglundina elegans, Melonis barleeanum, Nonion affine, Oridorsalis umbonatus, Pullenia bulloides, and Uvigerina proboscidea) is characterized by deep-water cosmopolitan faunas (deeper than 3000 mbsl). On the basis of a comparison with several environmental parameters, dissolved oxygen concentrations are likely to be the most effective factor controlling foraminiferal depth distributions in the eastern equatorial Pacific especially below the oxygen minimum zone (OMZ). Around OMZ, nitrate concentration also might be related with the benthic assemblage due to the nitrate respiration.